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Bioresorbable Polymers: Advanced Materials and 4D Printing for Tissue Engineering

M3 Health Industria e Comercio de Produtos Medicos, Odontologicos e Correlatos S.A., Jundiaí, Sao Paulo 13212-213, Brazil
Department of Periodontology and Oral Implantology, Dental Research Division, University of Guarulhos, Guarulhos, Sao Paulo 07023-070, Brazil
Author to whom correspondence should be addressed.
Polymers 2021, 13(4), 563;
Submission received: 19 January 2021 / Revised: 8 February 2021 / Accepted: 8 February 2021 / Published: 13 February 2021
(This article belongs to the Special Issue Applications of Biopolymer Scaffolds)


Three-dimensional (3D) printing is a valuable tool in the production of complexes structures with specific shapes for tissue engineering. Differently from native tissues, the printed structures are static and do not transform their shape in response to different environment changes. Stimuli-responsive biocompatible materials have emerged in the biomedical field due to the ability of responding to other stimuli (physical, chemical, and/or biological), resulting in microstructures modifications. Four-dimensional (4D) printing arises as a new technology that implements dynamic improvements in printed structures using smart materials (stimuli-responsive materials) and/or cells. These dynamic scaffolds enable engineered tissues to undergo morphological changes in a pre-planned way. Stimuli-responsive polymeric hydrogels are the most promising material for 4D bio-fabrication because they produce a biocompatible and bioresorbable 3D shape environment similar to the extracellular matrix and allow deposition of cells on the scaffold surface as well as in the inside. Subsequently, this review presents different bioresorbable advanced polymers and discusses its use in 4D printing for tissue engineering applications.

Graphical Abstract

1. Introduction

Tissue engineering/regenerative medicine is an interdisciplinary area, addressing cell-based therapies and the use of bioactive and porous materials with the objective of developing functional substitutes for the repair or replacement of tissues/organs affected by an injury or disease [1]. Tissue engineering is based on three elements that must be in synergy: 1. Matrix (scaffold), 2. cells (stem cells or primary lineages), and 3. signals (mechanical, physical, electrical and/or molecules: Proteins, peptides, and cytokines) [2]. The scaffold, being a key factor, is responsible for physical and structural support for cell growth and differentiation and transport of suitable nutrients [1]. Ideally, its topography and chemical composition should be similar to the characteristics of an extracellular matrix (extracellular matrix, ECM), mimicking an extracellular environment that favors cell-material interactions [3]. The formation of tissues inside the scaffolds (three-dimensional structures) is directly influenced by the porosity rate and pore size, and these factors must be specific for the tissue regeneration. These characteristics are essential in providing an adequate supply of oxygen to promote angiogenesis [4].
Several conventional methods have been used extensively to produce porous scaffolds over the past years, such as salt/particle leaching, foaming through gas or chemical reagents, molding by solvent or by fusion, phase separation, and lyophilization [5]. However, these methods have limitations in producing three-dimensional (3D) scaffolds. This is due to their lack of control in the formation of pores and lack of interconnected pores that favor the transport of nutrients, consequently contributing to an accelerated cell growth rate inside the scaffold. Additive manufacturing is a promising alternative to produce porous 3D scaffolds that overcome these disadvantages, especially for bone substitutes.
Additive manufacturing, also known as 3D printing, comprises a series of technologies that consists of the direct generation of objects layer-by-layer through computer-aided design (CAD, computer-aided design) and/or computer-aided manufacturing (CAM) [6]. The layer-by-layer construction process have a distinct feature that allows highly complex structures to be built quickly. This technique allows the fabrication of biological structures using tissue-like materials, such as hydrogels, to be applied for tissue regeneration [7].
Even though 3D printing poses several positive properties, printed scaffolds may not promote the necessary biological responses [8]. A native tissue exhibits constant morphological changes in response to various surrounding stimuli, while the printed structures cannot actively transform after printing. Therefore, materials capable of responding to different external stimuli over time (advanced materials) have been extensively studied to be applied in tissue regeneration processes. Stimuli-responsive biomaterials can be used in the concept of four-dimensional (4D) bioprinting, in which 3D printed scaffolds are designed to transform over time according to one or more environment stimuli [9,10]. Thereupon, this review highlights the different advanced biomaterials available for 3D printing and discusses the recent advances in 4D printing for tissue regeneration.

2. Printable Hydrogels

The hydrogel is a hydrophilic scaffold composed of covalent and non-covalent polymeric chains bonds, providing a 3D shape environment similar to the native extra-cellular matrix (ECM) [11,12]. Its cross-linked polymers form a porous 3D structure with a high hydration level (they swell up to 99% (w/w) concerning their dry weight) without dissolving, allowing the network to retain proteins and growing factors, as well as providing an environment for gaseous and nutrients exchange, being essential for cell growth and survival [13,14,15,16,17]. Furthermore, hydrogel 3D scaffolds are beneficial for cell transplantation and tissue engineering [18,19].
The methods used for fabrication of hydrogel scaffolds include solvent casting/leaching, gas foaming/leaching, photo-lithography, electrospinning, and 3D printing [17]. Regarding the development of printable hydrogels, the most challenging approach are the physicochemical and mechanical properties, which allow the hydrogel to hold minimally adequate mechanical properties after printing and quick gelation to ensure fidelity of form of the structure to be rebuilt [11,17,18]. The printed shape maintenance depends on the hydrogel’s rheological properties, which is related to its composition (polymer and crosslinking) [17,20].
Bioresorbability and biodegradability are required to allow scaffold degradation within the implantation site during tissue regeneration [21]. Bioresorbable polymers present four degradation stages in biological systems: Hydration, strength decrease, loss of mass integrity, and solubilization via phagocytosis [22]. The degradation rate relies on polymeric nature, quantity, pH, and environment temperature [23]. The resorption of polymers is desirable for biomedical applications once they perform their function, the polymer chain tend to break into small pieces that will be reabsorbed or eliminated from the body [24,25]. Additionally, the scaffold’s gradual degradation promotes an increase in pore size, allowing a higher rate of cell proliferation and migration [26] for subsequent replacement of newly formed tissue.
Living cells can be seeded onto 3D-printed hydrogel-scaffold or can be used in a bio ink formulation since hydrogels are biologically active components for 3D printing (bioprinting) (Figure 1A) [27]. The use of tissue-specific cells in materials for 3D printing allows the creation of multifaceted and 3D-mimicked tissues, which facilitates cell adhesion due to its cell-containing products, proliferation, and differentiation once they are seeded within the structure [28,29,30].
Bioink selection for 3D bioprinting relies on several requirements, including printability, viscoelasticity, biocompatibility with living cells, tissue regeneration, resorption, shear-thinning, permeability to oxygen, nutrients, and metabolic wastes [31,32]. Furthermore, other crucial characteristics for bioinks based on hydrogels include the reversibility of gelation (relevant for pre-culture before delivery), fast gelling, and the absence of volume modification during gelling [33]. The bioink’s rheological, mechanical, and biological properties will directly impact the functionality of the final printed tissues and organs [34].
The most common 3D bioprinting techniques are the inkjet printing, microextrusion, laser-assisted printing (SLS or SLM), and stereolithography (SLA). Inkjet printing (also known as a drop-on-demand printer and direct-writing) is a fast and low-cost method in which drops of bioink liquid are ejected through thermal, electrostatic or piezoelectric actuation onto a substrate to form 3D structures in a discontinuous process [31,35,36,37]. In the microextrusion method, the bioink is extruded by a pneumatic or mechanical (piston or screw) dispensing system (needles or nozzle) in a continuous process [38,39]. In the laser-assisted printing system, a focused laser is pulsed in an absorbing layer (titanium or gold) forcing a drop of the bioink layer to deposit on substrate and form the desired structure [40,41]. The SLA technique is based on photosensitive polymers (photopolymers), acting as feedstock, which are polymerized through a UV laser light in a layer-by-layer process [42].
The biopolymers (alginate, hyaluronic acid, collagen, fibrin, fibroin, gelatin, and chitosan), are the most used polymers for the production of printable hydrogels and hydrogel-based bioinks in addition to the synthetic polymers, which include the polyethylene glycol (PEG), the Polylactide acid (PLA), the poly(lactic-co-glycolic acid) (PLGA), the polycaprolactone (PCL), and the poloxamers.
Agarose is a linear polysaccharide polymer derived from red algae, and its gelation arises through the formation of intermolecular hydrogen bonds upon cooling [43,44]. Agarose hydrogels’ viscoelastic properties depend on the source, the purification method employed, the molecular weight, and in the solution concentration [45]. These hydrogels can elicit unfavored in vivo reactions [46] and are usually used as a fugitive ink or sacrificial material in tissue engineering [47,48].
Alginate is a water-soluble polysaccharide and consists of a linear (1–4)-linked β-d-mannuronic acid (M blocks) and its C5-epimer α-l-guluronic acid (G blocks) residues [49]. The gel’s viscosity and elasticity depend on the alginate source, concentration, and the G block content [50,51,52]. It can be extracted from a brown seaweed or can be synthesized using bacterial Pseudomonas or Azotobacter [53]. This polyanionic hydrophilic polysaccharide presents a relatively short cross-linking time and is compatible with several cell types [54,55]. The hydrogel is formed when multivalent cations (usually Ca2+) are added to an aqueous alginate solution. Although it is mechanically unstable for a prolonged culture, alginate hydrogels present low degradation rates and cannot be used alone [56]. Alginate-based hydrogels can be applied for vascular and cartilage tissues and are extensively studied in the area of tissue engineering. [57,58,59].
Chitosan is a deacetylated form of chitin derived from shells of crustaceans [60,61]. This natural cationic polysaccharide is insoluble in water and needs to be solubilized in acid solutions [62]. The hydrogel presents relatively good mechanical stability and may be easily mixed with other hydrogels. Due to its acidity, it needs to be neutralized and can reduce cell viability. In addition, presents a limited printability due to its low mechanical strength and low gelation speed and for that reason cannot be printed alone. There are only few studies of chitosan-based hydrogels for tissue engineering [63].
Collagen, the most abundant protein in the mammalian species and marine organisms, is the primary studied natural polymer for biomaterials [64]. Collagen hydrogels are considered a suitable cell carrier that may be easily mixed with other hydrogel materials; therefore, it presents low mechanical stability and a prolonged cross-linking time (slow gelation). Likewise, it is not indicated to be used alone, and it better performs when used in polymeric composites. Type I collagen (Col-I) can self-assemble to form fibrous hydrogels at 37 °C [65]. These hydrogels have been reported in various tissue engineering applications, but mainly have been mainly utilized in cartilage and skin tissues [66,67,68].
Gelatin is a partially hydrolyzed polypeptide and is considered a form of collagen. Its gelling property depends on its source. Gelatin hydrogels present a good cell viability, a low mechanical stability, and a high solubility at a physiological temperature [69]. The thermo-responsive property functions as a cell carrier and fugitive ink, making it a good choice to be used in tissue engineering. Gelatin methacryloyl (GelMA) is a modified gelatin with a low mechanical stability [70]. Although, GelMA is compatible to many cell types, cell viability in GelMA hydrogels depends on the photocrosslinking time, which is the intensity of light and photoinitiator used to induce polymerization [71]. There are several studies of vascular, cartilage, and liver tissue engineering using gelatin and GelMA based-hydrogels [72,73,74,75,76,77,78].
The hyaluronic acid is a linear non-sulfated glycosaminoglycan (GAG) polysaccharide that requires association to other polymers as a consequence of its low mechanical stability [79]. It is commonly used to increase cell viability through cell proliferation enhancement. Due to their properties, hyaluronic acid-based hydrogels have been studied for cardiovascular and cartilage tissue engineering [80,81].
Polyethylene glycol (PEG) is the most used synthetic polymer to produce biomedical hydrogels [82]. This hydrophilic polymer can be transformed into a gel by photopolymerization [83]. PEG hydrogels present good mechanical stability and their properties may be easily manipulated using chemical modification techniques, however, they do not provide biological cues for cell proliferation [84]. The photocrosslinking time, the intensity of the light, and the photoinitiator have a great influence on cell viability. PEG-based hydrogels can be applied in different approaches to tissue regeneration, such as vascular, bone, and cartilage tissues [48,85,86,87,88].
Poly(lactic acid) (PLA) is a biocompatible synthetic hydrophobic aliphatic polyester [89] commonly used in bone tissue engineering [86,87,88,89]. The stereoisomers distribution within the polymers chains (L/D ratios) and molecular weights determines the thermal stability and the degradation properties [90].
Poly(lactic-co-glycolic acid) (PLGA) is a synthetic copolymer composed of lactic acid (LA) and glycolic acid (GA), which polymerizes through the ester linkage of their monomers [91,92]. This copolymer can be degraded by hydrolysis and the degradation time is determined by the monomer’s ratio. Considering its good mechanical strengths and structural versatility, it is often used as support structures for cartilaginous and osteochondral tissue regeneration [93,94,95,96,97]. Nonetheless, it is commonly associated with other polymers (polymeric composites) [98,99,100,101,102] since it presents poor bioactivity characteristics.
Polycaprolactone (PCL) is a thermoplastic polyester obtained by ring-opening polymerization of ε-caprolactone monomers via anionic, cationic, coordination, or radical polymerization mechanism [103]. It is a bioresorbable polymer that degrades by hydrolysis of their ester linkages. PCL may be produced with different molecular weights and shape, impacting on the degradation rate and mechanical strength [104]. PCL hydrogels present good rheological and viscoelastic properties, regulable resorption, and controllable mechanical properties; nevertheless, PCL does not have biofunctional groups to promote better surface chemistry and favor a better cell adhesion in comparison to other bioactive polymers; hence, the PCL present a low biocompatibility [105,106]. It is consequently an excellent choice of use as a supporting device, especially for hard tissues. There are several reports of its use in cardiac, bone, and cartilage engineering [106,107,108,109,110,111,112,113].
Pluronic acid (or polaxamer) is a tri-block thermoplastic copolymer consisting of a hydrophobic poly(propylene oxide) (PPO) portion and two hydrophilic poly(ethylene oxide) (PEO) portions arranged in a PEO-PPO-PEO configuration. The non-ionic surfactant gelation temperature is dependent on its concentration and structure [114]. The main characteristics of this gel form are good biocompatibility, low cytotoxicity, weak mechanical properties, quick degradation rates, rapid dissolution in aqueous solutions, and poor cell viabilities [115,116]. In the area of tissue engineering, polaxamer hydrogels have been studied for diverse approaches in tissue regeneration [114,117,118,119,120].
Biocompatible and bioresorbable polymers can also be used to produce bio-based aerogels. Aerogels are materials synthesized from gels by replacement of the solvent with a gas [121]. This replacement is carried out, after gelation step, during a supercritical fluid drying process [122]. The result is a material with a high porosity (90–99%), comprising meso and micropores (50 nm), which provides a high internal surface area and low densities [123,124,125,126,127,128]. These scaffolds can be used for tissue engineering applications due to its nanofibrous structure that are suitable for cell adhesion, proliferation and migration [129]. However the traditional technologies for aerogel production lack reproducible customization of the 3D structures and do not allow the fabrication of complex structures [121]. 3D printing of aerogel can overcome the above-mentioned shortcomings, but it requires printable sol or gel with suitable viscosity and mechanical strength. Only a few studies have been reported using 3D printing techniques [121,130,131,132,133,134,135]. Maleki et al. [135] formulated a hybrid silica–silk fibroin aerogel with an excellent printability in the wet state using a micro-extrusion based 3D printing approach. Cheng et al. [121] described a new technique that integrates direct ink writing and freeze-casting with non-toxic solvent-based inks followed by special drying techniques. Taken together, these polymers, biopolymers, or synthetic polymers, could be divided into conventional and advanced (smart) polymers according to their response to environmental [3].

