Electrical stimulation using conductive polymer polypyrrole promotes differentiation of human neural stem cells: a biocompatible platform for translational neural tissue engineering

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.

Thermoresponsive and Conductive Chitosan-Polyurethane Biocompatible Thin Films with Potential Coating Application

Conductive thin films have great potential for application in the biomedical field. Herein, we designed thermoresponsive and conductive thin films with hydrophilicity, strain sensing, and biocompatibility. The crosslinked dense thin films were synthesized and prepared through a Schiff base reaction and ionic interaction from dialdehyde polyurethane, N-carboxyethyl chitosan, and double-bonded chitosan grafted polypyrrole. The thin films were air-dried under room temperature. These thin films showed hydrophilicity and conductivity (above 2.50 mS/cm) as well as responsiveness to the deformation. The tensile break strength (9.72 MPa to 15.07 MPa) and tensile elongation (5.76% to 12.77%) of conductive thin films were enhanced by heating them from 25 °C to 50 °C. In addition, neural stem cells cultured on the conductive thin films showed cell clustering, proliferation, and differentiation. The application of the materials as a conductive surface coating was verified by different coating strategies. The conductive thin films are potential candidates for surface modification and biocompatible polymer coating.

Modulation of Differentiation of Embryonic Stem Cells by Polypyrrole: The Impact on Neurogenesis

The active role of biomaterials in the regeneration of tissues and their ability to modulate the behavior of stem cells in terms of their differentiation is highly advantageous. Here, polypyrrole, as a representantive of electro-conducting materials, is found to modulate the behavior of embryonic stem cells. Concretely, the aqueous extracts of polypyrrole induce neurogenesis within embryonic bodies formed from embryonic stem cells. This finding ledto an effort to determine the physiological cascade which is responsible for this effect. The polypyrrole modulates signaling pathways of Akt and ERK kinase through their phosphorylation. These effects are related to the presence of low-molecular-weight compounds present in aqueous polypyrrole extracts, determined by mass spectroscopy. The results show that consequences related to the modulation of stem cell differentiation must also be taken into account when polypyrrole is considered as a biomaterial.

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.

Novel Electrode Designs for Neurostimulation in Regenerative Medicine: Activation of Stem Cells

Neural stem and progenitor cells (i.e., neural precursors) are found within specific regions in the central nervous system and have great regenerative capacity. These cells are electrosensitive and their behavior can be regulated by the presence of electric fields (EFs). Electrical stimulation is currently used to treat neurological disorders in a clinical setting. Herein we propose that electrical stimulation can be used to enhance neural repair by regulating neural precursor cell (NPC) kinetics and promoting their migration to sites of injury or disease. We discuss how intrinsic and extrinsic factors can affect NPC migration in the presence of an EF and how this impacts electrode design with the goal of enhancing tissue regeneration. We conclude with an outlook on future clinical applications of electrical stimulation and highlight technological advances that would greatly support these applications.

An Electroactive Oligo-EDOT Platform for Neural Tissue Engineering

The unique electrochemical properties of the conductive polymer poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) make it an attractive material for use in neural tissue engineering applications. However, inadequate mechanical properties, and difficulties in processing and lack of biodegradability have hindered progress in this field. Here, the functionality of PEDOT:PSS for neural tissue engineering is improved by incorporating 3,4-ethylenedioxythiophene (EDOT) oligomers, synthesized using a novel end-capping strategy, into block co-polymers. By exploiting end-functionalized oligoEDOT constructs as macroinitiators for the polymerization of poly(caprolactone), a block co-polymer is produced that is electroactive, processable, and bio-compatible. By combining these properties, electroactive fibrous mats are produced for neuronal culture via solution electrospinning and melt electrospinning writing. Importantly, it is also shown that neurite length and branching of neural stem cells can be enhanced on the materials under electrical stimulation, demonstrating the promise of these scaffolds for neural tissue engineering.

Conductive Biomaterials as Substrates for Neural Stem Cells Differentiation towards Neuronal Lineage Cells

The injuries and defects in the central nervous system are the causes of disability and death of an affected person. As of now, there are no clinically available methods to enhance neural structural regeneration and functional recovery of nerve injuries. Recently, some experimental studies claimed that the injuries in brain can be repaired by progenitor or neural stem cells located in the neurogenic sites of adult mammalian brain. Various attempts have been made to construct biomimetic physiological microenvironment for neural stem cells to control their ultimate fate. Conductive materials have been considered as one the best choices for nerve regeneration due to the capacity to mimic the microenvironment of stem cells and regulate the alignment, growth, and differentiation of neural stem cells. The review highlights the use of conductive biomaterials, e.g., polypyrrole, polyaniline, poly(3,4-ethylenedioxythiophene), multi-walled carbon nanotubes, single-wall carbon nanotubes, graphene, and graphite oxide, for controlling the neural stem cells activities in terms of proliferation and neuronal differentiation. The effects of conductive biomaterials in axon elongation and synapse formation for optimal repair of central nervous system injuries are also discussed.

