SATURN: Technical and Design Challenges of Building a Self-sustaining Sound and Vibration Sensing Material

Abstract

SATURN is a thin and flexible multi-layer material that can sense sound and other mechanical vibrations in the environment without any external power source. It is constructed of inexpensive materials (paper, copper, and plastic), so that it can be attached to a variety of objects and surfaces. When flat, SATURN's frequency response below 5000Hz is comparable to a powered microphone. When bent, SATURN has a comparable frequency response up to 3000Hz. As a sound power harvester, SATURN can harvest 7 microWatts, which allows the detection of loud sound events. We explore the space of potential applications for SATURN as part of self-sustaining interactive systems.

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[HIGHLIGHTS]
S
ATURN is a thin and exible multi-layer material that can sense
sound and other mechanical vibrations in the environment without
any external power source. It is constructed of inexpensive materials
(paper, copper, and plastic), so that it can be attached to a variety
of objects and surfaces. When at, SATURN’s frequency response below
5000Hz is comparable to a powered microphone. When bent, SATURN
has a comparable frequency response up to 3000Hz. As a sound power
harvester, SATURN can harvest 7 microWatts, which allows the detection
of loud sound events. We explore the space of potential applications for
SATURN as part of self-sustaining interactive systems.
Excerpted from “SATURN: A Thin and Flexible Self-powered Microphone Leveraging Triboelectric
Nanogenerator,” from Proceedings of the ACM on Interactive, Mobile, Wearable and Ubiquitous
Technologies with permission.https://dl.acm.org/citation.cfm?id=3214263 © ACM 2018
[HIGHLIGHTS]
Nivedita Arora, Jin Yu, HyunJoo Oh, Thad E. Starner and Gregory D. Abowd
Georgia Institute of Technology, Atlanta, GA.
Editors: Nic Lane and Xia Zhou
SATURN:
Technical and Design Challenges
of Building a Self-sustaining Sound
and Vibration Sensing Material
What if a paper-like material could sense
and transmit sound and other mechanical
vibrations wirelessly? Imagine an early
morning scene in Sal’s household. Sal has
installed a single voice home control device
in her living room with paper microphones
at dierent places to extend remote inter-
actions. Sal gets up hearing the alarm sound
from the smart home device and double
taps the sticky-note paper microphone on
her bedpost to stop it. She goes to her closet
to dress for the day, unsure yet about the
weather. Sal asks the microphone sticky note
on her closet wall about the temperature
outside. Next, as she comes out to the living
room, she hears the voice of her 7-year-old
child on the home assistant, calling for her
via the paper microphone placed on his toy
elephant. She goes to check on him as her
husband arrives from his workout to open
the main door using a unique password
combination of blows and taps. e smart
home device recognizes his arrival and
announces that he is home. To make this
scenario a reality, where a disposable paper-
like material can sense mechanical vibrations,
such as voice, taps, and blows, we built a
wireless audio sensing and communication
material: SATURN (Self-powered Audio
Triboelectric Ultra-thin Rollable Nano-
generator) [2]. SATURN is thin and exible
in form factor, cheap to manufacture and
self-sustaining in its power consumption.
It has comparable signal quality to active
microphones that consume power and are
more expensive and bulky.

