SIROF stabilized PEDOT/PSS allows biocompatible and reversible direct current stimulation capable of driving electrotaxis in cells

Electrodeposited coatings for neural electrodes: A review

Neural electrodes play a pivotal role in ensuring safe stimulation and high-quality recording for various bioelectronics such as neuromodulation devices and brain-computer interfaces. With the miniaturization of electrodes and the increasing demand for multi-functionality, the incorporation of coating materials via electrodeposition to enhance electrodes performance emerges as a highly effective strategy. These coatings not only substantially improve the stimulation and recording performance of electrodes but also introduce additional functionalities. This review began by outlining the application scenarios and critical requirements of neural electrodes. It then delved into the deposition principles and key influencing factors. Furthermore, the advancements in the electrochemical performance and adhesion stability of these coatings were reviewed. Ultimately, the latest innovative works in the electrodeposited coating applications were highlighted, and future perspectives were summarized.

Analysis of electrochemical impedance spectroscopy data for sputtered iridium oxide electrodes

Objective. Our objective was to perform a complete analysis ofin-vitroimpedance data for sputtered iridium oxide film (SIROF) micro-electrodes. The analysis included quantification of the stochastic and bias error structure and development of a process model that accounted for the chemistry and physics of the electrode-electrolyte interface.Approach. The measurement model program was used to analyze electrochemical impedance spectroscopy (EIS) data for SIROF micro-electrodes at potentials ranging from -0.4 to +0.6 V(Ag|AgCl). The frequency range used for the analysis was that determined to be consistent with the Kramers-Kronig relations. Interpretation of the data was enabled by truncating frequencies at which the ohmic impedance influenced the impedance.Main results. An interpretation model was developed that considered the impedance of the bare surface and the contribution of a porous component, based on the de Levie model of porous electrodes. The influence of iridium oxidation state on impedance was included. The proposed model fit all 36 EIS spectra well. The effective capacitance of the SIROF system ranged from 32 mF cm-2at -0.4 V(Ag|AgCl) to a maximum of 93 mF cm-2at 0.2 and 0.4 V(Ag|AgCl).Significance. The model developed to interpret the impedance response of neural stimulation electrodesin vitroguides model development forin-vivostudies.

Biodegradable stimulating electrodes for resident neural stem cell activation in vivo

Brain stimulation has been recognized as a clinically effective strategy for treating neurological disorders. Endogenous brain neural precursor cells (NPCs) have been shown to be electrosensitive cells that respond to electrical stimulation by expanding in number, undergoing directed cathodal migration, and differentiating into neural phenotypes in vivo, supporting the application of electrical stimulation to promote neural repair. In this study, we present the design of a flexible and biodegradable brain stimulation electrode for temporally regulated neuromodulation of NPCs. Leveraging the cathodally skewed electrochemical window of molybdenum and the volumetric charge transfer properties of conductive polymer, we engineered the electrodes with high charge injection capacity for the delivery of biphasic monopolar stimulation. We demonstrate that the electrodes are biocompatible and can deliver an electric field sufficient for NPC activation for 7 days post implantation before undergoing resorption in physiological conditions, thereby eliminating the need for surgical extraction. The biodegradable electrode demonstrated its potential to be used for NPC-based neural repair strategies.

Interfacing with the Brain: How Nanotechnology Can Contribute

Interfacing artificial devices with the human brain is the central goal of neurotechnology. Yet, our imaginations are often limited by currently available paradigms and technologies. Suggestions for brain-machine interfaces have changed over time, along with the available technology. Mechanical levers and cable winches were used to move parts of the brain during the mechanical age. Sophisticated electronic wiring and remote control have arisen during the electronic age, ultimately leading to plug-and-play computer interfaces. Nonetheless, our brains are so complex that these visions, until recently, largely remained unreachable dreams. The general problem, thus far, is that most of our technology is mechanically and/or electrically engineered, whereas the brain is a living, dynamic entity. As a result, these worlds are difficult to interface with one another. Nanotechnology, which encompasses engineered solid-state objects and integrated circuits, excels at small length scales of single to a few hundred nanometers and, thus, matches the sizes of biomolecules, biomolecular assemblies, and parts of cells. Consequently, we envision nanomaterials and nanotools as opportunities to interface with the brain in alternative ways. Here, we review the existing literature on the use of nanotechnology in brain-machine interfaces and look forward in discussing perspectives and limitations based on the authors' expertise across a range of complementary disciplines─from neuroscience, engineering, physics, and chemistry to biology and medicine, computer science and mathematics, and social science and jurisprudence. We focus on nanotechnology but also include information from related fields when useful and complementary.

