Electronic skin is the linchpin to the next generation of bionic limbs, especially bionic hands. The ability to sense contact, pressure, texture, etc., is one of the main prerequisites for more intuitive control and greatly improved dexterity. It may also hold the key to finally eliminating phantom limb pain.
The Miracle of Human Skin
Human skin is remarkable. It is mechanically tough, stretchable, and self-repairing. It plays a vital role in both temperature regulation and the immune system. And it has a sensory system capable of differentiating between thousands of sensations.
This short video elaborates on some of these unique capabilities:
Because human skin is so tightly integrated with some bodily functions (such as circulating 25 % of our blood supply at any one time), it is not practical at this stage to envision electronic skin performing all those functions. But it can play a vital role in connecting users of bionic limbs to the outside world.
Electronic Skin Requirements for Bionic Limbs
To make writing this section easier, we’re going to focus on electronic skin for bionic hands.
Contact and Pressure
Users of bionic hands need to know when they have come into contact with an object and how much pressure they are applying when they grasp that object.
Only a few commercial bionic hands currently offer this type of sensory feedback. One of these is the Psyonic Ability Hand, which embeds touch sensors in its fingertips. Contact with an object is relayed to users via a vibration unit in the hand’s arm socket. As pressure against the sensors increases, so too does the level of vibration.
This type of feedback is valuable to users because it allows them to regulate the amount of force they are applying to an object without relying solely on visual guidance.
However, this is still rudimentary compared to natural abilities. For one, sensor coverage is quite poor compared to the tens of thousands of sensory receptors on the human hand. What if you pick up an irregular-shaped object where the main contact point is not the fingertips but instead the underside of the knuckles? What if parts of the object are rigid but other parts are fragile, as might occur with certain electronic components? In such a case, users need sensory information for each point of contact.
You reach into a bowl of oranges and apples, seeking the latter but without looking. Using a natural hand, it is easy to distinguish between the two fruits based on surface texture alone. One is pitted; the other is smooth.
Tactile feedback is an intricate part of our skin sensory system. Being able to distinguish between two surfaces is almost as important to our non-visual dexterity as the ability to sense contact/pressure.
It is even more important emotionally because it helps create a more intimate connection with our surrounding world. Recreating that connection through electronic skin will make a bionic hand feel more like a natural, living hand. This may, in turn, play a vital part in eliminating the phantom limb pain often experienced by amputees, i.e. by fooling the brain into believing that the natural hand is still there.
There is no such thing as sensing shape, per se. Identifying a shape with one’s hands is primarily a function of the brain; sensory feedback merely provides some of the input data.
We are describing this subject separately because the ability to identify a shape and the location of one’s fingers on that shape is crucial to our dexterity.
Imagine for a moment that you pick up your house key from a hallway table to unlock your front door.
If you are using a natural hand, you probably won’t pick up the key exactly the way you intend to use it. Instead, you’ll glance at the key, grab it any way you can, and immediately turn your attention to the door lock.
Before getting there, your hand and your brain will repeatedly sense the position of your fingers on the key and adjust them until you achieve the desired grip.
This requires the ability to sense both contact/pressure and texture, as well as feedback on hand geometry, a huge database of known shapes, and some advanced spatial algorithms.
Imagine using a natural hand to pick up this smooth plastic ball about the size of a softball:
Because the ball is light, you could probably lift it using the same grip force that you used to grasp it.
But what if someone had secretly filled the ball with lead? In that case, it would likely start to slip from your grasp as you attempted to lift it. To maintain your hold, you would have to tighten your grip.
Electronic skin is essential to duplicating this response in a bionic hand. It has to detect when an object begins to slide across its surface, or perhaps when the object begins to stretch its skin pre-slide so that the user has a chance to react.
Having the ability to sense temperature through electronic skin is likely not as important as the other sensory capabilities described to this point, mainly because a) there are fewer use-case scenarios, b) there is no risk of medical injury to a bionic hand, and c) people tend to rely more heavily on visual guidance in dangerous situations.
