Modified soft material promises better bioelectronics, researchers say

Soft bioelectronic devices hold potential for many advances in the health care field, but researchers have faced hurdles in identifying materials that are biocompatible while still maintaining all necessary characteristics to operate effectively. A team co-led by Penn State researchers has now taken a step toward achieving such a material, modifying an existing biocompatible material to conduct electricity efficiently in wet environments, as well as send and detect ionic currents within biological media.

Their results were published in the journal Matter.

“Materials capable of simultaneous ionic and electrical conduction have the potential to interface with biosystems in unique ways,” said co-corresponding author Enrique Gomez, Penn State professor of chemical engineering and interim associate dean for equity and inclusion in the College of Engineering. ?

There is tremendous interest in creating organic bioelectronics and organic electrochemical transistors (OECTs), the researchers said, as they could have a wide range of biomedical applications. However, one limiting factor is identifying nontoxic materials that can conduct electricity, interact with ions —which is critical to functioning in biological environments — and operate efficiently in the aqueous, water-based environments of biological systems.

One material of interest has been PEDOT:PSS, which is a nontoxic polymer that is able to conduct electricity. PEDOT:PSS is used to create thin films that are effectively fiber networks that are only nanometers wide. Electrical current can run through the fibers, which are also sensitive to ions in their environment.

According to the researchers, because ions interact with the fibers — and affect their conductivity — PEDOT:PSS could be used to sense what is happening around the fibers.

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

However, PEDOT:PSS’s structural stability declines significantly when placed in aqueous environments — like biological systems. That’s because PEDOT:PSS is a single material made from two components: the PEDOT, which conducts electricity and is not soluble in water; and PSS, which responds to ions, but is water soluble. In other words, the PSS makes the material start to fall apart when it encounters water.

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

“Our work here is important, because we’ve found a new way to make a PEDOT:PSS that is structurally stable in wet environments and able to both interact with ions and conduct electricity very efficiently,” said George Malliaras, co-corresponding author and Prince Philip Professor of Technology at Cambridge University.

Specifically, the researchers start with PEDOT:PSS in solution and then add ionic salts. Given time, the ionic salts interact with the PEDOT:PSS, causing it to self-assemble into fibers with a unique structure that remains stable in wet environments. ?

“We already knew that ionic salts could affect PEDOT:PSS,” said Aram Amassian, co-corresponding author of the paper and a professor of materials science and engineering at NC State. “What’s new here is that by giving the ionic salts more time to see the full extent of those effects, we modified the crystalline structures of the PEDOT and the PSS to essentially lace themselves together at the molecular scale.”

This makes the PSS impervious to the water in the environment, according to the researchers, allowing the PEDOT:PSS to maintain its structural stability at the molecular level.

“The change is also hierarchical, meaning that there are shifts at the molecular level all the way up to macroscale,” said Yaroslava Yingling, co-author of the paper and Kobe Steel Distinguished Professor of Materials Science and Engineering at NC?State. “The ionic salts cause the PEDOT:PSS to essentially reorganize itself into a phase that resembles a web-like gel that is preserved in both dry and wet environments.”

In addition to being stable in aqueous environments, the resulting films retain their conductivity. What’s more, the researchers said, because the PEDOT and PSS are tightly interwoven, it is easy for ions to reach and interact with the PSS component of the material.

“Given that PEDOT:PSS is transparent, flexible, stretchable, conductive and biocompatible, the range of potential applications is exciting — extending well beyond the biomedical sector,” Gomez said.

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

The work was done with support from the Office of Naval Research.

Editor’s note: A version of this article originally appeared on North Carolina State University’s website.?