Electrical Stimulation of Human Mesenchymal Stem Cells on Conductive Nanofibers Enhances their Differentiation toward Osteogenic Outcomes

Controlled Sr(ii) ion release from in situ crosslinking electroactive hydrogels with potential for the treatment of infections

The development of electrochemical stimuli-responsive drug delivery systems is of both academic and industrial interest due to the ease with which it is possible to trigger payload release, providing drug delivery in a controllable manner. Herein, the preparation of in situ forming hydrogels including electroactive polypyrrole nanoparticles (PPy-NPs) where Sr2+ ions are electrochemically loaded for electrically triggered release of Sr2+ ions is reported. The hydrogels were characterized by a variety of techniques including Fourier-transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), thermogravimetric analysis (TGA), X-ray diffraction (XRD), cyclic voltammetry (CV), etc. The cytocompatibility towards human mesenchymal stem cells (MSCs) and fibroblasts were also studied. The Sr2+ ion loaded PEC-ALD/CS/PPy-NPs hydrogel showed no significant cytotoxicity towards human mesenchymal stem cells (MSCs) and fibroblasts. Sr2+ ions were electrochemically loaded and released from the electroactive hydrogels, and the application of an electrical stimulus enhanced the release of Sr2+ ions from gels by ca. 2-4 fold relative to the passive release control experiment. The antibacterial activity of Sr2+ ions against E. coli and S. aureus was demonstrated in vitro. Although these prototypical examples of Sr2+ loaded electroactive gels don't release sufficient Sr2+ ions to show antibacterial activity against E. coli and S. aureus, we believe future iterations with optimised physical properties of the gels will be capable of doing so.

Electrical stimulation: Effective cue to direct osteogenic differentiation of mesenchymal stem cells?

Mesenchymal stem cells (MSCs) play a major role in bone tissue engineering (BTE) thanks to their capacity for osteogenic differentiation and being easily available. In vivo, MSCs are exposed to an electroactive microenvironment in the bone niche, which has piezoelectric properties. The correlation between the electrically active milieu and bone's ability to adapt to mechanical stress and self-regenerate has led to using electrical stimulation (ES) as physical cue to direct MSCs differentiation towards the osteogenic lineage in BTE. This review summarizes the different techniques to electrically stimulate MSCs to induce their osteoblastogenesis in vitro, including general electrical stimulation and substrate mediated stimulation by means of conductive or piezoelectric cell culture supports. Several aspects are covered, including stimulation parameters, treatment times and cell culture media to summarize the best conditions for inducing MSCs osteogenic commitment by electrical stimulation, from a critical point of view. Electrical stimulation activates different signaling pathways, including bone morphogenetic protein (BMP) Smad-dependent or independent, regulated by mitogen activated protein kinases (MAPK), extracellular signal-regulated kinases (ERK) and p38. The roles of voltage gate calcium channels (VGCC) and integrins are also highlighted according to their application technique and parameters, mainly converging in the expression of RUNX2, the master regulator of the osteogenic differentiation pathway. Despite the evident lack of homogeneity in the approaches used, the ever-increasing scientific evidence confirms ES potential as an osteoinductive cue, mimicking aspects of the in vivo microenvironment and moving one step forward to the translation of this approach into clinic.

Biocompatibility of electrospun cell culture scaffolds made from balangu seed mucilage/PVA composites

Synthesis of Balangu (Lallemantia royleana) seed mucilage (BSM) solutions combined with polyvinyl alcohol (PVA) was studied for the purpose of producing 3D electrospun cell culture scaffolds. Production of pure BSM nanofibers proved to be difficult, yet integration of PVA contributed to a facile and successful formation of BSM/PVA nanofibers. Different BSM/PVA ratios were fabricated to achieve the desired nanofibrous structure for cell proliferation. It is found that the optimal bead-free ratio of 50/50 with a mean fiber diameter of ≈180 nm presents the most desirable scaffold structure for cell growth. The positive effect of PVA incorporation was approved by analyzing BSM/PVA solutions through physiochemical assays such as electrical conductivity, viscosity and surface tension tests. According to the thermal analysis (TGA/DSC), incorporation of PVA enhanced thermal stability of the samples. Successful fabrication of the nanofibers is verified by FT-IR spectra, where no major chemical interaction between BSM and PVA is detected. The crystallinity of the electrospun nanofibers is investigated by XRD, revealing the nearly amorphous structure of BSM/PVA scaffolds. The MTT assay is employed to verify the biocompatibility of the scaffolds. The cell culture experiment using epithelial Vero cells shows the affinity of the cells to adhere to their nanofibrous substrate and grow to form continuous cell layers after 72 h of incubation.

