The New Age of Bionic Limbs

Many images come to mind when we hear the word “prosthetic”. Some may picture a pirate’s peg leg or even a wounded warrior’s synthetic arm. Generally, the thought is a very rudimentary idea of what we as a species can come up with to replace a missing limb. While it is no simple task to replace something biological with something synthetic and expect to make it work, most have agreed that the advancements in this technology have been rather slow. The process of attaching something to the human body is surprisingly complex, even shoes still give us blisters from time to time. Yet, when we hear the words “bionic arm”, our minds instantly go to science fiction. Perhaps a cyborg laser arm or Anakin Skywalker’s synthetic arm that can feel when it is prodded. What most people don’t know is that the technology for that fancy sci-fi arm is here. Well, without being able to shoot powerful lasers, for now. The idea of replacing lost limbs with artificial ones has been around for hundreds of years. The first recorded prosthetic belonged to a mummy in Egypt who replaced their big toe with one made of wood and leather between 950-710 B.C.E.  While a big toe prosthetic is unexpected to be the first, it does serve a function as it was necessary to wear traditional Egyptian sandals. As we fast forward in time, more significant examples of prosthetics begin to emerge, such is the case of “Götz of the Iron Hand”. In 1504, his right hand had been severed in battle from an opposing cannonball. Götz Berlichingen soon replaced that very same hand with one made of iron and two hinges to allow four finger-like hooks to come inwards to the palm so that he may wield a sword (Morton). Undeterred by his disability, Götz Berlichingen fought for years with that iron hand until he replaced it with a newer and more advanced prosthetic (Morton). This one was equipped with joints at each knuckle for a tighter grip. Spring-loaded mechanisms lock the fingers in a ratchet-and-pawl system to keep the grip tight and sturdy (Morton). That very same hand is kept at a castle museum in Berlichingen’s native Jagsthausen. In recent studies, The World Health Organization estimates that 3.5 million people need prosthetics and orthotic devices, yet 75% of developing countries lack access to these devices. If knights and doctors in the Middle Ages were able to invent these intricate designs with less sophisticated tools, why did it take so long to advance this technology to a point where it would be more than just a simple leg or fake arm and why aren’t more people wearing them?

It simply boils down to money. The Veterans Affairs (VA) surveyed 581 veterans in 2010 to calculate just how much having a prosthetic would cost the veteran over 5 years. The unilateral upper limb average costs are $31,129 and $117,440, unilateral lower limb costs are $82,251 and $228,665, and multiple limb costs are $130,890 and $453,696 (Blough, David K et al.). The average lifetime costs with unilateral upper-limb loss were the lowest at $823,299. One of the main reasons these figures are so high is due to having to need the prosthetics repaired, refitted, and replaced. Pair this with a notoriously predatory healthcare system and those who are disabled are drained of their money. While the VA does supply and repair prosthetics for free for those wounded veterans, the rest of Americans are either stuck with copays from insurance or forced to pay out of pocket for inflated prices. Children in need of prosthetics must replace theirs more often which also brings the price higher and accessibility out of reach to most.

While the cost of entry barrier may seem high for those in need of prosthetics, companies such as Unlimited Tomorrow aim to break that very barrier to supply prosthetics that are not only more advanced than one would pay out of pocket for but much cheaper as well. Coming from humble beginnings, Easton LaChappelle had been developing robotic limbs since the age of 14 and since meeting a young girl at a science fair who had an expensive yet limiting prosthetic limb, he set out on a journey to create a cutting-edge artistic approach to building these innovative limbs. For the last 10 years, they have reached their goal by providing prosthetic arms that look just like their opposing limb while keeping their measuring and fitment process in-house to lower costs. Each specifically designed socket is equipped with 36 precise sensors to allow for a more responsive bionic limb as well as the addition of vibration cues when the prosthetic meets an object. These arms can function just like a normal arm with the benefits of being lightweight and much more advanced than most of the competition. Unlimited Tomorrow also recycles its technology into a replacement or refitted prosthetic to keep costs down just as well. The total price? $7,995 out of pocket and they work with insurance deductibles and co-pays.

It isn’t just startup companies who are trying to tackle this issue, researchers at the Massachusetts Institute of Technology (MIT), John Hopkins Medicine, and other independent companies are working to revolutionize the way people interact with their prosthetics. To make a bionic arm function, the prosthetic must be attached to the user in a way that allows it to read the electric signals from muscles and/or nerves to establish bidirectional control. This is generally categorized into three different types of limbs: Myoelectric, osseointegration, and mind-controlled bionic limbs.

