Electronic devices that can signal directly to living cells have a variety of medical applications, from targeted drug delivery to artificial eyes and ears. Pacemakers, which use electrical impulses to keep the heart beating steadily, are one example of such technology already in use. But getting rigid traditional electronics to interface well with soft tissue can be challenging. Now, an international team of researchers has used multiphoton lithography to create bioelectrodes from conducting polymers. Their results, published recently in the Journal of Materials Chemistry B (DOI: 10.1039/c5tb00104h), open the door for more customizable and precise bioelectronic devices.
“The tunable properties of conducting polymers make them attractive components of electroactive biomaterials such as drug delivery devices, electrodes, or tissue scaffolds,” writes John Hardy, a postdoctoral associate at the University of Florida and the first author on the article.
According to Hardy, conducting polymers are “softer than the inorganic conductors commonly used—for instance, metals—which diminishes mechanical mismatch with the tissues surrounding the implant.”
The researchers used a printing technique called multiphoton lithography to produce arrays of wires out of polypyrrole, a conducting polymer, on glass. Multiphoton lithography allows for precise design on the micrometer scale, the size scale of living cells, making the technique ideal for fabrication of biologically relevant structures.
The team tested the drug delivery capabilities of the array by loading a fluorescent dye and then applying a voltage to the wires. As hoped, the stimulation (0.6 volts applied over 30 seconds) caused release of the dye into a buffered saline solution.
The researchers also wanted to explore whether the conducting polymer structures could potentially be used as implantable neural interfaces. They used the conducting polymer array as an electrode to stimulate cells in an ex vivo slice of mouse brain. Again, when electrical current was applied to the wires, the targeted cells in the mouse brain showed activity, demonstrating that the conductive polymers could interface successfully with the nervous system.
The conducting polymer arrays will certainly require fine-tuning before being used clinically—passive leakage of the dye was one problem—and the amount of dye actively transported under electrical stimulation was fairly modest. However, once optimized, the technique can be used in a wide variety of medical applications.
“Conceptually, it should be possible to print de novo designed 3D [three-dimensional] structures to make patient-specific implants,” says Hardy, where this is a direction the research group plans to explore in the future. “In the long term, we can foresee prospects for such materials as drug delivery devices or long-term biointerfaces with bodily tissues.”
Polypyrrole (Ppy) electrode arrays could be used for (a) drug delivery or (b) neural stimulation. Credit: John George Hardy, University of Florida.
“Being able to precisely design stimulation and recording traces at micron resolution could bring unprecedented control to neural interfacing in vitro and in vivo,” says Jonathan Rivnay, a researcher at École des Mines de Saint-Étienne in France, who was not involved in the research.
Rivnay sees applications to both basic and applied research. “Their ability to deliver minute amounts of [a] drug on cue, and in a highly localized manner—potentially at sub-cellular resolution—could pave the way for both novel fundamental studies of controlled networks of neurons, as well as for clinical therapies based on combined chemical and electrical stimulation,” he says.
