Scientific Method / Science & Exploration

Organic conductors, nanostructures, and future medical implants

A perspective in today's issue of Science discusses how advances in renewable …

When it comes to materials science, two of the hottest areas of research are nanotechnology and organic circuitry, both of which we've covered extensively. Particles structured on the nanoscale often have distinct structural and surface area properties that provide them with structural and catalytic behavior that's distinct from that of the parent material. Organic conductors and semiconductors have attracted interest because they can potentially be manufactured largely out of abundant elements like nitrogen and carbon, and provide flexible electronics. Both of these materials have largely attracted interest in light of what they can do to move humanity onto a more sustainable path, but a perspective that will appear in today's issue of Science describes another field that they have the potential to revolutionize: medical implants.

The perspective, written by a trio of researchers from Australia's University of Wollongong, starts with a basic premise: we're in pretty good shape when it comes to basic, structural implants like knee replacements, but we're just starting to scratch the surface of the potential to create implants that communicate with the surrounding tissues. Devices like implanted pacemakers and defibrillators can afford to be relatively crude in how they interact with the heart, but we're starting to see implants that communicate with nerve cells, such as cochlear implants and the electrodes used for deep brain stimulation.

The problem here is typically not getting sufficient power to the nerve cells (which are fairly sensitive to electric currents), but ensuring that the current gets to the nerve cells in the first place. Scar tissue usually builds up around implanted tissue—not such a big problem when the implant is a replacement hip, but a serious problem when the scar tissue gets between an implanted electrode and the nerves it's supposed to interact with. The development of scar tissue has been observed around implants used for deep brain stimulation in Parkinson's patients.

It's precisely this problem that the new materials may target. It's possible to identify specific nanoscale structures that favor nerve attachment or disfavor the growth of the cells responsible for scar tissue; provided that traditional electrodes can be manufactured in a way that implants should be able to better integrate with the nervous system they're supposed to be communicating with.

Organic circuitry offers even better prospects, as conducting polymers could incorporate not only the structural features that work for standard electrodes, but also provide chemical structures that increase nerve cell attachment and/or inhibit the production of scar tissue. As noted above, the typical issue with organic polymers—the ability to carry significant currents— isn't much of an issue when it comes to interfacing with nerve cells.

The perspective also describes how some conductive polymers can be produced using reactions that run at room temperature using chemicals that aren't toxic to living cells. As a result, it's apparently possible to embed cells within these materials as they form, which makes it far easier to integrate the material with living tissue. Another option is to embed proteins or other molecules that act as nerve attractants or growth factors, which would increase the prospect of a good integration between the implant and the body's tissues. The authors also discuss the potential use of graphene, which is essentially an unrolled carbon nanotube, which has already been shown to form functional electrical connections with nerve cells.

The focus here is on nerves, because there are a variety of techniques like deep brain stimulation that appear poised to leave the lab. These include things like smart prosthetics that can be controlled by the same nerves that once maneuvered the missing body part, to human-computer interfaces for those with spinal cord injuries. But the authors also note that many of the same techniques could improve the performance of more traditional medical implants. For example, an organic coating that promotes bone formation could improve the integration of items like joint replacement hardware.

Overall, there appear to be plenty of opportunities for experimentation with different electrical, chemical, and mechanical properties when it comes to these technologies, so the real challenge may be in exhausting all the potential combinations in order to discover the optimal combination for a given application.

Science, 2009. DOI: 10.1126/science.1168346

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