Myoelectric Limbs

Myoelectric limbs are the next generation of prostheses after the cable-driven body-powered prosthetics. Instead of relying on the wearer’s body to manipulate the limb, the myoelectric limb is suction fitted and uses a battery to control various motors within the limb. These motors receive signals from microprocessors that use the electronic sensors attached to the host to read their muscle, nerve, and electrical activity (Al Muderis and Ridgewell). The pattern recognition software then translates those signals into movements with varying intensities and strengths depending on the users’ movements of their physical muscles. This allows them to grip an object as much or as little as they want as well as the speed at which they move their appendage. The movement is also refined enough to allow the users to tackle unexpectedly complex tasks, such as taking out credit cards from their wallets (Al Muderis and Ridgewell).

Myoelectric limbs come in different types as well depending on the user’s needs. A single motor hand is only capable of opening and closing while multi-articulating myoelectric hands allow for full control. Myoelectric hooks are also available for users who just need a limb to pinch, pick up, or for durability.

While these limbs are much more advanced, they do have their shortcomings that sometimes lead to frustration and even overall rejection of the limb. Movement can seem to have a lag from the time the user wants to use the limb to when it moves. The batteries and motors also are heavy and can make the arm feel fatiguing to the user.

Osseointegration

Osseointegration is the attachment of a metal implant to the bone for a permanent anchoring point. These metal implants are custom-made, porous, and coated with titanium to allow the bone to grow within the implant. This method allows for greater mobility, reduced nerve pain, and better proprioception, which is the sense that lets us perceive the motion, location, and action of our body parts. Since the prosthesis is directly attached to the bone, the weight-bearing is brought back to the femur, hip joint, tibia, or other bones for a reduction of muscle atrophy (Al Muderis and Ridgewell). Wearers of these prostheses would be able to feel the tactile feedback in their bones and some are even able to distinguish the tactile feeling of walking through various surface mediums, such as soil or concrete, through Osseo perception (“Osseointegrated Limb Replacement”). Osseointegration also helps to correct the angle at which the prosthetic is attached due to ill-fitted prosthetics. This method will last for many years; however, long-term results aren’t widely available as this procedure has started in 1990 in Sweden. Recovery time for this surgery typically lasts between three to four days while the growth of bone within the implant will take about three months. After this time patients can walk without crutches (“Osseointegrated Limb Replacement”).  

This method eliminates the inconveniences of having a socket-based prosthesis such as:

• Nerve pain
• Attachment difficulties
• Sweating
• Discomfort and irritation
• Poor-fitting

Mind Controlled Limbs

This is where bionic limbs blur the line between science fiction and reality. These prosthetics are highly advanced and are connected to the user in a much more intimate way by integrating with body tissues and reading signals and commands straight from the central nervous system to perform movements and actions much more realistically with less ‘lag time’. Due to highly successful experimental surgeries called targeted muscle reinnervation (TMR), these limbs also enable its user to feel the sense of touch through their prosthetics. Invented by Dr. Todd A. Kuiken and Dr. Gregory A. Dumanian, this surgery was developed to improve myoelectric upper limb prostheses by surgically transferring severed motor nerves to the motor points of denervated target muscles to allow them to contract in response to neural control signals for the missing limb. This allows for additional control sites to eliminate the need of switching the prosthesis between its different control modes for seamless articulation and movement. TMR has also been proven to alleviate painful amputation neuromas, which are tumor-like thickening of a nerve stump in the region of the scar after amputation of a limb (Kuiken, Barlow, et al. 2018).

While TMR has targeted upper arms for its treatment, legs have had their fair share of development well. On one fateful evening, Hugh Herr and his friend had fallen during a climbing incident in the snow. On their painful journey back to civilization, both he and his companion suffered frostbite that resulted in the loss of Herr’s legs. While normally such an injury would end someone’s climbing career, Herr had instead crafted specialized prostheses to allow him to climb even better than before while also opening his pathway to innovating his advanced prosthetics called EmPower. Herr describes these bionics as a “glorious interplay between biology and engineering design” to interconnect people with a machine. By linking the bionic limb to its user’s nervous system, which he calls “neuro-embodied design”, Herr was able to maximize bidirectional communication by allowing the limb and the nervous system to send and receive signals to one another, ultimately allowing for a limb that not only functions just by thinking of moving it but also feels just as their normal limb would (“Project Overview ‹ Agonist-antagonist Myoneural Interface (AMI) –”).