3. Advanced Polymers

Advanced or smart materials (also called “sensitive” materials) are materials that have one or more properties or functions with the ability of responding to one or multiple external stimuli, classified as physical (temperature, humidity, electric field, magnetic field, light), chemical (pH value and ion concentration), and/or biological (enzymes and peptides) [17]. When it comes advanced polymers, these stimuli promote changes in their microstructure (Figure 1B), in which the polymeric chains can be reversibly altered in relation to hydrophilic–hydrophobic balance, conformation, solubility, or degradation [13,136]. In a vivo environment, these materials are normally responsive to multiple stimuli, and their property can be advantageous in the development of scaffolds for biomedical applications.
Temperature is the most used stimulus for biomaterials [137,138,139,140]. The hydrogels synthesis based on thermosensitive polymers has been highlighted in applications for tissue engineering, drug release, gene therapy, or biosensing, due to the sol-gel phase transition behavior of these polymers at a critical temperature [136,141]. Besides, most thermoresponsive materials enable reversible deformation [142]. Thermosensitive polymers are classified into two types regarding the critical temperature, the lower critical solution temperature (LCST), and the upper critical solution temperature (UCST). In these materials, small variations close to the critical temperature abruptly influence hydrophilic–hydrophobic interactions, often leading to a phase transition [136,143]. Therefore, UCST polymers have high solubility with an increase in temperature above its critical point. Whereas LCST are known to have a decrease in solubility when there is an increase in temperature. Recently, copolymers containing LCST close to a physiological temperature have been highlighted for the development of new materials [13], since they are good candidates for injectable and printable hydrogels for tissue engineering.
Poly(N-isopropylacrylamide) (PNIPAM)-based hydrogels are the most used thermoresponsive materials. These biocompatible materials are swollen in the solution at low temperature and shrink upon increase temperature above 32–35 °C (LCST). Furthermore, they present reversible folding/unfolding and may be used for 3D printing [144,145,146,147]. The main disadvantage of PNIPAM is that it is not a bioresorbable polymer [148], although there are several strategies for the development of bioresorbable PNIPAM-based hydrogels with the introduction of bioresorbable cross linkers and/or natural polymers, such as polysaccharides [149,150,151] and proteins [152], and synthetic polymers, including polyesters [153,154], PCL [138,155,156], and PEG [157,158]. Methoxy poly(ethylene glycol)-poly(pyrrolidone-co-lactide) (mPDLA, P3L7) diblock copolymer [159], poly(propylene oxide)–poly(ethylene oxide) (PEO–PPO–PEO) triblock hydrogel [160], PLGA–PEG–PLGA triblock copolymers [161,162,163] and BOX copolymer [164] are examples of bioresorbable thermoresponsive hydrogels, which not are based on PNIPAM.
Photoresponsive materials undergo physical (conformation, polarity) or chemical (hydrophilicity, charge, bond strength) transformation upon exposure to light, which can consequently result in alterations on material wettability, solubility, optical properties, and/or degradability [136]. Optical stimuli may be applied to a localized region without contact, and its dose may be easily adjusted to control response [8,165,166]. Photoresponsive systems may be divided according to light source: visible (vis), ultraviolet (UV), and near-infrared (NIR) light. UV light is powerful, yet presents low tissue penetrability and high toxicity [167]. Visible light exhibit innocuous properties and high tissue penetrability, with a weak efficiency as stimulus [168]. NIR light is an efficient stimulus for optical-responsive materials and provides low toxicity and high tissue penetration [168,169].
Optical-responsive materials present chromophores on the polymer backbone that captures the optical signal and converts the photoirradiation into a photoreaction [169]. Depending on the type of chromophore present in the reaction, it can be reversible or irreversible [170]. The most utilized photochromic compounds in polymeric systems are azobenzenes, spiropyrans, spirooxazines, diarylethenes, and fulgides [171]. Polydopamine (PDA) is a biocompatible dopamine derived from synthetic eumelanin polymer that has been widely used in biomedical engineering due to its photothermal effect [172]. Optical stimuli can also be used to induce photodegradation to certain materials [173]. One approach to tune resorption rate by light is to add photodegradable moieties (e.g., coumarinyl or o-nitrobenzyl ester) to the hydrogel [174,175].
Electric field-responsive materials are often polyelectrolyte hydrogels that can swell, shrink, erode, or bend in response to an electric field. Additionally, an electric field can be applied on cells and tissues to stimulate several biological activities such as cell adhesion and orientation, and calcium deposition [176,177,178]. The electrical stimulus is relatively easy to generate and control [179], and as a result, electro-responsive materials have been studied for several biomedical approaches, including in drug delivery [180] and cardiac tissue engineering [181]. Conductive polymers, such as poly[3,4-(ethylenedioxy)thiophene] (PEDOT), polypyrrole (PPy), and polyaniline (PANi) have been extensively studied for this purposes [182,183,184,185,186], but there are some drawbacks in using them in tissue engineering, considering their poor processability, mechanical properties and cell interaction, and lack of resorption [187,188]. To overcome these issues electroactive polymer are mixed with other polymers such as PLA, PCL, PGLA, chitosan, gelatin, and collagen [189,190,191,192]. However, even minimizing the amounts of conductive polymers, they will remain in the patient’s body [193]. Thus, bioresorbable synthesis, electrically conducting polymers (BECPs), has been a solution to overcome this issue [187]. The following hydrogels are some examples of BECPs: Gelatin-g-polyaniline/genipin [194], aniline pentamer (AP) grafting gelatin (GA) (AP-g-GA) [195], GelMA/Bio-IL, and PEGDA/Bio-IL [196].
Different polymeric materials may be functionalized with magnetic-responsive additives (micro- and nanoparticles) to respond to magnetic-field stimuli, which control the polymeric scaffold’s physical, structural, and mechanical properties [197,198]. Magnetic field-responsive materials respond and actuate according to the magnetic field’s steering in a tunable and wireless manner [199]. The type of polymer and magnetic particles, and their ratio and distribution within the matrix will determine the material response [170]. The most used additive is Fe3O4 regarding its superparamagnetic features, biocompatibility, and lack of toxicity [200,201,202,203,204,205]. PEG [203,206], polyurethane acrylate (PUA) [207], polyvinyl alcohol (PVA) [205], GelMA [208], and alginate [204] are some examples of bioresorbable polymers used in these materials.
Some materials are prompt to change their shape and size (swell or shrink) in response to humidity variation [209]. Humidity-responsive materials are composed of highly hydrophilic expandable elements and non-active rigid elements that transform the sorption or desorption of moisture into driving forces for movement [10]. These materials can occasionally return to their original state upon the removal of the stimulus (reversible). Some examples of bioresorbable polymers are cellulose [210,211], polyurethane copolymers [212], poly(ethylene glycol) diacrylate (PEGDA) [213], and PEG-conjugated azobenzene derivative (PCAD) with agarose [214].
pH-responsive polymers contain chemical groups (carboxyl, pyridine, sulfonic, phosphate, and tertiary amines) that accept or release protons in response to surrounding pH changes, resulting in structural or property changes such as solubility, degradability, conformation, activity, and self-assembly [215,216]. Once pH disparities occur in various parts of the human body, the responsiveness of these materials can be further explored in the field tissue engineering [170]. Several biocompatible and bioresorbable natural and synthetic polymers have been studied for this purpose, such as chitosan [217,218,219], hyaluronic acid [220], gelatin [221], alginic acid [222,223], dextran [224], PLGA [225], poly(histidine) (PHIS) [226], and poly(aspartic acid) (PASA) [227,228].
Other materials can be responsive to different biological stimuli, such as enzymes, oligopeptides, and proteins [229,230]. Oligopeptides and recombinant proteins have emerged as an alternative in developing smart materials, since engineering makes it possible to develop and design new polymers with a complexity and a functionality not found in nature [231]. The synthesis of recombinant natural or artificial proteins promotes the adaptation of several properties in the material including its mechanics, degradation, porosity, cell interaction, cytocompatibility, and response to external stimuli (temperature, pH, ionic forces, etc.) [15,231]. Genetically modified recombinant proteins are most likely to achieve a defined molecular structure than synthetically produced materials and may be easily modified. An example would be the fusion proteins or hybrid proteins, which consist of combining a sequence or functional domains of a specific protein with another protein sequence of interest. In such way, recombinant proteins and hybrid fusion proteins, and polynucleotides emerge as viable alternatives to produce hydrogels for three-dimensional (3D) printing.
Amongst the genetically encoded polymeric sequences, ELPs (elastin-like polypeptides), also known as elastin-like recombinamers (ELRs), have been widely used for the development of thermosensitive block copolymers for several biomedical applications, mainly for release systems and tissue engineering [232,233,234,235,236,237,238]. Elastin-like polypeptides can undergo reversible phase transition induced by pH, temperature, or ionic strength, whose transition phase is directly dependent on the ELP’s sequence transition temperature (Tt). Transition temperature varies according to the ELP’s sequence, molecular weight, and concentration. These polymers are soluble at temperatures below Tt, and insoluble at temperatures above Tt, that is, they have LCST [232]. Additionally, the mechanical properties and swelling rate of ELP hydrogels are related to the concentration, molecular weight, and content of lysine or cysteine of the monomeric sequence [239].
Cell traction forces (CTF) are the tangential tension exerted by cells on the extracellular matrix (ECM) or underlying layer, a crucial biological stimulus. The most known example is the cylindrical tubes, fabricated using traction cell force of cells seeded on a flat microplate [240]. This technique, cell origami, is based on cell traction forces by which cells transform 2D (two-dimensional) surfaces on 3D structures by folding elements in pre-defined shapes. The contractile force exerted by the cells originated from actin polymerization and actomyosin interactions, often occur in various physiological processes and are responsible for the origami folding [241]. Different scaffold properties and different mechanical forces applied on this scaffold generate various effects on the cell phenotype and metabolism [242,243,244], and it will directly impact the extracellular matrix (ECM) production, composition [245,246], and consequently, cellular traction forces [247].
Surface tension (capillary force) can also transform membranes into 3D structures [248,249,250]. An example is a capillary origami, where a liquid can be droplet on a soft film, and after the liquid evaporates, surface tension drags the fil and changes its shape [251].
In this context, smart polymers have a wide advantage in the production of hydrogels, both for bioprinting and cells carriers, or injectable drugs [252], due to their versatility and adjustability sensitivity to the stimuli of the surrounding environment, making it possible to control the desired physical-chemical and mechanical properties for each particular application.

4. 4D Printing in Tissue Engineering

Three-dimensional bioprinting considers only the printed object’s initial state and assumes an inanimate and static scaffold; however, placing biocompatible materials and cells through printing is not enough to construct a tissue or an organ [253]. Four-dimensional (4D) printing adds time to the process as the fourth dimension, and considers and plans changes on printed objects shapes and/or functionalities when an external or internal stimulus is imposed following the 3D printing process (Figure 1A) [142,253,254]. 4D printing is influenced by five key factors: The additive manufacturing process, the responsive material, the type of stimulus, the interaction mechanism between stimulus, and the material, and the mathematical modeling of the material transformation [170]. When exposed to appropriate stimuli, responsive materials undergo physical or chemical changes, leading to macroscopic level transformations (dimension, secondary structure, solubility, degree of intermolecular association, sol-gel transition, chain breakage) that may be useful in tissue engineering [143]. Although there are several studies using resorbable smart materials in 4D-printing for tissue engineering approaches (Table 1), scaffold degradability is a crucial property for tissue regeneration.
4D biofabrication can be performed in three ways: (i) Scaffold production, followed by material transformation and by cell seeding; (ii) scaffold production, followed by cell seeding and by material transformation; (iii) scaffold production simultaneously with cell seeding-containing material followed by material transformation [148]. However, living cells interaction with the material and/or the stimulus and/or the material transformation needs to be considered [253,275]. The materials used in the process of biofabrication must be biocompatible and non-toxic and be favorable for cell adhesion and growth. When cells are seeded prior material transformation, stimuli and transformation should not affect the viability or cell type characteristics. Considering the third approach, scaffold manufacture (bioprinting) must be suitable for cell viability. Therefore, cell traction forces depend on the cell phenotype, cell density and cell adhesion, and should be optimized for controlled conformations in printed structures [276].
In tissue engineering, vascularization is the a key factor for engineer functional tissues since it is necessary to effectively supply nutrients and oxygen, and to remove metabolic products over a distance of 100–200 mm [277,278,279]. 4D bioprinting has been intensively studied to produce blood vessel structures and microfluidic channels in different scaffolds. Cylinder-shaped structures resembling vasculature can be produced by bioprinting sacrificial hydrogels containing cells [47], or by self-folding polymers in the presence of cells [240]. Sacrificial polymers (fugitive inks), are usually applied as temporary support of overhanging structures during 3D bioprinting, and are extensively used to construct microfluidic channels which allows the creation of vascularized tissues [280]. Agarose and gelatin are the most used polymers in sacrificial strategy, where the desired channel cavity into the material is filled with these temporarily polymers during the printing process and are subsequently removed by heating [48,262,263,269]. The surfactant Pluronic F127 can be utilized as a fugitive ink for microfluidic network constructs that can also be removed through temperature [264].
There are other applications for thermoresponsive materials in tissue engineering [261,265,266,267]. Moroni et al. [268] for example, described the fabrication of 3D shape memory polymer scaffold able to change their shape in time during culture. Cells were seeded onto polyurethane/collagen type I scaffolds in a temporary shape, and during culture, due to temperature increase, the permanent shape was recovered and allowed the adherent cells to present a significantly more elongated shape. Peeters et al. [147] developed a thermoresponsive bioresorbable polymer bilayer construct (PLA-b-PEG-b-PLA/NIPAAm) that swell and subsequently roll-up under low temperatures. A catheter could deliver these cell-laden “wrap” structures to impaired myocardium, where would unroll and expose the delivered cells to the damaged tissue in response to a temperature increase to 37 °C.
Magnetic-responsive scaffolds can be applied in tissue regeneration when alignment [207], mechanical stimulation [281], and stem cell differentiation are required [282]. Magnetic field direction and strength generate specific alterations on morphology and geometry of these materials. Magnetic field-responsive materials can also be used to manipulate cell-laden printed scaffolds [276]. For this purpose, magnetic field influence on cells also needs to be considered. Stem cells loaded with magnetic nanoparticles, for example, may form 3D aggregates under magnetic fields [283].
Magnetic particles are included in the polymeric hydrogel to produce the magnetic-sensitive material. These particles may leach from the material matrix in living systems and, depending on particle size (smaller than 50 nm) and cross biological membranes, it may negatively affect the tissue functionality [284]. Therefore, the biocompatibility of magnetic particles is of extreme importance to the magnetic field-responsive materials in tissue engineering, and for this reason, iron-based particles are the most used ones [256,257]. De Santis et al. [258] analyzed the behavior of human mesenchymal stem cells (hMSCs) seeded on a 3D additive-manufactured poly(ɛ-caprolactone)/iron-doped hydroxyapatite (PCL/FeHA) nanocomposite scaffolds under a magnetic field. They demonstrated that cell adhesion and proliferation can be enhanced by employing a sinusoidal magnetic (frequency of 70 Hz and intensity of 25–30 mT). For cartilage tissue engineering, Campos et al. [259] developed an advanced bioprinting strategy incorporating magnetic field into the 3D printer with the objective of generating complex multilayers tissues with aligned collagen fiber.
Materials that are responsive to other stimuli have been developed for use in tissue engineering. Park et al. [269] controlled changes in shape of 3D printed bilayer Sil-Ma hydrogels by modulating their properties in physiological conditions through osmolarity. Based on this technique, they constructed a trachea mimetic tissue using two cell types that successfully integrated rabbit damage trachea in vivo. Another group proposed 4D bioprinting to fabricate cell-laden bilayer constructs (alginate/GelMA:alginate/PDA ) with controlled curve structures by NIR-light that can be widely used in regenerative medicine [260].
In the human body, tissues respond to small biological molecules or bio-macromolecules, such as glucose, enzymes, nucleic acids, polypeptides, and proteins [285]. Several studies focus on the development of materials associated with the responsive behavior upon being exposed to these stimuli. Burdick et al. [271] produced mimicked blood-vessel structures by seeding cells into the microchannels of a support hydrogel with RGD peptides for adhesion HUVEC cells and protease-cleavable crosslinkers for cell-mediated degradation. Once cells were exposed to angiogenic factors, the hydrogel support was degraded, and a scaffold-free cell structure was formed. Enzymes can also be utilized as biological stimuli to enhance biological activities in the scaffolds. Marquette et al. [272] entrapped two different enzymes (alkaline phosphatase and thrombin) into a 3D printed structure to attribute multiple biological activities to the scaffold. The first enzyme enabled localized and pre-programmed calcification, while the thrombin permitted the formation of fibrin biofilm.

