Electroactive polymers for tissue regeneration: Developments and perspectives

Development and Utilization of Multifunctional Polymeric Scaffolds for the Regulation of Physical Cellular Microenvironments

Polymeric biomaterials exhibit excellent physicochemical characteristics as a scaffold for cell and tissue engineering applications. Chemical modification of the polymers has been the primary mode of functionalization to enhance biocompatibility and regulate cellular behaviors such as cell adhesion, proliferation, differentiation, and maturation. Due to the complexity of the in vivo cellular microenvironments, however, chemical functionalization alone is usually insufficient to develop functionally mature cells/tissues. Therefore, the multifunctional polymeric scaffolds that enable electrical, mechanical, and/or magnetic stimulation to the cells, have gained research interest in the past decade. Such multifunctional scaffolds are often combined with exogenous stimuli to further enhance the tissue and cell behaviors by dynamically controlling the microenvironments of the cells. Significantly improved cell proliferation and differentiation, as well as tissue functionalities, are frequently observed by applying extrinsic physical stimuli on functional polymeric scaffold systems. In this regard, the present paper discusses the current state-of-the-art functionalized polymeric scaffolds, with an emphasis on electrospun fibers, that modulate the physical cell niche to direct cellular behaviors and subsequent functional tissue development. We will also highlight the incorporation of the extrinsic stimuli to augment or activate the functionalized polymeric scaffold system to dynamically stimulate the cells.

Conductive Polymeric-Based Electroactive Scaffolds for Tissue Engineering Applications: Current Progress and Challenges from Biomaterials and Manufacturing Perspectives

The practice of combining external stimulation therapy alongside stimuli-responsive bio-scaffolds has shown massive potential for tissue engineering applications. One promising example is the combination of electrical stimulation (ES) and electroactive scaffolds because ES could enhance cell adhesion and proliferation as well as modulating cellular specialization. Even though electroactive scaffolds have the potential to revolutionize the field of tissue engineering due to their ability to distribute ES directly to the target tissues, the development of effective electroactive scaffolds with specific properties remains a major issue in their practical uses. Conductive polymers (CPs) offer ease of modification that allows for tailoring the scaffold's various properties, making them an attractive option for conductive component in electroactive scaffolds. This review provides an up-to-date narrative of the progress of CPs-based electroactive scaffolds and the challenge of their use in various tissue engineering applications from biomaterials perspectives. The general issues with CP-based scaffolds relevant to its application as electroactive scaffolds were discussed, followed by a more specific discussion in their applications for specific tissues, including bone, nerve, skin, skeletal muscle and cardiac muscle scaffolds. Furthermore, this review also highlighted the importance of the manufacturing process relative to the scaffold's performance, with particular emphasis on additive manufacturing, and various strategies to overcome the CPs' limitations in the development of electroactive scaffolds.

Rational design of a highly porous electronic scaffold with concurrent enhancement in cell behaviors and differentiation under electrical stimulation

Conductive polymers (CPs) have received increasing attention as promising materials for studying electrophysiological signals in cell and tissue engineering. The combination of CPs with electrical stimulation (ES) could possibly enhance neurogenesis, osteogenesis, and myogenesis. To date, research has been prioritized on capitalizing CPs as two-dimensional (2D) structures for guiding the differentiation. In contrast, relatively little is conducted on the implementation of 3D conductive scaffolds. In this research, we report the synergic assembly of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and multi-walled carbon nanotubes (MWCNTs) as a biocompatible, electrically conductive, mechanically robust and structurally porous 3D scaffold. To showcase the bioelectronic utilization, a proof-of-concept demonstration of electrically stimulated cell culture under ES is conducted. The ES effects coupled with the 3D scaffold are promising on pheochromocytoma 12 (PC12), a neuronal cell line, and the ES effect on osteogenesis of human adipose-derived stem cells (hASC) was further studied. PC12 cultured on this PEDOT:PSS/MWCNT 3D scaffolds was induced to differentiate toward a more mature neuronal phenotype with the ES treatment. Furthermore, hASC osteogenesis could be highly promoted in this conductive scaffold with ES. Calcium deposition concentration and osteo-differentiated gene markers were significantly higher with ES. The facile assembly of 3D conductive scaffolds sheds light on both platforms for investigating the 3D microenvironment for electrophysiological simulation of cells and tissues under the ES treatment of in vivo tissue engineering.

Experimental-numerical analysis of cell adhesion-mediated electromechanical stimulation on piezoelectric nanofiber scaffolds

Electrospun nanofibers exhibiting piezoelectricity are a specific class of smart materials which could provide electric stimulation to cells in a noninvasive way and contribute to tissue regeneration. During cell-material interaction, the materials display electromechanical behavior by transforming cell adhesion force into surface charge. In the process, how the cell adhesion states and the electromechanical properties of scaffolds determine the actual piezoelectric potential implemented on a cell is still unclear. Herein, we fabricated piezoelectric poly(vinylidene fluoride) (PVDF) nanofiber scaffolds with different topographies, and investigated their influences on cell morphology and cell adhesion-mediated electromechanical stimulation of mesenchymal stem cell (MSC). Our results demonstrated that MSC seeded on aligned piezoelectric nanofibers exhibited elongated morphology combined with higher intracellular calcium activity than those adhered on random nanofibers with rounded shape. The underlying mechanism was further quantitatively analyzed using a three-dimensional (3D) finite element method with respect to cell adhesion states and architecture parameters of nanofiber scaffolds. The results suggested that cell morphology and cell adhesion force influenced the piezoelectric output through modulating the location and magnification of force implemented on the scaffolds. In addition, the change of alignment, pore size and diameter of the nanofiber network could alter the mechanical property of the scaffolds, and then bias the actual piezoelectric output experienced by a cell. These findings provide new insights for probing the mechanism of cell self-stimulation on piezoelectric scaffolds, and pave the way for rational design of piezoelectric scaffolds for cell regulation and tissue regeneration.

Electroactive Smart Materials for Neural Tissue Regeneration

Repair in the human nervous system is a complex and intertwined process that offers significant challenges to its study and comprehension. Taking advantage of the progress in fields such as tissue engineering and regenerative medicine, the scientific community has witnessed a strong increase of biomaterial-based approaches for neural tissue regenerative therapies. Electroactive materials, increasingly being used as sensors and actuators, also find application in neurosciences due to their ability to deliver electrical signals to the cells and tissues. The use of electrical signals for repairing impaired neural tissue therefore presents an interesting and innovative approach to bridge the gap between fundamental research and clinical applications in the next few years. In this review, first a general overview of electroactive materials, their historical origin, and characteristics are presented. Then a comprehensive view of the applications of electroactive smart materials for neural tissue regeneration is presented, with particular focus on the context of spinal cord injury and brain repair. Finally, the major challenges of the field are discussed and the main challenges for the near future presented. Overall, it is concluded that electroactive smart materials play an ever-increasing role in neural tissue regeneration, appearing as potentially valuable biomaterials for regenerative purposes.

