3D printing of photocurable poly(glycerol sebacate) elastomers

Advanced optical assessment and modeling of extrusion bioprinting

In the context of tissue engineering, biofabrication techniques are employed to process cells in hydrogel-based matrices, known as bioinks, into complex 3D structures. The aim is the production of functional tissue models or even entire organs. The regenerative production of biological tissues adheres to a multitude of criteria that ultimately determine the maturation of a functional tissue. These criteria are of biological nature, such as the biomimetic spatial positioning of different cell types within a physiologically and mechanically suitable matrix, which enables tissue maturation. Furthermore, the processing, a combination of technical procedures and biological materials, has proven highly challenging since cells are sensitive to stress, for example from shear and tensile forces, which may affect their vitality. On the other hand, high resolutions are pursued to create optimal conditions for subsequent tissue maturation. From an analytical perspective, it is prudent to first investigate the printing behavior of bioinks before undertaking complex biological tests. According to our findings, conventional shear rheological tests are insufficient to fully characterize the printing behavior of a bioink. For this reason, we have developed optical methods that, complementarily to the already developed tests, allow for quantification of printing quality and further viscoelastic modeling of bioinks.

Multimodal Three-Dimensional Printing for Micro-Modulation of Scaffold Stiffness Through Machine Learning

The ability to precisely control a scaffold's microstructure and geometry with light-based three-dimensional (3D) printing has been widely demonstrated. However, the modulation of scaffold's mechanical properties through prescribed printing parameters is still underexplored. This study demonstrates a novel 3D-printing workflow to create a complex, elastomeric scaffold with precision-engineered stiffness control by utilizing machine learning. Various printing parameters, including the exposure time, light intensity, printing infill, laser pump current, and printing speed were modulated to print poly (glycerol sebacate) acrylate (PGSA) scaffolds with mechanical properties ranging from 49.3 ± 3.3 kPa to 2.8 ± 0.3 MPa. This enables flexibility in spatial stiffness modulation in addition to high-resolution scaffold fabrication. Then, a neural network-based machine learning model was developed and validated to optimize printing parameters to yield scaffolds with user-defined stiffness modulation for two different vat photopolymerization methods: a digital light processing (DLP)-based 3D printer was utilized to rapidly fabricate stiffness-modulated scaffolds with features on the hundreds of micron scale and a two-photon polymerization (2PP) 3D printer was utilized to print fine structures on the submicron scale. A novel 3D-printing workflow was designed to utilize both DLP-based and 2PP 3D printers to create multiscale scaffolds with precision-tuned stiffness control over both gross and fine geometric features. The described workflow can be used to fabricate scaffolds for a variety of tissue engineering applications, specifically for interfacial tissue engineering for which adjacent tissues possess heterogeneous mechanical properties (e.g., muscle-tendon).

Polyglycerol Sebacate Elastomer: A Critical Overview of Synthetic Methods and Characterisation Techniques

Poly (glycerol sebacate) is a widely studied elastomeric copolymer obtained from the polycondensation of two bioresorbable monomers, glycerol and sebacic acid. Due to its biocompatibility and the possibility to tailor its biodegradability rate and mechanical properties, PGS has gained lots of interest in the last two decades, especially in the soft tissue engineering field. Different synthetic approaches have been proposed, ranging from classic thermal polyesterification and curing to microwave-assisted organic synthesis, UV crosslinking and enzymatic catalysis. Each technique, characterized by its advantages and disadvantages, can be tailored by controlling the crosslinking density, which depends on specific synthetic parameters. In this work, classic and alternative synthetic methods, as well as characterisation and tailoring techniques, are critically reviewed with the aim to provide a valuable tool for the reproducible and customized production of PGS for tissue engineering applications.

Synthesis and Properties of Photocurable Polymers Derived from the Polyesters of Glycerol and Aliphatic Dicarboxylic Acids

The rapid development of 3D printing technology and the emerging applications of shape memory elastomer have greatly stimulated the research of photocurable polymers. In this work, glycerol (Gly) was polycondensed with sebacic, dodecanedioic, or tetradecanedioic acids to provide precursor polyesters with hydroxyl or carboxyl terminal groups, which were further chemically functionalized by acryloyl chloride to introduce sufficient, photocurable, and unsaturated double bonds. The chemical structures of the acrylated polyesters were characterized by FT IR and NMR spectroscopies. The photoinitiated crosslinking behavior of the acrylated polyesters under ultraviolet irradiation without the addition of any photoinitiator was investigated. The results showed that the precursor polyesters that had a greater number of terminated hydroxyls and a less branched structure obtained a relatively high acetylation degree. A longer chain of aliphatic dicarboxylic acids (ADCAs) and higher ADCA proportion lead to a relatively lower photopolymerization rate of acrylated polyesters. However, the photocured elastomers with a higher ADCA proportion or longer-chain ADCAs resulted in better mechanical properties and a lower degradation rate. The glass transition temperature (Tg) of the elastomer increased with the alkyl chain length of the ADCAs, and a higher Gly proportion resulted in a lower Tg of the elastomer due to its higher crosslinking density. Thermal gravimetric analysis (TGA) showed that the chain length of the ADCAs and the molar ratio of Gly to ADCAs had less of an effect on the thermal stability of the elastomer. As the physicochemical properties can be adjusted by choosing the alkyl chain length of the ADCAs, as well as changing the ratio of Gly:ADCA, the photocurable polyesters are expected to be applied in multiple fields.