3,4-Ethylenedioxythiophene (EDOT) End-Group Functionalized Poly-ε-caprolactone (PCL): Self-Assembly in Organic Solvents and Its Coincidentally Observed Peculiar Behavior in Thin Film and Protonated Media

End-group functionalization of homopolymers is a valuable way to produce high-fidelity nanostructured and functional soft materials when the structures obtained have the capacity for self-assembly (SA) encoded in their structural details. Herein, an end-functionalized PCL with a π-conjugated EDOT moiety, (EDOT-PCL), designed exclusively from hydrophobic domains, as a functional "hydrophobic amphiphile", was synthesized in the bulk ROP of ε-caprolactone. The experimental results obtained by spectroscopic methods, including NMR, UV-vis, and fluorescence, using DLS and by AFM, confirm that in solvents with extremely different polarities (chloroform and acetonitrile), EDOT-PCL presents an interaction- and structure-based bias, which is strong and selective enough to exert control over supramolecular packing, both in dispersions and in the film state. This leads to the diversity of SA structures, including spheroidal, straight, and helical rods, as well as orthorhombic single crystals, with solvent-dependent shapes and sizes, confirming that EDOT-PCL behaves as a "block-molecule". According to the results from AFM imaging, an unexpected transformation of micelle-type nanostructures into single 2D lamellar crystals, through breakout crystallization, took place by simple acetonitrile evaporation during the formation of the film on the mica support at room temperature. Moreover, EDOT-PCL's propensity for spontaneous oxidant-free oligomerization in acidic media was proposed as a presumptive answer for the unexpected appearance of blue color during its dissolution in CDCl3 at a high concentration. FT-IR, UV-vis, and fluorescence techniques were used to support this claim. Besides being intriguing and unforeseen, the experimental findings concerning EDOT-PCL have raised new and interesting questions that deserve to be addressed in future research.

The Effect of Physical Cues of Biomaterial Scaffolds on Stem Cell Behavior

Stem cells have been sought as a promising cell source in the tissue engineering field due to their proliferative capacity as well as differentiation potential. Biomaterials have been utilized to facilitate the delivery of stem cells in order to improve their engraftment and long-term viability upon implantation. Biomaterials also have been developed as scaffolds to promote stem cell induced tissue regeneration. This review focuses on the latter where the biomaterial scaffold is designed to provide physical cues to stem cells in order to promote their behavior for tissue formation. Recent work that explores the effect of scaffold physical properties, topography, mechanical properties and electrical properties, is discussed. Although still being elucidated, the biological mechanisms, including cell shape, focal adhesion distribution, and nuclear shape, are presented. This review also discusses emerging areas and challenges in clinical translation.

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.

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.

Continuous Electrical Stimulation Affects Initial Growth and Proliferation of Adipose-Derived Stem Cells

The aim of the study was to establish electrical stimulation parameters in order to improve cell growth and viability of human adipose-derived stem cells (hADSC) when compared to non-stimulated cells in vitro. hADSC were exposed to continuous electrical stimulation with 1.7 V AC/20 Hz. After 24, 72 h and 7 days, cell number, cellular surface coverage and cell proliferation were assessed. In addition, cell cycle analysis was carried out after 3 and 7 days. After 24 h, no significant alterations were observed for stimulated cells. At day 3, stimulated cells showed a 4.5-fold increase in cell numbers, a 2.7-fold increase in cellular surface coverage and a significantly increased proliferation. Via cell cycle analysis, a significant increase in the G2/M phase was monitored for stimulated cells. Contrastingly, after 7 days, the non-stimulated group exhibited a 11-fold increase in cell numbers and a 4-fold increase in cellular surface coverage as well as a significant increase in cell proliferation. Moreover, the stimulated cells displayed a shift to the G1 and sub-G1 phase, indicating for metabolic arrest and apoptosis initiation. In accordance, continuous electrical stimulation of hADSC led to a significantly increased cell growth and proliferation after 3 days. However, longer stimulation periods such as 7 days caused an opposite result indicating initiation of apoptosis.

