Wearable medical sensors enhanced with ultrathin mesh

On-skin medical sensors and wearable devices are important healthcare tools that must be flexible and ultrathin so they can move with the human body. The technology also needs to withstand bending and stretching and needs to be gas-permeable to prevent irritation and discomfort. The devices also require an overheat protection circuit, to prevent them from overheating and burning the wearer. Researchers have demonstrated how an important component of the sensors, called a thermistor, can be constructed using an ultrathin fiber-mesh. Thermistors are a type of resistor whose resistance varies with temperature. The research findings about the thermistor have been published in Advanced Science.

Chihiro Okutani, an assistant professor in the Department of Electrical and Computer Engineering at Shinshu University in Japan, said that an overheat protection is required to avoid burning biological tissues during the operation of flexible devices. “One candidate is a polymer positive temperature coefficient (PTC) thermistor, which has a large increase in resistance within a narrow temperature range. For such thermistors to be applied for on-skin medical sensors, they must be stretchable and bendable down to several hundred micrometres. However, it is still challenging to fabricate a thermistor whose temperature characteristics do not deteriorate when wrapped around a needle with a bending radius of less than 1 mm,” Okutani said.

It is important for this technology to be able to wrap around a needle because sometimes sensors are attached to needles or catheters while in use; in order to achieve this, the thermistor needs to be ultrathin. Researchers used a technique called electrospinning to create the ultrathin mesh-type polymer PTC thermistor. Electrospinning uses electricity to create tiny fibres; the fibres can be made out of different materials, but in this case the researchers used a solution of composite materials. The thermistor was then tested to ensure it achieved similar performance capabilities of existing technology. Like typical film-type thermistors, the mesh-type polymer PTC thermistor showed an increase in resistance of three orders of magnitude, an important characteristic for preventing overheating and burns.

By using a mesh structure, the thermistor also achieved transparency, which can help the sensors blend into the skin, and gas-permeability. The researchers also demonstrated the operation of the thermistor wrapped around a 280-micrometre needle by fabricating the fibres on a 1.4 micrometre ultrathin film. Even with this fibre layer, which serves to give the mesh structure and additional heat sensing, the thermistor remained very thin.

Though the thermistor technology is promising, more research is needed to make this a reliable alternative to the current thermistor technology on the market. A mesh-type thermistor has a high initial resistance value due to its limited number of conductive paths. The researchers proposed that reducing the spacing between fibres in the mesh or increasing the number of electrodes used could resolve some of these problems, but additional testing will need to be done.

“Our next step is practical applications of the developed thermistors. We believe that the ultra-flexible and gas-permeable thermistors can act as overheat prevention components for on-skin or implantable devices, which make flexible sensors safer to operate and more reliable,” Okutani said.

Image credit: Chihiro Okutani, Shinshu University