5. Prospects and Conclusions

In nature, tissues are non-static functional systems able to respond to different environment changes. Although 3D printing is an indispensable tool to produce complex-shape structures for tissue engineering, the resulting printed structures are static and not capable to actively alter in response to environment variations. 4D printing has emerged recently as a technology that confers predicted dynamic transformations to printed structures in a controllable manner using responsive-materials and/or cells. However, 4D printing is still in the stage of proof-of-concept, and as it is an emerging technology, it presents several many limitations as well as challenges to overcome, such as structural design, print techniques, and ink development. There is no consistent computational model to accurately predict the material transformation over time, and technological advances are required in software and mathematical modeling [265,286]. Printing techniques using cells are recent and are constantly being improved. Achievement of higher resolution for bioprinting is always a challenge since it requires higher shear forces, negatively impacting the cell viability [148].
Responsive materials have been studied for decades but only a few have been designed for 4D printing to be applied in tissue engineering. For this purpose, these materials must be biocompatible, noncytotoxic, and preferably biodegradable (resorbable). In addition, they must present certain mechanical strength and need to be capable of performing the dynamic process in physiological environment. A crucial consideration is that the stimulus used must be safe and easy to control if applied in the presence of cells, or in the body. Extreme pH values and high temperatures, for example, should be avoided. Due to such strict requirements, only a few dynamic polymers meet all the desired qualifications. Moreover, in nature, tissues are subjected to many different stimuli and so far, most described materials are responsive to only one stimulus. Therefore, greater efforts and expertise should be applied in the development novel and multifunctional 4D inks to improve 4D printing technique.
Additionally, there are few studies on the maturation of cell-laden printed tissues, and little is known about the effect of cells on the materials shape transformation. In conclusion, 4D printing is a visionary, promising, and powerful technology mimicking the organization and biological functionality of native tissues. However, there are necessary improvements before this technology is qualified for clinical applications.


This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

No new data were created or analyzed in this study. Data sharing is not applicable to this article.

Conflicts of Interest

The authors declare no conflict of interest.


