Bionic touch — the ability to recreate a sense of touch in bionic prostheses — is quickly becoming a reality. Mimicking the natural sensory signals seems to be the optimal path to success. Now the race is on to finalize methods of implementation. Should the signals be bi-directional, i.e. integrating touch with movement? Should they be conveyed through implanted electrodes or stimulated through the skin? Just a few more years of research should answer these key questions.
What is Bionic Touch?
By the simplest definition, bionic touch refers to establishing a connection between the brain and electronic sensors on a prosthesis to create a sense of touch.
Duplicating our full natural sense of touch is a daunting task. There are more than 3,000 touch receptors on a human fingertip. We have the natural ability to feel not only the size and shape of an object, but also its texture, pliability, temperature, and more.
Even blindfolded, most of us can identify common objects through touch alone.
Yet, it goes deeper than this. Your mind retains a sophisticated spatial map of your body. It can feel the current location of every body part relative to you as a whole, and also to your surroundings.
Ever reach out for a bedside glass of water in the dark? Ever touch your finger to your nose with your eyes closed? This innate sense of self is so strong that the brain can even feel a lost limb after amputation — the “phantom limb” effect.
The brain’s sensory cortex and motor cortex (responsible for movement) share an intimate relationship. Say you reach behind you, without looking, to retrieve an apple from a bowl, but you touch the side of the bowl first. Your mind and body will rapidly exchange sensory and movement information to make intuitive adjustments until you select an individual apple.
Amputees will never forget what it was like to have these capabilities, and they will never be satisfied until they get all of them back. Fulfilling this desire is the ultimate goal of bionic touch. But we need to take some baby steps first.
The Use of Pressure Sensors in Commercial Bionic Hands
In September 2019, Psyonic entered the bionic hand market with its Ability Hand.
They took the same sensors found in modern cell phones and repurposed them as highly sensitive touch sensors, which they placed in the Ability Hand’s fingertips.
When the fingers touch an object, they send a signal that stimulates the residual limb, informing the user of both contact and pressure.
This makes the Ability Hand far more intuitive to use, as demonstrated in this short video of a user picking up an eggshell while blindfolded:
This doesn’t replicate natural touch by any means. But it’s an important leap in practical functionality. And at $10 for the sensors, no one can argue with its cost-effectiveness.
We believe that the Vincent Evolution 3 offers similar capabilities, but have been unable to confirm this due to the lack of documents and videos available in English.
Regardless, we think it is only a matter of time before every commercial bionic hand incorporates some form of pressure feedback.
E-Dermis Electronic Skin
John Hopkins University (JHU) has created an electronic skin called “e-dermis”. This video from freethink.com provides an excellent overview:
This represents a giant technological leap over the user of simple pressure sensors. By embedding sensors in multiple layers, e-dermis has the potential to detect not only pressure but also pain. Eventually, it may even be able to sense temperature.
E-dermis also offers significant commercial advantages over other methods of sensory feedback. First, it conveys its sensory information by stimulating nerves in the user’s residual limb through the skin. This non-invasive technique, known as transcutaneous electrical nerve stimulation (TENS), is a lot less expensive and risky than surgically implanting electrodes.
Another advantage is that e-dermis is not tied to a specific prosthetic device. It can be layered on top of any prosthesis, making it a kind of development platform that can be used by scientific teams around the world for many purposes. This serves to pool resources and accelerate advancements.
Still, e-dermis on its own may not be enough. Fully re-establishing the intimate relationship between the brain’s sensory and motor cortices requires true two-way communication with living nerve bundles. The organization leading the charge on this initiative is the Defense Advanced Research Projects Agency, otherwise known as DARPA.
DARPA’s HAPTIX Initiative
In 2006, DARPA launched its Revolutionizing Prosthetics program. This resulted in two advanced bionic arms: the LUKE Arm and the Modular Prosthetic Limb (MPL). You can read more about them on our Understanding Bionic Arms & Hands page.
While advanced in many ways, both arms lacked a sense of touch. To address this, DARPA launched its Hand Proprioception and Touch Interfaces (HAPTIX) program in 2015. The goal of this program is to enable precision control of a bionic hand and sensory feedback via bi-directional nerve implants. In other words, true mind control and bionic touch.
One of the program participants is a biomechanical engineering team at the University of Utah. It’s objective was to upgrade the LUKE arm with these new capabilities:
The advantages of this approach are many. First, it helps amputees identify and manipulate objects much faster than could without sensory feedback. It also seems to reduce phantom pain from the missing limb — a matter of great importance to amputees.
There are, however, drawbacks. One of these is the cost. Implanting electrodes the residual limb is expensive. And at $100,000, the LUKE arm remains beyond the budget of most amputees. So most people will have to wait a while for this technology to trickle down into lower-cost prostheses.
Another drawback is portability. Until recently, the hand’s sensory feedback had to be channeled through a laptop, translated into the language of the brain, and then communicated to the target nerve bundle via the implanted electrodes. Scientists have come up with a portable device to handle the translation, but it is just now entering field trials.
Finally, there are potential complications with implanting electrodes in the human body, and not just the basic medical risks of invasive surgery, such as infection. Scar tissue can form around electrodes, or the electrodes can shift position — either of which may interfere with the reception or transmission of signals.
However, we are venturing past the general subject of bionic touch here and into more advanced neural interfaces. To help you out with that, see Advanced Neural Interfaces for Bionic Hands.
Bionic Touch for Lower Limb Amputees
A great deal of attention has been paid to creating a sense of touch for bionic hands. This probably reflects the fact that we experience so many of our tactile sensations with our natural hands.
But a sense of touch also important for our legs and feet.
This can be partially achieved with lower-limb osseointegration. Vibrations passed from a prosthesis through an osseointegrated implant are not only felt by the wearer; they are also heard. This gives the wearer the ability to identify the surface being walked on. For more information, see our article on Osseointegration for Bionic Limbs.
There is also every reason to believe that e-dermis or something similar might be adapted for lower limb use.
And there are other active projects underway, such as this one at ETH Zurich (the university responsible for the Cybathlon):
We will continue to monitor developments in this area and will periodically update this article as required.
For more information on e-dermis and other types of electronic skin, see A Quick Look at Electronic Skin.
For more information on upper-limb bionics, see Understanding Bionic Arms & Hands.
For more information on lower-limb bionics, see Understanding Bionic Legs & Feet.