SENSORS
Tough wearable flexible sensor
By Julien Happich
Researchers from the University of Waterloo have combined
3D-printing with in-mold graphene inclusion to
yield graphene-doped porous silicone pads usable as
tough flexible pressure sensors sensitive to the faintest deformations.
Three-dimensional flexible porous dielectrics treated with a
conductive surface are often used as force or pressure sensors,
whereby any mechanical deformation of the pores results in
detectable changes in the overall conductivity of the fabricated
conductor. But under the repetitive mechanical deformation of
their flexible substrate, the conductive surface can suffer from
delamination and decay.
In a paper titled “3D-Printed Ultra-Robust Surface-Doped
Porous Silicone Sensors for Wearable Biomonitoring“ published
in the ACS Nano journal, the researchers present a simple fabrication
process where graphene nanoplatelets are not directly
deposited within the connected pores surface of the flexible
shape, but instead are deposited on the surface of a sacrificial
mold (dip coated with a solution of graphene nanoplatelets).
When silicone is then
poured into the sacrificial
mold, the graphene nanoplatelets
naturally end up
being transferred and embedded
onto the surface
of the silicone material,
effectively surface-doping
the porous silicone pad.
Key in realizing this was
the design of a sacrificial
mold, using fused deposition
modelling (FDM)
A flexible porous silicone pad
surface-doped with graphene
nanoplatelets. Credit: University
of Waterloo.
3D-printing to yield a mold internally shaped with ordered,
interconnected, and tortuous internal geometries (with triply
periodic minimal surfaces). Once dip coated with the graphene
nanoplatelets and poured with silicone, the mold material is
removed to leave only the flexible sensor.
With this fabrication method, the authors report a stable
coating on various porous silicone samples, with long-term
electrical resistance durability over a 12 months period and high
resistance against harsh conditions, including the exposure to
organic solvents. They also observed that the sensors retained
their conductivity upon severe compressive deformations (over
75% compressive strain) with high strain-recoverability even
across cyclic deformations, temperature, and humidity.
For the sensors tested, the paper reports a gauge factor as
high as 10 within the compressive strain range of 2 to 10%,
making them suitable for detecting even the small deformations
resulted by the human pulse.
The biocompatible material and the 3-D printing process
enable custom-made devices to precisely fit the body shapes of
users, while also improving comfort compared to existing wearable
devices and reducing manufacturing costs due to simplicity,
claim the researchers.
Because the flexible sensor’s sensitivity can easily be tuned
(through the internal shape of the interconnected pore surfaces),
it could be engineered for many different applications, from
smart insoles to wearable wrist-worn pulse sensors or embedded
into smart garments for monitoring walking and running
activities.
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