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of applications. e acoustic sensitivity
of SATURN when bent reduces with
increasing bending angle due to the increase
in stiness of the structure which results in
lesser vibration of the layers. At a bending
angle of 45 degrees, SATURN is still a
usable microphone and comparable to an
active microphone till 3000 Khz, allowing
capture of more than 60% of sounds
associated with voice.
Sound Energy Harvester
SATURN in the presence of loud sound can
harvest enough energy that could be used
for doing computational tasks. We analyzed
the 4x4 cm SATURN microphone patch as
a power harvester under loud sound pressure
(100 dB). For a 1 Mohm load, SATURN
generates approximately 0.5 Vpp at 150 Hz,
which rises to a maximum at 2.5 Vpp at 250
Hz and then falls below 1.0 Vpp at 350 Hz.
e same behavior is shown in the power
curve, with 6499 nW being the maximum
power that can be harvested at 250 Hz.
SELFSUSTAINING
COMPUTATIONAL SYSTEM
In the previous section, we saw the
characterization of SATURN as both a
self-sustaining microphone and an energy
harvester. To use SATURN in a practical
application, it needs to be embedded in
a self-sustaining computational system
that can allow a transfer or storage of the
data. Figure 5 shows how a tag consisting
of SATURN, a transistor, and a exible
antenna can leverage radio backscatter
technology to passively communicate
sensed sound or vibration information [1].
Figure 6 demonstrates how SATURN, when
in the presence of a loud sound, generates
enough power that it can support ipping
of a bit in ash memory, which can be
interrogated later using radio waves [2].
In an alternate method, if the loud sound
is persistent for a few seconds, it can be
used to activate a low-power long-range
radio [3], which allows for real-time
communication of loud acoustic events.
EXPLORATION OF APPLICATIONS
e thin and exible form factor of SATURN
allows it to be placed on dierent surfaces
for self-sustaining audio sensing appli-
cations (Figure 7). We imagine scenarios
where many dierent inexpensive SATURN
patches in the home can extend the range of
audio input for home assistants. In addition
to speech sensing, SATURN can also be
used as a contact microphone to sense
simple input touches, such that dierent
force of taps could be detected. In more
industrial environments, SATURN can be
used for acoustic failure monitoring and
diagnostics in places where it is dicult
or dangerous for humans to access. For
example, SATURN patches might be
placed on turbines in a nuclear power
plant to monitor them for vibrations that
indicate damage or wear. SATURN as
a loud sound power harvester could be
used for inexpensive, battery-free ambient
monitoring of sources of noise pollution.
Applications include monitoring for sound
thresholds exceeding human hearing
tolerance, such as in construction zones,
mines, music venues, power stations,
airports, spaceports, and military environ-
ments. Similarly, SATURN-based sensors
might be used for monitoring events, such
as landslides and mine gas explosions.
CHALLENGES AND
FUTURE WORK
is section gives an overview of some of
the challenges faced and future work for
this project to make it more concrete and
deployable in real-life scenarios.
1. System Challenges: SATURN is an early
prototype for the paper microphone
scenario from Sal’s home. ere is a
need to work on the robustness of the
radio backscatter architecture for the
communication of data. Large scale
deployment of such sensors requires
us to create innovation on both the
hardware and the soware side to be
able to distinguish dierent devices
in a single room. In addition, there is
a need to incorporate privacy-aware
design principles and come up with both
innovative technological and social
approaches to ensure user control over
when and where a SATURN patch
senses and transmits data. For example,
[HIGHLIGHTS]
[HIGHLIGHTS]
FIGURE 2. Cycle of electricity generation process in SATURN under external acoustic excitation
due to the combined eect of triboelectrication and electrostatic induction.
FIGURE 3. Structural device design optimization.
HOW SATURN WORKS
Recent advances in materials science dem-
onstrate the possibility of self-powered, easy-
to-manufacture sensors that take advantage
of the triboelectric nanogenerator (TENG)
eect, which converts mechanical vibrations
into electrical energy [4,6]. When made
in the right form factor, these mechanical
energy generators could be manufactured as
self-sustaining sound and vibration sensors.
We use these principles for the design and
fabrication of SATURN, as explained below.
Principle of Working: Triboelectric
Nanogenerator (TENG)
When two dierent materials come into
contact and separate, or rub alongside
each other, they tend to either gain or lose
electrons, based on their position relative
to each other in the triboelectric series [8].
is common phenomenon of exchange of
electrons is called triboelectrication. e
redistribution of charge creates an electric
potential between the layers. If there is a
conductive path between the two layers,
the charge dierence will balance due to
electrostatic induction. Repeated contact
and separation, therefore, produces an
alternating current [5]. is multilayer
structure, consisting of dierent materials
that are both conductive on one side,
is called a Triboelectric Nanogenerator
(TENG).
Device Design
SATURN is an example of a TENG and
consists of two layers (Figure 1). e rst
is the copper that acts as a triboelectrically
positive material. is layer is coated onto
paper for mechanical support. Paper is low
cost, exible, light, and easy to perforate,
making it a favorable medium to support
vibration in the presence of sound waves.
e second layer is a dielectric plastic,
PTFE(Polytetrauoroethylene). It is a
triboelectrically negative material coated
with copper on one side. e rst and second
layers are placed with the copper side of the
paper touching the non-copper-coated side
of the PTFE. e layers are anchored to each
other using glue in a specic grid dot pattern.
A potential dierence is caused by vibration
and is measured between the two copper-
coated surfaces.
Mechanism
is section explains how the change in
air pressure due to sound vibrations causes
constant contact and separation in the
multilayer structure of SATURN. When the
two layers of SATURN, paper and PTFE,
come in contact with each other, charges
are induced in the copper and the PTFE
due to triboelectrication (Figure 2a).
PTFE has a greater electron anity and
gains electrons from the copper to become
negatively charged. In parallel, the copper
layer on the paper becomes positively
charged. e subsequent separation of the
paper and the PTFE (Figure 2b) induces a
potential dierence across the two copper
electrodes. Such a separation causes current
to ow from the paper towards the PTFE
layer when the device is connected to an
external load. is ow of current reverses
the polarity (Figure 2c) of charges on the
two copper electrodes (i.e., now the copper
on PTFE has more positive charge than
the copper layer on the paper). e next
compression results in a reversal of the
current ow (Figure 2d) from the paper
towards the PTFE layer to complete the
cycle of electricity generation.
Device Optimization
We have optimized dierent device design
parameters in SATURN’s structure to increase
its electrical response across a wide frequency
of the audible range. Dierent structural
design parameters (Figure 3) – the hole size
and spacing of holes in paper, the geometry
of the patch, and the glue points to attach the
two layers – are varied to understand their
eect on signal quality, to nally come up with
a design that is both reliable and replicable.
PERFORMANCE
Self-sustaining Sound Sensor
Aer optimizing SATURN’s structural
parameters, we are able to reach the best
acoustic sensitivity of -25.63 dB (re mV/Pa)
at 1000 Hz with a circular shape of 16 cm
area with a grid pattern of holes of 0.4 mm
diameter and 0.2 mm spacing glued at 8
equally distant points around the edges and
the center to the PTFE. In this conguration,
the SATURN microphone has a comparable
frequency response from 20-5000Hz to an
active microphone (Figure 4). Approximately
90% of the information related to human
voice is within this range, making SATURN
a good quality microphone for a variety
FIGURE 1. SATURN device design: Multiple layered structure of SATURN, consisting of paper with
holes coated with copper (triboelectrically positive material) and PTFE (triboelectrically negative
material) coated with copper.
SATURN IS AN
EXAMPLE OF A
SELFSUSTAINING
WIRELESS
MECHANICAL
VIBRATION AND
SOUND SENSING
COMPUTATIONAL
MATERIAL

