A new hydrogel electrolyte enables fast, low-power solid-state circuits for implantable bioelectronics, offering high precision, stability, and compatibility with complex neural systems.
(Nanowerk Spotlight) Electronic devices that can sense, process, and respond to biological signals are reshaping how researchers approach medicine, neuroscience, and human–machine interaction. These systems, often soft, flexible, and powered by organic materials, promise to monitor brain activity, stimulate nerves, and control prosthetics with a level of precision and integration that rigid silicon electronics cannot match. The ambition is clear: build circuits that are not just compatible with the body, but functionally embedded within it.
Yet at the core of many of these bioelectronic systems lies a persistent technical obstacle. Organic electrochemical transistors—or OECTs—have emerged as one of the most promising components for such interfaces. They operate at low voltages, work well in wet environments, and can amplify faint biological signals. But their performance has depended almost entirely on liquid electrolytes—saline-based solutions that shuttle ions in and out of the transistor channel. While effective at driving fast switching and strong responses, these liquids are difficult to confine. They spread, leak, evaporate, and cause interference between closely packed devices. They make miniaturization harder, circuit integration more complex, and long-term implantation more fragile.
Solid-state electrolytes have been explored as a replacement. Some are made from ionic gels or charged polymers, others from hydrogels with modified compositions. But each compromise has created new limitations: reduced ion mobility, patterning challenges, long response times, or incompatibility with both p-type and n-type transistor operation. These tradeoffs have made it difficult to build dense, fast, reliable circuits for real use in living systems.
Using a naturally derived polymer from seaweed and a light-activated crosslinker, they’ve built a platform that enables high-speed operation, micrometer-scale precision, and compatibility with flexible, implantable devices. The system supports both logic circuits and spiking neural mimics, all operating on a solid-state foundation—offering a solution to a long-standing bottleneck in bioelectronic circuit design.
This work introduces a solid-state hydrogel based on ι-carrageenan, a charged polysaccharide extracted from red seaweed, crosslinked with poly(ethylene glycol) diacrylate (PEGDA). When exposed to ultraviolet light, PEGDA forms a permanent network that locks the ι-carrageenan into a soft, water-stable gel. The result is a solid-state electrolyte that can be patterned with high precision, while maintaining ionic conductivity at levels comparable to liquid saline.
The hydrogel can be processed as a liquid and selectively hardened using light exposure. Before crosslinking, it spreads easily for coating or printing. After UV exposure, it forms a water-insoluble gel that can be patterned down to 15 micrometers. This resolution is sufficient for building densely packed circuits on flexible substrates. Crucially, the material retains ionic conductivity above 10 millisiemens per centimeter—on par with 0.1 molar sodium chloride. That conductivity enables fast ion movement through the gel, preserving the switching speed and signal fidelity expected of high-performance OECTs.
Design and performance characteristics of the photopatternable ι-carrageenan-based solid-state hydrogel electrolyte. (a) Schematic of the UV-induced crosslinking process: upon exposure to 365 nm light, PEGDA crosslinks in the presence of the photoinitiator PI-2959, forming a water-stable hydrogel network with ι-carrageenan; unexposed regions remain soluble and are removed through water development, enabling spatially defined patterning. (b) Molecular structures of the key components: ι-carrageenan (a sulfated polysaccharide), PEGDA (poly(ethylene glycol) diacrylate), photoinitiator PI-2959, and glycerol. (c) Reversible sol–gel transition behavior of the electrolyte as a function of temperature. (d) Optical microscopy images showing pixel arrays patterned at lateral dimensions of 100, 50, and 30 micrometers, as well as line arrays down to 15 micrometers. (e) Ionic conductivity measured across different ι-carrageenan concentrations before and after crosslinking, showing values exceeding 10 mS/cm. (f) Storage and loss moduli as a function of shear stress for various hydrogel formulations, indicating gel-to-flow transition behavior. (g) Yield stress dependence on polymer concentration, confirming tunability for coating or printing methods. (h) Comparative plot of ionic conductivity versus minimum achievable pattern size among reported solid-state electrolytes, demonstrating the unique combination of high conductivity and fine patternability achieved in this system. (Image: Reprinted from DOI:10.1002/adma.202509314, CC BY) (click on image to enlarge)
The team tested the electrolyte using both p-type and n-type OECTs made from common organic semiconductors: P(g32T-TT) and BBL. These devices were fabricated on glass and flexible substrates using standard photolithographic processes. The resulting transistors showed excellent electrical characteristics: low threshold voltages, high transconductance, and rapid on–off switching. Measured response times were under one millisecond in both transistor types. For the p-type device, switching occurred in 0.21 milliseconds on and 0.038 milliseconds off—well within the range needed for real-time bioelectronic processing.
The researchers also evaluated the system’s ability to support more complex functions. Using complementary pairs of p- and n-type OECTs, they constructed inverters and logic gates, including NAND, NOR, and four-input NAND gates. These circuits operated correctly at low supply voltages, consuming less than 2 microwatts during peak switching. Logic levels were stable, and circuit footprints were small, with the basic gates occupying just 0.4 square millimeters.
The team further integrated 18 transistors into a compact half-adder circuit measuring 2 square millimeters. All of these elements were made entirely using the same photopatternable hydrogel, confirming its compatibility with multi-transistor integration.
To move beyond digital logic, the researchers designed a circuit that mimics the behavior of a spiking neuron. This organic electrochemical neuron (OECN) was based on the leaky integrate-and-fire model used in artificial neural networks. It combines complementary OECTs with a reset transistor and integrates them into a spiking architecture that converts a continuous input into transient voltage pulses. The circuit was encapsulated using a biocompatible layer of parylene and fabricated on an ultrathin flexible substrate.
To demonstrate biological relevance, the team implanted this device in mice. They connected it to flexible stimulation electrodes coated with PEDOT:PSS, a conductive polymer that lowers electrode impedance. The system was wrapped around the cervical vagus nerve, a major nerve involved in autonomic regulation of the heart and digestive system. When inactive, the device produced no physiological effect. When activated to spike at frequencies between 1 and 20 hertz, it induced a measurable drop in heart rate of 2 to 4 percent—consistent with the known effects of vagus nerve stimulation.
Unlike previous systems based on liquid electrolytes, this device remained stable after implantation, with no fluid reservoirs or leakage pathways. Its function did not degrade after encapsulation, and spiking behavior remained consistent. The reduction in spiking frequency observed after implantation was attributed to the mouse acting as an external load, not to any failure of the circuit.
By combining high ionic conductivity, patternability, and biocompatibility, this hydrogel-based electrolyte platform overcomes the major constraints that have limited the development of integrated, implantable OECT-based circuits. It supports the construction of small, low-power digital logic components and neuromorphic spiking systems, using materials and fabrication processes that are compatible with soft-tissue environments. The use of naturally derived components like ι-carrageenan further supports its potential for long-term biointegration.
The platform introduced in this study enables a new level of complexity and stability in soft bioelectronics. It demonstrates that solid-state, hydrogel-based circuits can meet the electrical demands of real-world applications without sacrificing manufacturability or implant safety. By bridging the gap between ionic transport and scalable circuit design, this work sets the foundation for future generations of bioelectronic therapies and neural interfaces.
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