Conductive granular hydrogels: injectable electrodes as 3D-printable bioelectronics

A team from the McKelvey School of Engineering at Washington University in St. Louis reports on conductive granular hydrogels that can be injected, shaped, and processed via 3D printing. The goal is to replace rigid implants made of metals or silicon with soft, body-like electrodes that can measure and stimulate biological signals. The results were published on October 8 in the journal Small.

The group led by Alexandra Rutz developed spherical microparticles from the conductive polymer PEDOT:PSS. When densely packed, the particles behave like a pasty, microporous mass: under shear they flow through a needle or print nozzle; once the load is removed, they reestablish their contacts and form a dimensionally stable composite.

“Granular hydrogels have not been widely studied for these applications, but we have found that this material has the potential to be injected with a needle at the site,” Rutz said. “We’re trying to borrow techniques from tissue engineering to try to have these electronically conducting materials emulate properties of the body while being able to leverage the function of these materials to have more sophisticated ways of doing it.”

The temporary contact points between the particles create micron-scale pores and enable reversible switching between flowing and solid behavior. In printing trials, continuous strands were produced that conform to complex surfaces.

Proof of function was achieved in Barani Raman’s lab, where particles placed on the antenna tips of grasshoppers recorded local field potentials during odor stimulation.

“Because the particles’ connections aren’t permanent, they can move relative to each other, and the material will flow like a liquid when you apply a certain amount of force that allows them to be injected or extruded,” she said. “But when you remove that force, they recover those connections and become more of a paste-like solid again, so it’s a very adaptable material.”

“With further development, we envision these conducting granular hydrogels could be used as 3D printed customized electrodes that can conform to topographically diverse surfaces or completely encapsulate biological components, tissue engineering scaffolds or injectable therapies,” Rutz said.

Rutz and Goestenkors have filed a U.S. patent for the fabrication and applications of the particles and are working with the university’s Office of Technology Management on protection of rights and technology transfer into practice.