Engineers print wearable sensors directly on skin without heat
An international team of researchers developed a novel technique to
produce precise, high-performing biometric sensors

Date:
October 12, 2020
Source:
Penn State
Summary:
Wearable sensors are evolving from watches and electrodes
to bendable devices that provide far more precise biometric
measurements and comfort for users. Now, an international team of
researchers has taken the evolution one step further by printing
sensors directly on human skin without the use of heat.



FULL STORY ========================================================================== Wearable sensors are evolving from watches and electrodes to bendable
devices that provide far more precise biometric measurements and comfort
for users.

Now, an international team of researchers has taken the evolution one
step further by printing sensors directly on human skin without the use
of heat.


==========================================================================
Led by Huanyu "Larry" Cheng, Dorothy Quiggle Career Development Professor
in the Penn State Department of Engineering Science and Mechanics,
the team published their results in ACS Applied Materials & Interfaces.

"In this article, we report a simple yet universally applicable
fabrication technique with the use of a novel sintering aid layer to
enable direct printing for on-body sensors," said first author Ling
Zhang, a researcher in the Harbin Institute of Technology in China and
in Cheng's laboratory.

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 around 572 degrees Fahrenheit (300 degrees Celsius) 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 212 F (100 C).



========================================================================== "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, who noted that about 104 F (40 C) 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 --
which comprises eggshells. The layer reduces printing surface roughness
and allows for an ultrathin layer of metal patterns that can bend and
fold while maintaining electromechanical capabilities. When the sensor
is printed, the researchers use an air blower, such as a hair dryer set
on cool, to remove the water that is used as a solvent in the ink.

"The outcome is profound," Cheng said. "We don't need to rely on heat to sinter." The sensors are capable of precisely and continuously capturing temperature, humidity, blood oxygen levels and heart performance signals, according to Cheng. The researchers also linked the on-body sensors
into a network with wireless transmission capabilities to monitor the combination of signals as they progress.

The process is also environmentally friendly, Cheng said. The sensor
remains robust in tepid water for a few days, but a hot shower will
easily remove it.

"It could be recycled, since removal doesn't damage the device,"
Cheng said.

"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." Next, the researchers plan
to alter the technology to target specific applications as needed, such
as a precise on-body sensor network placed to monitor the particular
symptoms associated with COVID-19.


========================================================================== Story Source: Materials provided by Penn_State. Original written by
Ashley J. WennersHerron.

Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Ling Zhang, Hongjun Ji, Houbing Huang, Ning Yi, Xiaoming Shi,
Senpei Xie,
Yaoyin Li, Ziheng Ye, Pengdong Feng, Tiesong Lin, Xiangli
Liu, Xuesong Leng, Mingyu Li, Jiaheng Zhang, Xing Ma, Peng
He, Weiwei Zhao, Huanyu Cheng. Wearable Circuits Sintered
at Room Temperature Directly on the Skin Surface for Health
Monitoring. ACS Applied Materials & Interfaces, 2020; 12 (40):
45504 DOI: 10.1021/acsami.0c11479 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/10/201012103136.htm

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