Decellularized Biohybrid Nerve Promotes Motor Axon Projections

Developing nerve grafts with intact mesostructures, superior conductivity, minimal immunogenicity, and improved tissue integration is essential for the treatment and restoration of neurological dysfunctions. A key factor is promoting directed axon growth into the grafts. To achieve this, biohybrid nerves are developed using decellularized rat sciatic nerve modified by in situ polymerization of poly(3,4-ethylenedioxythiophene) (PEDOT). Nine biohybrid nerves are compared with varying polymerization conditions and cycles, selecting the best candidate through material characterization. These results show that a 1:1 ratio of FeCl3 oxidant to ethylenedioxythiophene (EDOT) monomer, cycled twice, provides superior conductivity (>0.2 mS cm-1), mechanical alignment, intact mesostructures, and high compatibility with cells and blood. To test the biohybrid nerve's effectiveness in promoting motor axon growth, human Spinal Cord Spheroids (hSCSs) derived from HUES 3 Hb9:GFP cells are used, with motor axons labeled with green fluorescent protein (GFP). Seeding hSCS onto one end of the conduit allows motor axon outgrowth into the biohybrid nerve. The construct effectively promotes directed motor axon growth, which improves significantly after seeding the grafts with Schwann cells. This study presents a promising approach for reconstructing axonal tracts in humans.

Electrical Microneedles for Wound Treatment

Electrical stimulation has been hotpot research and provoked extensive interest in a broad application such as wound closure, tissue injury repair, and nerve engineering. In particular, immense efforts have been dedicated to developing electrical microneedles, which demonstrate unique features in terms of controllable drug release, real-time monitoring, and therapy, thus greatly accelerating the process of wound healing. Here, a review of state-of-art research concerning electrical microneedles applied for wound treatment is presented. After a comprehensive analysis of the mechanisms of electrical stimulation on wound healing, the derived three types of electrical microneedles are clarified and summarized. Further, their applications in wound healing are highlighted. Finally, current perspectives and directions for the development of future electrical microneedles in improving wound healing are addressed.

Bioelectronics for electrical stimulation: materials, devices and biomedical applications

Bioelectronics is a hot research topic, yet an important tool, as it facilitates the creation of advanced medical devices that interact with biological systems to effectively diagnose, monitor and treat a broad spectrum of health conditions. Electrical stimulation (ES) is a pivotal technique in bioelectronics, offering a precise, non-pharmacological means to modulate and control biological processes across molecular, cellular, tissue, and organ levels. This method holds the potential to restore or enhance physiological functions compromised by diseases or injuries by integrating sophisticated electrical signals, device interfaces, and designs tailored to specific biological mechanisms. This review explains the mechanisms by which ES influences cellular behaviors, introduces the essential stimulation principles, discusses the performance requirements for optimal ES systems, and highlights the representative applications. From this review, we can realize the potential of ES based bioelectronics in therapy, regenerative medicine and rehabilitation engineering technologies, ranging from tissue engineering to neurological technologies, and the modulation of cardiovascular and cognitive functions. This review underscores the versatility of ES in various biomedical contexts and emphasizes the need to adapt to complex biological and clinical landscapes it addresses.

From electricity to vitality: the emerging use of piezoelectric materials in tissue regeneration

The unique ability of piezoelectric materials to generate electricity spontaneously has attracted widespread interest in the medical field. In addition to the ability to convert mechanical stress into electrical energy, piezoelectric materials offer the advantages of high sensitivity, stability, accuracy and low power consumption. Because of these characteristics, they are widely applied in devices such as sensors, controllers and actuators. However, piezoelectric materials also show great potential for the medical manufacturing of artificial organs and for tissue regeneration and repair applications. For example, the use of piezoelectric materials in cochlear implants, cardiac pacemakers and other equipment may help to restore body function. Moreover, recent studies have shown that electrical signals play key roles in promoting tissue regeneration. In this context, the application of electrical signals generated by piezoelectric materials in processes such as bone healing, nerve regeneration and skin repair has become a prospective strategy. By mimicking the natural bioelectrical environment, piezoelectric materials can stimulate cell proliferation, differentiation and connection, thereby accelerating the process of self-repair in the body. However, many challenges remain to be overcome before these concepts can be applied in clinical practice, including material selection, biocompatibility and equipment design. On the basis of the principle of electrical signal regulation, this article reviews the definition, mechanism of action, classification, preparation and current biomedical applications of piezoelectric materials and discusses opportunities and challenges for their future clinical translation.

