Heat-based stabilization of a conductive polymer simplifies bioelectronics fabrication

Recent advances in the field of materials science have opened new possibilities for the fabrication of bioelectronics, devices designed to be worn or implanted in the human body. Bioelectronics can help to track or support the function of organs, tissues and cells, which can contribute to the prevention and treatment of various diseases.

A promising material for the fabrication of bioelectronics is PEDOT:PSS, a polymer known for its high electrical conductivity, flexibility and compatibility with biological tissues. Despite its advantageous properties, PEDOT:PSS is known to gradually dissolve in biological fluids, a limitation that has so far been counteracted using chemical compounds and processes.

Researchers at Stanford University, the University of Cambridge and Rice University recently uncovered an easier and potentially safer strategy to stabilize this bio-compatible polymer using heat. Their proposed thermal treatment, outlined in the journal Advanced Materials, was found to make PEDOT:PSS films stable in water without the need for any chemical additives.

“This work started out from a serendipitous discovery during the course of my previous research, where I was using the responsive polymer, PEDOT:PSS to make shape-changing photonic devices,” Siddharth Doshi, co-first author of the paper, told Tech Xplore.

“I noticed that films of PEDOT:PSS that I’d accidentally baked at higher than usual temperatures didn’t dissolve in water. This was a huge surprise, as PEDOT:PSS is a widely studied conductive polymer, and to get around the fact that it delaminates in water, hundreds of bioelectronics papers use chemical cross-linkers to stabilize their devices.”

After making this surprising observation as part of their earlier research, Doshi and his colleagues set out to explore the possibility that heating up PEDOT:PSS films could also stabilize them in fluids. In addition, they wished to determine what exactly happened when the films were heated above certain temperatures, how this heating process affected their properties and whether a heat-based approach could replace existing chemical stabilization processes.

“The key advantage of our approach is its simplicity—you can simply heat up films of commercial, unmodified PEDOT:PSS on a hot-plate between 150°C and 200°C for 2 mins, and it no longer dissolves in water,” explained Doshi. “It works on different substrates, including stretchable plastics and even different fabrics, and avoids many of the complications of chemical crosslinkers, which affect the conductivity and reliability of the films.”

The heat treatment devised by the researchers could also enable the direct patterning of PEDOT:PSS simply by applying heat to specific sites on the films and thus removing the need for complex lithography techniques. As part of their study, Doshi and his colleagues also demonstrated the 3D printing of PEDOT:PSS at the micro-scale, using a focused femtosecond laser beam.

“The PEDOT:PSS absorbs strongly at the near-infrared laser wavelength we used, which causes local heating,” said Doshi. “Through layer-by-layer scanning of a focused laser spot, we could locally stabilize 3D patterns within the film which remain even after the unexposed parts of the film are washed away in water. As a nice additional benefit, this gives us an environmentally friendly way to pattern this material, using only water for processing instead of other toxic solvents.”

The initial tests run by the researchers yielded very promising results. Ultimately, their heat-based approach was found to make PEDOT:PSS films stable in water, while also improving their performance.

“Heat-treated bioelectronic devices such as transistors, spinal cord stimulators and electrocorticography (ECoG) arrays were easier to fabricate, more reliable, and equally high performing. And they proved to be robust in chronic in vivo experiments, maintaining stability for over 20 days post-implantation,” said Margaux Forner, Ph.D. student from the University of Cambridge and co-first author on the paper.

“Notably, the film maintained excellent electrical performance when stretched, highlighting its potential for resilient bioelectronic devices both inside and outside the body.”

“Our characterization suggests that heat-treatment drives phase separation of PEDOT and PSS-rich regions, helping stabilize the polymer blend by creating a network of a water-insoluble, PEDOT-rich phase,” added Scott Keene, Assistant Professor at Rice University and the senior author of the paper. “In addition to making the polymer water-stable, we found that phase separation improves both the conductivity and capacitance of our films, two critical parameters for bioelectronic devices.”

In addition, the simple heat-based treatment introduced by Doshi and his colleagues could be easily integrated with existing manufacturing processes. In the future, it could thus simplify the development of various PEDOT:PSS-based devices, including bioelectronics, as well as wearable electronics and electronic skins.

“We are also really excited about the ability to 3D-print the polymers at the microscale,” said Doshi. “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 photo-sensitive binders or resins; however, these additions affect the properties of the material or are challenging to scale down to micron-length scales.”

The researchers have successfully used their heat-based approach to create complex 3D structures, including blocks, textured surfaces and sculptural test-pieces with curves, bevels and recesses, made of PEDOT:PSS. They achieved this using femtosecond laser patterning, but it could eventually also be attained using other laser-based methods.

The researchers hope that other materials scientists and engineers will start experimenting with their thermal treatment and using it to stabilize PEDOT:PSS films without relying on chemical processes. In the future, their newly introduced approach could facilitate the use of these films for the development of implantable devices and other devices that are meant to be resistant to water or other fluids.

“One direction for future research will be to explore new ways to interface with the 3D world of biology through functional cell-interfaces,” said Doshi. “We are also interested in going back to our original motivation of exploring new ways of fabricating PEDOT:PSS, which is in making switchable 3D photonic devices that use the electro-optic tunability of PEDOT:PSS to dynamically change their optical properties.

“There is a lot of interest in the field of micro and nano-optics for devices that change their functionality on demand, and 3D devices could have a lot of advantages over 2D devices that have been most widely explored to date.”

In their next studies, Doshi and his colleagues also plan to further investigate the fundamental mechanisms underpinning the stabilization of PEDOT:PSS when it is heated above 150°C for over two minutes. To do this, they will employ advanced imaging and material characterization techniques.

“Techniques like in-situ transmission electron microscopy or in-situ X-ray diffraction could let us visualize what is exactly happening to the PEDOT and PSS chains and the overall microstructure of the material in real-time,” added Doshi.

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