Degradable biomedical elastomers: paving the future of tissue repair and regenerative medicine

Degradable biomedical elastomers (DBE), characterized by controlled biodegradability, excellent biocompatibility, tailored elasticity, and favorable network design and processability, have become indispensable in tissue repair. This review critically examines the recent advances of biodegradable elastomers for tissue repair, focusing mainly on degradation mechanisms and evaluation, synthesis and crosslinking methods, microstructure design, processing techniques, and tissue repair applications. The review explores the material composition and cross-linking methods of elastomers used in tissue repair, addressing chemistry-related challenges and structural design considerations. In addition, this review focuses on the processing methods of two- and three-dimensional structures of elastomers, and systematically discusses the contribution of processing methods such as solvent casting, electrostatic spinning, and three-/four-dimensional printing of DBE. Furthermore, we describe recent advances in tissue repair using DBE, and include advances achieved in regenerating different tissues, including nerves, tendons, muscle, cardiac, and bone, highlighting their efficacy and versatility. The review concludes by discussing the current challenges in material selection, biodegradation, bioactivation, and manufacturing in tissue repair, and suggests future research directions. This concise yet comprehensive analysis aims to provide valuable insights and technical guidance for advances in DBE for tissue engineering.

Photo-/thermo-responsive bioink for improved printability in extrusion-based bioprinting

Extrusion-based bioprinting has demonstrated significant potential for manufacturing constructs, particularly for 3D cell culture. However, there is a greatly limited number of bioink candidates exploited with extrusion-based bioprinting, as they meet the opposing requirements for printability with indispensable rheological features and for biochemical functionality with desirable microenvironment. In this study, a blend of silk fibroin (SF) and iota-carrageenan (CG) was chosen as a cell-friendly printable material. The SF/CG ink exhibited suitable viscosity and shear-thinning properties, coupled with the rapid sol-gel transition of CG. By employing photo-crosslinking of SF, the printability with Pr value close to 1 and structural integrity of the 3D constructs were significantly improved within a matter of seconds. The printed constructs demonstrated a Young's modulus of approximately 250 kPa, making them suitable for keratinocyte and myoblast cell culture. Furthermore, the high cell adhesiveness and viability (maximum >98%) of the loaded cells underscored the considerable potential of this 3D culture scaffold applied for skin and muscle tissues, which can be easily manipulated using an extrusion-based bioprinter.

Micromolded honeycomb scaffold design to support the generation of a bilayered RPE and photoreceptor cell construct

Age-related macular degeneration (AMD) causes blindness due to loss of retinal pigment epithelium (RPE) and photoreceptors (PRs), which comprise the two outermost layers of the retina. Given the small size of the macula and the importance of direct contact between RPE and PRs, the use of scaffolds for targeted reconstruction of the outer retina in later stage AMD and other macular dystrophies is particularly attractive. We developed microfabricated, honeycomb-patterned, biodegradable poly(glycerol sebacate) (PGS) scaffolds to deliver organized, adjacent layers of RPE and PRs to the subretinal space. Furthermore, an optimized process was developed to photocure PGS, shortening scaffold production time from days to minutes. The resulting scaffolds robustly supported the seeding of human pluripotent stem cell-derived RPE and PRs, either separately or as a dual cell-layered construct. These advanced, economical, and versatile scaffolds can accelerate retinal cell transplantation efforts and benefit patients with AMD and other retinal degenerative diseases.

Highly resilient and fatigue-resistant poly(4-methyl-ε-caprolactone) porous scaffold fabricated via thiol-yne photo-crosslinking/salt-templating for soft tissue regeneration

Elastomeric scaffolds, individually customized to mimic the structural and mechanical properties of natural tissues have been used for tissue regeneration. In this regard, polyester elastic scaffolds with tunable mechanical properties and exceptional biological properties have been reported to provide mechanical support and structural integrity for tissue repair. Herein, poly(4-methyl-ε-caprolactone) (PMCL) was first double-terminated by alkynylation (PMCL-DY) as a liquid precursor at room temperature. Subsequently, three-dimensional porous scaffolds with custom shapes were fabricated from PMCL-DY via thiol-yne photocrosslinking using a practical salt template method. By manipulating the M_n of the precursor, the modulus of compression of the scaffold was easily adjusted. As evidenced by the complete recovery from 90% compression, the rapid recovery rate of >500 mm min^-1, the extremely low energy loss coefficient of <0.1, and the superior fatigue resistance, the PMCL20-DY porous scaffold was confirmed to harbor excellent elastic properties. In addition, the high resilience of the scaffold was confirmed to endow it with a minimally invasive application potential. In vitro testing revealed that the 3D porous scaffold was biocompatible with rat bone marrow stromal cells (BMSCs), inducing BMSCs to differentiate into chondrogenic cells. In addition, the elastic porous scaffold demonstrated good regenerative efficiency in a 12-week rabbit cartilage defect model. Thus, the novel polyester scaffold with adaptable mechanical properties may have extensive applications in soft tissue regeneration.

Progress in Photocurable 3D Printing of Photosensitive Polyurethane: A Review

In recent years, as a class of advanced additive manufacturing (AM) technology, photocurable 3D printing has gained increasing attention. Based on its outstanding printing efficiency and molding accuracy, it is employed in various fields, such as industrial manufacturing, biomedical, soft robotics, electronic sensors. Photocurable 3D printing is a molding technology based on the principle of area-selective curing of photopolymerization reaction. At present, the main printing material suitable for this technology is the photosensitive resin, a composite mixture consisting of a photosensitive prepolymer, reactive monomer, photoinitiator, and other additives. As the technique research deepens and its application gets more developed, the design of printing materials suitable for different applications is becoming the hotspot. Specifically, these materials not only can be photocured but also have excellent properties, such as elasticity, tear resistance, fatigue resistance. Photosensitive polyurethanes can endow photocured resin with desirable performance due to their unique molecular structure including the inherent alternating soft and hard segments, and microphase separation. For this reason, this review summarizes and comments on the research and application progress of photocurable 3D printing of photosensitive polyurethanes, analyzing the advantages and shortcomings of this technology, also offering an outlook on this rapid development direction.

