A Systematic Review of Biopolymer Phase Change Materials for Thermal Energy Storage: Challenges, Opportunities, and Future Direction

Abstract

This article systematically reviews biopolymer phase change materials (PCMs) for TES applications. The review was conducted based on Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines using databases from Scopus, Web of Science, and Google Scholar. The biopolymer PCMs are categorized as natural, synthetic, and hybrid (a combination of natural and synthetic). A total of 82 articles were included in the analysis. Several thermal properties, mechanical properties, advancements, and challenges are discussed. This article aims to review biopolymer PCMs and identify research gaps for future development. Natural biopolymer PCMs include lipid, lignin, polysaccharides, proteins, etc. Synthetic biopolymer PCMs include supramolecular, polyethylene glycol, polyurethane, polyrotaxane, polylactic acid, etc. Hybrid biopolymer PCMs combine natural and synthetic polymers with conductive fillers, balancing high latent heat with improved thermal stability and durability, although issues, like leakage and low conductivity, persist. It is found that biopolymers can be used as the core and supporting matrix of PCMs. Several cases and configurations of core, supporting matrix, and fillers in the development of PCM from biopolymers are discussed. This article also demonstrates that several natural, synthetic, and hybrid biopolymer PCMs hold promise for demanding TES applications due to their tunable properties and reliability. Biopolymer PCMs offer a sustainable alternative to petroleum-derived substances by minimizing environmental harm, cutting carbon emissions, and promoting a circular economy. This review also highlights several challenges, such as feedstock selection, purification and encapsulation, system compatibility, and standardization, that future research might address to enable scalable, safe, and cost-effective biopolymer PCM solutions.

1. Introduction

An increased interest in the reliance on renewable energy sources faces a critical challenge; their inherent variability and intermittent availability often fail to align with the consistent and reliable energy supply required for residential, commercial, or national grid demands. To address this issue, an effective energy storage system can be taken into account. Among the emerging solutions for energy storage systems, thermal energy storage (TES) offers a promising approach [1]. By efficiently storing excess thermal energy during periods of surplus and releasing it during shortages, TES systems can bridge the gap between energy supply and demand.

TES can be classified into three main classifications based on their operational mechanisms: sensible TES, latent TES, and thermochemical TES [2]. Compared to thermochemical TES, latent TES, which uses phase change materials (PCM), is likely considered more practical for many applications [3], with several types already being commercially available [4]. Unlike sensible TES, which stores energy primarily by changing temperatures, PCMs store and release thermal energy through phase transitions. This mechanism enables PCMs to store a significant amount of energy per unit mass during phase changes. PCMs also offer flexibility in certain applications, which makes them a preferred choice for a wide range of TES solutions [5].

PCMs store thermal energy through physical transformations between states of matter, such as solid–solid, solid–liquid, solid–gas, or liquid–gas transitions. However, in TES applications, solid–gas and liquid–gas phase changes are rarely used because they involve drastic volume expansions or pressure changes during the transition, which makes them unstable and difficult to manage [6]. In contrast, the solid–solid phase transition occurs when a material’s internal structure (e.g., its crystalline arrangement) reorganizes at a fixed temperature. This process avoids complications, like leakage or visible physical changes, and does not require supercooling (cooling below the freezing point to initiate solidification) [6]. Despite these advantages, designing TES systems that effectively use solid–solid PCMs remains technically challenging due to limitations in material availability and energy storage capacity. For these reasons, PCMs with solid–liquid transitions are currently the most practical and widely adopted option for TES. These PCMs efficiently absorb and release large amounts of energy during melting and solidification, offering a reliable balance of high storage capacity, ease of integration, and operational stability.

Recent advancements in sustainable materials are steering attention toward biopolymer PCMs as eco-friendly alternatives to conventional synthetic options [7]. Unlike conventional PCMs, which often rely on petroleum-derived or inorganic components, biopolymer PCMs are produced from renewable or biodegradable sources. Many biopolymer PCMs have adopted solid–liquid phase change and solid–solid mechanisms. Their eco-friendly nature and low toxicity address growing environmental concerns associated with synthetic PCMs. Moreover, polymers can also be processed using scalable, cost-effective manufacturing techniques, which are critical for enabling mass production and widespread adoption in industries and for several applications. Several biopolymer PCMs are used in building materials to enhance thermal mass and provide passive thermal regulation (as building insulation). They are also used in some electronics cooling applications to manage heat dissipation. Some biopolymer PCMs are applied in transportation to regulate temperatures (as insulation). There are many more advantages of biopolymer PCM utilization. However, it seems that there is still a lack of studies on biopolymer PCMs.

Table 1. A summary of recent review articles discussing biopolymer PCMs.

Authors, Year	Focus Area of Studies	Research Gap
Prajapati et al., 2019 [8]	This review discussed the development and application of biodegradable polymers to stabilize PCM for TES.	A lack of systematic studies that compare different biopolymer PCMs in terms of their thermal performance, stability, mechanical integrity, and so on. There is limited exploration into the processing techniques, design, and flexibility of biopolymer PCMs.
Zhang et al., 2022 [9]	The study reviewed how biomass materials can be combined with conventional PCMs (like polyethylene glycols, paraffins, and fatty acids) to overcome common issues such as leakage during phase transition.	Although some additional functions, such as photothermal conversion, thermochromism, and magnetothermal conversion, were briefly discussed, there is a lack of studies on the flexibility of PCM based on biomass materials. There is limited work on integrating these additional features with biopolymer PCMs for TES. More studies are needed to improve encapsulation techniques for PCMs based on biomass materials.
Baylis and Cruickshank, 2023 [10]	The article discussed the biopolymer PCMs for passive TES in building applications. Some integration methods and performances were discussed.	There is a lack of studies in the experimental validation to assess the long-term stability, performance, and fire safety. The flexibility of biopolymer PCMs is not comprehensively discussed.
Dutta et al., 2023 [11]	The study briefly discussed developing and evaluating biopolymer PCMs for TES applications. A few techniques and composite designs were briefly reviewed.	There is a lack of comprehensive studies on biopolymer PCMs and their flexibility. The optimization of, and strategies to improve, biopolymer PCMs are not comprehensively discussed.
Pielichowska et al., 2024 [12]	The review focused on reviewing the state of the-art in applying biopolymers to develop more sustainable PCMs for TES.	Although many encapsulation and stabilization techniques have been developed at the laboratory level, methods to scale up these processes for industrial production and practical applications are still underexplored. There is also a lack of systematic studies on the flexibility offered by biopolymer PCMs. The compatibility between biopolymers and various additives is still underexplored.
Liu et al., 2025 [7]	The study integrates sustainable polymer with PCMs, focusing on preparation methods, encapsulation strategies, performance enhancement, advanced applications, and recyclable polymers.	There is a lack of information related to a comprehensive evaluation of how sustainable polymers can be optimally integrated with PCMs as TES. There is a lack of systematic studies on the flexibility offered by biopolymer PCMs.

provides a summary of recent review articles that explore biopolymer PCMs. The focus of each study and the research gaps are discussed, highlighting the progress of developments in biopolymer PCMs for TES. There appears to be limited exploration of biopolymer PCMs for TES integrated with thermal energy conversion systems such as the organic Rankine cycle or other thermal power plants. Interest in research in TES for waste heat recovery has increased, as this technology can be used to recover low-to-medium-temperature waste heat or other renewable energy (i.e., focused on thermal energy). Despite the potential of waste heat recovery to enhance energy efficiency, the role of biopolymer PCMs in optimizing these systems has not been thoroughly investigated. Therefore, this article aims to evaluate the potential of biopolymer PCMs for TES in advancing thermal energy conversion technologies. Moreover, this article systematically analyzes the advantages, challenges, and scalability of biopolymer PCMs for TES. This article explores future opportunities for integrating biopolymer PCMs into TES and several configurations of thermal energy conversion systems. Moreover, several challenges that are crucial to the development of biopolymer PCMs will be discussed. This study’s approach may help to give insights into the direction of future research on this topic.

2. The Methodology of Review

This review adopted the methodology of the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) [13]. It ensures a transparent and reproducible review process by systematically including all relevant studies, which is especially crucial for research on biopolymer PCMs. The thorough inclusion of relevant studies in the literature in this field is vital to uphold the accuracy and credibility of the review outcomes. Figure 1 illustrates the methodology used in this review.

This review conducted a comprehensive literature search using databases such as Scopus, Web of Science (WoS), Google Scholar, and other relevant sources. The targeted keyword search was: “phase change material AND biopolymer AND thermal energy storage,” aiming to identify relevant research papers. A total of 208 articles were initially collected (20 from Scopus, 58 from WoS, and 130 from Google Scholar and other sources). A deduplication process was performed to ensure accuracy, identifying 16 duplicate entries. After removing these, 192 unique articles remained for the initial screening. Based on abstract reviews, 69 articles were excluded for being irrelevant to the scope of this review, leaving 123 articles for further examination. Upon full-text reading, 41 articles were excluded because they did not provide significant insights into biopolymer PCMs. As a result, a total of 82 articles were considered relevant and included in the final review.

This review article is organized as follows. After the introduction, Section 2 describes the methodology of the review. Section 3 outlines the developments of biopolymer PCMs for TES. This section begins with an overview of biopolymer PCM classifications and highlights recently developed materials. Section 4, Section 5 and Section 6 explore natural, synthetic, and hybrid (natural–synthetic) biopolymer PCMs. In these sections, the flexibility parameters of biopolymer PCMs, such as adaptability to temperature fluctuations, compatibility with encapsulation methods, mechanical durability, and so on, are discussed. Their properties play a key role in their successful integration into TES, which enables solutions for diverse energy storage needs. Moreover, Section 7 analyzes how biopolymer PCMs can be incorporated into TES for thermal energy conversion systems. It discusses various TES configurations (e.g., encapsulated, composite-based systems, and so on) and technologies that emphasize their suitability for applications. Section 7 also discusses future research directions on this topic, addressing research gaps and highlighting opportunities to enhance thermal performance through several technologies. At the end of the study, Section 8 concludes the review article by summarizing some key information and reporting on the significance of the study.

3. Brief Description Related to Biopolymer Phase Change Materials and Their Classifications

PCMs, often referred to as latent TES materials, can store and release significant amounts of energy through the formation and breaking of molecular bonds [14]. Their elevated latent heat of fusion contributes to considerable TES density. The ability of these materials to store and release thermal energy while maintaining a nearly constant temperature makes them valuable across various commercial applications [15]. This includes their use in heating and cooling systems for buildings, thermal comfort textiles, solar energy systems, thermal shields of electronic gadgets, power generation systems, and food storage [8,15]. For instance, in smart textiles, PCMs absorb and release latent heat during phase transitions, maintaining a stable microclimate around the body. Because of this temperature-regulating ability, PCMs are well-suited for use in clothing and other items worn on the body, providing comfort even when the surrounding temperature changes [16].

However, one of the limitations of PCMs is the risk of liquid leakage during the phase transition process [17]. As PCMs undergo melting, insufficient encapsulation or stabilization may lead to leakage from the storage system. This leakage reduces thermal storage capacity, accelerates material degradation through corrosion or chemical interactions, and poses safety risks such as flammability or environmental contamination. In construction composites, it can also compromise structural integrity by weakening mechanical strength [18]. To address this issue, polymeric materials are utilized, serving as a supportive material when combined with the PCMs, improving their durability and ability to manage heat [19,20]. Polymers enhance the flexibility of PCMs through structural encapsulation, chemical crosslinking, and so on [21,22]. Flexibility, a defining property of PCMs employed in TES and management, is characterized by the material’s ability to undergo deformation without structural failure in response to external forces [23]. Flexible PCMs use the support matrix structure to control fluidity during phase changes and maintain a soft mechanical response [23].

Despite their benefits, conventional polymers present increasing challenges due to their reliance on non-renewable resources and their role in atmospheric carbon emissions. Furthermore, their ability to resist degradation results in considerable environmental pollution, negatively impacting ecosystems, wildlife, and human health [24]. In response, biopolymers derived from renewable biological resources have emerged as a promising sustainable alternative.

3.1. Classification of Biopolymers

The increasing need for sustainable and biodegradable solutions has led to a considerable focus on biopolymers across various applications. According to the definition, biopolymers are large molecules constructed from repeating structural units formed by living organisms [25,26]. Natural sources of biopolymers are derived from plant-based materials, animal derivatives, microbial products, algae, and waste from agricultural activities [25,27]. Biopolymers are recognized for their natural abundance, unique molecular architecture, desirable mechanical properties, and renewable origins. They also exhibit excellent biocompatibility and biodegradability while remaining lightweight, cost-effective, and non-toxic [28].

Biopolymers are classified into two categories: natural and synthetic. Natural biopolymers are polymers that are directly obtained from nature without significant chemical modification or in a minimally processed form [29,30]. Several examples of natural biopolymer include polysaccharides, polyphenolics, lignocellulose, proteins, and lipids. Lignocellulose is an abundant and renewable plant biomass composed mainly of cellulose (35–55%), hemicellulose (20–40%), and lignin (10–25%), along with other polar and non-polar compounds [31]. It is the most widely available biomass for fuel production and ranks as the fourth major energy source after coal, petroleum, and natural gas. While cellulose and hemicellulose can be hydrolyzed into sugars, such as glucose, xylose, arabinose, and galactose, lignin is more resistant to biodegradation [32]. Synthetic biopolymers are synthesized from renewable natural resources with chemical modifications, and biological fermentation [33,34]. Synthetic biopolymers can be divided into several classes, such as supramolecular, polyether, aliphatic polyester, polyolefin, thermosetting, branched, and synthetic eumelanin. Supramolecular polymers are self-assembled polymeric structures held together by non-covalent interactions such as hydrogen bonding, ionic interactions, metal–ligand coordination, π–π stacking, and host–guest complexation [35]. These interactions allow for the formation of large polymer-like aggregates with dynamic features, such as responsiveness to external stimuli and self-healing capabilities, making them ideal for developing adaptable materials [35,36]. Unlike conventional covalent polymers, supramolecular polymers combine desirable mechanical properties with unique characteristics such as reversibility, adaptability, and environmental responsiveness. These qualities have led to their application in diverse fields including drug delivery, molecular electronics, sensors, cell recognition, and sustainable materials [37].