Highly elastic, electroconductive, immunomodulatory graphene crosslinked collagen cryogel for spinal cord regeneration

Novel amino-functionalized graphene crosslinked collagen based nerve conduit having appropriate electric (3.8 ± 0.2 mSiemens/cm) and mechanical cues (having young modulus value of 100-347 kPa) for stem cell transplantation and neural tissue regeneration was fabricated using cryogelation. The developed conduit has shown sufficiently high porosity with interconnectivity between the pores. Raman spectroscopy analysis revealed the increase in orderliness and crosslinking of collagen molecules in the developed cryogel due to the incorporation of amino-functionalized graphene. BM-MSCs grown on graphene collagen cryogels have shown enhanced expression of CD90 and CD73 gene upon electric stimulation (100 mV/mm) contributing towards maintaining their stemness. Furthermore, an increased secretion of ATP from BM-MSCs grown on graphene collagen cryogel was also observed upon electric stimulation that may help in regeneration of neurons and immuno-modulation. Neuronal differentiation of BM-MSCs on graphene collagen cryogel in the presence of electric stimulus showed an enhanced expression of MAP-2 kinase and β-tubulin III. Immunohistochemistry studies have also demonstrated the improved neuronal differentiation of BM-MSCs. BM-MSCs grown on electro-conductive collagen cryogels under inflammatory microenvironment in vitro showed high indoleamine 2,3 dioxygenase activity. Moreover, macrophages cells grown on graphene collagen cryogels have shown high CD206 (M2 polarization marker) and CD163 (M2 polarization marker) and low CD86 (M1 polarization marker) gene expression demonstrating M2 polarization of macrophages, which may aid in tissue repair. In an organotypic culture, the developed cryogel conduit has supported cellular growth and migration from adult rat spinal cord. Thus, this novel electro-conductive graphene collagen cryogels have potential for suppressing the neuro-inflammation and promoting the neuronal cellular migration and proliferation, which is a major barrier during the spinal cord regeneration.

Microchip with Single-Cell Impedance Measurements for Monitoring Osteogenic Differentiation of Mesenchymal Stem Cells under Electrical Stimulation

Effective induction methods and in situ monitoring are essential for studying the mechanism of biological responses in stem cell differentiation. This article proposes an induction method incorporating electrical stimulation under an inhomogeneous field with single-cell impedance monitoring for studying osteogenic differentiation of mesenchymal stem cells (MSCs) using a microchip. The microchip contains an array of sextupole-electrode units for implementing a combination of controllable electrical stimulation and single-cell impedance measurements. MSCs are inducted to osteogenic differentiation under electrical stimulation using quadrupole electrodes and single-cell impedances are monitored in situ using a pair of microelectrodes at each unit center. The proposed microchip adopts an array design to monitor a number of MSCs in parallel, which improves measurement throughput and facilitates to carry out statistic tests. We perform osteogenic differentiation of MSCs on the microchip with and without electrical stimulation meanwhile monitoring single-cell impedance in real time for 21 days. The recorded impedance results show the detailed characteristic change of MSCs at the single-cell level during osteogenic differentiation, which demonstrates a significant difference between the conditions with and without electrical stimulation. The cell morphology and various staining analyses are also used to validate osteogenesis and correlate with the impedance expression. Correlation analysis of the impedance measurement, cell morphology, and various staining assays proves the great acceleration effect of the proposed electrical stimulation on osteogenic differentiation of MSCs. The proposed impedance method can monitor the dynamic process of cell development and study heterogeneity of stem cell differentiation at the single-cell level.

Electroconductive Hydrogels for Tissue Engineering: Current Status and Future Perspectives

Over the past decade, electroconductive hydrogels, integrating both the biomimetic attributes of hydrogels and the electrochemical properties of conductive materials, have gained significant attention. Hydrogels, three-dimensional and swollen hydrophilic polymer networks, are an important class of tissue engineering (TE) scaffolds owing to their microstructural and mechanical properties, ability to mimic the native extracellular matrix, and promote tissue repair. However, hydrogels are intrinsically insulating and therefore unable to emulate the complex electrophysiological microenvironment of cardiac and neural tissues. To overcome this challenge, electroconductive materials, including carbon-based materials, nanoparticles, and polymers, have been incorporated within nonconductive hydrogels to replicate the electrical and biological characteristics of biological tissues. This review gives a brief introduction on the rational design of electroconductive hydrogels and their current applications in TE, especially for neural and cardiac regeneration. The recent progress and development trends of electroconductive hydrogels, their challenges, and clinical translatability, as well as their future perspectives, with a focus on advanced manufacturing technologies, are also discussed.

Bioactive polymeric materials and electrical stimulation strategies for musculoskeletal tissue repair and regeneration

Electrical stimulation (ES) is predominantly used as a physical therapy modality to promote tissue healing and functional recovery. Research efforts in both laboratory and clinical settings have shown the beneficial effects of this technique for the repair and regeneration of damaged tissues, which include muscle, bone, skin, nerve, tendons, and ligaments. The collective findings of these studies suggest ES enhances cell proliferation, extracellular matrix (ECM) production, secretion of several cytokines, and vasculature development leading to better tissue regeneration in multiple tissues. However, there is still a gap in the clinical relevance for ES to better repair tissue interfaces, as ES applied clinically is ineffective on deeper tissue. The use of a conducting material can transmit the stimulation applied from skin electrodes to the desired tissue and lead to an increased function on the repair of that tissue. Ionically conductive (IC) polymeric scaffolds in conjunction with ES may provide solutions to utilize this approach effectively. Injectable IC formulations and their scaffolds may provide solutions for applying ES into difficult to reach tissue types to enable tissue repair and regeneration. A better understanding of ES-mediated cell differentiation and associated molecular mechanisms including the immune response will allow standardization of procedures applicable for the next generation of regenerative medicine. ES, along with the use of IC scaffolds is more than sufficient for use as a treatment option for single tissue healing and may fulfill a role in interfacing multiple tissue types during the repair process.