  1. Langer, R.; Vacanti, J.P. Tissue engineering. Science 1993, 260, 920–926. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  2. Lynch, S.E. Tissue Engineering: Applications in Oral and Maxillofacial Surgery and Periodontics; Quintessence Books; Quintessence Publishing: Berlin, Germany, 2008; ISBN 9780867154641. [Google Scholar]
  3. Luo, Y.; Engelmayr, G.; Auguste, D.T.; da Silva Ferreira, L.; Karp, J.M.; Saigal, R.; Langer, R. 3D Scaffolds. In Principles of Tissue Engineering; Lanza, R., Langer, R., Vacanti, J.B.T.-P., Atala, A., Eds.; Academic Press: Boston, MA, USA, 2014; pp. 475–494. ISBN 978-0-12-398358-9. [Google Scholar]
  4. Karageorgiou, V.; Kaplan, D. Porosity of 3D biomaterial scaffolds and osteogenesis. Biomaterials 2005, 26, 5474–5491. [Google Scholar] [CrossRef]
  5. Bose, S.; Vahabzadeh, S.; Bandyopadhyay, A. Bone tissue engineering using 3D printing. Mater. Today 2013, 16, 496–504. [Google Scholar] [CrossRef]
  6. Saska, S.; Mendes, L.S.; Gaspar, A.M.M.; Capote, T.S.D.O. Bone Substitute Materials in Implant Dentistry. In Current Concepts in Dental Implantology; Turkyilmaz, I., Ed.; Intech: London, UK, 2015; pp. 25–43. [Google Scholar]
  7. Christensen, K.; Davis, B.; Jin, Y.; Huang, Y. Effects of printing-induced interfaces on localized strain within 3D printed hydrogel structures. Mater. Sci. Eng. C 2018, 89, 65–74. [Google Scholar] [CrossRef] [PubMed]
  8. Li, Y.-C.; Zhang, Y.S.; Akpek, A.; Shin, S.R.; Khademhosseini, A. 4D bioprinting: The next-generation technology for biofabrication enabled by stimuli-responsive materials. Biofabrication 2016, 9, 12001. [Google Scholar] [CrossRef] [Green Version]
  9. Qasim, M.; Chae, D.S.; Lee, N. Advancements and frontiers in nano-based 3d and 4d scaffolds for bone and cartilage tissue engineering. Int. J. Nanomed. 2019, 14, 4333–4351. [Google Scholar] [CrossRef] [Green Version]
  10. Shie, M.-Y.; Shen, Y.-F.; Astuti, S.D.; Lee, A.K.-X.; Lin, S.-H.; Dwijaksara, N.L.B.; Chen, Y.-W. Review of Polymeric Materials in 4D Printing Biomedical Applications. Polymers 2019, 11, 1864. [Google Scholar] [CrossRef] [Green Version]
  11. Hoffman, A.S. Hydrogels for biomedical applications. Adv. Drug Deliv. Rev. 2002, 54, 3–12. [Google Scholar] [CrossRef]
  12. Xu, B.; Li, Y.; Deng, B.; Liu, X.; Wang, L.; Zhu, Q. Chitosan hydrogel improves mesenchymal stem cell transplant survival and cardiac function following myocardial infarction in rats. Exp. Ther. Med. 2017, 13, 588–594. [Google Scholar] [CrossRef] [Green Version]
  13. Ward, M.A.; Georgiou, T.K. Thermoresponsive Polymers for Biomedical Applications. Polymers 2011, 3, 1215–1242. [Google Scholar] [CrossRef] [Green Version]
  14. Skardal, A.; Atala, A. Biomaterials for Integration with 3-D Bioprinting. Ann. Biomed. Eng. 2015, 43, 730–746. [Google Scholar] [CrossRef] [PubMed]
  15. Jungst, T.; Smolan, W.; Schacht, K.; Scheibel, T.; Groll, J. Strategies and Molecular Design Criteria for 3D Printable Hydrogels. Chem. Rev. 2016, 116, 1496–1539. [Google Scholar] [CrossRef] [PubMed]
  16. Yue, K.; Santiago, G.T.-D.; Alvarez, M.M.; Tamayol, A.; Annabi, N.; Khademhosseini, A. Synthesis, properties, and biomedical applications of gelatin methacryloyl (GelMA) hydrogels. Biomaterials 2015, 73, 254–271. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  17. Nikolova, M.P.; Chavali, M.S. Recent advances in biomaterials for 3D scaffolds: A review. Bioact. Mater. 2019, 4, 271–292. [Google Scholar] [CrossRef] [PubMed]
  18. El-Sherbiny, I.M.; Yacoub, M.H. Hydrogel scaffolds for tissue engineering: Progress and challenges. Glob. Cardiol. Sci. Pract. 2013, 2013, 38. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  19. Guadalupe, E.; Ramos, D.; Shelke, N.B.; James, R.; Gibney, C.; Kumbar, S.G. Bioactive polymeric nanofiber matrices for skin regeneration. J. Appl. Polym. Sci. 2015, 132. [Google Scholar] [CrossRef]
  20. Qasim, M.; Baipaywad, P.; Udomluck, N.; Na, D.; Park, H. Enhanced therapeutic efficacy of lipophilic amphotericin B against Candida albicans with amphiphilic poly(N-isopropylacrylamide) nanogels. Macromol. Res. 2014, 22, 1125–1131. [Google Scholar] [CrossRef]
  21. Gopinathan, J.; Noh, I. Recent trends in bioinks for 3D printing. Biomater. Res. 2018, 22, 11. [Google Scholar] [CrossRef] [Green Version]
  22. Retzepi, M.; Donos, N. Guided Bone Regeneration: Biological principle and therapeutic applications. Clin. Oral Implant. Res. 2010, 21, 567–576. [Google Scholar] [CrossRef]
  23. Warrer, K.; Karring, T.; Nyman, S.; Gogolewski, S. Guided tissue regeneration using biodegradable membranes of polylactic acid or polyurethane. J. Clin. Periodontol. 1992, 19, 633–640. [Google Scholar] [CrossRef]
  24. Ulery, B.D.; Nair, L.S.; Laurencin, C.T. Biomedical Applications of Biodegradable Polymers. J. Polym. Sci. B Polym. Phys. 2011, 49, 832–864. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  25. Vroman, I.; Tighzert, L. Biodegradable Polymers. Materials 2009, 2, 307–344. [Google Scholar] [CrossRef]
  26. Unagolla, J.M.; Jayasuriya, A.C. Hydrogel-based 3D bioprinting: A comprehensive review on cell-laden hydrogels, bioink formulations, and future perspectives. Appl. Mater. Today 2020, 18, 100479. [Google Scholar] [CrossRef] [PubMed]
  27. Groll, J.; Burdick, J.A.; Cho, D.W.; Derby, B.; Gelinsky, M.; Heilshorn, S.C.; Jüngst, T.; Malda, J.; Mironov, V.A.; Nakayama, K.; et al. A definition of bioinks and their distinction from biomaterial inks. Biofabrication 2019, 11, 013001. [Google Scholar] [CrossRef] [PubMed]
  28. Majeed, S. Advancement of Bio inks in three Dimensional Bioprinting. Biomed. J. Sci. Tech. Res. 2018, 11, 4. [Google Scholar] [CrossRef] [Green Version]
  29. Guillemot, F.; Mironov, V.; Nakamura, M. Bioprinting is coming of age: Report from the International Conference on Bioprinting and Biofabrication in Bordeaux (3B’09). Biofabrication 2010, 2, 10201. [Google Scholar] [CrossRef]
  30. Ozbolat, I.T. Bioprinting scale-up tissue and organ constructs for transplantation. Trends Biotechnol. 2015, 33, 395–400. [Google Scholar] [CrossRef]
  31. Hospodiuk, M.; Dey, M.; Sosnoski, D.; Ozbolat, I.T. The bioink: A comprehensive review on bioprintable materials. Biotechnol. Adv. 2017, 35, 217–239. [Google Scholar] [CrossRef] [Green Version]
  32. Dorishetty, P.; Dutta, N.K.; Choudhury, N.R. Bioprintable tough hydrogels for tissue engineering applications. Adv. Colloid Interface Sci. 2020, 281, 102163. [Google Scholar] [CrossRef]
  33. You, F.; Wu, X.; Zhu, N.; Lei, M.; Eames, B.F.; Chen, X. 3D Printing of Porous Cell-Laden Hydrogel Constructs for Potential Applications in Cartilage Tissue Engineering. ACS Biomater. Sci. Eng. 2016, 2, 1200–1210. [Google Scholar] [CrossRef]
  34. Carrow, J.K.; Kerativitayanan, P.; Jaiswal, M.K.; Lokhande, G.; Gaharwar, A.K. Polymers for Bioprinting. In Essentials of 3D Biofabrication and Translation; Atala, A., Yoo, J.J., Eds.; Academic Press: Boston, MA, USA, 2015; pp. 229–248. ISBN 978-0-12-800972-7. [Google Scholar]
  35. Matai, I.; Kaur, G.; Seyedsalehi, A.; McClinton, A.; Laurencin, C.T. Progress in 3D bioprinting technology for tissue/organ regenerative engineering. Biomaterials 2020, 226, 119536. [Google Scholar] [CrossRef] [PubMed]
  36. Dogan, E.; Bhusal, A.; Cecen, B.; Miri, A.K. 3D Printing metamaterials towards tissue engineering. Appl. Mater. Today 2020, 20, 100752. [Google Scholar] [CrossRef] [PubMed]
  37. Bedell, M.L.; Navara, A.M.; Du, Y.; Zhang, S.; Mikos, A.G. Polymeric Systems for Bioprinting. Chem. Rev. 2020, 120, 10744–10792. [Google Scholar] [CrossRef] [PubMed]
  38. Khalil, S.; Sun, W. Biopolymer deposition for freeform fabrication of hydrogel tissue constructs. Mater. Sci. Eng. C 2007, 27, 469–478. [Google Scholar] [CrossRef]
  39. Kesti, M.; Müller, M.; Becher, J.; Schnabelrauch, M.; D’Este, M.; Eglin, D.; Zenobi-Wong, M. A versatile bioink for three-dimensional printing of cellular scaffolds based on thermally and photo-triggered tandem gelation. Acta Biomater. 2015, 11, 162–172. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  40. Ringeisen, B.R.; Kim, H.; Barron, J.A.; Krizman, D.B.; Chrisey, D.B.; Jackman, S.; Auyeung, R.Y.C.; Spargo, B.J. Laser Printing of Pluripotent Embryonal Carcinoma Cells. Tissue Eng. 2004, 10, 483–491. [Google Scholar] [CrossRef]
  41. Du, Y.; Liu, H.; Yang, Q.; Wang, S.; Wang, J.; Ma, J.; Noh, I.; Mikos, A.G.; Zhang, S. Selective laser sintering scaffold with hierarchical architecture and gradient composition for osteochondral repair in rabbits. Biomaterials 2017, 137, 37–48. [Google Scholar] [CrossRef]
  42. Sinha, S.K. Chapter 5—Additive manufacturing (AM) of medical devices and scaffolds for tissue engineering based on 3D and 4D printing. In 3D and 4D Printing of Polymer Nanocomposite Materials; Sadasivuni, K.K., Deshmukh, K., Almaadeed, M.A., Eds.; Elsevier: Amsterdam, The Netherlands, 2020; pp. 119–160. ISBN 978-0-12-816805-9. [Google Scholar]
  43. Velasco, D.; Tumarkin, E.; Kumacheva, E. Microfluidic Encapsulation of Cells in Polymer Microgels. Small 2012, 8, 1633–1642. [Google Scholar] [CrossRef]
  44. Fu, X.T.; Kim, S.M. Agarase: Review of Major Sources, Categories, Purification Method, Enzyme Characteristics and Applications. Mar. Drugs 2010, 8, 200–218. [Google Scholar] [CrossRef] [Green Version]
  45. Meena, R.; Siddhanta, A.K.; Prasad, K.; Ramavat, B.K.; Eswaran, K.; Thiruppathi, S.; Ganesan, M.; Mantri, V.A.; Rao, P.V.S. Preparation, characterization and benchmarking of agarose from Gracilaria dura of Indian waters. Carbohydr. Polym. 2007, 69, 179–188. [Google Scholar] [CrossRef]
  46. Hunziker, E.B. Articular cartilage repair: Basic science and clinical progress. A review of the current status and prospects. Osteoarthr. Cartil. 2002, 10, 432–463. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  47. Norotte, C.; Marga, F.S.; Niklason, L.E.; Forgacs, G. Scaffold-free vascular tissue engineering using bioprinting. Biomaterials 2009, 30, 5910–5917. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  48. Bertassoni, L.E.; Cecconi, M.; Manoharan, V.; Nikkhah, M.; Hjortnaes, J.; Cristino, A.L.; Barabaschi, G.; Demarchi, D.; Dokmeci, M.R.; Yang, Y.; et al. Hydrogel bioprinted microchannel networks for vascularization of tissue engineering constructs. Lab Chip 2014, 14, 2202–2211. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  49. Andersen, T.; Auk-Emblem, P.; Dornish, M. 3D Cell Culture in Alginate Hydrogels. Microarrays 2015, 4, 133–161. [Google Scholar] [CrossRef] [PubMed]
  50. Mancini, M.; Moresi, M.; Rancini, R. Mechanical properties of alginate gels: Empirical characterisation. J. Food Eng. 1999, 39, 369–378. [Google Scholar] [CrossRef]
  51. Drury, J.L.; Dennis, R.G.; Mooney, D.J. The tensile properties of alginate hydrogels. Biomaterials 2004, 25, 3187–3199. [Google Scholar] [CrossRef]
  52. Gomez, C.G.; Rinaudo, M.; Villar, M.A. Oxidation of sodium alginate and characterization of the oxidized derivatives. Carbohydr. Polym. 2007, 67, 296–304. [Google Scholar] [CrossRef]
  53. Remminghorst, U.; Rehm, B.H.A. Bacterial alginates: From biosynthesis to applications. Biotechnol. Lett. 2006, 28, 1701–1712. [Google Scholar] [CrossRef]
  54. Axpe, E.; Oyen, M.L. Applications of Alginate-Based Bioinks in 3D Bioprinting. Int. J. Mol. Sci. 2016, 17, 1976. [Google Scholar] [CrossRef] [Green Version]
  55. Rowley, J.A.; Madlambayan, G.; Mooney, D.J. Alginate hydrogels as synthetic extracellular matrix materials. Biomaterials 1999, 20, 45–53. [Google Scholar] [CrossRef]
  56. Jia, J.; Richards, D.J.; Pollard, S.; Tan, Y.; Rodriguez, J.; Visconti, R.P.; Trusk, T.C.; Yost, M.J.; Yao, H.; Markwald, R.R.; et al. Engineering alginate as bioink for bioprinting. Acta Biomater. 2014, 10, 4323–4331. [Google Scholar] [CrossRef] [Green Version]
  57. Attalla, R.; Ling, C.; Selvaganapathy, P. Fabrication and characterization of gels with integrated channels using 3D printing with microfluidic nozzle for tissue engineering applications. Biomed. Microdevices 2016, 18, 17. [Google Scholar] [CrossRef] [PubMed]
  58. Markstedt, K.; Mantas, A.; Tournier, I.; Ávila, H.M.; Hägg, D.; Gatenholm, P. 3D Bioprinting Human Chondrocytes with Nanocellulose–Alginate Bioink for Cartilage Tissue Engineering Applications. Biomacromolecules 2015, 16, 1489–1496. [Google Scholar] [CrossRef] [PubMed]
  59. Poldervaart, M.T.; Wang, H.; van der Stok, J.; Weinans, H.; Leeuwenburgh, S.C.G.; Öner, F.C.; Dhert, W.J.A.; Alblas, J. Sustained Release of BMP-2 in Bioprinted Alginate for Osteogenicity in Mice and Rats. PLoS ONE 2013, 8, e72610. [Google Scholar] [CrossRef]
  60. Levengood, S.K.L.; Zhang, M. Chitosan-based scaffolds for bone tissue engineering. J. Mater. Chem. B 2014, 2, 3161–3184. [Google Scholar] [CrossRef] [PubMed]
  61. Unagolla, J.M.; Alahmadi, T.E.; Jayasuriya, A.C. Chitosan microparticles based polyelectrolyte complex scaffolds for bone tissue engineering in vitro and effect of calcium phosphate. Carbohydr. Polym. 2018, 199, 426–436. [Google Scholar] [CrossRef]
  62. Park, S.Y.; Lee, B.I.; Jung, S.T.; Park, H.J. Biopolymer composite films based on κ-carrageenan and chitosan. Mater. Res. Bull. 2001, 36, 511–519. [Google Scholar] [CrossRef]
  63. Dong, L.; Wang, S.-J.; Zhao, X.-R.; Zhu, Y.-F.; Yu, J.-K. 3D-Printed Poly(ε-caprolactone) Scaffold Integrated with Cell-laden Chitosan Hydrogels for Bone Tissue Engineering. Sci. Rep. 2017, 7, 13412. [Google Scholar] [CrossRef] [Green Version]
  64. Gómez-Guillén, M.C.; Giménez, B.; López-Caballero, M.E.; Montero, M.P. Functional and bioactive properties of collagen and gelatin from alternative sources: A review. Food Hydrocoll. 2011, 25, 1813–1827. [Google Scholar] [CrossRef] [Green Version]
  65. Xing, R.; Liu, K.; Jiao, T.; Zhang, N.; Ma, K.; Zhang, R.; Zou, Q.; Ma, G.; Yan, X. An Injectable Self-Assembling Collagen–Gold Hybrid Hydrogel for Combinatorial Antitumor Photothermal/Photodynamic Therapy. Adv. Mater. 2016, 28, 3669–3676. [Google Scholar] [CrossRef]
  66. Park, J.Y.; Choi, J.-C.; Shim, J.-H.; Lee, J.-S.; Park, H.; Kim, S.W.; Doh, J.; Cho, D.-W. A comparative study on collagen type I and hyaluronic acid dependent cell behavior for osteochondral tissue bioprinting. Biofabrication 2014, 6, 35004. [Google Scholar] [CrossRef] [PubMed]
  67. Rhee, S.; Puetzer, J.L.; Mason, B.N.; Reinhart-King, C.A.; Bonassar, L.J. 3D Bioprinting of Spatially Heterogeneous Collagen Constructs for Cartilage Tissue Engineering. ACS Biomater. Sci. Eng. 2016, 2, 1800–1805. [Google Scholar] [CrossRef] [PubMed]