Bioelectronic Direct Current Stimulation at the Transition Between Reversible and Irreversible Charge Transfer

Many biological processes rely on endogenous electric fields (EFs), including tissue regeneration, cell development, wound healing, and cancer metastasis. Mimicking these biological EFs by applying external direct current stimulation (DCS) is therefore the key to many new therapeutic strategies. During DCS, the charge transfer from electrode to tissue relies on a combination of reversible and irreversible electrochemical processes, which may generate toxic or bio-altering substances, including metal ions and reactive oxygen species (ROS). Poly(3,4-ethylenedioxythiophene) (PEDOT) based electrodes are emerging as suitable candidates for DCS to improve biocompatibility compared to metals. This work addresses whether PEDOT electrodes can be tailored to favor reversible biocompatible charge transfer. To this end, different PEDOT formulations and their respective back electrodes are studied using cyclic voltammetry, chronopotentiometry, and direct measurements of H2O2 and O2. This combination of electrochemical methods sheds light on the time dynamics of reversible and irreversible charge transfer and the relationship between capacitance and ROS generation. The results presented here show that although all electrode materials investigated generate ROS, the onset of ROS can be delayed by increasing the electrode's capacitance via PEDOT coating, which has implications for future bioelectronic devices that allow longer reversibly driven pulse durations during DCS.

Flexible Triboelectric Nanogenerator for Promoting the Proliferation and Migration of Human Fibroblast Cells

Chronic wound healing is often a prolonged process with the migration and proliferation of fibroblast cells playing crucial roles. Electrical stimulation (ES) has emerged as a promising physical therapy modality to promote these key events. In this study, we address this issue by employing a triboelectric nanogenerator (TENG) as an electrical stimulator for both drug release and the stimulation of fibroblast cells. The flexible TENG with a sandwich structure was fabricated using a PCL nanofibrous layer, Kapton, and silicon rubber. The TENG could be folded to any degree and twisted, and it could return to its original shape when the force was removed. Cultured cells received ES twice and three times daily for 8 days, with a 30 min interval between sessions. By applying current in a safe range and appropriate time (twice daily), fibroblasts demonstrate an accelerated proliferation and migration rate. These observations were confirmed through cell staining. Additionally, in vitro tests demonstrated the TENG's ability to simultaneously provide ES and release vitamin C from the patch. After 2 h, the amount of released drug increased 2 times in comparison to the control group. These findings provide support for the development of a TENG for the treatment of wounds, which underlines the promise of this new technique for developing portable electric stimulation devices.

PEDOT:PSS-coated platinum electrodes for neural stimulation

Safe and long-term electrical stimulation of neurons requires charge injection without damaging the electrode and tissue. A common strategy to diminish adverse effects includes the modification of electrodes with materials that increases the charge injection capacity. Due to its high capacitance, the conducting polymer PEDOT:PSS is a promising coating material; however, the neural stimulation performance in terms of stability and safety remains largely unexplored. Here, PEDOT:PSS-coated platinum (Pt-PEDOT:PSS) microelectrodes are examined for neural stimulation and compared to bare platinum (Pt) electrodes. Microelectrodes in a bipolar configuration are used to deliver current-controlled, biphasic pulses with charge densities ranging from 64 to 255 μC cm-2. Stimulation for 2 h deteriorates bare Pt electrodes through corrosion, whereas the PEDOT:PSS coating prevents dissolution of Pt and shows no degradation. Acute stimulation of primary cortical cells cultured as neurospheres shows similar dependency on charge density for Pt and Pt-PEDOT:PSS electrodes with a threshold of 127 μC cm-2 and increased calcium response for higher charge densities. Continuous stimulation for 2 h results in higher levels of cell survival for Pt-PEDOT:PSS electrodes. Reduced cell survival on Pt electrodes is most profound for neurospheres in proximity of the electrodes. Extending the stimulation duration to 6 h increases cell death for both types of electrodes; however, neurospheres on Pt-PEDOT:PSS devices still show significant viability whereas stimulation is fatal for nearly all cells close to the Pt electrodes. This work demonstrates the protective properties of PEDOT:PSS that can be used as a promising approach to extend electrode lifetime and reduce cell damage for safe and long-term neural stimulation.