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.

Maneuvering the Migration and Differentiation of Stem Cells with Electrospun Nanofibers

Electrospun nanofibers have been extensively explored as a class of scaffolding materials for tissue regeneration, because of their unique capability to mimic some features and functions of the extracellular matrix, including the fibrous morphology and mechanical properties, and to a certain extent the chemical/biological cues. This work reviews recent progress in applying electrospun nanofibers to direct the migration of stem cells and control their differentiation into specific phenotypes. First, the physicochemical properties that make electrospun nanofibers well-suited as a supporting material to expand stem cells by controlling their migration and differentiation are introduced. Then various systems are analyzed in conjunction with mesenchymal, neuronal, and embryonic stem cells, as well as induced pluripotent stem cells. Finally, some perspectives on the challenges and future opportunities in combining electrospun nanofibers with stem cells are offered to address clinical issues.

Polypyrrole-Incorporated Conducting Constructs for Tissue Engineering Applications: A Review

Conductive polymers have recently attracted interest in biomedical applications because of their excellent intrinsic electrical conductivity and satisfactory biocompatibility. Polypyrrole (PPy) is one of the most popular among these conductive polymers due to its high conductivity under physiological conditions, and it can be chemically modified to allow biomolecules conjugation. PPy has been used in fabricating biocompatible stimulus-responsive scaffolds for tissue engineering applications, especially for repair and regeneration of electroactive tissues, such as the bone, neuron, and heart. This review provides a comprehensive overview of the basic properties and synthesis methods of PPy, as well as a summary of the materials that have been integrated with PPy. These composite scaffolds are comparatively evaluated with regard to their mechanical properties, biocompatibility, and usage in tissue engineering.

Electrically Conducting Hydrogels for Health care: Concept, Fabrication Methods, and Applications

Electrically conducting hydrogels are gaining increasing attention due to their potential application in smart patches, biosensors, functional tissue engineering scaffolds, wound management, and implants. The current review focuses on these novel materials, their synthesis routes, and their composites. Special attention is paid to fabrication routes to produce functional composites with organic and inorganic components. The design of conductive hydrogels leads to inheritance of the advantages of each component and offers new features from the synergistic effects between the components, thus opening new application areas. The review also discusses the emerging role of 3D printing as an advanced approach toward new design, functionality, and material combination possibilities. The issue of lack of the spatial control with current techniques is highlighted, and possible new routes to solve it are discussed. The review will provide readers with knowledge tool to select appropriate methodology for designing desired hydrogel material composites.

Regenerative medicine and drug delivery: Progress via electrospun biomaterials

Worldwide research on electrospinning enabled it as a versatile technique for producing nanofibers with specified physio-chemical characteristics suitable for diverse biomedical applications. In the case of tissue engineering and regenerative medicine, the nanofiber scaffolds' characteristics are custom designed based on the cells and tissues specific needs. This fabrication technique is also innovated for the production of nanofibers with special micro-structure and secondary structure characteristics such as porous fibers, hollow structure, and core- sheath structure. This review attempts to critically and succinctly capture the vast number of developments reported in the literature over the past two decades. We then discuss their applications as scaffolds for induction of cells growth and differentiation or as architecture for being used as graft for tissue engineering. The special nanofibers designed for improving regeneration of several tissues including heart, bone, central nerve system, spinal cord, skin and ocular tissue are introduced. We also discuss the potential of the electrospinning in drug delivery applications, which is a critical factor for cell culture, tissue formation and wound healing applications.

Electrical stimulation-induced osteogenesis of human adipose derived stem cells using a conductive graphene-cellulose scaffold

The versatile properties of graphene-based materials are enabling various tissue regeneration, towards meeting an ever increasing demand for replacement tissues due to injury through trauma and disease. In particular, an innate ability for graphene to promote osteogenic differentiation of stem cells, combined with the potential to enhance the biological activity of cells through electrical stimulation (ES) using graphene, supports its use for osteoinduction or reconstruction. In this paper, we describe a miniaturized graphene-cellulose (G-C) scaffold-based device that incorporates electroactive G-C 'paper' within a polystyrene chamber for concomitant cell culture and ES. The G-C electrodes possessed lower impedance and higher charge injection capacity than gold (Au) electrodes, with high stability. By coupling ES with previously reported properties of the G-C scaffolds, we have advanced the platform for improved adipose derived stem cell (ADSC) support and osteogenic differentiation. We anticipate using the G-C scaffold-based ES device for in vitro modelling of osteogenic induction, bone tissue engineering and in vivo bone regeneration towards new therapeutic strategies for bone injury and disease. Furthermore, the device could reasonably be used for ES and culture of other cell types and engineering other tissues.