  68. Yanez, M.; Rincon, J.; Dones, A.; De Maria, C.; Gonzales, R.; Boland, T. In Vivo Assessment of Printed Microvasculature in a Bilayer Skin Graft to Treat Full-Thickness Wounds. Tissue Eng. Part A 2014, 21, 224–233. [Google Scholar] [CrossRef] [Green Version]
  69. Raucci, M.G.; D’Amora, U.; Ronca, A.; Demitri, C.; Ambrosio, L. Bioactivation Routes of Gelatin-Based Scaffolds to Enhance at Nanoscale Level Bone Tissue Regeneration. Front. Bioeng. Biotechnol. 2019, 7, 27. [Google Scholar] [CrossRef] [Green Version]
  70. Van Den Bulcke, A.I.; Bogdanov, B.; De Rooze, N.; Schacht, E.H.; Cornelissen, M.; Berghmans, H. Structural and Rheological Properties of Methacrylamide Modified Gelatin Hydrogels. Biomacromolecules 2000, 1, 31–38. [Google Scholar] [CrossRef]
  71. Klotz, B.J.; Gawlitta, D.; Rosenberg, A.J.W.P.; Malda, J.; Melchels, F.P.W. Gelatin-Methacryloyl Hydrogels: Towards Biofabrication-Based Tissue Repair. Trends Biotechnol. 2016, 34, 394–407. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  72. Duan, B.; Hockaday, L.A.; Kang, K.H.; Butcher, J.T. 3D bioprinting of heterogeneous aortic valve conduits with alginate/gelatin hydrogels. J. Biomed. Mater. Res. A 2013, 101, 1255–1264. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  73. Kang, H.-W.; Lee, S.J.; Ko, I.K.; Kengla, C.; Yoo, J.J.; Atala, A. A 3D bioprinting system to produce human-scale tissue constructs with structural integrity. Nat. Biotechnol. 2016, 34, 312–319. [Google Scholar] [CrossRef]
  74. Skardal, A.; Devarasetty, M.; Kang, H.-W.; Mead, I.; Bishop, C.; Shupe, T.; Lee, S.J.; Jackson, J.; Yoo, J.; Soker, S.; et al. A hydrogel bioink toolkit for mimicking native tissue biochemical and mechanical properties in bioprinted tissue constructs. Acta Biomater. 2015, 25, 24–34. [Google Scholar] [CrossRef]
  75. Chen, Y.-C.; Lin, R.-Z.; Qi, H.; Yang, Y.; Bae, H.; Melero-Martin, J.M.; Khademhosseini, A. Functional Human Vascular Network Generated in Photocrosslinkable Gelatin Methacrylate Hydrogels. Adv. Funct. Mater. 2012, 22, 2027–2039. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  76. Zhang, Y.S.; Arneri, A.; Bersini, S.; Shin, S.-R.; Zhu, K.; Goli-Malekabadi, Z.; Aleman, J.; Colosi, C.; Busignani, F.; Dell’Erba, V.; et al. Bioprinting 3D microfibrous scaffolds for engineering endothelialized myocardium and heart-on-a-chip. Biomaterials 2016, 110, 45–59. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  77. Ma, X.; Qu, X.; Zhu, W.; Li, Y.-S.; Yuan, S.; Zhang, H.; Liu, J.; Wang, P.; Lai, C.S.E.; Zanella, F.; et al. Deterministically patterned biomimetic human iPSC-derived hepatic model via rapid 3D bioprinting. Proc. Natl. Acad. Sci. USA 2016, 113, 2206–2211. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  78. Schuurman, W.; Levett, P.A.; Pot, M.W.; van Weeren, P.R.; Dhert, W.J.A.; Hutmacher, D.W.; Melchels, F.P.W.; Klein, T.J.; Malda, J. Gelatin-Methacrylamide Hydrogels as Potential Biomaterials for Fabrication of Tissue-Engineered Cartilage Constructs. Macromol. Biosci. 2013, 13, 551–561. [Google Scholar] [CrossRef] [PubMed]
  79. Prestwich, G.D. Hyaluronic acid-based clinical biomaterials derived for cell and molecule delivery in regenerative medicine. J. Control. Release 2011, 155, 193–199. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  80. Gaetani, R.; Feyen, D.A.M.; Verhage, V.; Slaats, R.; Messina, E.; Christman, K.L.; Giacomello, A.; Doevendans, P.A.F.M.; Sluijter, J.P.G. Epicardial application of cardiac progenitor cells in a 3D-printed gelatin/hyaluronic acid patch preserves cardiac function after myocardial infarction. Biomaterials 2015, 61, 339–348. [Google Scholar] [CrossRef] [PubMed]
  81. Law, N.; Doney, B.; Glover, H.; Qin, Y.; Aman, Z.M.; Sercombe, T.B.; Liew, L.J.; Dilley, R.J.; Doyle, B.J. Characterisation of hyaluronic acid methylcellulose hydrogels for 3D bioprinting. J. Mech. Behav. Biomed. Mater. 2018, 77, 389–399. [Google Scholar] [CrossRef]
  82. Mawad, D.; Poole-Warren, L.A.; Martens, P.; Koole, L.H.; Slots, T.L.B.; van Hooy-Corstjens, C.S.J. Synthesis and Characterization of Radiopaque Iodine-containing Degradable PVA Hydrogels. Biomacromolecules 2008, 9, 263–268. [Google Scholar] [CrossRef]
  83. Zhu, J. Bioactive modification of poly(ethylene glycol) hydrogels for tissue engineering. Biomaterials 2010, 31, 4639–4656. [Google Scholar] [CrossRef] [Green Version]
  84. Zustiak, S.P.; Leach, J.B. Hydrolytically Degradable Poly(Ethylene Glycol) Hydrogel Scaffolds with Tunable Degradation and Mechanical Properties. Biomacromolecules 2010, 11, 1348–1357. [Google Scholar] [CrossRef] [Green Version]
  85. Jia, W.; Gungor-Ozkerim, P.S.; Zhang, Y.S.; Yue, K.; Zhu, K.; Liu, W.; Pi, Q.; Byambaa, B.; Dokmeci, M.R.; Shin, S.R.; et al. Direct 3D bioprinting of perfusable vascular constructs using a blend bioink. Biomaterials 2016, 106, 58–68. [Google Scholar] [CrossRef] [Green Version]
  86. Gao, G.; Schilling, A.F.; Yonezawa, T.; Wang, J.; Dai, G.; Cui, X. Bioactive nanoparticles stimulate bone tissue formation in bioprinted three-dimensional scaffold and human mesenchymal stem cells. Biotechnol. J. 2014, 9, 1304–1311. [Google Scholar] [CrossRef]
  87. Gao, G.; Yonezawa, T.; Hubbell, K.; Dai, G.; Cui, X. Inkjet-bioprinted acrylated peptides and PEG hydrogel with human mesenchymal stem cells promote robust bone and cartilage formation with minimal printhead clogging. Biotechnol. J. 2015, 10, 1568–1577. [Google Scholar] [CrossRef] [PubMed]
  88. Gao, G.; Schilling, A.F.; Hubbell, K.; Yonezawa, T.; Truong, D.; Hong, Y.; Dai, G.; Cui, X. Improved properties of bone and cartilage tissue from 3D inkjet-bioprinted human mesenchymal stem cells by simultaneous deposition and photocrosslinking in PEG-GelMA. Biotechnol. Lett. 2015, 37, 2349–2355. [Google Scholar] [CrossRef] [PubMed]
  89. Basu, A.; Kunduru, K.R.; Doppalapudi, S.; Domb, A.J.; Khan, W. Poly(lactic acid) based hydrogels. Adv. Drug Deliv. Rev. 2016, 107, 192–205. [Google Scholar] [CrossRef] [PubMed]
  90. Hamad, K.; Kaseem, M.; Yang, H.W.; Deri, F.; Ko, Y.G. Properties and medical applications of polylactic acid: A review. Express Polym. Lett. 2015, 9, 435–455. [Google Scholar] [CrossRef]
  91. Samadi, N.; Abbadessa, A.; Di Stefano, A.; van Nostrum, C.F.; Vermonden, T.; Rahimian, S.; Teunissen, E.A.; van Steenbergen, M.J.; Amidi, M.; Hennink, W.E. The effect of lauryl capping group on protein release and degradation of poly(D,L-lactic-co-glycolic acid) particles. J. Control. Release 2013, 172, 436–443. [Google Scholar] [CrossRef] [PubMed]
  92. Wang, X.; Sui, S. Pulsatile Culture of a Poly(DL-Lactic-Co-Glycolic Acid) Sandwiched Cell/Hydrogel Construct Fabricated Using a Step-by-Step Mold/Extraction Method. Artif. Organs 2011, 35, 645–655. [Google Scholar] [CrossRef]
  93. Uematsu, K.; Hattori, K.; Ishimoto, Y.; Yamauchi, J.; Habata, T.; Takakura, Y.; Ohgushi, H.; Fukuchi, T.; Sato, M. Cartilage regeneration using mesenchymal stem cells and a three-dimensional poly-lactic-glycolic acid (PLGA) scaffold. Biomaterials 2005, 26, 4273–4279. [Google Scholar] [CrossRef]
  94. Jazayeri, H.E.; Tahriri, M.; Razavi, M.; Khoshroo, K.; Fahimipour, F.; Dashtimoghadam, E.; Almeida, L.; Tayebi, L. A current overview of materials and strategies for potential use in maxillofacial tissue regeneration. Mater. Sci. Eng. C 2017, 70, 913–929. [Google Scholar] [CrossRef] [Green Version]
  95. Fan, H.; Hu, Y.; Zhang, C.; Li, X.; Lv, R.; Qin, L.; Zhu, R. Cartilage regeneration using mesenchymal stem cells and a PLGA–gelatin/chondroitin/hyaluronate hybrid scaffold. Biomaterials 2006, 27, 4573–4580. [Google Scholar] [CrossRef]
  96. Park, K.-S.; Kim, B.-J.; Lih, E.; Park, W.; Lee, S.-H.; Joung, Y.K.; Han, D.K. Versatile effects of magnesium hydroxide nanoparticles in PLGA scaffold–mediated chondrogenesis. Acta Biomater. 2018, 73, 204–216. [Google Scholar] [CrossRef] [PubMed]
  97. Gentile, P.; Chiono, V.; Carmagnola, I.; Hatton, P. V An Overview of Poly(lactic-co-glycolic) Acid (PLGA)-Based Biomaterials for Bone Tissue Engineering. Int. J. Mol. Sci. 2014, 15, 3640–3659. [Google Scholar] [CrossRef] [PubMed]
  98. Yan, B.; Zhang, Z.; Wang, X.; Ni, Y.; Liu, Y.; Liu, T.; Wang, W.; Xing, H.; Sun, Y.; Wang, J.; et al. PLGA–PTMC–Cultured Bone Mesenchymal Stem Cell Scaffold Enhances Cartilage Regeneration in Tissue-Engineered Tracheal Transplantation. Artif. Organs 2017, 41, 461–469. [Google Scholar] [CrossRef]
  99. Chang, N.-J.; Jhung, Y.-R.; Yao, C.-K.; Yeh, M.-L. Hydrophilic Gelatin and Hyaluronic Acid-Treated PLGA Scaffolds for Cartilage Tissue Engineering. J. Appl. Biomater. Funct. Mater. 2013, 11, 45–52. [Google Scholar] [CrossRef]
  100. Qian, Y.; Zhou, X.; Zhang, F.; Diekwisch, T.G.H.; Luan, X.; Yang, J. Triple PLGA/PCL Scaffold Modification Including Silver Impregnation, Collagen Coating, and Electrospinning Significantly Improve Biocompatibility, Antimicrobial, and Osteogenic Properties for Orofacial Tissue Regeneration. ACS Appl. Mater. Interfaces 2019, 11, 37381–37396. [Google Scholar] [CrossRef] [PubMed]
  101. García-García, P.; Reyes, R.; Segredo-Morales, E.; Pérez-Herrero, E.; Delgado, A.; Évora, C. PLGA-BMP-2 and PLA-17β-Estradiol Microspheres Reinforcing a Composite Hydrogel for Bone Regeneration in Osteoporosis. Pharmaceutics 2019, 11, 648. [Google Scholar] [CrossRef] [Green Version]
  102. Wu, S.; Zhou, R.; Zhou, F.; Streubel, P.N.; Chen, S.; Duan, B. Electrospun thymosin Beta-4 loaded PLGA/PLA nanofiber/ microfiber hybrid yarns for tendon tissue engineering application. Mater. Sci. Eng. C. Mater. Biol. Appl. 2020, 106, 110268. [Google Scholar] [CrossRef]
  103. Hoskins, J.N.; Grayson, S.M. Synthesis and Degradation Behavior of Cyclic Poly(ε-caprolactone). Macromolecules 2009, 42, 6406–6413. [Google Scholar] [CrossRef]
  104. Huang, Y.-T.; Wang, W.-C.; Hsu, C.-P.; Lu, W.-Y.; Chuang, W.-J.; Chiang, M.Y.; Lai, Y.-C.; Chen, H.-Y. The ring-opening polymerization of ε-caprolactone and l-lactide using aluminum complexes bearing benzothiazole ligands as catalysts. Polym. Chem. 2016, 7, 4367–4377. [Google Scholar] [CrossRef]
  105. Malikmammadov, E.; Tanir, T.E.; Kiziltay, A.; Hasirci, V.; Hasirci, N. PCL and PCL-based materials in biomedical applications. J. Biomater. Sci. Polym. Ed. 2018, 29, 863–893. [Google Scholar] [CrossRef]
  106. Buyuksungur, S.; Tanir, T.E.; Buyuksungur, A.; Bektas, E.I.; Kose, G.T.; Yucel, D.; Beyzadeoglu, T.; Cetinkaya, E.; Yenigun, C.; Tönük, E.; et al. 3D printed poly(ε-caprolactone) scaffolds modified with hydroxyapatite and poly(propylene fumarate) and their effects on the healing of rabbit femur defects. Biomater. Sci. 2017, 5, 2144–2158. [Google Scholar] [CrossRef]
  107. Van Rie, J.; Declercq, H.; Van Hoorick, J.; Dierick, M.; Van Hoorebeke, L.; Cornelissen, R.; Thienpont, H.; Dubruel, P.; Van Vlierberghe, S. Cryogel-PCL combination scaffolds for bone tissue repair. J. Mater. Sci. Mater. Med. 2015, 26, 123. [Google Scholar] [CrossRef]
  108. Bahcecioglu, G.; Hasirci, N.; Bilgen, B.; Hasirci, V. A 3D printed PCL/hydrogel construct with zone-specific biochemical composition mimicking that of the meniscus. Biofabrication 2019, 11, 25002. [Google Scholar] [CrossRef] [PubMed]
  109. Wang, S.-J.; Zhang, Z.-Z.; Jiang, D.; Qi, Y.-S.; Wang, H.-J.; Zhang, J.-Y.; Ding, J.-X.; Yu, J.-K. Thermogel-Coated Poly(ε-Caprolactone) Composite Scaffold for Enhanced Cartilage Tissue Engineering. Polymers 2016, 8, 200. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  110. Srinivasa Reddy, C.; Reddy Venugopal, J.; Ramakrishna, S.; Zussman, E. Polycaprolactone/oligomer compound scaffolds for cardiac tissue engineering. J. Biomed. Mater. Res. Part A 2014, 102, 3713–3725. [Google Scholar] [CrossRef] [PubMed]
  111. Pok, S.; Myers, J.D.; Madihally, S.V.; Jacot, J.G. A multilayered scaffold of a chitosan and gelatin hydrogel supported by a PCL core for cardiac tissue engineering. Acta Biomater. 2013, 9, 5630–5642. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  112. Hernandez, I.; Kumar, A.; Joddar, B. A Bioactive Hydrogel and 3D Printed Polycaprolactone System for Bone Tissue Engineering. Gels 2017, 3, 26. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  113. Kundu, J.; Shim, J.-H.; Jang, J.; Kim, S.-W.; Cho, D.-W. An additive manufacturing-based PCL—Alginate—chondrocyte bioprinted scaffold for cartilage tissue engineering. J. Tissue Eng. Regen. Med. 2015, 9, 1286–1297. [Google Scholar] [CrossRef]
  114. Müller, M.; Becher, J.; Schnabelrauch, M.; Zenobi-Wong, M. Nanostructured Pluronic hydrogels as bioinks for 3D bioprinting. Biofabrication 2015, 7, 35006. [Google Scholar] [CrossRef]
  115. Liu, F.; Wang, X. Synthetic Polymers for Organ 3D Printing. Polymers 2020, 12, 1765. [Google Scholar] [CrossRef]
  116. Doğan, A.; Yalvaç, M.E.; Şahin, F.; Kabanov, A.V.; Palotás, A.; Rizvanov, A.A. Differentiation of human stem cells is promoted by amphiphilic pluronic block copolymers. Int. J. Nanomed. 2012, 7, 4849–4860. [Google Scholar] [CrossRef] [Green Version]
  117. Temofeew, N.A.; Hixon, K.R.; McBride-Gagyi, S.H.; Sell, S.A. The fabrication of cryogel scaffolds incorporated with poloxamer 407 for potential use in the regeneration of the nucleus pulposus. J. Mater. Sci. Mater. Med. 2017, 28, 36. [Google Scholar] [CrossRef]
  118. Russo, E.; Villa, C. Poloxamer Hydrogels for Biomedical Applications. Pharmaceutics 2019, 11, 671. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  119. Wu, B.; Takeshita, N.; Wu, Y.; Vijayavenkataraman, S.; Ho, K.Y.; Lu, W.F.; Fuh, J.Y.H. Pluronic F127 blended polycaprolactone scaffolds via e-jetting for esophageal tissue engineering. J. Mater. Sci. Mater. Med. 2018, 29, 140. [Google Scholar] [CrossRef] [PubMed]
  120. Lee, J.; Kim, G. Three-Dimensional Hierarchical Nanofibrous Collagen Scaffold Fabricated Using Fibrillated Collagen and Pluronic F-127 for Regenerating Bone Tissue. ACS Appl. Mater. Interfaces 2018, 10, 35801–35811. [Google Scholar] [CrossRef]
  121. Cheng, Q.; Liu, Y.; Lyu, J.; Lu, Q.; Zhang, X.; Song, W. 3D printing-directed auxetic Kevlar aerogel architectures with multiple functionalization options. J. Mater. Chem. A 2020, 8, 14243–14253. [Google Scholar] [CrossRef]
  122. Nita, L.E.; Ghilan, A.; Rusu, A.G.; Neamtu, I.; Chiriac, A.P. New Trends in Bio-Based Aerogels. Pharmaceutics 2020, 12, 449. [Google Scholar] [CrossRef]
  123. Baumann, T.F.; Worsley, M.A.; Han, T.Y.-J.; Satcher, J.H. High surface area carbon aerogel monoliths with hierarchical porosity. J. Non. Cryst. Solids 2008, 354, 3513–3515. [Google Scholar] [CrossRef]
