Restoring The Sense Of Touch: Advances In Artificial Skin Technology
(Posted on Thursday, June 1, 2023)
The following is the first part of a series on brain-machine integration and biomechanical solutions to restore function to tissues damaged by disease, trauma, or time.
Researchers have developed haptic technology over the past several years to recreate touch sensation by applying forces to a user. Haptics have been implemented in virtual reality, telerobotics, computer simulations, and more.
Recent advances in artificial skin technology may take haptic technology to a new level, enabling those with prosthetic limbs to feel touch sensation once again. The skin is the largest organ in the body. It protects our more vulnerable organs from pathogens, debris, radiation, and other harm. Its other role, however, is far more complicated.
Touch sensation results from a network of neutrons embedded in the skin that relay signals from the outermost points of our body to our central nervous system. When you touch the phone, tablet, or computer in front of you, the feeling of touch is a series of electrical signals sent to the brain. When this connection is severed, such as in spinal injuries or loss of a limb, touch sensation is lost.
However, this loss may no longer be permanent. Researchers Wang et al. recently unveiled an advanced e-skin technology that enables detailed sensory feedback and soft interaction with our surroundings. The e-skin is soft, mimicking the physical characteristics of human skin, and can be encoded to sense touch, temperature changes, and pressure, all conveyed back to the brain by artificial neural networks. Essentially, they created artificial skin that could return the fifth sense to those who have lost it.
For the most part, electronic systems remain rigid and stiff. Recent years have brought the advent of more flexible electronic materials, such as soft device fabrication, which allows for something like e-skin to be constructed. An issue that remained for Wang et al. was that even the best flexible electronic materials were still high voltage (30 to 100 V). A wearable with such high voltage presents a significant risk to the wearer.
To overcome the high voltage problem, the researchers developed a trilayer insulator to add to the e-skin. The insulator is smooth and thin, keeping the e-skin easily wearable and flexible.
The most significant difficulty with e-skin is detecting contact with an external object, but rather the brain’s interpretation of sense and reaction to it. When you touch something scalding, your brain immediately detects danger, and you unconsciously pull back instantly. This is sensory feedback. The neural network connecting your skin to your brain enables a rapid response to touch, faster than we can consciously think. With artificial skin, the sensory feedback mechanism must be reconstructed.
When you touch something, analog signals are encoded into electrical signals and passed on to the brain through the nerves. For the e-skin, the researchers built a network of solid-state synaptic transistors to carry the electrical signals.
Wang et al. developed a closed-loop system connecting the e-skin to the somatosensory cortex in a live rat model. The somatosensory cortex is located in close proximity to the motor cortex, aiding the speed at which our motor responses can be so rapid. In vivo experiments showed pressure to the e-skin resulting in significant activation of the somatosensory cortex and, in turn, considerable muscle activation in the rat.
Following a significant round of animal and human testing for safety and efficacy, Wang et al.’s e-skin technology could be used in various applications. First and foremost, regeneratively, it could restore touch sensation to the nearly two million amputees in the United States alone, as well as all those with diseases or preexisting conditions impacting touch sensation.
Further, e-skin could be used industrially, whether applying it to human-operated machinery to equip the operator better or even to robots to receive data to better conduct their operation.
While this technology is at least years from widespread commercialization, with the rapid development of robotics, artificial intelligence, and other significant regenerative enhancements, we will soon see a proliferation of this and similar advances moving forward.