Schwann Cell-Like Cells: Origin and Usability for Repair and Regeneration of the Peripheral and Central Nervous System

Functional recovery after neurotmesis, a complete transection of the nerve fiber, is often poor and requires a surgical procedure. Especially for longer gaps (>3 mm), end-to-end suturing of the proximal to the distal part is not possible, thus requiring nerve graft implantation. Artificial nerve grafts, i.e., hollow fibers, hydrogels, chitosan, collagen conduits, and decellularized scaffolds hold promise provided that these structures are populated with Schwann cells (SC) that are widely accepted to promote peripheral and spinal cord regeneration. However, these cells must be collected from the healthy peripheral nerves, resulting in significant time delay for treatment and undesired morbidities for the donors. Therefore, there is a clear need to explore the viable source of cells with a regenerative potential similar to SC. For this, we analyzed the literature for the generation of Schwann cell-like cells (SCLC) from stem cells of different origins (i.e., mesenchymal stem cells, pluripotent stem cells, and genetically programmed somatic cells) and compared their biological performance to promote axonal regeneration. Thus, the present review accounts for current developments in the field of SCLC differentiation, their applications in peripheral and central nervous system injury, and provides insights for future strategies.

Heparin mimics and fibroblast growth factor-2 fabricated nanogold composite in promoting neural differentiation of mouse embryonic stem cells

The replacement therapy or transplantation using neural cells, which differentiated from stem cells, has emerged as a promising strategy for repairing damaged neural tissues and helping functional recovery in the treatment of neural system diseases. The challenge, however, is how to control embryonic stem cell fate so that neural differentiation can be efficiently directed to enrich a neuron cell population, and meanwhile to maintain their bioactivities. This is a key question and has a very significant impact in regenerative medicine. Here we proposed a new neural-differentiation inductive nanocomposite, containing gold nanoparticles (AuNPs), poly(2-methacrylamido glucopyranose-co-3-sulfopropyl acrylate) (PMS), and basic fibroblast growth factor (FGF2), for the high efficient directional neural-specific differentiation of mouse embryonic stem cells (mESCs). In this AuNP-PMS/FGF2 composite, PMS, playing as the high-active mimic of heparin/heparan sulfate (HS), is covalently anchored to AuNPs and bound with FGF2 on the surface of nanoparticles, forming a HS/FGF2 complex nanomimics to facilitate its binding to FGF receptor (FGFR) and promote high neural-inductive activity of mESCs. The stability, bioactivity and biocompatibility of the composite are investigated in this study. The results showed that the AuNP-PMS/FGF2 composite could maintain a long-term stability at room temperature for at least 8 days, and greatly promote the neural differentiation of mESCs. Compared with the other materials, the AuNP-PMS/FGF2 composite could significantly stimulate the expression of the specific neural differentiation markers (nestin and β3-tubulin), while obviously down-regulate the mRNA production of pluripotency marker Oct-4 in mESCs. Moreover, the promotion effect of the composite on neuronal maturation marker β3-tubulin expression achieved maximally at the low concentration of FGF2 (4 ng/mL), which suggested the high efficiency of AuNP-PMS/FGF2 composite in neural differentiation of mESCs. Meanwhile, both mESCs and L929 cells showed desirable growth during the incubation with AuNP-PMS/FGF2 composite. The AuNP-PMS/FGF2 system presents a new way to achieve HS/FGF2 complex nanomimics efficiently for the neural differentiation of mESCs.

Engineering the ABIO-BIO interface of neurostimulation electrodes using polypyrrole and bioactive hydrogels

Following spinal cord injury, the use of electrodes for neurostimulation in animal models has been shown to stimulate muscle movement, however, the efficacy of such treatment is impaired by increased interfacial impedance caused by fibrous encapsulation of the electrode. Sputter-deposited gold-on-polyimide electrodes were modified by potentiostatic electrodeposition of poly(pyrrole-co-3-pyrrolylbutyrate-conj-aminoethylmethacrylate): sulfopropyl methacrylate [P(Py-co-PyBA-conj-AEMA):SPMA] to various charge densities (0–100 mC/cm2) to address interfacial impedance and coated with a phosphoryl choline containing bioactive hydrogel to address biocompatibility at the ABIO-BIO interface. Electrodes were characterized with scanning electron microscopy (surface morphology), multiple-scan rate cyclic voltammetry (peak current and electroactive area), and electrochemical impedance spectroscopy (charge transfer resistance and membrane resistance). SEM analysis and electroactive area calculations identified films fabricated with a charge density of 50 mC/cm2 as well suited for neurostimulation electrodes. Charge transfer resistance demonstrated a strong inverse correlation (−0.83) with charge density of electrodeposition. On average, the addition of polypyrrole and hydrogel to neurostimulation electrodes decreased charge transfer resistance by 82 %. These results support the use of interfacial engineering techniques to mitigate high interfacial impedance and combat the foreign body response towards epidurally implanted neurostimulation electrodes.

