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Breakthrough in Conductive Polymers Enhances Biocompatibility and Device Fabrication
A remarkable discovery by a collaborative team from Rice University, the University of Cambridge, and Stanford University has simplified the production of a crucial material used extensively in medical research and computing.
For over twenty years, researchers have relied on a chemical crosslinker to stabilize a conductive polymer known as PEDOT:PSS in aqueous environments. While exploring new methods to pattern this material for biomedical optics, PhD student Siddharth Doshi from Stanford, in partnership with Rice materials scientist Scott Keene, opted to omit the crosslinker and increased the temperature during the material preparation process. The outcome was unexpected: the resulting samples exhibited stability without the need for the crosslinker.
“It was more of a serendipitous discovery because Siddharth was trying out processes very different from the standard recipe, yet the samples still turned out fine,” Keene explained. “We were like, ‘Wait! Really?’ This led us to investigate the underlying principles of the phenomenon.”
The findings revealed that when PEDOT:PSS is heated beyond typical levels, it not only remains stable without a crosslinker but also enhances the quality of devices produced from it. This innovative method, detailed in a recent publication in Advanced Materials, has the potential to simplify the manufacturing of bioelectronic devices, making them more reliable for uses in neural implants, biosensors, and advanced computing technologies.
PEDOT:PSS is composed of two polymers: one is non-water-soluble and conducts electronic charge, while the other is water-soluble and carries ionic charge. This dual capacity allows PEDOT:PSS to bridge the divide between biological systems and electronic interfaces.
“It allows you to essentially communicate with the brain,” noted Keene, who focuses his research on advanced materials aimed at creating smaller, high-resolution electrodes capable of precise recording and stimulation of neural activity.
The human nervous system uses ions—such as sodium and potassium—to transmit signals, whereas electronic devices rely on electron flow. Consequently, a material that accommodates both types of charge is essential for effective neural implants and other bioelectronic applications that need to convert biological signals into interpretable data while minimizing tissue damage.
By eliminating the need for a crosslinker, this research not only optimizes the production process for PEDOT:PSS but also enhances its effectiveness. The new technique yields a material with threefold increased electrical conductivity and improved batch consistency—attributes that are highly beneficial for medical use.
The traditional crosslinker functioned by chemically linking the strands of the two types of polymer in PEDOT:PSS, forming a network. However, it left certain water-soluble strands exposed, which contributed to stability challenges. Additionally, the crosslinker introduced variability and potential toxicity into the composition.
On the other hand, applying heat stabilizes PEDOT:PSS through a phase change. When subjected to elevated temperatures, the water-insoluble portion reorganizes, causing the water-soluble components to emerge on the surface and allowing for their removal. The leftover material is a thinner, purer, and more stable conductive film.
“This method effectively resolves many issues researchers face when working with PEDOT:PSS,” Keene emphasized. “It also removes the need for a potentially harmful chemical.”
Margaux Forner, a doctoral candidate at Cambridge and co-first author of the study alongside Doshi, remarked that bioelectronic devices created with heat-treated PEDOT:PSS, including transistors, spinal cord stimulators, and electrocorticography arrays—used to capture brain activity—proved easier to fabricate. They demonstrated reliability and performance comparable to those made with a crosslinker.
“The heat-treated devices remained stable for over 20 days post-implantation in chronic in vivo tests,” Forner shared. “Crucially, the film’s electrical performance remained excellent even when stretched, indicating its potential for durable bioelectronic systems both within and outside of the body.”
This advancement may illuminate the reasons behind earlier complications in using PEDOT:PSS for long-term neural implants, including instances noted with Neuralink. By enhancing the reliability of PEDOT:PSS, this discovery stands to propel the field of neurotechnology forward, paving the way for implants aimed at restoring mobility following spinal cord injuries and developing interfaces that connect the brain with external mechanisms.
Furthermore, the researchers developed techniques to pattern PEDOT:PSS into microscale 3D structures—a significant advancement that could lead to improved bioelectronic devices. Utilizing a high-precision femtosecond laser allows the selective heating of material sections, resulting in specialized textures that enhance cellular interactions with devices.
“We are incredibly enthusiastic about our ability to 3D print polymers at such a small scale,” Doshi stated. “This has been a crucial objective for the community, as crafting this functional material in 3D could facilitate better integration with the biological environment. Traditionally, this has involved combining PEDOT:PSS with various photosensitive materials that often alter its properties or present scaling challenges.”
Keene’s past research on patterning grooves in electrodes demonstrated that cells preferentially adhere to textured surfaces matching their scale. “A 20-micron cell prefers to attach to textures about 20 microns in size,” he explained.
This innovation could enable the design of neural interfaces that foster improved integration with surrounding tissues, enhancing signal quality and longevity.
Keene has also pursued the application of PEDOT:PSS in neuromorphic memory devices, which seek to enhance artificial intelligence algorithms. Neuromorphic memory aims to replicate how the brain retains information.
“It essentially mimics the synaptic plasticity of the brain,” Keene remarked. “We can manipulate the conductivity between two terminals, which resembles the brain’s process of learning by strengthening or weakening synaptic connections between neurons.”
This research not only disrupts long-held beliefs but also enhances the utility and strength of PEDOT:PSS—an evolution that could expedite the development of safer and more effective neural implants and bioelectronic systems.
The study received support from Stanford, Meta, the Wu Tsai Human Performance Alliance at Stanford, the Joe and Clara Tsai Foundation, the Wellcome Trust, the National Science Foundation, and several other organizations, ensuring a collaborative approach to innovation in bioelectronic materials.
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