Ionic liquids bestow stretch on biomedical sensors

Researchers based in Australia have developed a soft and stretchable device that recognises artery pulses or body movements, and relays the information to a smartphone.

The demand for wearable devices has surged in recent years with gadgets to monitor body movements, heart rate and sweat metabolites, among other things. However, most of the devices currently available are not truly wearable because they do not sit flush with the skin so do not deform as the body moves.

Wenlong Cheng and his team of materials scientists and chemical engineers at Monash University used an ionic liquid sealed in a silicon mould to counteract these problems. They created a durable, waterproof and lightweight device that senses body movements. When the sensor deforms, current running through the device changes while the voltage remains fixed. A smartphone is capable of picking up these variations in resistance, which are unique to each movement.

An ammeter indicates changes in the electric current running through the sensor as it is stretched © Royal Society of Chemistry

Ionic liquids generally have lower Young’s moduli than the elastomeric polymers typically used in electronic devices. These low moduli allow the sensors to bend without any signs of cracking or loss of conductivity. While Cheng has used a model ionic liquid in this study, his team is also testing a range of hydrophilic and hydrophobic ionic liquids for these sensors.

Cheng foresees a key application for the sensor in monitoring health, in particular for people working at desks on a daily basis, who may be at risk of cervical spondylosis – a form of neck pain. ‘From our perspective, stretchable electronics should be the ultimate electronic product.’