The development of advanced materials necessitates addressing inherent limitations within individual biopolymer classes. Addressing the limitations of individual polymer classes, the strategic development of hybrid biopolymers through the integration of natural and synthetic components is crucial for advanced materials. Natural biopolymers offer excellent biocompatibility, bioactivity, and hydrophilicity but often lack robust mechanical properties, consistent quality, and processability [38]. Contrarily, synthetic biopolymers provide tailored mechanical strength and processability but may exhibit poor bioactivity and cell interactions. By integrating these materials, hybrid biopolymers aim to create materials with enhanced overall performance, leveraging the strengths of both while mitigating their weaknesses [38]. This synergistic approach allows for the design of materials with improved mechanical integrity, controlled degradation, enhanced bioactivity, and better processability [38]. By strategically blending natural and synthetic components, researchers can tailor material properties to meet the demanding requirements for PCM applications. Figure 2 illustrates the classification of biopolymer materials into natural, synthetic, and hybrid (natural–synthetic combinations).

3.2. Biopolymer Materials

3.2.1. Natural Biopolymer Materials

As a widely accessible polysaccharide, starch is derived from plants cultivated across most temperate zones [39]. It is the second most abundant natural polymer after cellulose [40]. Starch offers several advantages: biodegradability, biocompatibility, non-toxicity, sustainability, flexibility, and low cost. Starch has emerged as a promising alternative to petroleum-based polymers for the development of environmentally friendly materials [41,42]. It is sourced from a wide range of edible plants such as potato, cassava, corn, rice, wheat, barley, and sweet potato [41,43].

Starch is a semicrystalline homopolysaccharide composed entirely of glucose units, structured into supramolecular granules with crystallinity ranging from 20–40% [41]. It consists primarily of two polysaccharide components: amylose and amylopectin [44]. Amylose is mostly linear, formed by α(1→4)-linked D-glucopyranosyl units, and typically accounts for 15–35% of starch granules. Although largely unbranched, some high molecular-weight amylose chains may contain limited branching [41,44]. In contrast, amylopectin is a highly branched molecule, composed mainly of α(1→4) linkages with approximately 5% α(1→6) bonds occurring every 24–30 glucose units. These structural differences influence their functional roles in starch granules; amylose forms amorphous lamellae, while amylopectin contributes to the crystalline regions [44]. In addition, cyclodextrins (CDs), also known as cycloamyloses, cyclomaltases, or Schardinger’s dextrins, are cyclic oligosaccharides derived from starch through enzymatic conversion [45,46]. These polymers feature a ring-like structure of α-D-glucopyranose units. The classification of CDs (α, β, γ) depends on the number of glucose units they contain: six for α-CD, seven for β-CD, and eight for γ-CD [45].

Cellulose is one of the most abundant natural polymers on Earth and is predominantly found in the cell walls of plants, as well as in algae and certain bacterial species [47,48]. Cellulose is a linear homopolysaccharide of β-D-glucopyranose units linked by β(1→4) bonds. These units, with three hydroxyl groups, form strong hydrogen bonds, leading to stiff chains that assemble into strong, insoluble, chemically reactive microfibrils and higher structures [49]. Its unique properties, such as hydrophilicity, chirality, biodegradability, and functionality, make it widely applicable in both traditional and advanced technologies [49].

Beyond its native form, cellulose can be chemically modified into various derivatives with improved solubility, film-forming ability, and functional flexibility, making it useful in a wide range of industrial applications. Common examples include carboxymethyl cellulose (CMC), a water-soluble thickener used in coatings and absorbent materials; methyl cellulose, which forms gels in cold water and is applied in paints and adhesives; hydroxyethyl cellulose, valued for its solubility and thickening properties in cleaning agents and lubricants; ethyl cellulose, which is water-insoluble and used in moisture-resistant coatings and composites; and cellulose acetate, a thermoplastic derivative used in films, molded items, and biodegradable products [50].

Another notable form is bacterial cellulose, a unique cellulose derived from microorganisms like Komagataeibacter xylinus. Bacterial cellulose features a nanofibrillar network with high crystallinity (80–90%) and a polymerization degree of 7000–16,000, forming a porous architecture that holds over 99% water, resulting in stable hydrogel structures. Its inherent mechanical strength, chemical stability, biodegradability, and biocompatibility support its fabrication into diverse forms such as films, fleeces, spheres, and hollow particles [51]. During biosynthesis, bacterial cells extrude cellulose chains that aggregate into elementary nanofibers (~1.5 nm in diameter), which further assemble into larger ribbons with widths ranging from 50 to 80 nm. This nanoscale width is crucial, as it influences the porosity, surface area, and interfacial interaction of bacterial cellulose with other materials [50].

Chitin, a naturally occurring polysaccharide and one of the most abundant biopolymers, is primarily found in the exoskeletons of arthropods (e.g., shrimps, crabs, lobsters, and insects), cell walls of fungi and yeast, and to a lesser extent, in organisms such as sponges, corals, and certain bacterial membranes [52,53]. Chemically, chitin is a linear polymer of N-acetyl-D-glucosamine, linked through β(1→4)-glycosidic bonds, structurally resembling cellulose with an acetamido group substituting the hydroxyl group at carbon-2 [54]. Chitin is a structured crystalline polymer in three forms (α, β, γ) that differ in unit cell size, packing, and the number of chains per cell. Hydration levels and the polarity of adjacent chains in successive layers also influence these variations [53]. Naturally occurring in ordered microfibrillar arrangements, chitin plays a structural and reinforcing role across both the animal and fungal kingdoms [53].

One of the most valuable derivatives of chitin is chitosan, which is obtained through alkaline or enzymatic deacetylation of chitin [52]. It offers key advantages, such as biocompatibility, biodegradability, and non-toxicity, which support its broad use in sustainable material development. In natural environments, chitosan can be broken down by enzymes like lysozymes into harmless byproducts, reinforcing its potential in eco-friendly applications [55].

Alginate, also known as alginic acid or algin, is a naturally derived, water-soluble, non-toxic, and non-irritant anionic polysaccharide found abundantly in the cell walls of brown algae [56,57]. Brown algae are cultivated across both hemispheres, with significant production regions, including Australia, Chile, China, Denmark, France, India, Indonesia, Ireland, South Africa, Spain, the United Kingdom, and the United States of America [58]. Alginate is extracted from various brown algae species such as Ascophyllum nodosum, Laminaria hyperborea, Laminaria digitata, Laminaria japonica, and Microcystis pyrifera [56].

Sodium alginate is a water-soluble, anionic polysaccharide extracted from the cell walls of brown algae by converting alginic acid into sodium salt using alkaline solutions such as sodium carbonate or sodium hydroxide [59,60]. Its hydroxyl and carboxylate groups provide non-toxicity, hydrophilicity, biocompatibility, and ion-exchange capability [59,61]. This renewable, biodegradable, and cost-effective biopolymer is easily modified and is now widely used beyond traditional roles, forming hydrogels, aerogels, foams, and films for water treatment and energy storage. These materials are shaped into beads, monoliths, or membranes for functional performance. Brown seaweeds, like Laminaria digitata and Ascophyllum nodosum, are preferred sources due to scalable extraction via washing, alkaline processing, and salt precipitation [61].

Alginate is a linear anionic polysaccharide composed of β-D-mannuronic acid (M) and α-L-guluronic acid (G) units linked by 1→4 glycosidic bonds. These uronic acid monomers are arranged in homopolymeric (MM, GG) and heteropolymeric (MG, GM) blocks, where the sequence and ratio vary depending on the biological source. The structural distinction at the C-5 position (equatorial in M and axial in G) results in different conformations; M-blocks form more flexible, linear chains, while G-blocks introduce buckling and stiffness. This block distribution governs key properties such as viscosity, gelation, and mechanical strength. The detailed stereochemical arrangements are illustrated in Figure 3.

Pectin is a naturally occurring plant-based polysaccharide with significant potential for application in biodegradable plastic development due to its abundance, low cost, renewability, and favorable physicochemical properties [62,63]. Pectin, a complex polysaccharide found in plant cell walls, is primarily extracted from citrus and apple peels or pulp due to their high yield and availability as byproducts of food processing [62]. However, various agricultural wastes, including banana, mango, and pomelo peels, as well as cacao waste and coffee pulp or grounds, have also been identified as potential alternative sources [62]. Recent studies have reported promising pectin yields from sugar beet pulp, reaching up to 37% using microwave-assisted extraction, as well as from Jabuticaba peel (22%) and Passiflora tripartita peel (23%) [62].

Structurally, pectin is a heterogeneous biopolymer composed primarily of α-(1→4)-linked D-galacturonic acid (GalA) residues, which form the backbone of the homogalacturonan region, often referred to as the “smooth” region. This domain accounts for approximately 60–65% of the molecule and is mostly linear. In contrast, the rhamnogalacturonan I (RG-I) region, also known as the “hairy” region, has a branched structure composed of repeating disaccharide units of GalA and rhamnose, often carrying neutral sugar side chains [63,64]. The degree of methyl esterification of the carboxyl groups in GalA residues significantly affects the solubility, gelling capacity, and film-forming ability of pectin, leading to its classification into high-methoxyl (DE > 50%) and low-methoxyl (DE < 50%) types, with high-methoxyl pectin being more commonly used in industry [62,65]. The characteristics and physicochemical properties of these pectins are illustrated in Figure 4.

Protein, alongside polysaccharides and polynucleotides, represents a class of renewable biopolymers derived from plant or animal sources and is increasingly utilized for sustainable material development [66]. Protein is composed of monomeric amino acid units linked by amide bonds into polypeptide chains [67]. Protein exhibits desirable properties such as biocompatibility, biodegradability, processability, and tunable mechanical strength, making it attractive for various material applications [67].

Both animal-derived proteins (e.g., silk, collagen, gelatin, keratin, whey protein) and plant-derived proteins (e.g., soy protein, corn zein, wheat protein) are employed in material production [67,68]. Plant proteins exhibit greater hydrophobicity, enabling their use in hydrogels and nanoparticle systems without toxic crosslinkers, and offer a cost advantage over animal proteins [66].

Collagen and gelatin are prominent animal-derived proteins widely applied in biopolymer-based materials. Collagen, a major structural protein in the extracellular matrix of vertebrates, constitutes approximately 30% of total body protein mass and is found in tissues such as skin, tendon, bone, cartilage, blood vessels, and cornea tissues [39,67]. Collagen, a primary protein in the animal extracellular matrix, is essential for the structural integrity and proper functioning of tissues [67]. Collagen can be extracted from animal by-products of the livestock industry and processed into biocompatible materials such as films, membranes, hydrogels, and sponges [67]. Gelatin is a natural and biodegradable protein derived from the partial hydrolysis of collagen, a structural protein abundantly found in animal connective tissues such as skin, bones, and tendons [67,69]. Gelatin’s biodegradability, its breakdown by microorganisms via enzymes, like proteases, into amino acids, is influenced by crosslinking, environmental conditions, and the presence of these enzymes. This process occurs in diverse environments such as soil, water, and the human body [69].

Silk is a natural protein polymer produced by Bombyx mori silkworms, orb-weaver spiders, and other Lepidoptera species [48,70]. Commercial silk is primarily derived from domesticated Bombyx mori (mulberry silk) and wild varieties such as Eri, Muga, and Tussar silks from Saturniidae species [70]. Spider silk offers superior mechanical strength but is commercially limited due to low yield, making B. mori silk the most widely used [70]. Silk mainly comprises two proteins: fibroin, which provides structural strength, and sericin, which acts as a binding agent [70].

Lipids are a diverse group of biomolecules primarily characterized by their solubility in nonpolar organic solvents and general hydrophobicity rather than by a shared structural motif [71]. They are typically extracted from biological tissues using solvents, such as chloroform or methanol, to separate them from proteins and carbohydrates due to their differential solubility [72]. Defined by both their solubility properties and structural variability, lipids encompass simple lipids (e.g., triacylglycerols, waxes), compound lipids (e.g., phospholipids, glycolipids), and derived lipids (e.g., sterols, fatty acids), each serving distinct biological roles [72,73].

Fatty acids, which vary in their hydrocarbon chain length and the number and location of double bonds, are frequently found as the building blocks of more complex lipids [74]. Fatty acids are typically classified by their degree of saturation and carbon chain length [75,76]. Saturated fatty acids have no double bonds in their hydrocarbon chains. Palmitic acid, stearic acid, and lauric acid are examples of saturated fatty acids [75,76]. Palmitic acid and stearic acid are the primary saturated fatty acids detected in various plant-derived oils, with palmitic acid ranging from 16% to 20% in olive and tomato seed oils, and stearic acid typically below 4% [76]. Monounsaturated fatty acids contain one double bond. Oleic acid is found in high concentrations in olive seed oil (54.4%) and tomato seed oil (up to 72%), representing the dominant monounsaturated fatty acids in these matrices [76]. Two or more double bonds characterize polyunsaturated fatty acids. Linoleic acid was reported as the major polyunsaturated fatty acid in several oils, including sesame and rice bran oils, while alpha-linolenic acid was found in notable levels in wheat bran (9.5%) and rye bran oils (7.6%) [76]. In marine lipid sources, long-chain polyunsaturated fatty acids, such as eicosapentaenoic acid and docosahexaenoic acid, were shown to be present in significant amounts, particularly in fish liver, viscera, and trimmings [75,76]. Short-chain fatty acids, such as acetate, propionate, and butyrate, contain fewer than six carbons and are mainly produced by gut bacteria fermenting dietary fiber [75]. The long-chain fatty acids (palmitic, oleic, linoleic, eicosapentaenoic acid, and docosahexaenoic acid) are consistently found across both plant-based and marine oils [75,76].

