Conductive granular hydrogels: 3D-printable electrodes as an alternative to rigid implants

Researchers at the McKelvey School of Engineering at Washington University in St. Louis report on conductive granular hydrogels that could serve as injectable and 3D-printable electrode materials. The goal is to eventually replace rigid, surgically implanted sensors made of metal or silicon with soft, tissue-compatible systems. The team led by biomedical engineer Alexandra Rutz and doctoral student Anna Goestenkors describes in Small granulates made of spherical PEDOT:PSS hydrogel particles that can be compacted into paste-like composites.

“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.”

In the compacted state, the particles form a network with micropores that favors cells and ion transport. The connections between particles are not permanent, allowing the material to be extruded like a fluid under sufficient force and then resolidify into a shape-stable composite afterward.

“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.”

From a technology perspective, the material is simple to produce via a water-in-oil emulsion: polymer droplets form under stirring and crosslink at elevated temperatures; 90 °C yielded the most reproducible particles after oil removal. In a feasibility study on grasshopper antennae, the researchers were able to measure local field potentials correlated with odor stimuli after small particle aggregates were applied.

“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.

The authors envision application scenarios ranging from 3D-printed, adaptable electrodes to biohybrid tissue scaffolds and injectable interfaces. An ongoing patent process is intended to secure further development and potential commercialization. For the 3D-printing community, this approach offers an option to manufacture conductive bioelectronics additively and compatibly with tissue without requiring large surgical access points.