Injectable and Conductive Granular Hydrogels for 3D Printing and Electroactive Tissue Support

Conductive hydrogels are attractive to mimic electrophysiological environments of biological tissues and toward therapeutic applications. Injectable and conductive hydrogels are of particular interest for applications in 3D printing or for direct injection into tissues; however, current approaches to add conductivity to hydrogels are insufficient, leading to poor gelation, brittle properties, or insufficient conductivity. Here, an approach is developed using the jamming of microgels to form injectable granular hydrogels, where i) hydrogel microparticles (i.e., microgels) are formed with water-in-oil emulsions on microfluidics, ii) microgels are modified via an in situ metal reduction process, and iii) the microgels are jammed into a solid, permitting easy extrusion from a syringe. Due to the presence of metal nanoparticles at the jammed interface with high surface area in this unique design, the granular hydrogels have greater conductivity than non-particle (i.e., bulk) hydrogels treated similarly or granular hydrogels either without metal nanoparticles or containing encapsulated nanoparticles. The conductivity of the granular hydrogels is easily modified through mixing conductive and non-conductive microgels during fabrication and they can be applied to the 3D printing of lattices and to bridge muscle defects. The versatility of this conductive granular hydrogel will permit numerous applications where conductive materials are needed.

Biomimetic Polymer-Based Engineered Scaffolds for Improved Stem Cell Function

Scaffolds are considered promising materials for tissue engineering applications due to their unique physiochemical properties. The high porosity and adequate mechanical properties of the scaffolds facilitate greater cell adhesion, proliferation, and differentiation. Stem cells are frequently applied in tissue engineering applications due to their excellent potential. It has been noted that cell functions are profoundly affected by the nature of the extracellular matrix (ECM). Naturally derived ECM contains the bioactive motif that also influences the immune response of the organism. The properties of polymer scaffolds mean they can resemble the native ECM and can regulate cellular responses. Various techniques such as electrospinning and 3D printing, among others, are frequently used to fabricate polymer scaffolds, and their cellular responses are different with each technique. Furthermore, enhanced cell viability, as well as the differentiation ability of stem cells on the surface of scaffolds, opens a fascinating approach to the formation of ECM-like environments for tissue engineering applications.

Conductive nanostructured Si biomaterials enhance osteogeneration through electrical stimulation

It is well known that the differentiation of stem cells is affected by the cell culture medium, the scaffold surface and electrochemical signals. However, stimulation of patterned biomaterials seeded with stem cell cultures has not been explored. Herein the effect of electrical stimulation on osteogenic differentiation of rat bone marrow-derived mesenchymal stem cells (rBMSCs) cultured on solid and nanoporous micropyramid patterned Si surfaces was evaluated. It was found that both stimulation and scaffold patterning significantly enhanced osteo-differentiation. The stimulated nanoporous micropyramid scaffolds were more promising compared to the stimulated solid micropyramid surfaces, as they significantly promoted the osteogenic differentiation of rBMSCs via BMP/Smad signaling pathway. Particularly, as compared to the unstimulated patterned biomaterials, the stimulated patterned scaffolds allowed for a significant increase in core binding factor alpha l, alkaline phosphatase, the alpha l chain of type I Col, osteocalcin, and osteonectin, all of which are characteristic for osteo-differentiation. The proposed combination of electrical stimulation with scaffold patterning may provide novel promising strategies for bone tissue engineering and regenerative medicine.