  124. Kistler, S. Coherent Expanded Aerogels and Jellies. Nature 1931, 127, 741. [Google Scholar] [CrossRef]
  125. Wang, J.; Liu, D.; Li, Q.; Chen, C.; Chen, Z.; Song, P.; Hao, J.; Li, Y.; Fakhrhoseini, S.; Naebe, M.; et al. Lightweight, Superelastic Yet Thermoconductive Boron Nitride Nanocomposite Aerogel for Thermal Energy Regulation. ACS Nano 2019, 13, 7860–7870. [Google Scholar] [CrossRef] [PubMed]
  126. Yao, B.; Chandrasekaran, S.; Zhang, H.; Ma, A.; Kang, J.; Zhang, L.; Lu, X.; Qian, F.; Zhu, C.; Duoss, E.B.; et al. 3D-Printed Structure Boosts the Kinetics and Intrinsic Capacitance of Pseudocapacitive Graphene Aerogels. Adv. Mater. 2020, 32, 1906652. [Google Scholar] [CrossRef] [Green Version]
  127. Hu, P.; Lyu, J.; Fu, C.; Gong, W.; Liao, J.; Lu, W.; Chen, Y.; Zhang, X. Multifunctional Aramid Nanofiber/Carbon Nanotube Hybrid Aerogel Films. ACS Nano 2020, 14, 688–697. [Google Scholar] [CrossRef]
  128. Cai, B.; Eychmüller, A. Promoting Electrocatalysis upon Aerogels. Adv. Mater. 2019, 31, 1804881. [Google Scholar] [CrossRef] [Green Version]
  129. Baldino, L.; Cardea, S.; Reverchon, E. Natural Aerogels Production by Supercritical Gel Drying. Chem. Eng. Trans. 2015, 43, 739–744. [Google Scholar] [CrossRef]
  130. Zhang, Q.; Zhang, F.; Medarametla, S.P.; Li, H.; Zhou, C.; Lin, D. 3D Printing of Graphene Aerogels. Small 2016, 12, 1702–1708. [Google Scholar] [CrossRef] [PubMed]
  131. Tang, X.; Zhou, H.; Cai, Z.; Cheng, D.; He, P.; Xie, P.; Zhang, D.; Fan, T. Generalized 3D Printing of Graphene-Based Mixed-Dimensional Hybrid Aerogels. ACS Nano 2018, 12, 3502–3511. [Google Scholar] [CrossRef] [PubMed]
  132. Li, V.C.-F.; Dunn, C.K.; Zhang, Z.; Deng, Y.; Qi, H.J. Direct Ink Write (DIW) 3D Printed Cellulose Nanocrystal Aerogel Structures. Sci. Rep. 2017, 7, 8018. [Google Scholar] [CrossRef] [PubMed]
  133. Li, V.C.F.; Mulyadi, A.; Dunn, C.K.; Deng, Y.; Qi, H.J. Direct Ink Write 3D Printed Cellulose Nanofiber Aerogel Structures with Highly Deformable, Shape Recoverable, and Functionalizable Properties. ACS Sustain. Chem. Eng. 2018, 6, 2011–2022. [Google Scholar] [CrossRef]
  134. Saeed, S.; Al-Sobaihi, R.M.; Bertino, M.F.; White, L.S.; Saoud, K.M. Laser induced instantaneous gelation: Aerogels for 3D printing. J. Mater. Chem. A 2015, 3, 17606–17611. [Google Scholar] [CrossRef]
  135. Maleki, H.; Montes, S.; Hayati-Roodbari, N.; Putz, F.; Huesing, N. Compressible, Thermally Insulating, and Fire Retardant Aerogels through Self-Assembling Silk Fibroin Biopolymers Inside a Silica Structure—An Approach towards 3D Printing of Aerogels. ACS Appl. Mater. Interfaces 2018, 10, 22718–22730. [Google Scholar] [CrossRef]
  136. Cabane, E.; Zhang, X.; Langowska, K.; Palivan, C.G.; Meier, W. Stimuli-Responsive Polymers and Their Applications in Nanomedicine. Biointerphases 2012, 7, 9. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  137. Wang, X.; Sun, Y.; Peng, C.; Luo, H.; Wang, R.; Zhang, D. Transitional Suspensions Containing Thermosensitive Dispersant for Three-Dimensional Printing. ACS Appl. Mater. Interfaces 2015, 7, 26131–26136. [Google Scholar] [CrossRef]
  138. Stoychev, G.; Puretskiy, N.; Ionov, L. Self-folding all-polymer thermoresponsive microcapsules. Soft Matter 2011, 7, 3277–3279. [Google Scholar] [CrossRef]
  139. Ding, Z.; Yuan, C.; Peng, X.; Wang, T.; Qi, H.J.; Dunn, M.L. Direct 4D printing via active composite materials. Sci. Adv. 2017, 3, e1602890. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  140. Ayano, E.; Kanazawa, H. Temperature-responsive Smart Packing Materials Utilizing Multi-functional Polymers. Anal. Sci. 2014, 30, 167–173. [Google Scholar] [CrossRef] [Green Version]
  141. Li, L.; Shan, H.; Yue, C.Y.; Lam, Y.C.; Tam, K.C.; Hu, X. Thermally Induced Association and Dissociation of Methylcellulose in Aqueous Solutions. Langmuir 2002, 18, 7291–7298. [Google Scholar] [CrossRef]
  142. Yang, G.H.; Yeo, M.; Koo, Y.W.; Kim, G.H. 4D Bioprinting: Technological Advances in Biofabrication. Macromol. Biosci. 2019, 19, 1800441. [Google Scholar] [CrossRef] [PubMed]
  143. Aguilar, M.R.; San Román, J. (Eds.) Introduction to smart polymers and their applications. In Smart Polymers and Their Applications; Woodhead Publishing: Sawston, UK, 2014; pp. 1–11. ISBN 978-0-85709-695-1. [Google Scholar]
  144. Han, D.; Lu, Z.; Chester, S.A.; Lee, H. Micro 3D Printing of a Temperature-Responsive Hydrogel Using Projection Micro-Stereolithography. Sci. Rep. 2018, 8, 1963. [Google Scholar] [CrossRef]
  145. Moon, H.J.; Ko, D.Y.; Park, M.H.; Joo, M.K.; Jeong, B. Temperature-responsive compounds as in situ gelling biomedical materials. Chem. Soc. Rev. 2012, 41, 4860–4883. [Google Scholar] [CrossRef]
  146. Khutoryanskaya, O.V.; Mayeva, Z.A.; Mun, G.A.; Khutoryanskiy, V.V. Designing Temperature-Responsive Biocompatible Copolymers and Hydrogels Based on 2-Hydroxyethyl(meth)acrylates. Biomacromolecules 2008, 9, 3353–3361. [Google Scholar] [CrossRef]
  147. Pedron, S.; van Lierop, S.; Horstman, P.; Penterman, R.; Broer, D.J.; Peeters, E. Stimuli Responsive Delivery Vehicles for Cardiac Microtissue Transplantation. Adv. Funct. Mater. 2011, 21, 1624–1630. [Google Scholar] [CrossRef]
  148. Ionov, L. 4D Biofabrication: Materials, Methods, and Applications. Adv. Healthc. Mater. 2018, 7, 1800412. [Google Scholar] [CrossRef] [PubMed]
  149. Gao, C.; Ren, J.; Zhao, C.; Kong, W.; Dai, Q.; Chen, Q.; Liu, C.; Sun, R. Xylan-based temperature/pH sensitive hydrogels for drug controlled release. Carbohydr. Polym. 2016, 151, 189–197. [Google Scholar] [CrossRef]
  150. Dumitriu, R.P.; Oprea, A.-M.; Cheaburu, C.N.; Nistor, M.-T.; Novac, O.; Ghiciuc, C.M.; Profire, L.; Vasile, C. Biocompatible and biodegradable alginate/poly(N-isopropylacrylamide) hydrogels for sustained theophylline release. J. Appl. Polym. Sci. 2014, 131. [Google Scholar] [CrossRef]
  151. Bakarich, S.E.; Gorkin, R., III; Panhuis, M.I.H.; Spinks, G.M. 4D Printing with Mechanically Robust, Thermally Actuating Hydrogels. Macromol. Rapid Commun. 2015, 36, 1211–1217. [Google Scholar] [CrossRef] [PubMed]
  152. Charan, H.; Kinzel, J.; Glebe, U.; Anand, D.; Garakani, T.M.; Zhu, L.; Bocola, M.; Schwaneberg, U.; Böker, A. Grafting PNIPAAm from β-barrel shaped transmembrane nanopores. Biomaterials 2016, 107, 115–123. [Google Scholar] [CrossRef]
  153. Li, Z.; Guo, X.; Matsushita, S.; Guan, J. Differentiation of cardiosphere-derived cells into a mature cardiac lineage using biodegradable poly(N-isopropylacrylamide) hydrogels. Biomaterials 2011, 32, 3220–3232. [Google Scholar] [CrossRef]
  154. Patenaude, M.; Hoare, T. Injectable, Degradable Thermoresponsive Poly(N-isopropylacrylamide) Hydrogels. ACS Macro Lett. 2012, 1, 409–413. [Google Scholar] [CrossRef]
  155. Gan, J.; Guan, X.; Zheng, J.; Guo, H.; Wu, K.; Liang, L.; Lu, M. Biodegradable, thermoresponsive PNIPAM-based hydrogel scaffolds for the sustained release of levofloxacin. RSC Adv. 2016, 6, 32967–32978. [Google Scholar] [CrossRef]
  156. Galperin, A.; Long, T.J.; Garty, S.; Ratner, B.D. Synthesis and fabrication of a degradable poly(N-isopropyl acrylamide) scaffold for tissue engineering applications. J. Biomed. Mater. Res. Part A 2013, 101A, 775–786. [Google Scholar] [CrossRef] [Green Version]
  157. Yang, J.; van Lith, R.; Baler, K.; Hoshi, R.A.; Ameer, G.A. A Thermoresponsive Biodegradable Polymer with Intrinsic Antioxidant Properties. Biomacromolecules 2014, 15, 3942–3952. [Google Scholar] [CrossRef]
  158. Zhu, Y.; Hoshi, R.; Chen, S.; Yi, J.; Duan, C.; Galiano, R.D.; Zhang, H.F.; Ameer, G.A. Sustained release of stromal cell derived factor-1 from an antioxidant thermoresponsive hydrogel enhances dermal wound healing in diabetes. J. Control. Release 2016, 238, 114–122. [Google Scholar] [CrossRef]
  159. Fu, T.-S.; Wei, Y.-H.; Cheng, P.-Y.; Chu, I.-M.; Chen, W.-C. A Novel Biodegradable and Thermosensitive Poly(Ester-Amide) Hydrogel for Cartilage Tissue Engineering. Biomed Res. Int. 2018, 2018, 2710892. [Google Scholar] [CrossRef] [Green Version]
  160. Dutta, S.; Cohn, D. Temperature and pH responsive 3D printed scaffolds. J. Mater. Chem. B 2017, 5, 9514–9521. [Google Scholar] [CrossRef]
  161. Shim, M.S.; Lee, H.T.; Shim, W.S.; Park, I.; Lee, H.; Chang, T.; Kim, S.W.; Lee, D.S. Poly(D,L-lactic acid-co-glycolic acid)-b-poly(ethylene glycol)-b-poly (D,L-lactic acid-co-glycolic acid) triblock copolymer and thermoreversible phase transition in water. J. Biomed. Mater. Res. 2002, 61, 188–196. [Google Scholar] [CrossRef] [PubMed]
  162. Zentner, G.M.; Rathi, R.; Shih, C.; McRea, J.C.; Seo, M.-H.; Oh, H.; Rhee, B.G.; Mestecky, J.; Moldoveanu, Z.; Morgan, M.; et al. Biodegradable block copolymers for delivery of proteins and water-insoluble drugs. J. Control. Release 2001, 72, 203–215. [Google Scholar] [CrossRef]
  163. Yu, L.; Zhang, Z.; Zhang, H.; Ding, J. Biodegradability and Biocompatibility of Thermoreversible Hydrogels Formed from Mixing a Sol and a Precipitate of Block Copolymers in Water. Biomacromolecules 2010, 11, 2169–2178. [Google Scholar] [CrossRef] [PubMed]
  164. Wu, M.-H.; Shih, M.-H.; Hsu, W.-B.; Dubey, N.K.; Lee, W.-F.; Lin, T.-Y.; Hsieh, M.-Y.; Chen, C.-F.; Peng, K.-T.; Huang, T.-J.; et al. Evaluation of a novel biodegradable thermosensitive keto-hydrogel for improving postoperative pain in a rat model. PLoS ONE 2017, 12, e0186784. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  165. Cui, J.; Del Campo, A. Photo-responsive polymers: Properties, synthesis and applications. In Smart Polymers and Their Applications; Aguilar, M.R., San Román, J., Eds.; Woodhead Publishing: Sawston, UK, 2014; pp. 93–133. ISBN 978-0-85709-695-1. [Google Scholar]
  166. Kaehr, B.; Shear, J.B. Multiphoton fabrication of chemically responsive protein hydrogels for microactuation. Proc. Natl. Acad. Sci. USA 2008, 105, 8850–8854. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  167. Martínez-Carmona, M.; Lozano, D.; Baeza, A.; Colilla, M.; Vallet-Regí, M. A novel visible light responsive nanosystem for cancer treatment. Nanoscale 2017, 9, 15967–15973. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  168. Gao, S.; Tang, G.; Hua, D.; Xiong, R.; Han, J.; Jiang, S.; Zhang, Q.; Huang, C. Stimuli-responsive bio-based polymeric systems and their applications. J. Mater. Chem. B 2019, 7, 709–729. [Google Scholar] [CrossRef]
  169. Bagheri, A.; Arandiyan, H.; Boyer, C.; Lim, M. Lanthanide-Doped Upconversion Nanoparticles: Emerging Intelligent Light-Activated Drug Delivery Systems. Adv. Sci. 2016, 3, 1500437. [Google Scholar] [CrossRef] [Green Version]
  170. Tamay, D.G.; Usal, T.D.; Alagoz, A.S.; Yucel, D.; Hasirci, N.; Hasirci, V. 3D and 4D Printing of Polymers for Tissue Engineering Applications. Front. Bioeng. Biotechnol. 2019, 7, 164. [Google Scholar] [CrossRef] [PubMed]
  171. Ercole, F.; Davis, T.P.; Evans, R.A. Photo-responsive systems and biomaterials: Photochromic polymers{,} light-triggered self-assembly{,} surface modification{,} fluorescence modulation and beyond. Polym. Chem. 2010, 1, 37–54. [Google Scholar] [CrossRef]
  172. Liu, Y.; Ai, K.; Lu, L. Polydopamine and Its Derivative Materials: Synthesis and Promising Applications in Energy, Environmental, and Biomedical Fields. Chem. Rev. 2014, 114, 5057–5115. [Google Scholar] [CrossRef] [PubMed]
  173. Brown, T.E.; Anseth, K.S. Spatiotemporal hydrogel biomaterials for regenerative medicine. Chem. Soc. Rev. 2017, 46, 6532–6552. [Google Scholar] [CrossRef] [PubMed]
  174. Griffin, D.R.; Kasko, A.M. Photodegradable macromers and hydrogels for live cell encapsulation and release. J. Am. Chem. Soc. 2012, 134, 13103–13107. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  175. Kloxin, A.M.; Kasko, A.M.; Salinas, C.N.; Anseth, K.S. Photodegradable Hydrogels for Dynamic Tuning of Physical and Chemical Properties. Science 2009, 324, 59–63. [Google Scholar] [CrossRef] [Green Version]
  176. Basser, P.J.; Roth, B.J. New Currents in Electrical Stimulation of Excitable Tissues. Annu. Rev. Biomed. Eng. 2000, 2, 377–397. [Google Scholar] [CrossRef] [Green Version]
  177. Sun, S.; Titushkin, I.; Cho, M. Regulation of mesenchymal stem cell adhesion and orientation in 3D collagen scaffold by electrical stimulus. Bioelectrochemistry 2006, 69, 133–141. [Google Scholar] [CrossRef]
  178. Zhang, J.; Neoh, K.G.; Kang, E.-T. Electrical stimulation of adipose-derived mesenchymal stem cells and endothelial cells co-cultured in a conductive scaffold for potential orthopaedic applications. J. Tissue Eng. Regen. Med. 2018, 12, 878–889. [Google Scholar] [CrossRef]
  179. Sahle, F.F.; Gulfam, M.; Lowe, T.L. Design strategies for physical-stimuli-responsive programmable nanotherapeutics. Drug Discov. Today 2018, 23, 992–1006. [Google Scholar] [CrossRef]
  180. Servant, A.; Methven, L.; Williams, R.P.; Kostarelos, K. Electroresponsive Polymer–Carbon Nanotube Hydrogel Hybrids for Pulsatile Drug Delivery In Vivo. Adv. Healthc. Mater. 2013, 2, 806–811. [Google Scholar] [CrossRef]
  181. Ahadian, S.; Huyer, L.D.; Estili, M.; Yee, B.; Smith, N.; Xu, Z.; Sun, Y.; Radisic, M. Moldable elastomeric polyester-carbon nanotube scaffolds for cardiac tissue engineering. Acta Biomater. 2017, 52, 81–91. [Google Scholar] [CrossRef] [PubMed]
  182. Stříteský, S.; Marková, A.; Víteček, J.; Šafaříková, E.; Hrabal, M.; Kubáč, L.; Kubala, L.; Weiter, M.; Vala, M. Printing inks of electroactive polymer PEDOT:PSS: The study of biocompatibility, stability, and electrical properties. J. Biomed. Mater. Res. Part A 2018, 106, 1121–1128. [Google Scholar] [CrossRef] [PubMed]
  183. Humpolíček, P.; Radaszkiewicz, K.A.; Capáková, Z.; Pacherník, J.; Bober, P.; Kašpárková, V.; Rejmontová, P.; Lehocký, M.; Ponížil, P.; Stejskal, J. Polyaniline cryogels: Biocompatibility of novel conducting macroporous material. Sci. Rep. 2018, 8, 135. [Google Scholar] [CrossRef]
  184. Mao, J.; Zhang, Z. Polypyrrole as Electrically Conductive Biomaterials: Synthesis, Biofunctionalization, Potential Applications and Challenges. In Cutting-Edge Enabling Technologies for Regenerative Medicine. Advances in Experimental Medicine and Biology; Chun, H.J., Park, C.H., Kwon, I.K., Khang, G., Eds.; Springer: Singapore, 2018; pp. 347–370. ISBN 978-981-13-0950-2. [Google Scholar]
  185. Nezakati, T.; Seifalian, A.; Tan, A.; Seifalian, A.M. Conductive Polymers: Opportunities and Challenges in Biomedical Applications. Chem. Rev. 2018, 118, 6766–6843. [Google Scholar] [CrossRef]
