Portable sensors evolve from watches and electrodes to foldable devices that provide users with more accurate biometric measurements and added comfort.
Now, an international team of researchers has taken the evolution one step further, printing sensors directly on human skin without using heat.
“We report a simple but universally applicable manufacturing technique, with the use of a new sintering layer to allow direct printing for sensors on the body,” said Ling Zhang, a researcher at Harbin Institute of Technology in China and Cheng’s lab.
How to print sensors?
Huanyu “Larry” Cheng, a professor in the Pennsylvania Department of State Engineering and Mechanics, and his colleagues have previously developed printed circuit boards for use in wearable sensors, but printing directly on the skin has been hampered by the gluing of metal components to the sensor.
Called sintering, this process usually requires temperatures of about 300 degrees Celsius to bind the silver nanoparticles of the sensor.
“The surface of the skin can’t withstand such a high temperature, obviously,” Cheng said.
“To overcome this limitation, we proposed an adjuvant layer of sintering – something that would not hurt the skin and could help the material to sinter at a lower temperature.”
By adding a nanoparticle to the mixture, the silver particles are sintered at a temperature lower than about 100 degrees C.
“This can be used to print sensors on clothes and paper, which is useful, but it’s still bigger than the skin can handle,” said Cheng, who noted that even a temperature of about 40 degrees C could it still burns the skin tissues.
“We changed the formula of the aid layer, we changed the printing material and we found that we can sinter at room temperature.”
An extra layer makes the whole process easier
Sintering aid layer at room temperature it consists of a paste of polyvinyl alcohol – the main ingredient in face masks – and calcium carbonate – found in eggshells.
The layer reduces the roughness of the printing surface and allows the printing of an ultra-thin layer of metal patterns that can bend and fold while maintaining their electromechanical capabilities.
The sensors are able to continuously and accurately capture temperature, humidity, blood oxygen levels and heart performance signals.
The researchers also linked the sensors on the body to a network with wireless transmission capabilities to monitor the signal combination as it progresses.
The process is also environmentally friendly, according to Cheng. The sensor stays robust in hot water for a few days, but a hot shower will easily remove it.
“It could be recycled because the removal does not damage the device,” Cheng said.
“And removing the sensor doesn’t affect the skin either. This is especially important for people with sensitive skin, such as the elderly and babies. The device can be useful without being an additional burden for the person using it or for the environment ”.
The researchers plan to modify the technology to target specific applications, such as a precise network of body sensors placed to monitor the symptoms associated with Covid-19.