In Vitro Cell Interactions on PVDF Films: Effects of Surface Morphology and Polar Phase Transition

In recent years, several studies have validated the use of piezoelectric materials for in situ biological stimulation, opening new interesting insights for bio-electric therapies. In this work, we investigate the morphological properties of polyvinylidene fluoride (PVDF) in the form of microstructured films after temperature-driven phase transition. The work aims to investigate the correlations between morphology at micrometric (i.e., spherulite size) and sub-micrometric (i.e., phase crystallinity) scale and in vitro cell response to validate their use as bio-functional interfaces for cellular studies. Morphological analyses (SEM, AFM) enabled evidence of the peculiar spherulite-like structure and the dependence of surface properties (i.e., intra-/interdomain roughness) upon process conditions (i.e., temperature). Meanwhile, chemical (i.e., FTIR) and thermal (i.e., DSC) analyses highlighted an influence of casting temperature and polymer solution on apolar to polar phases transition, thus affecting in vitro cell response. Accordingly, in vitro tests confirmed the relationship between micro/sub-microstructural properties and hMSC response in terms of adhesion and viability, thus suggesting a promising use of PVDF films to model, in perspective, in vitro functionalities of cells under electrical stimuli upon mechanical solicitation.

Conjugated Polymer for Implantable Electronics toward Clinical Application

Owing to their excellent mechanical flexibility, mixed-conducting electrical property, and extraordinary chemical turnability, conjugated polymers have been demonstrated to be an ideal bioelectronic interface to deliver therapeutic effect in many different chronic diseases. This review article summarizes the latest advances in implantable electronics using conjugated polymers as electroactive materials and identifies remaining challenges and opportunities for developing electronic medicine. Examples of conjugated polymer-based bioelectronic devices are selectively reviewed in human clinical studies or animal studies with the potential for clinical adoption. The unique properties of conjugated polymers are highlighted and exemplified as potential solutions to address the specific challenges in electronic medicine.

Development of Highly Crystalline Polylactic Acid with β-Crystalline Phase from the Induced Alignment of Electrospun Fibers

Polylactic acid (PLA) is one of the known synthetic polymers with potential piezoelectric activity but this property is directly related to both the crystalline structure and crystalline degree. Depending on the process conditions, PLA can crystallize in three different forms: α-, β-, and γ- form, with β-crystalline phase being the piezoelectric one. To obtain this crystalline structure, transformation of α to β is required. To do so, the strategies followed so far consisted in annealing or/and stretching of previously obtained PLA in the form of films or fibers, that is, additional post-processing steps. In this work, we are able to obtain PLA fibers with high macromolecular alignment, as demonstrated by SEM, and in the β polymorph, as detected by X-ray diffraction (XRD) without the requirement of post-processing. For that, PLA fibers were prepared by using an electrospinning coupled to a drum collector. This set up and the optimization of the parameters (voltage flow-rate, and drum collector speed) induced molecular stretching giving rise to uniaxially oriented and highly aligned fibers.

Biomedical Implants with Charge-Transfer Monitoring and Regulating Abilities

Transmembrane charge (ion/electron) transfer is essential for maintaining cellular homeostasis and is involved in many biological processes, from protein synthesis to embryonic development in organisms. Designing implant devices that can detect or regulate cellular transmembrane charge transfer is expected to sense and modulate the behaviors of host cells and tissues. Thus, charge transfer can be regarded as a bridge connecting living systems and human-made implantable devices. This review describes the mode and mechanism of charge transfer between organisms and nonliving materials, and summarizes the strategies to endow implants with charge-transfer regulating or monitoring abilities. Furthermore, three major charge-transfer controlling systems, including wired, self-activated, and stimuli-responsive biomedical implants, as well as the design principles and pivotal materials are systematically elaborated. The clinical challenges and the prospects for future development of these implant devices are also discussed.

Milestones and current achievements in development of multifunctional bioscaffolds for medical application

Tissue engineering (TE) is a rapidly growing interdisciplinary field, which aims to restore or improve lost tissue function. Despite that TE was introduced more than 20 years ago, innovative and more sophisticated trends and technologies point to new challenges and development. Current challenges involve the demand for multifunctional bioscaffolds which can stimulate tissue regrowth by biochemical curves, biomimetic patterns, active agents and proper cell types. For those purposes especially promising are carefully chosen primary cells or stem cells due to its high proliferative and differentiation potential. This review summarized a variety of recently reported advanced bioscaffolds which present new functions by combining polymers, nanomaterials, bioactive agents and cells depending on its desired application. In particular necessity of study biomaterial-cell interactions with in vitro cell culture models, and studies using animals with in vivo systems were discuss to permit the analysis of full material biocompatibility. Although these bioscaffolds have shown a significant therapeutic effect in nervous, cardiovascular and muscle, tissue engineering, there are still many remaining unsolved challenges for scaffolds improvement.

Electrical Stimulation and Conductive Polymers as a Powerful Toolbox for Tailoring Cell Behaviour in vitro

Electrical stimulation (ES) is a well-known method for guiding the behaviour of nerve cells in in vitro systems based on the response of these cells to an electric field. From this perspective, understanding how the electrochemical stimulus can be tuned for the design of a desired cell response is of great importance. Most biomedical studies propose the application of an electrical potential to cell culture arrays while examining the cell response regarding viability, morphology, and gene expression. Conversely, various studies failed to evaluate how the fine physicochemical properties of the materials used for cell culture influence the observed behaviours. Among the various materials used for culturing cells under ES, conductive polymers (CPs) are widely used either in pristine form or in addition to other polymers. CPs themselves do not possess the optimal surface for cell compatibility because of their hydrophobic nature, which leads to poor protein adhesion and, hence, poor bioactivity. Therefore, understanding how to tailor the chemical properties on the material surface will determine the obtention of improved ES platforms. Moreover, the structure of the material, either in a thin film or in porous electrospun scaffolds, also affects the biochemical response and needs to be considered. In this review, we examine how materials based on CPs influence cell behaviour under ES, and we compile the various ES setups and physicochemical properties that affect cell behaviour. This review concerns the culture of various cell types, such as neurons, fibroblasts, osteoblasts, and Schwann cells, and it also covers studies on stem cells prone to ES. To understand the mechanistic behaviour of these devices, we also examine studies presenting a more detailed biomolecular level of interaction. This review aims to guide the design of future ES setups regarding the influence of material properties and electrochemical conditions on the behaviour of in vitro cell studies.

In Vitro Effect of Replicated Porous Polymeric Nano-MicroStructured Biointerfaces Characteristics on Macrophages Behavior

In the last decades, optimizing implant properties in terms of materials and biointerface characteristics represents one of the main quests in biomedical research. Modifying and engineering polyvinylidene fluoride (PVDF) as scaffolds becomes more and more attractive to multiples areas of bio-applications (e.g., bone or cochlear implants). Nevertheless, the acceptance of an implant is affected by its inflammatory potency caused by surface-induced modification. Therefore, in this work, three types of nano-micro squared wells like PVDF structures (i.e., reversed pyramidal shape with depths from 0.8 to 2.5 microns) were obtained by replication, and the influence of their characteristics on the inflammatory response of human macrophages was investigated in vitro. FTIR and X-ray photoelectron spectroscopy analysis confirmed the maintaining chemical structures of the replicated surfaces, while the topographical surface characteristics were evaluated by AFM and SEM analysis. Contact angle and surface energy analysis indicated a modification from superhydrophobicity of casted materials to moderate hydrophobicity based on the structure's depth change. The effects induced by PVDF casted and micron-sized reversed pyramidal replicas on macrophages behavior were evaluated in normal and inflammatory conditions (lipopolysaccharide treatment) using colorimetric, microscopy, and ELISA methods. Our results demonstrate that the depth of the microstructured surface affects the activity of macrophages and that the modification of topography could influence both the hydrophobicity of the surface and the inflammatory response.