  186. Green, R.A.; Baek, S.; Poole-Warren, L.A.; Martens, P.J. Conducting polymer-hydrogels for medical electrode applications. Sci. Technol. Adv. Mater. 2010, 11, 14107. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  187. Guo, B.; Glavas, L.; Albertsson, A.-C. Biodegradable and electrically conducting polymers for biomedical applications. Prog. Polym. Sci. 2013, 38, 1263–1286. [Google Scholar] [CrossRef]
  188. Chan, E.W.C.; Bennet, D.; Baek, P.; Barker, D.; Kim, S.; Travas-Sejdic, J. Electrospun Polythiophene Phenylenes for Tissue Engineering. Biomacromolecules 2018, 19, 1456–1468. [Google Scholar] [CrossRef] [PubMed]
  189. Ghasemi-Mobarakeh, L.; Prabhakaran, M.P.; Morshed, M.; Nasr-Esfahani, M.H.; Ramakrishna, S. Electrical Stimulation of Nerve Cells Using Conductive Nanofibrous Scaffolds for Nerve Tissue Engineering. Tissue Eng. Part A 2009, 15, 3605–3619. [Google Scholar] [CrossRef] [PubMed]
  190. Zhang, Z.; Rouabhia, M.; Wang, Z.; Roberge, C.; Shi, G.; Roche, P.; Li, J.; Dao, L.H. Electrically Conductive Biodegradable Polymer Composite for Nerve Regeneration: Electricity-Stimulated Neurite Outgrowth and Axon Regeneration. Artif. Organs 2007, 31, 13–22. [Google Scholar] [CrossRef]
  191. Shi, G.; Rouabhia, M.; Meng, S.; Zhang, Z. Electrical stimulation enhances viability of human cutaneous fibroblasts on conductive biodegradable substrates. J. Biomed. Mater. Res. Part A 2008, 84A, 1026–1037. [Google Scholar] [CrossRef]
  192. Jeong, S.I.; Jun, I.D.; Choi, M.J.; Nho, Y.C.; Lee, Y.M.; Shin, H. Development of Electroactive and Elastic Nanofibers that contain Polyaniline and Poly(L-lactide-co-ε-caprolactone) for the Control of Cell Adhesion. Macromol. Biosci. 2008, 8, 627–637. [Google Scholar] [CrossRef] [PubMed]
  193. Palza, H.; Zapata, P.A.; Angulo-Pineda, C. Electroactive Smart Polymers for Biomedical Applications. Materials 2019, 12, 277. [Google Scholar] [CrossRef] [Green Version]
  194. Li, L.; Ge, J.; Guo, B.; Ma, P.X. In situ forming biodegradable electroactive hydrogels. Polym. Chem. 2014, 5, 2880–2890. [Google Scholar] [CrossRef]
  195. Liu, Y.; Hu, J.; Zhuang, X.; Zhang, P.; Wei, Y.; Wang, X.; Chen, X. Synthesis and Characterization of Novel Biodegradable and Electroactive Hydrogel Based on Aniline Oligomer and Gelatin. Macromol. Biosci. 2012, 12, 241–250. [Google Scholar] [CrossRef] [PubMed]
  196. Noshadi, I.; Walker, B.W.; Portillo-Lara, R.; Shirzaei Sani, E.; Gomes, N.; Aziziyan, M.R.; Annabi, N. Engineering Biodegradable and Biocompatible Bio-ionic Liquid Conjugated Hydrogels with Tunable Conductivity and Mechanical Properties. Sci. Rep. 2017, 7, 4345. [Google Scholar] [CrossRef] [Green Version]
  197. Filipcsei, G.; Csetneki, I.; Szilágyi, A.; Zrínyi, M. Magnetic Field-Responsive Smart Polymer Composites. In Oligomers—Polymer Composites—Molecular Imprinting. Advances in Polymer Science; Gong, B., Sanford, A.R., Ferguson, J.S., Eds.; Springer: Berlin/Heidelberg, Germany, 2007; pp. 137–189. ISBN 978-3-540-46830-1. [Google Scholar]
  198. Li, Y.; Huang, G.; Zhang, X.; Li, B.; Chen, Y.; Lu, T.; Lu, T.J.; Xu, F. Magnetic Hydrogels and Their Potential Biomedical Applications. Adv. Funct. Mater. 2013, 23, 660–672. [Google Scholar] [CrossRef]
  199. Shi, Q.; Liu, H.; Tang, D.; Li, Y.; Li, X.; Xu, F. Bioactuators based on stimulus-responsive hydrogels and their emerging biomedical applications. NPG Asia Mater. 2019, 11, 64. [Google Scholar] [CrossRef] [Green Version]
  200. Xu, F.; Wu, C.M.; Rengarajan, V.; Finley, T.D.; Keles, H.O.; Sung, Y.; Li, B.; Gurkan, U.A.; Demirci, U. Three-Dimensional Magnetic Assembly of Microscale Hydrogels. Adv. Mater. 2011, 23, 4254–4260. [Google Scholar] [CrossRef] [Green Version]
  201. Kim, J.A.; Choi, J.-H.; Kim, M.; Rhee, W.J.; Son, B.; Jung, H.-K.; Park, T.H. High-throughput generation of spheroids using magnetic nanoparticles for three-dimensional cell culture. Biomaterials 2013, 34, 8555–8563. [Google Scholar] [CrossRef]
  202. Okochi, M.; Takano, S.; Isaji, Y.; Senga, T.; Hamaguchi, M.; Honda, H. Three-dimensional cell culture array using magnetic force-based cell patterning for analysis of invasive capacity of BALB/3T3/v-src. Lab Chip 2009, 9, 3378–3384. [Google Scholar] [CrossRef] [PubMed]
  203. Zhang, Q.; Liu, J.; Yuan, K.; Zhang, Z.; Zhang, X.; Fang, X. A multi-controlled drug delivery system based on magnetic mesoporous Fe3O4nanopaticles and a phase change material for cancer thermo-chemotherapy. Nanotechnology 2017, 28, 405101. [Google Scholar] [CrossRef] [PubMed]
  204. Zhao, X.; Kim, J.; Cezar, C.A.; Huebsch, N.; Lee, K.; Bouhadir, K.; Mooney, D.J. Active scaffolds for on-demand drug and cell delivery. Proc. Natl. Acad. Sci. USA 2011, 108, 67–72. [Google Scholar] [CrossRef] [Green Version]
  205. Ramanujan, R.V.; Lao, L.L. The mechanical behavior of smart magnet–hydrogel composites. Smart Mater. Struct. 2006, 15, 952–956. [Google Scholar] [CrossRef] [Green Version]
  206. Tasoglu, S.; Kavaz, D.; Gurkan, U.A.; Guven, S.; Chen, P.; Zheng, R.; Demirci, U. Paramagnetic Levitational Assembly of Hydrogels. Adv. Mater. 2013, 25, 1137–1143. [Google Scholar] [CrossRef] [Green Version]
  207. Kokkinis, D.; Schaffner, M.; Studart, A.R. Multimaterial magnetically assisted 3D printing of composite materials. Nat. Commun. 2015, 6, 8643. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  208. Tasoglu, S.; Yu, C.H.; Gungordu, H.I.; Guven, S.; Vural, T.; Demirci, U. Guided and magnetic self-assembly of tunable magnetoceptive gels. Nat. Commun. 2014, 5, 4702. [Google Scholar] [CrossRef] [Green Version]
  209. De Haan, L.T.; Verjans, J.M.N.; Broer, D.J.; Bastiaansen, C.W.M.; Schenning, A.P.H.J. Humidity-Responsive Liquid Crystalline Polymer Actuators with an Asymmetry in the Molecular Trigger That Bend, Fold, and Curl. J. Am. Chem. Soc. 2014, 136, 10585–10588. [Google Scholar] [CrossRef]
  210. Mulakkal, M.C.; Trask, R.S.; Ting, V.P.; Seddon, A.M. Responsive cellulose-hydrogel composite ink for 4D printing. Mater. Des. 2018, 160, 108–118. [Google Scholar] [CrossRef]
  211. Zhang, K.; Geissler, A.; Standhardt, M.; Mehlhase, S.; Gallei, M.; Chen, L.; Thiele, C.M. Moisture-responsive films of cellulose stearoyl esters showing reversible shape transitions. Sci. Rep. 2015, 5, 11011. [Google Scholar] [CrossRef] [PubMed]
  212. Jung, Y.C.; Hwa So, H.; Whan Cho, J. Water-Responsive Shape Memory Polyurethane Block Copolymer Modified with Polyhedral Oligomeric Silsesquioxane. J. Macromol. Sci. Part B 2006, 45, 453–461. [Google Scholar] [CrossRef]
  213. Lv, C.; Sun, X.-C.; Xia, H.; Yu, Y.-H.; Wang, G.; Cao, X.-W.; Li, S.-X.; Wang, Y.-S.; Chen, Q.-D.; Yu, Y.-D.; et al. Humidity-responsive actuation of programmable hydrogel microstructures based on 3D printing. Sens. Actuators B Chem. 2018, 259, 736–744. [Google Scholar] [CrossRef]
  214. Zhang, L.; Liang, H.; Jacob, J.; Naumov, P. Photogated humidity-driven motility. Nat. Commun. 2015, 6, 7429. [Google Scholar] [CrossRef] [PubMed]
  215. Kocak, G.; Tuncer, C.; Bütün, V. pH-Responsive polymers. Polym. Chem. 2017, 8, 144–176. [Google Scholar] [CrossRef]
  216. Reyes-Ortega, F. pH-responsive polymers: Properties, synthesis and applications. In Smart Polymers and Their Applications; Aguilar, M.R., San Román, J., Eds.; Woodhead Publishing: Sawston, UK, 2014; pp. 45–92. ISBN 978-0-85709-695-1. [Google Scholar]
  217. Santos, J.R.; Alves, N.M.; Mano, J.F. New Thermo-responsive Hydrogels Based on Poly(N-isopropylacrylamide)/Hyaluronic Acid Semi-interpenetrated Polymer Networks: Swelling Properties and Drug Release Studies. J. Bioact. Compat. Polym. 2010, 25, 169–184. [Google Scholar] [CrossRef]
  218. Basu, A.; Kunduru, K.R.; Abtew, E.; Domb, A.J. Polysaccharide-Based Conjugates for Biomedical Applications. Bioconjug. Chem. 2015, 26, 1396–1412. [Google Scholar] [CrossRef] [PubMed]
  219. Zhou, T.; Xiao, C.; Fan, J.; Chen, S.; Shen, J.; Wu, W.; Zhou, S. A nanogel of on-site tunable pH-response for efficient anticancer drug delivery. Acta Biomater. 2013, 9, 4546–4557. [Google Scholar] [CrossRef]
  220. Miyazaki, M.; Yuba, E.; Hayashi, H.; Harada, A.; Kono, K. Development of pH-Responsive Hyaluronic Acid-Based Antigen Carriers for Induction of Antigen-Specific Cellular Immune Responses. ACS Biomater. Sci. Eng. 2019, 5, 5790–5797. [Google Scholar] [CrossRef] [PubMed]
  221. Samal, S.K.; Dash, M.; Van Vlierberghe, S.; Kaplan, D.L.; Chiellini, E.; van Blitterswijk, C.; Moroni, L.; Dubruel, P. Cationic polymers and their therapeutic potential. Chem. Soc. Rev. 2012, 41, 7147–7194. [Google Scholar] [CrossRef] [PubMed]
  222. Lin, Y.-H.; Liang, H.-F.; Chung, C.-K.; Chen, M.-C.; Sung, H.-W. Physically crosslinked alginate/N,O-carboxymethyl chitosan hydrogels with calcium for oral delivery of protein drugs. Biomaterials 2005, 26, 2105–2113. [Google Scholar] [CrossRef] [PubMed]
  223. Ju, H.K.; Kim, S.Y.; Kim, S.J.; Lee, Y.M. pH/temperature-responsive semi-IPN hydrogels composed of alginate and poly(N-isopropylacrylamide). J. Appl. Polym. Sci. 2002, 83, 1128–1139. [Google Scholar] [CrossRef]
  224. Zhang, R.; Tang, M.; Bowyer, A.; Eisenthal, R.; Hubble, J. A novel pH- and ionic-strength-sensitive carboxy methyl dextran hydrogel. Biomaterials 2005, 26, 4677–4683. [Google Scholar] [CrossRef] [PubMed]
  225. Rodríguez-Hernández, J.; Lecommandoux, S. Reversible Inside−Out Micellization of pH-responsive and Water-Soluble Vesicles Based on Polypeptide Diblock Copolymers. J. Am. Chem. Soc. 2005, 127, 2026–2027. [Google Scholar] [CrossRef]
  226. Jia, N.; Ye, Y.; Wang, Q.; Zhao, X.; Hu, H.; Chen, D.; Qiao, M. Preparation and evaluation of poly(l-histidine) based pH-sensitive micelles for intracellular delivery of doxorubicin against MCF-7/ADR cells. Asian J. Pharm. Sci. 2017, 12, 433–441. [Google Scholar] [CrossRef] [PubMed]
  227. Chang, C.J.; Swift, G. POLY(ASPARTIC ACID) HYDROGEL. J. Macromol. Sci. Part A 1999, 36, 963–970. [Google Scholar] [CrossRef]
  228. Zhao, Y.; Su, H.; Fang, L.; Tan, T. Superabsorbent hydrogels from poly(aspartic acid) with salt-, temperature- and pH-responsiveness properties. Polymer 2005, 46, 5368–5376. [Google Scholar] [CrossRef]
  229. Hu, Q.; Katti, P.S.; Gu, Z. Enzyme-responsive nanomaterials for controlled drug delivery. Nanoscale 2014, 6, 12273–12286. [Google Scholar] [CrossRef]
  230. Wang, X.; Qin, X.-H.; Hu, C.; Terzopoulou, A.; Chen, X.-Z.; Huang, T.-Y.; Maniura-Weber, K.; Pané, S.; Nelson, B.J. 3D Printed Enzymatically Biodegradable Soft Helical Microswimmers. Adv. Funct. Mater. 2018, 28, 1804107. [Google Scholar] [CrossRef]
  231. Rodríguez-Cabello, J.; Reguera, J.; Prieto, S.; Alonso, M. Protein-Based Smart Polymers. In Smart Polymers: Applications in Biotechnology and Biomedicine; Galaev, I., Mattiasson, B., Eds.; CRC Press: Boca Raton, FL, USA, 2007; pp. 177–209. ISBN 978-0-8493-9161-3. [Google Scholar]
  232. Hassouneh, W.; MacEwan, S.R.; Chilkoti, A. Fusions of Elastin-Like Polypeptides to Pharmaceutical Proteins. In Protein Engineering for Therapeutics, Part A; Wittrup, K.D., Verdine, G.L., Eds.; Academic Press: Cambridge, MA, USA, 2012; Volume 502, pp. 215–237. ISBN 0076-6879. [Google Scholar]
  233. Yeboah, A.; Cohen, R.I.; Rabolli, C.; Yarmush, M.L.; Berthiaume, F. Elastin-like polypeptides: A strategic fusion partner for biologics. Biotechnol. Bioeng. 2016, 113, 1617–1627. [Google Scholar] [CrossRef]
  234. Kowalczyk, T.; Hnatuszko-Konka, K.; Gerszberg, A.; Kononowicz, A.K. Elastin-like polypeptides as a promising family of genetically-engineered protein based polymers. World J. Microbiol. Biotechnol. 2014, 30, 2141–2152. [Google Scholar] [CrossRef] [Green Version]
  235. Dos Santos, B.P.; Garbay, B.; Pasqua, M.; Chevron, E.; Chinoy, Z.S.; Cullin, C.; Bathany, K.; Lecommandoux, S.; Amédée, J.; Oliveira, H.; et al. Production, purification and characterization of an elastin-like polypeptide containing the Ile-Lys-Val-Ala-Val (IKVAV) peptide for tissue engineering applications. J. Biotechnol. 2019, 298, 35–44. [Google Scholar] [CrossRef]
  236. Chen, Z.; Ding, Z.; Zhang, G.; Tian, L.; Zhang, X. Construction of Thermo-Responsive Elastin-Like Polypeptides (ELPs)-Aggregation-Induced-Emission (AIE) Conjugates for Temperature Sensing. Molecules 2018, 23, 1725. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  237. Araújo, A.; Olsen, B.D.; Machado, A.V. Engineering Elastin-Like Polypeptide-Poly(ethylene glycol) Multiblock Physical Networks. Biomacromolecules 2018, 19, 329–339. [Google Scholar] [CrossRef] [PubMed]
  238. Rodríguez-Cabello, J.C.; de Torre, I.G.; Ibañez-Fonseca, A.; Alonso, M. Bioactive scaffolds based on elastin-like materials for wound healing. Adv. Drug Deliv. Rev. 2018, 129, 118–133. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  239. Meyer, D.E.; Chilkoti, A. Genetically Encoded Synthesis of Protein-Based Polymers with Precisely Specified Molecular Weight and Sequence by Recursive Directional Ligation:  Examples from the Elastin-like Polypeptide System. Biomacromolecules 2002, 3, 357–367. [Google Scholar] [CrossRef]
  240. Kuribayashi-Shigetomi, K.; Onoe, H.; Takeuchi, S. Cell Origami: Self-Folding of Three-Dimensional Cell-Laden Microstructures Driven by Cell Traction Force. PLoS ONE 2012, 7, e51085. [Google Scholar] [CrossRef]
  241. Tan, J.L.; Tien, J.; Pirone, D.M.; Gray, D.S.; Bhadriraju, K.; Chen, C.S. Cells lying on a bed of microneedles: An approach to isolate mechanical force. Proc. Natl. Acad. Sci. USA 2003, 100, 1484–1489. [Google Scholar] [CrossRef] [Green Version]
  242. Carosi, J.A.; Eskin, S.G.; McIntire, L.V. Cyclical strain effects on production of vasoactive materials in cultured endothelial cells. J. Cell. Physiol. 1992, 151, 29–36. [Google Scholar] [CrossRef]
  243. Chien, S.; Li, S.; Shyy, J.Y.-J. Effects of Mechanical Forces on Signal Transduction and Gene Expression in Endothelial Cells. Hypertension 1998, 31, 162–169. [Google Scholar] [CrossRef]
  244. Ives, C.L.; Eskin, S.G.; McIntire, L.V. Mechanical effects on endothelial cell morphology: In vitro assessment. Vitr. Cell. Dev. Biol. 1986, 22, 500–507. [Google Scholar] [CrossRef] [PubMed]
  245. Vernon, R.B.; Sage, E.H. A Novel, Quantitative Model for Study of Endothelial Cell Migration and Sprout Formation within Three-Dimensional Collagen Matrices. Microvasc. Res. 1999, 57, 118–133. [Google Scholar] [CrossRef] [PubMed]