That having been said, temperature sensing could still be quite valuable, especially when cooking or handing off a hot plate or mug to an unsuspecting recipient. It may also have a positive emotional impact by allowing the wearer to feel the warmth of a loved one’s hand.
Our natural ability to sense pain helps us avoid injury. Duplicating this capability in electronic skin could similarly help users avoid damaging a bionic limb. But the sensing mechanisms here are mainly just variations in pressure and temperature sensing.
Stretchability and Durability
A hand by its very nature must be able to twist itself into a myriad of shapes and grips. Some of these movements, such as completely curling or extending one’s fingers, requires significant skin elasticity.
It’s not enough for skin to be able to just fold and stretch. Any embedded sensors must be both numerous enough and sufficiently pliable so that they continue to work in any hand position.
They must also be durable enough so that they don’t degrade over time and change the input data on which programming algorithms and/or the brain rely.
Existing or Pending Electronic Skin Options
Now that we know the bionic hand requirements for electronic skin, let’s examine the current leading options for this important technology.
E-dermis is the brainchild of scientists at John Hopkins University (JHU). The following video provides a good overview of e-dermis and how it integrates with both a bionic hand and its wearer as of mid-2018:
As the video notes, e-dermis’s technical focus at this point was still very much on contact and pressure sensing, with a special emphasis on distinguishing between objects that are pointy (likely painful) vs round (not painful).
Note, however, that this is not as limited as it sounds. The ability to distinguish between sharp and curved surfaces allows for some shape identification (e.g. the ability to distinguish between three objects). And there has already been some 3rd-party work to incorporate temperature sensors.
What we could not find, after combing research papers, etc., was any indication that e-dermis can or will support slip detection or feedback on texture. But other research groups have made tremendous progress in these areas.
ACES (Asynchronous Coded Electronic Skin)
On the other side of the world from John Hopkins University, a team at the National University of Singapore (NUS) has been hard at work developing an electronic skin that can distinguish between textures and read Braille.
This video provides a quick overview:
Note, however, there is only a passing reference in this video to future work on sensing temperature and pain. A research group in Australia is much further along in these two capabilities.
RMIT Electronic Skin Prototype
There is no trade name for this prototype yet; RMIT is the name of a university in Melbourne, Australia.
Researchers there have developed another novel approach to electronic skin, as briefly described in this video:
Note, also, the crossover use for this technology from prosthetics to medical uses, such as skin grafts.
VATES (Visually Aided Tactile Enhancement System)
Lastly, we would be remiss if we didn’t mention new research on electronic skin conducted by the Beijing Institute of Nanoenergy and Nanosystems at the Chinese Academy of Sciences.
VATES, as it is known, was inspired by spiders that have sensing organs on their legs. The organs have a pattern of slits that allow the spiders to detect the faintest movements in their vicinity.
The VATES team has mimicked this capability using multiple layers of polymer film coated with silver in a way that allows cracks to form in the silver. These cracks become parallel channels that conduct electricity and are extremely sensitive to movement.
For example, VATES can detect the light brush of a feather, the touch of a flower petal, and even the touch of a wire too small to see.
A Quick Summary
As you can see from this article, we don’t yet have one form of electronic skin that can do everything required by bionic limbs, especially bionic hands. But we are getting closer with each passing month.
If any new developments occur with electronic skin, we’ll update this article and re-post it.
For more information on bionic touch in general, see Understanding Bionic Touch.
To learn more about the neural interfaces needed to take advantage of electronic skin’s advanced sensory capabilities, see Mind-Controlled Bionic Limbs and Advanced Neural Interfaces for Bionic Hands.
For more information on upper limb bionics, see Understanding Bionic Arms & Hands.
For more information on lower limb bionics, see Understanding Bionic Legs & Feet.