Conjugated Polymer Actuators and Devices: Progress and Opportunities

Conjugated polymers (CPs), as exemplified by polypyrrole, are intrinsically conducting polymers with potential for development as soft actuators or "artificial muscles" for numerous applications. Significant progress has been made in the understanding of these materials and the actuation mechanisms, aided by the development of physical and electrochemical models. Current research is focused on developing applications utilizing the advantages that CP actuators have (e.g., low driving potential and easy to miniaturize) over other actuating materials and on developing ways of overcoming their inherent limitations. CP actuators are available as films, filaments/yarns, and textiles, operating in liquids as well as in air, ready for use by engineers. Here, the milestones made in understanding these unique materials and their development as actuators are highlighted. The primary focus is on the recent progress, developments, applications, and future opportunities for improvement and exploitation of these materials, which possess a wealth of multifunctional properties.

Combining Stem Cells and Biomaterial Scaffolds for Constructing Tissues and Cell Delivery

Combining stem cells with biomaterial scaffolds serves as a promising strategy for engineering tissues for both in vitro and in vivo applications. This updated review details commonly used biomaterial scaffolds for engineering tissues from stem cells. We first define the different types of stem cells and their relevant properties and commonly used scaffold formulations. Next, we discuss natural and synthetic scaffold materials typically used when engineering tissues, along with their associated advantages and drawbacks and gives examples of target applications. New approaches to engineering tissues, such as 3D bioprinting, are described as they provide exciting opportunities for future work along with current challenges that must be addressed. Thus, this review provides an overview of the available biomaterials for directing stem cell differentiation as a means of producing replacements for diseased or damaged tissues.

The antibacterial effect of potassium-sodium niobate ceramics based on controlling piezoelectric properties

The implant infection is one of the most serious postsurgical complications of medical device implantation. Therefore, the development of biocompatible materials with improved antibacterial properties is of great importance. It might be a new insight to apply the intrinsic electrical properties of biomaterials to solve this problem. Here, potassium-sodium niobate piezoceramics (K_0.5Na_0.5NbO₃, KNN) with different piezoelectric constants were prepared, and the microstructures and piezoelectric properties of these piezoceramics were evaluated. Moreover, the antibacterial effect and biocompatibility of these piezoceramics were assayed. Results showed that these piezoceramics were able to decrease the colonies of bacteria staphylococcus aureus (S. aureus), favor the rat bone marrow mesenchymal stem cells (rBMSCs) proliferation and promote the cell adhesion and spreading. The above effects were found closely related to the surface positive charges of the piezoceramics, and the sample bearing the most positive charges on its surface (sample 80KNN) had the best performance in both antibacterial effect and biocompatibility. Based on our work, it is feasible to develop biocompatible antibacterial materials by controlling piezoelectric properties.

Amine-Functionalized Electrically Conductive Core-Sheath MEH-PPV:PCL Electrospun Nanofibers for Enhanced Cell-Biomaterial Interactions

In the present study, a conducting polymer, poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) along with a biodegradable polymer poly(ε-caprolactone) (PCL) was used to prepare an electrically conductive, biocompatible, bioactive, and biodegradable nanofibrous scaffold for possible use in neural tissue engineering applications. Core-sheath electrospun nanofibers of PCL as the core and MEH-PPV as the sheath, were surface-functionalized with (3-aminopropyl) triethoxysilane (APTES) and 1,6-hexanediamine to obtain amine-functionalized surface to facilitate cell-biomaterial interactions with the aim of replacing the costly biomolecules such as collagen, fibronectin, laminin, and arginyl-glycyl-aspartic acid (RGD) peptide for surface modification. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) confirmed the formation of core-sheath morphology of the electrospun nanofibers, whereas Fourier-transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) revealed successful incorporation of amine functionality after surface functionalization. Adhesion, spreading, and proliferation of 3T3 fibroblasts were enhanced on the surface-functionalized electrospun meshes, whereas the neuronal model rat pheochromocytoma 12 (PC12) cells also adhered and differentiated into sympathetic neurons on these meshes. Under a constant electric field of 500 mV for 2 h/day for 3 consecutive days, the PC12 cells displayed remarkable improvement in the neurite formation and outgrowth on the surface-functionalized meshes that was comparable to those on the collagen-coated meshes under no electrical signal. Electrical stimulation studies further demonstrated that electrically stimulated PC12 cells cultured on collagen I coated meshes yielded more and longer neurites than those of the unstimulated cells on the same scaffolds. The enhanced neurite growth and differentiation suggest the potential use of these scaffolds for neural tissue engineering applications.

