A Comparative Review of Natural and Synthetic Biopolymer Composite Scaffolds

Tissue engineering (TE) and regenerative medicine integrate information and technology from various fields to restore/replace tissues and damaged organs for medical treatments. To achieve this, scaffolds act as delivery vectors or as cellular systems for drugs and cells; thereby, cellular material is able to colonize host cells sufficiently to meet up the requirements of regeneration and repair. This process is multi-stage and requires the development of various components to create the desired neo-tissue or organ. In several current TE strategies, biomaterials are essential components. While several polymers are established for their use as biomaterials, careful consideration of the cellular environment and interactions needed is required in selecting a polymer for a given application. Depending on this, scaffold materials can be of natural or synthetic origin, degradable or nondegradable. In this review, an overview of various natural and synthetic polymers and their possible composite scaffolds with their physicochemical properties including biocompatibility, biodegradability, morphology, mechanical strength, pore size, and porosity are discussed. The scaffolds fabrication techniques and a few commercially available biopolymers are also tabulated.


Introduction
Tissue engineering (TE) is the in vitro construction of bioartificial tissues and in vivo modification of cell growth and function through the implantation of appropriate cells isolated from donor tissues to generate biocompatible scaffold materials [1]. This approach specifically focuses on the vital imbalance between the rising number of patients waiting for organ transplantation due to end-stage failure and a limited number of donated organs available for those procedures [2]. TE and regenerative medicine integrate information and technology from various fields such as genetics, engineering, pharmaceutics, medicine, chemistry, and materials sciences to perform treatments or to restore or replace damaged tissues and organs [3][4][5]. It holds the promise of sustainable development due to ever-going improvement in biomaterials and implies the procedure of fusing scaffolds, molecules, and cells that are biologically active into functional tissues. The ultimate goal is to completely monitor, create a functional structure/support to repair, preserve, or improve damaged tissues or entire organs and to implement "enhanced and sustainable quality of life (QOL) with health" as stated in the prime goal of the World Health Organization (WHO) [6,7].
In this field, two primary approaches are used to generate engineered tissues. Primarily, scaffolding is used as a cell supporting system for seeding cells in vitro, and further cells In TE along with regenerative medicine, scaffolds may act as delivery vectors or as cellular systems for drugs and cells. The other choice is to combine scaffolds with different cell types that can enhance osteogenic lineage tissue formation in vivo or release unique soluble lineage molecules. Before being implanted into the target site, these cells can be expanded selectively ex vivo. Scaffolds in clinical medicine are upcoming areas of considerable significance. They are typically associated with organ disease or failure conditions and used to repair organs to restore normal functionality [29,30]. It is well-known that scaffolds support and promote growth of regenerative cells and perform a major role in TE efficiency. Besides, the scaffolding biomaterial facilitates proliferation, differentiation, cell adhesion, offers mass transport and temporary 3D mechanical support, and finally causes the formation of neo-tissue (newly formed tissue built around a scaffold) [31]. In TE along with regenerative medicine, scaffolds may act as delivery vectors or as cellular systems for drugs and cells. The other choice is to combine scaffolds with different cell types that can enhance osteogenic lineage tissue formation in vivo or release unique soluble lineage molecules. Before being implanted into the target site, these cells can be expanded selectively ex vivo. Scaffolds in clinical medicine are upcoming areas of considerable significance. They are typically associated with organ disease or failure conditions and used to repair organs to restore normal functionality [29,30]. It is well-known that scaffolds support and promote growth of regenerative cells and perform a major role in TE efficiency. Besides, the scaffolding biomaterial facilitates proliferation, differentiation, cell adhesion, offers mass transport and temporary 3D mechanical support, and finally causes the formation of neo-tissue (newly formed tissue built around a scaffold) [31].
In TE applications, the biological crosstalk between the scaffold and the cells is controlled by the properties of the materials and final scaffold characteristics. Materials used for scaffold manufacturing must have intrinsic biofunctionality and appropriate chemistry to stimulate molecular biorecognition from cells to induce proliferation, cell adhesion, and activation. The mechanical properties of the scaffold and kinetics of decomposition in selected materials must be adjusted to the TE application, specifically to ensure the essential structural features and to achieve the rate of new tissue formation. The final effectiveness of the regenerative process plays a major role in scaffolding, exposed surface area, pore Depending on the intended application, scaffold materials can be natural or synthetic, degradable or nondegradable. The polymer's properties depend on their constituent macromolecules' structure, composition, and arrangement. The principal forms of polymers used as biomaterials are biologically natural polymers, synthetic biodegradable and nonbiodegradable polymers as shown in Figure 2. Because of their specific characteristics, such as a wide range of biodegradation rates, high porosity with various pore sizes, high surface-to-volume ratio, and mechanical property, polymeric scaffolds attract great interest. They offer distinct benefits of biofunctionality, flexibility, and biological properties that are essential in TE and biomedical applications [31,[41][42][43].

Natural Biopolymer-Based Scaffolds
Natural biopolymers have resurged over the past few decades as primary bioactive substances used in the applications of medical materials. Based on their monomeric units and structure, biopolymers are categorized roughly into three classes [27,44]:

Natural Biopolymer-Based Scaffolds
Natural biopolymers have resurged over the past few decades as primary bioactive substances used in the applications of medical materials. Based on their monomeric units and structure, biopolymers are categorized roughly into three classes [27,44]: • Polypeptide-and protein-based: collagen, fibrin, fibrinogen, gelatin, silk, elastin, myosin, keratin, and actin.
polyampholyte surface property. However, gelatin is negative at higher pH and positive at lower pH [59]. Gelatin polymer is primarily limited by its high biodegradation rate due to enzyme digestion and high physiological solubility which describes its low mechanical stability, leading to a disparity between the new formation of bone and the degeneration of scaffolds. Alongside this, the role of higher-order gelatin structures and bioactivity of scaffolds is still raising many unanswered questions. The most important thing is whether the cells are sensitive to the secondary and higher-order gelatin structures in the scaffolds [60]. At the same time, due to the existence of active chemical groups (e.g., NH 2 and COOH), chemical treatments may be performed to increase the degradation period [61].
Silk is a natural protein-based polymer derived from various Lepidoptera larvae, such as spiders, as well as silkworms. In nature, silk displays numerous combinations, structures, and functions. This complex behavior of silk is induced by its source and atmosphere. Silk fiber is a perfect blend of high strength, low weight, excellent durability, and elasticity (the strongest natural fiber). Silk is made up of two separate main proteins; one is silk fibroin (SF) made from the fibrous portion of the filament and another is sericin, which is a gluelike and water-soluble protein containing 18 different kinds of amino acids [62]. Studies have shown that, in addition to a tenable degradation rate and mechanical properties, the manufacturing of silk fibroin-based scaffolds of varying configurations yields attractive biocompatibility. Fibroin has been shown to have predictable proteolytic degradation in comparison to other biological materials by modifying fibroin diameter, several failure intervals, failure strength, and mass degradation [63]. Apart from the extraordinary mechanical properties, silk is biocompatible, thermostable (up to~250 • C), and processable in a wide range of temperatures. Several scientists have tested composite silk scaffolding to achieve the necessary characteristics by changing the blended materials, the silk-producing source, or the material concentration in that composite, and it was proven very suitable for the TE. [64]. The versatility of silk fibroin can be seen in its various applications, from silk as a bulk part to silk as a coating or reinforcement of non-cytocompatible scaffolds. Silk fiber inclusion increases the compression strength in both in vivo and in vitro tests, minimizing setting time without adverse impact on injectability and cytocompatibility. Silk fibroin is one of the products that are favorable for bone tissue scaffolding applications due to its specific moderate mechanical properties, more controllable degradation rate than of many natural polymers, and high biological compatibility [62].
Fibrinogenic and fibrin-based scaffolds can provide an adequate environment for the natural matrix. Provided that these primary materials are available widely as the main coagulation proteins in blood, native biochemical associations with damaged tissues and cells can be easily communicated [65]. Similar to collagen scaffolds, fibrinogenic and fibrinbased scaffolds may achieve high efficiency of cell seeding and uniform cell distribution by proliferating, migrating, and differentiating into specific tissues/organs through the secretion of the ECM. There are also several drawbacks such as weak mechanical properties for the regeneration of skeletal tissues, the potential for transmission of diseases through unpredictable biological affinities, and fibrin deformation [66]. Fibrinogen provides significant healing benefits as it provides an attractive proliferation surface, cellular attachment, 3D fibrous structural support, and nanotextured surfaces consisting of a fibrous cell signaling network and cell-cell interactions. [67]. Fibrinogen-and fibrin-based scaffolds induce ECM development in TE for supporting connective tissues like nerves, blood vessels, skin, ligaments, bones, cartilages, and tendons. Fibrin-based scaffolds promote and offer enough time for neo-matrix development while resorbing gradually due to the action of proteases. These therapeutic assets facilitate wound healing and reduce the formation of scars for more natural, functional, and esthetic characteristics [68].