Lignin, recognized as one of the most abundant renewable biopolymers on Earth, plays a vital structural role in plant cell walls and is one of the largest natural sources of aromatic compounds [77,78]. It is primarily generated as a by-product in the pulp and paper industry [77]. Lignin is a polyphenolic biopolymer composed of three monolignol units: p-coumaryl, coniferyl, and sinapyl alcohols. These units form a crosslinked three-dimensional network consisting of p-hydroxyphenyl, guaiacyl, and syringyl structures [78,79]. This structural complexity imparts high thermal stability, antioxidant activity, and mechanical strength, supporting its potential for chemical modification and valorization [78,80].

3.2.2. Synthetic Biopolymer Materials

Polyethylene glycol (PEG) is a water-soluble polymer composed of repeating ethylene oxide units. It is non-toxic, chemically inert, non-corrosive, and widely used in several industrial applications due to its tunable properties based on weight (e.g., PEG 10,000 refers to a molecular weight of ~10,000 g/mol). The molecular weight is determined by the reaction time and the catalyst. PEG exhibits a high phase change enthalpy [81,82]. It remains stable over repeated phase transitions and has low vapor pressure. However, pure PEG suffers from low thermal conductivity and leakage during phase transitions. Several thermal properties of pure PEGs are summarized in

Table 2. Thermal properties of several PEG samples taken from [83].

PEG Molecular Weight	Tm [°C]	hm [J/g]	k [W/m·K]
PEG 1000	34.75	154.40	0.29
PEG 1500	47.23	161.43	0.31
PEG 2000	50.77	165.43	0.31
PEG 4000	55.95	173.62	0.33
PEG 6000	59.54	179.70	0.34
PEG 8000	59.74	177.53	0.33
PEG 10,000	58.01	182.86	0.33
PEG 12,000	60.93	173.40	0.32
PEG 20,000	62.27	168.50	0.32

.

Polyurethanes (PUs) are a class of versatile polymeric materials formed by the reaction between diisocyanates and polyols. They can be tailored for a wide range of applications by varying the raw materials and processing techniques. PU is a versatile, durable polymer used in foams, coatings, adhesives, and so on [84]. It offers mechanical strength, toughness, elasticity, and so on. PU is synthesized primarily via polyaddition reactions between diisocyanates and polyols, often with the help of catalysts, chain extenders, surfactants, and other additives, depending on the desired application. There are classifications of PUs, which are illustrated in Figure 5. Classifications includes thermoplastic, flexible, rigid, adhesives, ionomers, coatings, binders, sealants, elastomers, and so on [85].

The classification of PUs offers advantages for PCM and TES applications. Rigid PUs provide structural support and insulation for building-integrated TES, while flexible and thermoplastic PUs enable PCM encapsulation in wearable and electronic systems. Pus, with their shape-memory and thermal responsiveness, show promise for advanced innovative TES applications. The PU’s tunable properties and compatibility with various fillers make it a good candidate for enhancing PCM and TES.

Polyrotaxane (PLR) is a type of supramolecular polymer structure made by threading cyclic molecules, such as α-CD, onto a linear polymer chain such as polyethylene oxide (PEO) [86]. The synthesis of PLR illustrated in Figure 6 involves a green, one-step method that avoids complex chemical modifications. In water, α-CD molecules self-assemble onto the PEO backbone through host–guest interactions driven by hydrogen bonding and hydrophobic effects. Unlike traditional PLRs, no end-capping agents are used, simplifying the process and reducing environmental impacts. Physical entanglement and hydrogen bonds between α-CD and PEO stabilize the structure, eliminating the need for additional crosslinking steps.

PLR can be a good candidate for PCM due to its enhanced mechanical properties, good form stability, and high TES capacity, of up to 104 J/g [86]. It also shows minimal heat loss (<2.07%) and stable performance over 60 thermal cycles. PLR integrates smart functionality, including shape memory properties with ~90% strain fixity and ~99% recovery, ideal for heat-responsive applications in flexible electronics or smart packaging.

Polylactic acid (PLA) is a biodegradable synthetic biopolymer derived from lactic acid, a naturally occurring compound obtained by fermenting sugars from starch-rich biomass like corn and sugarcane [87,88]. PLA is a linear aliphatic thermoplastic polyester with a renewable origin and complete biodegradability [88,89]. This bio-based origin and biodegradability position PLA as a greener alternative to conventional petrochemical-based polymers such as polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), and polystyrene (PS) [88,89].

PLA is a good candidate for diverse applications due to its environmentally friendly profile, thermoplasticity, non-toxicity, and reliable mechanical performance. Its advantages include high recyclability, biocompatibility, low energy consumption during manufacturing, and versatile processability into various shapes. Moreover, PLA exhibits good transparency, making it suitable for sustainable and aesthetically appealing packaging solutions [67].

3.2.3. Functional Roles of Biopolymers in PCM Systems

PCM systems are developed by combining either organic or inorganic, with additional materials, such as support matrices and functional fillers, to overcome the inherent limitations of pure ones. These composite systems typically comprise three key components: the core PCM, which stores and releases thermal energy during phase transitions; a support matrix that prevents phase leakage; and filler or enhancer materials that improve thermal conductivity and mechanical stability [90].

Core PCM serves as the primary substance in TES systems, responsible for absorbing and releasing latent heat during the solid–liquid phase transition. These materials play a key role in determining the thermal performance of the system, with selection criteria based on properties such as melting point, latent heat capacity, and thermal reliability. However, despite their inherent advantages, core PCMs often suffer from limitations. To mitigate these challenges and enhance overall functionality, composite PCM systems are typically engineered by integrating core PCMs with structural support matrices and thermally conductive fillers.

Shape stabilization is a promising approach to prevent leakage in PCMs by incorporating them into a support matrix, forming what are known as shape-stabilized PCMs [91]. This support matrix is crucial as it encapsulates the PCM, preventing it from leaking even when it melts during phase transformation. In addition to acting as a containment, the materials forming this support matrix can also enhance the PCM’s thermal conductivity, thereby improving the efficiency of heat storage and release [91]. Diverse materials are employed to create this support matrix, with common categories including polymers, porous materials, and nanomaterials [92].

PCMs often suffer from inherently low thermal conductivity and diffusivity. To overcome this limitation and enhance their thermal performance, particularly within composite systems, thermally conductive fillers are frequently incorporated [93]. Among the most common fillers are carbon-based materials, prized for their high thermal conductivity and low density. Examples include carbon nanotubes, carbon nanofibers, natural graphite, expanded graphite (EG), nano-graphite [14], and exfoliated graphite [93]. Various ceramic materials are also utilized as fillers such as silicon carbide, alumina (Al₂O₃), aluminum nitride, and boron nitride (BN) [94].

4. Natural Biopolymers PCM for TES

Natural lipid-based materials have garnered research interest in the last few years as sustainable candidates for biopolymer material PCMs for TES due to their renewability, inherent safety, and desirable phase transition characteristics. Mert et al. [95] developed PCMs using coconut oil. Pure coconut oil leaks during melting; the challenge was preventing this without losing thermal storage ability. Therefore, a porous support structure with activated carbon was introduced. The study observed that higher activated carbon loading (5%) reduced mechanical strength due to heterogeneous filler distribution.

Wang et al. [96] used fatty acids (CA-SA) and Tussah silk as natural biopolymers. Tussah silk was selected because of its unique and superior natural properties compared to other fibers like mulberry silk or cotton. Tussah silk offers a natural bio-protein matrix with high porosity, hydrophilicity, and skin compatibility (as the application is for smart textiles). Tussah silk’s structure allowed for higher PCM loading and better thermal-regulating performance after treatment, as illustrated in Figure 7. This developed PCM was still stable after 1000 cycles. The optimal ratio of CA to SA was 92.89:7.11. When combined with waterborne polyurethane (WPU) as a binder (the combination through a simple pad dry-cure method), the ideal ratio of the CA-SA mixture to WPU was 1.5:1. It is important to note that some migration of fatty acids may still occur if the WPU coating is not optimized.

Montanari et al. [97] designed a fully bio-based transparent wood biocomposite with integrated phase-change functionality for TES. As illustrated in Figure 8, birch wood veneers are delignified using peracetic acid and succinylated with bio-based succinic anhydride, then infused with a mixture of 1-dodecanol and bio-based limonene acrylate, followed by in situ polymerization. The resulting transparent wood composite showseda high latent heat of melting, h_m = 89.4 J/g, at T_m = 24.3 °C. Above the melting point (T > T_m), the transparent wood maintained a bending strength of 39.4 MPa and a modulus of 9.6 GPa. It also exhibited thermo-responsive optical transmittance, increasing from 59% to 86%, and excellent shape stability without PCM leakage, as confirmed by leakage and thermal cycling tests.

Wang et al. [98] developed a novel class of cellulose-based phase change nanocrystals incorporating a core–shell architecture, with a cellulose nanocrystal core and a phase-change octadecyl-functionalized shell. The research demonstrated that C18-UCNCs form flaky nanostructures capable of reversible thermal responses, with consistent performance during repeated heating and cooling. Experimental results confirmed that C18-UCNCs spontaneously assemble into hierarchical architectures through their engineered core-shell structure. The cellulose nanocrystal core ensures structural integrity, while the octadecyl-modified shell facilitates reversible phase transitions under thermal stimuli. This organization enhances both thermal conductivity and photothermal efficiency. Critically, the system’s thermoresponsive nature enabled dynamic control over surface morphology, optical properties, and superhydrophobicity—features rigorously validated in the study. These capabilities position C18-UCNCs as promising candidates for advanced applications like thermal imaging and innovative coatings.

Cao et al. [99] used LA, SA, and CMC to create a shape-stabilized PCM. In this case, CMC enabled fast thermal exchange due to its high surface area. The CMC also offered a porous structure, which provided good mechanical support, thermal insulation, and leakage resistance. The developed PCM could maintain performance for 100 thermal cycles. Udangawa et al. [100] introduced a combination of oil and cellulose from spruce pulp as a natural biopolymer PCM. The opposing polarity of coconut oil and cellulose aided in creating a stable core–shell structure via a wet–wet coaxial electrospinning configuration, wherein a molten coconut oil core was surrounded by a cellulose solution dissolved in 1-ethyl-3-methylimidazolium acetate, and extruded into an ethanol–water coagulation bath, followed by freeze-drying to yield shape-stabilized fibrous mats. As an alternative, Németh et al. [101] combined coconut oil with CA (which was derived from SA and crosslinked with calcium chloride). Thermal analysis showed that the optimized microcapsules could achieve a high PCM loading of approximately 81.1%, with latent heat values of 84.7 J/g (during melting). Later, this PCM was optimized to reduce excessive gelation during the formation of the second CA layer and enable recycling of partially gelated bulk alginate solution. The results showed that the remaining alginate (72.3%) could be recycled for emulsion/core formation, which minimized waste [102].

Konuklu et al. [103] explored PCM using myristic acid and cellulose as a matrix/support. The cellulose matrices provide physical entrapment of myristic acid, which prevents leakage and offers structural stability. The study used the direct impregnation method to produce PCM, which started by heating myristic acid and cellulose, stirring for 15 min, and then cooling it at room temperature. Some PCM combinations were observed to be unstable and excluded from testing. Later, Lin et al. [104] combined myristic acid and ethyl cellulose using the emulsification–solvent evaporation method in PCM production. In this case, ethyl cellulose formed a robust and biodegradable shell that prevented leakage and stabilized myristic acid. It is important to note that ethanol was used as a solvent during the production.

Zhang et al. [105] attempted to combine coconut oil (as PCM core) with PS and cellulose nanocrystals. In this study, PS offered mechanical stability and a robust shell, while cellulose nanocrystals acted as both an emulsifier and part of the shell. In this study, pickering emulsion polymerization was introduced to produce the PCM, where polymerization at T = 70 °C was used to form a PS shell around emulsion droplets.

Yoo et al. [106] used the combination of methyl laurate, poly(urea-urethane), and hydrophobized cellulose nanocrystals as a PCM via the interfacial polymerization method. Methyl laurate was used as the PCM core while poly(urea-urethane) and hydrophobized cellulose nanocrystals were the shells. In this study, poly(urea-urethane) offered toughness and a flexible shell while hydrophobized cellulose nanocrystal improved barrier properties, thermal conductivity, and mechanical strength. Although the results showed a high encapsulation efficiency of up to 97%, the higher viscosity complicated emulsification.

