Conductive Hydrogel Electrodes Research Articles

Preparation and characterization of neural stem cell-loaded conductive hydrogel cochlear implant electrode coatings

Sensorineural deafness is a hearing impairment resulting from damage to the auditory nerve or inner ear hair cells. Currently, cochlear implants (CIs) are widely used as hearing aids for sensorineural deafness patients. A fundamental limitation of cochlear implants (CIs) is that spiral ganglion neurons (SGNs) cannot be replenished. This greatly restricts the rehabilitation of sensorineural deafness. Additionally, the insertion of CIs can cause secondary cochlear damage, worsening the condition of the patients' cochlear. Therefore, a new type of neural stem cells (NSCs) loaded graphene oxide-polyaniline/GelMA (GO-PAni/GelMA) conductive hydrogel electrode for cochlear implant was fabricated via in-situ radical polymerization and cyclic UV curing technique. On the one hand, the hydrogel electrode, as a direct contact layer, helps to avoid the physical hurt for cochlear. On the other hand, NSCs were supplemented via the hydrogel carrier and neuronal differentiation was induced by electrical stimulation, which was validated by the experimental results of immunofluorescence, Phalloidin Staining and RT-qPCR. Furthermore, based on RNA sequencing and transcriptome analysis, we hypothesized that the neuronal differentiation of NSCs was adjusted by the calcium signaling pathway and GABAergic synapse. Overall, our cell loading conductive hydrogel electrode may be an effective solution to sensorineural deafness. The revelation of the mechanism of neuronal differentiation promoted by electrical stimulation provides a basis for further sensorineural deafness treatment using conductive hydrogel CI electrode.

Body-worn and self-powered flexible optoelectronic device for metronomic photodynamic therapy

Photodynamic therapy (PDT) as a clinical method relies on appropriate light delivery to activate photosensitizers, usually necessitates the utilization of cumbersome surgical instruments and high irradiation intensity, along with the requirement for hospitalization. To extend the applicability of PDT beyond hospital for better patient mobility, we design a wearable and self-powered metronomic PDT (mPDT) system for chronic wound infection treatment. A flexible alternative current electroluminescent (ACEL) device is constructed through sandwiching an emissive layer between conductive hydrogel electrodes. This ACEL device works as a therapeutic patch by loading photosensitizer (PS) in its bottom hydrogel electrode, thus aviods the intravenous administration to patients. Under the triboelectric nanogenerator generated AC pulse, the electroluminescence produced from emissive layer can be absorbed by the PS-loaded electrode to generate reactive oxygen species for mPDT. Benefited from its arbitrary tailorability, this device can be customized into on-demand shapes and sizes. Using diabetic infected wound as a model condition, this ACEL mPDT device effectively eliminates drug-resistant bacteria and accelerates wound healing. Thus, the body-worn optoelectronic device successfully avoids the utilization of extracorporeal physical light and power sources, providing a promising strategy for convenient, user-friendly, and prolonged treatment of superficial diseases.

Self-powered strain sensing devices with wireless transmission: DIW-printed conductive hydrogel electrodes featuring stretchable and self-healing properties

With rapid advancements in health and human–computer interaction, wearable electronic skins (e-skins) designed for application on the human body provide a platform for real-time detection of physiological signals. Wearable strain sensors, integral functional units within e-skins, can be integrated with Internet of Things (IoT) technology to broaden the applications for human body monitoring. A significant challenge lies in the reliance of most existing wearable strain sensors on rigid external power supplies, limiting their practical flexibility. In this study, we present an innovative strategy to fabricate glutaraldehyde (GA)-poly(vinyl alcohol) (PVA)/cellulose nanocrystals (CNC)/Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) conductive hydrogels through multiple hydrogen bonding systems. Combining the advantageous rheological properties of the precursor solution and the high specific surface area after freeze–thaw cycling, we have created a self-powered sensing system prepared by large-area printing using direct ink writing (DIW) printing. The resulting conductive hydrogel exhibits commendable mechanical properties (411 KPa), impressive stretchability (580 %), and robust self-healing capabilities (>98.3 %). The strain sensor, derived from the conductive hydrogel, demonstrates a gauge factor (GF) of 2.5 within a stretching range of 0–580 %. Additionally, the resultant supercapacitor displays a peak energy density of 0.131 mWh/cm3 at a power density of 3.6 mW/cm3. Benefiting from its elevated strain response and remarkable power density features, this self-powered strain sensing system enables the real-time monitoring of human joint motion. The incorporation of a 5G transmission module enhances its capabilities for remote data monitoring, thereby contributing to the progress of wireless tracking technologies for self-powered electronic skin.