Polysaccharide-Based Scaffolds
Another group of naturally occurring polymers is polysaccharides made of different units of monosaccharide or disaccharide chains (e.g., starch, cellulose, etc.). The effect is an incredibly large number of structurally diverse polysaccharides as numerous distinct saccharide isomers are mixed by utilizing a range of chemical bonds. The polysaccharide chemistry is as rich as of proteins in terms of diversity and heterogeneity. Therefore, it is not unexpected that different saccharides and polysaccharides perform a major role in finely tuning cell environmental response [69]. It is possible to categorize polysaccharides into structural and storage polysaccharides. Cellulose in plants and chitin in crustacean shells are examples of structural polysaccharides, while starch and glycogen can be included in storage polysaccharides [70]. Despite these benefits, there are some restrictions on the use of natural polysaccharides to prepare scaffolds. Their distribution, branching, and sequence of molecular weight are not consistent. These differences may be deleterious for biorecognition events as well as affect rheology. Generally, many naturally occurring polysaccharides are not biodegradable when introduced in mammalian species because of the absence of digestive enzymes. As a result, these are not a primary material option in the biomedical application without further chemical alteration [71].
Chitin and chitosan are interesting materials for biomedical and pharmaceutical applications because they have positive properties that make them ideal in the biomedical field, such as non-toxicity, biodegradability, and biocompatibility [72]. These materials often reflect a wide range of proprieties owing to their reactive hydroxy and amino groups, high charge density, as well as their broad hydrogen-bonding capacities and the single chemical structure. The combination of diverse physicochemical and biological features allows a vast variety of biomedical uses [73]. Chitin is generally found in shells of crustaceans and its derivative chitosan is obtained by deacetylation of chitin. These are glycosaminoglycanlike natural cationic polysaccharides [74]. Applicability of chitosan includes implantable and injectable orthopedic and periodontal devices, wound healing agents, lung surfactant additives, drug delivery systems, and TE scaffolds due to its high biodegradability and biocompatibility along with its unique interactions with ECM components and growth factors [75]. Owing to the excess of their reactive amino and hydroxy groups and cations, chitin and chitosan are coupled with other molecules to boost the biological functions of other materials in implant products. For instance, it is established that the hydrophilicity of other biomaterials and their biocompatibility are improved by chitosan coating. These chitosan-coated composites can promote cell proliferation and adherence [76,77]. The key route for the in vivo breakdown of chitin and chitosan is known to be lysozymes which slowly act to depolymerize the polysaccharide. The biodegradation rate depends on the acetyl content quantity, which is an easily variable parameter. Chitin and chitosan modification producing significant products with enhanced properties as required for scaffolds has been explored, and research in this field of biomaterials will continue to be pursued [78].
Hyaluronic acid (HA) is a linear polysaccharide, ubiquitous and extremely biologically compatible in the ECM of mammals. HA is a glycosaminoglycan found in many areas of the body in the extracellular tissue [79,80]. It is an increasingly important material for the study of biomaterials and finds applications in different fields stretching from tissue culture scaffolds to cosmetic materials. Its physical and biochemical properties both in the solution and hydrogel forms are highly desirable to different body repair technologies [81]. HA is an essential part of connective tissue where it plays a major role in cell growth, cell differentiation, and lubrication. HA includes functional groups such as carboxylic acids and alcohols that can be used for the implementation of functional domains or the development of a hydrogel by connecting them. HA can form a new type of TE scaffold which is both bioactive and biodegradable. It shows low non-specific protein adsorption and can be tailored to facilitate growth and repair of tissues via cell receptors [82].

