Technology developed at Purdue University is expected to soon change the lives of amputees with bionic limbs. These robotic prosthetics have traditionally been controlled by electrodes that sit on the skin to record signals from the muscle; for example, a bionic lower arm could rely on electrodes that sit on top of the still-existing bicep. Purdue researchers have developed tiny electrodes that, instead, are buried inside the muscle; it’s a development a Chicago-based startup describes as “game-changing” for bionic limbs—so much so that it purchased the exclusive license and aims to give amputees freedom of movement that was previously impossible.

Traditionally, bionic limbs rely on signals from the brain, nerves or muscles to control their movement. Purdue Biomedical Engineering Professor Dr. Pedro Irazoqui says the brain option requires risky surgery. Although less dangerous, the nerve method also requires surgery.

“The folks trying to record signals from the brain or the nerves have been in the papers for three decades, but there isn’t a single product out there that is reliably being implanted in patients,” says Irazoqui, “because getting those signals is just too hard to do.”

That’s why most bionic limbs rely on signals from muscles to control movement of the prosthetic, but those signals are limited. Researchers at the Rehabilitation Institute of Chicago recently discovered a method to make muscle-controlled operation of limbs even better, and in 2013, founded a company called Coapt to bring it to the masses.

“You can take the nerve that used to go down the missing arm, and splice it into the pectoral and lateral muscles that are left behind,” says Irazoqui. “After that nerve regenerates—which is a pretty straightforward process—when you just think about moving your missing arm, those [pectoral and lateral muscles] are going to contract. [Those muscles] act as a biological amplifier, making the signal from the nerve much easier to record.”

In addition to re-routing the nerve, Coapt developed what it describes as an “upgrade kit” that can be used with any manufacturers’ bionic limb. The company say it’s the first method to successfully control a robotic limb “intuitively,” or with thoughts. Coapt’s device—added to any prosthetic—records the now- amplified signal from the muscle and uses a signal-processing algorithm to translate it into movement of the bionic limb. The company has been relying on external electrodes sitting on top of the amputee’s skin to record the signals.

“If the electrode is stationary on your skin, and the muscle is moving around inside your body, then your signal source is moving with respect to your recording electrode,” says Irazoqui. “That means your signal is going to be pretty hard to keep track of.”

Coapt learned about Purdue biomedical engineers capable of making internal electrodes, and soon realized the potential for a breakthrough technology. After months of collaboration, Coapt has purchased the exclusive license for Purdue’s internal electrodes, which the company says will revolutionize bionic limbs.

“Some of the muscles we use to control our fingers or our wrists are deep—far inside the body. And the signals that are produced are very small; you can’t detect them from the skin’s surface,” says Coapt Co-founder Dr. Levi Hargrove. “For example, the muscle that controls rotation is very deep in your forearm…but with an implant, we can put [the electrode] right inside that specific muscle, and it’s going to give us a very clean signal that’s going to allow the person to rotate the limb whenever they want to.”

Comparable to the size of Tylenol capsules, multiple electrodes could be injected using local anesthetic—about eight for a typical patient, dependent on how many degrees of movement the amputee wants. For example, bending and extending the elbow are two separate movements that would require two electrodes.

“[Coapt] is the first group to come up with a way of controlling a robotic limb with thoughts that actually works and is clinically relevant to the patient—it’s a major, major achievement,” says Irazoqui. “Purdue’s contribution was to say, ‘If you need that muscle signal, we can make an implantable device to acquire that signal smaller than anybody else can and make your solution more robust and reliable.’”

The company will use the Purdue technology to focus on bionic arms initially, but believes leg prosthetics could be next. While other developers are working on similar internal electrodes, Hargrove says “Purdue’s is the best,” and Coapt believes that will translate into newfound freedom of movement for amputees. 

Hargrove says internal electrodes also eliminate common problems such as having to remove and re-attach existing external electrodes to shower, for example.

Irazoqui says, in addition to amputees in general, knowing the technology would change the lives of wounded soldiers helped motivate him.

Irazoqui says internal electrodes will resolve issues with external ones, such as fat or certain physical attributes interfering with recording the muscle signal.

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