Just ten years ago, bionic limbs like those seen in science fiction were little more than flashy technological novelties. Most prosthetics at the time were mechanical “body-powered” devices – bulky and heavy, requiring users to strain their muscles just to perform basic arm movements. These often relied on large torso harnesses and offered very limited mobility. Even the high-tech options came with major drawbacks: users had to carry heavy backpack-mounted computers to control the limbs, sacrificing practicality and comfort for the sake of showcasing innovation. But now, especially through a surge in demand for prosthetics due to the ongoing war in Ukraine, a new technology has come to light which may just revolutionise the entire industry, addressing all of the inherent flaws that past products fell short of.

Let’s look at the first problem of prosthetic limbs. Connection. How exactly does an artificial structure connect to the body to perform a function of a limb? As mentioned above, body powered limbs require large harnesses which wrap around the torso to distribute the weight. Other prosthetic limbs, especially legs which are specialised for sport (think Paralympics) use ‘sockets’ which consist of nylon socks and sleeves with connection parts. These distribute the weight of the prosthetic to the skin and rely on the skin entirely. This results in skin irritation and friction burns1 as a result of the skin having to deal with the excess pressure usually supported by the bone. This is worked around using osseointegration – direct connection of the prosthetic to the bone using titanium, a method commonly used in dental implants, which was brought into the industry by Per-Ingvar Brånemark2 in the 1960s. 

Titanium is a good example of a biocompatible material – a material which isn’t toxic to the body – due to it forming a stable oxide layer which enacts as an organic substrate, allowing tissue growth3. As a result, molecules, cells, and eventually bone will attach around a titanium implant if in contact with the bone. When applied in prosthetics, this means that if a titanium implant is inserted into the bone with a connection to the limb, bone grows around it over a few months, creating an incredibly strong attachment to the bionic limb that replicates and even enhances the strength of bone itself in supporting the body. This is even used to replicate the movement of the radius and ulna by connecting the two bones outside the arm using an artificial joint that mimics the natural ligament connections of the bone.

But with structural integrity itself being covered, what about the issue of function? How exactly can the arm be controlled? Well, when an amputation takes place, the peripheral neurones and receptors are severed, leaving a cut connection in the muscle. This confuses the brain, which sends signals to the area thinking that the arm is still there but just frozen or dislocated. The brain can send signals to the area for decades after the amputation, resulting in long term phantom pain (when pain is felt despite the arm not being there). The signals can be used to power the arm by being detected through electrodes which are surgically implanted to prevent noise interference of sound frequencies into the muscle. When nerves which originally control the hand are severed above the muscle, they can be redirected to different muscles present in the shoulder (such as the deltoids and trapezius so that control is still achieved.

However, the body’s electrical signals are much weaker than the signals used in modern technology (evident as high operating voltages can kill very easily by causing lethal current to flow) so need to be amplified. This is done by using Regenerative Peripheral Nerve Interfaces (RPNIs), which consist of grafts of healthy muscle taken from elsewhere in the body and placed around the severed nerve endings to act as amplifiers for the signal. Wires are then used to connect the RPNIs to the inserted electrodes, which as a result feed signals to the prosthetic4. This connection requires no battery supply, almost serving as a USB port between the arm and the prosthetics. Additionally, the nerves can support additional functions in order to ensure that no function of existing muscles is lost by splitting the individual fascicles of each nerve and assigning different RPNIs to each fascicle. This is especially important as there are numerous muscles in the forearm controlling complex finger movement, and in order to replicate the movement of the hand each muscle and nerve must be replaced.

Each prosthesis has a CPU that can be trained to assign different signals, much like a modern LLM (large language model). The patient has their prosthesis wired to a computer and is told to try to perform a specific function with the arm. While they obviously cannot, this request stimulates a certain set of signals to be sent from the brain down to the arm. The artificial intelligence in the arm is programmed to assign this set of signals to a predetermined task (the one requested in the first place) allowing the patient to use their arm properly.

While cyborg technology such as this appears to be straight out of Cyberpunk, it is very close to becoming a reality. The only problem present so far is the risk of infection due to the metal implant piercing the skin (the body’s natural disease barrier). But so far this is solved with regular doctor checkups, and is overshadowed by the towering prospects of regaining full limb function with prosthetics, bridging the gap for people with disabilities and helping them (literally) regain their footing in the world.

References

1. Issues faced by people with amputation(s) during lower limb prosthetic rehabilitation: A thematic analysis - PMC

2. Intraosseous anchorage of dental prostheses: an early 20th century contribution - PubMed

3. Tillotson_Marcus_Thesis.pdf – 1.2, properties of titanium

4. [Long-term upper-extremity prosthetic control using regenerative peripheral nerve interfaces and implanted EMG electrodes - PMC]

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