Finite element analysis of electric field distribution during direct current stimulation of the spinal cord: Implications for device design

Spinal cord injury (SCI) arises from damage to the spinal cord, often caused by trauma or disease. The resulting sensorimotor dysfunction is variable and dependent on the extent of the injury. Despite years of research, curative options for SCI remain limited. However, recent advancements in electric field stimulated axonal regrowth have shown promise for neuronal regeneration. One roadblock in the development of therapeutic treatments based on this is a lack of understanding of the exogenous electric field distribution in the injured tissue, and in particular, how this is influenced by electrode geometry and placement. To better understand this electric field, and provide a means by which it can be optimized, we have developed a finite element model of such spinal cord treatment. We investigate the impact of variations in electrode geometry, spinal cord size, and applied current magnitude as well as looking at several injury models in relation to clinically observed outcomes. Through this, we show that electrode shape has little effect on the induced electric field, that the placement of these electrodes has a noticeable influence on the field distribution, and that the magnitude of this field is governed by both the applied current and the spinal cord morphology. We also show that the injury modality influences the induced field distribution and that a stronger understanding of the injury will help decide treatment parameters. This work provides guidance in the design of electrodes for future clinical application in direct current electric field stimulation for axonal regeneration.

Brain stimulation-on-a-chip: a neuromodulation platform for brain slices

Electrical stimulation of ex vivo brain tissue slices has been a method used to understand mechanisms imparted by transcranial direct current stimulation (tDCS), but there are significant direct current electric field (dcEF) dosage and electrochemical by-product concerns in conventional experimental setups that may impact translational findings. Therefore, we developed an on-chip platform with fluidic, electrochemical, and magnetically-induced spatial control. Fluidically, the chamber geometrically confines precise dcEF delivery to the enclosed brain slice and allows for tissue recovery in order to monitor post-stimulation effects. Electrochemically, conducting hydrogel electrodes mitigate stimulation-induced faradaic reactions typical of commonly-used metal electrodes. Magnetically, we applied ferromagnetic substrates beneath the tissue and used an external permanent magnet to enable in situ rotational control in relation to the dcEF. By combining the microfluidic chamber with live-cell calcium imaging and electrophysiological recordings, we showcased the potential to study the acute and lasting effects of dcEFs with the potential of providing multi-session stimulation. This on-chip bioelectronic platform presents a modernized yet simple solution to electrically stimulate explanted tissue by offering more environmental control to users, which unlocks new opportunities to conduct thorough brain stimulation mechanistic investigations.

Cryogel-based neurostimulation electrodes to activate endogenous neural precursor cells

The delivery of electrical pulses to the brain via penetrating electrodes, known as brain stimulation, has been recognized as an effective clinical approach for treating neurological disorders. Resident brain neural precursor cells (NPCs) are electrosensitive cells that respond to electrical stimulation by expanding in number, migrating and differentiating which are important characteristics that support neural repair. Here, we report the design of a conductive cryogel brain stimulation electrode specifically developed for NPC activation. The cryogel electrode has a modulus switching mechanism permitting facile penetration and reducing the mechanical mismatch between brain tissue and the penetrating electrode. The cryogel demonstrated good in vivo biocompatibility and reduced the interfacial impedance to deliver the stimulating electric field with lower voltage under charge-balanced current controlled stimulation. An ex vivo assay reveals that electrical stimulation using the cryogel electrodes results in significant expansion in the size of NPC pool. Hence, the cryogel electrodes have the potential to be used for NPC activation to support endogenous neural repair. STATEMENT OF SIGNIFICANCE: The objective of this study is to develop a cryogel-based stimulation electrode as an alternative to traditional electrode materials to be used in regenerative medicine applications for enhancing neural regeneration in brain. The electrode offers benefits such as adaptive modulus for implantation, high charge storage and injection capacities, and modulus matching with brain tissue, allowing for stable delivery of electric field for long-term neuromodulation. The electrochemical properties of cryogel electrodes were characterized in living tissue with an ex vivo set-up, providing a deeper understanding of stimulation capacity in brain environments. The cryogel electrode is biocompatible and enables low voltage, current-controlled stimulation for effective activation of endogenous neural precursor cells, revealing their potential utility in neural stem cell-mediated brain repair.