Electrochemically Enhanced Drug Delivery Using Polypyrrole Films

The delivery of drugs in a controllable fashion is a topic of intense research activity in both academia and industry because of its impact in healthcare. Implantable electronic interfaces for the body have great potential for positive economic, health, and societal impacts; however, the implantation of such interfaces results in inflammatory responses due to a mechanical mismatch between the inorganic substrate and soft tissue, and also results in the potential for microbial infection during complex surgical procedures. Here, we report the use of conducting polypyrrole (PPY)-based coatings loaded with clinically relevant drugs (either an anti-inflammatory, dexamethasone phosphate (DMP), or an antibiotic, meropenem (MER)). The films were characterized and were shown to enhance the delivery of the drugs upon the application of an electrochemical stimulus in vitro, by circa (ca.) 10⁻30% relative to the passive release from non-stimulated samples. Interestingly, the loading and release of the drugs was correlated with the physical descriptors of the drugs. In the long term, such materials have the potential for application to the surfaces of medical devices to diminish adverse reactions to their implantation in vivo.

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.

Combining electrical stimulation and tissue engineering to treat large bone defects in a rat model

Bone Tissue engineering (BTE) has recently been introduced as an alternative to conventional treatments for large non-healing bone defects. BTE approaches mimic autologous bone grafts, by combining cells, scaffold, and growth factors, and have the added benefit of being able to manipulate these constituents to optimize healing. Electrical stimulation (ES) has long been used to successfully treat non-healing fractures and has recently been shown to stimulate bone cells to migrate, proliferate, align, differentiate, and adhere to bio compatible scaffolds, all cell behaviors that could improve BTE treatment outcomes. With the above in mind we performed in vitro experiments and demonstrated that exposing Mesenchymal Stem Cells (MSC) + scaffold to ES for 3 weeks resulted in significant increases in osteogenic differentiation. Then in in vivo experiments, for the first time, we demonstrated that exposing BTE treated rat femur large defects to ES for 8 weeks, caused improved healing, as indicated by increased bone formation, strength, vessel density, and osteogenic gene expression. Our results demonstrate that ES significantly increases osteogenic differentiation in vitro and that this effect is translated into improved healing in vivo. These findings support the use of ES to help BTE treatments achieve their full therapeutic potential.

Charge injection based electrical stimulation on polypyrrole planar electrodes to regulate cellular osteogenic differentiation

This study reveals that the Q_inj on electrodes is a more significant factor than applied voltage for electrical stimulation to regulate cellular osteogenic differentiation, and the charge injection capacity can be tuned by thickness of Ppy.

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.

Silicone Substrate with Collagen and Carbon Nanotubes Exposed to Pulsed Current for MSC Osteodifferentiation

Autologous human adipose tissue-derived mesenchymal stem cells (MSCs) have the potential for clinical translation through their induction into osteoblasts for regeneration. Bone healing can be driven by biophysical stimulation using electricity for activating quiescent adult stem cells. It is hypothesized that application of electric current will enhance their osteogenic differentiation, and addition of conductive carbon nanotubes (CNTs) to the cell substrate will provide increased efficiency in current transmission. Cultured MSCs were seeded and grown onto fabricated silicone-based composites containing collagen and CNT fibers. Chemical inducers, namely, glycerol phosphate, dexamethasone, and vitamin C, were then added to the medium, and pulsatile submilliampere electrical currents (about half mA for 5 cycles at 4 mHz, twice a week) were applied for two weeks. Calcium deposition indicative of MSC differentiation and osteoblastic activity was quantified through Alizarin Red S and spectroscopy. It was found that pulsed current significantly increased osteodifferentiation on silicone-collagen films without CNTs. Under no external current, the presence of 10% (m/m) CNTs led to a significant and almost triple upregulation of calcium deposition. Both CNTs and current parameters did not appear to be synergistic. These conditions of enhanced osteoblastic activities may further be explored ultimately towards future therapeutic use of MSCs.