Electrochemical quartz crystal microbalance with dissipation investigation of fibronectin adsorption dynamics driven by electrical stimulation onto a conducting and partially biodegradable copolymer

Functional surface coatings are a key option for biomedical applications, from polymeric supports for tissue engineering to smart matrices for controlled drug delivery. Therefore, the synthesis of new materials for biological applications and developments is promising. Hence, biocompatible and stimuli-responsive polymers are interesting materials, especially when they present conductive properties. PEDOT-co-PDLLA graft copolymer exhibits physicochemical and mechanical characteristics required for biomedical purposes, associated with electroactive, biocompatible, and partially biodegradable properties. Herein, the study of fibronectin (FN) adsorption onto PEDOT-co-PDLLA carried out by an electrochemical quartz crystal microbalance with dissipation is reported. The amount of FN adsorbed onto PEDOT-co-PDLLA was higher than that adsorbed onto the Au surface, with a significant increase when electrical stimulation was applied (either at +0.5 or -0.125 V). Additionally, FN binds to the copolymer interface in an unfolded conformation, which can promote better NIH-3T3 fibroblast cell adhesion and later cell development.

Conducting Polymer Mediated Electrical Stimulation Induces Multilineage Differentiation with Robust Neuronal Fate Determination of Human Induced Pluripotent Stem Cells

Electrical stimulation is increasingly being used to modulate human cell behaviour for biotechnological research and therapeutics. Electrically conductive polymers (CPs) such as polypyrrole (PPy) are amenable to in vitro and in vivo cell stimulation, being easy to synthesise with different counter ions (dopants) to augment biocompatibility and cell-effects. Extending our earlier work, which showed that CP-mediated electrical stimulation promotes human neural stem cell differentiation, here we report using electroactive PPy containing the anionic dopant dodecylbenzenesulfonate (DBS) to modulate the fate determination of human induced pluripotent stem cells (iPSCs). Remarkably, the stimulation without conventional chemical inducers resulted in the iPSCs differentiating to cells of the three germ lineages-endoderm, ectoderm, and mesoderm. The unstimulated iPSC controls remained undifferentiated. Phenotypic characterisation further showed a robust induction to neuronal fate with electrical stimulation, again without customary chemical inducers. Our findings add to the growing body of evidence supporting the use of electrical stimulation to augment stem cell differentiation, more specifically, pluripotent stem cell differentiation, and especially neuronal induction. Moreover, we have shown the versatility of electroactive PPy as a cell-compatible platform for advanced stem cell research and translation, including identifying novel mechanisms of fate regulation, tissue development, electroceuticals, and regenerative medicine.

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.

Three-dimensional directional nerve guide conduits fabricated by dopamine-functionalized conductive carbon nanofibre-based nanocomposite ink printing

Directional growth induced by dopamine-functionalized CNF-based nanocomposite ink printing.

Controlling properties of human neural progenitor cells using 2D and 3D conductive polymer scaffolds

Human induced pluripotent stem cell-derived neural progenitor cells (hNPCs) are a promising cell source for stem cell transplantation to treat neurological diseases such as stroke and peripheral nerve injuries. However, there have been limited studies investigating how the dimensionality of the physical and electrical microenvironment affects hNPC function. In this study, we report the fabrication of two- and three-dimensional (2D and 3D respectively) constructs composed of a conductive polymer to compare the effect of electrical stimulation of hydrogel-immobilized hNPCs. The physical dimension (2D vs 3D) of stimulating platforms alone changed the hNPCs gene expression related to cell proliferation and metabolic pathways. The addition of electrical stimulation was critical in upregulating gene expression of neurotrophic factors that are important in regulating cell survival, synaptic remodeling, and nerve regeneration. This study demonstrates that the applied electrical field controls hNPC properties depending on the physical nature of stimulating platforms and cellular metabolic states. The ability to control hNPC functions can be beneficial in understanding mechanistic changes related to electrical modulation and devising novel treatment methods for neurological diseases.

Electrical stimulation as a novel tool for regulating cell behavior in tissue engineering

Recently, electrical stimulation as a physical stimulus draws lots of attention. It shows great potential in disease treatment, wound healing, and mechanism study because of significant experimental performance. Electrical stimulation can activate many intracellular signaling pathways, and influence intracellular microenvironment, as a result, affect cell migration, cell proliferation, and cell differentiation. Electrical stimulation is using in tissue engineering as a novel type of tool in regeneration medicine. Besides, with the advantages of biocompatible conductive materials coming into view, the combination of electrical stimulation with suitable tissue engineered scaffolds can well combine the benefits of both and is ideal for the field of regenerative medicine. In this review, we summarize the various materials and latest technologies to deliver electrical stimulation. The influences of electrical stimulation on cell alignment, migration and its underlying mechanisms are discussed. Then the effect of electrical stimulation on cell proliferation and differentiation are also discussed.

