  Any two objects in contact with each other exert a force that could be simply
due to gravity or mechanical contact, such as a robotic arm gripping an object
or even the contact between two bones at our knee joints. The ability to
naturally measure and monitor these contact forces allows a plethora of
applications from warehouse management (detect faulty packages based on
weights) to robotics (making a robotic arms' grip as sensitive as human skin)
and healthcare (knee-implants). It is challenging to design a ubiquitous force
sensor that can be used naturally for all these applications. First, the sensor
should be small enough to fit in narrow spaces. Next, we don't want to lay
cumbersome cables to read the force values from the sensors. Finally, we need
to have a battery-free design to meet the in-vivo applications. We develop
WiForceSticker, a wireless, battery-free, sticker-like force sensor that can be
ubiquitously deployed on any surface, such as all warehouse packages, robotic
arms, and knee joints. WiForceSticker first designs a tiny
$4$~mm~$\times$~$2$~mm~$\times$~$0.4$~mm capacitative sensor design equipped
with a $10$~mm~$\times$~$10$~mm antenna designed on a flexible PCB substrate.
Secondly, it introduces a new mechanism to transduce the force information on
ambient RF radiations that can be read by a remotely located reader wirelessly
without requiring any battery or active components at the force sensor, by
interfacing the sensors with COTS RFID systems. The sensor can detect forces in
the range of $0$-$6$~N with sensing accuracy of $<0.5$~N across multiple
testing environments and evaluated with over $10,000$ varying force level
presses on the sensor. We also showcase two application case studies with our
designed sensors, weighing warehouse packages and sensing forces applied by
bone joints.
