This Sensor Is a Cut Above the Rest

To make the leap from a circuit board to the human body, electronic components need a serious makeover. As wearable electronic devices increase in popularity for a wide range of applications from health monitoring to virtual reality, we find ourselves in need of soft, stretchable sensors that can conform to, and move with, the body. The rigid designs traditionally used in these devices cannot flex with the body, and they are uncomfortable for long term use in a wearable.

One type of sensor that is particularly important for wearable applications is the strain sensor. These devices provide a great deal of precise information about how their wearer is moving, and the position that their body is in. From health to fitness tracking, this is essential information. Fortunately, many soft, flexible sensors of this sort already exist. Unfortunately, they tend to be fragile and prone to breaking, leaving these poor little sensors in the e-waste heap long before their time. How sad!

But now a new and better option exists. A group led by researchers at Vrije Universiteit Brussel has developed a stretchy, self-healing strain sensor. The thin, flexible device can move with the body without causing discomfort while recording accurate strain data. But unlike existing systems, it is not easily damaged. And even if it is cut completely in half, it can be mended by just sticking it back together.

The key to the design is a self-healing polymer and a liquid metal conductor. The polymer is based on a chemical bonding process called Diels-Alder crosslinking. Unlike traditional silicone substrates, which tear and stay broken, this polymer can “remember” how it was structured. When the material is damaged, the chemical bonds break apart but remain reactive. If the pieces are pressed back together, those bonds re-form, essentially knitting the sensor back into a single piece. Left at room temperature, the sensor can heal in about 24 hours. With a little help from heat, the healing process takes just four hours.

Inside this self-healing skin is a conductive liquid metal alloy called Galinstan, made of gallium, indium, and tin. Galinstan flows easily, but it is also biocompatible and safe to use near the human body. One might assume that cutting the sensor in half would cause the liquid metal to leak out, but the team found otherwise. When exposed to air, Galinstan quickly forms a thin oxide layer that acts like a temporary seal, much like clotting blood. When the broken pieces of the polymer are pressed together again, the oxide barrier dissolves, reconnecting the liquid metal channel and restoring electrical conductivity.

In tests, the sensor was stretched and broken multiple times, then healed without problems. Even after six cycles of complete rupture, the sensor still performed at about 80 percent of its original capacity.

The team has already spun out a company, Valence Technologies, to commercialize the design. Their vision includes applications in rehabilitation medicine, sports performance tracking, and even soft robotics. Eventually, they hope to scale up the technology so that full-body movement can be tracked with an array of these resilient sensors.