Fabrication and desired properties of conductive hydrogel dressings for wound healing

Conductive hydrogels are platforms recognized as constituting promising materials for tissue engineering applications. This is because such conductive hydrogels are characterized by the inherent conductivity properties while retaining favorable biocompatibility and mechanical properties. These conductive hydrogels can be particularly useful in enhancing wound healing since their favorable conductivity can promote the transport of essential ions for wound healing via the imposition of a so-called transepithelial potential. Other valuable properties of these conductive hydrogels, such as wound monitoring, stimuli-response etc., are also discussed in this study. Crucially, the properties of conductive hydrogels, such as 3D printability and monitoring properties, suggest the possibility of its use as an alternative wound dressing to traditional dressings such as bandages. This review, therefore, seeks to comprehensively explore the functionality of conductive hydrogels in wound healing, types of conductive hydrogels and their preparation strategies and crucial properties of hydrogels. This review will also assess the limitations of conductive hydrogels and future perspectives, with an emphasis on the development trend for conductive hydrogel uses in wound dressing fabrication for subsequent clinical applications.

Smart Hydrogel Sensors with Antifreezing, Antifouling Properties for Wound Healing

Flexible electronic devices with biological therapeutic and sensing properties are one of the current research directions. Here, a multifunctional hydrogel for stress sensing and wound healing was prepared by a simple one-pot method and a solution replacement method. Among them, zwitterionic polymers promote wound healing by promoting the polarization of M2 macrophages, collagen deposition, and blood vessel formation. Glycerin can significantly improve the resilience and frost resistance of the hydrogel, ensuring that a sensor made using the hydrogel can work normally in a cold environment. In addition, zwitterionic polymers are highly biocompatible, providing excellent antibacterial adhesion to aid the wound healing process, and good electrical conductivity enhances sensing sensitivity and stability. Based on these properties, multifunctional hydrogels could detect human vital activities while promoting wound healing, providing new ideas for the fields of diagnosis and wound dressing.

Biocompatible and Electroconductive Nanocomposite Scaffolds with Improved Piezoelectric Response for Bone Tissue Engineering

Electroactive scaffolds are relatively new tools in tissue engineering that open new avenue in repairing damaged soft and hard tissues. These scaffolds can induce electrical signaling while providing an ECM-like microenvironment. However, low biocompatibility and lack of biodegradability of piezoelectric and conductive polymers limits their clinical translation. In the current study, we have developed highly biocompatible, electroconductive nanofibrous scaffolds based on poly-L-lactic acid/polyaniline/carbon nanotube (PLLA/polyaniline/CNT). Physical and chemical properties of fabricated scaffolds were tested using various techniques. Biological characteristics of the scaffolds are also examined to check cellular attachment as well as differentiation of cultured (progenitor) cells. Scaffolds were optimized to direct osteogenic differentiation of mesenchymal stem cells. Such scaffolds can offer new strategies for the regeneration of damaged/lost bone.

Conductive photothermal non-swelling nanocomposite hydrogel patch accelerating bone defect repair

Bone defect repair is one of the most common issue in clinic. Developmental multifunctional scaffolds have become a promising strategy to effectively promote bone defect repair. Here, a series of...

Mechanistically Scoping Cell-Free and Cell-Dependent Artificial Scaffolds in Rebuilding Skeletal and Dental Hard Tissues

Rebuilding mineralized tissues in skeletal and dental systems remains costly and challenging. Despite numerous demands and heavy clinical burden over the world, sources of autografts, allografts, and xenografts are far limited, along with massive risks including viral infections, ethic crisis, and so on. Per such dilemma, artificial scaffolds have emerged to provide efficient alternatives. To date, cell-free biomimetic mineralization (BM) and cell-dependent scaffolds have both demonstrated promising capabilities of regenerating mineralized tissues. However, BM and cell-dependent scaffolds have distinctive mechanisms for mineral genesis, which makes them methodically, synthetically, and functionally disparate. Herein, these two strategies in regenerative dentistry and orthopedics are systematically summarized at the level of mechanisms. For BM, methodological and theoretical advances are focused upon; and meanwhile, for cell-dependent scaffolds, it is demonstrated how scaffolds orchestrate osteogenic cell fate. The summary of the experimental advances and clinical progress will endow researchers with mechanistic understandings of artificial scaffolds in rebuilding hard tissues, by which better clinical choices and research directions may be approached.

