Synthesis and characterization of conductive, biodegradable, elastomeric polyurethanes for biomedical applications

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.

Synergetic Effect of Electrical and Topographical Cues in Aniline Trimer-Based Polyurethane Fibrous Scaffolds on Tissue Regeneration

Processibility and biodegradability of conductive polymers are major concerns when they are applied to tissue regeneration. This study synthesizes dissolvable and conductive aniline trimer-based polyurethane copolymers (DCPU) and processes them into scaffolds by using electrospinning with different patterns (random, oriented, and latticed). The effects of topographic cue changes on electrical signal transmission and further regulation of cell behaviors concerning bone tissue are researched. Results show that DCPU fibrous scaffolds possessed good hydrophilicity, swelling capacity, elasticity, and fast biodegradability in enzymatic liquid. In addition, the conductivity and efficiency of electrical signal transmission can be tuned by changing the surface’s topological structure. Among them, oriented DCPU scaffolds (DCPU-O) showed the best conductivity with the lowest ionic resistance value. Furthermore, the viability and proliferation results of bone mesenchymal stem cells (BMSCs) demonstrate a significant increase on three DCPU scaffolds compared to AT-free scaffolds (DPU-R). Especially, DCPU-O scaffolds exhibit superior abilities to promote cell proliferation because of their unique surface topography and excellent electroactivity. Concurrently, the DCPU-O scaffolds can synergistically promote osteogenic differentiation in terms of osteogenic differentiation and gene expression levels when combined with electrical stimulation. Together, these results suggest a promising use of DCPU-O fibrous scaffolds in the application of tissue regeneration.

Elastomeric Trilayer Substrates with Native-like Mechanical Properties for Heart Valve Leaflet Tissue Engineering

Heart valve leaflets have a complex trilayered structure with layer-specific orientations, anisotropic tensile properties, and elastomeric characteristics that are difficult to mimic collectively. Previously, trilayer leaflet substrates intended for heart valve tissue engineering were developed with nonelastomeric biomaterials that cannot deliver native-like mechanical properties. In this study, by electrospinning polycaprolactone (PCL) polymer and poly(l-lactide-co-ε-caprolactone) (PLCL) copolymer, we created elastomeric trilayer PCL/PLCL leaflet substrates with native-like tensile, flexural, and anisotropic properties and compared them with trilayer PCL leaflet substrates (as control) to find their effectiveness in heart valve leaflet tissue engineering. These substrates were seeded with porcine valvular interstitial cells (PVICs) and cultured for 1 month in static conditions to produce cell-cultured constructs. The PCL/PLCL substrates had lower crystallinity and hydrophobicity but higher anisotropy and flexibility than PCL leaflet substrates. These attributes contributed to more significant cell proliferation, infiltration, extracellular matrix production, and superior gene expression in the PCL/PLCL cell-cultured constructs than in the PCL cell-cultured constructs. Further, the PCL/PLCL constructs showed better resistance to calcification than PCL constructs. Trilayer PCL/PLCL leaflet substrates with native-like mechanical and flexural properties could significantly improve heart valve tissue engineering.

Rational design of biodegradable thermoplastic polyurethanes for tissue repair

As a type of elastomeric polymers, non-degradable polyurethanes (PUs) have a long history of being used in clinics, whereas biodegradable PUs have been developed in recent decades, primarily for tissue repair and regeneration. Biodegradable thermoplastic (linear) PUs are soft and elastic polymeric biomaterials with high mechanical strength, which mimics the mechanical properties of soft and elastic tissues. Therefore, biodegradable thermoplastic polyurethanes are promising scaffolding materials for soft and elastic tissue repair and regeneration. Generally, PUs are synthesized by linking three types of changeable blocks: diisocyanates, diols, and chain extenders. Alternating the combination of these three blocks can finely tailor the physio-chemical properties and generate new functional PUs. These PUs have excellent processing flexibilities and can be fabricated into three-dimensional (3D) constructs using conventional and/or advanced technologies, which is a great advantage compared with cross-linked thermoset elastomers. Additionally, they can be combined with biomolecules to incorporate desired bioactivities to broaden their biomedical applications. In this review, we comprehensively summarized the synthesis, structures, and properties of biodegradable thermoplastic PUs, and introduced their multiple applications in tissue repair and regeneration. A whole picture of their design and applications along with discussions and perspectives of future directions would provide theoretical and technical supports to inspire new PU development and novel applications.

