New Flexible Material Technology Creates E-Skin for Prosthetics

More than two million Americans are currently living with a lost limb. The use of artificial limbs has been around for millennia, dating back to ancient Egyptians. However, it wasn't until 1912 that lighter, aluminum prostheses were available. The 21st century will no doubt be remembered as a significant step forward in prosthetic functionality as well. Through the combination of flexible materials that mimic human skin and sensory-enabled technologies, researchers are helping amputees become more functional and improve their overall quality of life.

Flexible Materials Lay the Foundation

E-skin mimics the functionality and mechanical properties of natural skin. Polyamine-based materials are self-healing due to a reversible bond exchange. This means the bonds that hold the polymer network together can break and reform under certain conditions, such as heating. The thermoset materials irreversibly form crosslinked networks through covalent bonds. One research group took this material and made it self-healing, fully recyclable, and malleable. They explained this was done by "doping the dynamic covalent thermoset with conductive silver nanoparticles."

Hybrid materials have also proven to be successful foundations. Incorporating inorganic particles expands the functionality of polymer-based materials. For example, micro-structured nickel particles are sometimes incorporated into the polymer network, promoting self-healing. In fact, one study showed these hybrid materials were able to regain conductivity within 15 seconds of breakage and reestablish their mechanical properties in 10 minutes at room temperature with no added stimulus. 

Technologies Create Sensory Illusion

With a natural skin foundation, scientists have been able to create an e-dermis with sensory illusion, in effect giving amputees a sense of touch. The e-skin is laced with sensors that act as nerve endings, recreating the sense of touch and pain. The e-dermis is connected to the participant by using a non-invasive method called transcutaneous electrical nerve stimulation (TENS). The sensors detect stimuli and relay the impulses back to the peripheral nerves. The e-skin can be fitted onto the amputee's existing prosthetic. An engineering team from John's Hopkins was able to create a sensory function that was so defined that wearers could detect the shape of an object they were picking up, as well as if it was sharp or dull. "For the first time, a prosthesis can provide a range of perceptions from fine touch to noxious to an amputee, making it more like a human hand," explained one researcher. 

This new technology counters what is often referred to as a phantom limb, a sensory illusion when amputees often feel their missing limb is still there. The research team created a "neuromorphic model" that mimics touch and pain receptors in a natural nervous system. The e-dermis electronically encodes sensations just as receptors in natural skin would. The participant's brain activity was tracked on an EEG to confirm sensations in the phantom limb. 

The Impact

The combination of flexible materials and sensory technology has shown to have a significant impact on the lives of amputees. In one study published in the journal Scientific Reports, amputees said it was "like losing my hand all over again" when the study ended, and they had to give the device back. Participants reported their prosthetic became more than just a tool. They used it more often and for more daily tasks when it was combined with the sensory technology. Participants also said the new technologies gave them greater confidence when performing tasks and when interacting socially with peers. Researchers concluded the emotional impacts of increased sensation will have a dramatic effect on the quality of life of amputees. 

Conclusion

Novel flexible materials coupled with recent advances in connecting to sensory devices have bridged the gap in prosthetic technology, allowing people living with limb loss to regain sensation and awareness in their prosthesis.  The expectation is that this field will only continue to become more advanced in the coming years.