Electrically Conductive Materials: Opportunities and Challenges in Tissue Engineering

Tissue engineering endeavors to regenerate tissues and organs through appropriate cellular and molecular interactions at biological interfaces. To this aim, bio-mimicking scaffolds have been designed and practiced to regenerate and repair dysfunctional tissues by modifying cellular activity. Cellular activity and intracellular signaling are performances given to a tissue as a result of the function of elaborated electrically conductive materials. In some cases, conductive materials have exhibited antibacterial properties; moreover, such materials can be utilized for on-demand drug release. Various types of materials ranging from polymers to ceramics and metals have been utilized as parts of conductive tissue engineering scaffolds, having conductivity assortments from a range of semi-conductive to conductive. The cellular and molecular activity can also be affected by the microstructure; therefore, the fabrication methods should be evaluated along with an appropriate selection of conductive materials. This review aims to address the research progress toward the use of electrically conductive materials for the modulation of cellular response at the material-tissue interface for tissue engineering applications.

Human Neural Tissues from Neural Stem Cells Using Conductive Biogel and Printed Polymer Microelectrode Arrays for 3D Electrical Stimulation

Electricity is important in the physiology and development of human tissues such as embryonic and fetal development, and tissue regeneration for wound healing. Accordingly, electrical stimulation (ES) is increasingly being applied to influence cell behavior and function for a biomimetic approach to in vitro cell culture and tissue engineering. Here, the application of conductive polymer (CP) poly(3,4-ethylenedioxythiophene)-polystyrenesulfonate (PEDOT:PSS) pillars is described, direct-write printed in an array format, for 3D ES of maturing neural tissues that are derived from human neural stem cells (NSCs). NSCs are initially encapsulated within a conductive polysaccharide-based biogel interfaced with the CP pillar microelectrode arrays (MEAs), followed by differentiation in situ to neurons and supporting neuroglia during stimulation. Electrochemical properties of the pillar electrodes and the biogel support their electrical performance. Remarkably, stimulated constructs are characterized by widespread tracts of high-density mature neurons and enhanced maturation of functional neural networks. Formation of tissues using the 3D MEAs substantiates the platform for advanced clinically relevant neural tissue induction, with the system likely amendable to diverse cell types to create other neural and non-neural tissues. The platform may be useful for both research and translation, including modeling tissue development, function and dysfunction, electroceuticals, drug screening, and regenerative medicine.

Neural responses to electrical stimulation in 2D and 3D in vitro environments

Electrical stimulation (ES) to manipulate the central (CNS) and peripheral nervous system (PNS) has been explored for decades, recently gaining momentum as bioelectronic medicine advances. The application of ES in vitro to modulate a variety of cellular functions, including regenerative potential, migration, and stem cell fate, are being explored to aid neural degeneration, dysfunction, and injury. This review describes the materials and approaches for the application of ES to the PNS and CNS microenvironments, towards an improved understanding of how ES can be harnessed for beneficial clinical applications. Emphasized are some recent advances in ES, including conductive polymers, methods of charge transfer, impact on neural cells, and a brief overview of alternative methodologies for cellular targeting including magneto, ultrasonic, and optogenetic stimulation. This review will examine how heterogenous cell populations, including neurons, glia, and neural stem cells respond to a wide range of conductive 2D and 3D substrates, stimulation regimes, known mechanisms of response, and how cellular sources impact the response to ES.

Patterned iridium oxide film as neural electrode interface: Biocompatibility and improved neurite outgrowth with electrical stimulation

Iridium (Ir) thin film was deposited on patterned titanium substrate by direct-current (DC) magnetron sputtering, and then activated in sulfuric acid (H₂SO₄) through repetitive potential sweeps to form iridium oxide (IrO_x) as neural electrode interface. The resultant IrO_x film showed a porous and open morphology with aligned microstructure, exhibited superior electrochemical performance and excellent stability. The IrO_x film supported neural stem cells (NSCs) attachment, proliferation and improved processes without causing toxicity. The patterned IrO_x films offered a unique system to investigate the synergistic effects of topographical cue and electrical stimulation on neurite outgrowth. Electrical stimulation, when applied through patterned IrO_x films, was found to further increase the neurite extension of neuron-like cells and significantly reorient the neurite alignment towards to the direction of stimulation. These results indicate that IrO_x film, as electrode-tissue interface is highly stable and biocompatible with excellent electrochemical properties.

A Mussel-Inspired Persistent ROS-Scavenging, Electroactive, and Osteoinductive Scaffold Based on Electrochemical-Driven In Situ Nanoassembly

Conductive polymers are promising for bone regeneration because they can regulate cell behavior through electrical stimulation; moreover, they are antioxidative agents that can be used to protect cells and tissues from damage originating from reactive oxygen species (ROS). However, conductive polymers lack affinity to cells and osteoinductivity, which limits their application in tissue engineering. Herein, an electroactive, cell affinitive, persistent ROS-scavenging, and osteoinductive porous Ti scaffold is prepared by the on-surface in situ assembly of a polypyrrole-polydopamine-hydroxyapatite (PPy-PDA-HA) film through a layer-by-layer pulse electrodeposition (LBL-PED) method. During LBL-PED, the PPy-PDA nanoparticles (NPs) and HA NPs are in situ synthesized and uniformly coated on a porous scaffold from inside to outside. PDA is entangled with and doped into PPy to enhance the ROS scavenging rate of the scaffold and realize repeatable, efficient ROS scavenging over a long period of time. HA and electrical stimulation synergistically promote osteogenic cell differentiation on PPy-PDA-HA films. Ultimately, the PPy-PDA-HA porous scaffold provides excellent bone regeneration through the synergistic effects of electroactivity, cell affinity, and antioxidative activity of the PPy-PDA NPs and the osteoinductivity of HA NPs. This study provides a new strategy for functionalizing porous scaffolds that show great promise as implants for tissue regeneration.

