The Bionic Limb Revolution? Not So Fast

By Sarah Zhang | March 26, 2012 9:52 am

A monkey controls his robotic arm with a brain-machine interface. 

If this monkey can eat marshmallows with his robotic arm, mind-controlled prosthetics for humans can’t be far off, right? Well, that’s true if all you ever wanted to do with your prosthetic was sit strapped in a chair reaching for marshmallows. But as Michael Chorost explains in a recent feature for Wired, challenges abound when building an arm that works in everyday life.

Over the course of a day, you might use your arm to pick up a chair, unzip your jacket, or scratch your neck—each one of these actions are unique. But statistical algorithms used now can translate the firing of neurons into only a few stereotyped motions. And it’s not just about writing better algorithms; it’s an input problem too. Getting electrodes to pick up signals from the same neurons over time is a continuous battle against the body’s natural defenses:

Electrodes are made of metal. The body is loaded with water, salt, and a dizzying array of other chemicals. Putting them together is like trying to bond a fork and a steak. And the steak fights back by trying to dissolve the fork.

The steak treats the fork as a threat—which, of course, it is. Confronted with foreign bodies, the brain mounts an inflammatory response called gliosis, wrapping cells like astrocytes and microglia around the electrodes to wall them off. Over time, the electrodes become encapsulated in a sheath of scar tissue that acts as an insulator.

Prosthetics that avoid electrodes and attach sensors to the skin are also bedeviled by a lack of specificity. Muscles grow and shrink, the body changes, and sensors end up in a slightly different place each time the arm is strapped on, so movements have to be repeatedly recalibrated. Chorost is ultimately optimistic about the future as scientists develop techniques to record from hundreds of thousands of neurons at once and prevent scar tissue build-up around electrodes. It’s just that no one getting a prosthetic today should expect it to behave like a real limb.

  • kirk

    Graphene semiconductor sensors with carbon nanotube signal routes. I’ve been talking about this with my son the biomedical engineer. Carbon based materials for carbon based meat puppets.

  • http://kforcounter.blogspot.com Cody

    What about non-invasive headsets? Is motor control not accessible by EEG or something?
    http://www.ted.com/talks/tan_le_a_headset_that_reads_your_brainwaves.html

  • Geoffrey.Frasz

    I hope the monkey in the video did not have its arm surgically removed for the experiment.

  • Jay Fox

    @3: Probably not. As I recall, the arm(s) was/were strapped down and immobilized. They just intercepted the signals being sent to the real arm.

  • floodmouse

    @ Kirk: Has the graphene stuff been tested? Does it interact with tissue or is it inert?

  • Aidan

    I wouldn’t suggest using carbon nanotubes in that way. All nanotubes are inherently small and pointy [and] – like asbestos particulates- have the potential to cause cancer, but I wouldn’t be too worried about the future efficiency of all the fancy technologies coming out. How long did it take for cars to be more than a mere novelty for the rich?

  • sardonic

    What I don’t get is why the scientists dont have some bio-organic interface that they graft onto the region of skin before in turn attaching the arm?

    It’s relatively easy to take an artificial skin mesh (already used and in-production for burn-victims), take a blood sample, break it down and filter it for genetic markers, soak the mesh in the markers and then graft it on.

    Alternatively scientists could take scar tissue samples from the person in question, put the receiver in a growth medium and culture the tissue to grow over the nodes to a thickness just sufficient to transmit/receive a signal without causing a immune system response.

    I understand the way the receivers typically work by directly touching tissue other than skin, but what most people don’t realize is when a muscle or nerve signal is being sent for volunteer movement the electrical resistance of an adjacent patch of skin oscillates, and this oscillation is unique per movement, easily detectable, and does not change much with age.

  • Tony

    Actually, I went to a seminar not too long ago where a researcher (case western I think) is experimenting with a kind of semiconductor that turns fluid-like at normal body temp. That would help against a lot of the scar tissue development caused by injury in the metal electrodes.

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