Flexible Electronics Promise New Neurotech Applications

by James Cavuoto, editor

Researchers at a number of institutions in the U.S. and Europe are currently working on applications for flexible electronics, including several that may find their way into neurotechnology devices. Flexible electronic structures offer neural engineers the potential to bend, expand, or otherwise manipulate sensors and stimulation electrodes in nerve or muscle tissue in a more natural manner than is possible with rigid devices.

The National Institutes of Health has funded a project to develop flexible arrays of microelectrodes for brain research, which includes investigators at Columbia University and Princeton University. The goal of the project is to develop microelectrode arrays capable of simultaneous mechanical stimulation and electrophysiological recording of living brain tissue in vitro.

The stretchable microelectrode array will be a powerful new tool in traumatic brain injury research, complementing current in vivo studies. Flexible arrays will allow researchers to replicate injuries in the lab without destroying the electrodes that monitor how brain cells respond to physical trauma. The devices feature microelectrodes that are able to withstand the sudden stretching that is used to simulate severe head trauma. The systems could allow far more nuanced studies of brain injury than previously possible and may lead to better treatments in the minutes and hours immediately following the injury. The work also has implications for other areas of medicine, including next-generation prosthetics.

“This is an immediate application of the electronics of the future,” said Sigurd Wagner, a Princeton professor of electrical engineering. Led by Barclay Morrison III, an assistant biomedical engineering professor at Columbia, members of the team presented their work at the recent conference of the Materials Research Society in San Francisco.

Existing techniques to study traumatic brain injury have been limited because it is almost impossible to insert an electrode into a cell to obtain a recording, remove the probe, injure the cell, and then reinsert the probe into the same cell, Morrison said. Because of this limitation, researchers rely on other surrogate markers of injury, such as cell death.

“In terms of traumatic brain injury, there can be a lot of functional damage to the brain in other ways than just killing a cell,” Morrison said. “Neurons can still be alive, but not properly firing,” which leads to problems ranging from comas to epilepsy.

These improperly functioning neurons can now be assessed by the electrodes in the stretchable membranes. After brain cells have been placed on the flexible surface and allowed to grow, the researchers measure their normal activity. The membrane is then suddenly stretched and returned to its original form. Having withstood the shock, the electrodes embedded in the membrane continue to monitor the cellular activity, providing a before and after picture of traumatic brain injury.

Future work will continue to refine these measurements and also attempt to obtain readings from cells during the injury events themselves, Morrison said. The flexible electrodes also can be used to provide electrical input to brain tissue and may one day be used to induce learning in brain cells damaged by trauma. This technology also has promising applications for the engineering of nervous, muscular, and skeletal tissue. For instance, Morrison said, the electrodes could potentially be used to train heart tissue grown in the lab to contract appropriately when stimulated.

The engineers created the first working stretchable circuits by linking tiny pieces of traditional semiconductors mounted on a rubbery membrane with thin pieces of gold. Even when stretched, the circuits maintained their ability to conduct electricity.

Research on the flexible membranes also is likely to contribute to the longstanding challenge of connecting electronic devices to the human nervous system, Wagner said. Prosthetic devices, for example, could be coated with electronic “skin” that senses touch and temperature and sends that information back to the brain like any natural human limb.

“A basic problem with the interface between electronics and living tissue is that electronics are hard and tissues are soft,” he said, noting that nerve cells quickly become irritated when in contact with the hard electrodes of today. The hope is that the devices of the future will flex with living tissue, maintaining a connection without damaging the human cells.

Researchers at Argonne National Laboratory and the University of Illinois at Urbana-Champaign are also working on flexible electronic structures. Investigators there believe the flexible structures could have useful applications as sensors and as electronic devices integrated into artificial muscles or biological tissues.

“Flexible electronics are typically characterized by conducting plastic-based liquids that can be printed onto thin, bendable surfaces,” said Yugang Sun of Argonne National Lab. “The objective of our work was to generate a concept along with subsequent technology that would allow for electronic wires and circuits to stretch like rubber bands and accordions leading to sensor-embedded covers for aircraft and robots, and even prosthetic skin for humans.

The team has been successful in fabricating thin ribbons of silicon and designing them to bend, stretch and compress like an accordion without losing their ability to function.