A high-power glucose fuel cell for potential application in implant surfaces

The glucose fuel cells (GFCs) leverage the implant surface as an electrode, representing an optimal approach for miniaturizing implantable power sources. A conductive hydrogel electrode membrane is crafted through in situ reduction and electrochemical co-deposition techniques by employing bacterial cellulose as a scaffold. This process enables the seamless integration of the GFC directly onto the implant surface. The integrated GFC exhibits an impressive open-circuit voltage peak of 0.894 V and a peak power density of 94.7 μW cm−2. Additionally, the fuel cell demonstrates resistance to chloride ion toxicity under simulated interstitial fluid conditions. Although performance within horse serum is moderate, the methodology presents a viable strategy for developing GFCs on implant surfaces.

A novel CoS/Ti3C2Tx MXene conductive filler effectively improves the pseudocapacitive performance of conductive hydrogel electrode materials for all-solid-state supercapacitors

It is currently an essential research direction to develop a low-cost, cost-effective, and environmentally friendly high-performance flexible electrode material. The electrode materials prepared from Ti3C2Tx MXene based hydrogels showed favorable physical and chemical properties (e.g., extreme flexibility, excellent mechanical strength, and electrical conductivity), and CoS (cobaltous sulfide) nanoparticles were grown by in-situ hydrothermal growth on the “accordion-like” Ti3C2Tx MXene surfaces, which were then synthesised into PCCT (PAA/chitosan/CoS/Ti3C2Tx) conductive hydrogels with physical crosslinked networks in a two-step process. This 3D porous structure containing highly conductive materials can provide suitable geometrical space and electronic structure, which can help to suppress the accumulation of active material at high mass loading and improve the specific capacitance of the electrode material. With a current density of 4 mA/g, the specific capacitance of the PCCT conductive hydrogel electrode was 3.6F/g. At a current density of 10 mA/g, the cycling stability was still 80 % after 2000 cycles. The elongation at break of PCCT conductive hydrogel wass about 349 %. After 48 h of healing at room temperature, the self-healing efficiency can reach 66.88 %. A foundation has been laid for obtaining flexible all-solid-state supercapacitors with stable structures and excellent electrochemical properties.

Injectable conductive hydrogel electrodes for minimally invasive neural interfaces.

Soft bioelectronic neural interfaces have great potential as mechanically favourable alternatives to implantable metal electrodes. In this pursuit, conductive hydrogels (CHs) are particularly viable, combining tissue compliance with the required electrochemical characteristics. Physically-aggregated CHs offer an additional advantage by their facile synthesis into injectable systems, enabling minimally invasive implantation, though they can be impeded by a lack of control over their particle size and packing. Guided by these principles, an injectable PEDOT:PSS/acetic acid-based hydrogel is presented herein whose mechanical and electrochemical properties are independently tuneable by modifying the relative acetic acid composition. The fabrication process further benefits from employing batch emulsion to decrease particle sizes and facilitate tighter packing. The resulting material is stable and anatomically compact upon injection both in tissue phantom and ex vivo, while retaining favourable electrochemical properties in both contexts. Biphasic current stimulation yielding voltage transients well below the charge injection limit as well as the gel's non-cytotoxicity further underscore its potential for safe and effective neural interfacing applications.