Sustainable Approach for the Synthesis of a Semicrystalline Polymer with a Reversible Shape-Memory Effect

Shape-memory polymers (SMPs) have demonstrated potential for use in automotive, biomedical, and aerospace industries. However, ensuring the sustainability of these materials remains a challenge. Herein, a sustainable approach to synthesize a semicrystalline polymer using biomass-derivable precursors via catalyst-free polyesterification is presented. The synthesized biodegradable polymer, poly(1,8-octanediol-co-1,12-dodecanedioate-co-citrate) (PODDC), exhibits excellent shape-memory properties, as evidenced by good shape fixity and shape recovery ratios of 98%, along with a large reversible actuation strain of 28%. Without the use of a catalyst, the mild polymerization enables the reconfiguration of the partially cured two-dimensional (2D) film to a three-dimensional (3D) geometric form in the middle process. This study appears to be a step forward in developing sustainable SMPs and a simple way for constructing a 3D structure of a permanent shape.

Different methods of synthesizing poly(glycerol sebacate) (PGS): A review

Poly(glycerol sebacate) (PGS) is a biodegradable elastomer that has attracted increasing attention as a potential material for applications in biological tissue engineering. The conventional method of synthesis, first described in 2002, is based on the polycondensation of glycerol and sebacic acid, but it is a time-consuming and energy-intensive process. In recent years, new approaches for producing PGS, PGS blends, and PGS copolymers have been reported to not only reduce the time and energy required to obtain the final material but also to adjust the properties and processability of the PGS-based materials based on the desired applications. This review compiles more than 20 years of PGS synthesis reports, reported inconsistencies, and proposed alternatives to more rapidly produce PGS polymer structures or PGS derivatives with tailor-made properties. Synthesis conditions such as temperature, reaction time, reagent ratio, atmosphere, catalysts, microwave-assisted synthesis, and PGS modifications (urethane and acrylate groups, blends, and copolymers) were revisited to present and discuss the diverse alternatives to produce and adapt PGS.

Nanocellulose as a promising substrate for advanced sensors and their applications

Nanocellulose has broad and promising applications owing to its low density, large specific surface area, high mechanical strength, modifiability, renewability. Recently, nanocellulose has been widely used to fabricate flexible, durable and environmental-friendly sensor substrates. In this contribution, the construction and characteristics of nanocellulose-based sensors are comprehensively reviewed. Various nanocellulose-based sensors are summarized and divided into colorimetric, fluorescent, electronic, electrochemical and SERS types according to the sensing mechanism. This review also introduces the applications of nanocellulose-based sensors in the fields of biomedicine, environmental monitoring, food safety, and wearable devices.

Optimisation of Glycerol and Itaconic Anhydride Polycondensation

Glycerol polyesters have recently become objects of interest in tissue engineering. Barely known so far is poly(glycerol itaconate) (PGItc), a biocompatible, biodegradable polyester. Due to the presence of a C=C electron-acceptor moiety, it is possible to post-modify the product by Michael additions to change the properties of PGItc. Thus, using PGItc as one of the elements of cellular scaffold crosslinked in situ for bone tissue regeneration seems to be a very attractive yet unexplored solution. This work aims to optimize the synthesis of PGItc to obtain derivatives with a double bond in the side chain with the highest conversion rates. The experiments were performed with itaconic anhydride and glycerol using mathematical planning of experiments according to the Box-Behnken plan without solvent and catalyst. The input variables of the process were the ratio of the OH/COOH, temperature, and reaction time. The optimised output variables were: the degree of esterification (EDtitr), the degree of esterification calculated from the analysis of 1H NMR spectra (EDNMR), and the degree of itaconic anhydride conversion—calculation based on 13C NMR spectra (%X13CNMR). In each of statistical models, the significance of the changed synthesis parameters was determined. Optimal conditions are when OH/COOH ratio is equal to 1.5, temperature is 140 °C and time of reaction is 5 h. The higher OH/COOH ratio, temperature and longer the experiment time, the higher the value of the degree of esterification and the degree of anhydride conversion.

Synthesis and Characterization of Alkyne-Functionalized Photo-Cross-Linkable Polyesters

An alkyne-functionalized elastomer derived from sebacic acid, 1,3-propanediol, and alkyne-functionalized serinol is synthesized via melt condensation. A low-power UV lamp triggers the cross-linking rapidly via thiol-yne click chemistry. The cross-linking behavior is studied by photorheology and NMR spectroscopy. The resultant elastomer possesses mechanical properties similar to those of human soft tissues and exhibits in vitro degradability and good cytocompatibility.

Degradable, Photochemically Printable Poly(propylene fumarate)-Based ABA Triblock Elastomers

Additive manufacturing is rapidly advancing tissue engineering, but the scope of its clinical translation is limited by a lack of materials designed to meet specific mechanical properties and resorption timelines. Materials that are printable via photochemical cross-linking, fully degradable, and elastomeric have proven to be particularly challenging to develop. Herein, we report the synthesis of a series of poly(propylene fumarate-b-γ-methyl-ε-caprolactone-b-propylene fumarate) ABA triblock polymers using sequential ring-opening polymerization and ring-opening copolymerization. When cross-linked photochemically using a continuous liquid interface production digital light processing Carbon M2 printer, these ABA-type triblock copolymers are durable elastomers with tunable degradation and elastic properties. The polymers are shown to undergo slow, hydrolytic degradation in vitro with minimal loss of mechanical performance during degradation.

Biodegradable Elastomers and Gels for Elastic Electronics

Biodegradable electronics are considered as an important bio-friendly solution for electronic waste (e-waste) management, sustainable development, and emerging implantable devices. Elastic electronics with higher imitative mechanical characteristics of human tissues, have become crucial for human-related applications. The convergence of biodegradability and elasticity has emerged a new paradigm of next-generation electronics especially for wearable and implantable electronics. The corresponding biodegradable elastic materials are recognized as a key to drive this field toward the practical applications. The review first clarifies the relevant concepts including biodegradable and elastic electronics along with their general design principles. Subsequently, the crucial mechanisms of the degradation in polymeric materials are discussed in depth. The diverse types of biodegradable elastomers and gels for electronics are then summarized. Their molecular design, modification, processing, and device fabrication especially the structure-properties relationship as well as recent advanced are reviewed in detail. Finally, the current challenges and the future directions are proposed. The critical insights of biodegradability and elastic characteristics in the elastomers and gel allows them to be tailored and designed more effectively for electronic applications.