Electrical stimulation affects neural stem cell fate and function in vitro

Electrical stimulation (ES) has been applied in cell culture system to enhance neural stem cell (NSC) proliferation, neuronal differentiation, migration, and integration. According to the mechanism of its function, ES can be classified into induced electrical (EFs) and electromagnetic fields (EMFs). EFs guide axonal growth and induce directional cell migration, whereas EMFs promote neurogenesis and facilitates NSCs to differentiate into functional neurons. Conductive nanomaterials have been used as functional scaffolds to provide mechanical support and biophysical cues in guiding neural cell growth and differentiation and building complex neural tissue patterns. Nanomaterials may have a combined effect of topographical and electrical cues on NSC migration and differentiation. Electrical cues may promote NSC neurogenesis via specific ion channel activation, such as SCN1α and CACNA1C. To accelerate the future application of ES in preclinical research, we summarized the specific setting, such as current frequency, intensity, and stimulation duration used in various ES devices, as well as the nanomaterials involved, in this review with the possible mechanisms elucidated. This review can be used as a checklist for ES work in stem cell research to enhance the translational process of NSCs in clinical application.

Fabrication of a Biocompatible Liquid Crystal Graphene Oxide-Gold Nanorods Electro- and Photoactive Interface for Cell Stimulation

For decades, electrode-tissue interfaces are pursued to establish electrical stimulation as a reliable means to control neuronal cells behavior. However, spreading of electrical currents in tissues limits its spatial precision. Thus, optical cues, such as near-infrared (NIR) light, are explored as alternatives. Presently, NIR stimulation requires higher energy input than electrical methods despite introduction of light absorbers, e.g., gold nanoparticles. As potential solution, NIR and electrical costimulation are proposed but with limited interfaces capable of sustaining this stimulation technique. Here, a novel electroactive nanocomposite with photoactive properties in the NIR range is constructed by N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide hydrochloride/N-hydroxysulfosuccinimide sodium (EDC)/NHS conjugation of liquid crystal graphene oxide (LCGO) to protein-coated gold nanorods (AuNR). The liquid crystal graphene oxide-gold nanorod nanocomposite (LCGO-AuNR) is fabricated into a hydrophilic electrode-coating via drop-casting, making it appropriate for versatile electrode-tissue interface fabrication. UV-vis spectrophotometry results demonstrate that LCGO-AuNR presents an absorbance peak at 798 nm (NIR range). Cyclic voltammetry measurements further confirm its electroactive capacitive properties. Furthermore, LCGO-AuNR coating supports cell adhesion, proliferation, and differentiation of NG108-15 neuronal cells. This biocompatible interface is anticipated, with ideal electrical and optical properties for NIR and electrical costimulation, to enable further development of the technique for energy-efficient and precise neuronal cell modulation.

Effect of monophasic pulsed stimulation on live single cell de-adhesion on conducting polymers with adsorbed fibronectin as revealed by single cell force spectroscopy

The force required to detach a single fibroblast cell in contact with the conducting polymer, polypyrrole doped with dodecylbenzene, was quantified using the Atomic Force Microscope-based technique, Single Cell Force Spectroscopy. The de-adhesion force for a single cell was 0.64 ± 0.03 nN and predominately due to unbinding of α5β1 integrin complexes with surface adsorbed fibronectin, as confirmed by blocking experiments using antibodies. Monophasic pulsed stimulation (50 μs pulse duration) superimposed on either an applied oxidation (+500) or reduction (-500 mV) constant voltage caused a significant decrease in the de-adhesion force by 30%-45% to values ranging from 0.34 to 0.43 nN (±0.02 nN). The electrical stimulation caused a reduction in the molecular-level jump and plateau interactions, while an opposing increase in nonspecific interactions was observed during the cell de-adhesion process. Due to the monophasic pulsed stimulation, there is an apparent change or weakening of the cell membrane properties, which is suggested to play a role in reducing the cell de-adhesion. Based on this study, pulsed stimulation with optimized threshold parameters represents a possible approach to tune cell interactions and adhesion on conducting polymers.

Conductive polymers to modulate the post-stroke neural environment

Despite the prevalence of stroke, therapies to augment recovery remain limited. Here we focus on the use of conductive polymers for cell delivery, drug release, and electrical stimulation to optimize the post-stroke environment for neural recovery. Conductive polymers and their interactions with in vitro and in vivo neural systems are explored. The ability to continuously modify the neural environment utilizing conductive polymers provides applications in directing stem cell differentiation and increasing neural repair. This exciting class of polymers offers new approaches to optimizing the post-stroke brain to improve functional recovery.

