Sep 02 2021

Bionic Arms

The term “bionics” was coined by Jack E. Steele in August 1958. It is a portmanteau of biologic and electronic. Martin Caidin used the word in his 1972 novel, Cyborg (which is another portmanteau of cybernetic organism). But the term really became popularized in the 1970s TV show, The Six Million Dollar Man. Of course, at the time bionic limbs seemed futuristic, perhaps something we would see in a few decades. Thirty years always feels like far enough in the future that any imagined technology should be ready by then. But here we are, almost 50 years later, and we are nowhere near the technology Steve Austin was sporting. Bionics, as depicted, was more like 100 or more years premature. This is tech more appropriate to Luke Skywalker’s hand in Star Wars, rather than some secret government project in the 1970s.

We are, however, making progress, which I have been writing about periodically here. Now a team at Cleveland Clinic has produced a robot arm tested in two subjects, and they are breaking out the term “bionic” to describe their technology. They achieve their level of functionality by combining three aspects of a brain-machine interface connecting to a robotic limb – intuitive motor control, touch sensation, and kinesthetic sensation (simulating proprioception with vibration). The kinesthetic sensation allows the user to feel the robotic limb’s movements. The authors write:

Here, we show that the neurorobotic fusion of touch, grip kinesthesia, and intuitive motor control promotes levels of behavioral performance that are stratified toward able-bodied function and away from standard-of-care prosthetic users.

Motor control in this case is achieved through the peripheral nerves in the amputated limb. There are several ways to achieve a brain-machine interface for a robotic limb. One is to connect the brain directly to the limb, through some kind of electrodes reading brain activity. The advantage of this approach is that it bypasses the spinal cord, and if that is the location of the damage this is really the only option. But if the nervous system is intact, and the limb itself is missing through amputation, then the pathway from brain to nerve ending can be intact. These surviving nerve endings were surgically grafted into existing patches of muscle. These muscles are not able to perform any action by themselves, but activity in those muscle can be read and translated into robotic movements. The advantage of this over direct nerve connection is that the muscle activation amplifies the signal significantly.

The researchers also grafted sensory nerve onto patches of skin, which can then be activated by sensors in the robotic limb, including surface touching receptors, and kinesthetic receptors that activate when the robotic limb moves (such as closing or opening the hand). All of these elements have been described before. What is new here is brining all three together into one advanced “bionic” limb.

The research so far is only on two subjects, and so much more evaluation will be necessary. The technology can also benefit from continued incremental improvements in the engineering. But the existing limbs do work. What the research found is that the users were able to control them well, and their control was similar to normal upper extremities. Without the sensory feedback users of robotic limbs have to constantly look at the limb, to see what it is doing. It is also difficult for them to make corrections, if they are applying too little or too much force, for example.

However, with the bionic limb, control was much more natural. The sensory feedback allowed the users to control the limb without looking at it. It also allowed them to make more fine corrections, because they could sense the action of the limb. This is still not even close to The Six Million Dollar Man level. No one would confuse these bionic limbs for real arms. But they do have the potential for restoring significant function to those with amputated limbs. This is a great milestone advance.

What would it take to get to full bionic limbs? The brain-machine interface needs to get much more sophisticated, allowing for finer level of control of individual fingers. At the biological end, this will require improved surgical techniques, and perhaps even some biological intervention to improve nerve and muscle function. It remains to be seen if direct brain connections will eventually result in better control, but that technology has a long way to go also. Probably something like the microwire electrodes will be necessary. On the robotic end we will need to further develop soft robotics Рactuators that function more like natural muscle. These will need to be small enough to allow for the complex arrangement of muscles that create the fine motor control of hands.

In between the biologic and robotic components, overall functioning may be optimized with AI computer control, helping to translate the desired movements into robotic action. The whole thing needs to be powered as well. This will benefit from improved battery technology, and also from harvesting different sources of ambient energy.

I do wonder how long it will actually take before we have a fully bionic limb that is indistinguishable from a natural limb (for the user and for outside observers). There are too many variables for a confident prediction. I would say, however, at least another 100 years seems likely. Of course this will depend on how we define success. It’s a fuzzy boundary. But we may get at least close by this time.

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