Some combinations of protein and polysaccharide as biopolymer PCM were investigated. Konuklu and Paksoy [107] explored the development of bio-based microcapsules and microcomposites. In this case, chitosan and gelatin were used as shell materials, while caprylic acid and decanoic acid were used as PCM cores. Chitosan provides film-forming, antimicrobial, and pH-sensitive characteristics, while gelatin contributes elasticity and biocompatibility. Their electrostatic interaction in acidic conditions facilitated coacervation, which formed stable microcapsules. The materials showed good thermal aging and reliability. Moreover, Singh et al. [108] introduced a green and renewable microencapsulation method using gelatin and gum arabic to encapsulate capric acid. The electrostatic interaction between positively charged gelatin and negatively charged gum arabic created a stable shell via complex coacervation. The developed PCM was stable for at least 50 cycles and had no leakage up to 80 °C. Some porosity from freeze-drying affects evaporation behavior. The shell may not fully resist evaporation above 100 °C.

McCord et al. [109] combined several natural biopolymers as PCMs, which included SA, flax fiber, polypyrrole, and fatty acids. SA and flax fiber (as a reinforcement) offered structural integrity and porosity, while polypyrrole (as a conductive polymer) enhances electrical and thermal conductivity, as well as light/electric-to-heat conversion. The mechanical properties improved when flax fiber was blended with SA to form the composite sponge. This enhancement was attributed to the entanglement of a high aspect ratio of flax fiber with SA chains, which improved interfacial bonding and stress distribution within the matrix. The improved mechanical properties were observed when the sponge was impregnated with fatty acids. Moreover, the combination with fatty acids offered high latent heat and biodegradability.

Several of the natural biopolymer PCMs discussed above exhibit a range of different thermal properties, which makes them suitable for different TES applications. These PCM properties include melting enthalpy, melting temperature, thermal conductivity, and density, as well as various mechanical characteristics. For ease of comparison, these thermal and mechanical properties are comprehensively summarized in

Table 3. Summarized thermal and mechanical properties of natural biopolymer PCMs.

No	Biopolymer PCM	Ratio	Type of Polymer	hm [J/g]	Tm [°C]	k [W/m·K]	ρ [kg/m3]	Mechanical Properties	Refs.
1	Coconut oil/(activated carbon, another supporting matrix)	31.34/68.66 wt%	lipid	42.92	24.02	n.a.	n.a.	Ec = 7.2 MPa and σc = 0.19 MPa	[95]
2	CA/SA	92.89/7.11 wt%	lipid	170.2	26.30	n.a.	n.a.	n.a.	[96]
3	1-Dodecanol/PLIM/succinylated birch wood	52/23/24 wt%	lignocellulosic	89.4	24.3	n.a.	1001	σm = 86 MPa, σb = 39.4 MPa, and Ec = 9600 MPa	[97]
4	C18/UCNC	n.a.	polysaccharide	40.8	60	n.a.	n.a.	σf = 9.71 MPa	[98]
5	(LA+SA)/CMC	70.4/29.6 wt%	lipid-polysaccharide	114.6	32.2	n.a.	n.a.	n.a.	[99]
6	Coconut oil/Cellulose	76/24 wt%	lipid-polysaccharide	134.9	22	n.a.	n.a.	n.a.	[100]
7	Coconut oil/CA	81.1/18.9 wt%	lipid-polysaccharide	84.7	24.7	n.a.	n.a.	n.a.	[101]
8	Coconut oil/CA	75/25 wt%	lipid-polysaccharide	81.8	~25	n.a.	n.a.	n.a.	[102]
9	Myristic Acid/Methyl Cellulose	50/50 wt%	lipid-polysaccharide	71.3	53.2	n.a.	n.a.	n.a.	[103]
10	Myristic Acid/Ethyl Cellulose	66.7/33.3 wt%	lipid-polysaccharide	122.61	55.3	n.a.	n.a.	n.a.	[104]
11	Coconut oil/CNC	n.a.	lipid-polysaccharide	n.a.	23.4	n.a.	n.a.	n.a.	[105]
12	Methyl laurate/(CNC and other components)	60/40 wt%	lipid-polysaccharide	113.18	6.12	n.a.	n.a.	E = 0.4–0.8 MPa	[106]
13	Caprylic Acid/(Gelatin+Chitosan)	49.98/50.02 wt%	protein-polysaccharide	79.18	11.53	n.a.	n.a.	n.a.	[107]
14	Gelatin/Gum Arabic	50/50 wt%	protein-polysaccharide	86.4	31.74	n.a.	n.a.	n.a.	[108]
15	Decanoic acid/(Flax fiber+SA+Polypyrrole)	59.75/40.25 wt%	lipid-polysaccharide-lignocellulosic	100.98	32.85	0.31	71.3	σc30% strain = ~0.23 MPa and σc70% strain = ~1 MPa	[109]
16	Palmatic acid/(Flax fiber+SA+Polypyrrole)	73.84/26.16 wt%	lipid-polysaccharide-lignocellulosic	154.52	63.33	0.28	98.02	n.a.	[109]

.

5. Synthetic Biopolymer PCMs for TES

Several synthetic biopolymer PCMs have been studied and developed using different types of materials. Yin et al. [86] used PLR (a biopolymer-based supramolecular) and PEO. When PLR is bent or stretched, it behaves like a flexible rubber band, not a brittle plastic. Figure 9 shows the twisted, folded, and stretched sample using 30% PLR (added with red pigment). It also illustrates the mechanical enhancement mechanism. In this case, the PEO was a string and the α-CDs were beads threaded onto it. The α-CDs can slide along the string when stretched but cannot detach due to end-capping stoppers. The hydroxyl groups on α-CDs formed multiple hydrogen bonds with PEO and water molecules. These bonds break and reform reversibly, acting like sacrificial bonds. The PLR chains physically entangle, which adds network density and cohesion to the matrix. One study [110] proposed a sustainable, shape-stable, and biocompatible PCM using PLR and PPP. PLR is a supramolecular structure that is composed of interlocked molecules held together by non-covalent interactions like hydrogen bonds and steric hindrance. PLR provided functional shape stability and phase change, while PPP improved fire resistance and thermal conductivity. PPP also acted as a plasticizer for flexibility. The combination of these materials enhanced dispersion, thermal conductivity, and compatibility due to hydrogen bonding and miscibility. However, PCM may increase supercooling and reduce crystallinity and enthalpy at a higher PPP ratio.

Trigui et al. [111] studied the combination of PEG/LDPE/eggshell powder/graphite (70/20/5/5 wt%) as a synthetic biopolymer PCM. LPDE acted as a supporting matrix or shape stabilizer. In this case, eggshell and graphite were used as fillers and thermal conductivity enhancers, respectively. The porous CaCO₃ taken from eggshell powder may enhance structure and reduce leakage. Pereira et al. [112] explored the oligomeric form (OBS) as a PCM. Succinic acid + butanediol formed the OBS backbone. In this study, glycerin was added to modify the chain length and reduce crystallinity. Phosphoric acid was used as a catalyst, with the concentration having the most impact on melting-point tuning. Botlhoko et al. [113] demonstrated blends of PLA and PCL as PCMs. In this case, PCL acted as a flexible soft segment and nucleating agent for PLA. It was observed that the blend showed higher activation energy than pure PLA, which is important for thermal cycling durability in PCM applications. The high elongation and impact resistance due to PCL meant that the PCM remained mechanically intact even after repeated phase changes. Moreover, Lan et al. [114] introduced a novel biopolymer PCM that had h_m = ~43.9 J/g at T_m = ~35.7 °C. This PCM was a blend of poly(butylene adipate-co-hexamethylene adipate)/PBHA with CHDM and PBT. PBHA (from Adipic acid, 1,4-butanediol, and 1,6-hexanediol) formed the flexible polyester PCM matrix. 1,4-cyclohexanedimethanol was added to introduce good stability and processability, while PBT offered a tough support matrix to provide mechanical strength during melt-spinning. CDHM and PBT support may reduce leakage risk. However, crystallinity and the latent heat of PCM may decrease with increasing CDHM.

Yang et al. [115] introduced Poly(EUA/CO₂)-g-CnH₂n+1 as a synthetic biopolymer PCM, with epoxy-functionalized 10-undecenoic acid (EUA) derived from castor oil as a bio-based monomer and CO₂ as copolymerization (introducing carbonate units for thermal stability). Various alkyl thiols (CnH₂n+1SH) were grafted to form pendant side chains for phase transition tuning.

The various synthetic biopolymer PCMs discussed above offer diverse thermal characteristics, making them applicable to TES systems. Several properties contribute to their performance such as melting enthalpy, melting temperature, thermal conductivity, density, and several mechanical characteristics. For convenient comparison and evaluation, these thermal and mechanical properties are listed in

Table 4. Summarized thermal and mechanical properties of synthetic biopolymer PCMs.

No	Biopolymer PCM	Ratio	Type of Polymer	hm [J/g]	Tm [°C]	k [W/m·K]	ρ [kg/m3]	Mechanical Properties	Refs.
1	PLR/Pentaerythritol Phosphate	90/10 wt%	supramolecular	81.86	~45.9	0.36	n.a.	E = 1193 MPa and σt = 31.5 MPa	[110]
2	PEO/PLR	70/30 wt%	supramolecular	88.57	60.12	n.a.	1420	E = 826.7 MPa and σt = 13.9 MPa	[86]
3	PEG/low-density PE/eggshell powder/graphite	70/20/5/5 wt%	polyether	120.1	62.5	n.a.	n.a.	n.a.	[111]
4	Oligo(butylene succinate)	n.a.	aliphatic polyester	n.a.	48	n.a.	n.a.	n.a.	[112]
5	PLA/PCL	60/40 wt%	aliphatic polyester	32.53 and 17.23	176.42 and 56.13	n.a.	n.a.	σt = ~66 MPa	[113]
6	PBHA/C5/PBT	33/67 wt%	aliphatic polyester	43.9	35.7	n.a.	n.a.	σmax27% = ~2.07 cN/dtex, σmax30% = ~1.78 cN/dtex, and σmax33% = ~1.62 cN/dtex	[114]
7	Comb-like	n.a.	branched	108.5	70.3	n.a.	n.a.	n.a.	[115]

.

Yin et al. [81] developed a novel hybrid biopolymer PCM using PEG, PLR, and boron nitride, which aimed to maintain flexibility and shape stability, leakage resistance, improve thermal conductivity, and enable eco-friendly processing with only water as a solvent. The PLR was derived from α-CD, which provided biocompatibility, processability, and physical crosslinking points for shape retention. Some variations of the BN content were tested to improve the heat conductivity. In this study, adding 37.5% of BN content significantly improved the thermal conductivity from 0.30 W/m·K to 2.72 W/m·K. The developed PCM, PLR-PEG-60BN, which has 37.5% BN content, offers T_m,onset = 23.62 °C.

Fredi et al. [116] demonstrated that photo-crosslinked PEO can function as a shape-stable, leak-free PCM. The study used photo-crosslinking (UV-curing) to create covalent bonds between polyethylene oxide (PEO) chains, effectively locking the structure in place. This process enhances water resistance, mechanical strength, and thermal integrity above the melting point. However, an increased charging/discharging time was observed due to the structural additives.

Sundararajan et al. [117] developed a solid–solid PCM using hyperbranched PU architecture incorporating PEG. The material achieved a high fusion enthalpy of 146.6 J/g at T_m = ~56 °C. It also demonstrated excellent thermal stability up to 300 °C and no chemical degradation after 100 cycles. In this study, the A₂ + B₃ method was employed as a strategic approach to synthesize hyperbranched polyurethanes. The method involved step-growth polymerization between an A₂-type monomer, which contains two reactive isocyanate (–NCO) groups, and a B₃-type monomer, which contains three hydroxyl (–OH) groups. This method is advantageous due to its simplicity, scalability, and ability to produce materials with desirable thermal and mechanical properties.

Lee and Kim [118] aimed to improve the crosslinking and performance of biopolymer PCM using castor oil-derived hyperbranched polyols to replace petroleum-based ones. The study focused on the solid–solid PCMs, which use castor oil-based hyperbranched bio-PU as the matrix to confine PEG and prevent phase leakage. It was observed that the hybrid biopolymer forms a stable network that limits PEG mobility while enhancing crystallinity and durability.

Feng et al. [119] combined PEG and PLA as a synthetic PCM using graphitic carbon nitride (g-C₃N₄) to enhance the thermal conductivity of the blend. The g-C₃N₄ was synthesized via the low-cost calcination of melamine. The PCM with PLA/PEG/g-C₃N₄ of 3:6:1 has h_m = 106.1 J/g, modulus at a maximum load of 3044 MPa, and hardness at a maximum load of 96 MPa. Jia et al. [120] demonstrated the selection of crystallinity and glass transition temperature of the PLA matrix to maximize the latent heat of PEG. Crystalline PLA stabilizes fiber morphology and suppresses PEG leakage. It was observed that tuning the glass transition temperature below PEG’s melting temperature may create a rubbery matrix. In this case, rubbery semi-crystalline PLA might be optimal for maximizing PEG crystallization and melting enthalpy. Liu et al. [121] introduced poly(L-lactic acid) as a matrix or shell/support structure and demonstrated coaxial electrospinning for encapsulating poly(ethylene oxide) (PEO), as illustrated in Figure 10. Coaxial electrospinning allows for the fabrication of core–shell nanofibers. In this method, two different polymer solutions (or one polymer and one additive) were fed simultaneously through a specialized needle setup made of two concentrically aligned nozzles. In this study, the latent TES of PEO was similar to PEG but more viscous, which reduced leakage. Poly(L-lactic acid) provided structural stability and reduced thermal conductivity due to the effect of the glass transition temperature. The melting enthalpy of 58.79 J/g was obtained with 49 wt% PEO. Moreover, Yin et al. [122] proposed a synthetic biopolymer PCM with no solvents. They used Poly(Glycerol-Itaconic acid) or PGI as a support matrix and benzoyl peroxide 0.5 wt% as a crosslink initiator. PGI was formed from glycerol and itaconic acid, creating compatible support with good hydrogen bonding with PEG, tunable crosslinking for physical entrapment, and solid–solid phase change behavior. This PCM could maintain melting enthalpy up to 86.93 J/g at T_m = 41.92 °C; however, supercooling ~13 °C was observed. In this study, leakage occurred due to reduced crosslinking at a high PEG content (>72.67%). Moreover, Soo et al. [123] introduced mPEG/PLA, which maintained melting enthalpy up to 56 J/g. This block copolymer demonstrated good thermal stability after 100 heating/cooling cycles, retaining over 94% of its original latent heat.