Conductive, Self-Healing, Adhesive, and Antibacterial Hydrogels Based on Lignin/Cellulose for Rapid MRSA-Infected Wound Repairing

The abuse of antibiotics induces the emergence of drug-resistant bacteria, which greatly increases the difficulty of clinical treatment of infected wounds. It is urgent to design a multifunctional wound dressing independent of antibiotics. In this work, we designed multifunctional hydrogels based on lignin and cellulose in natural polymers. Lignin with antioxidant properties could reduce silver nanoparticles in situ and could also be used as a crosslinking agent to construct hydrogels between hydroxypropyl cellulose modified with phenylboric acid by a dynamic borate bond. Hydrogels have excellent properties such as self-healing, shape adaptability, biocompatibility, blood compatibility, antioxidant properties, excellent broad-spectrum antimicrobial properties, good tissue adhesion, and electrical conductivity. The tissue adhesion of hydrogels endows them with an excellent hemostasis property in a rat liver injury model. In vivo experiments demonstrated that hydrogels can maintain a moist healing environment, reduce inflammatory cell infiltration, promote M2 macrophage polarization, accelerate collagen deposition, promote the regeneration of new blood vessels, and significantly speed up the wound healing of methicillin-resistant Staphylococcus aureus (MRSA)-infected wounds. Therefore, these multifunctional hydrogels are an excellent candidate to treat multiple stages of wound healing and have a broad application prospect in the medical field.

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.

Functionalization of an Electroactive Self-Healing Polypyrrole-Grafted Gelatin-Based Hydrogel by Incorporating a Polydopamine@AgNP Nanocomposite

Hydrogels are considered a promising wound dressing owing to their ability to absorb wound exudates and their moist network structure for skin regeneration. It is of great significance to give added multiple functions to hydrogels for wound healing. In this paper, we present a gelatin-based hydrogel with self-healing ability, conductivity, and antibacterial and antioxidant activities. Dopamine was added into an alkaline solution to polymerize into polydopamine (PDA), which was used to reduce AgNO₃ into Ag nanoparticles (AgNPs) to gain a PDA@AgNP composite. Polypyrrole-grafted gelatin (PPyGel) was dissolved in a PDA@AgNP solution and ferric ions were used as a cross-linking agent to form PDA@AgNPs-PPyGel-Fe hydrogels. The as-prepared hydrogels are soft and ductile and exhibit porous structures with pore sizes from 20 to 50 μm. The hydrogels have high water absorption ability, indicating the potential to absorb wound exudates. PPy and Fe³⁺ endow the hydrogels with slightly higher conductivity than that of skin tissue, indicating the ability to effectively transmit bioelectric signals for skin regeneration. The ionic interactions and hydrogen bonding in hydrogels make them possess self-healing ability, and the self-healing process can be completed in 30 min. PDA confers hydrogels with effective antioxidant activities, while AgNPs endow hydrogels with good antibacterial activities. Moreover, the hydrogels possess good blood compatibility and cytocompatibility. In sum, the developed hydrogel has potential applications as wound dressings.

Redox Polymers for Tissue Engineering

This review will focus on the targeted design, synthesis and application of redox polymers for use in regenerative medicine and tissue engineering. We define redox polymers to encompass a variety of polymeric materials, from the multifunctional conjugated conducting polymers to graphene and its derivatives, and have been adopted for use in the engineering of several types of stimulus responsive tissues. We will review the fundamental properties of organic conducting polymers (OCPs) and graphene, and how their properties are being tailored to enhance material - biological interfacing. We will highlight the recent development of high-resolution 3D fabrication processes suitable for biomaterials, and how the fabrication of intricate scaffolds at biologically relevant scales is providing exciting opportunities for the application of redox polymers for both in-vitro and in-vivo tissue engineering. We will discuss the application of OCPs in the controlled delivery of bioactive compounds, and the electrical and mechanical stimulation of cells to drive behaviour and processes towards the generation of specific functional tissue. We will highlight the relatively recent advances in the use of graphene and the exploitation of its physicochemical and electrical properties in tissue engineering. Finally, we will look forward at the future of organic conductors in tissue engineering applications, and where the combination of materials development and fabrication processes will next unite to provide future breakthroughs.

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.