Extruded poly (glycerol sebacate) and polyglycolic acid vascular graft forms a neoartery

In the ongoing search for the optimal biomaterial for tissue engineered vascular grafts (TEVGs), poly (glycerol sebacate) (PGS) has emerged as a new potential candidate. We have utilized a novel method to create unique, pore-free, extruded PGS grafts with and without a supportive exterior layer of polyglycolic acid (PGA). The 1 mm diameter by 5 mm length TEVGs were implanted in a rat model of infrarenal abdominal aorta interposition grafting. Three months after implantation, TEVGs comprised of extruded PGS with an external PGA braid demonstrated a patency rate of 9/10 (90%) with no signs of dilatation, dehiscence, or rupture. The PGS/PGA graft was remodeled into a neoartery with complete endothelialization of the neoartery lumen and formation of smooth muscle actinin multilayers as demonstrated via immunohistochemistry. Formation and maturation of extracellular matrix material were also observed, with amounts of elastin and collagen comparable to native rat aorta. No significant host inflammatory response was observed. These findings suggest the combination of an extruded PGS tube with an external reinforcing PGA braid is a promising material for small diameter TEVGs.

Lipase-Catalyzed Synthesis and Characterization of Poly(glycerol sebacate)

This study demonstrated that immobilized Candida antarctica lipase B (N435) catalysis in bulk leads to higher molecular weight poly(glycerol sebacate), PGS, than self-catalyzed condensation polymerization. Since the glass-transition temperature, fragility, modulus, and strength for rubbery networks are inversely dependent on the concentration of chain ends, higher molecular weight PGS prepolymers will enable the preparation of cross-linked PGS matrices with unique mechanical properties. The evolution of molecular species during the prepolymerization step conducted at 120 °C for 24 h, prior to enzyme addition, revealed regular decreases in sebacic acid and glycerol-sebacate dimer with corresponding increases in oligomers with chain lengths from 3 to 7 units such that a homogeneous liquid substrate has resulted. At 67 h, for N435-catalyzed PGS synthesis, the carboxylic acid conversion reached 82% without formation of a gel fraction, and number-average molecular weight (M_n) and weight-average molecular weight (M_w) values reached 6000 and 59 400 g/mol, respectively. In contrast, self-catalyzed PGS condensation polymerizations required termination at 55 h to avoid gelation, reached 72% conversion, and M_n and M_w values of 2600 and 13 800 g/mol, respectively. We also report the extent that solvent fractionation can enrich PGS in higher molecular weight chains. The use of methanol as a nonsolvent increased M_n and M_w by 131.7 and 18.3%, respectively, and narrower dispersity (Đ) decreased by 47.7% relative to the nonfractionated product.

Review of Materials and Fabrication Methods for Flexible Nano and Micro-Scale Physical and Chemical Property Sensors

The use of flexible sensors has tripled over the last decade due to the increased demand in various fields including health monitoring, food packaging, electronic skins and soft robotics. Flexible sensors have the ability to be bent and stretched during use and can still maintain their electrical and mechanical properties. This gives them an advantage over rigid sensors that lose their sensitivity when subject to bending. Advancements in 3D printing have enabled the development of tailored flexible sensors. Various additive manufacturing methods are being used to develop these sensors including inkjet printing, aerosol jet printing, fused deposition modelling, direct ink writing, selective laser melting and others. Hydrogels have gained much attention in the literature due to their self-healing and shape transforming. Self-healing enables the sensor to recover from damages such as cracks and cuts incurred during use, and this enables the sensor to have a longer operating life and stability. Various polymers are used as substrates on which the sensing material is placed. Polymers including polydimethylsiloxane, Poly(N-isopropylacrylamide) and polyvinyl acetate are extensively used in flexible sensors. The most widely used nanomaterials in flexible sensors are carbon and silver due to their excellent electrical properties. This review gives an overview of various types of flexible sensors (including temperature, pressure and chemical sensors), paying particular attention to the application areas and the corresponding characteristics/properties of interest required for such. Current advances/trends in the field including 3D printing, novel nanomaterials and responsive polymers, and self-healable sensors and wearables will also be discussed in more detail.

A novel engineered vascular construct of stem cell-laden 3D-printed PGSA scaffold enhances tissue revascularization

Development of transplantable engineered tissue has been hampered by lacking vascular network within the engineered tissue. Three-dimensional (3D) printing has emerged as a new technology with great potential in fabrication and customization of geometric microstructure. In this study, utilizing digital light processing system, we manufactured a recently designed novel 3D architecture scaffold with poly(glycerol sebacate) acrylate (PGSA). Vascular construct was subsequently generated by seeding stem cells within this scaffold. PGSA provided inductive substrate in terms of supporting three-germ layer differentiation of embryonic stem cells (ESCs) and also promoting ESCs-derived vascular progenitor cells (VPCs) differentiation into endothelial cells (ECs). Furthermore, the differentiation efficiency of VPCs into ECs on PGSA was much higher than that on collagen IV or fibronectin. The results from seeding VPCs in the rotating hexagonal PGSA scaffold suggest that this architectural framework is highly efficient for cell engraftment in 3D structures. After long-term suspension culture of the VPCs in scaffold under directed EC differentiation condition, VPC-differentiated ECs were populated in the scaffold and expressed EC markers. Transplantation of the vascular construct in mice resulted in formation of new vascular network and integration of the microvasculature within the scaffold into the existing vasculature of host tissue. Importantly, in a mouse model of wound healing, ECs from the transplanted vascular construct directly contributed to revascularization and enhanced blood perfusion at the injured site. Collectively, this transplantable vascular construct provides an innovative alternative therapeutic strategy for vascular tissue engineering.