Synthetic Biopolymer-Based Scaffolds
Synthetic polymers are advantageous in a few characteristics such as tunable properties, endless forms, and established structures over natural polymers. The support offered by synthetic biomaterials can enable restoration of damaged or diseased tissue structure Polymers 2021, 13, 1105 9 of 51 and function. Polymerization, interlinkage, and functionality (changed by block structures, by combining them, by copolymerization) of their molecular weight, molecular structure, physical and chemical features make them easily synthesized as compared to naturally occurring polymers [83,84]. The disadvantages of synthetic biomaterials are that they lack cell adhesion sites and require chemical modifications to enhance cell adhesion. Many commercially available synthetic polymers exhibit similar physicochemical and mechanical characteristics to biological tissues. In biodegradable polymers, synthetic polymers are a major category and can be produced under controlled conditions. In a broad spectrum, the mechanical and physical characteristics are predictable and reproducible, such as strength, Young's modulus, and degradation rate. Poly(α-hydroxy esters) including PCL, PGA, PLA, and their copolymer PLGA and poly(ethers) including PEO and PEG, PVA, and PU are the most widely studied degradable synthetic materials. These are probably the most popular examples, although there are currently many other synthetic materials being sought [85][86][87]. These polymers have various levels of biodegradability, biocompatibility, and mechanical properties, but no single polymer holds all three of these critical properties at the optimum level [88].
PLA is a gradually crystallizing semicrystalline polymer [89]. Due to its host tissue biocompatibility, hydrophobic nature, relatively simple processability, and biodegradability, PLA is one of the unsurpassed choices for numerous biomedical applications without the need for a second intervention [90,91]. PLA is largely prepared from the lactic acid (LA) monomer through the fermentation process of natural resources such as wheat and grain or by various routes of polymerization as a petrochemical derivative. PLA degradation products, specifically, water and CO 2 , are neither carcinogenic to the human body nor harmful [92]. This substance can be available in many forms, for example, as poly(L-lactic acid) (PLLA), poly(D,L-lactic acid) (PDLA), and poly(D,L-lactic acid) (PDLLA), that can be used for various tenacities, such as for the manufacture of screws, pins, rods, plates, including for biomedical implants, and is suitable for multiple purposes [93]. The easiest linear aliphatic polyester is polyglycolic acid (PGA). It is not soluble in the majority of organic solvents due to its high degree of crystallinity. By random cleavage of its ester linkages in the backbone, it undergoes bulk erosion. Under physiological conditions, PGA breaks down into glycolic acids that can join the tricarboxylic acid cycle and be expelled from the body as water and CO 2 [94]. Polylactic-co-glycolic acid (PLGA) is known as a random ring-opening copolymer of PLA and PGA. PLGA is a biodegradable polymer thanks to its non-toxicity, high cell adhesion, controllable degradation rates, and favorable mechanical properties [95]. The pendant methyl side group on the structure of the PLGA chain causes the hydrophobic surface similar to PLA [96]. In this regard, the degradation rate of PLGA products can be regulated by varying the percentage of these two polymers [97,98].
In TE, PLA, PGA, and their copolymer PLGA are commonly used to treat patients with organs or tissues that have been damaged or destroyed. They have demonstrated their biocompatibility, their deterioration into non-toxic products, and a long history of use in degradable surgical sutures [99]. PLA, PGA, and PLGA degrade via hydrolysis of ester bonds. When degraded, natural pathways eliminate the monomeric components. The body includes highly regulated pathways to fully eliminate lactic and glycolic acid monomeric components. While PGA is converted or removed by other pathways into metabolites, PLA is cleared through the cycle of tricarboxylic acid. Because of these properties, PLA and PGA are used in products and devices approved by the US FDA, including in degradable sutures [99]. PLA and PGA can be simply processed and their physical and mechanical properties and degradation rates can be modified using different molecular weights and compositions of copolymers over a wide range [100,101].
PCL is a semicrystalline and aliphatic polymer that is extremely tough and demonstrates sufficient biocompatibility. PCL's hydrophobic nature prevents cell adhesion and cell proliferation [102]. Initially, PCL degradation takes place in amorphous substance domains, which means that crystal dominances remain untouched. At this stage, non-enzymatic bulk hydrolysis of ester connections catalyzed by the carboxylic acid end groups is carried out [103]. A foreign body response consisting of giant cells and macrophages with a few neutrophils occurs after the material becomes very brittle with extensive hydrolysis [104]. Copolymerization, surface functionalization, or blend formulation are some of the approaches to enhance its bioactivity. The rate of deterioration is relatively slow (2-4 years) and it is degraded by the hydrolysis of its ester linkages under physiological conditions [105].
Polyethylene glycol (PEG)-based polymers, in terms of biomedical applications, are non-ionic, biocompatible, and have optimal physicochemical and biological properties. After implantation, PEG is minimally immunogenic. A variety of cross-link methods are used to manufacture hydrophilic PEG scaffolds. The selected cross-linking process can affect the scaffold's physiochemical characteristics, including permeability, molecular diffusion, elasticity, modulus, or rate of degradation [106]. Poly(ethylene oxide) (PEO) is a hydrophilic polymer that is usually inert with minimal antigenicity, immunogenicity, cell adhesion, and protein binding [107]. Its inhibition of binding proteins is caused by a lack of groups contributing hydrogen. In the 1970s, PEG, the shorter molecular form of PEO, became famous when scientists discovered that the polymer inhibits absorption of proteins. The photopolymerization capabilities of both PEO and PEG include adaptable mechanical features, as well as simple control of the architecture of scaffolds and chemical composition, which all make them appealing scaffold materials for the creation of 3D tissue regeneration templates [108].
Another cross-linkable, biodegradable, high-strength polymeric biomaterial engineered for orthopedic applications is co-polyester poly(propylene) fumarate (PPF). It is a linear polyester with repeating units containing ester bonds [109]. The double bonds of fumarate in PPF can be crosslinked to form polymer networks at low temperatures. They are particularly suitable for orthopedic applications because of high mechanical strength. The ester bond hydrolysis allows PPF to degrade, and degradation time can be affected by several factors (such as molecular weight, curing agent types, and cross-link density) [110]. In the presence of water, PPF degrades into propylene glycol and fumaric acid, products of degradation that are quickly removed from the human body by natural metabolic processes. PPF is commonly used to improve PLA, PGA, or PCL hydrophobicity.
Polyurethane (PU) contains a urethane moiety in its repeating units. The reaction of diisocyanate with polyol normally produces these polymers. In the manufacture of blood-contacting devices such as artificial veins and arteries or heart valves, polyurethanes are the most widely used materials, and have also been used to engineer tissues such as bones, heart muscles, heart valves, blood vessels, skin, skeletal muscles, and cartilages [111]. They provide a large family of materials with the only common feature of urethane links in large molecular chains. Urethane links typically formed by isocyanate and alcohol reactions. In the preparation and treatment of polyurethanes, in addition to the formation of urethane bonds, several other reactions lead to the development of various bonds such as allophane, biuret, acyl urea, or isocyanurate, which may result in further branching or crosslinking affecting the overall physical and chemical properties and biocompatibility [112,113]. Alternative diissocyanin compounds are required to design biodegradable polyurethanes because conventional aromatic diisocyanate is toxic and presumed carcinogenic. Biodegradable diisocyanates, such as lysine diisocyanate or hexamethylene diisocyanate, release non-toxic products while degrading [114].
In general, synthetic polymers, when used as a scaffold, PGA and its copolymers, such as PLGA, degrade too rapidly since their tensile strength decreases by half within two weeks. PLLA, on the other hand, degrades too slowly, taking about 3-6 years for maximum resorption. In recent tissue engineering studies, lactide copolymers, such as lactide-ε-caprolactone copolymers (LA-CL cop), were given preference because of this unsatisfactory resorption property of PGA and PLLA. According to various studies, the degradation rates of some synthetic biodegradable polymers decrease in the following order: PGA~PLGA > PDLLA > PLLA > PCL~PPF > polybutylene succinate (PBS). When comparing, the degradation rate of PCL happens at a considerably slower pace than that of PLA, PGA, and PLGA. This slow degradation makes PCL less attractive for soft tissue engineering applications but is more desirable for long-term implants and controlled release applications [8,13,115] such as bone TE where mechanical and other physical properties should be maintained for at least 6 months by scaffolding. On the other hand, PPF and other biodegradable polymers lack the mechanical strength that is needed for load-bearing applications such as bone TE [116][117][118][119].
While some of the polymer scaffolds mentioned above degrade enzymatically and/or hydrolytically, several other polymers, including PEO, PEG (i.e., the low molecular weight variant of PEO), and PVA, are used to induce more rapidly degrading or instantly soluble characteristics in scaffold materials [85,87]. The mechanical characteristics of PLA differ depending on their molecular weight and optical purity. High molecular weight increases the Young's modulus and strength and decreases the elongation at break. Regularly used PLA (approximately 5% d-lactic acid) is a brittle material, with little elongation at break (6% to 11%), 900-1300 MPa elastic modulus, and 61-73 MPa strength. In contrast to PLLA and PDLA, PLA has great mechanical characteristics [120]. PLA has mechanical properties identical to polyethylene terephthalate (PET), but the maximum continuous usage temperature is significantly lower. Polyhydroxybutyrate (PHB) has little elongation, is brittle, and quickly tears under mechanical pressure [121].
Synthetic polymers are produced from hydrocarbon building blocks in the laboratory setting. Although the intrinsic cell interaction moieties of the biopolymers may be lacking, their capacity to be specifically controlled in structure and reproducibility make them useful along with natural polymers in biomaterial composites for TE applications [122,123].
A variety of polymeric substances used for biomedical scaffolding applications particularly towards regenerative engineering, both naturally and synthetically fabricated, are tabulated in Table 1 along with their advantages and disadvantages.   • Prolonged presence of silk may induce degradation, which releases certain residues or degraded products that may prompt the immune response. [64,126,127]  • Prolonged presence of silk may induce degradation, which releases certain residues or degraded products that may prompt the immune response. • Prolonged presence of silk may induce degradation, which releases certain residues or degraded products that may prompt the immune response.
• Suitable medium for cell encapsulation.
• Nondegradability due to the absence of an appropriate enzyme in the body.
• Mimicking function of the extracellular matrix of body tissue.
• Suitable medium for cell encapsulation.
• Nondegradability due to the absence of an appropriate enzyme in the body.
• Suitable medium for cell encapsulation.
• Nondegradability due to the absence of an appropriate enzyme in the body. [
• Suitable medium for cell encapsulation.
• Nondegradability due to the absence of an appropriate enzyme in the body. [
• Suitable medium for cell encapsulation.
• Nondegradability due to the absence of an appropriate enzyme in the body. [
• Suitable medium for cell encapsulation.
• Nondegradability due to the absence of an appropriate enzyme in the body.
• Have multiple regulatory functions, e.g., in the anticoagulation of blood, inhibition of tumor growth, and metastasis. • Control the inflammatory processes.
• Have multiple regulatory functions, e.g., in the anticoagulation of blood, inhibition of tumor growth, and metastasis.
• Control the inflammatory processes.
• Controls cell proliferation and angiogenesis.
• Slow degradation rate (lower than that of PLA and PLGA).
• Good mechanical properties. • Degraded by hydrolysis or bulk erosion.
• Hydrophobicity of PCL is another major issue that hinders wound healing application.
• Some problems related to withstanding mechanical loads.
• Good degradation rate.
• PLA-based materials suffer from the lack of ideal surface chemistry that could aid cell adhesion and proliferation.
• Excellent cell adhesion and proliferation.
• Good mechanical properties.
• Features faster degradation than either PGA or PLA.
• Wide range of degradation rates.
• High sensitivity to hydrolysis.