Biodegradable polyurethane scaffolds in regenerative medicine: Clinical translation review

Early explorations of tissue engineering and regenerative medicine concepts commonly utilized simple polyesters such as polyglycolide, polylactide, and their copolymers as scaffolds. These biomaterials were deemed clinically acceptable, readily accessible, and provided processability and a generally known biological response. With experience and refinement of approaches, greater control of material properties and integrated bioactivity has received emphasis and a broadened palette of synthetic biomaterials has been employed. Biodegradable polyurethanes (PUs) have emerged as an attractive option for synthetic scaffolds in a variety of tissue applications because of their flexibility in molecular design and ability to fulfill mechanical property objectives, particularly in soft tissue applications. Biodegradable PUs are highly customizable based on their composition and processability to impart tailored mechanical and degradation behavior. Additionally, bioactive agents can be readily incorporated into these scaffolds to drive a desired biological response. Enthusiasm for biodegradable PU scaffolds has soared in recent years, leading to rapid growth in the literature documenting novel PU chemistries, scaffold designs, mechanical properties, and aspects of biocompatibility. Despite the enthusiasm in the field, there are still few examples of biodegradable PU scaffolds that have achieved regulatory approval and routine clinical use. However, there is a growing literature where biodegradable PU scaffolds are being specifically developed for a wide range of pathologies and where relevant pre-clinical models are being employed. The purpose of this review is first to highlight examples of clinically used biodegradable PU scaffolds, and then to summarize the growing body of reports on pre-clinical applications of biodegradable PU scaffolds.

3D Printed Biodegradable Polyurethaneurea Elastomer Recapitulates Skeletal Muscle Structure and Function

Effective skeletal muscle tissue engineering relies on control over the scaffold architecture for providing muscle cells with the required directionality, together with a mechanical property match with the surrounding tissue. Although recent advances in 3D printing fulfill the first requirement, the available synthetic polymers either are too rigid or show unfavorable surface and degradation profiles for the latter. In addition, natural polymers that are generally used as hydrogels lack the required mechanical stability to withstand the forces exerted during muscle contraction. Therefore, one of the most important challenges in the 3D printing of soft and elastic tissues such as skeletal muscle is the limitation of the availability of elastic, durable, and biodegradable biomaterials. Herein, we have synthesized novel, biocompatible and biodegradable, elastomeric, segmented polyurethane and polyurethaneurea (TPU) copolymers which are amenable for 3D printing and show high elasticity, low modulus, controlled biodegradability, and improved wettability, compared to conventional polycaprolactone (PCL) and PCL-based TPUs. The degradation profile of the 3D printed TPU scaffold was in line with the potential tissue integration and scaffold replacement process. Even though TPU attracts macrophages in 2D configuration, its 3D printed form showed limited activated macrophage adhesion and induced muscle-like structure formation by C2C12 mouse myoblasts in vitro, while resulting in a significant increase in muscle regeneration in vivo in a tibialis anterior defect in a rat model. Effective muscle regeneration was confirmed with immunohistochemical assessment as well as evaluation of electrical activity produced by regenerated muscle by EMG analysis and its force generation via a custom-made force transducer. Micro-CT evaluation also revealed production of more muscle-like structures in the case of implantation of cell-laden 3D printed scaffolds. These results demonstrate that matching the tissue properties for a given application via use of tailor-made polymers can substantially contribute to the regenerative outcomes of 3D printed tissue engineering scaffolds.