Light-induced osteogenic differentiation of BMSCs with graphene/TiO2 composite coating on Ti implant

Light-induced surface potential have been demonstrated as an effective bone marrow mesenchymal stem cells (BMSCs) osteogenic differentiation regulator. However, traditional bone repair implants almost were weak or no light-responsive. Fortunately, surface modification was a feasible strategy to realize its light functionalization for bone implants. Herein, a graphene oxide (GO)/titanium dioxide (TiO₂) nanodots composite coating on the surface of titanium (Ti) implant was constructed, and GO was reduced to reduced graphene oxide (rGO) with the method of UV-assisted photocatalytic reduction. After rGO deposited on the surface of TiO₂, a heterojunction formed at the interface of rGO and TiO₂. With visible light illumination, positive charges accumulated on the surface of rGO/TiO₂ film, and performed as a positive surface potential change. The light-induced surface potential which was generated under proper light intensity is harmless to the cell adhesion and proliferation behavior, but presented a good BMSCs osteogenic differentiation promoting effect, and the activation of the voltage-gated calcium channels through surface potential and the promotion of the adsorption of osteogenic growth factors could be the reason. This work given a new insight of the modification for Ti implant with a light-induced surface potential, and shows potential application for bone regeneration on the clinical practice through light stimulation.

Integrating Emerging Polymer Chemistries for the Advancement of Recyclable, Biodegradable, and Biocompatible Electronics

Through advances in molecular design, understanding of processing parameters, and development of non-traditional device fabrication techniques, the field of wearable and implantable skin-inspired devices is rapidly growing interest in the consumer market. Like previous technological advances, economic growth and efficiency is anticipated, as these devices will enable an augmented level of interaction between humans and the environment. However, the parallel growing electronic waste that is yet to be addressed has already left an adverse impact on the environment and human health. Looking forward, it is imperative to develop both human- and environmentally-friendly electronics, which are contingent on emerging recyclable, biodegradable, and biocompatible polymer technologies. This review provides definitions for recyclable, biodegradable, and biocompatible polymers based on reported literature, an overview of the analytical techniques used to characterize mechanical and chemical property changes, and standard policies for real-life applications. Then, various strategies in designing the next-generation of polymers to be recyclable, biodegradable, or biocompatible with enhanced functionalities relative to traditional or commercial polymers are discussed. Finally, electronics that exhibit an element of recyclability, biodegradability, or biocompatibility with new molecular design are highlighted with the anticipation of integrating emerging polymer chemistries into future electronic devices.

Recent advances of polymer-based piezoelectric composites for biomedical applications

Over the past decades, electronics have become central to many aspects of biomedicine and wearable device technologies as a promising personalized healthcare platform. Lead-free piezoelectric materials for converting mechanical into electrical energy through piezoelectric transduction are of significant value in a diverse range of technological applications. Organic piezoelectric biomaterials have attracted widespread attention as the functional materials in the biomedical devices due to their advantages of excellent biocompatibility. They include synthetic and biological polymers. Many biopolymers have been discovered to possess piezoelectricity in an appreciable amount, however their investigation is still preliminary. Due to their piezoelectric properties, better known synthetic fluorinated polymers have been intensively investigated and applied in biomedical applications including controlled drug delivery systems, tissue engineering, microfluidic and artificial muscle actuators, among others. Piezoelectric polymers, especially poly (vinylidene fluoride) (PVDF) and its copolymers are increasingly receiving interest as smart biomaterials due to their ability to convert physiological movements to electrical signals when in a controllable and reproducible manner. Despite possessing the greatest piezoelectric coefficients among all piezoelectric polymers, it is often desirable to increase the electrical outputs. The most promising routes toward significant improvements in the piezoelectric response and energy-harvesting performance of such materials is loading them with various inorganic nanofillers and/or applying some modification during the fabrication process. This paper offers a comprehensive review of the principles, properties, and applications of organic piezoelectric biomaterials (polymers and polymer/ceramic composites) with special attention on PVDF-based polymers and their composites in sensors, drug delivery and tissue engineering. Subsequently focuses on the most common fabrication routes to produce piezoelectric scaffolds, tissue and sensors which is electrospinning process. Promising upcoming strategies and new piezoelectric materials and fabrication techniques for these applications are presented to enable a future integration among these applications.

The influence of reduced graphene oxide on stem cells: a perspective in peripheral nerve regeneration

Graphene and its derivatives are fascinating materials for their extraordinary electrochemical and mechanical properties. In recent decades, many researchers explored their applications in tissue engineering and regenerative medicine. Reduced graphene oxide (rGO) possesses remarkable structural and functional resemblance to graphene, although some residual oxygen-containing groups and defects exist in the structure. Such structure holds great potential since the remnant-oxygenated groups can further be functionalized or modified. Moreover, oxygen-containing groups can improve the dispersion of rGO in organic or aqueous media. Therefore, it is preferable to utilize rGO in the production of composite materials. The rGO composite scaffolds provide favorable extracellular microenvironment and affect the cellular behavior of cultured cells in the peripheral nerve regeneration. On the one hand, rGO impacts on Schwann cells and neurons which are major components of peripheral nerves. On the other hand, rGO-incorporated composite scaffolds promote the neurogenic differentiation of several stem cells, including embryonic stem cells, mesenchymal stem cells, adipose-derived stem cells and neural stem cells. This review will briefly introduce the production and major properties of rGO, and its potential in modulating the cellular behaviors of specific stem cells. Finally, we present its emerging roles in the production of composite scaffolds for nerve tissue engineering.

Injectable muscle-adhesive antioxidant conductive photothermal bioactive nanomatrix for efficiently promoting full-thickness skeletal muscle regeneration

The completed skeletal muscle regeneration resulted from severe injury and muscle-related disease is still a challenge. Here, we developed an injectable muscle-adhesive antioxidant conductive bioactive photothermo-responsive nanomatrix for regulating the myogenic differentiation and promoting the skeletal muscle regeneration in vivo. The multifunctional nanomatrix was composed of polypyrrole@polydopamine (PPy@PDA, 342 ± 5.6 nm) nanoparticles-crosslinked Pluronic F-127 (F127)-polycitrate matrix (FPCP). The FPCP nanomatrix demonstrated inherent multifunctional properties including excellent photothermo-responsive and shear-thinning behavior, muscle-adhesive feature, injectable ability, electronic conductivity (0.48 ± 0.03 S/m) and antioxidant activity and photothermal function. The FPCP nanomatrix displayed better photothermal performance with near-infrared irradiation, which could provide the photo-controlled release of protein (91% ± 2.6% of BSA was released after irradiated 3 times). Additionally, FPCP nanomatrix could significantly enhance the cell proliferation and myogenic differentiation of mouse myoblast cells (C2C12) by promoting the expressions of myogenic genes (MyoD and MyoG) and myosin heavy chain (MHC) protein with negligible cytotoxicity. Based on the multifunctional properties, FPCP nanomatrix efficiently promoted the full-thickness skeletal muscle repair and regeneration in vivo, through stimulating the angiogenesis and myotube formation. This study firstly indicated the vital role of multifunctional PPy@PDA nanoparticles in regulating myogenic differentiation and skeletal muscle regeneration. This work also suggests that rational design of bioactive matrix with multifunctional feature would greatly enhance the development of regenerative medicine.

