A sticky problem
Pittsburgh scientists find a way to connect electrodes to the squishy material of the brain
By connecting electrodes to the nervous system, scientists are able to restore limited vision or hearing, prevent and control seizures, modulate hormone levels or reduce pain. Patients with brain-machine interfaces have even been able to control video games with their brains. Electronic implants have been used clinically for a half-century.
But the electronics neuroengineers generally have had access to were made the same way as the ones found in your computer: large integrated circuits of rigid metals and silicon.
For the heart, this works; the heart is large and reasonably firm, so large electrodes can safely be implanted on it. But to implant electrodes on the brain, spine or individual nerves, this becomes a problem. An electrode has to remain connected to the same area even as that area shifts and flexes, and rigidity can also amplify the immune systems’ natural reaction to any foreign implant.
“The brain and the spinal cord have the mechanical properties of Jell-O, really soft and squishy, so for a [brain-machine interface] to interrogate that structure, they have to kind of match that tissue,” said Chris Bettinger, associate professor of materials science and biomedical engineering at Carnegie Mellon University.
With that in mind, Mr. Bettinger and his team have developed a process for fabricating soft, adhesive hydrogels and printing an ultra-thin micro-electrode array onto them. Working with bioengineer Robert Gaunt of the University of Pittsburgh, who was able to show the gel functioning as a neural interface in a cat, they published a paper on the gel in May in the scientific journal Advanced Functional Materials.
Mr. Bettinger trained as a material scientist, getting his doctorate at MIT, and worked in designing bodycompatible polymers before getting interested in materials that “do something,” as he put it.
“Electronics are more fun in general,” he explained. In recent years, his lab, the Bettinger Group, has worked on electronics that can be swallowed, materials that can be made to disintegrate by ultrasound, and neural interfaces. The group’s latest paper presents the process to marry adhesive flexible material to a micron-thin layer of electronic circuitry to interface with the nervous system.
Neural interfaces that hook computers up to the nervous system are already in clinical uses. Caleb Kemere, a neuroengineer at Rice University, spoke about a company called Livanova that stimulates the vagus nerve to prevent and control seizures in epileptic patients.
The advantage of Mr. Bettinger’s work, Mr. Kemere said, is that existing implants “often move after implantation, which ends up causing a loss of function.”
You could compare this to a Band-Aid, for example. When it gets wet, it moves or comes off.
The researchers’ material uses a naturally occurring organic compound called “catechols” for all three core properties: flexibility, adhesion and connectivity. A hydrogel is a “polymer plus a solvent, water,” like gelatin but in this case with catechols. Furthermore, catechols are adhesive, so they both “pick up the electronics” and “stick to the tissue.” “Chemically similar” materials are used for glues in internal medicine, Mr. Bettinger said.
The flexibility and adhesion also help with the main side effect researchers worry about with electronic implants: immune response. “Flexible materials produce less of an immune response,” said Jacob Robinson, a colleague of Mr. Kemere’s at Rice. Implantation, via invasive surgery, will always produce an immune response, but having an implant that acts like tissue can minimize the problem.
Mr. Gaunt of Pitt works with neural interfaces for prosthetics generally. To test the material, he and his team implanted it on the dorsal root ganglia of a cat; it was able to record neural activity, as intended.
Mr. Gaunt hastened to say that the hydrogel is in early research stages, “and the translational pathway for an implantable medical device is long and complicated and expensive.” He could not predict how long it would take for the gel to be clinically ready and approved by regulators. But he and other researchers were very optimistic about the eventual application of materials like it for an array of medical uses.
The paper focuses on use of the gel for sensors, but Mr. Kemere pointed out that the electrode arrays can both record and transmit electronic signals.
Treating chronic pain is one potential clinical use, he said. “The problem with chronic pain is that you have increasing tolerance to opioids.
“Treatment of pain directly at the nervous system will become only more and more common because of the problem of pain medications,” Mr. Kemere said.
Mr. Bettinger’s hydrogel would “fit nicely under the concept of electronic medicine or electroceuticals,” said Mr. Robinson. “For instance, for organs that regulate hormone levels, instead of a pill, you modulate the nerve that controls the organ that regulates that hormone.”
In that case you’d be affecting only the target organ rather than the whole body, reducing potential problems of systemic side effects, he said.
There are other uses, too, Mr. Bettinger said. “Paraplegics, if you ask them what they need, they won’t necessarily say they want the use of their legs back. They want sexual function and bladder function. So if you could void the bladder in a controlled way that would be very helpful to people who need that capability.”
Mr. Bettinger’s next priority is showing specific applications for the hydrogel, he said.
Mr. Gaunt in his own lab is interested in neural interfaces for prosthetics, currently exploring how information can travel back from the prosthetic to the nervous system. “If you touch something with a prosthetic, can you feel it?”