Mineralized DNA-collagen complex-based biomaterials for bone tissue engineering

DNA is a highly polyanionic biomolecule that complexes with both collagen and hydroxyapatite. By combining these complexes, we synthesized nucleic-acid collagen complexes (NACC) mineralized with hydroxyapatite. The composite complexes were made using a short, monodisperse single-stranded DNA, type I collagen, and mineralizing medium. They rapidly self-assembled into both mineralized NACC microfibers and 3D NACC gels. At the nanoscale, these complexes are hierarchical, interwoven, curly nanofibrils resembling native extracellular matrix, which mineralized an interpenetrating nanocrystalline hydroxyapatite phase. Mineralization was able to be done either before or after NACC formation enabling temporal control of the process. In response to the NACC material, primary human osteoblasts took on an osteocyte-like morphology. Moreover, the cells agglomerated and remodeled the NACC gels into densified, tissue-like structures within 3 days. NACC fibers and gels have promise not only as osteoconductive coatings and scaffolds, but as coatings and scaffolds for any tissue using this new form of naturally-derived biomaterials.

Nanostructured Biomaterials for Bone Regeneration

This review article addresses the various aspects of nano-biomaterials used in or being pursued for the purpose of promoting bone regeneration. In the last decade, significant growth in the fields of polymer sciences, nanotechnology, and biotechnology has resulted in the development of new nano-biomaterials. These are extensively explored as drug delivery carriers and as implantable devices. At the interface of nanomaterials and biological systems, the organic and synthetic worlds have merged over the past two decades, forming a new scientific field incorporating nano-material design for biological applications. For this field to evolve, there is a need to understand the dynamic forces and molecular components that shape these interactions and influence function, while also considering safety. While there is still much to learn about the bio-physicochemical interactions at the interface, we are at a point where pockets of accumulated knowledge can provide a conceptual framework to guide further exploration and inform future product development. This review is intended as a resource for academics, scientists, and physicians working in the field of orthopedics and bone repair.

Injectable thermosensitive hybrid hydrogel containing graphene oxide and chitosan as dental pulp stem cells scaffold for bone tissue engineering

Here, we fabricated thermosensitive injectable hydrogel containing poly (N-isopropylacrylamide) (PNIPAAm)-based copolymer/graphene oxide (GO) composite with different feed ratio to chitosan (CS) as a natural polymer through physical and chemical crosslinking for the proliferation and differentiation of the human dental pulp stem cells (hDPSCs) to the osteoblasts. The PNIPAAm copolymer/GO composite was synthesized by free-radical copolymerization of (N-isopropylacrylamide) (NIPAAm), itaconic acid (IA) and maleic anhydride-modified poly(ethylene glycol) (PEG) in the presence of GO and used for the preparation of the hydrogels. The formulated hydrogels were evaluated for the porous architecture, rheological behavior, compressive strength, swelling property, in vitro degradation, hemocompatibility, biocompatibility, and differentiation. The hydrogel could enhance the deposition of minerals and the activity of alkaline phosphatase (ALP), in large part attributable to the oxygen and amine-containing functional groups of GO and CS. The engineered hydrogel could also upregulate the expression of the Runt-related transcription factor 2 and osteocalcin in the hDPSCs cultivated in both the normal and osteogenic media. It seems to promote the absorption of osteogenic inducer too. Based on our findings, the engineered hydrogel demonstrated the osteogenic potential, upon which it is proposed as a constructing scaffold in bone tissue engineering for the transplantation of hDPSCs.

Three dimensional printed degradable and conductive polymer scaffolds promote chondrogenic differentiation of chondroprogenitor cells

We report a conductive and biodegradable 3D printed polymer scaffold that promotes chondrogenic differentiation of chondroprogenitor cells. The conductive material consists of tetraniline-b-polycaprolactone-b-tetraaniline and polycaprolactone.