Electroactive Polymeric Composites to Mimic the Electromechanical Properties of Myocardium in Cardiac Tissue Repair

Due to the limited regenerative capabilities of cardiomyocytes, incidents of myocardial infarction can cause permanent damage to native myocardium through the formation of acellular, non-conductive scar tissue during wound repair. The generation of scar tissue in the myocardium compromises the biomechanical and electrical properties of the heart which can lead to further cardiac problems including heart failure. Currently, patients suffering from cardiac failure due to scarring undergo transplantation but limited donor availability and complications (i.e., rejection or infectious pathogens) exclude many individuals from successful transplant. Polymeric tissue engineering scaffolds provide an alternative approach to restore normal myocardium structure and function after damage by acting as a provisional matrix to support cell attachment, infiltration and stem cell delivery. However, issues associated with mechanical property mismatch and the limited electrical conductivity of these constructs when compared to native myocardium reduces their clinical applicability. Therefore, composite polymeric scaffolds with conductive reinforcement components (i.e., metal, carbon, or conductive polymers) provide tunable mechanical and electroactive properties to mimic the structure and function of natural myocardium in force transmission and electrical stimulation. This review summarizes recent advancements in the design, synthesis, and implementation of electroactive polymeric composites to better match the biomechanical and electrical properties of myocardial tissue.

Composites made of polyorganophosphazene and carbon nanotube up-regulating osteogenic activity of BMSCs under electrical stimulation

Bone is an electrically responsive tissue, so electroactive materials that can deliver electrical cues to bone are helpful for enhancing regeneration under electrical stimulation (ES), and conductive materials are crucial in ES transmission to determine osteogenesis. Compared with polyesters, biodegradable polyorganophosphazenes (POPPs) show superiority in the field of bone tissue engineering thanks to their rich phosphorus/nitrogen contents, suggesting that the combination of POPPs-based conductive substrates with ES may achieve synergistic enhancements on osteogenesis. Herein, conductive composite films were fabricated by blending poly[(alanine ethyl ester)-(glycine ethyl ester)]phosphazene (PAGP) with carbon nanotubes (CNTs). After surface modification with polydopamine (PDA), bone marrow mesenchymal stromal cells (BMSCs) were cultured on the films under ES, using the cells cultured on conductive films composed of poly(L-lactide) (PLLA) and CNTs as controls. The BMSCs on PAGP/CNT films demonstrated significantly faster proliferation rates and stronger osteogenic differentiation potentials than those on PLLA/CNT films, while cell attachments on the two PDA-coated substrates were similar. Under appropriate ES, further increases in the expressions of osteogenic markers as alkaline phosphatase, collagen I and calcium deposition were identified in comparison with the cases without ES. The contributions of the osteocompatible POPPs, the substrate conductivity and the ES treatment to enhanced osteogenesis suggested new strategies for the design of bone repair materials.

Micropatterned Silk-Fibroin/Eumelanin Composite Films for Bioelectronic Applications

There has been growing interest in the use of natural bionanomaterials and nanostructured systems for diverse biomedical applications. Such materials can confer unique functional properties as well as address concerns pertaining to sustainability in production. In this work, we propose the biofabrication of micropatterned silk fibroin/eumelanin composite thin films to be used in electroactive and bioactive applications in bioelectronics and biomedical engineering. Eumelanin is the most common form of melanin, naturally derived from the ink of cuttlefish, having antioxidant and electroactive properties. Another natural biomaterial, the protein silk fibroin, is modified with photoreactive chemical groups, which allows the formation of electroactive eumelanin thin films with different microstructures. The silk fibroin/eumelanin composites are fabricated to obtain thin films as well as electroactive microstructures using UV curing. Here, we report for the first time the preparation, characterization, and physical, electrochemical, and biological properties of these natural silk fibroin/eumelanin composite films. Higher concentrations of eumelanin incorporated into the films exhibit a higher charge storage capacity and good electroactivity even after 100 redox cycles. In addition, the microscale structure and the cellular activity of the fibroin/eumelanin films are assessed for understanding of the biological properties of the composite. The developed micropatterned fibroin/eumelanin films can be applied as natural electroactive substrates for bioapplications (e.g., bioelectronics, sensing, and theranostics) because of their biocompatible properties.

Advances in Molecularly Imprinted Polymers Based Affinity Sensors (Review)

Recent challenges in biomedical diagnostics show that the development of rapid affinity sensors is very important issue. Therefore, in this review we are aiming to outline the most important directions of affinity sensors where polymer-based semiconducting materials are applied. Progress in formation and development of such materials is overviewed and discussed. Some applicability aspects of conducting polymers in the design of affinity sensors are presented. The main attention is focused on bioanalytical application of conducting polymers such as polypyrrole, polyaniline, polythiophene and poly(3,4-ethylenedioxythiophene) ortho-phenylenediamine. In addition, some other polymers and inorganic materials that are suitable for molecular imprinting technology are also overviewed. Polymerization techniques, which are the most suitable for the development of composite structures suitable for affinity sensors are presented. Analytical signal transduction methods applied in affinity sensors based on polymer-based semiconducting materials are discussed. In this review the most attention is focused on the development and application of molecularly imprinted polymer-based structures, which can replace antibodies, receptors, and many others expensive affinity reagents. The applicability of electrochromic polymers in affinity sensor design is envisaged. Sufficient biocompatibility of some conducting polymers enables to apply them as "stealth coatings" in the future implantable affinity-sensors. Some new perspectives and trends in analytical application of polymer-based semiconducting materials are highlighted.

Direct Current Stimulation for Improved Osteogenesis of MC3T3 Cells Using Mineralized Conductive Polyaniline

Hydroxyapatites (HAPs) are usually coated on the surface of an implant to improve the osseointegration with defect bone tissue. Besides, conducting polymers have the advantages of good conductivity, reasonable biocompatibility, and easy of modification, which endow them applicable to electrical stimulation therapy. However, it still remains a great challenge to fabricate hybrid coating combing HAP with conducting polymer on implant surface efficiently. In this work, phytic acid-doped polyaniline (PANI) were successfully synthesized on medical titanium (Ti) sheets. By virtue of the abundant anodic phosphoric groups of phytic acid, HAP nanocrystals were biomineralized on PANI. The PANI-HAP hybrid layer exhibits good cell compatibility with MC3T3 cells. More importantly, HAP nanocrystals and PANI operate synergistically on cell proliferation and osteogenesis through electrical stimulation. Alkaline phosphatase activity and extracellular calcium contents of cells on PANI-HAP display 3-fold and 2.6-fold increases, compared with bare Ti sheets, respectively. The valid integration of mineralization and electrical stimulation in this work renders an efficient strategy for implant coating, which might have potential applications in bone-related defects.