In situ thermo-co-electroresponsive mucogel for controlled release of bioactive agent

The purpose of this work was to develop an in situ thermosensitive electro-responsive mucoadhesive gel loaded with bioactive agent (nanocomposite) meant for nose to brain delivery in a controllable manner when electric stimulation is applied. Nanocomposite was developed using a combinatorial blending of chitosan, hydroxypropylmethylcellulose, pluronic F127 and polyaniline which was then loaded with BCNU-Nano-co-Plex (the bioactive agent). The nanocomposite was a liquid at room temperature but formed an in situ mucogel at a temperature of 27.5 ± 0.5 °C. Furthermore, the nanocomposite possessed a redox element which makes it responsive to electrical stimulation (ES). The stimuli responsiveness enabled the formulation to release the bioactive agent when electrical potential was applied and demonstrated a desired 10.28% release of nanoparticles per application cycle. The results further revealed pore formation within the formulation which accommodated the loaded nanoparticles. The release profile also demonstrated a pulsatile release of the bioactive material when subjected to ES. This formulation may therefore be useful as a nose to brain drug delivery system that can be modulated to deliver bioactive agents to the brain via electro-actuation in an "on-off" drug release kinetics by means of an external ES for a controlled nose-to-brain delivery.

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.

Matrices, scaffolds & carriers for cell delivery in nerve regeneration

Nerve injuries can be life-long debilitating traumas that severely impact patients' quality of life. While many acellular neural scaffolds have been developed to aid the process of nerve regeneration, complete functional recovery is still very difficult to achieve, especially for long-gap peripheral nerve injury and most cases of spinal cord injury. Cell-based therapies have shown many promising results for improving nerve regeneration. With recent advances in neural tissue engineering, the integration of biomaterial scaffolds and cell transplantation are emerging as a more promising approach to enhance nerve regeneration. This review provides an overview of important considerations for designing cell-carrier biomaterial scaffolds. It also discusses current biomaterials used for scaffolds that provide permissive and instructive microenvironments for improved cell transplantation.

State-of-the-Art Techniques in Treating Peripheral Nerve Injury

BACKGROUND

Peripheral nerve injuries remain a major clinical concern, as they often lead to chronic disability and significant health care expenditures. Despite advancements in microsurgical techniques to enhance nerve repair, biological approaches are needed to augment nerve regeneration and improve functional outcomes after injury.

METHODS

Presented herein is a review of the current literature on state-of-the-art techniques to enhance functional recovery for patients with nerve injury. Four categories are considered: (1) electroceuticals, (2) nerve guidance conduits, (3) fat grafting, and (4) optogenetics. Significant study results are highlighted, focusing on histologic and functional outcome measures.

RESULTS

This review documents the current state of the literature. Advancements in neuronal stimulation, tissue engineering, and cell-based therapies demonstrate promise with regard to augmenting nerve regeneration and appropriate rehabilitation.

CONCLUSIONS

The future of treating peripheral nerve injury will include multimodality use of electroconductive conduits, fat grafting, neuronal stimulation, and optogenetics. Further clinical investigation is needed to confirm the efficacy of these technologies on peripheral nerve recovery in humans, and how best to implement this treatment for a diverse population of nerve-injured patients.

Characterization of rabbit urine-derived stem cells for potential application in lower urinary tract tissue regeneration

Tissue-engineered urethra with autologous cells seeded on biodegradable scaffolds offers an alternative for lower urinary tract reconstruction. Rabbit is most commonly used as an animal model in urethra and bladder tissue repair. The goal of this study is to characterize rabbit urine-derived stem cells (rUSC) and induce these cells to differentiate into urothelial and smooth muscle cells as an autologous cell source for potential use in lower urinary tract tissue regeneration in a rabbit model. We successfully cultured rUSC from 12 urine samples and 13 bladder wash samples of six rabbits. rUSC colonies appeared more in the bladder wash solution (2-4/15 ml) than those in the urine samples (1-2 clones/15 ml urine). The cells displayed rice grain-like in morphology. Mean population doubling of rUSC was 48.5 ± 6.2 and average doubling time was 25.7 ± 8.4 h, indicating that a single of rUSC clone generated about 4 × 10¹⁴ cells in 50 days. The rUSC were positive for CD29, CD90 and CD105 but negative for CD31, CD34 and CD45 in flow cytometry. When exposed to PDGF-BB and TGF-β1, these cells could differentiate into spindle-like cells, expressing smooth muscle-specific proteins, including α-smooth muscle action, desmin and myosin. Urothelially differentiated rUSC expressed urothelial-specific proteins, i.e., AE1/AE3 and E-cadherin when exposed to epidermal growth factor (EGF). Osteogenic-differentiated rUSC expressed osteogenic marker, i.e., alkaline phosphatase when exposed to serum containing DMEM low-glucose medium with osteogenic supplements. In conclusion, rUSC can be isolated from bladder wash or urine samples and cultured in vitro. There stem cells possess strong proliferative ability and are capable of differentiating in urothelial, myogenic and osteogenic lineages. Thus, rUSC are a potential alternative autologous cell source for lower urinary tract repair with tissue engineering technology in a rabbit model.