Mussel-Inspired Conducting Copolymer with Aniline Tetramer as Intelligent Biological Adhesive for Bone Tissue Engineering

Electrically conducting polymers have been emerging as intelligent bioactive materials for regulating cell behaviors and bone tissue regeneration. Additionally, poor adhesion between conventional implants and native bone tissue may lead to displacement, local inflammation, and unnecessary secondary surgery. Thus, a conductive bioadhesive with strong adhesion performance provides an effective approach to fulfill fixation and regeneration of comminuted bone fracture. Inspired by mussel chemistry, we designed the conductive copolymers poly{[aniline tetramer methacrylamide]-co-[dopamine methacrylamide]-co-[poly(ethylene glycol) methyl ether methacrylate]} [poly(ATMA-co-DOPAMA-co-PEGMA); AT:conductive aniline tetramer; DOPA:dopamine; PEG:poly(ethylene glycol))] with AT content 3.0, 6.0, and 9.0 mol %, respectively. The adhesive strength of this copolymer was enhanced during tensile process perhaps due to the synergistic effects of H-bonding, π-π interactions, and polymer long-chain entanglement, reaching up to 1.28 MPa with 6 mol % AT. Biological characterizations of preosteoblasts indicated that the bioadhesives exhibited desirable biocompatibility. In addition, the osteogenic differentiation was synergistically enhanced by the conductive substrate and electrical stimulation with a square wave, frequency of 100 Hz, 50% duty cycle, and electrical potential of 500 mV, as indicated by ALP activity, calcium deposition, and expression of osteogenic genes. The ALP activity at 14 days and calcium deposition at 28 days on the 9 mol % AT group were significantly higher than that on PLGA under electrical stimulation. The expression value of OPN for 9 mol % AT group was notably upregulated by 5.9-fold compared with PLGA at 7 days under electrical stimulation. Overall, the conductive polymers with strong adhesion can synergistically upregulate the cellular activity combining with electrical stimulation and might be a promising bioadhesive for orthopedic and dental applications.

Polypyrrole Nanocones and Dynamic Piezoelectric Stimulation-Induced Stem Cell Osteogenic Differentiation

Imitating the physiological microenvironment of living cell and tissues opens new avenues of research into the application of electricity to medical therapies. In this study, dynamic piezoelectric stimulation is generated in a dynamic culture because of the piezoelectric effect of the poly(vinylidene fluoride)-polypyrrole (PVDF-PPy) electroactive composite. Combined with PPy nanocones, dynamic piezoelectric signals are effectively and continuously provided to cells. In the presence of dynamic piezoelectric stimulation and PPy nanocones, PPy-PVDF NS samples show promoted bone mesenchymal stem cell (BMSCs) adhesion, spreadin, and osteogenic differentiation. On the basis of the results of this study, PPy nanocones and dynamic piezoelectric stimulation can be administered to modulate cell behavior, paving the way for the exploration of cellular responses to dynamic electrical stimulation.

Hybrid polymer biomaterials for bone tissue regeneration

Native tissues possess unparalleled physiochemical and biological functions, which can be attributed to their hybrid polymer composition and intrinsic bioactivity. However, there are also various concerns or limitations over the use of natural materials derived from animals or cadavers, including the potential immunogenicity, pathogen transmission, batch to batch consistence and mismatch in properties for various applications. Therefore, there is an increasing interest in developing degradable hybrid polymer biomaterials with controlled properties for highly efficient biomedical applications. There have been efforts to mimic the extracellular protein structure such as nanofibrous and composite scaffolds, to functionalize scaffold surface for improved cellular interaction, to incorporate controlled biomolecule release capacity to impart biological signaling, and to vary physical properties of scaffolds to regulate cellular behavior. In this review, we highlight the design and synthesis of degradable hybrid polymer biomaterials and focus on recent developments in osteoconductive, elastomeric, photoluminescent and electroactive hybrid polymers. The review further exemplifies their applications for bone tissue regeneration.

Degradable conductive self-healing hydrogels based on dextran-graft-tetraaniline and N-carboxyethyl chitosan as injectable carriers for myoblast cell therapy and muscle regeneration