The Effect of Heat Treatment toward Glycerol-Based, Photocurable Polymeric Scaffold: Mechanical, Degradation and Biocompatibility

Photocurable polymers have become increasingly important for their quick prototyping and high accuracy when used in three dimensional (3D) printing. However, some of the common photocurable polymers are known to be brittle, cytotoxic and present low impact resistance, all of which limit their applications in medicine. In this study, thermal treatment was studied for its effect and potential applications on the mechanical properties, degradability and biocompatibility of glycerol-based photocurable polymers, poly(glycerol sebacate) acrylate (PGSA). In addition to the slight increase in elongation at break, a two-fold increase in both Young's modulus and ultimate tensile strength were also observed after thermal treatment for the production of thermally treated PGSA (tPGSA). Moreover, the degradation rate of tPGSA significantly decreased due to the increase in crosslinking density in thermal treatment. The significant increase in cell viability and metabolic activity on both flat films and 3D-printed scaffolds via digital light processing-additive manufacturing (DLP-AM) demonstrated high in vitro biocompatibility of tPGSA. The histological studies and immune staining indicated that tPGSA elicited minimum immune responses. In addition, while many scaffolds suffer from instability through sterilization processes, it was proven that once glycerol-based polymers have been treated thermally, the influence of autoclaving the scaffolds were minimized. Therefore, thermal treatment is considered an effective method for the overall enhancement and stabilization of photocurable glycerol-based polymeric scaffolds in medicine-related applications.

A Review: Optimization for Poly(glycerol sebacate) and Fabrication Techniques for Its Centered Scaffolds

Poly(glycerol sebacate) (PGS), an emerging promising thermosetting polymer synthesized from sebacic acid and glycerol, has attracted considerable attention due to its elasticity, biocompatibility, and tunable biodegradation properties. But it also has some drawbacks such as harsh synthesis conditions, rapid degradation rates, and low stiffness. To overcome these challenges and optimize PGS performance, various modification methods and fabrication techniques for PGS-based scaffolds have been developed in recent years. Outlining the current modification approaches of PGS and summarizing the fabrication techniques for PGS-based scaffolds are of great importance to accelerate the development of new materials and enable them to be appropriately used in potential applications. Thus, this review comprehensively overviews PGS derivatives, PGS composites, PGS blends, processing for PGS-based scaffolds, and their related applications. It is envisioned that this review could instruct and inspire the design of the PGS-based materials and facilitate tissue engineering advances into clinical practice.

Poly(Glycerol Sebacate) in Biomedical Applications-A Review of the Recent Literature

Poly(glycerol sebacate) (PGS) continues to attract attention for biomedical applications owing to its favorable combination of properties. Conventionally polymerized by a two-step polycondensation of glycerol and sebacic acid, variations of synthesis parameters, reactant concentrations or by specific chemical modifications, PGS materials can be obtained exhibiting a wide range of physicochemical, mechanical, and morphological properties for a variety of applications. PGS has been extensively used in tissue engineering (TE) of cardiovascular, nerve, cartilage, bone and corneal tissues. Applications of PGS based materials in drug delivery systems and wound healing are also well documented. Research and development in the field of PGS continue to progress, involving mainly the synthesis of modified structures using copolymers, hybrid, and composite materials. Moreover, the production of self-healing and electroactive materials has been introduced recently. After almost 20 years of research on PGS, previous publications have outlined its synthesis, modification, properties, and biomedical applications, however, a review paper covering the most recent developments in the field is lacking. The present review thus covers comprehensively literature of the last five years on PGS-based biomaterials and devices focusing on advanced modifications of PGS for applications in medicine and highlighting notable advances of PGS based systems in TE and drug delivery.

A perfusable, multifunctional epicardial device improves cardiac function and tissue repair

Despite advances in technologies for cardiac repair after myocardial infarction (MI), new integrated therapeutic approaches still need to be developed. In this study, we designed a perfusable, multifunctional epicardial device (PerMed) consisting of a biodegradable elastic patch (BEP), permeable hierarchical microchannel networks (PHMs) and a system to enable delivery of therapeutic agents from a subcutaneously implanted pump. After its implantation into the epicardium, the BEP is designed to provide mechanical cues for ventricular remodeling, and the PHMs are designed to facilitate angiogenesis and allow for infiltration of reparative cells. In a rat model of MI, implantation of the PerMed improved ventricular function. When connected to a pump, the PerMed enabled targeted, sustained and stable release of platelet-derived growth factor-BB, amplifying the efficacy of cardiac repair as compared to the device without a pump. We also demonstrated the feasibility of minimally invasive surgical PerMed implantation in pigs, demonstrating its promise for clinical translation to treat heart disease.

4D Printing of shape-memory polymeric scaffolds for adaptive biomedical implantation

4D printing has shown great potential in a variety of biomedical applications due to the adaptability and minimal invasiveness of fabricated devices. However, commonly employed shape memory polymers (SMPs) possess undesirable transition temperatures (T_transs), leading to complications in implantation operations. Herein, we demonstrate 4D printing of a new SMP named poly(glycerol dodecanoate) acrylate (PGDA) with a T_trans in a range of 20 °C - 37 °C making it appropriate for shape programming at room temperature and then shape deployment within the human body. In addition, the material possesses suitable rheological properties to allow for the fabrication of a variety of delicate 3D structures such as "triangular star", "six-petal flower", "honeycomb", "tube", tilted "truncated hollow cones", as well as overhanging "bridge", "cage", and "mesh". The printed 3D structures show shape memory properties including a large fixity ratio of 100% at 20 °C, a large recovery ratio of 98% at 37 °C, a stable cyclability of > 100 times, and a fast recovery speed of 0.4 s at 37 °C. Moreover, the Young's moduli of the printed structures can be decreased by 5 times due to the phase transition of PGDA, which is compatible with biological tissues. Finally, in vitro stenting and in vivo vascular grafting demonstrated the geometrical and mechanical adaptivity of the printed constructs for biomedical implantation. This newly developed PGDA SMP based 4D printing technology has the potential to pave a new route to the fabrication of shape memory scaffolds for personalized biomedical applications.

