Stanford University chemical engineering professor Zhenan Bao and his research team have developed a plastic “skin” that can deliver sensory inputs to the brain, based on electrical signals generated from how hard it is pressed.
“This is the first time a flexible, skin-like material has been able to detect pressure and also transmit a signal to a component of the nervous system,” said Bao, according to Science Daily.
The “skin” works through a two-ply plastic construct. A top layer creates a sensing mechanism and a bottom layer transports electrical signals that translate sensory input into biochemical stimuli that can be processed by nerve cells. Now, the top layer can detect pressure over the same range as human skin — from a light finger-tap to a firm handshake.
Human skin, according to National Geographic, is made up of three layers. The outermost layer is the epidermis, which is composed primarily of protein cells called keratinocytes. Keratinocytes form multiple layers growing outward as exterior cells die and flake off. The epidermis also harbors defensive Langerhans cells that alert the immune system to viruses and bacteria.
The second layer is the dermis, whose collagen and elastin fibers give skin its strength and elasticity. Blood vessels within the dermis regulate body temperature by increasing blood flow to the skin to allow heat to escape. The third layer is the subcutis, which cushions people from knocks and falls while providing a layer of fat as a fuel reserve, in case of food shortage.
The second layer, the dermis, houses the network of nerve fibers and receptors that Bao’s plastic “skin” is trying to recreate. This network picks up sensations such as touch, temperature and pain, and relays them to the brain.
According to the University of Pennsylvania, mechanoreceptors, sensory receptors that can be found in the dermis, respond to sensations such as touch, pressure, stretching and movement. Different types of receptors sense different stimuli. For example, free nerve endings sense pain, while specialized receptors like Merkel’s discs and Meissner’s corpuscles sense touch.
Plastic is naturally springy, and by introducing a waffle pattern, Bao’s team made it more sensitive to pressure, since the waffle pattern compresses the plastic’s molecular springs. To further exploit this, Bao’s team scattered billions of carbon nanotubes, which form a spherical shape, through the plastic.
“There are two things happening when we press the material,” said Bao. The carbon nanotubes within the waffled plastic form a spherical shape, which increases when pressure is applied. “At the same time,” said Bao, “the spherical particles are pressed closer to each other, and the improved contact between the spheres also contributes to the high sensitivity.”
The team hopes to develop different sensors to replicate more intricate sensations. For example, the sensors would be able to distinguish between corduroy and silk, or a cold glass of water and a hot cup of coffee. The current two-ply system will allow the team to add more sensations as it develops new mechanisms.
“We have a lot of work to take this from experimental to practical applications,” said Bao. “But after spending many years in this work, I now see a clear path where we can take our artificial skin.”