Polyhydroxybutyrate (PHB)
• It is a homopolymer having a stereoregular structure with high crystallinity.
• Advantages over PLA and PGA.
• Slow rate of degradation.
• Can be obtained naturally.
• Thermal instability during melt processing impedes its commercial application. • Low bioactivity.
• Hydrophobicity of PCL is another major issue that hinders wound healing application. • Some problems related to withstanding mechanical loads.
• Have multiple regulatory functions, e.g., in the anticoagulation of blood, inhibition of tumor growth, and metastasis.
• Control the inflammatory processes.
• Controls cell proliferation and angiogenesis.
• Slow degradation rate (lower than that of PLA and PLGA).
• Good mechanical properties. • Degraded by hydrolysis or bulk erosion.
• Hydrophobicity of PCL is another major issue that hinders wound healing application.
• Some problems related to withstanding mechanical loads.
• Good degradation rate.
• PLA-based materials suffer from the lack of ideal surface chemistry that could aid cell adhesion and proliferation.
• Excellent cell adhesion and proliferation.
• Good mechanical properties.
• Features faster degradation than either PGA or PLA.
• Wide range of degradation rates.
• High sensitivity to hydrolysis.

Polyhydroxybutyrate (PHB)
• It is a homopolymer having a stereoregular structure with high crystallinity.
• Advantages over PLA and PGA.
• Slow rate of degradation.
• Can be obtained naturally.
• Thermal instability during melt processing impedes its commercial application. • Biocompatibility.
• Have multiple regulatory functions, e.g., in the anticoagulation of blood, inhibition of tumor growth, and metastasis.
• Control the inflammatory processes.
• Controls cell proliferation and angiogenesis.
• Slow degradation rate (lower than that of PLA and PLGA).
• Good mechanical properties. • Degraded by hydrolysis or bulk erosion.
• Hydrophobicity of PCL is another major issue that hinders wound healing application.
• Some problems related to withstanding mechanical loads.
• Good degradation rate.
• PLA-based materials suffer from the lack of ideal surface chemistry that could aid cell adhesion and proliferation.
• Excellent cell adhesion and proliferation.
• Good mechanical properties.
• Features faster degradation than either PGA or PLA.
• Wide range of degradation rates.
• High sensitivity to hydrolysis.

Polyhydroxybutyrate (PHB)
• It is a homopolymer having a stereoregular structure with high crystallinity.
• Advantages over PLA and PGA.
• Slow rate of degradation.
• Can be obtained naturally.
• Thermal instability during melt processing impedes its commercial application. • Poor osteoconductivity.

Polyglycolic acid (PGA)
Glycosaminoglycans • Consist of repeating disaccharides linked by glycosidic bonds creating individual complex structures.
• Have multiple regulatory functions, e.g., in the anticoagulation of blood, inhibition of tumor growth, and metastasis.
• Control the inflammatory processes.
• Controls cell proliferation and angiogenesis.
• Slow degradation rate (lower than that of PLA and PLGA).
• Good mechanical properties. • Degraded by hydrolysis or bulk erosion.
• Hydrophobicity of PCL is another major issue that hinders wound healing application.
• Some problems related to withstanding mechanical loads.
• Good degradation rate.
• PLA-based materials suffer from the lack of ideal surface chemistry that could aid cell adhesion and proliferation. • Controls cell proliferation and angiogenesis.
• Slow degradation rate (lower than that of PLA and PLGA).
• Good mechanical properties. • Degraded by hydrolysis or bulk erosion.
• Hydrophobicity of PCL is another major issue that hinders wound healing application.
• Some problems related to withstanding mechanical loads.
• Good degradation rate.
• PLA-based materials suffer from the lack of ideal surface chemistry that could aid cell adhesion and proliferation. • Controls cell proliferation and angiogenesis.
• Slow degradation rate (lower than that of PLA and PLGA).
• Good mechanical properties. • Degraded by hydrolysis or bulk erosion.
• Hydrophobicity of PCL is another major issue that hinders wound healing application.
• Some problems related to withstanding mechanical loads.
• Good degradation rate.
• PLA-based materials suffer from the lack of ideal surface chemistry that could aid cell adhesion and proliferation. • Lack of cell-adhesive property.
• Less ingrowth of bone cells.
• Biodegradable amorphous polymer because of the aliphatic polycarbonate ester structure on its backbone.
• No inflammatory response.
• PPC has shortcomings such as viscous flow at room temperature and a relatively large brittleness at low temperature.
• Poor thermal and processing properties.
• Cell attachment to PPC is very limited due to its highly hydrophobic nature.

Natural-Natural Biopolymer Composites
As such, because of the broad disadvantage of allografts and alloplastic implants, including the lack of donors for donating tissues and organs, the possibility of immunological transplant rejection, tiredness, and fatigue, TE offers the opportunity to rebuild damaged or destroyed tissues without any complications [45,78,175]. The particular interest is the natural biological macromolecules due to their excellent biocompatibility, low immunogenicity, and cytocompatibility, as well as the antigenic nature that makes them popular for TE scaffolding applications [31].  • PPC has shortcomings such as viscous flow at room temperature and a relatively large brittleness at low temperature.
• Poor thermal and processing properties.
• Cell attachment to PPC is very limited due to its highly hydrophobic nature.

Natural-Natural Biopolymer Composites
As such, because of the broad disadvantage of allografts and alloplastic implants, including the lack of donors for donating tissues and organs, the possibility of immunological transplant rejection, tiredness, and fatigue, TE offers the opportunity to rebuild damaged or destroyed tissues without any complications [45,78,175]. The particular interest is the natural biological macromolecules due to their excellent biocompatibility, low immunogenicity, and cytocompatibility, as well as the antigenic nature that makes them popular for TE scaffolding applications [31].  • PPC has shortcomings such as viscous flow at room temperature and a relatively large brittleness at low temperature.
• Poor thermal and processing properties.
• Cell attachment to PPC is very limited due to its highly hydrophobic nature.

Natural-Natural Biopolymer Composites
As such, because of the broad disadvantage of allografts and alloplastic implants, including the lack of donors for donating tissues and organs, the possibility of immunological transplant rejection, tiredness, and fatigue, TE offers the opportunity to rebuild damaged or destroyed tissues without any complications [45,78,175]. The particular interest is the natural biological macromolecules due to their excellent biocompatibility, low immunogenicity, and cytocompatibility, as well as the antigenic nature that makes them popular for TE scaffolding applications [31].  • PPC has shortcomings such as viscous flow at room temperature and a relatively large brittleness at low temperature.
• Poor thermal and processing properties.
• Cell attachment to PPC is very limited due to its highly hydrophobic nature.

