Conformable Electrode Arrays for Wearable Neuroprostheses.

Wearable electrode arrays can selectively stimulate muscle groups by modulating their shape, size, and position over a targeted region. They can potentially revolutionize personalized rehabilitation by being noninvasive and allowing easy donning and doffing. Nevertheless, users should feel comfortable using such arrays, as they are typically worn for an extended time period. Additionally, to deliver safe and selective stimulation, these arrays must be tailored to a user's physiology. Fabricating customizable electrode arrays needs a rapid and economical technique that accommodates scalability. By leveraging a multilayer screen-printing technique, this study aims to develop personalizable electrode arrays by embedding conductive materials into silicone-based elastomers. Accordingly, the conductivity of a silicone-based elastomer was altered by adding carbonaceous material. The 1:8 and 1:9 weight ratio percentages of carbon black (CB) to elastomer achieved conductivities between 0.0021-0.0030 S cm-1 and were suitable for transcutaneous stimulation. Moreover, these ratios maintained their stimulation performance after several stretching cycles of up to 200%. Thus, a soft, conformable electrode array with a customizable design was demonstrated. Lastly, the efficacy of the proposed electrode arrays to stimulate hand function tasks was evaluated by in vivo experiments. The demonstration of such arrays encourages the realization of cost-effective, wearable stimulation systems for hand function restoration.

Towards biodegradable conducting polymers by incorporating seaweed cellulose for decomposable wearable heaters.

Thermotherapy shows significant potential for pain relief and enhanced blood circulation in wildlife rehabilitation, particularly for injured animals. However, the widespread adoption of this technology is hindered by the lack of biodegradable, wearable heating pads and concerns surrounding electronic waste (E-waste) in natural habitats. This study addresses this challenge by investigating an environmentally-friendly composite comprising poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS), seaweed cellulose, and glycerol. Notably, this composite exhibits remarkable biodegradability, losing half of its weight within one week and displaying noticeable edge degradation by the third week when placed in soil. Moreover, it demonstrates impressive heating performance, reaching a temperature of 51 °C at a low voltage of 1.5 V, highlighting its strong potential for thermotherapy applications. The combination of substantial biodegradability and efficient heating performance offers a promising solution for sustainable electronic applications in wildlife rehabilitation and forest monitoring, effectively addressing the environmental challenges associated with E-waste.

Design of Transcutaneous Stimulation Electrodes for Wearable Neuroprostheses.

Ease of use and non-invasiveness has made transcutaneous stimulation a pervasive approach for restoration of hand function. Besides, limited targetability and induced discomfort pose a significant impediment for its clinical translation. By modifying the electrode geometry, we aim to improve the stimulation performance of small surface area electrodes that are suited for forearm muscles. Accordingly, the stimulation performance of twelve electrode geometries was assessed using a computational model and subsequent experimentation on healthy participants. Several metrics quantified their stimulation performance in terms of selectivity, comfort, and safety. Systematic analysis showed that electrode geometries and their underlying currents distribution influence selectivity and comfort, allowing for better stimulation performance. Ranking the electrode geometries identified the concentric serpentine, and the fractal-based Sierpinski and Hibert-types to outperform the circular electrodes. At a comfortable level, these electrodes provoked selective and substantial muscle contraction. Ideally, these geometries can be a reference for optimal electrode designs. The novelty of this study lies with both model-based and experimental assessments on a wide range of electrode geometries and the introduction of a computational model for electrode performance evaluation. Implications from this study can aid with easy to fabricate and personalized electrode designs. By integrating these optimized electrode designs with advanced material technologies, the applicability of wearable neuroprostheses can be improved.

Enabling Free-Standing 3D Hydrogel Microstructures with Microreactive Inkjet Printing.

Reactive inkjet printing holds great prospect as a multimaterial fabrication process because of its unique advantages involving customization, miniaturization, and precise control of droplets for patterning. For inkjet printing of hydrogel structures, a hydrogel precursor (or cross-linker) is printed onto a cross-linker (or precursor) bath or a substrate. However, the progress of patterning and design of intricate hydrogel structures using the inkjet printing technique is limited by the erratic interplay between gelation and motion control. Accordingly, microreactive inkjet printing (MRIJP) was applied to demonstrate a spontaneous 3D printing of hydrogel microstructures by using alginate as the model system. In addition, a printable window within the capillary number-Weber number for the MRIJP technique demonstrated the importance of velocity to realization of in-air binary droplet collision. Finally, systematic analysis shows that the structure and diffusion coefficient of hydrogels are important factors that affect the shape of printed hydrogels over time. Based on such a fundamental understanding of MRIJP of hydrogels, the fabrication process and the structure of hydrogels can be controlled and adapt for 2D/3D microstructure printing of any low-viscosity (<40 cP) reactive inks, with a representative tissue-mimicking structure of a ∼200 µm diameter hollow tube presented in this work.

Direct Patterning of Highly Conductive PEDOT:PSS/Ionic Liquid Hydrogel via Microreactive Inkjet Printing.

The gelation of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) has gained popularity for its potential applications in three dimensions, while possessing tissue-like mechanical properties, high conductivity, and biocompatibility. However, the fabrication of arbitrary structures, especially via inkjet printing, is challenging because of the inherent gel formation. Here, microreactive inkjet printing (MRIJP) is utilized to pattern various 2D and 3D structures of PEDOT:PSS/IL hydrogel by in-air coalescence of PEDOT:PSS and ionic liquid (IL). By controlling the in-air position and Marangoni-driven encapsulation, single droplets of the PEDOT:PSS/IL hydrogel as small as a diameter of ≈260 µm are fabricated within ≈600 µs. Notably, this MRIJP-based PEDOT:PSS/IL has potential for freeform patterning while maintaining identical performance to those fabricated by the conventional spin-coating method. Through controlled deposition achieved via MRIJP, PEDOT:PSS/IL can be transformed into different 3D structures without the need for molding, potentially leading to substantial progress in next-generation bioelectronics devices.

Highly Stretchable and Highly Conductive PEDOT:PSS/Ionic Liquid Composite Transparent Electrodes for Solution-Processed Stretchable Electronics.

Stretchable conductive materials have received great attention owing to their potential for realizing next-generation stretchable electronics. However, the simultaneous achievement of excellent mechanical stretchability and high electrical conductivity as well as cost-effective fabrication has been a significant challenge. Here, we report a highly stretchable and highly conducting polymer that was obtained by incorporating an ionic liquid. When 1-ethyl-3-methylimidazolium tetracyanoborate (EMIM TCB) was added to an aqueous conducting polymer solution of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), it was found that EMIM TCB acts not only as a secondary dopant but also as a plasticizer for PEDOT:PSS, resulting in a high conductivity of >1000 S cm-1 with stable performance at tensile strains up to 50% and even up to 180% in combination with the prestrained substrate technique. Consequently, by exploiting the additional benefits of high transparency and solution-processability of PEDOT:PSS, we were able to fabricate a highly stretchable, semitransparent, and wholly solution-processed alternating current electroluminescent device with unimpaired performance at 50% strain by using PEDOT:PSS/EMIM TCB composite films as both bottom and top electrodes.