Citrate Crosslinked Poly(Glycerol Sebacate) with Tunable Elastomeric Properties

Poly(glycerol-sebacate) (PGS) is a biodegradable elastomer known for its mechanical properties and biocompatibility for soft tissue engineering. However, harsh thermal crosslinking conditions are needed to make PGS devices. To facilitate the thermal crosslinking, citric acid is explored as a crosslinker to form poly(glycerol sebacate citrate) (PGSC) elastomers. The effects of varying citrate contents and curing times are investigated on the mechanical properties, elasticity, degradation, and hydrophilicity. To examine the potential presence of unreacted citric acid, material acidity is monitored in relation to the citrate content and curing times. It is discovered that a low citrate content and a short curing time produce PGSC with tunable mechanical characteristics similar to PGS with enhanced elasticity. The materials demonstrate good cytocompatibility with human umbilical vein endothelial cells similar to the PGS control. The research study suggests that PGSC is a potential candidate for large-scale biomedical applications because of the quick thermal crosslink and tunable elastomeric properties.

Review on the Impact of Polyols on the Properties of Bio-Based Polyesters

Bio-based polyol polyesters are biodegradable elastomers having potential utility in soft tissue engineering. This class of polymers can serve a wide range of biomedical applications. Materials based on these polymers are inherently susceptible to degradation during the period of implantation. Factors that influence the physicochemical properties of polyol polyesters might be useful in achieving a balance between durability and biodegradability. The characterization of these polyol polyesters, together with recent comparative studies involving creative synthesis, mechanical testing, and degradation, have revealed many of their molecular-level differences. The impact of the polyol component on the properties of these bio-based polyesters and the optimal reaction conditions for their synthesis are only now beginning to be resolved. This review describes our current understanding of polyol polyester structural properties as well as a discussion of the more commonly used polyol monomers.

Toward a neurospheroid niche model: optimizing embedded 3D bioprinting for fabrication of neurospheroid brain-like co-culture constructs

A crucial step in creating reliablein vitroplatforms for neural development and disorder studies is the reproduction of the multicellular three-dimensional (3D) brain microenvironment and the capturing of cell-cell interactions within the model. The power of self-organization of diverse cell types into brain spheroids could be harnessed to study mechanisms underlying brain development trajectory and diseases. A challenge of current 3D organoid and spheroid models grown in petri-dishes is the lack of control over cellular localization and diversity. To overcome this limitation, neural spheroids can be patterned into customizable 3D structures using microfabrication. We developed a 3D brain-like co-culture construct using embedded 3D bioprinting as a flexible solution for composing heterogenous neural populations with neurospheroids and glia. Specifically, neurospheroid-laden free-standing 3D structures were fabricated in an engineered astrocyte-laden support bath resembling a neural stem cell niche environment. A photo-crosslinkable bioink and a thermal-healing supporting bath were engineered to mimic the mechanical modulus of soft tissue while supporting the formation of self-organizing neurospheroids within elaborate 3D networks. Moreover, bioprinted neurospheroid-laden structures exhibited the capability to differentiate into neuronal cells. These brain-like co-cultures could provide a reproducible platform for modeling neurological diseases, neural regeneration, and drug development and repurposing.

Human-Recombinant-Elastin-Based Bioinks for 3D Bioprinting of Vascularized Soft Tissues

Bioprinting has emerged as an advanced method for fabricating complex 3D tissues. Despite the tremendous potential of 3D bioprinting, there are several drawbacks of current bioinks and printing methodologies that limit  the ability to print elastic and highly vascularized tissues. In particular, fabrication of complex biomimetic structure that are entirely based on 3D bioprinting is still challenging primarily due to the lack of suitable bioinks with high printability, biocompatibility, biomimicry, and proper mechanical properties. To address these shortcomings, in this work the use of recombinant human tropoelastin as a highly biocompatible and elastic bioink for 3D printing of complex soft tissues is demonstrated. As proof of the concept, vascularized cardiac constructs are bioprinted and their functions are assessed in vitro and in vivo. The printed constructs demonstrate endothelium barrier function and spontaneous beating of cardiac muscle cells, which are important functions of cardiac tissue in vivo. Furthermore, the printed construct elicits minimal inflammatory responses, and is shown to be efficiently biodegraded in vivo when implanted subcutaneously in rats. Taken together, these results demonstrate the potential of the elastic bioink for printing 3D functional cardiac tissues, which can eventually be used for cardiac tissue replacement.

Photopolymerizable Biomaterials and Light-Based 3D Printing Strategies for Biomedical Applications

Since the advent of additive manufacturing, known commonly as 3D printing, this technology has revolutionized the biofabrication landscape and driven numerous pivotal advancements in tissue engineering and regenerative medicine. Many 3D printing methods were developed in short course after Charles Hull first introduced the power of stereolithography to the world. However, materials development was not met with the same enthusiasm and remained the bottleneck in the field for some time. Only in the past decade has there been deliberate development to expand the materials toolbox for 3D printing applications to meet the true potential of 3D printing technologies. Herein, we review the development of biomaterials suited for light-based 3D printing modalities with an emphasis on bioprinting applications. We discuss the chemical mechanisms that govern photopolymerization and highlight the application of natural, synthetic, and composite biomaterials as 3D printed hydrogels. Because the quality of a 3D printed construct is highly dependent on both the material properties and processing technique, we included a final section on the theoretical and practical aspects behind light-based 3D printing as well as ways to employ that knowledge to troubleshoot and standardize the optimization of printing parameters.