Natural-Natural Biopolymer Composites
As such, because of the broad disadvantage of allografts and alloplastic implants, including the lack of donors for donating tissues and organs, the possibility of immunological transplant rejection, tiredness, and fatigue, TE offers the opportunity to rebuild damaged or destroyed tissues without any complications [45,78,175]. The particular interest is the natural biological macromolecules due to their excellent biocompatibility, low immunogenicity, and cytocompatibility, as well as the antigenic nature that makes them popular for TE scaffolding applications [31].

Natural-Natural Biopolymer Composites
As such, because of the broad disadvantage of allografts and alloplastic implants, including the lack of donors for donating tissues and organs, the possibility of immunological transplant rejection, tiredness, and fatigue, TE offers the opportunity to rebuild damaged or destroyed tissues without any complications [45,78,175]. The particular interest is the natural biological macromolecules due to their excellent biocompatibility, low immunogenicity, and cytocompatibility, as well as the antigenic nature that makes them popular for TE scaffolding applications [31].
Polymer mixes describe a polymer material consisting of at least two or more polymers resulting in improved physicochemical properties compared to different individual polymers. Each one of the polymers holds its particular biological and physicochemical properties in a blend. It allows improving strength and rigidity while ensuring low density and lower weight compared to monocomponent polymers [41]. Though many studies stated their minimal mechanical effectiveness and superior sensitivity to environmental factors, such as temperature and humidity, blend growth is the major downside of bulk natural polymers [31]. Some of the natural biopolymer blends are tabulated in Table 2. • The highest stimulating effect was seen on gene expression and cartilaginous matrix protein production and also on cartilage regeneration. • The findings of vivo implantation showed that the pore size had no apparent effect on the proliferation of cells.
• Obviates the need to remove the material later or leave materials in the body. • The efficiency of the antimicrobial activity decreases over time.
• Decrement in the rate of degradation compared to a pristine polymer.
• Collagen-HA scaffolds that favor the differentiation of neural stem cells into neuronal cells in vitro in tandem with some mechanical behaviour of brain tissue.
Brain tissue engineering [184] Alginate/cellulose nanocrystals-chitosangelatin Layer-by-layer assembly and then freeze-drying • A strong 3D architecture with a well-defined porous structure improves compressive strength and controlled biodegradation.

Natural-Synthetic Biopolymer Composites
There are major advantages to natural biopolymers over synthetic materials, including lower/no toxicity, better bioactivity, enhanced cell response when associated with cells, excellent biocompatibility, extreme hydrophilicity, and effective biological function. However, their weak engineering properties often limit the utility of natural biopolymers. Significant drawbacks of natural biopolymers are as follows: • High batch-to-batch inconsistency owing to complicated isolation techniques from inconsistent sources.

•
Poor processability and solubility blocking the utilization of industrial fabrication processes. In contrast, synthetic polymers have shown many advantages, improved control over chemical composition, especially in terms of processability and good mechanical properties, but in scaffold products, there is insufficient bioactivity, low cell attachment capacity, hydrophobicity, and limited surface cell recognition.
It is proposed that not one substance gathers all the criteria for tissue replacement. Instead, a scaffold made from a composite containing both natural and synthetic biopolymers can provide a tissue substitute that satisfies all clinical requirements comprising the specific size and kind of wound, the age of the patient, and the procedure of preparation available [123]. Several researchers have investigated the use of a particular combination of natural and synthetic materials for manufacturing tissue scaffolds to take advantage of the intrinsic biocompatibility of natural materials and the physicochemical properties of synthetic polymers. A few natural-synthetic polymer composites along with their properties, biological assessment, and characteristics are tabulated in Table 3. Greater control of degradation rate is made possible with the use of different synthetic vs. natural product formulations [123]. The combination of natural and synthetic polymers (bioartificial combination) is a multipurpose method to design more successful biomaterials that enhance physical and biological features (for example, biocompatibility) [31]. They have been combined to take advantage of their favorable properties to overcome the disadvantages of each particular type of material [186].  • Enhancement of mechanical strength. • The adhesion and proliferation of osteoblast cells and the fiber alignment are induced to increase the metabolic activity of the cells.
• Cell attachment analysis.
• Hemostatic effect in vitro.
• Biodegradability analysis in vivo.
• The composite scaffolds had good blood coagulation abilities because of the hemostatic properties of CS and the porous structure. • Filaments and tiny pores in composite CS-PG scaffolds may serve as effective barriers and prevent cell infiltration.
• The composite PCL/PVP scaffolds are printed with the controllable diameter of the filament (~10 µm) that is close to living cells. • Experiments in cell culture found that printed scaffolds have excellent biocompatibility and support in vitro cell proliferation.
• Increased hydrophilicity and biological activity. • The properties of high water absorption, hierarchical cellular structure, and rapid recovery from 80 percent strain are presented by PLA/RC nanofiber-reconfigured scaffolds.
• The strong in vitro biocompatibility has been demonstrated and can provide effective cell attachment and proliferation environment.

Properties of Polymer Scaffolds
When dealing with the polymers for scaffolding materials, the properties of the polymer can be classified into three groups based on their intrinsic nature, processing conditions, and final product. The intrinsic properties are inherent characteristics of the polymer itself that depend primarily on its chemical structure and composition (e.g., crystallinity, density, solubility, transition temperatures, mechanical, electromagnetic, gas barrier properties, transparency, etc.) [201]. Processing characteristics include the melt flow index, viscosity, and the strength of the melt. While these properties suggest the material's behavior during the forming process (e.g., in extrusion), the practitioner requires additional details like the working conditions of each material at the different production stages. In general, the properties of the product are defined by combinations of both intrinsic properties, mechanical behavior, water resistance, heat resistance, esthetic properties, and environmental behavior including degradation conditions [202,203].
The structures should fulfill certain criteria that are necessary for use as scaffolds in TE. The key constraints to consider when constructing a scaffold are biological properties and physical properties. According to the tissue characteristics, certain properties can differentiate the right material, and a processing method must be chosen to manufacture these scaffolds with different characteristics. Some of the desired properties are biocompatibility, biodegradability, morphology, pore size and porosity, and mechanical strength [204].

Bioactivity
Bioactivity refers to a material's ability to impact its biological surroundings. Since the invention of "tissue engineering", [205] biomaterials have historically been used to offer a bioactive environment, in which cells adhere and propagate [206]. Three-dimensional (3D) scaffolds may actively interact with cellular components of the engineered tissues to promote and control their activities. Biomaterials may contain biological signs such as cell-adhesive ligands to reinforce attachment or physical signs like topography to influence cell morphology and orientation [207]. The scaffold may also act as a delivery vehicle or a repository for exogenous growth-stimulating signals like growth factors to speed up regeneration. In this regard, biomaterials must be compatible with biomolecules for the controlled release of bioactivity-retained biomolecules and compatible with the encapsulation technique.

