Technology That Feels

While this may seem like a trivial thought for most, those who have replaced their lost limbs with a prosthesis may never experience the same sensation you just felt.

Reading Time: 4 minutes

Cover Image
By Semoi Khan

Pick up a pencil and note the first sensation that comes to mind. You may notice its temperature, weight, and texture as you hold it between your fingers. Now, gently press the tip of the pencil against one of your fingers; you should immediately feel the change in texture. Continue experimenting with different levels of pressure, taking note of the level of discomfort you feel at varying pressures. At what point does this mundane experience become painful?

It may be difficult to pinpoint the exact moment touch becomes pain because two mechanisms are at play during this experiment. Our brains process touch and pain through low-threshold mechanoreceptors (LTMRs) and nociceptors, respectively. Just as the name implies, LTMRs can translate weak mechanical pressure into electrical signals to be processed by the brain. Nociceptors, or high-threshold mechanoreceptors, only activate when stronger pressure is administered (other nociceptors can also process thermal and chemical stimuli). Thus, the job of a nociceptor is to send a pain signal to the brain when a stimulus causes tissue damage. As such, pain allows us to avoid serious damage to our bodies.

Recall yourself picking up that pencil. If you imagined how it felt to hold the pencil and press the tip on your finger, you will be able to imagine that sharp sensation. While this may seem like a trivial thought for most, those who have replaced their lost limbs with a prosthesis may never experience the same sensation you just felt. Though amputees understand what it means for something to be round, rigid, or sharp, mass-produced prostheses currently do not transmit touch or pain signals to the brain. However, results from recent studies suggest otherwise.

In 2018, researchers at Johns Hopkins University implemented a sensory system in a modified prosthesis to mimic the cellular response pathway in our bodies. The prosthesis has a top layer called the e-dermis, a play on the word “epidermis,” the top layer of our skin. The e-dermis is laced with pressure sensors, including ones that act like nociceptors near the surface and mechanoreceptors below. When the hand prosthesis touches an object, the sensors transmit an electrical signal to the nerves at the site of amputation, allowing the brain to perceive “pain” and the shape of various objects.

The e-dermis technology may redefine prostheses from cosmetic or functional augmentations to treatments for phantom pain, a perception of feeling in a body part that is no longer there. This occurs as a result of decreased blood flow and nerve activity in the amputated area. While amputation removes the neurons that transmit information from the limb, it keeps the receiving end intact. As a result, the inactive neurons in the brain continue to receive signals from other sources. Prosthetic limbs that provide sensory feedback can increase the neuron activity and blood flow from the amputated area, decreasing the likelihood of phantom pain. Though the success of this treatment is not yet verified by experiments, experience with sensory prosthetics has generally been positive.

The e-dermis provides strong sensory feedback at the cost of a more invasive mechanism. Whereas non-sensory prosthetics are easily attached, the e-dermis requires a separate stimulator component that connects to peripheral neurons. This direct connection from an electrode to a neuron creates a strong electric signal that is directly controlled by sensors in the fingers. Though it provides more definite feedback, it is not without its own limitations. For example, an e-dermis device developed by the University of Illinois researchers utilizes an external module that connects to an existing prosthesis, but the module attaches on the outside, making the whole device bulky. They also found that sweat impaired the effectiveness of the module, making it inefficient. In addition, the e-dermis is only capable of tactile sensations and lacks the ability to provide feedback on heat. Furthermore, the signal that the e-dermis transmits is far slower and less acute than that of a neuron. These limitations translate to major differences between simulated pain and actual pain.

In one study published by Frontiers in Neuroscience this year, amputees who were instructed to use a prosthesis over four weeks reported some positive results. The prosthesis is a simple device that uses sensory bulbs at the fingertips to stimulate the neurons in the upper arm using air pressure, as opposed to the direct stimulation approach of the e-dermis. All testers reported that the mechanism was successful in helping them feel objects. However, responses varied when assessing the practicality of the feedback. Some users claimed that the prosthesis helped them feel whole again and that their movements became more natural. Others argued that the feedback delivery was too slow or weak, adding that they still had difficulty feeling small objects. While this experiment proves that stimulation through a prosthesis is possible, it also demonstrates that further research is needed to better personalize the technology.

In developing sensory prosthetic technologies, researchers must keep in mind the ethics of forcibly inducing pain. While it’s true that humans require the sense of pain to survive, prosthetic limbs do not bleed. As a result, critics argue that the idea of adding sensory capabilities to prosthetics is ridiculous. However, this choice should lie in the users themselves. Perhaps one day this feature will merely be a customization on more advanced technology, letting the users take charge of the ways they use their prosthetic limbs.

In addition, as many functional prosthetic limbs can cost upwards of $100 thousand, it becomes even more important for engineers to work around the current limitations in prosthetics. Perhaps the most cost-effective solution is something similar to that of the device used in the study published by Frontiers in Neuroscience, where stimulation occurs externally. While cost is a major concern for the average person, it may not be an issue for scientists and engineers who want to implement this technology into robotics. Integrating sensory technology into the field of robotics allows for more precise measurements and more complex actions. A NASA rover equipped with this technology may be able to explore like a human and return more detailed descriptions of the landscape. Robots with commercial applications may also benefit from this by gaining the ability to distinguish dangerous objects and the ability to assess the damage on themselves. Sensory technology can also be applied to virtual reality to create more immersive virtual worlds. Despite being a novel development, sensory technology not only shows promise in restoring the sense of pain in amputees through prosthetics but also has endless applications in various other fields.