Injectable conductive hydrogels have great potential as tissue engineering scaffolds and delivery vehicles for electrical signal sensitive cell therapy. In this work, we present the synthesis of a series of injectable electroactive degradable hydrogels with rapid self-healing ability and their potential application as cell delivery vehicles for skeletal muscle regeneration. Self-healable conductive injectable hydrogels based on dextran-graft-aniline tetramer-graft-4-formylbenzoic acid and N-carboxyethyl chitosan were synthesized at physiological conditions. The dynamic Schiff base bonds between the formylbenzoic acid and amine group from N-carboxyethyl chitosan endowed the hydrogels with rapid self-healing ability, which was verified by rheological test. Equilibrated swelling ratio, morphology, mechanical strength, electrochemistry and conductivity of the injectable hydrogels were fully investigated. The self-healable conductive hydrogels showed an in vivo injectability and a linear-like degradation behavior. Two different kinds of cells (C2C12 myoblasts and human umbilical vein endothelial cells (HUVEC)) were encapsulated in the hydrogels by self-healing effect. The L929 fibroblast cell culture results indicated the biocompatibility of the hydrogels. Moreover, the C2C12 myoblast cells were released from the conductive hydrogels with a linear-like profile. The in vivo skeletal muscle regeneration was also studied in a volumetric muscle loss injury model. All these data indicated that these biodegradable self-healing conductive hydrogels are potential candidates as cell delivery vehicles and scaffolds for skeletal muscle repair. STATEMENT OF SIGNIFICANCE: Injectable hydrogels with self-healing and electrical conductivity properties are excellent candidates as tissue-engineered scaffolds for myoblast cell therapy and skeletal muscle regeneration. The self-healing property of these hydrogels can prolong their lifespan. However, most of the reported conductive hydrogels are not degradable or do not have the self-healing ability. Herein, we synthesized antibacterial conductive self-healing hydrogels as a cell delivery carrier for cardiac cell therapy based on chitosan-grafted-tetraaniline hydrogels synthesized in our previous work. However, an acid solution was used to dissolve the polymers in that study, which may induce toxicity to cells. In this work, we synthesized a series of injectable electroactive biodegradable hydrogels with rapid self-healing ability composed of N-carboxyethyl chitosan (CECS) and dextran-graft-aniline oligomers, and these hydrogel precusor can dissolve in PBS solution of pH 7.4; we further demonstrated their potential application as cell delivery vehicles for skeletal muscle regeneration.

Bioactive Silk-Based Nerve Guidance Conduits for Augmenting Peripheral Nerve Repair

Repair of peripheral nerve injuries depends upon complex biology stemming from the manifold and challenging injury-healing processes of the peripheral nervous system. While surgical treatment options are available, they tend to be characterized by poor clinical outcomes for the injured patients. This is particularly apparent in the clinical management of a nerve gap whereby nerve autograft remains the best clinical option despite numerous limitations; in addition, effective repair becomes progressively more difficult with larger gaps. Nerve conduit strategies based on tissue engineering approaches and the use of silk as scaffolding material have attracted much attention in recent years to overcome these limitations and meet the clinical demand of large gap nerve repair. This review examines the scientific advances made with silk-based conduits for peripheral nerve repair. The focus is on enhancing bioactivity of the conduits in terms of physical guidance cues, inner wall and lumen modification, and imbuing novel conductive functionalities.

Versatile biomimetic conductive polypyrrole films doped with hyaluronic acid of different molecular weights

Electrically conductive polypyrrole (PPy) is an intriguing biomaterial capable of efficient electrical interactions with biological systems. Especially, biomimetic PPy-based biomaterials incorporating biomolecules, such as hyaluronic acid (HA), can impart the characteristic biological interactions with living cells/tissues to the conductive biomaterials. Here we report the effects of the molecular weight (MW) of HA on PPy-based biomaterials. We utilized HA of a wide range of MW (35 × 10³ Da-3 × 10⁶ Da) as dopants during the electrochemical production of PPy/HA films and their characterization of materials and cellular interactions. With increases in the MWs of HA dopants, PPy/HA exhibited more hydrophilic, higher electrochemical activity and lower impedance. In vitro studies revealed that PPy films doped with low MW HA were supportive to cell adhesion and growth, while PPy films doped with high MW HA were resistant to cell attachment. Subcutaneous implantation of the PPy/HA films for 4 weeks revealed that all the PPy/HA films were tissue compatible. We successfully demonstrate the importance of HA dopant MWs in modulating the chemical and electrical properties of the materials and cellular responses to the materials. Such materials have potential for various biomedical applications, including as tissue engineering scaffolds and as electrodes for neural recording and neuromodulation. STATEMENT OF SIGNIFICANCE: Hyaluronic acid (HA)-doped polypyrrole (PPy) films were electrochemically synthesized as novel biomimetic conductive materials capable of efficient electrical signaling and preferential biological interactions. Molecular weights (MWs) of HA varied in a wide range (35 × 10³-2 × 10⁶ Da) and critically determine chemical, electrochemical, and biological properties of PPy/HA. Especially, PPy films with low MW HA markedly support cell adhesion and growth, while PPy films with high MW HA are resistant to cell attachment. Furthermore, PPy/HA exhibits greatly improved tissue compatibility and in vivo EMG signal recording ability. We for the first time demonstrate that biomimetic PPy/HA-based biomaterials can serve as versatile and effective platforms for various biomedical applications, such as tissue engineering scaffolds and bioelectrodes.