Poly (acrylic acid)/Chitosan hydrogel with excellent self-healing properties for supercapacitor electrodes using PbS/Ti3C2Tx heterostructure as conductive fillers

Being a natural polymer with good biocompatibility and biodegradability, chitosan (CS) is often designed as a dual-network conductive hydrogel material. This is especially promising in the field of stretchable and flexible supercapacitors. The PbS/Ti3C2Tx heterostructure is incorporated into the PAA/CS (PC) hydrogel using a two-step method, in which multiple hydrogen bonding interactions are constructed enable the PAA/CS/PbS/Ti3C2Tx (PCPT) conductive hydrogel electrode material to possess a self-healing efficiency of up to 93% at room temperature. It also has a rich 3D pore structure that allows the PCPT conductive hydrogel electrode to reach a specific capacitance of 77.05 F/g at a current density of 10 mA/g. Even after 2000 cycles at a high current density of 200 mA/g, the specific capacitance remains at 89% of its original value. It has offered a new idea for the preparation of multiple-action self-healing electrode materials.

Silver Nanoparticle-Embedded Conductive Hydrogels for Electrochemical Sensing of Hydroquinone.

In this work, a conductive hydrogel was successfully synthesized, taking advantage of the high number density of active amino and hydroxyl groups in carboxymethyl chitosan and sodium carboxymethyl cellulose. These biopolymers were effectively coupled via hydrogen bonding with the nitrogen atoms of the heterocyclic rings of conductive polypyrrole. The inclusion of another biobased polymer, sodium lignosulfonate (LS), was effective to achieve highly efficient adsorption and in-situ reduction of silver ions, leading to silver nanoparticles that were embedded in the hydrogel network and used to further improve the electro-catalytic efficiency of the system. Doping of the system in the pre-gelled state led to hydrogels that could be easily attached to the electrodes. The as-prepared silver nanoparticle-embedded conductive hydrogel electrode exhibited excellent electro-catalytic activity towards hydroquinone (HQ) present in a buffer solution. At the optimum conditions, the oxidation current density peak of HQ was linear over the 0.1-100 μM concentration range, with a detection limit as low as 0.12 μM (signal-to-noise of 3). The relative standard deviation of the anodic peak current intensity was 1.37% for eight different electrodes. After one week of storage in a 0.1 M Tris-HCl buffer solution at 4 °C, the anodic peak current intensity was 93.4% of the initial current intensity. In addition, this sensor showed no interference activity, while the addition of 30 μM CC, RS, or 1 mM of different inorganic ions does not have a significant impact on the test results, enabling HQ quantification in actual water samples.

Preparation and Properties of Nano Platinum/Carbon Nanotubes/Bacterial Cellulose Conductive Hydrogel Electrode Films

Using the natural nanomesh structure of bacterial cellulose (BC), the good conductivity of multi-walled carbon nanotubes (MWCNTs) and the good electrocatalytic activity of nano platinum (PtNPs), a three-way porous conductive hydrogel of PtNPs and BC was designed. The infiltrating and doping of MWCNTs on BC thin films was realized by ultrasonic assisted electrophoretic deposition process, which ensured the dual characteristics of ionic and electronic conduction of electrode films. The adsorption capacity of BC for chloroplatinic acid and the strong reductivity of sodium borohydride were also used to realize the high-load PtNPs recombination on MWCNTs/BC porous layered conductive hydrogel electrode films. By testing the electrochemical performance of PtNPs/MWCNTs/BC conductive hydrogel electrode film and characterization of microstructure, the electrode film not only has high electroactive area, low surface charge transfer resistance and good diffusion permeability and other electrochemical characteristics, but also shows high catalytic activity for glucose in PBS solution. The most important finding is that the electrode film can selectively catalyze glucose in oxygen-rich PBS solution. The porous layered structure of PtNPs/MWCNTs/BC electrode membrane and the high load dispersion of PtNPs are the reasons for the selective catalytic ability of PtNPs/MWCNTs/BC conductive hydrogel electrode membrane, which can be used for the implanted surface glucose fuel cell.