In the literature, it is found that some researchers used additional high-density polyethylene (HDPE) (~50%) for the development of biopolymer PCMs. It is important to note that this was a petro-based thermoplastic and not a biopolymer. Lu et al. [124] demonstrated PLA/HDPE as a synthetic biopolymer PCM. HDPE softens at around 100 °C and collapses when it melts (leading to leakage). PLA has a higher melting point, up to 175 °C, than PLA and forms a solid (unmelted support structure). A combination of 50/50 PLA/HDPE allows for the HDPE to undergo a phase change while the PLA maintains the structural shape. This blend has latent heat up to 100 J/g at T_m = 136.6 °C. It was observed that increasing the PLA content may reduce thermal stability. In addition, if the PLA content is reduced below 50%, the shape stability might be compromised. Lu et al. [125] introduced e-glass fiber as a reinforcement material and an ethylene-butyl acrylate-glycidyl methacrylate terpolymer in a PLA/HDPE blend to enhance the mechanical and thermal properties. The results showed that toughness was improved, with a tensile strength of 36.9 MPa and an impact strength of up to 4.4 kJ/m². It was observed that the ethylene-butyl acrylate-glycidyl methacrylate terpolymer improved interfacial adhesion between immiscible PLA and HDPE.

Studies in the literature show that there is significant potential in combining two or more different synthetic biopolymers to develop PCMs with tailored thermal and mechanical properties. Such combinations can result in synergistic effects, enhancing overall performance for specific TES applications. The synthetic biopolymer PCM blends discussed above demonstrate a wide range of thermal behaviors, including variations in melting enthalpy, melting temperature, thermal conductivity, and density, along with diverse mechanical characteristics. These properties directly influence the efficiency, stability, and application potential of the materials. For ease of comparison and evaluation, the thermal and mechanical properties of these combinations are summarized in

Table 5. Summarized thermal and mechanical properties of combined synthetic–synthetic biopolymer PCMs.

No	Biopolymer PCM	Ratio	Type of Polymer	hm [J/g]	Tm [°C]	k [W/m·K]	ρ [kg/m3]	Mechanical Properties	Refs.
1	PEG/PLR/BN	66/22/12 wt%	supramolecular, polyether	91.16	35.98 and 56.79	2.72	n.a.	n.a.	[81]
2	PEO/Trimethylolpropane triacrylate	75/25 wt%	polyether-thermosetting	112.2	58.8	n.a.	n.a.	σt = 0.74 MPa	[116]
3	PEG/supporting matrix	n.a.	polyether-thermosetting	146.6	56.15	n.a.	n.a.	n.a.	[117]
4	PEG/PU	86/14 wt%	polyether-thermosetting	126.5	52.54	n.a.	n.a.	n.a.	[118]
5	PLA/PEG/g-C3N4	30/60/10 wt%	aliphatic polyester-polyether	106.1	64.54 and 174.15	0.32	n.a.	E = 3044 MPa and hardness at max load = 96 MPa	[119]
6	PLA/PEG	50/50 wt%	aliphatic polyester-polyether	100.3	63.2	n.a.	n.a.	n.a.	[120]
7	PLA/PEG	51/49 wt%	aliphatic polyester-polyether	58.79	60.95	~0.026	n.a.	E = 502 MPa and σy = 12 MPa	[121]
8	PEG/Poly(Glycerol-Itaconic acid)	72.67/27.33 wt%	aliphatic polyester-polyether	86.93	41.92	n.a.	n.a.	n.a.	[122]
9	PLA/mPEG	50/50 wt%	aliphatic polyester-polyether	55.8	50.5	n.a.	n.a.	n.a.	[123]
10	PLA/HDPE	50/50 wt%	aliphatic polyester-polyolefin	100.1	136.6	n.a.	n.a.	n.a.	[124]
11	PLA/HDPE	60/40 wt%	aliphatic polyester-polyolefin	PLA: 24.5 HDPE: 83.2	PLA: 168.5 HDPE: 132.6	n.a.	n.a.	σt = 36.9 MPa and a = ~4.4 kJ/m2	[125]

.

6. Hybrid or Composite Biopolymer PCMs for TES

Sarkar et al. [126] developed hybrid biopolymer PCMs using PU reinforced with leather waste. They synthesized the PU matrix from PCL and castor oil and blended it with leather waste and poly(glycidyl methacrylate)/PGMA. The PGM was formed in situ from GMA using a radical initiator (AIBN). Two composite types were developed, PL(PU+leather waste) and PLG(PU+leather waste+PGMA), with PLG designed to enhance crosslinking and shape stability. The PLG composite demonstrated good shape recovery, shape fixity (60–80%), and a rapid response of <30 s in water. Self-healing was achieved at just 60 °C, with full recovery observed within 1–2 h.

Moreover, Chen et al. [127] proposed solid–solid PCM using PEG and MDI as crosslinking agents. The synthetic route involves first reacting PEG with MDI in dimethylformamide at 70 °C to form a PEG/MDI prepolymer, followed by the dropwise addition of glucose to create a crosslinked PCM network. Extra PEG (50–70 wt%) was then added and the mixture was thermally cured at 80 °C to form the final PCM composite. The synthetic route of the proposed PCM is illustrated in Figure 11. It was observed that a latent heat of 131.9 J/g could be achieved using PEG 70 wt% without leakage. This PCM offers reusability after 500 cycles. Yang et al. [128] proposed a solid–solid PCM using PEG. In this case, xylitol was used as a curing agent or crosslinking agent while MDI was used as a coupling agent. This proposed method was used to introduce solvent-free synthesis in making the PCM. However, the latent heat h_m = 76.37 °C was lower than pure PEG due to the crosslinking limiting PEG mobility. A slight reduction in crystallinity and in the melting point (T_m = 41.65 °C) were also observed compared to pure PEG.

Liu et al. [129] attempted to eliminate toxic solvents and improve environmental safety while enhancing thermal storage by developing a PCM with h_m = 117.7 J/g at T_m = 51.4 °C. The study used castor oil in the solvent-free synthesis of PCM, which aimed for high latent heat and biodegradability. PEG4000 and PEG6000 were used as TES components, castor oil as a biodegradable support skeleton, and MDI and HDI as coupling reagents. The combination of natural and synthetic biopolymers (hybrid) offered flexibility, biodegradability, and structural integrity. In this case, HDI’s flexible chain lowered steric hindrance, which enhanced crystallinity.

In 2023, Bragaglia et al. [130] explored the use of waste fat from cooking pork sausages as an eco-sustainable PCM for TES. The waste fat, composed predominantly of oleic (43.1%), palmitic (16.8%), linoleic (11.5%), and stearic acids (7%), exhibited a gel-like consistency due to its mixed saturated/unsaturated nature. The phase transition characteristics were confirmed via DSC, revealing a melting peak at 32 °C with a melting enthalpy of 20.29 J/g when impregnated into a polypropylene (PP) mat sourced from waste surgical masks. This composite, referred to as Mask filter-BioPCM, retained stable performance over multiple cycles and did not exhibit leakage, even at 60 °C, showcasing excellent shape stability and capillary retention. In addition, its thermal conductivity was measured at 0.213 W/m·K, suggesting moderate heat transfer capability. Mechanically, the PP mat demonstrated a tensile strength of 3 MPa, suitable for flexible containment applications. The study highlights an effective circular economic strategy for waste fat materials for building-integrated passive cooling systems. In the same year, Zheng et al. [131] synthesized a new class of bio-based PCMs, known as fatty acid amides, through the solvent-free amidation of soybean oil with various alkyl amines, employing a non-toxic DABCO catalyst. Among the synthesized samples, the most promising was the FAAm derived from fully hydrogenated soybean oil and diaminobutane. This material demonstrated a melting temperature of 151.75 °C and an exceptionally high melting enthalpy of 202.3 J/g, placing it among the top-performing organic PCMs in terms of latent heat. The sample maintained a single, sharp, phase transition peak, indicating high purity and structural uniformity. It also showed excellent mechanical integrity, with a tensile strength of 3 MPa, and exhibited superior thermal stability. These characteristics highlight the potential of efficient high-temperature bio-PCMs for textile processing, steam generation, and industrial waste heat recovery applications.

Perez-Arce et al. [132] and Wang et al. [133] have significantly advanced lignin-based biopolymer PCMs for TES through innovative material design. Perez-Arce et al. [132] synthesized lignin-based polyols (LBPs) via cationic ring-opening polymerization, identifying LBP-BO-5 as the most promising, with the highest melting enthalpy (53.7 J/g), and a melting point of 19.9 °C. This temperature is well-suited for building applications, particularly for enhancing energy efficiency through night-time cooling or off-peak electricity use. LBP-BO-5 also had the highest tetrahydrofuran content (79.2 wt%), contributing to its distinct thermal behavior. Meanwhile, Wang et al. [133] developed microencapsulated 1-tetradecanol using pickering emulsion polymerization stabilized by lignin nanoparticles. It also demonstrated excellent thermal stability (81.4% encapsulation ratio, >97% efficiency) and no leakage after prolonged heating at 60 °C.

Unlike PU, which involves harmful chemicals, such as diisocyanate, and exhibits low phase change enthalpy, Shilpa et al. [134] introduced PEG-based PCM with PGMA, leather waste, fluorescein, and rhodamine B dyes, which maintain thermal enthalpy up to h_m = 152.6 J/g at T_m = 56.3 °C and avoid such toxic chemicals. It is important to highlight that leather waste provides hydrogen bonding with PEG, while PGMA enables crosslinking for shape stability. The combination of hybrid biopolymer enhanced thermal stability, mechanical performance (tensile strength of 3.08 MPa) and shape memory. In this case, it offered multifunctionality for thermal storage and other applications.

Fashandi and Leung [135] developed a PCM using palmitic acid (PA) and PLA. PA melts during heat absorption and can leak without encapsulation. Encapsulating PA in PLA prevents leakage and improves heat transfer and handling. In this study, PVA/SDS were used as emulsifiers to aid in forming uniform microcapsules. This PCM offers a melting enthalpy h_m = 70.1 J/g at T_m = 62.1 °C and good thermal reliability (maintaining performance after 50 thermal cycles). However, it is important to note that too much emulsifier might cause micelle formation and an uneven surface; precise control of the multi-core morphology is required to improve encapsulation. Baştürk and Kahraman [136] proposed a biopolymer PCM to solve leakage problems using UV-curing to embed fatty alcohols inside a bio-derived network. Several acrylated soybean oils containing different fatty alcohols were developed to observe the shape stability and leak-proofness. Doracure 1173 was used to ensure effective crosslinking via UV light. The results also indicated that both the phase change temperatures and the melting and freezing enthalpies rose as the alkyl chain length of the fatty alcohols increased.

Wu et al. [137] introduced a new castor oil-based polyurethane-acrylate (COPUA) as a bio-based encapsulation matrix, which had not been previously explored in PCM applications. The combination of PA and COPUA offered biodegradable, form-stable PCMs with good thermal storage capabilities. The developed PCM has h_m = 141.2 J/g at T_m = 66.6 °C. There is restricted crystallinity due to the crosslinked matrix and there is a slight decrease in enthalpy value after 1000 cycles. One study [138] introduced PCM using a reversible Diels–Alder (D–A) reaction. The D–A reaction is a [4+2] cycloaddition between a diene (furan) and a dienophile (maleimide), forming a covalent bond that is thermally reversible. The D–A network allows for the breaking and reforming of the crosslinked structure by heating/cooling.

Furthermore, several studies have attempted to develop a new biopolymer PCM based on the combination of polysaccharides with aliphatic polyester or polysaccharides with polyether. He et al. [139] developed a biopolymer PCM using PEG, cellulose nanofibers (CNF), EG, and BN with h_m = 79.46 J/g at T_m = 59.51 °C. CNF forms hydrogen bonds with PGE, while EG and BN enhance the thermal conductivity up to k = 10.83 W/m·K and prevent leakage. Jia et al. [140] introduced a PCM using a combination of PEG, chitosan, and BN to solve the PEG’s leakage, poor thermal conductivity, and shape instability. Chitosan was used as a bio-based structural scaffold; BN was used as a thermal conductive filler. The PCM remained stable for over 50 thermal cycles (no degradation in DSC/FTIR). It was observed that the denser scaffold resulted in some crystallinity loss and a drop in latent heat with increased BN content. Wan et al. [141] also developed PEG and CNF as a combination; however, in this case, a functionalized synthetic of L-glutamine-grafted BN nanosheets (BNNSs-g) was introduced to improve thermal conductivity. Compared to BN, BNNSs-g has good dispersibility (>30 mg/mL in water) due to the carboxyl and amine groups. Moreover, BNNSs-g offers good hydrogen bonds with CNF (BNNSs-g forms a hydrogel when mixed with CNF via hydrogen bonding, as illustrated in Figure 12) and PEG, which leads to good shape stability.