3D printing of a biocompatible double network elastomer with digital control of mechanical properties

The majority of 3D-printed biodegradable biomaterials are brittle, limiting their potential application to compliant tissues. Poly (glycerol sebacate) acrylate (PGSA) is a synthetic biodegradable and biocompatible elastomer, compatible with light-based 3D printing. In this work we employed digital-light-processing (DLP)-based 3D printing to create a complex PGSA network structure. Nature-inspired double network (DN) structures with two geometrically interconnected segments with different mechanical properties were printed from the same material in a single shot. Such capability has not been demonstrated by any other fabrication technique. The biocompatibility of PGSA after 3D printing was confirmed via cell-viability analysis. We used a finite element analysis (FEA) model to predict the failure of the DN structure under uniaxial tension. FEA confirmed the soft segments act as sacrificial elements while the hard segments retain structural integrity. The simulation demonstrated that the DN design absorbs 100% more energy before rupture than the network structure made by single exposure condition (SN), doubling the toughness of the overall structure. Using the FEA-informed design, a new DN structure was printed and the FEA predicted tensile test results agreed with tensile testing of the printed structure. This work demonstrated how geometrically-optimized material design can be easily and rapidly achieved by using DLP-based 3D printing, where well-defined patterns of different stiffnesses can be simultaneously formed using the same elastic biomaterial, and overall mechanical properties can be specifically optimized for different biomedical applications.

Bioprinting 101: Design, Fabrication, and Evaluation of Cell-Laden 3D Bioprinted Scaffolds

3D bioprinting is an additive manufacturing technique that recapitulates the native architecture of tissues. This is accomplished through the precise deposition of cell-containing bioinks. The spatiotemporal control over bioink deposition permits for improved communication between cells and the extracellular matrix, facilitates fabrication of anatomically and physiologically relevant structures. The physiochemical properties of bioinks, before and after crosslinking, are crucial for bioprinting complex tissue structures. Specifically, the rheological properties of bioinks determines printability, structural fidelity, and cell viability during the printing process, whereas postcrosslinking of bioinks are critical for their mechanical integrity, physiological stability, cell survival, and cell functions. In this review, we critically evaluate bioink design criteria, specifically for extrusion-based 3D bioprinting techniques, to fabricate complex constructs. The effects of various processing parameters on the biophysical and biochemical characteristics of bioinks are discussed. Furthermore, emerging trends and future directions in the area of bioinks and bioprinting are also highlighted. Graphical abstract [Figure: see text] Impact statement Extrusion-based 3D bioprinting is an emerging additive manufacturing approach for fabricating cell-laden tissue engineered constructs. This review critically evaluates bioink design criteria to fabricate complex tissue constructs. Specifically, pre- and post-printing evaluation approaches are described, as well as new research directions in the field of bioink development and functional bioprinting are highlighted.

3D Printing of Vascular Tubes Using Bioelastomer Prepolymers by Freeform Reversible Embedding

Bioelastomers have been extensively used in tissue engineering applications because of favorable mechanical stability, tunable properties, and chemical versatility. As these materials generally possess low elastic modulus and relatively long gelation time, it is challenging to 3D print them using traditional techniques. Instead, the field of 3D printing has focused preferentially on hydrogels and rigid polyester materials. To develop a versatile approach for 3D printing of elastomers, we used freeform reversible embedding of suspended prepolymers. A family of novel fast photocrosslinakble bioelastomer prepolymers were synthesized from dimethyl itaconate, 1,8-octanediol, and triethyl citrate. Tensile testing confirmed their elastic properties with Young's moduli in the range of 11-53 kPa. These materials supported cultivation of viable cells and enabled adhesion and proliferation of human umbilical vein endothelial cells. Tubular structures were created by embedding the 3D printed microtubes within a secondary hydrogel that served as a temporary support. Upon photocrosslinking and porogen leaching, the polymers were permeable to small molecules (TRITC-dextran). The polymer microtubes were assembled on the 96-well plates custom made by hot-embossing, as a tool to connect multiple organs-on-a-chip. The endothelialization of the tubes was performed to confirm that these microtubes can be utilized as vascular tubes to support parenchymal tissues seeded on them.

3D Printing Mechanically Robust and Transparent Polyurethane Elastomers for Stretchable Electronic Sensors

Advanced stretchable electronic sensors with a complex structure place higher requirements on the mechanical properties and manufacturing process of the stretchable substrate materials. Herein, three kinds of polyurethane acrylate oligomers were synthesized successfully and mixed with a commercial acrylate monomer (isobornyl acrylate) to prepare photocurable resins with a low viscosity for a digital light processing three-dimensional (3D) printer without custom equipment. Results showed that the resin containing poly(tetrahydrofuran) units (PPTMGA-40) exhibited optimal mechanical properties and shape recoverability. The tensile strength and elongation at break of PPTMGA-40 were 15.7 MPa and 414.3%, respectively. The unprecedented fatigue resistance of PPTMGA-40 allowed it to withstand 100 compression cycles at 80% strain without fracture. The transmittance of PPTMGA-40 reached 89.4% at 550 nm, showing high transparency. An ionic hydrogel was coated on the surface of 3D-printed structures to fabricate stretchable sensors, and their conductivity, transparency, and mechanical performance were characterized. A robust piezoresistive strain sensor with a high strength (∼6 MPa) and a wearable finger guard sensor were fabricated, demonstrating that this hydrogel-elastomer system can meet the requirements of applications for advanced stretchable electronic sensors and expand the usage scope.

Formation of highly elastomeric and property-tailorable poly(glycerol sebacate)-co-poly(ethylene glycol) hydrogels through thiol–norbornene photochemistry

Nor_PGS-co-PEG is a new type of hydrophilic PGS-based copolymer with definable properties for scaffold manufacturing as well as for biomedical applications.