Graphene Family Materials in Bone Tissue Regeneration: Perspectives and Challenges

We have witnessed abundant breakthroughs in research on the bio-applications of graphene family materials in current years. Owing to their nanoscale size, large specific surface area, photoluminescence properties, and antibacterial activity, graphene family materials possess huge potential for bone tissue engineering, drug/gene delivery, and biological sensing/imaging applications. In this review, we retrospect recent progress and achievements in graphene research, as well as critically analyze and discuss the bio-safety and feasibility of various biomedical applications of graphene family materials for bone tissue regeneration.

Conducting Polymers for Tissue Engineering

Electrically conducting polymers such as polyaniline, polypyrrole, polythiophene, and their derivatives (mainly aniline oligomer and poly(3,4-ethylenedioxythiophene)) with good biocompatibility find wide applications in biomedical fields including bioactuators, biosensors, neural implants, drug delivery systems, and tissue engineering scaffolds. This review focuses on these conductive polymers for tissue engineering applications. Conductive polymers exhibit promising conductivity as bioactive scaffolds for tissue regeneration, and their conductive nature allows cells or tissue cultured on them to be stimulated by electrical signals. However, their mechanical brittleness and poor processability restrict their application. Therefore, conductive polymeric composites based on conductive polymers and biocompatible biodegradable polymers (natural or synthetic) were developed. The major objective of this review is to summarize the conductive biomaterials used in tissue engineering including conductive composite films, conductive nanofibers, conductive hydrogels, and conductive composite scaffolds fabricated by various methods such as electrospinning, coating, or deposition by in situ polymerization. Furthermore, recent progress in tissue engineering applications using these conductive biomaterials including bone tissue engineering, muscle tissue engineering, nerve tissue engineering, cardiac tissue engineering, and wound healing application are discussed in detail.

Electroactive polymers for tissue regeneration: Developments and perspectives

Human body motion can generate a biological electric field and a current, creating a voltage gradient of -10 to -90 mV across cell membranes. In turn, this gradient triggers cells to transmit signals that alter cell proliferation and differentiation. Several cell types, counting osteoblasts, neurons and cardiomyocytes, are relatively sensitive to electrical signal stimulation. Employment of electrical signals in modulating cell proliferation and differentiation inspires us to use the electroactive polymers to achieve electrical stimulation for repairing impaired tissues. Electroactive polymers have found numerous applications in biomedicine due to their capability in effectively delivering electrical signals to the seeded cells, such as biosensing, tissue regeneration, drug delivery, and biomedical implants. Here we will summarize the electrical characteristics of electroactive polymers, which enables them to electrically influence cellular function and behavior, including conducting polymers, piezoelectric polymers, and polyelectrolyte gels. We will also discuss the biological response to these electroactive polymers under electrical stimulation. In particular, we focus this review on their applications in regenerating different tissues, including bone, nerve, heart muscle, cartilage and skin. Additionally, we discuss the challenges in tissue regeneration applications of electroactive polymers. We conclude that electroactive polymers have a great potential as regenerative biomaterials, due to their ability to stimulate desirable outcomes in various electrically responsive cells.

Oligoaniline-based conductive biomaterials for tissue engineering

The science and engineering of biomaterials have improved the human life expectancy. Tissue engineering is one of the nascent strategies with an aim to fulfill this target. Tissue engineering scaffolds are one of the most significant aspects of the recent tissue repair strategies; hence, it is imperative to design biomimetic substrates with suitable features. Conductive substrates can ameliorate the cellular activity through enhancement of cellular signaling. Biocompatible polymers with conductivity can mimic the cells' niche in an appropriate manner. Bioconductive polymers based on aniline oligomers can potentially actualize this purpose because of their unique and tailoring properties. The aniline oligomers can be positioned within the molecular structure of other polymers, thus painter acting with the side groups of the main polymer or acting as a comonomer in their backbone. The conductivity of oligoaniline-based conductive biomaterials can be tailored to mimic the electrical and mechanical properties of targeted tissues/organs. These bioconductive substrates can be designed with high mechanical strength for hard tissues such as the bone and with high elasticity to be used for the cardiac tissue or can be synthesized in the form of injectable hydrogels, particles, and nanofibers for noninvasive implantation; these structures can be used for applications such as drug/gene delivery and extracellular biomimetic structures. It is expected that with progress in the fields of biomaterials and tissue engineering, more innovative constructs will be proposed in the near future. This review discusses the recent advancements in the use of oligoaniline-based conductive biomaterials for tissue engineering and regenerative medicine applications.