Porous Electroactive and Biodegradable Polyurethane Membrane through Self-Doping Organogel

In order to improve the processability of conductive polyurethane (CPU) containing aniline oligomers, a new CPU containing aniline trimer (AT) and l-lysine (PUAT) are designed and synthesized. Further, the 3D porous PUAT membranes have been prepared by a simple gel cooperated with freeze-drying method. Chemical testings and conductive properties testify a self- doping model of PUAT based on the rich electronic l-lysine and electroaffinity AT moities. The self-doping behavior further endows the PUAT copolymers specific characteristics such as high electrical conductivity and the formation of the polaron lattice like-structure in good solvent dimethyl sulfoxide. The combination of organogel and freeze-drying could prevent the collapse of pore structure when the copolymers are molded as membranes. The synergistic effect of l-lysine and AT components has a strong influence on the dissolution, degradation, thermal stability, and mechanical properties of PUAT. The excellent properties of PUAT would broad the application of conductive polymers in biomedicine field.

Preparation of phospholipid-based polycarbonate urethanes for potential applications of blood-contacting implants

Polyurethanes are widely used in interventional devices due to the excellent physicochemical property. However, non-specific adhesion and severe inflammatory response of ordinary polyurethanes may lead to severe complications of intravenous devices. Herein, a novel phospholipid-based polycarbonate urethanes (PCUs) were developed via two-step solution polymerization by direct synthesis based on functional raw materials. Furthermore, PCUs were coated on biomedical metal sheets to construct biomimetic anti-fouling surface. The results of stress–strain curves exhibited excellent tensile properties of PCUs films. Differential scanning calorimetry results indicated that the microphase separation of such PCUs polymers could be well regulated by adjusting the formulation of chain extender, leading to different biological response. In vitro blood compatibility tests including bovine serum albumin adsorption, fibrinogen adsorption and denaturation, platelet adhesion and whole-blood experiment showed superior performance in inhibition non-specific adhesion of PCUs samples. Endothelial cells and smooth muscle cells culture tests further revealed a good anti-cell adhesion ability. Finally, animal experiments including ex vivo blood circulation and subcutaneous inflammation animal experiments indicated a strong ability in anti-thrombosis and histocompatibility. These results high light the strong anti-adhesion property of phospholipid-based PCUs films, which may be applied to the blood-contacting implants such as intravenous catheter or antithrombotic surface in the future.

Recent advances in tissue engineering scaffolds based on polyurethane and modified polyurethane

Organ repair, regeneration, and transplantation are constantly in demand due to various acute, chronic, congenital, and infectious diseases. Apart from traditional remedies, tissue engineering (TE) is among the most effective methods for the repair of damaged tissues via merging the cells, growth factors, and scaffolds. With regards to TE scaffold fabrication technology, polyurethane (PU), a high-performance medical grade synthetic polymer and bioactive material has gained significant attention. PU possesses exclusive biocompatibility, biodegradability, and modifiable chemical, mechanical and thermal properties, owing to its unique structure-properties relationship. During the past few decades, PU TE scaffold bioactive properties have been incorporated or enhanced with biodegradable, electroactive, surface-functionalised, ayurvedic products, ceramics, glass, growth factors, metals, and natural polymers, resulting in the formation of modified polyurethanes (MPUs). This review focuses on the recent advances of PU/MPU scaffolds, especially on the biomedical applications in soft and hard tissue engineering and regenerative medicine. The scientific issues with regards to the PU/MPU scaffolds, such as biodegradation, electroactivity, surface functionalisation, and incorporation of active moieties are also highlighted along with some suggestions for future work.

Electroconductive Nanobiomaterials for Tissue Engineering and Regenerative Medicine

Regenerative medicine aims to engineer tissue constructs that can recapitulate the functional and structural properties of native organs. Most novel regenerative therapies are based on the recreation of a three-dimensional environment that can provide essential guidance for cell organization, survival, and function, which leads to adequate tissue growth. The primary motivation in the use of conductive nanomaterials in tissue engineering has been to develop biomimetic scaffolds to recapitulate the electrical properties of the natural extracellular matrix, something often overlooked in numerous tissue engineering materials to date. In this review article, we focus on the use of electroconductive nanobiomaterials for different biomedical applications, particularly, very recent advancements for cardiovascular, neural, bone, and muscle tissue regeneration. Moreover, this review highlights how electroconductive nanobiomaterials can facilitate cell to cell crosstalk (i.e., for cell growth, migration, proliferation, and differentiation) in different tissues. Thoughts on what the field needs for future growth are also provided.

