Wearable sensors is a rapidly evolving field, with smart watches leading the way towards electrodes on bendable devices that provide extra precision and comfort for users. Printed sensors could be the ultimate realisation of this technology, providing temporary, custom-made monitoring of parameters such as blood oxygen or temperature.
Now a team led by Huanyu ‘Larry’ Cheng at Penn State University in Pennsylvania has taken the evolution one step further by printing sensors directly onto the skin.
Cheng and his colleagues previously developed flexible printed circuit boards for use in wearable sensors, but printing directly on skin has been hindered by the bonding process for the metallic components in the sensor. Called sintering, this process typically requires temperatures of about 300ºC to bond the sensor's silver nanoparticles together.
“The skin surface cannot withstand such a high temperature, obviously,” Cheng said. “To get around this limitation, we proposed a sintering aid layer – something that would not hurt the skin and could help the material sinter together at a lower temperature.”
By adding a nanoparticle to the mix, the silver particles sinter at a lower temperature of about 100ºC.
“That can be used to print sensors on clothing and paper, which is useful, but it's still higher than we can stand at skin temperature,” Cheng said. Temperatures of about 40ºC could still burn skin tissue. “We changed the formula of the aid layer, changed the printing material and found that we could sinter at room temperature.”
The room temperature sintering aid layer consists of polyvinyl alcohol paste – the main ingredient in peelable face masks – and calcium carbonate from eggshells. The layer reduces printing surface roughness and allows for an ultrathin layer of metal patterns, which can bend and fold while maintaining electromechanical capabilities. When the sensor is printed, the researchers use a cool air blower to remove water solvent from the ink.
The sensors are reportedly capable of precisely and continuously capturing temperature, humidity, blood oxygen levels and heart performance signals. The researchers also linked the on-body sensors into a network with wireless transmission capabilities, to monitor the combination of signals.
The printed sensors can withstand tepid water for several days, but are easily removed in a hot shower.
“It could be recycled, since removal doesn't damage the device,” said Cheng. “And, importantly, removal doesn't damage the skin either. That's especially important for people with sensitive skin, like the elderly and babies. The device can be useful without being an extra burden to the person using it, or to the environment.”
The researchers plan to develop the technology to target specific applications, such as an on-body sensor network to monitor symptoms associated with Covid-19.
The work was published in ACS Applied Materials & Interfaces.
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