Some supporting materials prevent PEG leakage; however, they typically reduce energy density or lack biocompatibility. Liu et al. [142] addressed the gap by using Ca²⁺-crosslinked sodium alginate (SA) to support PEG without compromising thermal or biological performance. The porous SA hydrogel traps the PEG physically, which restricts its flow when it is melted. This is a solution to PEG’s leakage problem in solid–liquid phase transitions. Wei et al. [143] proposed SA and polydopamine-modified zirconium phosphate (PDA@ZrP) with PEG as a biopolymer PCM to achieve high latent heat up to h_m = 0159.8 J/g. The combination helps to boost light–thermal conversion without toxic materials. As an alternative, Zhou et al. [144] introduced Cs_xWO₃ to improve latent heat and light-to-heat conversion. The study combined sweet potato and Cs_xWO₃ with PEG, where it helped to align PEG chains, boost crystallinity (relative crystallinity up to 97.7%) and latent heat up to 137.7 J/g, and increase the photothermal efficiency up to 91.1%.

Baniasadi [145] developed a hybrid biopolymer PCM using PEG, gum tragacanth, biochar, cloisite Na⁺, and octadecyl isocyanate. In this study, the biochar added mechanical durability and aided in heat distribution. Cloisite Na⁺ (which is sodium-modified clay), when it is well-dispersed, can increase thermal durability and prevent matrix collapse. It also creates a barrier effect that slows degradation and enhances flame resistance. Octadecyl isocyanate had the role of a hydrophobic surface coating. Combined with gum tragancanth and PEG as a hybrid biopolymer PCM, it had a melting enthalpy up to 110.5 J/g at T_m = 58.1 °C, thermal conductivity k = 0.035 W/m·K, and density up to 120 kg/m³.

Yadzani et al. [146] studied a combination of PEG with CNF as a PCM, which has h_m = 146.2 J/g at T_m = 65.4 °C. CNFs were added to prevent leakage via gelation and entrapment and to provide mechanical stability. In this case, the CNF acted as an organogelator, which created a physical matrix that retained molten PEG. Chen et al. [147] introduced polypyrrole into CNF/PEG as a PCM to add a continuous conductive network, which enabled Joule heating. In addition, it stiffened the sponge, which improved structural integrity and prevented leakage during the phase change. Polypyrroles form hydrogen bonds with CNF, creating a uniform, well-integrated coating that does not block pores or reduce flexibility. It was observed that polypyrrole enables electro-thermal conversion (up to 85.1% efficiency at 1.9 V).

Zhou et al. [148] created a cellulose nanocrystal (CNC)-supported 3D porous structure that enabled high PEG loading while maintaining mechanical integrity, even under compression. Although PEG is synthetic, it is hydrophilic and compatible with CNC. CNC provides a biodegradable porous support; PEG brings latent heat capacity. In the development of the PCM, the use of specific chemical agents, like 4-pentenoic acid (PA), 1-Ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC·HCl), 4-Dimethylaminopyridine (DMAP), and pentaerythritol tetra(3-mercaptopropionate), played a crucial role in forming a highly stable and efficient 3D supporting structure. The resulting porous and elastic matrix also improved the thermal reliability and mechanical strength of the PCM. This developed PCM has h_m = 158 J/g at T_m = 60.2 °C with k = 0.44 W/m·K. Cheng et al. [149] fabricated CNC and PEG as PCM where CNC–CHO (aldehyde-modified cellulose nanocrystals), along with melamine and acetic acid, were introduced to achieve a shape-stable, mechanically strong, and thermally reliable PCM. CNC–CHO provides reactive aldehyde groups that can chemically bond with melamine’s amine groups to form a stable, crosslinked 3D aerogel network. The fabricated PCM has h_m = 145.8 J/g at T_m = 57.3 °C, k = 0.42 W/m·K, compressive strength at 80%, and strain = 0.1145 MPa.

One study [150] combined PA from plant oils and PLA from crops. The PLA encapsulated PA to form a solid microsphere that prevented leakage during melting, solving one of the issues of using PEG or PA alone. Lauric acid (LA) has ideal phase change properties but suffers from leakage and odor issues. Guo et al. [151] aimed to stabilize LA by chemically bonding it with CMC and mixing it with PLA. The research demonstrated that the hybrid biopolymer PCM offers an effective, biodegradable, and thermally stable solution for TES. This PCM offers melting enthalpy up to h_m = 86.4 J/g and T_m = 40.1 °C. CMC has good compatibility and is renewable and biodegradable; however, it lacks sufficient structural support. PLA offers mechanical strength and thermal stability. This hybrid biopolymer PCM offers a tensile strength of 27.61 MPa and a flexural strength of 50.02 MPa. Peng et al. [152] developed a solid–solid biopolymer PCM using β-CD, MDI, and PEG. In this study, β-CD was used as a molecular framework, as it provided biocompatibility and abundant hydroxyl groups. MDI was used as a synthetic linker to ensure durable crosslinking. Different types of PCM were synthesized with variations of molar ratios to control the crosslinking density. The developed hybrid biopolymer PCM features a molar ratio of 4/7:2:1, and h_m = 115.2 J/g at T_m = 60.2 °C. It is also important to note that this PCM offers good reusability over 500 charge–discharge cycles with thermally stable performance up to 400 °C. However, the synthesis requires tight control of molar ratios to balance performance and flexibility.

In one study, Gao et al. [153] combined chitosan, hexamethylene diisocyanate (HMDI), chitosan-based polyurethane (c-PU), butyl stearate (BS), and spiropyran (SP) for a hybrid biopolymer PCM. Chitosan allows for chemical bonding with HMDI, forming a robust PU network. A c-PU shell was produced from HMDI, which reacted with chitosan and generated urethane and urea linkages. In this case, BS was the core material PCM, while SP imparted light-responsive color-changing behavior to the capsules, switching between colorless and pink when exposed to UV and visible light, respectively. Together, these components formed a synergistic system where the PCM has h_m = 104.1 J/g at T_m = ~24 °C.

Furthermore, one study [154] proposed the use of microcrystalline cellulose (MCC) and graphene nanoplatelets to form an anisotropic 3D structure, boosting thermal conductivity, mechanical stability, and encapsulation efficiency. A low filler content of graphene nanoplatelets (1.51 wt%) was used to improve PEG as a PCM. The proposed PCM achieved high latent heat, h_m = 182.6 J/g, and thermal conductivity, k = 1.03 W/m·K.

Sundararajan et al. [82] combined cellulose acetate (CA) and PEG as a hybrid biopolymer PCM, with h_m = 155.35 J/g at T_m = 60.56 °C. The study introduced a 2 min microwave process with a low amount of solvent for ease of production. Liu et al. [155] proposed a dual-functional composite that retained both the aerogel’s insulation and the PCM’s energy storage capacity using a one-step synthesis. In this work, PEG, PAM, and CA were combined. PAM formed the first polymer network, which offered mechanical strength. CA created the second ionic network, which improved flexibility and formed the aerogel. Moreover, Li et al. [156] proposed a solid–solid PCM using PEG and cellulose as a hybrid biopolymer PCM. Cellulose was used as a support matrix (or structural matrix). The hybrid biopolymer PCM has h_m = 182.6 J/g at T_m = ~67.6 °C. It was observed that PEG crystallization was partially hindered by being grafted onto cellulose. Although this slightly lowered enthalpy, it enhanced structural integrity, which is crucial for repeatable solid–solid phase transitions. In this study, small amounts of EG were also added to improve the thermal response time and thermal conductivity. The study showed that the EG integrated homogeneously into the biopolymer matrix, preventing aggregation and ensuring uniform heat transfer, which is often a significant challenge in composite PCMs.

Fan et al. [157] combined PEG and cellulose nanocrystals to produce green biopolymer PCM via green aqueous radical polymerization. Maleic anhydride was used as a crosslinker, which allows for grafting of the PEG into cellulose nanocrystals. It was observed that excessive grafting may decrease enthalpy due to crosslinking hindrance. Zhou et al. [158] developed waterproof PCM using cellulose nanocrystals and a poly(N-isopropylacrylamide) network. In this study, poly(N-isopropylacrylamide) offered a thermo-responsive waterproof shell and network crosslinking for shape stability.

Wang et al. [159] used cellulose aerogel to provide a porous network and encapsulate PEG. Dopamine was used to modify cellulose aerogel to reduce hydrogen bonding, which otherwise restricts PEG crystallization. In this study, a facile in situ method using freeze casting was used. PEG, at 65 °C, was added to the cellulose hydrogel and dopamine mixture. Then, liquid nitrogen was used to freeze it at −50 °C, after which it was heated at 80 °C for four hours. It appears that the method used is a solvent-free process; however, freeze-drying equipment is required.

Bao et al. [160] developed PLR with pentaerythritol phosphate for high flexibility, shape stability, and fire resistance in biopolymers for electronics. Pentaerythritol phosphate enhanced fire safety, improves thermal conductivity, and supports flexibility. The study also introduced the synthesis of pentaerythritol phosphate via a solvent-free method. It is important to note that thermal conductivity was enhanced by 68% and that flexibility was high, with elongation >1000%. Moreover, it was observed that high pentaerythritol phosphate may reduce crystallinity and increase supercooling.

Tang et al. [161] introduced bacterial cellulose and MXene as supporting matrices to encapsulate PEG 4000. The bacterial cellulose formed a strong 3D scaffold that held the PEG and prevented leakage, while MXene added photothermal conversion capacity and enhanced crystallization. The results showed that the PCM has good shape retention up to 120 °C, and it has high PEG loading (up to 97.9 wt%). However, it was noted that the production of this PCM required freeze-drying equipment.

Balderrama et al. [162] presented a sustainable method for encapsulating PCMs using polysaccharide matrices. They developed monolithic materials by emulsifying butyl stearate within a water-soluble polysaccharide, pullulan, which was crosslinked using sodium trimetaphosphate. Pullulan, a biodegradable polysaccharide derived from microbial fermentation, formed the continuous aqueous phase, while butyl stearate formed the dispersed internal oil phase. The resulting emulsions were stabilized using a combination of hydrophilic (Tween 80) and hydrophobic (Span 85) surfactants. STMP crosslinking under alkaline conditions formed solid monoliths, referred to as polyLIPEs, with up to 30–35% PCM content. The study identified that a pullulan concentration of 24% (w/w) offered the best compromise between emulsion stability and high PCM loading.

Several lignocellulosic-polyethers can also be used for hybrid biopolymer PCMs. Liang et al. [163] introduced wood flour to PEG as a PCM, where the wood flour provided a rigid, porous matrix that helped to maintain the physical form of the PCM during heating/cooling cycles. Wood flour contains hydroxyl groups (-OH) that can form hydrogen bonds with PEG, enhancing the retention and distribution of PEG within the matrix. The reduced crystallinity of PEG was observed due to confinement in the wood flour pores. Moreover, some latent heat was lost compared to the pure PEG. Zheng et al. [164] used bamboo flour as a natural support for PEG, which was prepared via dry ball milling. The porous matrix from bamboo fibers and parenchyma cells enhanced encapsulation (without the need for chemical crosslinkers, which effectively prevented leakage during melting), mechanical strength, and thermal conductivity. Using 30 wt% of bamboo flour, the PCM had a h_m = 113 J/g at T_m = 64.19 °C.

Gao et al. [165] demonstrated a natural carbon matrix from pinecones to offer a highly porous structure, thermal conductivity, and physical encapsulation via capillary force and hydrogen bonding to PEG/octadecane as a PCM. A rigid PU foam composite in which 25 wt% of PEG/PCC or OD/PCC shape-stabilized PCMs (PCMs/RPUF-25) were embedded formed a dual-functional building insulation material. The PCMs/RPUF-25 stored and released thermal energy, which buffered ambient temperature fluctuations more effectively than pure RPUF. The PCMs/RPUF-25 had a melting enthalpy of h_m = 51.99 J/g.

Hu et al. [166] introduced reduced graphene oxide-modified spent coffee grounds as a novel bio-based porous scaffold for PEG to achieve 60.3% PEG loading, good photothermal conversion, leakage prevention, and good cycling stability (100 cycles). The porous structure of spent coffee grounds enabled it to hold molten PEG through capillary action and hydrogen bonding, which minimized leakage during transitions. It was shown that when modified with reduced graphene oxide, spent coffee grounds improve thermal conductivity and photothermal conversion efficiency.

One study [167] chemically grafted PEG onto wood powder to prevent leakage and improve the structure, which created solid–solid PCMs. Wood powder was chosen due to its renewable, porous, and reactive surface. HDI was added as a crosslink, which enabled covalent bonding for shape stability. In this study, the results showed that the addition of wood powder can lead to reduced supercooling (i.e., faster and more consistent solidification). However, the latent heat drops slightly due to addition of wood powder. Zhang et al. [168] introduced pomelo peel flour to stabilize PEG during melting, prevent leakage, and maintain the PCM structure. Pomelo peel flour was used due to its biodegradable, porous, and hydrophilic properties, which are helpful for physical and chemical entrapment. It was observed that thermal reliability was maintained over 100 cycles with no leakage or deformation. Sheng et al. [169] aimed to solve leakage, poor light absorption, and low conductivity in PEG by combining it with pomelo peel flour and titanium carbide nanosheets (MXene). This study chose MXene as an inorganic nanofiller to enhance light absorption, conductivity, and PEG loading. The investigation considered variations in MXene content. There was a limited increase in conductivity due to the low MXene. Crystallinity was also reduced by the pomelo peel flour/MXene scaffold. The hybrid biopolymer PCM of 96.2 wt% PEG and pomelo peel flour (impregnated with MXene at 4 mg/mL concentration) offered h_m = 158.1 J/g at T_m = 62.7 °C. The TGA showed that this composition has a high thermal stability of T_max = ~405.9 °C.