Current advances in biodegradable synthetic polymer based cardiac patches

The number of people affected by heart disease such as coronary artery disease and myocardial infarction increases at an alarming rate each year. Currently, the methods to treat these diseases are restricted to lifestyle change, pharmaceuticals, and eventually heart transplant if the condition is severe enough. While these treatment options are the standard for caring for patients who suffer from heart disease, limited regenerative ability of the heart restricts the effectiveness of treatment and may lead to other heart-related health problems in the future. Because of the increasing need for more effective therapeutic technologies for treating diseased heart tissue, cardiac patches are now a large focus for researchers. The cardiac patches are designed to be integrated into the patients' natural tissue to introduce mechanical support and healing to the damaged areas. As a promising alternative, synthetic biodegradable polymer based biomaterials can be easily manipulated to customize material properties, as well as possess certain desired characteristics for cardiac patch use. This comprehensive review summarizes recent works on synthetic biodegradable cardiac patches implanted into infarcted animal models. In addition, this review describes the basic requirements that should be met for cardiac patch development, and discusses the inspirations to designing new biomaterials and technologies for cardiac patches.

Conductive biomaterials for muscle tissue engineering

Muscle tissues are soft tissues that are of great importance in force generation, body movements, postural support and internal organ function. Muscle tissue injuries would not only result in the physical and psychological pain and disability to the patient, but also become a severe social problem due to the heavy financial burden they laid on the governments. Current treatments for muscle tissue injuries all have their own severe limitations and muscle tissue engineering has been proposed as a promising therapeutic strategy to treat with this problem. Conductive biomaterials are good candidates as scaffolds in muscle tissue engineering due to their proper conductivity and their promotion on muscle tissue formation. However, a review of conductive biomaterials function in muscle tissue engineering, including the skeletal muscle tissue, cardiac muscle tissue and smooth muscle tissue regeneration is still lacking. Here we reviewed the recent progress of conductive biomaterials for muscle regeneration. The recent synthesis and fabrication methods of conductive scaffolds containing conductive polymers (mainly polyaniline, polypyrrole and poly(3,4-ethylenedioxythiophene), carbon-based nanomaterials (mainly graphene and carbon nanotube), and metal-based biomaterials were systematically discussed, and their application in a variety of forms (such as hydrogels, films, nanofibers, and porous scaffolds) for different kinds of muscle tissues formation (skeletal muscle, cardiac muscle and smooth muscle) were summarized. Furthermore, the mechanism of how the conductive biomaterials affect the muscle tissue formation was discussed and the future development directions were included.

Enhancing anti-thrombogenicity of biodegradable polyurethanes through drug molecule incorporation

Sufficient and sustained anti-thrombogenicity is essential for blood-contacting materials, because blood coagulation and thrombosis caused by platelet adhesion and activation on material surfaces may lead to functional failure and even fatal outcomes. Covalently conjugating antithrombogenic moieties into polymer, instead of surface modifying or blending, can maintain the anti-thrombogenicity of polymer at a high level over a time range. In this study, series of randomly crosslinked, elastic, biodegradable polyurethanes (PU-DPA) were synthesized through a one-pot and one-step method from polycaprolactone (PCL) diol, hexamethylene diisocyanate (HDI) and anti-thrombogenic drug, dipyridamole (DPA). The mechanical properties, hydrophilicity, in vitro degradation, and anti-thrombogenicity of the resultant PU-DPA polymers can be tuned by altering the incorporated DPA amount. The surface and bulk hydrophilicity of the polyurethanes decreased with increasing hydrophobic DPA amount. All PU-DPA polymers exhibited strong mechanical properties and good elasticity. The degradation rates of the PU-DPAs decreased with increasing DPA content in both PBS and lipase/PBS solutions. Covalently incorporating DPA into the polyurethane significantly reduced the platelet adhesion and activation compared to the polyurethane without DPA, and also can achieve sustained anti-thrombogenicity. The PU-DPA films also supported the growth of human umbilical vein endothelial cells. The attractive mechanical properties, blood compatibility, and cell compatibility of this anti-thrombogenic biodegradable polyurethane indicate that it has a great potential to be utilized for blood-contacting devices, and cardiovascular tissue repair and regeneration.