  246. Krishnan, L.; Hoying, J.B.; Nguyen, H.; Song, H.; Weiss, J.A. Interaction of angiogenic microvessels with the extracellular matrix. Am. J. Physiol. Heart Circ. Physiol. 2007, 293, H3650–H3658. [Google Scholar] [CrossRef] [Green Version]
  247. Underwood, C.J.; Edgar, L.T.; Hoying, J.B.; Weiss, J.A. Cell-generated traction forces and the resulting matrix deformation modulate microvascular alignment and growth during angiogenesis. Am. J. Physiol. Circ. Physiol. 2014, 307, H152–H164. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  248. Li, M.; Yang, Q.; Liu, H.; Qiu, M.; Lu, T.J.; Xu, F. Capillary Origami Inspired Fabrication of Complex 3D Hydrogel Constructs. Small 2016, 12, 4492–4500. [Google Scholar] [CrossRef] [PubMed]
  249. Antkowiak, A.; Audoly, B.; Josserand, C.; Neukirch, S.; Rivetti, M. Instant fabrication and selection of folded structures using drop impact. Proc. Natl. Acad. Sci. USA 2011, 108, 10400–10404. [Google Scholar] [CrossRef] [Green Version]
  250. Van Honschoten, J.W.; Berenschot, J.W.; Ondarçuhu, T.; Sanders, R.G.P.; Sundaram, J.; Elwenspoek, M.; Tas, N.R. Elastocapillary fabrication of three-dimensional microstructures. Appl. Phys. Lett. 2010, 97, 14103. [Google Scholar] [CrossRef]
  251. Py, C.; Reverdy, P.; Doppler, L.; Bico, J.; Roman, B.; Baroud, C. Capillary origami. Phys. Fluids 2007, 19, 91104. [Google Scholar] [CrossRef] [Green Version]
  252. Gómez-Mascaraque, L.G.; Palao-Suay, R.; Vázquez, B. The use of smart polymers in medical devices for minimally invasive surgery, diagnosis and other applications. In Smart Polymers and Their Applications; Aguilar, M.R., San Román, J., Eds.; Woodhead Publishing: Sawston, UK, 2014; pp. 359–407. ISBN 978-0-85709-695-1. [Google Scholar]
  253. Gao, B.; Yang, Q.; Zhao, X.; Jin, G.; Ma, Y.; Xu, F. 4D Bioprinting for Biomedical Applications. Trends Biotechnol. 2016, 34, 746–756. [Google Scholar] [CrossRef]
  254. Wan, Z.; Zhang, P.; Liu, Y.; Lv, L.; Zhou, Y. Four-dimensional bioprinting: Current developments and applications in bone tissue engineering. Acta Biomater. 2020, 101, 26–42. [Google Scholar] [CrossRef] [PubMed]
  255. Dong, S.-L.; Han, L.; Du, C.-X.; Wang, X.-Y.; Li, L.-H.; Wei, Y. 3D Printing of Aniline Tetramer-Grafted-Polyethylenimine and Pluronic F127 Composites for Electroactive Scaffolds. Macromol. Rapid Commun. 2017, 38, 1600551. [Google Scholar] [CrossRef] [PubMed]
  256. Wei, H.; Zhang, Q.; Yao, Y.; Liu, L.; Liu, Y.; Leng, J. Direct-Write Fabrication of 4D Active Shape-Changing Structures Based on a Shape Memory Polymer and Its Nanocomposite. ACS Appl. Mater. Interfaces 2017, 9, 876–883. [Google Scholar] [CrossRef] [PubMed]
  257. Zhang, J.; Zhao, S.; Zhu, M.; Zhu, Y.; Zhang, Y.; Liu, Z.; Zhang, C. 3D-printed magnetic Fe3O4/MBG/PCL composite scaffolds with multifunctionality of bone regeneration{,} local anticancer drug delivery and hyperthermia. J. Mater. Chem. B 2014, 2, 7583–7595. [Google Scholar] [CrossRef]
  258. D’Amora, U.; Russo, T.; Gloria, A.; Rivieccio, V.; D’Antò, V.; Negri, G.; Ambrosio, L.; De Santis, R. 3D additive-manufactured nanocomposite magnetic scaffolds: Effect of the application mode of a time-dependent magnetic field on hMSCs behavior. Bioact. Mater. 2017, 2, 138–145. [Google Scholar] [CrossRef]
  259. Betsch, M.; Cristian, C.; Lin, Y.-Y.; Blaeser, A.; Schöneberg, J.; Vogt, M.; Buhl, E.M.; Fischer, H.; Duarte Campos, D.F. Incorporating 4D into Bioprinting: Real-Time Magnetically Directed Collagen Fiber Alignment for Generating Complex Multilayered Tissues. Adv. Healthc. Mater. 2018, 7, 1800894. [Google Scholar] [CrossRef]
  260. Luo, Y.; Lin, X.; Chen, B.; Wei, X. Cell-laden four-dimensional bioprinting using near-infrared-triggered shape-morphing alginate/polydopamine bioinks. Biofabrication 2019, 11, 45019. [Google Scholar] [CrossRef]
  261. Seo, J.W.; Shin, S.R.; Park, Y.J.; Bae, H. Hydrogel Production Platform with Dynamic Movement Using Photo-Crosslinkable/Temperature Reversible Chitosan Polymer and Stereolithography 4D Printing Technology. Tissue Eng. Regen. Med. 2020, 17, 423–431. [Google Scholar] [CrossRef]
  262. Skardal, A.; Zhang, J.; McCoard, L.; Xu, X.; Oottamasathien, S.; Prestwich, G.D. Photocrosslinkable Hyaluronan-Gelatin Hydrogels for Two-Step Bioprinting. Tissue Eng. Part A 2010, 16, 2675–2685. [Google Scholar] [CrossRef] [Green Version]
  263. Lee, W.; Lee, V.; Polio, S.; Keegan, P.; Lee, J.-H.; Fischer, K.; Park, J.-K.; Yoo, S.-S. On-demand three-dimensional freeform fabrication of multi-layered hydrogel scaffold with fluidic channels. Biotechnol. Bioeng. 2010, 105, 1178–1186. [Google Scholar] [CrossRef]
  264. Kolesky, D.B.; Truby, R.L.; Gladman, A.S.; Busbee, T.A.; Homan, K.A.; Lewis, J.A. 3D Bioprinting of Vascularized, Heterogeneous Cell-Laden Tissue Constructs. Adv. Mater. 2014, 26, 3124–3130. [Google Scholar] [CrossRef]
  265. Miao, S.; Zhu, W.; Castro, N.J.; Nowicki, M.; Zhou, X.; Cui, H.; Fisher, J.P.; Zhang, L.G. 4D printing smart biomedical scaffolds with novel soybean oil epoxidized acrylate. Sci. Rep. 2016, 6, 27226. [Google Scholar] [CrossRef] [Green Version]
  266. Miao, S.; Zhu, W.; Castro, N.J.; Leng, J.; Zhang, L.G. Four-Dimensional Printing Hierarchy Scaffolds with Highly Biocompatible Smart Polymers for Tissue Engineering Applications. Tissue Eng. Part C Methods 2016, 22, 952–963. [Google Scholar] [CrossRef] [Green Version]
  267. Zarek, M.; Mansour, N.; Shapira, S.; Cohn, D. 4D Printing of Shape Memory-Based Personalized Endoluminal Medical Devices. Macromol. Rapid Commun. 2017, 38, 1600628. [Google Scholar] [CrossRef]
  268. Hendrikson, W.J.; Rouwkema, J.; Clementi, F.; van Blitterswijk, C.A.; Farè, S.; Moroni, L. Towards 4D printed scaffolds for tissue engineering: Exploiting 3D shape memory polymers to deliver time-controlled stimulus on cultured cells. Biofabrication 2017, 9, 31001. [Google Scholar] [CrossRef]
  269. Kim, S.H.; Seo, Y.B.; Yeon, Y.K.; Lee, Y.J.; Park, H.S.; Sultan, M.T.; Lee, J.M.; Lee, J.S.; Lee, O.J.; Hong, H.; et al. 4D-bioprinted silk hydrogels for tissue engineering. Biomaterials 2020, 260, 120281. [Google Scholar] [CrossRef] [PubMed]
  270. Cui, C.; Kim, D.-O.; Pack, M.Y.; Han, B.; Han, L.; Sun, Y.; Han, L.-H. 4D printing of self-folding and cell-encapsulating 3D microstructures as scaffolds for tissue-engineering applications. Biofabrication 2020, 12, 45018. [Google Scholar] [CrossRef] [PubMed]
  271. Song, K.H.; Highley, C.B.; Rouff, A.; Burdick, J.A. Complex 3D-Printed Microchannels within Cell-Degradable Hydrogels. Adv. Funct. Mater. 2018, 28, 1801331. [Google Scholar] [CrossRef]
  272. Devillard, C.D.; Mandon, C.A.; Lambert, S.A.; Blum, L.J.; Marquette, C.A. Bioinspired Multi-Activities 4D Printing Objects: A New Approach Toward Complex Tissue Engineering. Biotechnol. J. 2018, 13, 1800098. [Google Scholar] [CrossRef] [PubMed]
  273. Miao, S.; Cui, H.; Nowicki, M.; Lee, S.; Almeida, J.; Zhou, X.; Zhu, W.; Yao, X.; Masood, F.; Plesniak, M.W.; et al. Photolithographic-stereolithographic-tandem fabrication of 4D smart scaffolds for improved stem cell cardiomyogenic differentiation. Biofabrication 2018, 10, 35007. [Google Scholar] [CrossRef]
  274. Raman, R.; Cvetkovic, C.; Uzel, S.G.M.; Platt, R.J.; Sengupta, P.; Kamm, R.D.; Bashir, R. Optogenetic skeletal muscle-powered adaptive biological machines. Proc. Natl. Acad. Sci. USA 2016, 113, 3497–3502. [Google Scholar] [CrossRef] [Green Version]
  275. Choi, J.; Kwon, O.-C.; Jo, W.; Lee, H.J.; Moon, M.-W. 4D Printing Technology: A Review. 3D Print. Addit. Manuf. 2015, 2, 159–167. [Google Scholar] [CrossRef]
  276. Ashammakhi, N.; Ahadian, S.; Zengjie, F.; Suthiwanich, K.; Lorestani, F.; Orive, G.; Ostrovidov, S.; Khademhosseini, A. Advances and Future Perspectives in 4D Bioprinting. Biotechnol. J. 2018, 13, 1800148. [Google Scholar] [CrossRef]
  277. Malda, J.; Klein, T.J.; Upton, Z. The Roles of Hypoxia in the In Vitro Engineering of Tissues. Tissue Eng. 2007, 13, 2153–2162. [Google Scholar] [CrossRef] [PubMed]
  278. Jain, R.K.; Au, P.; Tam, J.; Duda, D.G.; Fukumura, D. Engineering vascularized tissue. Nat. Biotechnol. 2005, 23, 821–823. [Google Scholar] [CrossRef]
  279. Yang, G.; Mahadik, B.; Choi, J.Y.; Fisher, J.P. Vascularization in tissue engineering: Fundamentals and state-of-art. Prog. Biomed. Eng. 2020, 2, 12002. [Google Scholar] [CrossRef]
  280. Huang, G.Y.; Zhou, L.H.; Zhang, Q.C.; Chen, Y.M.; Sun, W.; Xu, F.; Lu, T.J. Microfluidic hydrogels for tissue engineering. Biofabrication 2011, 3, 12001. [Google Scholar] [CrossRef] [PubMed]
  281. Sapir-Lekhovitser, Y.; Rotenberg, M.Y.; Jopp, J.; Friedman, G.; Polyak, B.; Cohen, S. Magnetically actuated tissue engineered scaffold: Insights into mechanism of physical stimulation. Nanoscale 2016, 8, 3386–3399. [Google Scholar] [CrossRef] [PubMed]
  282. Fuhrer, R.; Hofmann, S.; Hild, N.; Vetsch, J.R.; Herrmann, I.K.; Grass, R.N.; Stark, W.J. Pressureless Mechanical Induction of Stem Cell Differentiation Is Dose and Frequency Dependent. PLoS ONE 2013, 8, e81362. [Google Scholar] [CrossRef]
  283. Souza, G.R.; Molina, J.R.; Raphael, R.M.; Ozawa, M.G.; Stark, D.J.; Levin, C.S.; Bronk, L.F.; Ananta, J.S.; Mandelin, J.; Georgescu, M.-M.; et al. Three-dimensional tissue culture based on magnetic cell levitation. Nat. Nanotechnol. 2010, 5, 291–296. [Google Scholar] [CrossRef] [Green Version]
  284. Adedoyin, A.A.; Ekenseair, A.K. Biomedical applications of magneto-responsive scaffolds. Nano Res. 2018, 11, 5049–5064. [Google Scholar] [CrossRef]
  285. Lui, Y.S.; Sow, W.T.; Tan, L.P.; Wu, Y.; Lai, Y.; Li, H. 4D printing and stimuli-responsive materials in biomedical aspects. Acta Biomater. 2019, 92, 19–36. [Google Scholar] [CrossRef] [PubMed]
  286. Yang, Q.; Gao, B.; Xu, F. Recent Advances in 4D Bioprinting. Biotechnol. J. 2020, 15, 1900086. [Google Scholar] [CrossRef]
Figure 1. (A) Schematic of the different 3D or 4D printing technologies using conventional or smart materials for cell-free or cell-laden (Bioprinting) scaffolds production. Smart materials scaffolds can change their size, shape, and/or functionality in response to one or more stimulus; (B) types of environment stimuli and responses observed in dynamic smart materials.
Figure 1. (A) Schematic of the different 3D or 4D printing technologies using conventional or smart materials for cell-free or cell-laden (Bioprinting) scaffolds production. Smart materials scaffolds can change their size, shape, and/or functionality in response to one or more stimulus; (B) types of environment stimuli and responses observed in dynamic smart materials.
Polymers 13 00563 g001
Table 1. Examples of 4D-printed resorbable materials in tissue engineering.
Table 1. Examples of 4D-printed resorbable materials in tissue engineering.
StimulusMaterial CompositionFabrication MethodCellsTissue Engineering ApplicationReference
PhysicalElectric fieldPluronic F127/AT-PEIMicroextrusionNo cells were testedMuscle and cardiac and nerve tissue [255]
Magnetic fieldFe3O4/BP/PLAInkjetNo cells were testedCardiovascular implant[256]
Magnetic fieldFe3O4/MBG/PCLMicroextrusionhBMSCs
Bone regeneration[257]
Magnetic fieldPCL/FeHA 80/20MicroextrusionhMSCs
(seeded after printing, before stimulus)
Bone regeneration[258]
Magnetic fieldCell-laden
Cartilage regeneration[259]
NIR light (808 nm) Cell-laden alginate/GelMA: alginate/PDAMicroextrusion293T
Vascularized scaffolds[260]
TemperatureHBC-MASLANo cells were testedVascularized scaffolds[261]
Vascularized scaffolds[48]
TemperatureCell-laden HA-MA:GE-MAMicroextrusionHepG2/C3A
Vascularized scaffolds[262]
(seeded after stimulus)
Vascularized scaffolds[263]
GelMa/Pluronic F127
Vascularized scaffolds[264]
Biomedical scaffolds[265]
TemperatureCastor oil-based polymersMicroextrusionhMSCs
Biomedical scaffolds[266]
SLSRCm and H9C2(2–1)
(seeded after printing, before stimulus)
Heart Failure treatment[147]
TemperatureMethacrylated PCLSLANo cells were testedTracheal stent[267]
TemperaturePU/collagen type IInkjethMSCs
(seeded after printing, before the stimulus)
Biomedical scaffolds[268]
PhysicochemicalOsmolarityCell-laden Sil-MASLSTBSCs and Chondrocytes
Trachea tissue[269]
(seeded after printing, before the stimulus)
Biomedical Scaffolds[270]
Growth Factors
Ad-HA or CD-HAInkjetHUVECs
(seeded after printing, before stimulus)
Vascularized tissues[271]
EnzymaticPEG/thrombin/alkaline phosphataseSLS NIH-3T3
(seeded within stimulus)
Biomedical Scaffolds[272]
MultipleUV light and temperatureSOEASLS-SLA-tandem hMSCs
(after stimulus)
Cardiac Regeneration[273]
Light (470 nm)
and electrical field
PEGDA700 + Irgacure 2959 photoinitiatorSLAC2C12
(seeded after printing, before stimuli)
Engineering Biological machines (bio-bots)[274]
Ad-HA: Hyaluronic acid macromer; AT-PEI: Aniline tetramer-grafted-polyethylenimine; BP: Benzophenone; CD-HA: Hyaluronic acid macromer; FeHA: Iron with hydroxyapatite; GelMA: Gelatin methacryloyl; Gel-COOH-MA: Gelatin methacryloyl with amine groups converted into carboxyl groups; HA-MA:GE-MA: Methacrylated hyaluronic acid with gelatin ethanolamide methacrylate; MBG: Mesoporous bioactive glass; NIPAAm: N-isopropyacrylamide; PCL: Polycaprolactone; PDA: Polydopamine; PEG: Polyethylene glycol; PEGDA700: Poly(ethylene glycol) diacrylate 700; PLA: Polylactic acid; PU: Polyurethane; Sil-MA: Synthesized from silk fibroin (SF) and glycidyl methacrylate solution (GMA); SOEA: Soybean oil epoxidized acrylate. 293T: human cell line, derived from the HEK 293; C2C12: immortalized mouse myoblast cell line; C3H/10T1/2: Mouse embryo cell line; H9C2(2–1): Rat BDIX heart myoblast; hBMSCs: Human bone marrow stem cells; hKAC: Human knee articular cells; HepG2/C3A: Human liver cancer cell line; hMSCs: Human mesenchymal stem cells; HUVECs: Human umbilical vein endothelial cells; NIH-3T3: NIH Swiss mouse embryo cells; RCm: Rat Cardiomyocytes; TBSCs: Trophoblast stem cells.
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Saska, S.; Pilatti, L.; Blay, A.; Shibli, J.A. Bioresorbable Polymers: Advanced Materials and 4D Printing for Tissue Engineering. Polymers 2021, 13, 563.

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Saska S, Pilatti L, Blay A, Shibli JA. Bioresorbable Polymers: Advanced Materials and 4D Printing for Tissue Engineering. Polymers. 2021; 13(4):563.

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Saska, Sybele, Livia Pilatti, Alberto Blay, and Jamil Awad Shibli. 2021. "Bioresorbable Polymers: Advanced Materials and 4D Printing for Tissue Engineering" Polymers 13, no. 4: 563.

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