As an alternative option, several hybrid biopolymer PCMs can be made from polyphenolic-polyether, polyphenolic-aliphatic polyester, and supramolecular-polyether. Lee et al. [170] proposed that chemically grafting PCL onto lignin offers better integration than just physical blending. PCL offers the ability to change phases and provide flexibility but with poor thermal stability. While the lignin provides an eco-friendly matrix, it is brittle and hydrophilic. Combining these materials offers several advantages, particularly when they are chemically grafted. This can offer good flexibility, with rubbery and stretchable characteristics. This hybrid biopolymer PCM has h_m = 61.16 J/g at T_m = 51.33 °C, and it can maintain its shape under T = ~100 °C. As an alternative option, Deshpande et al. [171] introduced a hybrid biopolymer PCM using 70 wt% PEG and 30 wt% lignin. An ethanol+DI water solvent mixture (3:1 ratio) was used to facilitate microwave blending. This combination of PEG and lignin offers a low-cost product. It is important to note that there is a slight hysteresis, a 22% enthalpy drop, and that high PEG loading (more than 70%) might cause leakage.

Some studies have proposed PCMs using a combination of three different polymers. Yin et al. [172] developed a hybrid biopolymer PCM with h_m = 189.5 J/g with a density of up to 1130 kg/m³. This hybrid biopolymer PCM consists of three components: PLR, chitosan, and sodium alginate, to produce form-stability with no leakage and sustainability through biopolymer use and solvent-free processing. Chitosan acts as a structural stabilizer, while sodium alginate acts as a 3D foam stabilizer. Combining PLR with natural biopolymers, such as chitosan and sodium alginate, offers improved porosity and shape control. However, chitosan foams may shrink during freeze-drying and sodium alginate may allow leakage without PLR due to large pores. Although the proposed PCM offered high latent heat in this study, the thermal conductivity has not yet been optimized. Xie et al. [173] introduce a bio-based, biodegradable, and non-carbonized PCM system using radish as a porous substrate and PDA as a functional interlayer. The composite consists of PGE, PDA, and radish, which offer leakage prevention and high latent heat at h_m = 161.52 J/g at T_m = 64.77 °C. The hybrid biopolymer PCM was proposed to address PEG leakage, avoid carbonization damage, improve PEG encapsulation, and enhance light absorption for solar thermal conversion. In this case, PDA functioned as an interlayer to bind PEG and enhance light absorption, while radish was used as a porous biomass substrate for PEG loading (i.e., porous support). However, no conductivity enhancement materials were added.

There is growing interest in developing PCMs through the combination of different types of biopolymers, particularly in the context of TES systems. Among these, combinations of natural and synthetic biopolymers present a promising approach for engineering PCMs with balanced and enhanced thermal and mechanical properties. While natural–natural polymer combinations are environmentally friendly and biodegradable, they often face limitations such as lower thermal conductivity, narrower thermal operating ranges, reduced long-term stability, and leakage during melting. On the other hand, synthetic–synthetic polymer combinations generally offer improved thermal stability, mechanical strength, and greater control over processing conditions, but may lack biodegradability and sustainability. The combination of synthetic–synthetic biopolymers may also suffer from leakage during melting. In contrast, natural–synthetic biopolymer combinations offer promising approaches. By blending the renewability and eco-friendliness of natural polymers with the functional versatility and structural integrity of synthetic ones, these hybrid systems provide greater design flexibility and wider property tunability. This allows researchers to tailor PCMs to meet specific application requirements, such as desired melting temperature, energy storage capacity, thermal conductivity, and mechanical durability. The natural–synthetic PCM combinations discussed above demonstrate a diverse range of thermal behaviors—including melting enthalpy, melting temperature, thermal conductivity, and density—as well as mechanical performance characteristics. These combinations can be fine-tuned to optimize both energy efficiency and material sustainability. For a comparison and evaluation, the thermal and mechanical properties of these materials are summarized in

Table 6. Summarized thermal and mechanical properties of combined natural–synthetic (hybrid) biopolymer PCMs.

No	Biopolymer PCM	Ratio	Type of Polymer	hm [J/g]	Tm [°C]	k [W/m·K]	ρ [kg/m3]	Mechanical Properties	Refs.
1	PU/leather waste/glycidyl methacrylate/AIBN	60/10/30/0.001 wt%	protein-thermosetting	36.73	46.9	n.a.	n.a.	n.a.	[126]
2	PEG/(PEG+4,4’-diphenylmethane diisocyanate/glucose)	70/30 wt%	polyether-glucose	131.9	61.11	n.a.	n.a.	n.a.	[127]
3	PEG/4,4’-diphenylmethane diisocyanate/Xylitol	n.a.	polyether-xylitol	76.37	41.65	n.a.	n.a.	n.a.	[128]
4	PEG/(Castor oil+HDI)	85.43/14.57 wt%	polyether-thermosetting-lipid hybrid	117.7	51.4	n.a.	n.a.	n.a.	[129]
5	Waste cooking fat/PP	75/25 wt%	lipid-synthetic polymer	20.29	32	0.213	n.a.	σt = 3 MPa	[130]
6	FHS/D4	66.67/33.33 mol%	lipid-siloxane	202.3	151.75	n.a.	n.a.	n.a.	[131]
7	Organosolv Lignin/Butylene 3Oxide/Co-polymerized Tetrahydrofuran	15.7/5.1/79.2 wt%	synthetic polymer-polyphenolic	53.7	19.9	n.a.	n.a.	n.a.	[132]
8	(1-Tetradecanol/(PMMA + PETRA))/Lignin nanoparticles	(66.7/33.3)/3% wt%	synthetic polymer-polyphenolic	190	43.84	n.a.	n.a.	n.a.	[133]
9	PEG/Poly(glycidyl methacrylate)/leather waste/AIBN	70/20/10/1.5 wt%	protein-polyether	152.6	56.3	n.a.	n.a.	σt = 3.08 MPa	[134]
10	PA/PLA	~40/~60 wt%	lipid-aliphatic polyester	70.1	62.1	n.a.	n.a.	n.a.	[135]
11	(ASO+fatty acids)/Darocure 1173	(67 + 33 wt%)/3 wt%	lipid-acrylate	67.51	73.07	n.a.	n.a.	n.a.	[136]
12	PA/COPUA	70/30 wt%	lipid-thermosetting	141.2	66.6	n.a.	n.a.	n.a.	[137]
13	Beeswax/DGEM-18/FA/MA crosslinked polymer network	52.4/47.6 wt%	lipid-thermosetting	119.1	48.3	n.a.	n.a.	σt = ~2 MPa	[138]
14	PEG/CNF/EG/BN	63/15/7/15 wt%	polysaccharide-polyether	79.46	59.51	10.83	n.a.	n.a.	[139]
15	MA/PU	30/70 wt%	polysaccharide-polyether	136.9	64.6	2.78	n.a.	n.a.	[140]
16	PEG/BNNSs-g	n.a.	polysaccharide-polyether	150.1	45.2	0.59	n.a.	n.a.	[141]
17	PEG/SA	93/7 wt%	polysaccharide-polyether	156.8	59	n.a.	n.a.	n.a.	[142]
18	PEG/(SA+PDA@ZrP)	92.75/7.25 wt%	polysaccharide-polyether	159.8	n.a.	n.a.	n.a.	n.a.	[143]
19	PEG/sweet potato foam/CsxWO3	70.1/29/0.99 wt%	polysaccharide-polyether	137.7	61.9	n.a.	n.a.	n.a.	[144]
20	PEG/Gum Tragacanth/biochar	63/27/10 wt%	polysaccharide-polyether	110.5	58.1	0.038	126	σc = 0.702 MPa	[145]
21	PEG/Cellulose Acetate	96.5/3.5 wt%	polysaccharide-polyether	155.35	60.56	n.a.	n.a.	n.a.	[82]
22	PEG/TOCNF	75/25 wt%	polysaccharide-polyether	137.2	58.8	n.a.	n.a.	E = 3000 MPa and σm = 39 MPa	[150]
23	PEG/cellulose	90.1/9.9 wt%	polysaccharide-polyether	151.8-170.5	58.9-59	00.21	n.a.	n.a.	[156]
24	PEG/CNC	97/3 wt%	polysaccharide-polyether	151.8	33.5	0.44	n.a.	n.a.	[148]
25	PEG/CNF	85/15 wt%	polysaccharide-polyether	146.2	65.4	0.040	n.a.	σt = 28 MPa	[146]
26	PEG/CNC	86/14 wt%	polysaccharide-polyether	140.3	30.6	0.42	35	σc80% strain = 0.1145 MPa	[149]
27	PEG/Polypyrrole-coated CNF	94/6 wt%	polysaccharide-polyether	169.7	57.4	n.a.	n.a.	σc = 4.5 MPa	[147]
28	PEG/CA	n.a.	polysaccharide-polyether	91.5	37.9	0.068	200	n.a.	[155]
30	PEG/CNC	n.a.	polysaccharide-polyether	82.3	47.1	n.a.	n.a.	n.a.	[157]
31	PEG/(CNC+PNIPAM)	92/8 wt%	polysaccharide-polyether	178.4	54.2	n.a.	n.a.	σc = 0.0362 MPa and Ec = 0.1829 MPa	[158]
32	PEG/Cellulose/Dopmine	90/9.4/0.6 wt%	polysaccharide-polyether	194.3	59.3	n.a.	n.a.	n.a.	[159]
33	mPEG+CHO/Chitosan	75/25 wt%	polysaccharide-polyether	49.03	46.42	n.a.	n.a.	σt = 1.36 cN/dtex	[160]
34	PEG/(Bacterial cellulose+Mxene)	97.9/2.1 wt%	polysaccharide-polyether	196.7	67	n.a.	n.a.	n.a.	[161]
35	PEG/Microcystalline Cellulose/Graphene Nanoplatelets	97.47/1.02/1.51 wt%	polyether-polysaccharide	182.6	~67.6	1.03	n.a.	n.a.	[154]
36	PEG/4,4′-Diphenylmethane diisocyanate/β-CD	28/56/16 mol%	cyclic oligosaccharide-polyether	115.2	60.2	n.a.	n.a.	n.a.	[152]
37	Butyl stearate/hexamethylene diisocyanate/Chitosan	66.67/30.67/2.66 wt%	polysaccharide-thermosetting	104.1	~24	n.a.	n.a.	n.a.	[153]
38	(LA+CMC)/PLA	85/15 wt%	polysaccharide-aliphatic polyester	86.4	40.1	n.a.	n.a.	σt = 27.61 MPa and σflex = 50.02 MPa	[151]
39	Butyl Stearate/crosslinked pullulan matrix	n.a.	aliphatic ester-polysaccharide	33	26.2	N/A	880	E = 15 MPa	[162]
40	Wood flour/PEG	25/75 wt%	lignocellulosic-polyether	108.6	47.7	n.a.	n.a.	n.a.	[163]
41	Pomelo Peel Foam/PEG	3.8/96.2 wt%	lignocellulosic-polyether	158.1	62.7	0.35	n.a.	n.a.	[169]
42	PEG/Wood powder	97/3 wt%	lignocellulosic-polyether	134.2	63.5	n.a.	n.a.	n.a.	[167]
43	PCC/PEG/RPUF	12.5/12.5/75 wt%	lignocellulosic-polyether	51.99	n.a.	0.0542	n.a.	σc = 1.154 MPa	[165]
44	SCGs/PEG	39.7/60.3 wt%	lignocellulosic-polyether	104.7	63	0.336	n.a.	n.a.	[166]
45	Bamboo flour/PEG	30/70 wt%	lignocellulosic-polyether	113	64.19	0.5	n.a.	n.a.	[164]
46	Pomelo Peel Flour/PEG	10.1/89.9 wt%	lignocellulosic-polyether	143.2	66.4	n.a.	n.a.	n.a.	[168]
47	Lignin/PCL	12.6/87.4 wt%	polyphenolic-aliphatic polyester	61.16	51.33	n.a.	n.a.	n.a.	[170]
48	Lignin/PEG	30/70 wt%	polyphenolic-polyether	100.91	60.33	0.31	n.a.	n.a.	[171]
49	PEG/(SAT+PLR)	95.22/4.78 wt%	polysaccharide-supramolecular-polyether	178.4	n.a.	n.a.	1120	n.a.	[172]
50	PEG/(Radish+PDA)	95/5 wt%	polysaccharide-eumelanin-polyether	161.52	64.77	n.a.	n.a.	n.a.	[173]

.

7. Discussion and Future Directions of Research

Natural biopolymer PCMs have shown potential for sustainable TES applications. Lipid-based biopolymer PCMs, such as coconut oil, waste animal fats, and soybean oil, offer good latent heat value, a tunable melting point, and eco-friendly sources. Fatty acids synthesized from soybean oil demonstrate high latent heat, which may be suitable for high-temperature TES, while waste fats from food processing and coconut oil provide viable options for low- and medium-temperature applications with good shape stabilization when embedded in polymer matrices. Moreover, lignin-derived biopolymer PCMs and lignocellulosic composites bring promising structural integrity, optical tunability, and good latent heat capacities, which make them suitable for building materials and other applications.

