Skin — the body’s largest organ — is constantly monitoring for pain. It can trigger pain-avoiding actions automatically through reflexes, for example when we touch something dangerously hot.
Now, researchers at RMIT University in Melbourne, Australia, have created an artificial skin that mimics this mechanism and reacts to pain stimuli.
Made of silicone rubber, it has the texture of real skin and is also “very similar to skin in its mechanical properties,” says Madhu Bhaskaran, an engineering professor at RMIT University and the project’s lead researcher.
It could lead to groundbreaking innovations in prosthetics and robotics.
Just like real skin, the artificial version is designed to react when pressure, heat or cold surpass a pain threshold. Its outer layers sandwich electronic circuits studded with sensors, which respond to stimuli.
“The fascinating thing about our body is that it works by sending electrical signals into a central nervous system,” says Bhaskaran. Electronic circuits work in a similar way, and are just as fast, she explains.
When we touch something burning hot, pain receptors in our skin send an electrical signal through our nerves to the brain. The brain sends its own electrical signal to initiate a response — for example a withdrawal reflex to move the affected limb away from the heat.
In much the same way, when one of the sensors in the artificial skin detects a pain stimulus, it sends an electrical signal to the brain-mimicking parts of the structure, Bhaskaran says. This can be programmed to trigger a movement.
“The key here is thresholds,” says Bhaskaran. She explains that although we sense stimuli constantly, we only react when the stimulus exceeds a threshold, “like touching something very hot.” The brain and the skin compare stimuli and identify which are dangerous, she says. When creating the artificial skin, the scientists set those thresholds for the brain-mimicking electronics.
The result is artificial skin that can differentiate between the gentle touch of a pin or a painful stab.
The artificial skin could help to create smart prosthetics covered with a functional skin that reacts to pain like human limbs, allowing the wearer to know if they’re touching something that could cause damage.
“We’ve come a long way with prosthetics, but the focus has been heavily on the motor actions that the prosthetic limb can perform,” says Bhaskaran. Because conventional artificial limbs don’t have skin, they don’t sense external dangers. “Having a skin-like layer would make it much more lifelike,” she says.
Steve Collins, an expert in prostheses and exoskeletons at Stanford University in the US, who is not involved with the project, agrees: “When a person loses a portion of their limb to amputation, they lose not only bone and muscle but also somatosensory organs,” he says, referring to organs that provide sensations like heat or pressure. While structure and movement have received a lot of attention in prosthesis design, sensing may be the key to obtaining human-like performance with artificial limbs, he says.
Peter Kyberd, an engineering professor at the University of Portsmouth who created the world’s first microprocessor-controlled prosthetic limb in 1998, says he finds the work interesting: “Most current attempts to provide feedback from prostheses to the user have not resulted in any improvements in function in everyday use,” he says. A big part of the problem is that “the type and quality of the feedback signals is very poor,” he says, adding that there is a long road ahead before the skin can be routinely used in prostheses.
The artificial skin also has the potential to be used for skin grafts, says Bhaskaran. The artificial graft could be a temporary measure that’s eventually replaced by real skin, or might be applied as a permanent graft when the use of real skin isn’t viable. The skin could also be used to create smart surgical gloves — replacing the sensitivity that is usually lost when wearing hand coverings.
The most forward-looking of all applications is robotics, where pain-sensing artificial skin would not only provide lifelike functionality, but also imbue a potential humanoid robot with the ability to feel pain — an intriguing step not just technologically, but philosophically as well.
Bhaskaran says the idea has potential, but she’s focusing on more immediate goals: “For now, we need to work closely with biomedical researchers to see how we can take this to the next step. The goal is to make it much more lifelike and integrate it into real-life applications.”