Biocompatibility
Biocompatibility is one of the essential attributes to be considered when designing soft and hard tissue scaffolds [92,208]. Biocompatibility describes the capacity of a biomaterial to execute its intended purpose concerning medical therapy without affecting the client or beneficiary of that therapy from suffering any adverse local or systemic effects. In that case, it should produce the most suitable cellular or tissue response and maximize the clinically relevant efficiency of that therapy. Molecular processes should not be poisonous, causing the host tissue to be immunologically rejected. As part of the protective system, the body develops immunological responses to foreign substances entering the body and triggers the rejection of scaffolds or implants. Therefore, only negligent immune reaction should be triggered by any bioengineered structures, so that the inflammation caused by this does not hinder the healing process or trigger any in vivo toxicity [204]. Besides, the structure must have biomimetic binding sites to conform to the cells, which can contribute to proliferation and differentiation.
Some main factors which define biocompatibility of the scaffolds are shown in Figure 3. Adsorption and desorption activities of polymeric materials of various types of mammalian cells depend on surface characteristics such as surface roughness, rigidity, hydrophilicity/hydrophobicity ratio, bulk chemistry, surface charge, and charge distribution. In order to improve biocompatibility of surfaces in contact with living tissue, a wide range of surface treatments are available to seal unwanted residues or additives by means of a coating and to control excretion and/or absorption using a selectively permeable surface. Polymers 2021, 13, x FOR PEER REVIEW 24 of 47 Figure 3. The essential variables that define the scaffold's biocompatibility.

Biodegradability
Excellent biocompatibility over time is essential for biomaterials that are biodegradable, as the physicochemical, mechanical, and biological characteristics of a biodegradable material change over time, and degradation products that are tissue-compatible to the original material vary. Degradation materials that are non-toxic and quickly metabolized and cleared from the body should provide an ideal biodegradable biomaterial [209]. The scaffold's absorption kinetics is essential and depend on the regenerating tissue. If a scaffold is employed for skeletal system TE, the biomaterial deterioration of the scaffold can be relatively gradual, as the mechanical strength must be preserved before the tissue reconstruction is nearly complete. In contrast, the scaffold does not need to last more than one month for skin TE. If the scaffold leftovers remain in the body for a longer period than needed, the residual material can retard rather than facilitate the regeneration of the tissue.
The gradual breakdown of a material mediated by a specific biological activity applies to the term biodegradation. More specifically, if this breakdown is due to cells and/or tissue activity, the substance should be described as biodegradable. The term "biodegradation" is often used to describe materials which are less independent of the degradation process after implantation in a bodily location. However, the fact that biodegradation products are metabolized or removed from the body is essential.
Due to their improved overall interactions with different cell types and lower or lack of immune response, natural polymers were among the first biodegradable scaffolding agents to be used clinically. However, despite the possibility for immune response or toxicity, synthetic polymers were later found to be cheaper and more effective than natural polymers, particularly, several polymer combinations [206].
Scaffolds can be classified into two types based on their degradation property, permanent/nondegradable and biodegradable. A permanent scaffold must not decay and the properties of the soft tissue it substitutes should be consistent [204]. However, the important factor requiring an in vivo analysis is the rate of biodegradation. The rate of biodegradation of a polymer is mostly contingent on the polymer's intrinsic properties, including the chemical structure, hydrophilicity/hydrophobicity level, crystalline/amorphous morphology, the existence of hydrolytically unstable bonds, glass transition tem-

Biodegradability
Excellent biocompatibility over time is essential for biomaterials that are biodegradable, as the physicochemical, mechanical, and biological characteristics of a biodegradable material change over time, and degradation products that are tissue-compatible to the original material vary. Degradation materials that are non-toxic and quickly metabolized and cleared from the body should provide an ideal biodegradable biomaterial [209]. The scaffold's absorption kinetics is essential and depend on the regenerating tissue. If a scaffold is employed for skeletal system TE, the biomaterial deterioration of the scaffold can be relatively gradual, as the mechanical strength must be preserved before the tissue reconstruction is nearly complete. In contrast, the scaffold does not need to last more than one month for skin TE. If the scaffold leftovers remain in the body for a longer period than needed, the residual material can retard rather than facilitate the regeneration of the tissue.
The gradual breakdown of a material mediated by a specific biological activity applies to the term biodegradation. More specifically, if this breakdown is due to cells and/or tissue activity, the substance should be described as biodegradable. The term "biodegradation" is often used to describe materials which are less independent of the degradation process after implantation in a bodily location. However, the fact that biodegradation products are metabolized or removed from the body is essential.
Due to their improved overall interactions with different cell types and lower or lack of immune response, natural polymers were among the first biodegradable scaffolding agents to be used clinically. However, despite the possibility for immune response or toxicity, synthetic polymers were later found to be cheaper and more effective than natural polymers, particularly, several polymer combinations [206].
Scaffolds can be classified into two types based on their degradation property, permanent/nondegradable and biodegradable. A permanent scaffold must not decay and the properties of the soft tissue it substitutes should be consistent [204]. However, the important factor requiring an in vivo analysis is the rate of biodegradation. The rate of biodegradation of a polymer is mostly contingent on the polymer's intrinsic properties, including the chemical structure, hydrophilicity/hydrophobicity level, crystalline/amorphous morphology, the existence of hydrolytically unstable bonds, glass transition temperatures (T g ), molecular weight, and copolymer ratio, so that a wide variety of maxillofacial applications can be manipulated within a week or months or years. The byproducts, without inducing any cytotoxicity, can easily exit the body. For inflammatory responses, the regulated activity of macrophages is required so that degradation can happen beside the growth of a new tissue. After its function is served, a biodegradable scaffold is required to degrade on its own in order to replace it with new cell growth. When constructing a scaffold, degradation rate and the degradation mechanism are important factors to be studied [210].
The degradation of scaffolds can happen via mechanisms involving physical or chemical processes and/or biological procedures that are intermediated by biological agents, such as tissue remodeling enzymes. Based on their method of preparation, biodegradable polymers are categorized into two main classes: stepwise polycondensation or ringopening polymerization. Agro-polymers such as polysaccharides and proteins comprise the first group [211]. Biodegradable polyesters, including aliphatic polyesters and aromatic polyesters, are in the second group. Biodegradable polymers have a wide variety of properties and can be substituted for nonbiodegradable polymers in a variety of applications, such as biomedical, textile, and packaging applications [208].
In biodegradable polymers, non-biological processes such as hydrolysis and erosion or biological processes such as enzymatic action or intervention by microorganisms such as bacteria, yeast, and fungi may initiate the degradation process. The majority of natural polymers have been documented to be enzymatically degraded. In the case of protein-based biomaterials, enzymes such as collagenases and metalloproteinases degrade peptide bonds in vivo [204]. The biomaterials dependent on polysaccharides are degraded by lysosomes and amylases within the body. On the other hand, most synthetic biodegradable polymers include hydrolyzable linkages that are degraded by a hydrolytic process, such as ester, urea, and urethane linkages. In parallel, these processes act to speed up the degradation of these polymers. The polymers' hydrophobic/hydrophilic nature substantially changes their biodegradability. In general, polymers with more polar groups are easily biodegradable. The entire process of biodegradation can range from days to months to years depending on the kind of polymer. Cell biomass and other intermediates can inevitably be mineralized to CO 2 over a long period [212,213].
In non-biological processes, chemical splitting is largely responsible for deterioration, along with physical erosion. In semicrystalline biodegradable polymers, amorphous domains are highly susceptible to water molecular diffusion [214]. In this case, the hydrolytic degradation arises first in the polymer's amorphous regions, heading to chain splitting. After that, the hydrolytic degradation takes place in crystalline parts. It should be noted that the degree of hydrolysis depends significantly on the relative hydrophilicity of the polyesters engaged. Physical erosion is associated with hydrolysis that assists with degradation. In physical erosion, the two processes resulting in the breakdown of the scaffold and resorption/dissolution of the material involved are bulk erosion and surface erosion. Although bulk erosion is associated with mass loss all over the material, the erosion of the surface is restricted only to the particular surfaces subjected and continues via an erosion front. Bulk erosion is prominent in the case of biodegradable aliphatic polyesters, leading to sample fragility and compromising the materials' mechanical and functional capabilities [214]. Therefore, while the scaffold size turns out to be smaller, the bulk structure is retained. These forms of degrading scaffolds provide the tissue to regenerate with more mechanical stability. The biodegradation behavior of different polymers in both biological process and non-biological process is illustrated in Figure 4.  The cleavage of hydrolytically or enzymatically sensitive bonds in polymers leads to polymer erosion and biomaterials degradation. On the other hand, most synthetic biodegradable polymers contain hydrolyzable linkages that are degraded by a hydrolytic process, such as ester, urea, and urethane linkages [215,216]. Biologically stable, nonbiodegradable polymeric scaffolds can deliver permanent support over time and should work best during the patient's lifetime. Future research will illustrate several parameters that are needed to optimize and monitor desired applications, such as polymerization conditions, compositions, and scaffolding techniques [208]. Degradation mechanism, duration, and solvent in the decay of some of the biodegradable polymers including both natural and synthetic polymers and their composites are tabulated in Table 4.  The cleavage of hydrolytically or enzymatically sensitive bonds in polymers leads to polymer erosion and biomaterials degradation. On the other hand, most synthetic biodegradable polymers contain hydrolyzable linkages that are degraded by a hydrolytic process, such as ester, urea, and urethane linkages [215,216]. Biologically stable, nonbiodegradable polymeric scaffolds can deliver permanent support over time and should work best during the patient's lifetime. Future research will illustrate several parameters that are needed to optimize and monitor desired applications, such as polymerization conditions, compositions, and scaffolding techniques [208]. Degradation mechanism, duration, and solvent in the decay of some of the biodegradable polymers including both natural and synthetic polymers and their composites are tabulated in Table 4.

