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Dec 2020 - Biomimetic design: making medical prosthetics more human
Author: Elliot Jackson-Smith, Innovation Programme Officer
Resulting from a misadventure as a young ice-climber, Dr Hugh Herr lost both of his legs below the knee to gangrene. Frustrated by the available state-of-the-art and driven to return to his sport, he began fabricating his own prostheses. Some years later, he now leads the Biomechatronics Research Group at MIT and is responsible for multiple ground-breaking innovations – including the world’s first bionic lower limb (the BiOM Ankle).
This system is a stepwise improvement on current passive lower limb prostheses. The incumbent technology has a number of key failings, not limited to the fact that designers must find a sweet spot of stiffness: where too much will fail to attenuate shock and too little will dissipate a considerable portion of the user’s energy. The former results in a high prevalence of lower back pain and joint pathology, whilst the latter leads to fatigue and subsequently limits activity.
Dr Herr’s system employs a biomimetic design. He and his team closely studied human movement and sought to mirror it mechanically. In doing so, they paired a linear actuator with a composite spring, which under computerised control mimics the calf muscle and achilles tendon. Using this technology, the system constantly adjusts to the terrain and the user’s gait, providing the appropriate stiffness and power assist to facilitate more natural and efficient movement. Subsequently, the user does not experience the energy losses associated with traditional passive prostheses; nor do their joints (particularly knees and spine) experience the adverse loading mechanics that are so conducive to osteoarthritis. It has been reported that amputees can regain mobility far quicker with this technology, as the device permits the use of their own pre-amputation motor patterns.
This is just one example of how intelligent design by biomimicry can facilitate stepwise progress in medical technology and provide a marked increase in quality of life to thousands of people (cost permitting, at $50,000 per device). Other foci of promising rapid development may be found in the field of Targeted Muscle Reinnervation (TMR). Here, surgeons take residual nerves from amputated limbs and connect them to functional muscles near the operative site. This has two key advantages: 1) providing neural control of prosthetic limbs, and 2) providing sensory feedback.
Neuronal control of prosthetic limbs is every bit as incredible as it sounds. By connecting the motor nerves that supply key agonists and antagonists at a given joint (i.e., the main muscles that facilitate a given joint movement e.g., wrist flexion) to a localised area of muscle, their signal is essentially amplified. Electromyography is used to isolate this neural signal, which can then be interpreted by computer programs and used to affect movement in an artificial limb. That is, a person can think about moving, and a machine will move for them.
Similarly, sensory nerves that would previously have supplied the skin overlying an amputated area may be reconnected to regions near the operative site. This alone has been proven to reduce phantom limb pain: a condition that can be particularly debilitating and pervasive to treat. It has been demonstrated that these reinnervated sites may be stimulated by sensory input from pressure transducers. That is, a person can use their prosthetic limb to reach out and touch an object, and the machine will feel for them.
These technologies are currently the preserve of research institutes, the rich, and the well-insured. There is a significant opportunity for ambitious engineers and scientists to develop them into widely attainable devices, which can provide incredible gain in function to hundreds of thousands of people worldwide. After all, healthcare technologies should be accessible and affordable to those who would receive significant benefit from them.
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