A nano-conductive osteogenic hydrogel to locally promote calcium influx for electro-inspired bone defect regeneration

Conductive nano-materials and electrical stimulation (ES) have been recognized as a synergetic therapy for ordinary excitable tissue repair. It is worth noting that hard tissues, such as bone tissue, possess bioelectrical properties as well. However, insufficient attention is paid to the synergetic therapy for bone defect regeneration via conductive biomaterials with ES. Here, a novel nano-conductive hydrogel comprising calcium phosphate-PEDOT:PSS-magnesium titanate-methacrylated alginate (CPM@MA) was synthesized for electro-inspired bone tissue regeneration. The nano-conductive CPM@MA hydrogel has demonstrated excellent electroactivity, biocompatibility, and osteoinductivity. Additionally, it has the potential to enhance cellular functionality by increasing endogenous transforming growth factor-beta1 (TGF-β1) and activating TGF-β/Smad2 signaling pathway. The synergetic therapy could facilitate intracellular calcium enrichment, resulting in a 5.8-fold increase in calcium concentration compared to the control group in the CPM@MA ES + group. The nano-conductive CPM@MA hydrogel with ES could significantly promote electro-inspired bone defect regeneration in vivo, uniquely allowing a full repair of rat femoral defect within 4 weeks histologically and mechanically. These results demonstrate that our synergistic strategy effectively promotes bone restoration, thereby offering potential advancements in the field of electro-inspired hard tissue regeneration using novel nano-materials with ES.

Generation of direct current electrical fields as regenerative therapy for spinal cord injury: A review

Electrical stimulation (ES) shows promise as a therapy to promote recovery and regeneration after spinal cord injury. ES therapy establishes beneficial electric fields (EFs) and has been investigated in numerous studies, which date back nearly a century. In this review, we discuss the various engineering approaches available to generate regenerative EFs through direct current electrical stimulation and very low frequency electrical stimulation. We highlight the electrode-tissue interface, which is important for the appropriate choice of electrode material and stimulator circuitry. We discuss how to best estimate and control the generated field, which is an important measure for comparability of studies. Finally, we assess the methods used in these studies to measure functional recovery after the injury and treatment. This work reviews studies in the field of ES therapy with the goal of supporting decisions regarding best stimulation strategy and recovery assessment for future work.

Electrotaxis evokes directional separation of co-cultured keratinocytes and fibroblasts

Bioelectric communication plays a significant role in several cellular processes and biological mechanisms, such as division, differentiation, migration, cancer metastasis, and wound healing. Ion flow across cellular walls leads to potential gradients and subsequent formation of constant or time-varying electric fields(EFs), which regulate cellular processes. An EF is natively generated towards the wound center during epithelial wound healing, aiming to align and guide cell migration, particularly of macrophages, fibroblasts, and keratinocytes. While this phenomenon, known as electrotaxis or galvanotaxis, has been extensively investigated across many cell types, it is typically explored one cell type at a time, which does not accurately represent cellular interactions during complex biological processes. Here we show the co-cultured electrotaxis of epidermal keratinocytes and dermal fibroblasts with a salt-bridgeless microfluidic approach for the first time. The electrotactic response of these cells was first assessed in mono-culture to establish a baseline, resulting in the characteristic cathodic migration for keratinocytes and anodic for fibroblasts. Both cell types retained their electrotactic properties in co-culture leading to clear cellular partition even in the presence of cellular collisions. The methods leveraged here pave the way for future co-culture electrotaxis experiments where the concurrent influence of cell types can be thoroughly investigated.

