A Glance at the Present State of Limb Prosthetics

The body is a life support system, carriage, and collection of useful accessories for the brain. Insofar as you as an individual are concerned, you are your brain. The rest of the body is only indispensable because we don't have other ways to provide the same capabilities. That will change, though likely not in any easily predicted way: the decades ahead are a time of great uncertainty in technological details because the pace of change is so rapid. Small differences in research today are magnified over the years leading up to future applications of that science. We might set out to draw a smooth line of progression between today's prosthetics - artificial limbs with simple nervous system interfaces, bioartificial dialysis devices, vision substitutes for the blind via implanted electrode grids, and so forth - and the integrated artificial brain carriage of tomorrow, a collection of technologies and capabilities that will bear roughly the same relationship to the natural human body and biological systems as a car bears to a horse. Some people, such as those involved in the 2045 Initiative see this as a goal to be pursed more aggressively and directly than is presently the case. But I suspect that the line will be anything but smooth and direct.

Prosthetics are just one approach to tackling the results of disability, disease, and the damage of aging, after all. If the goal is function where function is lost, then prosthetics are in competition with regenerative medicine. The two sides will tend to ebb and flow in funding for any of the thousands of potential applications as they do better or worse than one another in providing the ability to regain what was lost. My suspicion is that prosthetics will fade as as an active line of development in the decades ahead due to progress in tissue engineering and regenerative medicine. Artificial limbs, perhaps the least complex of all possible prosthetics, will soon be potentially better than the real thing in a range of capabilities. Yet I imagine that the average fellow short a limb would nonetheless jump at the chance of regrowing a biological replacement in the fashion of a salamander if that was a possibility, as it may well be twenty years from now.

Meanwhile, however, it is interesting to watch progress in this field. Some of the work on prosthetics is potentially applicable to augmentation devices such as wearable exoskeletons that might provide the frail elderly with far greater freedom of action. But again, this is no substitute for the creation of repair biotechnologies that might restore lost capabilities and health. Much of today's prosthetics development is a matter of substitution and compensation; increasingly useful, and the only game in town until there is more progress in medicine, but a phase of technology that will pass, or transition to augmentation of those without disability. Perhaps it will return in earnest at the end of the biological period of medicine when "prosthetics" will mean producing discrete systems of diamondoid nanomachinery to replace slices of our biology: artificial immune cells; artificial oxygen stores in blood; artificial ATP-producing nanofactories to augment mitochondria. All of these are machines that we can consider and design in theory today, and that might be hundreds of times more efficient than our evolved biology. Building an industry to create and maintain such things lies a few steps beyond present endeavors in medicine and materials science, however. It or something similar might be the new new thing for the 2040s and later.

Returning to the present day, here are a few articles noting the present state of work on limb prosthetics. Perhaps the most interesting aspect of this is work on integration with the nervous system. That is a technology that has wide-ranging applications beyond prosthetics, and we'll be seeing a lot more of in the years ahead.

Is This the Future of Robotic Legs?

"When you view the human being in terms of its locomotory function, some aspects are quite impressive," Herr said. "Our limbs are very versatile: We can go over very rough terrain, we can dance, we can stand still. But...our muscles, when they do positive work, 75 percent is thrown out as heat and only a quarter is mechanical work. So we're pretty inefficient, we're pretty slow and we're not terribly strong. These are weaknesses we can fix."

The next frontier for bionics, Herr believes, is neurally controlled devices. For now, the BiOM [prosthetic foot] works independently from the brain, with an algorithm and a processor governing the prosthetic's movement. But Herr is working on sensors that can tap into the body's nervous system - eventually we could see a prosthetic controlled by the brain, muscles and nerves.

Mind-controlled prosthetic arms that work in daily life are now a reality

The novel osseointegrated (bone-anchored) implant system gives patients new opportunities in their daily life and professional activities. "We have used osseointegration to create a long-term stable fusion between man and machine, where we have integrated them at different levels. The artificial arm is directly attached to the skeleton, thus providing mechanical stability. Then the human's biological control system, that is nerves and muscles, is also interfaced to the machine's control system via neuromuscular electrodes. This creates an intimate union between the body and the machine; between biology and mechatronics."

The patient is also one of the first in the world to take part in an effort to achieve long-term sensation via the prosthesis. Because the implant is a bidirectional interface, it can also be used to send signals in the opposite direction - from the prosthetic arm to the brain. This is the researchers' next step, to clinically implement their findings on sensory feedback.

"Reliable communication between the prosthesis and the body has been the missing link for the clinical implementation of neural control and sensory feedback, and this is now in place. So far we have shown that the patient has a long-term stable ability to perceive touch in different locations in the missing hand. Intuitive sensory feedback and control are crucial for interacting with the environment, for example to reliably hold an object despite disturbances or uncertainty. Today, no patient walks around with a prosthesis that provides such information, but we are working towards changing that in the very short term."

Amputees discern familiar sensations across prosthetic hand

The system, which is limited to the lab at this point, uses electrical stimulation to give the sense of feeling. But there are key differences from other reported efforts. First, the nerves that used to relay the sense of touch to the brain are stimulated by contact points on cuffs that encircle major nerve bundles in the arm, not by electrodes inserted through the protective nerve membranes. Second, to provide more natural sensations, the research team has developed algorithms that convert the input from sensors taped to a patient's hand into varying patterns and intensities of electrical signals. The sensors themselves aren't sophisticated enough to discern textures, they detect only pressure.

The different signal patterns, passed through the cuffs, are read as different stimuli by the brain. The [researchers believe] that everyone creates a map of sensations from their life history that enables them to correlate an input to a given sensation. "I don't presume the stimuli we're giving is hitting the spots on the map exactly, but they're familiar enough that the brain identifies what it is."


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