Endogenous Electric Signaling as a Blueprint for Conductive Materials in Tissue Engineering

Bioelectricity plays an important role in cell behavior and tissue modulation, but is understudied in tissue engineering research. Endogenous electrical signaling arises from the transmembrane potential inherent to all cells and contributes to many cell behaviors, including migration, adhesion, proliferation, and differentiation. Electrical signals are also involved in tissue development and repair. Synthetic and natural conductive materials are under investigation for leveraging endogenous electrical signaling cues in tissue engineering applications due to their ability to direct cell differentiation, aid in maturing electroactive cell types, and promote tissue functionality. In this review, we provide a brief overview of bioelectricity and its impact on cell behavior, report recent literature using conductive materials for tissue engineering, and discuss opportunities within the field to improve experimental design when using conductive substrates.

Graphene-Based Scaffolds for Regenerative Medicine

Leading-edge regenerative medicine can take advantage of improved knowledge of key roles played, both in stem cell fate determination and in cell growth/differentiation, by mechano-transduction and other physicochemical stimuli from the tissue environment. This prompted advanced nanomaterials research to provide tissue engineers with next-generation scaffolds consisting of smart nanocomposites and/or hydrogels with nanofillers, where balanced combinations of specific matrices and nanomaterials can mediate and finely tune such stimuli and cues. In this review, we focus on graphene-based nanomaterials as, in addition to modulating nanotopography, elastic modulus and viscoelastic features of the scaffold, they can also regulate its conductivity. This feature is crucial to the determination and differentiation of some cell lineages and is of special interest to neural regenerative medicine. Hereafter we depict relevant properties of such nanofillers, illustrate how problems related to their eventual cytotoxicity are solved via enhanced synthesis, purification and derivatization protocols, and finally provide examples of successful applications in regenerative medicine on a number of tissues.

Tunable Substrate Functionalities Direct Stem Cell Fate toward Electrophysiologically Distinguishable Neuron-like and Glial-like Cells

Engineering cellular microenvironment on a functional platform using various biophysical cues to modulate stem cell fate has been the central theme in regenerative engineering. Among the various biophysical cues to direct stem cell differentiation, the critical role of physiologically relevant electric field (EF) stimulation was established in the recent past. The present study is the first to report the strategy to switch EF-mediated differentiation of human mesenchymal stem cells (hMSCs) between neuronal and glial pathways, using tailored functional properties of the biomaterial substrate. We have examined the combinatorial effect of substrate functionalities (conductivity, electroactivity, and topography) on the EF-mediated stem cell differentiation on polyvinylidene-difluoride (PVDF) nanocomposites in vitro, without any biochemical inducers. The functionalities of PVDF have been tailored using conducting nanofiller (multiwall-carbon nanotube, MWNT) and piezoceramic (BaTiO₃, BT) by an optimized processing approach (melt mixing-compression molding-rolling). The DC conductivity of PVDF nanocomposites was tuned from ∼10^-11 to ∼10^-4 S/cm and the dielectric constant from ∼10 to ∼300. The phenotypical changes and genotypical expression of hMSCs revealed the signatures of early differentiation toward neuronal pathway on rolled-PVDF/MWNT and late differentiation toward glial lineage on rolled-PVDF/BT/MWNT. Moreover, we were able to distinguish the physiological properties of differentiated neuron-like and glial-like cells using membrane depolarization and mechanical stimulation. The excitability of the EF-stimulated hMSCs was also determined using whole-cell patch-clamp recordings. Mechanistically, the roles of intracellular reactive oxygen species (ROS), Ca²⁺ oscillations, and synaptic and gap junction proteins in directing the cellular fate have been established. Therefore, the present work critically unveils complex yet synergistic interaction of substrate functional properties to direct EF-mediated differentiation toward neuron-like and glial-like cells, with distinguishable electrophysiological responses.

In vivo study of conductive 3D printed PCL/MWCNTs scaffolds with electrical stimulation for bone tissue engineering

Critical bone defects are considered one of the major clinical challenges in reconstructive bone surgery. The combination of 3D printed conductive scaffolds and exogenous electrical stimulation (ES) is a potential favorable approach for bone tissue repair. In this study, 3D conductive scaffolds made with biocompatible and biodegradable polycaprolactone (PCL) and multi-walled carbon nanotubes (MWCNTs) were produced using the extrusion-based additive manufacturing to treat large calvary bone defects in rats. Histology results show that the use of PCL/MWCNTs scaffolds and ES contributes to thicker and increased bone tissue formation within the bone defect. Angiogenesis and mineralization are also significantly promoted using high concentration of MWCNTs (3 wt%) and ES. Moreover, scaffolds favor the tartrate-resistant acid phosphatase (TRAP) positive cell formation, while the addition of MWCNTs seems to inhibit the osteoclastogenesis but present limited effects on the osteoclast functionalities (receptor activator of nuclear factor κβ ligand (RANKL) and osteoprotegerin (OPG) expressions). The use of ES promotes the osteoclastogenesis and RANKL expressions, showing a dominant effect in the bone remodeling process. These results indicate that the combination of 3D printed conductive PCL/MWCNTs scaffold and ES is a promising strategy to treat critical bone defects and provide a cue to establish an optimal protocol to use conductive scaffolds and ES for bone tissue engineering.

Theoretical analysis of the electrochemical systems used for the application of direct current/voltage stimuli on cell cultures

Endogenous electric fields drive many essential functions relating to cell proliferation, motion, differentiation and tissue development. They are usually mimicked in vitro by using electrochemical systems to apply direct current or voltage stimuli to cell cultures. The many studies devoted to this topic have given rise to a wide variety of experimental systems, whose results are often difficult to compare. Here, these systems are analysed from an electrochemical standpoint to help harmonize protocols and facilitate optimal understanding of the data produced. The theoretical analysis of single-electrode systems shows the necessity of measuring the Nernst potential of the electrode and of discussing the results on this basis rather than using the value of the potential gradient. The paper then emphasizes the great complexity that can arise when high cell voltage is applied to a single electrode, because of the possible occurrence of anode and cathode sites. An analysis of two-electrode systems leads to the advice to change experimental practices by applying current instead of voltage. It also suggests that the values of electric fields reported so far may have been considerably overestimated in macro-sized devices. It would consequently be wise to revisit this area by testing considerably lower electric field values.