Microstructure and Mechanical Properties Analysis of Multi-walled Carbon Nanotubes/Polyvinyl Alcohol Conducting Hydrogel Electrode Film

According to the good conductivity and electrocatalytic activity of nano platinum and multi-walled carbon nanotubes (MWCNTs) and the hydrogel properties of polyvinyl alcohol (PVA), MWCNTs and PVA double-network porous hydrogels were designed and prepared. The surface of the conductive hydrogel electrode film formed a three-dimensional porous structure, and the stacking of the PVA-coated MWCNTs formed a nanoscale micropore structure. However, the micropore network structure inside the electrode film was not obvious, which was the reason why the electrode film did not selectively catalyzed glucose. The elastic modulus of PtNPs/MWCNTs/PVA electrode film is consistent with that of PVA. Its modulus of elasticity is close to that of human tissue.

Glucose Electrocatalysis of Multi-walled Carbon Nanotubes/Polyvinyl Alcohol Conducting Hydrogel Composite Platinum Electrode

A porous conductive hydrogel electrode film of multi-walled carbon nanotubes (MWCNTs)/polyvinyl alcohol (PVA) was prepared by electrophoretic deposition technology, and the selective catalytic ability of MWCNTs/PVA on glucose was verified by cyclic voltammetry and open-circuit voltage. The electrochemical and physicochemical properties of MWCNTs/PVA porous conductive hydrogel electrode films were also characterized. The synergistic catalytic effects of MWCNTs/PVA porous conductive hydrogel electrode films and platinum electrode on the electrochemical oxidation of glucose in different PH solutions were studied.

Preparation and Characterization of Conductive Hydrogel Electrode Films of Nano Platinum/Multi-Walled Carbon Nanotubes/Polyvinyl Alcohol

Nano platinum were prepared on multi-walled carbon nanotubes/polyvinyl alcohol conducting hydrogel electrode film by electrostatic adsorption principle and mild in-situ reduction. The microstructure, conductivity, electrochemical performance and electrocatalytic activity of PtNPs/MWCNTs/PVA conducting hydrogel electrode film were studied. And the selective catalytic ability of the electrode film to glucose in oxygen-rich PBS solution, and the relationship between the selective catalytic ability and the preparation technology of the electrode film was analyzed.

Prestretching‐Enhanced Conductive Hydrogel Electrode for Supercapacitors with High Areal Capacitance and Excellent Stretching Stability

The challenge to prepare conductive hydrogel electrodes for stretchable supercapacitors with high areal capacitance and excellent stability during stretching has attracted tremendous attention from researchers. Herein, a method to enhance the performance of a conductive hydrogel electrode is reported. The in situ oxidative polymerization of aniline monomer is performed in the prestretched polyacrylamide (PAAm) hydrogel. After releasing the hydrogel, the polyaniline (PANI) nanoparticles can shrink with the PAAm substrate to form a dense and continuous conductive network. Due to the enhanced conductive networks, the prestretched PAAm‐PANI conductive hydrogel with prestrain of 500% (PPCH5) electrode shows high areal capacitance of 509.9 mF cm−2 at 0.5 mA cm−2. The symmetrical stretchable supercapacitor based on PPCH5 electrodes exhibits a high areal capacitance of 219.7 mF cm−2. The device shows high capacitance retention of 68.5% even when stretched to 200% strain and noncapacitance decays after 2000 stretching cycles at 100% strain. This strategy can be expanded to the carbon nanotubes (CNT)‐filled hydrogel systems. The PAAm‐PANI‐CNT conductive hydrogel with prestrain of 500%‐based supercapacitor shows specific capacitance of 1429.87 mF cm−2. The results demonstrate that prestretching is a highly effective method to enhance capacitance and stretching stability of PANI hydrogel electrode for stretchable supercapacitors.