STATEMENT OF SIGNIFICANCE

The tissue engineering applications of aniline oligomers and their derivatives have recently attracted an increasing interest due to their electroactive and biodegradable properties. However, no reports have systematically reviewed the critical role of oligoaniline-based conductive biomaterials in tissue engineering. Research on aniline oligomers is growing today opening new scenarios that expand the potential of these biomaterials from "traditional" treatments to a new era of tissue engineering. The conductivity of this class of biomaterials can be tailored similar to that of tissues/organs. To the best of our knowledge, this is the first review article in which such issue is systematically reviewed and critically discussed in the light of the existing literature. Undoubtedly, investigations on the use of oligoaniline-based conductive biomaterials in tissue engineering need further advancement and a lot of critical questions are yet to be answered. In this review, we introduce the salient features, the hurdles that must be overcome, the hopes, and practical constraints for further development.

Conductive nanofibrous composite scaffolds based on in-situ formed polyaniline nanoparticle and polylactide for bone regeneration

Conducting polymers and biodegradable polylactide (PLA) scaffolds are both promising biomaterials applied in bone tissue engineering. It is necessary to develop a composite scaffold combining their properties of osteogenic differentiation promotion and three-dimension matrix. To conquer the problem of poor processability of conductive polymers, we use a novel in-situ polymerization/thermal induced phase separation (TIPS) method to fabricate conductive nanofibrous PLA scaffolds with well-distributed polyaniline (PANI) nano-structures. The simple preparation technique provides the possibility to scale-up production of these conductive nanofibrous composite scaffolds. The scaffold structure and content of in-situ formed polyaniline nanoparticles was thoroughly characterized with ¹H NMR, FT-IR, XPS, TGA, SEM and UV-vis, and the conductivity/electrochemical properties of the composite scaffolds were controlled with varied feed ratios of aniline to PLA. Meanwhile, the good cytocompatibility of these composite scaffolds was evaluated by culturing bone marrow derived mesenchymal stem cells (BMSCs) on them. The effect of conductive nanofibrous scaffolds on osteogenic differentiation was studied with expression levels of alkaline phosphatase (Alp), osteocalcin (Ocn) and runt-related transcription factor 2 (Runx2) during the culture of BMSCs for three weeks. The calcium mineralization of BMSCs is determined by alizarin red staining. These results indicated that a moderate content of PANI in the conductive nanofibrous scaffolds significantly promoted osteogenic differentiation of BMSCs for engineering bone tissues.

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.

Electrospun conductive nanofibrous scaffolds for engineering cardiac tissue and 3D bioactuators

Mimicking the nanofibrous structure similar to extracellular matrix and conductivity for electrical propagation of native myocardium would be highly beneficial for cardiac tissue engineering and cardiomyocytes-based bioactuators. Herein, we developed conductive nanofibrous sheets with electrical conductivity and nanofibrous structure composed of poly(l-lactic acid) (PLA) blending with polyaniline (PANI) for cardiac tissue engineering and cardiomyocytes-based 3D bioactuators. Incorporating of varying contents of PANI from 0wt% to 3wt% into the PLA polymer, the electrospun nanofibrous sheets showed enhanced conductivity while maintaining the same fiber diameter. These PLA/PANI conductive nanofibrous sheets exhibited good cell viability and promoting effect on differentiation of H9c2 cardiomyoblasts in terms of maturation index and fusion index. Moreover, PLA/PANI nanofibrous sheets enhanced the cell-cell interaction, maturation and spontaneous beating of primary cardiomyocytes. Furthermore, the cardiomyocytes-laden PLA/PANI conductive nanofibrous sheets can form 3D bioactuators with tubular and folding shapes, and spontaneously beat with much higher frequency and displacement than that on cardiomyocytes-laden PLA nanofibrous sheets. Therefore, these PLA/PANI conductive nanofibrous sheets with conductivity and extracellular matrix like nanostructure demonstrated promising potential in cardiac tissue engineering and cardiomyocytes-based 3D bioactuators.

