(Nanowerk Spotlight) The seamless integration of electronic devices with living tissue remains one of the most significant challenges in bioelectronics. Traditional electronic components are rigid and brittle, while biological tissues are soft and flexible. This fundamental mismatch has limited the development of advanced medical devices that can maintain stable performance while conforming to the dynamic nature of living systems.
Organic electrochemical transistors (OECTs) have emerged as promising candidates for bridging this gap between electronic devices and biological systems. These devices use organic materials that can conduct both ions and electrons, making them particularly suitable for interfacing with living tissues, which communicate through ionic signals. The most widely used material for these transistors is PEDOT:PSS, a conducting polymer that can efficiently convert ionic signals to electronic ones. However, PEDOT:PSS becomes brittle when formed into thin films, with stretchability limited to less than 5% before breaking. This limitation has severely restricted the practical application of OECTs in medical devices that need to move and flex with the body.
Previous attempts to create stretchable OECTs have yielded mixed results. Some approaches used pre-stretched substrates or patterned metallic interconnections, while others experimented with porous semiconductors. While these methods showed some promise, they either compromised electrical performance or achieved only modest improvements in stretchability. The field needed a fundamental redesign to overcome these limitations.
Researchers from the University of Hong Kong and Tongji University have developed a novel approach that fundamentally reimagines how these transistors are built. Instead of trying to make the materials themselves more stretchy, they completely reconceptualized the transistor’s architecture, creating what they call a vertical intrinsically stretchable organic electrochemical transistor (VIS-OECT).
Comparison between VIS-OECTs and IS-OECTs. a) Schematics of the IS-OECTs and their 2D conductive pathways in (i) the original state and (ii) the stretched state. b) The schematic shows the transfer curves of the IS-OECTs. c) Schematics of the VIS-OECTs and their 3D conductive pathways in (iii) the original state and (iv) the stretched state. d) The schematic shows the transfer curves of the VIS-OECTs. (Image: Reproduced with permission by Wiley-VCH Verlag)
Traditional OECTs use a flat, horizontal layout where electrical current must travel across a thin film of conducting material, similar to water flowing across a flat surface. When this film stretches, cracks form like breaks in a frozen pond, completely disrupting the flow of electricity. The research team’s new design stacks the components vertically, creating multiple parallel pathways for electricity to flow through the device’s thickness. This three-dimensional arrangement means that even when cracks form, electricity can still find alternative routes through the material – much like water can still flow through a cracked sponge.
To make this vertical design work, the team developed specialized electrodes using a precise manufacturing technique. They deposit a thin layer of gold onto a carefully selected elastic polymer and heat the combination. During heating, gold atoms migrate into the polymer structure, creating a hybrid material that maintains electrical conductivity even when stretched. While effective in laboratory settings, scaling this delicate process for mass production presents significant challenges, particularly in maintaining consistent gold penetration across larger surfaces.
The performance improvements far exceeded expectations. While previous stretchable OECTs typically fail when stretched beyond 20%, the new vertical devices continue working up to 50% strain – well beyond what the individual materials should theoretically allow. Even more significant, the devices showed dramatic improvement in transconductance – their ability to amplify weak biological signals into clear electronic outputs. This property jumped from 0.2 to 27 millisiemens, a more than hundredfold increase that enables the detection of much subtler biological signals than previously possible.
To demonstrate practical applications, the researchers created an array of these vertical transistors to process electrical signals from muscles. The system successfully identified different hand gestures by analyzing these biological signals, maintaining over 80% accuracy even when stretched to 30%. This high signal sensitivity, combined with mechanical flexibility, could enable new types of wearable medical sensors that reliably monitor subtle muscle activity while conforming naturally to body movements.
However, several technical challenges must be addressed before these devices can be widely used in medical applications. The manufacturing process needs refinement for industrial-scale production, particularly in ensuring consistent quality across larger areas. The adhesion between different layers of the device must be improved to ensure long-term reliability under repeated stretching. Perhaps most importantly for medical applications, future versions need to be designed to allow gas exchange through the device and demonstrate long-term biocompatibility with living tissue.
The vertical architecture represents a fundamental advance in stretchable electronics, demonstrating how innovative design can overcome material limitations that have long constrained the field. This architectural approach marks a departure from previous solutions that often compromised device performance to achieve flexibility. Instead, the vertical design enhances both electrical characteristics and stretchability simultaneously.
The implications of this work extend far beyond medical devices. This new approach to designing flexible electronics could influence the development of next-generation wearable computers, smart textiles, and human-machine interfaces. As manufacturing challenges are addressed, this vertical architecture could become a fundamental building block for electronics that seamlessly integrate with the human body, opening new possibilities in fields ranging from medical monitoring to augmented human capabilities.
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