Porosity and Pore Size
The pore size, porosity, and mechanical properties of scaffolds play a vital role in TE. A scaffold's porosity relies on its pore quantity, pore size, form, connectivity, and orientation. Porosity typically promotes proliferation and migration of cells, delivering an atmosphere for the transmission of nutrients to the structures underlying or near the scaffolds. Porous scaffolds ensure cell growth, uniform distribution of cells, and vascularization [27,234,235]. Some of the polymer scaffold materials with good porosity and biomedical applications are tabulated in Table 5. Scaffolds must have a highly porous structure with an open, completely interconnected geometry. The porosity and, sequentially, the surface-to-volume ratio of the scaffold should not be so high that its mechanical strength is weakened [236]. 3D scaffolds face two major restrictions for TE applications-a scaffold can neither be too porous (due to compromised mechanical strength) nor significantly lack porosity (due to the lack of cellular ingress, vascularization, and signaling) [206]. The major parameters to consider while constructing a scaffold are average pore size, pore size distribution, pore length, pore interconnectivity, pore shape, pore throat size, and pore wall roughness. It establishes a biocompatible porous network from which the surrounding tissue is induced and serves as a temporary model for the development and reorganization of the new tissue. The pore size should not be less than 100 µm in diameter for the full diffusion of oxygen and nutrients to promote cell survival [237][238][239]. However, some specific pore sizes are defined for hard tissues that are in the range of 200-350 µm; some of them can be seen in Figure 5. Furthermore, scaffolds should have a suitable surface area with optimum porosity. Reduction of compressive and tensile properties is one of the drawbacks of increasing porosity [208,219].

Porosity and Pore Size
The pore size, porosity, and mechanical properties of scaffolds play a vital role in TE. A scaffold's porosity relies on its pore quantity, pore size, form, connectivity, and orientation. Porosity typically promotes proliferation and migration of cells, delivering an atmosphere for the transmission of nutrients to the structures underlying or near the scaffolds. Porous scaffolds ensure cell growth, uniform distribution of cells, and vascularization [27,234,235]. Some of the polymer scaffold materials with good porosity and biomedical applications are tabulated in Table 5. Scaffolds must have a highly porous structure with an open, completely interconnected geometry. The porosity and, sequentially, the surface-to-volume ratio of the scaffold should not be so high that its mechanical strength is weakened [236]. 3D scaffolds face two major restrictions for TE applications-a scaffold can neither be too porous (due to compromised mechanical strength) nor significantly lack porosity (due to the lack of cellular ingress, vascularization, and signaling) [206]. The major parameters to consider while constructing a scaffold are average pore size, pore size distribution, pore length, pore interconnectivity, pore shape, pore throat size, and pore wall roughness. It establishes a biocompatible porous network from which the surrounding tissue is induced and serves as a temporary model for the development and reorganization of the new tissue. The pore size should not be less than 100 µm in diameter for the full diffusion of oxygen and nutrients to promote cell survival [237,238,239]. However, some specific pore sizes are defined for hard tissues that are in the range of 200-350 µm; some of them can be seen in Figure 5. Furthermore, scaffolds should have a suitable surface area with optimum porosity. Reduction of compressive and tensile properties is one of the drawbacks of increasing porosity [208,219].

Morphology
Both chemical and topographical characteristics are involved in surface properties, which can control and influence cellular adhesion and proliferation. The surface of the scaffold is the original and principal interface site surrounding cells and tissue. Since most cells used for TE are based on anchorage, it has been suggested that their attachment may be facilitated by the scaffold. Therefore, scaffolds with a wide and functional surface area are favorable [246].
Scaffolds should be constructed to be consistent with the structure of the tissue and have a large surface area, high porosity, fully interconnected geometry, structural strength, and a particular three-dimensional shape [247]. Moreover, it has to be biocompatible in order to enable long-term substitution for a newly developed tissue. A given structure matrix design with particular material properties is necessary for every tissue. Along with the size of the pores, the performance of the implanted matrix and the rate of tissue ingrowth can be significantly affected by morphology [248]. The optimum porosity is strictly linked to the type of tissue, and a different microenvironment can be associated with diverse tissue architectures. Cell dimensions must be considered when designing a scaffold for TE, together with phenotypic expression, cell activity, and ECM production [247].

Mechanical Properties
To preserve integrity of the scaffold during implantation, mechanical strength is determined by the impact resistance of the final goods. Tensile and compressive tests include the most common mechanical tests to assess scaffolds. For the performance of the implant, the sufficient mechanical properties for a biomaterial to be used in a TE application are important. Factors such as elasticity, strength, and absorption at the material interface and their degradation depend on the biostability of several scaffolds and are important. In general, biopolymer scaffolds should have the mechanical properties appropriate for the site of implantation and should have the strength that is needed for any implantation requiring a surgical procedure; some of them are seen in Figure 6. The mechanical characteristics of a scaffold depend on the material and the process of production that influences structural parameters such as pore geometry, size, and form [204,249]. The properties of the scaffold are chosen corresponding to the application needed. To preserve their structural integrity, scaffolds must also be able to withstand the environmental stresses encountered during the operation. In addition, it is important to balance the ultimate strength and compliance with the surrounding tissue when constructing scaffolds for load-bearing applications, such as in bone TE. The scaffold should therefore have similar mechanical characteristics to the bone [250,251]. If the scaffold's mechanical strength exceeds that of the healthy bone underlying it, stress shielding may occur, causing atrophy or tissue loss. The mechanical characteristics of a polymer scaffold depend closely on the molecular weight and the crystalline properties of the polymer that also affect the degradation rate and mechanism [252].
To facilitate quick tissue regeneration, the scaffold should have the right mechanical features and degradation rate with the bioactive surface. After implantation, it is extremely important to preserve the mechanical strength of the scaffold system for the reconstruction of rough load-bearing tissues such as bones and cartilages [253,254]. Therefore, the scaffold should be of equal mechanical strength to that of the tissue. For biode- The properties of the scaffold are chosen corresponding to the application needed. To preserve their structural integrity, scaffolds must also be able to withstand the environmental stresses encountered during the operation. In addition, it is important to balance the ultimate strength and compliance with the surrounding tissue when constructing scaffolds for load-bearing applications, such as in bone TE. The scaffold should therefore have similar mechanical characteristics to the bone [250,251]. If the scaffold's mechanical strength exceeds that of the healthy bone underlying it, stress shielding may occur, causing atrophy or tissue loss. The mechanical characteristics of a polymer scaffold depend closely on the molecular weight and the crystalline properties of the polymer that also affect the degradation rate and mechanism [252].
To facilitate quick tissue regeneration, the scaffold should have the right mechanical features and degradation rate with the bioactive surface. After implantation, it is extremely important to preserve the mechanical strength of the scaffold system for the reconstruction of rough load-bearing tissues such as bones and cartilages [253,254]. Therefore, the scaffold should be of equal mechanical strength to that of the tissue. For biodegradable scaffolds, this strength is destined to reduce overtime, but the combined strength of the newly grown tissue and the decaying scaffold should be comparable [210], since the regenerated tissue fills the degraded scaffold place. It is, therefore, necessary that one or more of the following rheological parameters is evaluated: tensile strength/compressive strength, Young's modulus, maximum strain, and flexural modulus [255]. Table 6 illustrates a few of the biopolymer scaffold's mechanical properties including strength, Young's modulus, and elongation at break.

