Stretchable band does it all for biomedical wearables
Led by Prof. Wenlong Cheng, Director of Research at the Department of Chemical Engineering, the researchers took a novel approach to sensing motion and vital signs by devising a highly stretchable and very sensitive elastic strain sensor that could closely conform to the wearer’s skin.
Rather than rely on wavy patterns of conductive inks or other metallic/semiconductor particles compounded into an elastomeric material, the researchers embedded a channel filled with ionic liquid (IL) as a variable conductor. Interestingly, the fabrication process is very simple, and very cheap too.
The researchers relied on 3D printing to build a simple mould before pouring and curing Ecoflex silicone material to obtain a band featuring the imprinted microchannel. Once filled with ionic liquid, the millimetre-thin channel was sealed with another layer of silicone.
Because the ionic liquid flows freely within its stretchable channel, it doesn’t suffer from the fatigue, cracking or local delamination issues that make alternative stretchable electronics eventually fail under repeated flexures.
As the band is bent or stretched, the micro-channel is elongated and its width reduced (the liquid’s volume remains constant), affecting the distribution of the conductive ions and decreasing the channel’s overall electrical conductivity.
Once applied to the skin, monitoring the resistance change in the elastic band is akin to measuring skin motion and strain. The researchers demonstrated that they could tune the sensitivity of the band by adjusting the dimensions of the microchannel, from sub-millimetre to just under 2mm.
Applying a voltage of 3V across the band, they were able to detect a wide range of strains from 0.1 to 500% with repeatability and long-term stability with negligible variations. In fact, the band was so sensitive that even woven into a bracelet, it could detect minute deformations, such as identifying wrist pulses in real time, or monitoring different hand gestures such as clenching, palm bending or the bending of individual fingers.
But that’s not all, the researchers also noted that increased temperature contributed to enhanced conductivity sensitivity. They incorporated the device in a LED circuit and observed how its brightness could be regulated by the bracelet’s temperature, hence monitoring current resistance changes based on temperature. The current for human body temperature (37ºC) was around 10.2uA, making the elastic strain sensor potentially suitable as a wearable thermometer.
Another interesting observation is that this cheap strain sensor is not only waterproof, but remains operational under water and in fact, is pressure sensitive too. An interpretation is that upon submersion, water pressure shrinks the embedded channel and thus concentrates the ions, decreasing overall channel resistance.
To complete their extensive battery of tests, the Australian team performed washing tests at different temperatures, they also tested the bands to 50 000 life cycles under both low (5%) and high (100%) strains at frequencies of 1, 2 and 7Hz, always observing a high signal–noise-ratio, reliable dynamic responses and negligible peak losses (0.26%). Six month of shelf storage didn’t alter these results.
The complete results are detailed in the paper "Volume-invariant ionic liquid microbands as highly durable wearable biomedical sensors" published in the journal Materials Horizons from the Royal Society of Chemistry.
Check out Monash University at www.monash.edu.au