Our definition of bionic touch goes beyond sensory feedback, i.e. where sensors must make contact with the external world to generate information. It also includes the brain’s awareness of body position and movement, known as “proprioception”, and the improved integration of bionic limbs with our natural bodies using techniques like osseointegration. We have written this article to help readers understand the distinction between these capabilities, their relationship to each other, and their role in the future of bionic limbs.
What is Bionic Touch?
We define bionic touch as a combination of sensory feedback and other forms of awareness, such as proprioception and osseoperception.
Sensory feedback is the easiest to understand because it matches the traditional definition of “sense to touch”. When your body comes into contact with an object, skin sensors convey significant information about that object to your brain, which compares it to a library of known objects to identify what you’re touching, where you’re touching it, etc.
Proprioception is your brain’s ability to construct a mental map of your body and to use feedback from agonist-antagonist pairs of joint muscles to track your movement in space. The best way to experience this is to close your eyes and touch your forefinger to your nose. Until the moment your finger makes contact, all of this action is guided by proprioception.
Now let’s combine these two capabilities. Imagine awakening in a dark room and reaching out to turn on a nightstand light. Your brain uses a stored 3D map of your environment to get its bearings, proprioception to steer your hand to the general area of the light switch, and sensory feedback to identify what you’re touching so that you can make the necessary adjustments.
Finally, osseointegration involves implanting a metal rod into the bone of an amputee’s residual limb to function as the connector to a prosthesis. This makes the prosthesis an extension of the user’s skeleton, which improves both control and range of motion. It also allows the user to both feel and hear the vibrations passed from the prosthesis into the bone, the same way that you can feel and hear important information when a bony part of your natural body makes contact with an object.
Amputees will never be fully satisfied with bionic limbs until they restore all of these capabilities.
One capability essential to all bionic limbs, both upper and lower, is proprioception. Want to be able to bound up stairs or walk on uneven terrain without visually guiding each step? You need to be able to track each foot’s position to do so. Want to manipulate an object that you’re grasping without looking at it? You need to know where you’re fingers are and where you’re moving them to have success.
Proprioception is retained for the joints still present in the residual limb. It’s from the point of amputation onward that amputees lose this ability.
Until recently, it wasn’t possible to restore proprioception for lost joints, but a new surgical technique is changing this. The foundation of this technique is the recreation of agonist-antagonist muscle pairs in the residual limb:
The brain naturally equates one muscle pair to one phantom joint. Try to move the phantom limb and the muscles will respond even without using a bionic prosthesis. It is the sensory feedback generated by the movement of these muscles that allows the brain to track the phantom limb’s position the same way it tracks an intact limb.
Once this natural mechanism is restored, all that’s needed is to detect the myoelectric signals generated by the muscle movement, translate them into bionic limb commands, and, voila, you have extended the brain’s proprioceptive sense to a bionic device.
This is a gross over-simplification but you can read the full details in our article on Agonist-Antagonist Myoneural Interface (AMI).
Currently, AMI is only being used for below-the-knee amputees. However, the expectation is that the same technique should work for most lower- and upper-limb amputees.
Osseointegration and Osseoperception
We have an entire article on Osseointegration, supported by a 4-part, in-depth interview with one of the world’s leading experts on the subject. Rather than repeat that information here, please see Osseointegration for Bionic Limbs.
Sensory Feedback From Bionic Feet
After years of neglect, there has been a recent movement toward providing sensory feedback from bionic feet. The following video describes an example of this:
There may still be good reasons to pursue this type of solution but it will likely depend on the user’s situation, as follows:
- If the user has undergone AMI, he will already receive an indirect form of sensory feedback from reconstructed muscle pairs. Any time that those muscles move, including in response to a shift in leg stance, the brain will be aware of this movement.
- If the user has an osseointegrated implant, he will already benefit from osseoperception.
- If either AMI or osseointegration is present, it may be sufficient to augment the user’s implicit sensory feedback with non-invasive feedback mechanisms like small vibrators or transcutaneous electrical nerve stimulation (TENS) of the skin underneath the socket.
- For bionic feet, it may even be sufficient to use non-invasive feedback without the presence of AMI or osseointegration.
- If none of the preceding solutions is adequate, achieving sensory feedback through invasive surgery, as described in the video, may be the only option. This type of solution is called a “neural interface”.
You may notice that we’re treating neural interfaces as a last resort here. This is because surgically implanting electrodes to directly stimulate nerve bundles has the following drawbacks:
- cost of surgery;
- the pain of recovery;
- potential nerve damage;
- risk of infection;
- risk of scarring, which may eventually inhibit the ability to stimulate the nerve.
Until some of these drawbacks are addressed, it is difficult to envision this as a mainstream solution when other options may suffice.
For more information, see our article on Sensory Feedback for Bionic Feet.
Sensory Feedback From Bionic Hands
Far more attention has been paid to obtaining sensory feedback from bionic hands than from feet. With hands, and in particular fingers, it is not enough to know where the hand is or whether it made contact with an object. Hands perform finer skills and, as such, require more detailed sensory feedback.
Providing this level of feedback is currently only possible by using a neural interface, complete with all the drawbacks described in the preceding section.
Unfortunately, there is an additional limitation when it comes to bionic hands: a lack of precision. It is difficult to precisely stimulate a nerve with an electrode. As a result, touching an object with a bionic forefinger may be accidentally interpreted as touching it with the pinky instead, which could lead to a certain amount of clumsiness. To make matters worse, recent research indicates that the brain is not capable of adjusting its sensory map to overcome such errors.
There is hope for a less invasive procedure. Researchers at the University of Pittsburgh recently discovered that existing spinal cord stimulators can be used to produce a sense of touch in missing limbs. As a simple outpatient procedure, this is far less invasive than surgically implanting electrodes, but it is also highly experimental at this stage.
If you use a bionic limb, you are eventually going to want to restore as much of your natural sense of touch as possible. But with a rapidly changing technology landscape, plotting your path forward can be tricky.
Our advice is to deal with foundational issues first. So, if you’re a good candidate for osseointegration and you’re comfortable with that solution, embrace it. If AMI becomes available for your type of amputation, explore it.
After that, you may want to test non-invasive options for sensory feedback before subjecting yourself to additional surgery. This is especially true given that we are quite early in the evolution of neural interfaces. It would be unfortunate to incur the high cost, risks, and discomfort of surgery only to find out that something simpler, cheaper, and less invasive had come along.
For a comprehensive description of all current upper-limb technologies, devices, and research, see A Complete Guide to Bionic Arms & Hands.
For a comprehensive description of all current lower-limb technologies, devices, and research, see A Complete Guide to Bionic Legs & Feet.