Advances in Controlling Differentiation of Adult Stem Cells for Peripheral Nerve Regeneration

Adult stems cells, possessing the ability to grow, migrate, proliferate, and transdifferentiate into various specific phenotypes, constitute a great asset for peripheral nerve regeneration. Adult stem cells' ability to undergo transdifferentiation is sensitive to various cell-to-cell interactions and external stimuli involving interactions with physical, mechanical, and chemical cues within their microenvironment. Various studies have employed different techniques for transdifferentiating adult stem cells from distinct sources into specific lineages (e.g., glial cells and neurons). These techniques include chemical and/or electrical induction as well as cell-to-cell interactions via co-culture along with the use of various 3D conduit/scaffold designs. Such scaffolds consist of unique materials that possess controllable physical/mechanical properties mimicking cells' natural extracellular matrix. However, current limitations regarding non-scalable transdifferentiation protocols, fate commitment of transdifferentiated stem cells, and conduit/scaffold design have required new strategies for effective stem cells transdifferentiation and implantation. In this progress report, a comprehensive review of recent advances in the transdifferentiation of adult stem cells via different approaches along with multifunctional conduit/scaffolds designs is presented for peripheral nerve regeneration. Potential cellular mechanisms and signaling pathways associated with differentiation are also included. The discussion with current challenges in the field and an outlook toward future research directions is concluded.

Electrical Stimulation with a Conductive Polymer Promotes Neurite Outgrowth and Synaptogenesis in Primary Cortical Neurons in 3D

Deficits in neurite outgrowth and synaptogenesis have been recognized as an underlying developmental aetiology of psychosis. Electrical stimulation promotes neuronal induction including neurite outgrowth and branching. However, the effect of electrical stimulation using 3D electrodes on neurite outgrowth and synaptogenesis has not been explored. This study examined the effect of 3D electrical stimulation on 3D primary cortical neuronal cultures. 3D electrical stimulation improved neurite outgrowth in 3D neuronal cultures from both wild-type and NRG1-knockout (NRG1-KO) mice. The expression of synaptophysin and PSD95 were elevated under 3D electrical stimulation. Interestingly, 3D electrical stimulation also improved neural cell aggregation as well as the expression of PSA-NCAM. Our findings suggest that the 3D electrical stimulation system can rescue neurite outgrowth deficits in a 3D culturing environment, one that more closely resembles the in vivo biological system compared to more traditionally used 2D cell culture, including the observation of cell aggregates as well as the upregulated PSA-NCAM protein and transcript expression. This study provides a new concept for a possible diagnostic platform for neurite deficits in neurodevelopmental diseases, as well as a viable platform to test treatment options (such as drug delivery) in combination with electrical stimulation.

Polypyrrole increases branching and neurite extension by Neuro2A cells on PBAT ultrathin fibers

We present a methodology for production and application of electrospun hybrid materials containing commercial polyester (poly (butylene adipate-co-terephthalate; PBAT), and a conductive polymer (polypyrrole; PPy) as scaffold for neuronal growth and differentiation. The physical-chemical properties of the scaffolds and optimization of the electrospinning parameters are presented. The electrospun scaffolds are biocompatible and allow proper adhesion and spread of mesenchymal stem cells (MSCs). Fibers produced with PBAT with or without PPy were used as scaffold for Neuro2a mouse neuroblastoma cells adhesion and differentiation. Neuro2a adhered to PBAT and PBAT/PPy2% scaffolds without laminin coating. However, Neuro2a failed to differentiate in PBAT when stimulated by treatment with retinoic acid (RA), but differentiated in PBAT/PPy2% fibers. We hypothesize that PBAT hydrophobicity inhibited proper spreading and further differentiation, and inhibition was overcome by coating the PBAT fibers with laminin. We conclude that fibers produced with the combination of PBAT and PPy can support neuronal differentiation.

Use of conducting polymers to facilitate neurite branching in schizophrenia-related neuronal development

Schizophrenia (SCZ) is a debilitating mental disorder which results in high healthcare and loss of productivity costs to society. This disease remains poorly understood, however there is increasing evidence suggesting a role for oxidative damage in the disease etiology. We aimed to examine the effect of the conducting polymer polypyrrole on the growth and morphology of both wildtype and neuregulin-1 knock out (NRG-1 +/-) explant cells. Polypyrrole is an organic conducting polymer known to be cytocompatible and capable of acting as a platform for effective stimulation of neurons. Here we demonstrate for the first time the ability of this material to mediate processes occurring in disease affected neurons: schizophrenic model cortical neurons. Prefrontal cortical cells were grown on conducting polymer scaffolds of specific composition and showed significantly increased neurite branching and outgrowth length on the polymers compared to controls. Concurrently, a more significant enhancement was seen in both parameters in the NRG-1 +/- model cells. This finding implies that conducting polymers such as polypyrrole may be utilised to overcome neuro-functional deficits associated with neurological disease in humans.

PEDOT doped with algal, mammalian and synthetic dopants: polymer properties, protein and cell interactions, and influence of electrical stimulation on neuronal cell differentiation

PEDOT films were electrochemically polymerised with synthetic and biological dopants, characterised, and their interactions with proteins and neuronal cells investigated.