Several biopolymer PCMs face several material and processing challenges. These include low inherent thermal conductivity, mechanical fragility, and leakage risks under long-term thermal cycling. The degradation of PCM performance over cycles is often attributed to phase segregation, polymer softening, and diffusion of the PCM out of the matrix. To address these issues, reinforcement strategies, such as incorporating lignin nanoparticles, activated carbon, or high-aspect-ratio natural fibers like flax, have been implemented to improve matrix rigidity, reduce phase separation, and increase encapsulation efficiency. The combining of several natural–natural biopolymers, such as proteins, polysaccharides, and lipids, has been developed to take advantage of the complementary properties of each material (e.g., the flexibility and film-forming ability of gelatin, the porosity and thermal conductivity of cellulose, and the high latent heat of fatty acids) to produce multifunctional, robust, and reusable PCM composites. However, there is still a lack of studies on the combination of pure natural–natural biopolymers, such as PCMs, in this case. Further investigations into this combination are required to advance the use of natural biopolymer PCMs.

Based on studies in the literature, supramolecular systems, like PLR combined with PEO or PPP, use sacrificial hydrogen bonds and physical entanglement to achieve high latent heat and mechanical flexibility, while aliphatic polyesters, such as PLA–PCL blends, utilize PCL’s soft segments to improve thermal cycling durability and impact resistance. Polyether-based PCMs, including UV-crosslinked PEO and PEG–MDI networks, achieve leak-free operation and high enthalpy through covalent crosslinking, although challenges, like supercooling (e.g., ~13 °C in PGI–PEG) and reduced crystallinity at high additive ratios, persist. The most widely developed PCMs center on PEG and PLA due to their versatility, high latent heat, and compatibility with functional additives.

Despite progress, synthetic biopolymer PCMs face some challenges. Material-level limitations include inherent trade-offs between latent heat, crystallinity, and mechanical stability; for instance, high additive ratios (e.g., PPP, CDHM) reduce enthalpy and phase change efficiency, while excessive PEG content (>70%) compromises structural integrity, causing leakage. Future research directions emphasize mitigating supercooling via nucleating agents and enhancing thermal conductivity. Advanced manufacturing techniques, such as 3D printing and coaxial electrospinning, offer pathways for structural integration and leakage prevention, while stimuli-responsive networks (e.g., D–A reactions) could enable self-healing or tunable phase transitions. Prioritizing eco-friendly scalability and long-term durability testing (1000+ thermal cycles) will be critical for commercial viability, particularly in applications like wearable textiles, building insulation, and medical devices.

The combining of natural biopolymers with synthetic biopolymers creates a hybrid/composite that enhances performance and sustainability. Natural biopolymers, such as cellulose, chitosan, and starch, contribute functional groups (–OH or –NH₂) that form hydrogen bonds with PEG, strengthening structural integrity and improving shape stability while enhancing energy retention during phase transitions. These natural components serve as nucleating agents that enable controlled crystallization, reduce supercooling, and ensure durability across repeated thermal cycles. Synthetic biopolymers, including PLA, PU, and PGMA, contribute mechanical strength, chemical crosslinking potential, and structural support, thus complementing the flexibility and eco-friendliness of natural materials.

Moreover, integrating functional fillers, such as BN, EG, and graphene nanoplatelets, addresses the polymer matrices’ inherent low thermal conductivity, improving heat transfer and system efficiency. In addition to thermal performance, these hybrid biopolymer PCMs often possess multifunctional capabilities, such as light-triggered responsiveness, shape memory, or electro-thermal conversion, expanding their potential in smart textiles, solar energy systems, and biomedical applications. From a sustainability perspective, the use of renewable natural materials reduces reliance on fossil-based inputs, while water-based processing methods using dispersible biopolymers (e.g., wood flour, pomelo peel, or coffee grounds) offer an eco-friendly and cost-effective alternative to harsh chemical synthesis routes. Despite these advantages, several challenges remain. Leakage during phase transition is still of concern, especially in systems with high PCM content, where physical confinement methods may sacrifice latent heat capacity; in addition, chemical crosslinking can suppress crystallinity. Balancing thermal conductivity improvements with material flexibility and phase change efficiency is another hurdle, as excessive filler loading can lead to aggregation and performance loss. Moreover, processing challenges arise with rigid natural polymers, like cellulose and lignin, due to their strong internal bonding and limited solubility, which require specialized solvents or complex modifications. Hybrid systems also face issues related to recyclability and biodegradability when synthetic components or toxic crosslinkers are introduced, which potentially compromise end-of-life sustainability. Ensuring interfacial compatibility between components is critical to avoid phase separation and maintain mechanical integrity. Therefore, the suitable selection of modifications to all components is necessary.

Furthermore, another significant concern is long-term reliability. While many systems demonstrate good thermal properties, which maintain performance over hundreds of heating/cooling cycles without degradation, this remains a promising research focus. Oxidation, structural breakdown, and reduced latent heat due to crystallinity loss are some concerns, especially for lipid-rich or porous biopolymer matrices. Antioxidants and more stable encapsulants are required, although these can add to the cost and processing complexity. In addition, some encapsulation strategies, particularly those involving non-biodegradable or chemically crosslinked materials, may compromise the overall environmental safety and biodegradability of the system. This challenge emphasizes the need to develop green encapsulation approaches that ensure thermal stability without sacrificing sustainability. While hybrid biopolymer PCMS show potential by combining environmental benefits with advanced functionality, further innovation is needed to optimize compatibility, scalability, and long-term stability. Future research efforts should focus on refining green fabrication processes, improving filler dispersion, enhancing interfacial bonding, and redesigning hybrid networks that achieve both mechanical robustness and high thermal performance for TES applications.

Figure 13 compares the thermal properties of several natural, synthetic, and hybrid biopolymer PCMs from current studies in the literature on melting temperature and enthalpy. Natural biopolymer PCMs, represented by green circles, exhibit melting temperatures from 6.1 °C to 63.3 °C. Their materials are characterized by varied melting enthalpies, from 40.8 J/g to 170.2 J/g, showing that some of the materials with high latent heat are suitable for TES applications. In contrast, the synthetic biopolymer PCMs, shown as red triangles, display a broader range of melting temperatures, from 35.7 °C to nearly 176.4 °C. Their melting enthalpies are between 24.5 J/g and 146.60 J/g. Moreover, hybrid biopolymer PCMs, represented by black squares, appear to offer a compromise between natural and synthetic ones. The hybrid biopolymer PCMs, which can be developed using dual or ternary natural–synthetic polymers, offer melting temperatures in the range of about 19.9 °C to 151.75 °C while their melting enthalpies fall between 20.29 J/g and 202.3 J/g. Several hybrid biopolymers offer significant advantages in terms of improved thermal stability, structural integrity, and tunable properties. By combining natural and synthetic components, hybrid PCMs are engineered to overcome the limitations of each individual type. The natural component contributes to biodegradability and higher latent heat, while the synthetic portion enhances the material’s resistance to thermal degradation, leakage, and phase separation during repeated thermal cycles. As a result, hybrids tend to maintain their performance over more extended periods and under varying operational conditions. This makes them particularly suitable for applications where reliability, long-term durability, and consistent phase change behavior are critical, such as TES, building materials, thermal management in electronics, smart textiles, and so on.

There is still a lack of development in hybrid biopolymer PCMs designed for applications requiring operating temperatures above 100 °C. Most current biopolymer PCMs are optimized for moderate temperature ranges, typically between 30 °C and 70 °C, which align well with applications like building insulation and thermal management. However, it limits their suitability for moderate- and high-temperature TES applications, such as industrial waste heat recovery via ORC, concentrated solar power, and advanced electronics cooling, where materials must withstand repeated thermal cycling, elevated temperatures, and potential thermal degradation. Therefore, developing biopolymer PCMs that maintain thermal reliability, structural integrity, and high latent heat capacity at temperatures exceeding 100 °C is critical for expanding their applicability into more demanding high-energy applications.

Furthermore, the industrial adoption of biopolymer PCMs may face significant scalability challenges. Key constraints include reliable feedstock supply, cost-effective processing (especially purification and encapsulation), integration into existing systems, and safety considerations. Further barriers are the lack of standardized testing methods and validated performance data, which creates uncertainty for manufacturers and end-users. Many of the biopolymer PCMs mentioned above remain only at the laboratory scale. Broad industrial uptake will require addressing several interlinked areas: cost, processing efficiency, integration engineering, safety considerations, standardization, and full life-cycle performance validation.

Biopolymer PCMs have gained increasing attention, not only for TES in renewable energy systems, but also for their biocompatibility and biodegradability, two properties that are essential for biomedical fields. These materials are increasingly utilized in biomedical systems that require precise and safe thermal control, including smart drug delivery, thermal regulation dressings, vaccine cold-chain logistics, and medical thermal insulation.

In cancer therapy, PCMs offer a promising platform for temperature-triggered smart drug delivery. They enable the release of encapsulated drugs precisely at hyperthermic tumor sites, minimizing off-target toxicity and enhancing therapeutic efficacy. Natural fatty acid–based PCMs are particularly attractive due to their low toxicity, bioresorbability, and capacity to maintain localized heating in thermo-chemotherapy. Bao et al. [174] emphasized that PCMs can serve as efficient thermal gatekeepers for antitumor nanocarriers, ensuring drug stability during circulation and controlled release at elevated temperatures.

In wound healing, thermal regulation dressings incorporating PCMs maintain local wound temperatures within the optimal healing range (33–42 °C), which supports cellular activity and reduces inflammation. Unlike conventional dressings, these systems actively buffer temperature fluctuations. Xu et al. [175] developed bilayer-shell nano-encapsulated PCMs using eutectic fatty acids encapsulated with poly(methyl methacrylate) and chitosan, achieving high enthalpy (~99 J/g) and anti-permeability efficiency (>96%), while providing stable thermoregulation in hydrogel-based wound dressings.

In the context of vaccine cold-chain logistics, biopolymer-based eutectic PCMs can stabilize temperature-sensitive biological products in the critical 2 to 8 °C range. Liu et al. [176] engineered a composite phase change material composed of decyl alcohol and LA supported by EG, which achieved a high latent heat (~188.7 J/g) and enhanced thermal conductivity by incorporating expanded graphite, ensuring rapid thermal response and long-duration cold storage, which make it highly suitable for vaccine transport. Another notable application lies in biomedical thermal insulation. Liu et al. [142] designed a PEG/sodium alginate-based PCM composite with high PEG content and excellent form stability, maintaining structural integrity without leakage up to 95 °C. The material shows reversible thermal behavior, making it suitable for reusable medical pads, implantable thermal buffers, and wearable therapeutic devices that require stable and biocompatible thermal regulation.

8. Conclusions

This article provides a comprehensive review of various biopolymer PCMs. These materials are categorized into three main types: natural biopolymer PCMs, synthetic biopolymer PCMs, and hybrid biopolymer PCMs. This article discusses the characterization of the biopolymer PCMs and existing research gaps in this field, and aims to offer a systematic review of the current state of knowledge regarding biopolymer use in PCMs as TES working substances. Several key findings from this review are summarized as follows:

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This article provides a comprehensive classification of natural and synthetic biopolymer materials, highlighting their suitability and potential in developing PCMs. The natural biopolymer materials include polysaccharides, polyphenolics, lignocellulose, proteins, lipids, and so on. Several naturally derived materials were described. The synthetic biopolymer materials include supramolecular, polyether, aliphatic polyester, polyolefin, and others. Several examples of materials based on the classification have been briefly described;

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Several natural biopolymers, such as lipids, lignin, polysaccharides, protein, and others, offer a sustainable and promising approach for developing high-performance PCMs for TES. Some innovations, such as solvent-free synthesis of fatty acid amides, lignin-stabilized microencapsulation, electrospun core-shell fibers, and transparent wood composites, represent state-of-the-art approaches that improve thermal reliability, structural integrity, and multifunctionality. Despite challenges like limited thermal conductivity and long-term stability, these advancements highlight the growing potential and interest of biopolymers in next-generation applications, including smart textiles, passive building cooling, and industrial waste heat recovery;

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Furthermore, synthetic biopolymer PCMs demonstrate significant potential for sustainable TES. Innovations, like supramolecular PLR–PEO networks, PLA–PCL blends, and PEG–MDI crosslinked systems achieve high latent heat, mechanical flexibility, and leakage resistance. The combination of synthetic PCMs also demonstrates promising solutions with enhanced thermal conductivity. However, challenges, such as supercooling, trade-offs between crystallinity and additive ratios, and the scalability of solvent-free methods, remain barriers in this case. More studies on synthetic biopolymer PCMs are still required for further improvements;

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Hybrid biopolymer PCMs offer sustainability and high-performance solutions for TES by combining the structural and functional benefits of natural and synthetic polymers with conductive fillers. While they demonstrate enhanced energy retention, shape stability, and multifunctionality, challenges, such as leakage, low conductivity, and long-term reliability, still persist;

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For comparison, hybrid biopolymer PCMs offer a balanced combination of high latent heat from natural polymers and enhanced thermal stability and durability from synthetic components. Their melting enthalpy may be slightly higher than some natural PCMs. In addition, their tunable properties and long-term reliability make them well-suited for demanding TES applications.

Future research might focus on enhancing long-term thermal reliability, improving filler dispersion and interfacial bonding, and developing scalable, green fabrication methods. In addition, an increase in the use of biodegradable, low-cost bio sources and exploring novel ternary or multifunctional hybrid systems can further optimize PCM performance for broader industrial and environmental applications. In addition, advancing biopolymer PCMs that can maintain thermal stability and energy storage performance above 100 °C is promising for broader applications in industrial, solar, and electronic thermal management systems. The advancements not only offer improved thermal efficiency but also align with global goals for energy sustainability. Biopolymer PCMs represent an alternative option to petroleum-based materials by reducing environmental impact, lowering carbon emissions, and supporting circular economy initiatives.