Adjustable Polyurethane Foam as Filling Material for a Novel Spondyloplasty: Biomechanics and Biocompatibility

OBJECTIVE

To investigate the biomechanics and biocompatibility of polyurethane (PU) foam with adjustable stiffness as a filling material for a novel spondyloplasty that is designed to reduce the risk of postoperative adjacent level fractures.

METHODS

Sixty individual porcine lumbar vertebrae were randomly split into 4 groups: A, B, C, and D. Group A served as unmodified vertebral body controls. Groups B, C, and D consisted of hollowed vertebral bodies. Vertebrae of groups C and D were filled with adjustable PU foams of different stiffness. The compressive strength and stiffness of vertebrae from groups A-D were recorded and analyzed. 3T3 mouse fibroblasts were cultured with preformed PU foams for 4 days to test biocompatibility.

RESULTS

The strength and stiffness of the hollowed groups were lower than in group A. However, the differences were not statistically significant between group A and group C (P > 0.05), and were obviously different between group A and group B or group D (P < 0.01 and <0.05, respectively). Moreover, the strength and stiffness after filling foams in group C or group D were significantly greater than in group B (P < 0.01 and <0.05, respectively). Live/dead staining of 3T3 cells confirmed the biocompatibility of the PU foam.

CONCLUSIONS

The new PU foam shows adaptability regarding its stiffness and excellent cytocompatibility in vitro. The results support the clinical translation of the new PU foams as augmentation material in the development of a novel spondyloplasty.

A Novel Electroactive Agarose-Aniline Pentamer Platform as a Potential Candidate for Neural Tissue Engineering

Neuronal disorder is an important health challenge due to inadequate natural regeneration, which has been responded by tissue engineering, particularly with conductive materials. A bifunctional electroactive scaffold having agarose biodegradable and aniline pentamer (AP) conductive parts was designed that exhibits appropriate cell attachment/compatibility, as detected by PC12 cell seeding. The developed carboxyl-capped aniline-pentamer improved agarose cell adhesion potential, also the conductivity of scaffold was in the order 10-5 S/cm reported for cell membrane. Electrochemical impedance spectroscopy was applied to plot the Nyquist graph and subsequent construction of the equivalent circuit model based on the neural model, exhibiting an appropriate cell signaling and an acceptable consistency between the components of the scaffold model with neural cell model. The ionic conductivity was also measured; exhibiting an enhanced ionic conductivity, but lower activation energy upon a temperature rise. Swelling behavior of the sample was measured and compared with pristine agarose; so that aniline oligomer due to its hydrophobic nature decreased water uptake. Dexamethasone release from the developed electroactive scaffold was assessed through voltage-responsive method. Proper voltage-dependent drug release could be rationally expected because of controllable action and elimination of chemically responsive materials. Altogether, these characteristics recommended the agarose/AP biopolymer for neural tissue engineering.

Dopamine-Incorporated Dual Bioactive Electroactive Shape Memory Polyurethane Elastomers with Physiological Shape Recovery Temperature, High Stretchability, and Enhanced C2C12 Myogenic Differentiation

