This document discusses developing a microstructured elastomeric layer to increase the pressure sensitivity of soft optical sensors. The researchers fabricated an elastomer layer with an array of micro hemispheres and found that it responded linearly to pressures from 0-2 psi by increasing contact area. They characterized the material optically and found that optical transmission through it increased with applied pressure. Incorporating such a pressure-tuned layer could improve the sensitivity of applications like time-of-flight optical touchpads (TOFPADs) by providing more continuous pressure information rather than just an on/off response.

1. Microstructured
Materials for
Controlling the
Pressure Response of
Soft Optical Skins
Michael Portaro, Rio Brittany, Cindy
Harnett
University of Louisville ECE

2. • Goal: map human-scale pressures at soft surfaces
by inter-fiber optical coupling
• Problem: finding fiber materials that meet both
optical and mechanical requirements
• Solution: A micromolded elastomer layer with
pressure-tuned contact area
• Methods: Fabrication, area-vs-force
characterization, optical characterization in a
“pressure refractometer”
• Results: micro hemisphere array responds linearly
to pressures in 0-2 psi range

3. Connections between soft optical
sensors and haptics
Soft optical sensors can measure
& map:
• human-generated forces for
telehaptics
• forces at prosthetics or at
robotic manipulators for device-
to-human haptic feedback
• forces exerted by
electromagnetic haptic arrays
on skin, without
electromagnetic interference
Zhao et al., Science Robotics 2016
Reality Labs Research 2021
Yu et al., Nature 2019

4. Goal: map pressure in a soft
optical network
• Fiber-coupling methods give pressure
location—for example, by scanning a grid.
• Intensity of the cross-coupled light gives
pressure amplitude. It depends on fiber
crossing angle and fiber contact area.
• Fibers with the right optical properties might
not be soft enough to make good contact at
the desired pressure range.

5. Fiber junctions filter for higher
order modes
Compared to low-order, straight-
through modes, higher order modes:
• “zig zag” more than low order
modes
• interact with the fiber surface more
often
• experience more attenuation
• are more likely to get into the other
fiber than lower order modes
• travel more slowly

6. Time-of-flight of
optical pulse contains
spatial information
• Path length differences of 1
mm can be discriminated
• We can also measure
amplitudes in multiple
branches, if branch lengths
differ by > 20 cm
• Look at time of flight (TOF)
for spatial information, and
amplitudes for pressure,
stretch and bending
information
Lin and Harnett, IEEE
Sens. Lett. 2020

7. TOFPAD needs a more pressure-sensitive layer
• Time-of-flight optical touchpad
has soft urethane fibers
embedded in silicone
• Thin (125 micron) Teflon
interlayer is a good optical
cladding, but it isn’t
elastomeric
• TOFPAD is digital. There’s
not much pressure
information available here, it’s
ON or OFF.

8. • Time-of-flight optical touchpad
has soft urethane fibers
embedded in silicone
• Thin (125 micron) Teflon
interlayer is a good optical
cladding, but it isn’t
elastomeric
• TOFPAD is digital. There’s
not much pressure
information available here, it’s
ON or OFF.
TOFPAD needs a more pressure-sensitive layer

9. How about a microstructured elastomer layer?

10. Fabricating an elastomeric micro
hemisphere array
• Rowlux polycarbonate sheets are
covered in lenticular arrays
• Cast silicone on it (SmoothOn
MoldStar 20T) and cure, then
peel.
• Apply mold release (10:1 mineral
spirits:petroleum jelly) to the
silicone.
• Cast and cure optically clear
silicone (SmoothOn Solaris) onto
the silicone mold.
Same shape
as original
But elastomeric
and clear
Original (side view)
Replica (top view)
250 micron

11. Optical characterization in a “pressure refractometer”
• Large diameter (1 mm) soft
waveguides have multiple
guided modes with a range
of incidence angles
• Need to examine how angle
affects escape from the
compressed cladding
material to compare to ray
optics simulations
• Let’s measure transmission
from 0 to 90 degree
incidence Sensor
Total internal reflection
at 70 degrees for silicone
slab (n=1.41)
laser
power meter
sample
semicircular
glass prisms
(n=1.55)
Rio Brittany

12. Optical characterization in a “pressure refractometer”
• Let’s also squeeze the sample
and keep track of pressure
during these experiments. Sensor
Load cell
calibration
Translation
stage
Load cell
Michael Portaro

13. Area-vs-force
measurements
• And let’s point a microscope
camera at it
• Contact area increases as
pressure increases linearly from 0
to 3 PSI (0-20 kPa)
• Voids collapse together at
approximately 1.5 PSI (10 kPa),
halfway through the video

14. More pressure = more contact area at laser spot

15. Optical transmission increases with pressure
Transmitted
Intensity
(microWatts)
Rotation angle (degrees)
300
250
200
150
100
50
0
0 10 20 30 40 50 60
1.9 PSI (~ 13 kPa)
0.5 PSI
1.5 PSI
1.1 PSI
0 PSI
Glass-air
critical angle
45 degrees
70
Glass-silicone
critical angle
69 degrees
Silicone
Calculated transmission vs angle
curve for each material
Air

16. Optical transmission increases with pressure
Transmitted
Intensity
(microWatts)
Rotation angle (degrees)
300
250
200
150
100
50
0
0 10 20 30 40 50 60
1.9 PSI (~ 13 kPa)
0.5 PSI
1.5 PSI
1.1 PSI
0 PSI
Glass-air
critical angle
45 degrees
70
Glass-silicone
critical angle
69 degrees
200
150
100
50
0
At 55 degree
rotation angle
Pressure (kPa)
0 2 4 6 8 10 12 14 16
2 PSI
1 PSI

17. To sum it up
• Microstructured elastomer layer tunes the
pressure response over 0-2 PSI range.
• Further work: incorporate into TOFPAD
application to increase pressure
sensitivity range, study hysteresis and
effect of contact shape
Supported in part by NSF Awards 1849213 and 1935324