A stretchable radio frequency (RF) radiation sensor that combines a microfluidic antenna and rigid electronic circuits has been developed by scientists in Sweden. This could open the way to reliable and durable second skin sensors for monitoring health. 

Flexible electronics are used in applications such as cameras, computer keyboards and photovoltaic cells. Some success has been found with stretchable antennas but the connection between the stretchable material and the rigid circuits still results in strain and loss of device sensitivity. To make wearable devices, electronics not only need to be flexible but they also need to be stretchable to truly conform to skin. Unfortunately, development from a flexible to a stretchable device has remained an elusive goal.

Now, Shi Cheng and Zhigang Wu from Uppsala University have developed a hybrid technology that combines conventional rigid circuitry with a substrate making a device that can bend, twist and stretch. 

The researchers formed a stretchable antenna by injecting a liquid alloy into microfluidic channels of an elastic substrate, polydimethylsiloxane. Flexible printed circuits are attached to toughened areas of the elastic substrate so they are protected from stretching and twisting. The connection between the liquid alloy and the circuit is made using tin-plated contact pins and a semi-spherical solder ball that allows the connection to be maintained even if it is twisted or stretched. 

The technology to make circuits entirely out of microfluidics isn't yet available, explains Wu, the team used established components, the microfluidic antenna, rigid circuits and flexible printed circuits, to produce the hybrid device. 

The team used their device as an RF sensor by connecting to a LED indicator that turned on at a certain level of RF radiation. Stretching the sensor 15 per cent along either axis does not destroy the sensitivity to the RF radiation, says Wu. 

Steve Haswell, an expert on miniaturised systems at the University of Hull in the UK, comments: 'the work fills a missing link in the integration of lab on a chip systems.' And he was impressed that the sensor 'not only stretches, but also bends and twists.' 

Cheng and Wu now plan to improve the manufacturing process of their hybrid circuits as well as increasing the functionality of the sensors. 

Patricia Pantos 

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