Researchers from imec’s
associated lab at Ghent University have invented an innovative way to fabricate
shear sensors for use on flexible surfaces, such as human or robotic skin. The
new sensors are based on optical technology embedded in thin flexible substrates.
Shear sensors measure shear stress, i.e. stress that is applied parallel to
the surface of a material, as opposed to a stress that is applied perpendicularly.
Most shear sensors are based on micro-electromechanical systems (MEMS), and
are fabricated on rigid Si substrates. These sensors can have a high density
and sensitivity, but they are fairly thick and lack flexibility. Moreover, their
activity is based on electrical measurements, so they are sensitive to electromagnetic
interference.
Schema of the sensor stack, with the silicone layer deformed through shear stress.
There is a substantial demand for sensors that can measure shear stresses unobtrusively.
These would have to be compact and flexible, so they can be used on moving body
parts and wrapped around curved surfaces. The demand comes especially from the
medical community, which would use shear sensors, for example, for measuring
the skin friction between a prosthesis and stump. But they could also be used
in other application fields. In robotics, for example, flexible shear sensors
could help to create a sensitive artificial skin.
Imec researchers have recently reported on a way to fabricate such flexible
shear sensors using optical technology. Optical sensors have a high sensitivity,
a large dynamic range, and are not susceptible to electromagnetic interference
noise. Furthermore they can be embedded in flexible substrates, making them
potentially very compact, robust and flexible.
The innovative sensors are made with a process that allows embedding optical
components into very thin and flexible substrates (down to 50µm). One
such sensor is a stack consisting of a vertical-cavity surface-emitting laser
(VCSEL) source and a photodiode separated by a transparent deformable layer
made of silicone. In the stack, the VCSEL and photodiode face each other and
are aligned so that the photodiode captures most laser light when there is no
shear stress applied to the stack. When shear stress is applied, the laser and
the photodiode move relative to each other. The change in laser light captured
by the photodiode is a measure for the shear stress.
The prototype sensors were made using a layer of the silicone material Sylgard
184. The deformation of the silicone material showed a linear response to the
applied shear stress. In a next step, the researchers are working to adapt the
sensor design so that it can also indicate the direction of the shear stress.