Breakage-resistant hydrogel electrode enables ultrahigh mechanical reliability for triboelectric nanogenerators

Hydrogel based triboelectric nanogenerators (H-TENGs) are deemed as one of most promising flexible energy harvesting and self-powered systems due to its multifunctionality, but the poor mechanical properties of hydrogel electrode bring a major risk of shortening the service life of H-TENGs in case of huge physical impact. Herein, a durable conductive hydrogel electrode with breakage-resistant capacity is developed utilizing the Hofmeister effect on starch polymers through solvent-exchange strategy. Owing to bundled starch chains within hydrogel, the breakage-resistant hydrogel electrode possesses excellent mechanical reliability, including outstanding modulus, fracture energy, anti-puncture capacity and long-term stability. Furthermore, the reliable hydrogel electrode endows triboelectric nanogenerator with good electrical output performances and superb damage immunity, prolonging its service life under accidental physical impact. More significantly, this fabricated triboelectric nanogenerator shows great potential in high-impact application. This investigation offers a versatile and effective way to design next generation hydrogel-based triboelectric nanogenerator with superior damage immunity and long-service life.

Stretchable mesh microelectronics for the biointegration and stimulation of human neural organoids

Advances in tridimensional (3D) culture approaches have led to the generation of organoids that recapitulate cellular and physiological features of domains of the human nervous system. Although microelectrodes have been developed for long-term electrophysiological interfaces with neural tissue, studies of long-term interfaces between microelectrodes and free-floating organoids remain limited. In this study, we report a stretchable, soft mesh electrode system that establishes an intimate in vitro electrical interface with human neurons in 3D organoids. Our mesh is constructed with poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) based electrically conductive hydrogel electrode arrays and elastomeric poly(styrene-ethylene-butylene-styrene) (SEBS) as the substrate and encapsulation materials. This mesh electrode can maintain a stable electrochemical impedance in buffer solution under 50% compressive and 50% tensile strain. We have successfully cultured pluripotent stem cell-derived human cortical organoids (hCO) on this polymeric mesh for more than 3 months and demonstrated that organoids readily integrate with the mesh. Using simultaneous stimulation and calcium imaging, we show that electrical stimulation through the mesh can elicit intensity-dependent calcium signals comparable to stimulation from a bipolar stereotrode. This platform may serve as a tool for monitoring and modulating the electrical activity of in vitro models of neuropsychiatric diseases.

CuS/Ti3C2Tx homostructure with enhanced hydrogel performance for supercapacitor electrode materials

Hydrogel has a similar structure and modulus to human tissue, making it an ideal material for use as artificial skin or robotic electronic skin. CuS/Ti3C2Tx homostructures were prepared using an one-pot hydrothermal method for the conductive hydrogel electrodes. The homostructures material improves the electron transport from the active material to the collector and contributes more effectively to the pseudocapacitance. With the improved electrochemical performance of the hydrogel electrode material, the specific capacitance value is as high as 33.48F/g at a current density of 10 mA/g. The capacitance retention rate was 88.54% after 2000 cycles at a current density of 150 mA/g. Moreover, the multiple hydrogen bonding interaction between CuS/Ti3C2Tx and the hydrogel substrate gives the hydrogel electrode excellent mechanical properties with an elongation at break of 663.5%. This provides a way to construct conductive hydrogel electrode materials as flexible electronic energy storage devices.

Flexible patch with printable and antibacterial conductive hydrogel electrodes for accelerated wound healing

Electrical stimulation can facilitate wound healing with high efficiency and limited side effects. However, current electrical stimulation devices have poor conformability with wounds due to their bulky nature and the rigidity of electrodes utilized. Here, a flexible electrical patch (ePatch) made with conductive hydrogel as electrodes to improve wound management was reported. The conductive hydrogel was synthesized using silver nanowire (AgNW) and methacrylated alginate (MAA), with the former chosen as the electrode material considering its antibacterial properties, and the latter used due to its clinical suitability in wound healing. The composition of the hydrogel was optimized to enable printing on medical-grade patches for personalized wound treatment. The ePatch was shown to promote re-epithelization, enhance angiogenesis, mediate immune response, and prevent infection development in the wound microenvironment. In vitro studies indicated an elevated secretion of growth factors with enhanced cell proliferation and migration ability in response to electrical stimulation. An in vivo study in the Sprague-Dawley rat model revealed a rapid wound closure within 7 days compared to 20 days of usual healing process in rodents.

