In a nutshell, bionic limbs are artificial limbs that are designed to be intuitive for the wearer. The fancy name is a combination of the prefix ‘bio’, meaning life, and the shortened word ‘nics’, as in like ‘electronics’. Bionic limbs try to interpret muscle signals to recreate what a real limb would actually do, making the wearer use the bionic limb in the same way as the lost body part. This is unlike body-powered prosthetics, which often use unnatural movements and adjustments to compensate for function and dexterity.
For the longest time, the idea of a perfectly usable artificial limb indistinguishable from a natural one has been exclusively within the realm of science fiction. Today, we are getting closer and closer to the reality of such a concept, with current bionics applications, in general, making a drastic positive impact on the lives of millions of people.
Types of Bionic Limbs via Signal Input Type
Depending on the target input and stimulus, there are several ways of achieving the intuitive use and function of bionic limbs, such as:
1. Myoelectric limbs
‘Myo’ is a prefix used in anatomy to refer to anything muscle-related. As such, ‘myoelectric’ means anything that has to do with the electric properties of muscles. This is perhaps the most common type of bionic limb. They are the direct antithesis of the primitive design philosophy of body-powered prosthetics.
Most myoelectric limbs have a set of electronic sensors that are in direct contact with the attached body part. Since part of these muscles also respond to the movement done originally on the lost limb, the smallest traces of muscle, nerve, and electrical activity can be used to conjure patterns that would predict the intended motion of the user. In other words, related body information is sent to an integrated circuit of microprocessors, which then signal the appropriate motors and actuators on the custom prosthetic.
One inherent disadvantage of myoelectric limbs is that they are custom-made for each limb, and the wearer usually needs to train the system first before achieving an acceptable level of (original) dexterity. There is also the current limitation of reaction speed and very minute movement, particularly for bionic limbs that include the hand. It may be completely functional, but it is noticeably still clunkier and slower than actual palms and fingers.
2. Targeted muscle reinnervation-based bionic limbs
Named directly after a post-amputation medical procedure, this type of bionic limb leverages the organization and fixing of nerve endings for direct integration instead of just processing what the nerves indirectly do to the muscles near the lost body part.
routing severed or injured nerves to new muscle targets. This is because nerves, when severed or injured, can form into disorganized masses called neuromas if the original body part is no longer there for them to regenerate into.
With a targeted muscle reinnervation-based bionic limb, sensors can be attached “more directly” to the muscle tissues of the rerouted nerves, amplifying the natural movement control that can already be done to myoelectric limbs. The result, of course, is much more refined control, as well as a significantly shortened adaptation period using a special (stimulus) pattern recognition system.
The most obvious drawback of this technology is the pre-emptive nature of targeted muscle reinnervation itself. If you want bionic limbs this way, it has to be planned from the very beginning and not later. This means that people who have been living with lost limbs for a longer period can’t undergo such a procedure.
3. Myoelectric limbs using implanted sensors
This is a derivative of the original myoelectric method, but instead of tracking muscle movements and other nerve impulse-related patterns, there are actual sensors implanted directly where the bionic limbs are connected to the body. It is officially known today as implanted myoelectric sensor technology (IMES).
As complex as this sounds, the only condition to use this technology is to strategically place the sensors near the vicinity of the specific muscle. It doesn’t even need to be a deep incision and, once inserted, the sensors stay there semi-permanently.
This type of bionic limb can be viewed as a middle point between standard myoelectric and the more modern targeted muscle reinnervation-based one. You don’t need to do an immediate nerve rerouting procedure at the moment of amputation, but it is not as “crude” or as generic as memorizing muscle patterns either.
One very different version of this forgoes simply putting sensors on the point of contact. Instead, the sensors are wired directly to the brain. In particular, the sensory cortex and motor cortex are connected to an array of electrodes, which is then wired directly to the bionic hand. Because of the complexity and relative infancy of this technique, it is yet to be widely used and is not available commercially.
Hurdles and Challenges of Bionic Limbs
Other than the inherent limitations of current internal circuitry and robotics design, the widespread use of bionic limbs is still hampered by the following:
- Older amputations might not be eligible for bionic limbs – as mentioned earlier, some types of bionic sensory input are incompatible with amputees that have undergone older methods, or were not given more specific function-preserving medical procedures, or even had their limbs simply cut-off from a portion that is less practical for bionic limb use.
- Damage over time is not always perfectly predictable – bionic limbs are still essentially foreign objects introduced to the human body. Thus, they will be constantly subjected to potential issues such as bone fracturing due to (constant) physical impact and infections around the attached part, especially if the prosthetic is not designed securely enough to prevent small abrasions over time.
- Perfect replication of finger impulse and dexterity – while breakthroughs in bionic legs have made them almost indistinguishable from actual legs in terms of practical use and dexterity, we have yet to develop a perfect mechanical analog for the fast, reactive, and accurate movements of each individual finger. In time, perhaps, we’ll get there. But not today (as of the publishing of this article).
- Cost of advanced technologies – 3D printing and other cost-reducing, availability-boosting innovations have made high-quality prosthetics available to the common man. However, most of these do not use the advanced technologies we mentioned earlier. They are generally still only accessible to more affluent members of society.
- Can never have standardized products – prosthetics are never a one-size-fits-all thing. Every single bionic limb is custom-made specifically for the person using them. This further adds to the overall cost of more advanced versions, as well as making it impossible to apply economies of scale to such a user-restricted, tailor-made application.
Some Notable Breakthroughs in Bionic Limbs
Difficulties in implementation aside, scientists have made some very remarkable achievements over the past two decades regarding the level of performance and reliability of current generation bionic limbs:
- Osseointegration, or the incorporation of metal-based components directly into the human bone, goes back to the late 20th century. With improvements in material engineering and bone surgery methods, products such as the OPRA Implant System came to life. OPRA bionic limbs do not use any muscular input tracking of sorts. Instead, the leg is fused with a titanium rod, which becomes the foundation for an externally attached custom prosthesis.
- Hugh Herr, rock climbing enthusiast and engineer, pioneered one of the most naturally-functioning bionic limbs to ever exist today. After experiencing a frostbite accident that took away both his legs, he sought a way to make bionic limbs follow the natural human gait as closely as possible. As shown by this old TED video, the results were quite remarkable, and still continue to inspire a lot of people with similar disabilities today.
- The Johns Hopkins Applied Physics Laboratory (APL) has integrated the fundamental principles of osseointegration and partial IMES to create an adequately dexterous robotic arm that can be controlled directly using the mind. The DARPA-funded prosthetic prototype was first tested on Johnny Matheny in 2015, who has been demonstrating and showcasing both the marvels and troubles of the bionic arm’s technology to this very day.
Is it important for bionic limbs to look human?
One last aspect of designing bionic limbs today is the preference of whether the artificial body part should look like a natural arm, leg, or hand. This is definitely something that is outside its technological scope. But bionic limbs that look something like this may have implications on everyday life, especially if we get to the point where these things start to become functionally indistinguishable from natural limbs.
So to answer the question… no. At least not right now. Perhaps the better question to ask would be: is it important for bionic limbs of the future to stay at a human level? That is another branch of bionic limb design philosophy that we may start to consider as early as the next decade.