Editing the Shape Morphing of Monocomponent Natural Polysaccharide Hydrogel Films

Shape-morphing hydrogels can be widely used to develop artificial muscles, reconfigurable biodevices, and soft robotics. However, conventional approaches for developing shape-morphing hydrogels highly rely on composite materials or complex manufacturing techniques, which limit their practical applications. Herein, we develop an unprecedented strategy to edit the shape morphing of monocomponent natural polysaccharide hydrogel films via integrating gradient cross-linking density and geometry effect. Owing to the synergistic effect, the shape morphing of chitosan (CS) hydrogel films with gradient cross-linking density can be facilely edited by changing their geometries (length-to-width ratios or thicknesses). Therefore, helix, short-side rolling, and long-side rolling can be easily customized. Furthermore, various complex artificial 3D deformations such as artificial claw, horn, and flower can also be obtained by combining various flat CS hydrogel films with different geometries into one system, which can further demonstrate various shape transformations as triggered by pH. This work offers a simple strategy to construct a monocomponent hydrogel with geometry-directing programmable deformations, which provides universal insights into the design of shape-morphing polymers and will promote their applications in biodevices and soft robotics.

Stimuli-Responsive Biomaterials: Scaffolds for Stem Cell Control

Stem cell fate is closely intertwined with microenvironmental and endogenous cues within the body. Recapitulating this dynamic environment ex vivo can be achieved through engineered biomaterials which can respond to exogenous stimulation (including light, electrical stimulation, ultrasound, and magnetic fields) to deliver temporal and spatial cues to stem cells. These stimuli-responsive biomaterials can be integrated into scaffolds to investigate stem cell response in vitro and in vivo, and offer many pathways of cellular manipulation: biochemical cues, scaffold property changes, drug release, mechanical stress, and electrical signaling. The aim of this review is to assess and discuss the current state of exogenous stimuli-responsive biomaterials, and their application in multipotent stem cell control. Future perspectives in utilizing these biomaterials for personalized tissue engineering and directing organoid models are also discussed.

Composite Ferroelectric Membranes Based on Vinylidene Fluoride-Tetrafluoroethylene Copolymer and Polyvinylpyrrolidone for Wound Healing

Wound healing is a complex process and an ongoing challenge for modern medicine. Herein, we present the results of study of structure and properties of ferroelectric composite polymer membranes for wound healing. Membranes were fabricated by electrospinning from a solution of vinylidene fluoride/tetrafluoroethylene copolymer (VDF-TeFE) and polyvinylpyrrolidone (PVP) in dimethylformamide (DMF). The effects of the PVP content on the viscosity and conductivity of the spinning solution, DMF concentration, chemical composition, crystal structure, and conformation of VDF-TeFE macromolecules in the fabricated materials were studied. It was found that as PVP amount increased, the viscosity and conductivity of the spinning solutions decreased, resulting in thinner fibers. Using FTIR and XRD methods, it was shown that if the PVP content was lower than 50 wt %, the VDF-TeFE copolymer adopted a flat zigzag conformation (TTT conformation) and crystalline phases with ferroelectric properties were formed. Gas chromatography results indicated that an increase in the PVP concentration led to a higher residual amount of DMF in the material, causing cytotoxic effects on 3T3L1 fibroblasts. In vivo studies demonstrated that compared to classical gauze dressings impregnated with a solution of an antibacterial agent, ferroelectric composite membranes with 15 wt % PVP provided better conditions for the healing of purulent wounds.

Computational study on electromechanics of electroactive hydrogels for cartilage-tissue repair

BACKGROUND AND OBJECTIVE

The self-repair capability of articular cartilage is limited because of non-vascularization and low turnover of its extracellular matrix. Regenerating hyaline cartilage remains a significant clinical challenge as most non-surgical and surgical treatments provide only mid-term relief. Eventually, further pain and mobility loss occur for many patients in the long run due to further joint deterioration. Repair of articular cartilage tissue using electroactive scaffolds and biophysical stimuli like electrical and osmotic stimulation may have the potential to heal cartilage defects occurring due to trauma, osteoarthritis, or sport-related injuries. Therefore, the focus of the current study is to present a computational model of electroactive hydrogels for the cartilage-tissue repair as a first step towards an optimized experimental design.

METHODS

The multiphysics transport model that mainly includes the Poisson-Nernst-Planck equations and the mechanical equation is used to find the electrical stimulation response of the polyelectrolyte hydrogels. Based upon this, a numerical model on electromechanics of electroactive hydrogels seeded with chondrocytes is presented employing the open-source software FEniCS, which is a Python library for finite-element analysis.

RESULTS

We analyzed the ionic concentrations and electric potential in a hydrogel sample and the cell culture medium, the osmotic pressure created due to ionic concentration variations and the resulting hydrogel displacement. The proposed mathematical model was validated with examples from literature.

CONCLUSIONS

The presented model for the electrical and osmotic stimulation of a hydrogel sample can serve as a useful tool for the development and analysis of a cartilaginous scaffold employing electrical stimulation. By analyzing various parameters, we pave the way for future research on a finer scale using open-source software.

Engineering an Injectable Electroactive Nanohybrid Hydrogel for Boosting Peripheral Nerve Growth and Myelination in Combination with Electrical Stimulation

Electrical stimulation (ES) can be used to manipulate recovery after peripheral nerve injuries. Although biomaterial-based strategies have already been implemented to gain momentum for ES and engineer permissive microenvironments for neural regeneration, the development of biomaterials for specific stimuli-responsive modulation of neural cell properties remains a challenge. Herein, we homogeneously incorporate pristine carbon nanotubes into a functional self-assembling peptide to prepare a hybrid hydrogel with good injectability and conductivity. Two-dimensional (on the surface) and three-dimensional (within the hybrid hydrogel) culturing experiments demonstrate that ES promotes axon outgrowth and Schwann cell (SC) migration away from dorsal root ganglia spheres, further revealing that ES-enhanced interactions between SCs and axons result in improved myelination. Thus, our study not only advances the development of tailor-made materials but also provides useful insights into comprehensive approaches for promoting nerve growth and presents a practical strategy of repairing peripheral nerve injuries.

Gadolinium-Doped BTO-Functionalized Nanocomposites with Enhanced MRI and X-ray Dual Imaging to Simulate the Electrical Properties of Bone

Physicochemical properties of biomaterials play a regulatory role in osteoblast proliferation and differentiation. Inspired by the electrical properties of natural bone, the electroactive composites applied to osteogenesis have gradually become the hotspot of research. In this work, an electroactive biocomposite of poly(lactic-co-glycolic acid) mixed with gadolinium-doped barium titanate nanoparticles (Gd-BTO NPs) was investigated to establish the structure-activity relationship between electrical property, especially surface potential, and osteogenic activity. Furthermore, the potential mechanism was also explored. The results showed that the introduction of Gd-BTO NPs was more conducive to improve the elastic modulus and beneficial to utilize MRI and X-ray dual imaging. The electrical characteristics of composites indicate that the introduction of Gd-BTO NPs can effectively improve the electrical properties of materials including dielectricity, piezoelectricity, and surface potential. Moreover, adjusting the amount of gadolinium doping could optimize electrical activity and enhance MRI compatibility. The surface potential of 0.2Gd-BTO/PLGA could reach -58.2 to -60.9 mV at pH values from 7 to 9. Functional studies on cells revealed that the negative surface potential of poled Gd-BTO/PLGA enhanced cell attachment and osteogenic differentiation significantly. Furthermore, the negative surface potential could induce intracellular Ca²⁺ ion concentration oscillation and improve osteogenic differentiation via the calcineurin/NFAT signal pathway. These findings suggest that electroactive Gd-BTO/PLGA nanocomposites may have huge potential for bone regeneration and be expected to have wide applications in the field of bone tissue engineering.