Herr and his team have also developed their own surgical practice called agonist-antagonist myoneural interface (AMI) after revisiting surgical practices, which he calls “Civil War-era amputations”, to give more thought to how the limb was to be removed and how it would affect the prostheses the patient would soon be wearing (Zia). AMI is a method to restore proprioception and to achieve this, an AMI is made up of two muscles, the agonist and the antagonist, connected mechanically to allow them to counteract each other. When the agonist contracts, the antagonist is stretched, and vice versa to control and interpret proprioception to its user from a prosthetic joint. During the surgery, multiple muscle pairs are liked together to form AMIs within the amputated residuum (“Mind-controlled Arm Prostheses That ‘feel’ Are Now a Part of Everyday Life”). Electrodes are then placed onto each AMI muscle group to communicate with the prosthetics computer system to control its movements. This allows the user to feel the position and movements of their prosthetic as if it were their real limb. Because of this, those who use this technology see learning curves that last around 10 minutes before it feels natural (Zia).

Having these incredible advancements in prosthetic technology often leads one to wonder where we could take it next. While emerging technologies in 3D printing make rapid prototyping of these limbs fast, efficient, and cost-effective, there is much more we can do to what some may see as a blank canvas for a body part. While the goal of these limbs is to look and function as real as possible, what if we can take it further? To allow humans to run faster, jump higher, achieve physical results once thought impossible, or even arm themselves with a custom set of tools for their everyday life? To have a mechanic whose arm allows them to ratchet a bolt with a simple flex of their phantom finger, or perhaps a prosthetic eye that allows their user to zoom in and see much further than they normally would or even in infrared. These ideas are not outside of the realm of possibilities as Exo suits have also been in production to increase the potential we as humans have. This technology would boost the upper body strength of workers in construction, maintenance, and even agriculture or help rehabilitate those who are struggling to walk. Lockheed Martin has their own example of this called the Onyx Exoskeleton which uses AI to gather movement data from the users’ feet, hip, and knee sensors to allow their exoskeleton to help a soldier traverse landscapes with heavy packs of ammo and equipment on their back. Lockheed Martin has also developed the Fortis, a waist-mounted third arm capable of carrying and stabilizing tools and equipment up to 36 pounds, making it feel weightless for the operator. Exosuits are also seen in the medical sector as rehabilitation devices to help patients who suffered from spinal cord injuries as well as cerebrovascular diseases. An example is the Hybrid Assistive Limb or HAL from Cyberdyne. This product uses sensors to capture signals going to its users’ leg muscles to feed information to HAL, which assists them with walking while the wearer only must think about moving their legs.  

The future of mobility for not only disabled personnel but workers and even enthusiasts alike is bright. With access to newer and much more advanced technology for manufacturing and biology, we have grown into a new age where bionics will soon be seen as something that is normal and even sought after. Many brilliant minds are coming together to bring this technology out for those who need it the most and in a staggeringly short amount of time. This act of human enlightenment will surely replace the notion of being ‘disabled’ with being ‘superable’ and take us to levels once only thought of existing in Marvel movies.

References

Al Muderis, Munjed, and Emily Ridgewell. “Bionic Limbs.” Australian Academy of Science, edited by APC Prosthetics, 26 Sept. 2017, www.science.org.au/curious/people-medicine/bionic-limbs.

Blough, David K et al. “Prosthetic cost projections for servicemembers with major limb loss from Vietnam and OIF/OEF.” Journal of rehabilitation research and development vol. 47,4 (2010): 387-402. doi:10.1682/jrrd.2009.04.0037

“Mind-controlled Arm Prostheses That ‘feel’ Are Now a Part of Everyday Life.” ScienceDaily, www.sciencedaily.com/releases/2020/04/200430110321.htm.

Morton, Ella. “Object of Intrigue: The Prosthetic Iron Hand of a 16th-Century Knight.” Atlas Obscura, 22 Feb. 2016, www.atlasobscura.com/articles/object-of-intrigue-the-prosthetic-iron-hand-of-a-16thcentury-knight.

“Osseointegrated Limb Replacement.” Hospital for Special Surgery, 4 Nov. 2022, www.hss.edu/condition-list_osseointegration.asp.

“Project Overview ‹ Agonist-antagonist Myoneural Interface (AMI) –.” MIT Media Lab, www.media.mit.edu/projects/agonist-antagonist-myoneural-interface-ami/overview.

Zia, Shafaq. “Mind-controlled Prosthetic Arm Enables Patients to Feel the Objects They Grip.” STAT, 13 Sept. 2022, www.statnews.com/2020/04/30/mind-controlled-prosthetic-arm-patients-feel-objects.

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