STATEMENT OF SIGNIFICANCE

Cardiomyocytes-based bioactuators have been paid more attention due to their spontaneous motion by integrating cardiomyocytes into polymer structures, but developing suitable scaffolds for bioactuators remains challenging. Electrospun nanofibrous scaffolds have been widely used in cardiac tissue engineering because they can mimic the extracellular matrix of myocardium. Developing conductive nanofibrous scaffolds by electrospinning would be beneficial for cardiomyocytes-based bioactuators, but such scaffolds have been rarely reported. This work presented a conductive nanofibrous sheet based on polylactide and polyaniline via electrospinning with tunable conductivity. These conductive nanofibrous sheets performed the ability to enhance cardiomyocytes maturation and spontaneous beating, and further formed cardiomyocytes-based 3D bioactuators with tubular and folding shapes, which indicated their great potential in cardiac tissue engineering and bioactuators applications.

The Horizon of Materiobiology: A Perspective on Material-Guided Cell Behaviors and Tissue Engineering

Although the biological functions of cell and tissue can be regulated by biochemical factors (e.g., growth factors, hormones), the biophysical effects of materials on the regulation of biological activity are receiving more attention. In this Review, we systematically summarize the recent progress on how biomaterials with controllable properties (e.g., compositional/degradable dynamics, mechanical properties, 2D topography, and 3D geometry) can regulate cell behaviors (e.g., cell adhesion, spreading, proliferation, cell alignment, and the differentiation or self-maintenance of stem cells) and tissue/organ functions. How the biophysical features of materials influence tissue/organ regeneration have been elucidated. Current challenges and a perspective on the development of novel materials that can modulate specific biological functions are discussed. The interdependent relationship between biomaterials and biology leads us to propose the concept of "materiobiology", which is a scientific discipline that studies the biological effects of the properties of biomaterials on biological functions at cell, tissue, organ, and the whole organism levels. This Review highlights that it is more important to develop ECM-mimicking biomaterials having a self-regenerative capacity to stimulate tissue regeneration, instead of attempting to recreate the complexity of living tissues or tissue constructs ex vivo. The principles of materiobiology may benefit the development of novel biomaterials providing combinative bioactive cues to activate the migration of stem cells from endogenous reservoirs (i.e., cell niches), stimulate robust and scalable self-healing mechanisms, and unlock the body's innate powers of regeneration.

Self-Healing Conductive Injectable Hydrogels with Antibacterial Activity as Cell Delivery Carrier for Cardiac Cell Therapy

Cell therapy is a promising strategy to regenerate cardiac tissue for myocardial infarction. Injectable hydrogels with conductivity and self-healing ability are highly desirable as cell delivery vehicles for cardiac regeneration. Here, we developed self-healable conductive injectable hydrogels based on chitosan-graft-aniline tetramer (CS-AT) and dibenzaldehyde-terminated poly(ethylene glycol) (PEG-DA) as cell delivery vehicles for myocardial infarction. Self-healed electroactive hydrogels were obtained after mixing CS-AT and PEG-DA solutions at physiological conditions. Rapid self-healing behavior was investigated by rheometer. Swelling behavior, morphology, mechanical strength, electrochemistry, conductivity, adhesiveness to host tissue and antibacterial property of the injectable hydrogels were fully studied. Conductivity of the hydrogels is ∼10(-3) S·cm(-1), which is quite close to native cardiac tissue. Proliferation of C2C12 myoblasts in the hydrogel showed its good biocompatibility. After injection, viability of C2C12 cells in the hydrogels showed no significant difference with that before injection. Two different cell types were successfully encapsulated in the hydrogels by self-healing effect. Cell delivery profile of C2C12 myoblasts and H9c2 cardiac cells showed a tunable release rate, and in vivo cell retention in the conductive hydrogels was also studied. Subcutaneous injection and in vivo degradation of the hydrogels demonstrated their injectability and biodegradability. Together, these self-healing conductive biodegradable injectable hydrogels are excellent candidates as cell delivery vehicle for cardiac repair.