Mimicking the electrophysiological microenvironment of bone tissue using electroactive materials to promote its regeneration

The process of bone tissue repair and regeneration is complex and requires a variety of physiological signals, including biochemical, electrical and mechanical signals, which collaborate to ensure functional recovery. The inherent piezoelectric properties of bone tissues can convert mechanical stimulation into electrical effects, which play significant roles in bone maturation, remodeling and reconstruction. Electroactive materials, including conductive materials, piezoelectric materials and electret materials, can simulate the physiological and electrical microenvironment of bone tissue, thereby promoting bone regeneration and reconstruction. In this paper, the structures and performances of different types of electroactive materials and their applications in the field of bone repair and regeneration are reviewed, particularly by providing the results from in vivo evaluations using various animal models. Their advantages and disadvantages as bone repair materials are discussed, and the methods for tuning their performances are also described, with the aim of providing an up-to-date account of the proposed topics.

Numerical Simulations as Means for Tailoring Electrically Conductive Hydrogels Towards Cartilage Tissue Engineering by Electrical Stimulation

Cartilage regeneration is a clinical challenge. In recent years, hydrogels have emerged as implantable scaffolds in cartilage tissue engineering. Similarly, electrical stimulation has been employed to improve matrix synthesis of cartilage cells, and thus to foster engineering and regeneration of cartilage tissue. The combination of hydrogels and electrical stimulation may pave the way for new clinical treatment of cartilage lesions. To find the optimal electric properties of hydrogels, theoretical considerations and corresponding numerical simulations are needed to identify well-suited initial parameters for experimental studies. We present the theoretical analysis of a hydrogel in a frequently used electrical stimulation device for cartilage regeneration and tissue engineering. By means of equivalent circuits, finite element analysis, and uncertainty quantification, we elucidate the influence of the geometric and dielectric properties of cell-seeded hydrogels on the capacitive-coupling electrical field stimulation. Moreover, we discuss the possibility of cellular organisation inside the hydrogel due to forces generated by the external electric field. The introduced methodology is easily reusable by other researchers and allows to directly develop novel electrical stimulation study designs. Thus, this study paves the way for the design of future experimental studies using electrically conductive hydrogels and electrical stimulation for tissue engineering.

Bioactive biodegradable polycitrate nanoclusters enhances the myoblast differentiation and in vivo skeletal muscle regeneration via p38 MAPK signaling pathway

Complete skeletal muscle repair and regeneration due to severe large injury or disease is still a challenge. Biochemical cues are critical to control myoblast cell function and can be utilized to develop smart biomaterials for skeletal muscle engineering. Citric acid-based biodegradable polymers have received much attention on tissue engineering, however, their regulation on myoblast cell differentiation and mechanism was few investigated. Here, we find that citrate-based polycitrate-polyethylene glycol-polyethylenimine (POCG-PEI600) nanoclusters can significantly enhance the in vitro myoblast proliferation by probably reinforcing the mitochondrial number, promote the myotube formation and full-thickness skeletal muscle regeneration in vivo by activating the myogenic biomarker genes expression of Myod and Mhc. POCG-PEI600 nanoclusters could also promote the phosphorylation of p38 in MAP kinases (MAPK) signaling pathway, which led to the promotion of the myoblast differentiation. The in vivo skeletal muscle loss rat model also confirmed that POCG-PEI600 nanoclusters could significantly improve the angiogenesis, myofibers formation and complete skeletal muscle regeneration. POCG-PEI600 nanocluster could be also biodegraded into small molecules and eliminated in vivo, suggesting their high biocompatibility and biosafety. This study could provide a bioactive biomaterial-based strategy to repair and regenerate skeletal muscle tissue.

In-situ electromechanical testing and loading system for dynamic cell-biomaterial interaction study

The mechanical and electrical properties of biomaterials are essential in cell function regulation during cell-biomaterial interaction. However, previous studies focused on probing cell regulation mechanisms under one type of stimulus, and a platform that enables the study of electromechanical coupling effects of a biomaterial on cells is still lacking. Here, we present an in-situ electromechanical testing and loading system to image live cells when co-cultured with electroactive biomaterials. The system can provide accurate and repeatable stretch on biomaterials and cells to mimic in vivo tension microenvironment. Besides, the integrated displacement transducer, force sensor, and electrical signal detector enable the real time detection of electromechanical signals on electroactive biomaterials under various stretch loading. Combined with a microscope, live cell imaging can be realized to probe cell behavior. The feasibility of the system is validated by culturing mesenchymal stem cells on piezoelectric nanofiber and conductive hydrogel. Experiment results show the device as a reliable and accurate tool to investigate electromechanical properties of biomaterials and probe essential features of live cells. Our system provides a way to correlate cell behavior with electromechanical cues directly and is useful for exploration of cell function during cell-biomaterial interaction.

Mussel-Inspired Redox-Active and Hydrophilic Conductive Polymer Nanoparticles for Adhesive Hydrogel Bioelectronics

Conductive polymers (CPs) are generally insoluble, and developing hydrophilic CPs is significant to broaden the applications of CPs. In this work, a mussel-inspired strategy was proposed to construct hydrophilic CP nanoparticles (CP NPs), while endowing the CP NPs with redox activity and biocompatibility. This is a universal strategy applicable for a series of CPs, including polyaniline, polypyrrole, and poly(3,4-ethylenedioxythiophene). The catechol/quinone contained sulfonated lignin (LS) was doped into various CPs to form CP/LS NPs with hydrophilicity, conductivity, and redox activity. These CP/LS NPs were used as versatile nanofillers to prepare the conductive hydrogels with long-term adhesiveness. The CP/LS NPs-incorporated hydrogels have a good conductivity because of the uniform distribution of the hydrophilic NPs in the hydrogel network, forming a well-connected electric path. The hydrogel exhibits long-term adhesiveness, which is attributed to the mussel-inspired dynamic redox balance of catechol/quinone groups on the CP/LS NPs. This conductive and adhesive hydrogel shows good electroactivity and biocompatibility and therefore has broad applications in electrostimulation of tissue regeneration and implantable bioelectronics.

Stimuli-Responsive Materials for Tissue Engineering and Drug Delivery

Smart or stimuli-responsive materials are an emerging class of materials used for tissue engineering and drug delivery. A variety of stimuli (including temperature, pH, redox-state, light, and magnet fields) are being investigated for their potential to change a material's properties, interactions, structure, and/or dimensions. The specificity of stimuli response, and ability to respond to endogenous cues inherently present in living systems provide possibilities to develop novel tissue engineering and drug delivery strategies (for example materials composed of stimuli responsive polymers that self-assemble or undergo phase transitions or morphology transformations). Herein, smart materials as controlled drug release vehicles for tissue engineering are described, highlighting their potential for the delivery of precise quantities of drugs at specific locations and times promoting the controlled repair or remodeling of tissues.

