Bionic limbs are primarily driven by residual or other movements of the body in response to the motion that the user intends to do. Fundamentally, each muscle group coordinates with another muscle group to achieve a motion. As such, if we can reference the other adjustments your body makes to move your limbs, we can more or less estimate the motion that it would have done if the limbs were still intact.
We have briefly introduced each major discipline or method used to achieve this objective in our first article about bionic limbs. For this article, we’ll go a bit deeper, organize the information a bit differently, and ultimately assess if each method will remain relevant in the next few decades.
Interpreting Small Twitches: Myoelectric Bionic Limbs
Myoelectric is a generic/blanket term for anything that has to do with both muscles and electricity. In the case of bionic limbs, myoelectric refers to the method of trying to infer the correct intended action by analyzing “residual movement” of the remaining part. In simpler terms, sensors attached to the end of the amputated part look for small muscle twitches related to motions of the limb when it was still there.
With proper training, both by the user and the computer built to analyze the patterns, it becomes just a matter of repeating the same set of muscular impulses for the bionic limb to recreate the motion consistently. Of course, there is the issue of dexterity, something that is still not perfectly replicated for bionic arms due to the intricacy of natural hand and finger movements. But for the most part, all standard functions of the original limb are effectively restored.
Examples of Myoelectric Bionic Limbs
- Hero Arm by Open Bionics
- Esper Hand by Esper Bionics
- i-Limb Quantum Bionic Hand by Ossur (originally by Touch Bionics)
Advantages
- Mechanically simple enough, easily adaptable to general mechatronic movements
- Doesn’t require special medical procedures, especially prior to losing the limb
- Can still work well even if already on normal prosthetics for years
- Highest level of development and adoption among commercially available products
Disadvantages
- Mechanically too simple, dexterity improvement potential is limited
- Residual movement may not be detected properly if there aren’t enough nerves left
- Potential skin irritations, inflammations, infections
Refinements/Improvements
- Higher voltage sensitivity can lead to better dexterity levels
- Basic tactile feedback potentially reduces the need to measure motion via sight
- Artificial muscle fibers lead to the eventual restoration of natural finger movement?
Would myoelectric bionic limbs remain in use in the far future?
Even if we presume that other special medical procedures related to bionic limbs were to become more mainstream, myoelectric prostheses are still expected to remain popular. Like ICE automobiles, there is already a huge commercial industry already backing up the technology. It is also the least complex to design and use, as well as having virtually no restrictions on who can use them.
As the old adage says, if it ain’t broke, don’t fix it.
Tingle to the Bone: Osseointegrated Bionic Limbs
Osseointegration, on its own, does not really have anything to do with bionic limbs. But, due to the intermediary nature of the procedure in relation to the technology, they are often used as another alternative method of designing sensor-based prosthetics. The main objective is to create direct contact between the human bone, and an inert, sturdy, and dense metal, which is mostly titanium. This serves as the base onto which the entire bionic arm is installed. Sensors around the remaining body part then send signals wirelessly to the arm’s systems to translate the residual motion accordingly.
The benefit of osseointegrated bionic limbs is that the stresses involved in using standard prosthetics are greatly eliminated. Stability and control are also significantly improved, since the whole bionic limb is attached directly to the basic framework of the body. And most importantly, maintenance is simply a matter of detaching the whole mechatronic apparatus from the base titanium implant.
Examples for Osseointegrated Bionic Limbs
- Osseointegrated Bionic Legs by the University of Colorado Hospital
- Osseointegrated Bionic Arms by Johns Hopkins University Applied Physics Laboratory (APL)
Advantages
- Very tight, secure fit without the need for suction, suspension, or grip supports
- Almost zero probability for skin irritations, inflammations, infections
- Weight-bearing is brought back to the bones near the bionic limb, retraining them once more for everyday use
Disadvantages
- Most definitely NOT optimal for children
- Requires an additional specific medical procedure, albeit not time-sensitive
- Balancing the total weight of the prosthetic can be tricky
Refinements/Improvements
- Needs a way to shorten the training period for heavy activities (usually takes a whole year)
- Additional surface tactile feedback may greatly boost inherent osseoperception properties
- Direct brain interfacing may be an option in the future
Would osseointegrate bionic limbs remain in use in the far future?
Osseointegration on its own is here to stay. After all, despite the lack of significant financial backing as a commercial product from medical companies, it remains a critical procedure with applications outside bionic limb development. That said, osseointegrated bionic limbs will probably not achieve widespread adoption in the future due to specific usage requirements, as well as economic competition. It will be there only for the select few individuals who could benefit from its more optimized design.
Rewired and Repurposed: Targeted Muscle-Reinnervation-Based Prosthetics
Rearranging nerves isn’t just a sub-procedure for osseointegrated bionic limbs. It is an entire medical procedure created for the purpose of keeping the surviving limb’s functions as intact as possible. Technically named targeted muscle reinnervation, it is the method of reassigning severed or damaged nerves to new muscle targets. The objective is two-fold. First, the new nerves are now directly associated with the muscle twitches on the rerouted path. Second, the injured nerves are prevented from turning into disorganized (therefore unusable, or even harmful) masses called neuromas.
Because the nerves themselves are rearranged beforehand, strategically placing sensors around the amputated part becomes easier. This then results in wider ranges of adjustment and higher levels of (theoretical) dexterity. Coordinating the placement of sensors and nerves can even be done so that nearly identical muscle twitches react (more or less) to the same intended movement for the bionic limb as the original.
Examples for Osseointegrated Bionic Limbs
- Prototype reinnervated bionic arm by the Cleveland Clinic
- Osseointegrated (and reinnervated) Bionic Arms by Johns Hopkins University APL
Advantages
- Natural behaviors (pre-accident) can potentially be restored
- Higher level of granular movement and refined levels of adjustment
- Probably the most intuitive bionic limb type out of all past, present, and perhaps even future sensory prosthetics
Disadvantages
- Requires an additional specific medical procedure that is very time sensitive
- Reinnervation procedure must be done immediately before using/designing the bionic leg
- Exorbitantly expensive
Refinements/Improvements
- Can benefit greatly from an advanced (future) free finger movement system
- Basic tactile feedback potentially reduces the need to measure motion via sight
- Higher adoption levels may lower cost in the future
Would reinnervated bionic limbs remain in use in the far future?
As one of the most promising bionic arm types in terms of dexterity focus that we have right now, it is almost certain that it will remain in use in the far future. In fact, such a procedure could be incorporated by default on all medical emergencies requiring amputation. This is so that a speedy recovery is only just a matter of when the physical bionic limb would be attached.
Oh, but do watch the costs. The technology must become cheaper before it can seriously compete with the already-popular myoelectric bionic limbs.
And the Ultimate Bionic Limb Type Goes to…
To quote yet another boring answer to such a divisive topic… it depends. As circumstances and relative severity of the medical emergency varies significantly, so does the number of optimal solutions each individual case could have. In most cases, the patient’s preferences are also a factor. But for some options, such as when considering to undergo targeted muscle reinnervation, you only have a small window of opportunity to decide.
There is also the huge practical divide between bionic arms/hands and bionic legs/feet. While the most advanced bionic legs today are nearly indistinguishable from regular legs, bionic hands are still quite far from being remotely near the astounding dexterity of the original. As such, actual usability still remains quite limited to this day.
Featured image credit by British High Commission via Flickr.