Electrically conductive graphene/polyacrylamide hydrogels produced by mild chemical reduction for enhanced myoblast growth and differentiation

Graphene and graphene derivatives, such as graphene oxide (GO) and reduced GO (rGO), have been extensively employed as novel components of biomaterials because of their unique electrical and mechanical properties. These materials have also been used to fabricate electrically conductive biomaterials that can effectively deliver electrical signals to biological systems. Recently, increasing attention has been paid to electrically conductive hydrogels that have both electrical activity and a tissue-like softness. In this study, we synthesized conductive graphene hydrogels by mild chemical reduction of graphene oxide/polyacrylamide (GO/PAAm) composite hydrogels to obtain conductive hydrogels. The reduced hydrogel, r(GO/PAAm), exhibited muscle tissue-like stiffness with a Young's modulus of approximately 50kPa. The electrochemical impedance of r(GO/PAAm) could be decreased by more than ten times compared to that of PAAm and unreduced GO/PAAm. In vitro studies with C2C12 myoblasts revealed that r(GO/PAAm) significantly enhanced proliferation and myogenic differentiation compared with unreduced GO/PAAm and PAAm. Moreover, electrical stimulation of myoblasts growing on r(GO/PAAm) graphene hydrogels for 7days significantly enhanced the myogenic gene expression compared to unstimulated controls. As results, our graphene-based conductive and soft hydrogels will be useful as skeletal muscle tissue scaffolds and can serve as a multifunctional platform that can simultaneously deliver electrical and mechanical cues to biological systems.

STATEMENT OF SIGNIFICANCE

Graphene-based conductive hydrogels presenting electrical conductance and a soft tissue-like modulus were successfully fabricated via mild reduction of graphene oxide/polyacrylamide composite hydrogels to study their potential to skeletal tissue scaffold applications. Significantly promoted myoblast proliferation and differentiation were obtained on our hydrogels. Additionally, electrical stimulation of myoblasts via the graphene hydrogels could further upregulate myogenic gene expressions. Our graphene-incorporated conductive hydrogels will impact on the development of new materials for skeletal muscle tissue engineering scaffolds and bioelectronics devices, and also serve as novel platforms to study cellular interactions with electrical and mechanical signals.

In vitro effect of direct current electrical stimulation on rat mesenchymal stem cells

Background

Electrical stimulation (ES) has been successfully used to treat bone defects clinically. Recently, both cellular and molecular approaches have demonstrated that ES can change cell behavior such as migration, proliferation and differentiation.

Methods

In the present study we exposed rat bone marrow- (BM-) and adipose tissue- (AT-) derived mesenchymal stem cells (MSCs) to direct current electrical stimulation (DC ES) and assessed temporal changes in osteogenic differentiation. We applied 100 mV/mm of DC ES for 1 h per day for three, seven and 14 days to cells cultivated in osteogenic differentiation medium and assessed viability and calcium deposition at the different time points. In addition, expression of osteogenic genes, Runx2, Osteopontin, and Col1A2 was assessed in BM- and AT-derived MSCs at the different time points.

Results

Results showed that ES changed osteogenic gene expression patterns in both BM- and AT-MSCs, and these changes differed between the two groups. In BM-MSCs, ES caused a significant increase in mRNA levels of Runx2, Osteopontin and Col1A2 at day 7, while in AT-MSCs, the increase in Runx2 and Osteopontin expression were observed after 14 days of ES.

Discussion

This study shows that rat bone marrow- and adipose tissue-derived stem cells react differently to electrical stimuli, an observation that could be important for application of electrical stimulation in tissue engineering.

Pigmented Silk Nanofibrous Composite for Skeletal Muscle Tissue Engineering

Skeletal muscle tissue engineering (SMTE) employs designed biomaterial scaffolds for promoting myogenic differentiation of myoblasts to functional myotubes. Oxidative stress plays a significant role in the biocompatibility of biomaterials as well as in the fate of myoblasts during myogenesis and is also associated with pathological conditions such as myotonic dystrophy. The inherent electrical excitability of muscle cells inspired the use of electroactive scaffolds for SMTE. Conducting polymers attracted the attention of researchers for their use in muscle tissue engineering. However, poor biocompatibility, biodegradability and development of oxidative stress associated immunogenic response limits the extensive use of synthetic conducting polymers for SMTE. In order to address the limitations of synthetic polymers, intrinsically electroactive and antioxidant silk fibroin/melanin composite films and electrospun fiber mats were fabricated and evaluated as scaffolds for promoting myogenesis in vitro. Melanin incorporation modulated the thermal stability, electrical conductivity of scaffolds, fiber alignment in electrospun mats and imparted good antioxidant properties to the scaffolds. The composite electrospun scaffolds promoted myoblast assembly and differentiation into uniformly aligned high aspect ratio myotubes. The results highlight the significance of scaffold topography along with conductivity in promoting myogenesis and the potential application of silk nanofibrous composite as electoractive platform for SMTE.