Free-standing conductive hydrogel electrode for potentiometric glucose sensing.

Flexible conductive polymer hydrogels are attracting attention as an electrode material. Electrochemical biosensors with conductive polymer hydrogels have been developed because they have some advantages such as biocompatibility, high conductivity, 3D nanostructure, solvated surface, and enlarged interface. Conductive polymer hydrogels bearing receptor molecules such as enzymes in its 3D nanostructure enable the detection of target analytes with high sensitivity. However, because such hydrogels are fragile, they cannot stand on their own and a supporting substrate is required to fabricate them. This means that the loss of mechanical toughness is detrimental for their application to flexible biosensors. In this study, we have proposed a free-standing conductive hydrogel electrode with no coating on a substrate, which is composed of polyaniline with phenyl boronic acid including polyvinyl alcohol, for potentiometric glucose sensing. In addition, its electrical responsivity to glucose has been confirmed by investigating its mechanical properties at various glucose concentrations, considering the hydrogel compositions.

Underwater Bending Actuator Based on Integrated Anisotropic Textile Materials and a Conductive Hydrogel Electrode

Electroactive polymers (EAPs), especially dielectric elastomer actuators (DEAs), belong to a very promising and emerging class of functional materials. While DEAs are mostly utilized to rely on carbon-based electrodes, there are certain shortcomings of the use of carbon electrodes in the field of soft robotics. In this work we present a fish-like bending structure to serve as possible propulsion element, completely avoiding carbon-based electrodes. The presented robot is moving under water, using a particularly tailored conductive hydrogel as inner electrode and a highly anisotropic textile material to manipulate the bending behavior of the robot. The charge separation to drive two DEAs on the outsides of the robot is provided by the conductive hydrogel while the surrounding water serves as counter electrode. To characterize the hydrogel, tensile tests and impedance spectroscopy are used as measurement methods of choice. The performance of the robot was evaluated using a digital image correlation (DIC) measurement for its bending deflections under water. The developed fish-like robot was able to perform a dynamic bending movement, based on a tri-stable actuator setup. The performed measurements underpin the sufficient characteristics for an underwater application of conductive hydrogel electrodes as well as the applicability of the robotic concept for under water actuations.

Electrochemical and biological performance of chronically stimulated conductive hydrogel electrodes

Objective. Evaluate electrochemical properties, biological response, and surface characterization of a conductive hydrogel (CH) coating following chronic in vivo stimulation. Approach. Coated CH or uncoated smooth platinum (Pt) electrode arrays were implanted into the cochlea of rats and stimulated over a 5 week period with more than 57 million biphasic current pulses. Electrochemical impedance spectroscopy (EIS), charge storage capacity (CSC), charge injection limit (CIL), and voltage transient (VT) impedance were measured on the bench before and after stimulation, and in vivo during the stimulation program. Electrically-evoked auditory brainstem responses were recorded to monitor neural function. Following explant, the cochleae were examined histologically and electrodes were examined using scanning electron microscopy. Main results. CH coated electrodes demonstrated a bench-top electrochemical advantage over Pt electrodes before and after the electrical stimulation program. In vivo, CH coated electrodes also had a significant advantage over Pt electrodes throughout the stimulation program, exhibiting higher CSC (p= 0.002), larger CIL (p = 0.002), and lower VT impedance (p < 0.001). The CH cohort exhibited a greater tissue response (p= 0.003) with small deposits of particulate material within the tissue capsule. There was no loss in auditory neuron density or change in neural response thresholds in any cochleae. Examination of the electrode surface revealed that most CH electrodes exhibited some coating loss; however, there was no evidence of corrosion in the underlying Pt. Significance. CH coated electrodes demonstrated significant electrochemical advantages on the bench-top and in vivo and maintained neural function despite an increased tissue response and coating loss. While further research is required to understand the cause of the coating loss, CH electrodes provide promise for use in neural prostheses.