Modified soft material promises better bioelectronics

The scientific community has long been attracted by the potential of flexible bioelectronic devices, but faces obstacles in identifying materials that are biocompatible and have all the characteristics necessary to function effectively. Researchers have now taken a step in the right direction, modifying an existing biocompatible material so that it efficiently conducts electricity in humid environments and can send and receive ionic signals from biological media.

Their paper, “Electrostatic Self-Assembly Induces Efficient Mixed Transport and Water Stability in PEDOT:PSS for High Performance OECTs,” is published in the journal Matter.

“We’re talking about an order of magnitude improvement in the ability of soft bioelectronic materials to function effectively in biological environments,” says Aram Amassian, co-corresponding author of a paper on the work and professor of science and technology. materials engineering. at North Carolina State University. “This is not incremental progress.”

There is enormous interest in creating organic bioelectronic and organic electrochemical transistors (OECTs), with a wide range of biomedical applications. However, the identification of non-toxic materials capable of conducting electricity and interacting with ions, which is essential for operation in biological environments and for effective operation in water-based aqueous environments of biological systems , constitutes a limiting factor.

One interesting material is PEDOT:PSS, a non-toxic polymer capable of conducting electricity. PEDOT:PSS is used to create thin films that are actually networks of fibers only a few nanometers wide. Electric current can pass through fibers, which are also sensitive to ions in their environment.

“The idea is that, because the ions interact with the fibers and affect their conductivity, PEDOT:PSS can be used to detect what is happening around the fibers,” explains Laine Taussig, co-first author of the paper and recent Ph.D. D. graduate of NC State who now works at the Air Force Research Laboratory.

“Essentially, PEDOT:PSS would be able to monitor its biological environment. But we could also use electric current to influence the ions surrounding the PEDOT:PSS, sending signals to that biological environment,” explains Masoud Ghasemi, co-first author and a former postdoctoral fellow at NC State who is now a postdoctoral fellow at Penn State.

However, the structural stability of PEDOT:PSS decreases significantly when placed in aqueous environments, such as biological systems. Indeed, PEDOT:PSS is a unique material composed of two components: PEDOT, which conducts electricity and is not soluble in water; and PSS, which responds to ions but is soluble in water. In other words, PSS causes the material to disintegrate upon contact with water.

Previous efforts to stabilize the structure of PEDOT:PSS allowed the material to withstand aqueous environments, but both harmed PEDOT:PSS’s performance as a conductor and made it more difficult for ions to interact with the components. PSS of the material.

“Our work here is important because we have found a new way to create a PEDOT:PSS that is structurally stable in humid environments and capable of both interacting with ions and conducting electricity very efficiently,” explains George Malliaras, co-corresponding author. and Prince Philip Professor of Technology at the University of Cambridge.

Specifically, researchers start with PEDOT:PSS in solution, then add ionic salts. Over time, ionic salts interact with PEDOT:PSS, causing it to self-assemble into fibers whose unique structure remains stable in humid environments. This modified PEDOT:PSS is then dried and the ionic salts rinsed away.

“We already knew that ionic salts could affect PEDOT:PSS,” says Amassian. “What’s new here is that by giving the ionic salts more time to see the full extent of these effects, we modified the crystal structures of PEDOT and PSS so that they essentially intertwine at the molecular scale. This makes PSS impermeable to water in the environment, allowing PEDOT:PSS to maintain its structural stability at the molecular level.

“The change is also hierarchical, meaning there are changes at the molecular level up to the macro scale,” says Yaroslava Yingling, co-author of the paper and professor emeritus of human science and engineering. materials at Kobe Steel at NC State. “The ionic salts cause the PEDOT:PSS to essentially reorganize into a phase that resembles a web-like gel that is preserved in dry and humid environments.”

In addition to being stable in an aqueous medium, the films obtained retain their conductivity. Additionally, because PEDOT and PSS are closely bonded, it is easy for ions to reach and interact with the PSS component of the material.

“This new phase of PEDOT:PSS was used to create OECTs by our Cambridge collaborators,” says Amassian. “And these OECTs set a new state-of-the-art standard for volumetric capacity and mobility of electronic media. In other words, it is the new benchmark for conductivity and ionic reactivity in bio-electronics. friendly.”

“Since PEDOT:PSS is transparent, flexible, stretchable, conductive and biocompatible, the range of potential applications is exciting and extends well beyond the biomedical sector,” says Enrique Gomez, co-corresponding author and professor at Penn State.

The article was co-authored by Albert Kwansa, assistant research professor of materials science and engineering at NC State; Nathan Woodward, a Ph.D. student at NC State; Sanggil Han and Scott Keene of Cambridge; and Ruipeng Li of Brookhaven National Laboratory.