Mechanistic contribution of electroconductive hydroxyapatite–titanium disilicide composite on the alignment and proliferation of cells

We elucidate here the mechanistic contribution of a novel electroconductive hydroxyapatite-20 wt.% titanium disilicide (HA–TiSi2) composite system in favorably modulating osteoblast functions in relation to the monolithic HA. The higher electrical conductivity of HA–TiSi2 (σDC ∼ 67.117 ± 3.57 S/m) in comparison to glass sample effectively guided the electroactive myoblast, leading to their significant alignment and proliferation. This favorable behavior is attributed to the formation of small electrochemical cells between HA and TiSi2 phase, which produce a small electric field, directing the electroactive myoblast to migrate and grow in a particular direction. In contrast, no impact of TiSi2 on osteoblast function was observed because of their inability to respond to small electric field. However, the in vitro bioactivity in simulated body fluid indicated the nucleation and growth of apatite crystals. Moreover, in the context of load-bearing capability, the presence of 20 wt.% TiSi2 in HA led to increase in the fracture toughness by ∼100%. This study underscores the effectiveness of HA–TiSi2 in favorably modulating the cellular activity, myoblast in particular.

Designing a novel nanocomposite for bone tissue engineering using electrospun conductive PBAT/polypyrrole as a scaffold to direct nanohydroxyapatite electrodeposition

Electrospinning is a well-recognized technique for producing nanostructured fibers with different functionalities, generating materials that are able to support cell adhesion and further proliferation.

Synthesis and application of a novel poly-l-phenylalanine electroactive macromonomer as matrix for the biosensing of ‘Abused Drug’ model

The synthesis and biosensing application of a novel poly-l-phenylalanine-bearing electroactive macromonomer has been carried out.

Built-in microscale electrostatic fields induced by anatase-rutile-phase transition in selective areas promote osteogenesis

Bone has a built-in electric field because of the presence of piezoelectric collagen. To date, only externally applied electric fields have been used to direct cell behavior; however, these fields are not safe or practical for in vivo use. In this work, for the first time, we use a periodic microscale electric field (MEF) built into a titanium implant to induce osteogenesis. Such a MEF is generated by the periodic organization of a junction made of two parallel semiconducting TiO₂ zones: anatase and rutile with lower and higher electron densities, respectively. The junctions were formed through anatase-rutile-phase transition in selective areas using laser irradiation on the implants. The in vitro and in vivo studies confirmed that the built-in MEF was an efficient electrical cue for inducing osteogenic differentiation in the absence of osteogenic supplements and promoted bone regeneration around the implants. Our work opens up a new avenue toward bone repair and regeneration using built-in MEF.

Conducting polymer-based multilayer films for instructive biomaterial coatings

Aim:

To demonstrate the design, fabrication and testing of conformable conducting biomaterials that encourage cell alignment.

Materials & methods:

Thin conducting composite biomaterials based on multilayer films of poly(3.4-ethylenedioxythiophene) derivatives, chitosan and gelatin were prepared in a layer-by-layer fashion. Fibroblasts were observed with fluorescence microscopy and their alignment (relative to the dipping direction and direction of electrical current passed through the films) was determined using ImageJ.

Results:

Fibroblasts adhered to and proliferated on the films. Fibroblasts aligned with the dipping direction used during film preparation and this was enhanced by a DC current.

Conclusion:

We report the preparation of conducting polymer-based films that enhance the alignment of fibroblasts on their surface which is an important feature of a variety of tissues.

Lay abstract: Cells inhabit environments known as the extracellular matrix (ECM) which consists of a mixture of different biomolecules, and the precise composition and topographical properties are different in different tissues (e.g., bone, brain, muscle, skin). Cells interact intimately with the ECM, not only constructing the biomolecules, but assist its organization in 3D space, and its degradation (which is important for tissue remodeling); reciprocally, cells respond to the ECM (e.g., by modifying their size, shape, etc). Cellular alignment is observed in organs and tissues such as bones, muscles and skin, and this alignment is important for the healthy functioning of the organ/tissue. Here, we present a novel method of aligning cells on biomaterials, simply by applying an electrical current through the biomaterial.