Significant advancements of 4D printing in the field of orthopaedics

Researchers, engineers and doctors are continuously focusing on the development of orthopaedics parts characterised by the required responses. So, advanced manufacturing technologies are introduced to fulfil various previously faced challenges. 4D printing provides rapid development with its capability of customization of smart orthopaedics implants and appropriate surgical procedure. This technology opens up the making of innovative, adaptable internal splints, stents, replacement of tissues and organs. Thus, to write this review based article, relevant papers on 4D printing in medical/orthopaedics and smart materials are identified and studied. 4D printed parts show the capability of shape-changing and self-assembly to perform the required functions, which otherwise manufactured parts are not providing. Smart orthopaedics implants are used for spinal deformities, fracture fixation, joint, knee replacement and other related orthopaedics applications. This paper briefs about the 4D printing technology with its major benefits for orthopaedics applications. Today various smart materials are available, which could be used as raw material in 4D printing, and we have discussed capabilities of some of them. Due to the ability of shape-changing, smart implants can change their shape after being implanted in the patient body. Finally, twelve significant advancements of 4D printing in the field of orthopaedics are identified and briefly provided. Thus, 4D printing help to provide a significant effect on personalised treatments.

Electric Conductivity on Aligned Nanofibers Facilitates the Transdifferentiation of Mesenchymal Stem Cells into Schwann Cells and Regeneration of Injured Peripheral Nerve

Schwann cells (SCs) are the most promising seed cells for peripheral nerve tissue engineering, but clinical applications are limited by the lack of cell sources. Existing data demonstrate that bone marrow mesenchymal stem cells (BMSCs) can be induced to differentiate into Schwann-like cells and aligned nanofibers can enhance the differentiation. Considering that SCs are living along with the electrical conductive axons, it is hypothesized that conductivity properties may play roles in SCs differentiation and then facilitate nerve regeneration. To verify this hypothesis, amine functionalized multi-walled carbon nanotubes (MWCNTs) are incorporated with polycaprolactone and gelatin to fabricate aligned or random conductive nanofibers by electrospinning. Current data demonstrate that MWCNTs can dramatically increase the electrical conductive properties but do not alter the biocompatibility of the nanofibers. It is found that endowing conductive properties into the aligned nanofibers can significantly enhance their capability to promote the SCs differentiation. Furthermore, the aligned and conductive nanofibers with induced BMSCs can dramatically promote peripheral axonal regeneration. Collectively, the present study demonstrates that the conductive properties in the aligned nanofiber plays significant roles in SCs differentiation and the aligned and conductive nanofibers can be used as a promising scaffold for SCs differentiation and peripheral nerve tissue engineering.

Poly-l-Lactic Acid Nanotubes as Soft Piezoelectric Interfaces for Biology: Controlling Cell Attachment via Polymer Crystallinity

It has become increasingly evident that the mechanical and electrical environment of a cell is crucial in determining its function and the subsequent behavior of multicellular systems. Platforms through which cells can directly interface with mechanical and electrical stimuli are therefore of great interest. Piezoelectric materials are attractive in this context because of their ability to interconvert mechanical and electrical energy, and piezoelectric nanomaterials, in particular, are ideal candidates for tools within mechanobiology, given their ability to both detect and apply small forces on a length scale that is compatible with cellular dimensions. The choice of piezoelectric material is crucial to ensure compatibility with cells under investigation, both in terms of stiffness and biocompatibility. Here, we show that poly-l-lactic acid nanotubes, grown using a melt-press template wetting technique, can provide a "soft" piezoelectric interface onto which human dermal fibroblasts readily attach. Interestingly, by controlling the crystallinity of the nanotubes, the level of attachment can be regulated. In this work, we provide detailed nanoscale characterization of these nanotubes to show how differences in stiffness, surface potential, and piezoelectric activity of these nanotubes result in differences in cellular behavior.

Synthesis and 3D Printing of Conducting Alginate-Polypyrrole Ionomers

Hydrogels composed of calcium cross-linked alginate are under investigation as bioinks for tissue engineering scaffolds due to their variable viscoelasticity, biocompatibility, and erodibility. Here, pyrrole was oxidatively polymerized in the presence of sodium alginate solutions to form ionomeric composites of various compositions. The IR spectroscopy shows that mild base is required to prevent the oxidant from attacking the alginate during the polymerization reaction. The resulting composites were isolated as dried thin films or cross-linked hydrogels and aerogels. The products were characterized by elemental analysis to determine polypyrrole incorporation, electrical conductivity measurements, and by SEM to determine changes in morphology or large-scale phase separation. Polypyrrole incorporation of up to twice the alginate (monomer versus monomer) provided materials amenable to 3D extrusion printing. The PC12 neuronal cells adhered and proliferated on the composites, demonstrating their biocompatibility and potential for tissue engineering applications.

Electroactive poly (p-phenylene sulfide)/r-graphene oxide/chitosan as a novel potential candidate for tissue engineering

Designing novel biomaterials for tissue engineering purpose is an obvious necessary considering ever increasing need for appropriate biocompatibility and properties to achieve the maximum regeneration. In this research, a new type of biomaterial based on poly (phenylene sulfide) (PPS) and reduced graphene oxide (rGO) was synthesized and applied within chitosan based hydrogel to evaluate its performance as a wound dressing potentially. Fourier transform infrared spectroscopy (FTIR), X-ray diffraction spectrometry (XRD), scanning electron microscopy (SEM) and compression tests were performed to assess suitability of composite biomaterial. Thermal behavior of the PPS/rGO composite was evaluated by differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA). The PPS/rGO composition of 90: 10 (w/w) was selected because of having the highest biocompatibility and utilized in chitosan hydrogel. Chitosan hydrogel swelling ratio was declined from 800 to 200% by PPS/rGO addition; likewise, water vapor transition rate (WVTR) was dropped. A proper biocompatibility and cell attachment was confirmed, where porosity of ca. 80% appeared promising for tissue engineering uses. Overall, the result confirmed the appropriateness of PPS/rGO for tissue engineering uses.

Piezoelectric Scaffolds as Smart Materials for Neural Tissue Engineering

Injury to the central or peripheral nervous systems leads to the loss of cognitive and/or sensorimotor capabilities, which still lacks an effective treatment. Tissue engineering in the post-injury brain represents a promising option for cellular replacement and rescue, providing a cell scaffold for either transplanted or resident cells. Tissue engineering relies on scaffolds for supporting cell differentiation and growth with recent emphasis on stimuli responsive scaffolds, sometimes called smart scaffolds. One of the representatives of this material group is piezoelectric scaffolds, being able to generate electrical charges under mechanical stimulation, which creates a real prospect for using such scaffolds in non-invasive therapy of neural tissue. This paper summarizes the recent knowledge on piezoelectric materials used for tissue engineering, especially neural tissue engineering. The most used materials for tissue engineering strategies are reported together with the main achievements, challenges, and future needs for research and actual therapies. This review provides thus a compilation of the most relevant results and strategies and serves as a starting point for novel research pathways in the most relevant and challenging open questions.