Mechanical performance of elastomeric PGS scaffolds under dynamic conditions

In developing novel scaffolds, addressing mechanical properties is essential especially when future applications involve cyclic mechanical loading. Therefore, it is important to understand the behaviour of its physical properties with the evolution of its weight loss. Poly(glycerol sebacate) (PGS) is a promising material for tissue and biomedical engineering applications due to its biocompatibility, biodegradability and mechanical properties. To understand the impact of the hydrolytic degradation on the density, cross-linking degree and porosity; scaffolds with an average porosity of 93 ± 2% were synthetized by salt leaching technique and submitted to hydrolytic degradation. The scaffold showed a Young modulus of 17.3 ± 3.4 kPa, with a negligible energy loss during the mechanical solicitation. Moreover, a weight loss of 28 ± 2% followed by an increase in the swelling ratio of the scaffold was observed after 8 weeks of hydrolytic degradation. When submitted to cyclic mechanical loading-unloading, the PGS scaffolds present an outstanding fatigue behaviour under dry and wet conditions, with a remarkable resilience to the cyclic mechanical solicitation, and even after 1000 mechanical cycles, the construct was able to recover to its initial geometry. Overall, the PGS scaffolds demonstrate promising mechanical properties for biomedical applications, especially under dynamic conditions.

Bioprinting of a Cell-Laden Conductive Hydrogel Composite

Bioprinting has gained significant attention for creating biomimetic tissue constructs with potential to be used in biomedical applications such as drug screening or regenerative medicine. Ideally, biomaterials used for three-dimensional (3D) bioprinting should match the mechanical, hydrostatic, bioelectric, and physicochemical properties of the native tissues. However, many materials with these tissue-like properties are not compatible with printing techniques without modifying their compositions. In addition, integration of cell-laden biomaterials with bioprinting methodologies that preserve their physicochemical properties remains a challenge. In this work, a biocompatible conductive hydrogel composed of gelatin methacryloyl (GelMA) and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) was synthesized and bioprinted to form complex, 3D cell-laden structures. The biofabricated conductive hydrogels were formed by an initial cross-linking step of the PEDOT:PSS with bivalent calcium ions and a secondary photopolymerization step with visible light to cross-link the GelMA component. These modifications enabled tuning the mechanical properties of the hydrogels, with Young's moduli ranging from ∼40-150 kPa, as well as tunable conductivity by varying the concentration of PEDOT:PSS. In addition, the hydrogels degraded in vivo with no substantial inflammatory responses as demonstrated by haematoxylin and eosin (H&E) and immunofluorescent staining of subcutaneously implanted samples in Wistar rats. The parameters for forming a slurry of microgel particles to support 3D bioprinting of the engineered cell-laden hydrogel were optimized to form constructs with improved resolution. High cytocompatibility and cell spreading were demonstrated in both wet-spinning and 3D bioprinting of cell-laden hydrogels with the new conductive hydrogel-based bioink and printing methodology. The synergy of an advanced fabrication method and conductive hydrogel presented here is promising for engineering complex conductive and cell-laden structures.

Polyester-based ink platform with tunable bioactivity for 3D printing of tissue engineering scaffolds

In this work, we synthesized a novel polymeric biomaterial platform with tunable functionalizability for extrusion-based 3D printing. Biodegradable polymers were synthesized using 4-hydroxyphenethyl 2-(4-hydroxyphenyl)acetate (HTy), which is derived from Tyrosol and 2-(4-hydroxyphenyl)acetic acid. p-Phenylenediacetic acid (PDA) was introduced to enhance crystallinity. To enable functionalizability without deteriorating printability, glutamic acid derivatives were introduced into the polymer design, forming copolymers including poly(HTy-co-45%PDA-co-5%Gluhexenamide ester) (HP5GH), poly(HTy-co-45%PDA-co-5%Glupentynamide ester) (HP5GP), and poly(HTy-co-45%PDA-co-5%BocGlu ester) (HP5BG). The resulting polymers have: two melting temperatures (125-131 °C and 141-147 °C), Young's moduli of 1.9-2.4 GPa, and print temperatures of 170-190 °C. The molecular weight (Mw) loss due to hydrolytic degradation was gradual with ∼30% Mw retained after 25 weeks for HP5BG, whereas it was much faster for HP5GP and HP5GH with only 18% Mw retained after 8 weeks. HP5GH and HP5GP were successfully functionalized in solution (bulk) or on the surface using click-based chemistry. Finally, the utilization of this novel platform was demonstrated by studying osteogenic differentiation of human mesenchymal stem cells (hMSCs) using 3D printed scaffolds from HP5GP. Scaffolds were functionalized with azide-Heparin (az-Heparin) to bind and deliver bone morphogenetic protein 2 (BMP-2). This sample group significantly enhanced osteogenic differentiation of hMSCs as compared to unfunctionalized scaffolds incubated directly with az-Heparin or BMP-2 prior to cell culture. This novel polymer platform with tunable functionalizability could be utilized for additive manufacturing of biodegradable devices and scaffolds with tailored mechanical and bioactive properties for a wide range of medical applications including bone fixation devices and scaffolds for bone regeneration.

3D printing individualized heel cup for improving the self-reported pain of plantar fasciitis

BACKGROUND

To explore the therapeutic effect and the biomechanical mechanism of 3D printing individualized heel cup in treating of plantar heel pain.

METHODS

The clinical effect was evaluated by plantar pressure analysis and pain assessment in participants. Its biomechanical mechanism of protecting the plantar heel was explored using finite element simulation.

RESULTS

The individualized heel cup could support and protect the osseous structure and soft tissue of plantar heel while walking and jogging, as well as significantly reduce the self-reported pain after being worn for 4 weeks. The nylon heel cup could alter the load concentration of the heel as well as decrease the load affected on plantar fascia and calcaneus bone. It also provided an obvious support for heel pad.

CONCLUSION

To summarize, the 3D printed individualized heel cup can be used as an effective method for the treatment of plantar heel pain.