Commercial Status of Biopolymers
Currently, various biopolymers competing successfully in the global market due to their unique characteristic properties have a huge demand in biomedical applications. Besides, there is no surprise that improving human health and lifespan contribute to one of the fastest-growing markets for TE and regenerative medicine products. To help with this, the industry has been developing new biomaterial-based products, which include both synthetic and naturally derived materials [203,[272][273][274]. A few commercial polymeric materials along with their trade name available for different biomedical applications are summarized in Table 7.

Scaffold Fabrication Techniques
Numerous techniques have been developed over the years for biodegradable polymers processing and fabrication of different types of scaffolds. The conventional as well as new advanced technologies are widely used for scaffold fabrication as shown in Figure 7.

Scaffold Fabrication Techniques
Numerous techniques have been developed over the years for biodegradable polymers processing and fabrication of different types of scaffolds. The conventional as well as new advanced technologies are widely used for scaffold fabrication as shown in Figure  7. Conventional techniques comprise solvent casting, particulate leaching, melt molding, gas foaming, freeze-drying, etc. Advanced techniques including electrospinning, stereolithography, selective laser sintering, fused deposition modelling, 3D printing, and 3D bioprinting can be used for fabricating natural as well as synthetic polymers and their composite scaffolds. Table 8 describes some of the basic advantages and disadvantages of these techniques along with a few possible scaffold materials [161,273,[275][276][277][278]. The assessment of scaffold fabrication techniques should be performed by contemplating both potential advantages and disadvantages of each technique and the final product properties should match the needs of specific tissues to be regenerated. Conventional techniques comprise solvent casting, particulate leaching, melt molding, gas foaming, freeze-drying, etc. Advanced techniques including electrospinning, stereolithography, selective laser sintering, fused deposition modelling, 3D printing, and 3D bioprinting can be used for fabricating natural as well as synthetic polymers and their composite scaffolds. Table 8 describes some of the basic advantages and disadvantages of these techniques along with a few possible scaffold materials [161,273,[275][276][277][278]. The assessment of scaffold fabrication techniques should be performed by contemplating both potential advantages and disadvantages of each technique and the final product properties should match the needs of specific tissues to be regenerated. • Limited mechanical properties, residual solvents, and porogen material. • Longer processing time.
• This technique is mainly applied to produce thin membranes.
Different classes of synthetic polymers (e.g., PLLA, PLGA, or PEG) and natural polymers [279,280] Melt molding: Both polymers and a suitable porogen are melted together, then by cooling the polymer mixture the scaffold is obtained. In this process, the porosity is attained by dissolving the porogen in water • Independent control over porosity, pore size, pore interconnectivity, and geometry.
• The requirement of high temperature for the non-amorphous polymer. • Requires a residual porogen. • Longer processing time.
PLA, PGA, PLGA-gelatin, PA [281] Gas foaming: Polymer gel paste along with sieved effervescent salt particles poured into a mold and immersed into hot water.
Formation of the porous matrix after the evolution of ammonia and carbon dioxide gas from salt particles of the solidifying polymer matrix  • Pore anisotropy and the geometry of pore connectivity are substantially limited due to the continuous deposition process. • FDM is typically limited to synthetic thermoplastic polymers, thereby eliminating many natural biomaterials and thermoset synthetic polymers.
Biodegradable materials used for this method include PCL, PLGA, polycarbonate, polypropylene, and various polyesters [290,293,294] 3D printing: It is a process of reconstruction of a 3D physical model by the successive addition of material layers resulting in a 3D solid object based on CAD model design • Able to create almost any shape or geometric feature, allows defined internal architectures for implants.
• The addition of a chemical binder. • Post-fabrication efforts to remove the residual solvent such as vacuum drying are not completely effective; therefore, the issue of cytotoxicity in 3D printing (3DP)-fabricated scaffolds remains.
PEO, PCL, and PLGA [290,295,296] 3D bioprinting: It is the 3D printing process of generating layer-by-layer 3D tissue-like structures using viable cells, an encapsulation biomaterial, and growth and differentiation factors to create a bio-printed pre-tissue that is further transferred to an incubator where it matures into a tissue • Biomimicry. • Autonomous self-assembly. • Small tissue building blocks.
• The development of biomaterials for 3D bioprinting is still in its early stages.

Conclusions
In biomedical applications, scaffolds can be used ranging from regenerative engineering to controlled drug delivery and immunomodulation, and for this purpose, biomaterials have become an indispensable instrument as a scaffold material. The materials used for scaffold manufacturing must satisfy some criteria such as intrinsic biofunctionality and appropriate chemistry to stimulate molecular biorecognition by cells to induce proliferation, cell adhesion, and activation. The mechanical properties of scaffolds and kinetics of decomposition in selected materials must be adjusted to the TE application specifically to ensure the essential structural functions and to achieve the rate of formation of new tissues. The geometrical features like exposed surface area, pore distribution and porosity, and distribution affect the rate of cell penetration within the scaffold volume, the architecture of the ECM formed. The final effectiveness of the regenerative process plays a major role in scaffolding. Many biodegradable polymers of natural and synthetic origin have been established for use as biomaterials and careful consideration of the cellular environment and interactions needed is required to select a polymer for a given application. Despite advantages and disadvantages of individual materials, it is proposed that not one substance features all the perfect properties for a tissue replacement. Instead, a scaffold made from a composite containing both natural and synthetic biopolymers can permit tissue substitutes to be produced that satisfy all clinical requirements, including the specific size and kind of wound, the age of the patient, and the procedure of preparation available. Besides, scaffold design for tissue engineering includes several specifications. Many of these are dynamic and not yet well comprehended. In addition, being both bulk and biocompatible degraded, these scaffolds should possess sufficient mechanical properties to provide the neo-tissues with the necessary stress environment. To enable the entry of cells and nutrients, the scaffolds should also be porous and permeable and should demonstrate the required surface structure and chemistry for cell attachment. Funding: The author, R. Choudhary, gratefully acknowledges the financial support from the European Union's Horizon 2020 research and innovation program under the grant agreement No. 857287. The author presents sincere thanks to the Ministry of Science and Higher Education of the Russian Federation in the framework of the "Increase Competitiveness" Program of National University of Science and Technology (NUST) «MISiS», Moscow, Russia, for providing the financial help to carry out this research. This publication was also supported by Qatar University internal grant No. IRCC-2020-013. The findings herein are solely the responsibility of the authors.