Soft tissue engineering needs elastic biomaterials not only mimicking the elasticity of soft tissue but also possessing multiple bioactivity to promote cell adhesion, proliferation, and differentiation, which still remain ongoing challenges. Herein, we synthesized a series of dopamine-incorporated dual bioactive electroactive shape memory polyurethane elastomers by combining the properties of elastomeric poly(citric acid-co-polycaprolactone) (CA-PCL) polyurethane elastomer, bioactive dopamine (DA), and electroactive aniline hexamer (AH). The chemical structures, electroactivity, conductivity, thermal properties, hydrophilicity and hydration ability, mechanical properties, and degradability of the polyurethane elastomers were systematically characterized. The elastomers showed excellent shape fixity ratio and shape recovery ability under physiological conditions. The elastomers' elongation and stress were tailored by the AH content, whereas the hydrophilicity and hydration ability of the elastomers were adjusted by the content of DA and AH, as well as the doping state of AH. The viability and proliferation results of C2C12 cells seeded on the elastomers showed their excellent cytocompatibility. Additionally, by analyzing the protein and gene level, the promotion effect on myogenic differentiation of C2C12 cells by these elastomers compared to that by control groups (PCL80 000, CA-PCL elastomer, and CA-PCL elastomer with the DA segment) was demonstrated. Furthermore, the results from subcutaneous implantation confirmed the elastomers' mild host response in vivo. These results represent that these dopamine-incorporated dual bioactive electroactive shape memory polyurethane elastomers are promising candidates for soft tissue regeneration that is sensitive to electrical signals.

Low-Initial-Modulus Biodegradable Polyurethane Elastomers for Soft Tissue Regeneration

The mechanical match between synthetic scaffold and host tissue remains challenging in tissue regeneration. The elastic soft tissues exhibit low initial moduli with a J-shaped tensile curve. Suitable synthetic polymer scaffolds require low initial modulus and elasticity. To achieve these requirements, random copolymers poly(δ-valerolactone-co-ε-caprolactone) (PVCL) and hydrophilic poly(ethylene glycol) (PEG) were combined into a triblock copolymer, PVCL-PEG-PVCL, which was used as a soft segment to synthesize a family of biodegradable elastomeric polyurethanes (PU) with low initial moduli. The triblock copolymers were varied in chemical components, molecular weights, and hydrophilicities. The mechanical properties of polyurethanes in dry and wet states can be tuned by altering the molecular weights and hydrophilicities of the soft segments. Increasing the length of either PVCL or PEG in the soft segments reduced initial moduli of the polyurethane films and scaffolds in dry and wet states. The polymer films are found to have good cell compatibility and to support fibroblast growth in vitro. Selected polyurethanes were processed into porous scaffolds by a thermally induced phase-separation technique. The scaffold from PU-PEG_1K-PVCL_6K had an initial modulus of 0.60 ± 0.14 MPa, which is comparable with the initial modulus of human myocardium (0.02-0.50 MPa). In vivo mouse subcutaneous implantation of the porous scaffolds showed minimal chronic inflammatory response and intensive cell infiltration, which indicated good tissue compatibility of the scaffolds. Biodegradable polyurethane elastomers with low initial modulus and good biocompatibility and processability would be an attractive alternative scaffold material for soft tissue regeneration, especially for heart muscle.

Development of dopant-free conductive bioelastomers

Conductive biodegradable materials are of great interest for various biomedical applications, such as tissue repair and bioelectronics. They generally consist of multiple components, including biodegradable polymer/non-degradable conductive polymer/dopant, biodegradable conductive polymer/dopant or biodegradable polymer/non-degradable inorganic additives. The dopants or additives induce material instability that can be complex and possibly toxic. Material softness and elasticity are also highly expected for soft tissue repair and soft electronics. To address these concerns, we designed a unicomponent dopant-free conductive polyurethane elastomer (DCPU) by chemically linking biodegradable segments, conductive segments, and dopant molecules into one polymer chain. The DCPU films which had robust mechanical properties with high elasticity and conductivity can be degraded enzymatically and by hydrolysis. It exhibited great electrical stability in physiological environment with charge. Mouse 3T3 fibroblasts survived and proliferated on these films exhibiting good cytocompatibility. Polymer degradation products were non-toxic. DCPU could also be processed into a porous scaffold and in an in vivo subcutaneous implantation model, exhibited good tissue compatibility with extensive cell infiltration over 2 weeks. Such biodegradable DCPU with good flexibility and elasticity, processability, and electrical stability may find broad applications for tissue repair and soft/stretchable/wearable bioelectronics.