Prototype of Artificial Electronic Skin Demonstrates Pain Response

September 11, 2020 by Luke James

Australian researchers claim their e-skin prototype can react to pain, similarly to human skin. If developed, this could lead to major leaps in robotics and medical prosthetics.

Skin is the largest sensory organ composed of millions of complex sensors that detect stimuli of temperature, pressure, and touch. Upon detection of these stimuli, the skin instantaneously sends signals to the central nervous systems to initiate a motor response, and it's this complexity that makes it difficult to mimic in technology. 

Now, researchers at RMIT University in Australia claim that they’ve made a prototype of an electronic device that’s able to mimic the human body’s near-instant feedback response, reacting to painful sensations just as quickly as nerve signals traveling to the brain.


The prototype device

The prototype device is described as "skin-like," made from stretchable electronics. Image used courtesy of RMIT University

The pressure sensor prototype combines stretchable electronics and long-term memory cells while a heat sensor combines temperature-reactive coatings and memory. A pain sensor integrates all three technologies.


Transparent and Unbreakable Wearables

In the study, the researchers describe how they combined stretchable electronics that incorporate oxide materials with biocompatible silicon to create transparent, unbreakable, and wearable electronics. They also created temperature-reactive coatings that transform in response to heat and electronic memory cells that mimic the way the brain uses long-term memory to recall information. 


Three different technologies were integrated into the prototype

Three different technologies—a Pacinian corpuscle (for pressure), thermoreceptors (for temperature), and nociceptors (for pain)—were integrated into the prototype. Image used courtesy of RMIT University

Md Ataur Rahman, a doctoral researcher working on the study, says that the memory cells in each artificial skin prototype were able to trigger a response when pressure, heat, or pain reached a defined threshold. “We’ve essentially created the first electronic somatosensors—replicating the key features of the body’s complex system of neurons, neural pathways, and receptors that drive our perception of sensory stimuli,” he said.


The Artificial Nociceptor for Pain Sensing

The most important skin receptors relate to pressure (the Pacinian corpuscle), temperature (thermoreceptors), and pain (nociceptors). All of these receptors detect stimuli, measure it, and then transmit signals to the brain, triggering reactions.  


Conceptual framework of how biological receptors inspired artificial receptors

Conceptual framework of how biological receptors inspired artificial receptors. Image used courtesy of Advanced Intelligent Systems

In this study, the RMIT researchers report artificial electronic receptors that mimic receptors using a combination of several functional units. These include:

  • An oxygen-deficient, strontium titanate-based decision-making memristor
  • A gold, stretchable elastomer-based pressure sensor
  • A phase-change oxide-based temperature trigger, which acts as both a thermoreceptor and a nociceptor


Using Temperature as a Stimuli

To observe the behaviors of allodynia and hyperalgesia in their artificial thermoreceptor—which produce responses at underthreshold and overthreshold, respectively—a gold and stretchable elastomer‐based pressure sensor is switched to a low resistance state (LRS). A train of temperature pulse stimuli ranging from 66°C to 82°C is then applied.


Artificial nociceptor response characteristics

Artificial nociceptor response characteristics. Image used courtesy of Advanced Intelligent Systems


The response signal relative to the applied temperature is demonstrated (bottom left of the above figure) with the nociceptor turning on at roughly 68°C, which is the transition temperature of vanadium oxide (VO2) used in the temperature trigger.

When the VO2 goes to the LRS due to the temperature-induced transition, higher current begins to flow through the whole circuit; this is similar to the threshold behavior of a biological nociceptor, which generates brain-triggering action potential above the critical stimulus value.

Any further increases of stimulus intensity simply result in a larger current, consistent with a biological nociceptive neuron. 


What's Next for the Prototype?

In addition to the pain-sensing prototype, the research team claims to have also developed devices that use stretchable electronics to detect and respond to changes in pressure and temperature. With further development, they believe that this “stretchable artificial skin” could be an alternative to current invasive skin grafts. 

“We need further development to integrate this technology into biomedical applications but the fundamentals—biocompatibility, skin-like stretchability—are already there,” said lead researcher Professor Madhu Bhaskaran.