HOUSTON – (March 31, 2025) – A chance discovery led a team of scientists from Rice University, University of Cambridge and Stanford University to streamline the production of a material widely used in medical research and computing applications.
For over two decades, scientists working with a composite material known as PEDOT:PSS, used a chemical crosslinker to make the conductive polymer stable in water. While experimenting with ways to precisely pattern the material for applications in biomedical optics, Siddharth Doshi, a doctoral student at Stanford collaborating with Rice materials scientist Scott Keene, skipped adding the crosslinker and used a higher temperature while prepping the material. To his surprise, the resulting sample turned out to be stable on its own ⎯ no crosslinker needed.
"It was more of a serendipitous discovery because Siddharth was trying out processes very different to the standard recipe, but the samples still turned out fine," Keene said. "We were like, 'Wait! Really?' This prompted us to look into why and how this worked."
What Keene and his team found was that heating PEDOT:PSS beyond the usual threshold not only makes it stable without needing any crosslinker, but it also creates higher quality devices. This method, described in a recent study published in Advanced Materials, could make bioelectronic devices easier and more reliable to manufacture with potential applications in neural implants, biosensors and next-generation computing systems.
PEDOT:PSS is a blend of two polymers: one that conducts electronic charge and does not dissolve in water and another that conducts ionic charge and is water-soluble. Because it conducts both types of charges, PEDOT:PSS bridges the gap between living tissue and technology.
"It allows you to essentially talk the language of the brain," said Keene, who researches advanced materials for smaller, high-resolution electrodes capable of both recording and stimulating neural activity with precision.
The human nervous system relies on ions ⎯ charged particles like sodium and potassium ⎯ to transmit signals, while electronic devices work with electrons. A material that can handle both is crucial for neural implants and other bioelectronic devices that need to translate biological activity into readable data and send signals without damaging sensitive tissue.
By eliminating the crosslinker, the research findings not only streamline the PEDOT:PSS fabrication process but also improve its performance. The new method produces a material with three times higher electrical conductivity and more consistent stability between batches ⎯ key advantages for medical applications.
The crosslinker worked by chemically bonding the two types of polymer strands in PEDOT:PSS together, creating an interconnected mesh. However, it still left some of the water-soluble strands exposed ⎯ a likely cause for the stability issues. Moreover, the crosslinker introduced variability and potential toxicity in the material.
In contrast, the higher heat stabilizes PEDOT:PSS by causing a phase change in the material. When heated beyond a certain temperature, the water-insoluble polymer reorganizes internally, pushing the water-soluble components to the surface, where they can be washed away. What remains is a thinner, purer and more stable conducting film.
"This method pretty much simplifies a lot of these problems that people have working with PEDOT:PSS," Keene said. "It also essentially eliminates a potentially toxic chemical."
Margaux Forner, a doctoral student at Cambridge who is a first author on the paper along with Doshi, said that heat-treated bioelectronic devices such as transistors, spinal cord stimulators and electrocorticography arrays ⎯ implanted grids or strips of neuroelectrodes used to record brain activity ⎯ were easier to fabricate, more reliable and equally high performing to those fabricated using the crosslinker.
"The devices made from heat-treated PEDOT:PSS proved to be robust in chronic in vivo experiments, maintaining stability for over 20 days postimplantation," Forner said. "Notably, the film maintained excellent electrical performance when stretched, highlighting its potential for resilient bioelectronic devices both inside and outside the body."
The finding may help explain why previous efforts to use PEDOT:PSS in long-term neural implants, including those by Neuralink, ran into stability issues. By making PEDOT:PSS more reliable, this discovery could help advance neurotechnology, including implants to restore movement after spinal cord injuries and interfaces that link the brain to external devices.
Beyond simplifying fabrication, the team found a way to pattern PEDOT:PSS into microscopic 3D structures ⎯ a breakthrough that could further improve bioelectronic devices. Using a high-precision femtosecond laser, the researchers can selectively heat sections of the material, creating custom textures that enhance how cells interact with the devices.
"We are really excited about the ability to 3D-print the polymers at the microscale," Doshi said. "This has been a major goal for the community as writing this functional material in 3D could let you interface with the 3D world of biology. Typically, this is done by combining PEDOT:PSS with different photosensitive binders or resins; however, these additions affect the properties of the material or are challenging to scale down to micron-length scales."
In past research, Keene explored patterning grooves onto electrodes, finding that cells preferentially adhere to grooves on the same order as their length scale. In other words, "a 20 micron cell likes to grab on to 20 micron-sized textures," he said.
This technique could be used to design neural interfaces that encourage better integration with surrounding tissue, improving signal quality and longevity.
Keene had also previously researched PEDOT:PSS in the context of neuromorphic memory devices used to accelerate artificial intelligence algorithms. Neuromorphic memory is a type or artificial memory that mimics how the brain retains information.
"It basically emulates the synaptic plasticity of your brain," Keene said. "We can modify the connection between two terminals by controlling how conductive this material is; this is very similar to how your brain learns by strengthening or weakening synaptic connections between individual neurons."
By unseating a long-standing assumption, the research not only made PEDOT:PSS easier to work with but also more powerful ⎯ a shift that could accelerate the development of safer, more effective neural implants and bioelectronic systems.
The research was supported by Stanford, Meta, the Wu Tsai Human Performance Alliance at Stanford, the Joe and Clara Tsai Foundation, the Wellcome Trust, the National Science Foundation (2026822, 1542152), the Henry Royce Institute (EP/P024947/1, EP/R00661X/1), the United Kingdom Engineering and Physical Sciences Research Council (EP/W017091/1) and the European Union (Marie Skłodowska-Curie grant agreement No. 101022365). The content herein is solely the responsibility of the authors and does not necessarily represent the official views of the funders.