Bioelectronic microfluidic wound healing: a platform for investigating direct current stimulation of injured cell collectives

Upon cutaneous injury, the human body naturally forms an electric field (EF) that acts as a guidance cue for relevant cellular and tissue repair and reorganization. However, the direct current (DC) flow imparted by this EF can be impacted by a variety of diseases. This work delves into the impact of DC stimulation on both healthy and diabetic in vitro wound healing models of human keratinocytes, the most prevalent cell type of the skin. The culmination of non-metal electrode materials and prudent microfluidic design allowed us to create a compact bioelectronic platform to study the effects of different sustained (12 hours galvanostatic DC) EF configurations on wound closure dynamics. Specifically, we compared if electrotactically closing a wound's gap from one wound edge (i.e., uni-directional EF) is as effective as compared to alternatingly polarizing both the wound's edges (i.e., pseudo-converging EF) as both of these spatial stimulation strategies are fundamental to the eventual translational electrode design and strategy. We found that uni-directional electric guidance cues were superior in group keratinocyte healing dynamics by enhancing the wound closure rate nearly three-fold for both healthy and diabetic-like keratinocyte collectives, compared to their non-stimulated respective controls. The motility-inhibited and diabetic-like keratinocytes regained wound closure rates with uni-directional electrical stimulation (increase from 1.0 to 2.8% h^-1) comparable to their healthy non-stimulated keratinocyte counterparts (3.5% h^-1). Our results bring hope that electrical stimulation delivered in a controlled manner can be a viable pathway to accelerate wound repair, and also by providing a baseline for other researchers trying to find an optimal electrode blueprint for in vivo DC stimulation.

A microfluidic perspective on conventional in vitro transcranial direct current stimulation methods

Transcranial direct current stimulation (tDCS) is a promising non-invasive brain stimulation method to treat neurological and psychiatric diseases. However, its underlying neural mechanisms warrant further investigation. Indeed, dose-response interrelations are poorly understood. Placing explanted brain tissue, mostly from mice or rats, into a uniform direct current electric field (dcEF) is a well-established in vitro system to elucidate the neural mechanism of tDCS. Nevertheless, we will show that generating a defined, uniform, and constant dcEF throughout a brain slice is challenging. This article critically reviews the methods used to generate and calibrate a uniform dcEF. We use finite element analysis (FEA) to evaluate the widely used parallel electrode configuration and show that it may not reliably generate uniform dcEF within a brain slice inside an open interface or submerged chamber. Moreover, equivalent circuit analysis and measurements inside a testing chamber suggest that calibrating the dcEF intensity with two recording electrodes can inaccurately capture the true EF magnitude in the targeted tissue when specific criteria are not met. Finally, we outline why microfluidic chambers are an effective and calibration-free approach of generating spatiotemporally uniform dcEF for DCS in vitro studies, facilitating accurate and fine-scale dcEF adjustments. We are convinced that improving the precision and addressing the limitations of current experimental platforms will substantially improve the reproducibility of in vitro experimental results. A better mechanistic understanding of dose-response relations will ultimately facilitate more effective non-invasive stimulation therapies in patients.

Poly(3,4-ethylenedioxythiophene)-Based Neural Interfaces for Recording and Stimulation: Fundamental Aspects and In Vivo Applications

Next-generation neural interfaces for bidirectional communication with the central nervous system aim to achieve the intimate integration with the neural tissue with minimal neuroinflammatory response, high spatio-temporal resolution, very high sensitivity, and readout stability. The design and manufacturing of devices for low power/low noise neural recording and safe and energy-efficient stimulation that are, at the same time, conformable to the brain, with matched mechanical properties and biocompatibility, is a convergence area of research where neuroscientists, materials scientists, and nanotechnologists operate synergically. The biotic-abiotic neural interface, however, remains a formidable challenge that prompts for new materials platforms and innovation in device layouts. Conductive polymers (CP) are attractive materials to be interfaced with the neural tissue and to be used as sensing/stimulating electrodes because of their mixed ionic-electronic conductivity, their low contact impedance, high charge storage capacitance, chemical versatility, and biocompatibility. This manuscript reviews the state-of-the-art of poly(3,4-ethylenedioxythiophene)-based neural interfaces for extracellular recording and stimulation, focusing on those technological approaches that are successfully demonstrated in vivo. The aim is to highlight the most reliable and ready-for-clinical-use solutions, in terms of materials technology and recording performance, other than spot major limitations and identify future trends in this field.