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Acknowledgment
We would like to thank our collaborators
at Georgia Tech’s Nanotechnology Lab:
Prof. Zhong Lin Wang, Yi-Cheng Wang,
Steven L. Zhang and Zhengjun Wang for
their guidance on the original version of the
SATURN paper. We thank Diego Osorio
and Fereshteh Shahmiri for their help in
making diagrams for the original SATURN
paper, some of which have been adopted in
this article. We would like to thank GVU
Prototyping Lab and Georgia Tech IEN
Cleanroom facility for letting us use their
equipment and space for experiments.
Nivedita Arora is a PhD student at the School
of Interactive Computing at Georgia Institute
of Technology. She combines learnings from
HCI, material science, chemical, electrical
and mechanical engineering to develop
computational material that can self-sustainably
sense, compute, actuate and communicate.
Jin Yu is a master’s student at the School of
Architecture at Georgia Institute of Technology.
Her work focuses on developing a new medium
that bridges art, engineering, and computing
for expanding the creativity of individuals.
HyunJoo Oh is an assistant professor with a
joint appointment at the School of Industrial
Design and the School of Interactive Computing
at Georgia Institute of Technology. Her work
explores computational design tools that
integrate everyday materials with computing
and how these combinations can broaden
creative possibilities for designers and learners.
Thad E. Starner is a professor in the School
of Interactive Computing at Georgia Institute
of Technology and a sta research scientist at
Google Research and Machine Intelligence. His
work led to the Google Glass family of devices,
and his research continues to investigate how
wearable computing can assist users in face-
to-face conversations and everyday situations.
Gregory D. Abowd is Regents’ Professor and
J.Z. Liang Chair at the School of Interactive
Computing at Georgia Institute of Technology.
His research interests focus on how the
advanced information technologies of
ubiquitous computing (or ubicomp) impact
our everyday lives when they are integrated
seamlessly into our living spaces.
[HIGHLIGHTS] [HIGHLIGHTS]
REFERENCES
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Zero energy ubiquitous sound sensing surface
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nanogenerator for self-powered exible
electronics and Internet of ings. In Meeting
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[7] M. Weiser. e computer for the 21st century.
(1991). Scientic American, 265(3), 94–105.
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FIGURE 7. SATURN is exible and can be made in dierent shapes and sizes, allowing
instrumentation of everyday objects, such as a soda bottle, shirt, and paper crafts for interaction,
control, context sensing, and event detection applications.
SATURN microphone patches could
be constructed at a physical level that
requires tapping a nger on it to prime
the backscatter circuit before audio could
be transmitted (e.g., when a speaker
taps a microphone to ask, “Is this mic
on?”). Furthermore, the patch can be
constructed to have a limited sensing
range for the human voice. Another way
to design SATURN for privacy is to tune
the resonant frequencies of the patch to
focus on only certain types of sound or
vibration, excluding the human voice.
2. Design Challenges: SATURN is a
computational material that looks more
like paper and inspires creative uses,
such as being used as part of papercra.
Computation that looks and feels more
like everyday objects will change the way
that we as humans experience, understand
and build relationships with that techno-
logy. It also creates opportunities and
challenges for infusing computation with
values, such as sustainability, through
the deliberate choice of materials used
to create a computational eect.
3. Manufacturing C hallenges: In an ideal
situation, we would like these paper
microphones to be cheap and disposable,
so we would not worry if they are lost
or stolen. Currently, the bill of materials
for a single sticky note-size SATURN
microphone is less than a cent, but its
manufacturing cost is still high due to the
way we are depositing copper on paper
and PTFE. is pushes us to think about
the manufacturing process, so it can be
scaled. When SATURN is placed in a
self-sustaining computational scenario,
the cost gets higher, depending on the
active transistor component being used.
is expense begs the fundamental
question: does our traditional semi-
conductor industry provide the support
needed for large-scale ubiquitous sensing?
How can we change our traditional manu-
facturing techniques to be able to support
applications where objects and surfaces
have computation embedded in them?
4. Need for Multi-disciplinary Mindset:
Building SATURN involves solving
technical, system-level and design
challenges, which span many elds.
Materials science is required to design
SATURN patches; mechanical
engineering helps characterize the
eect of vibration on a patch; wireless,
low-power electronics is necessary for
building self-sustaining communication;
exible electronics are required for
manufacturing the prototype, and design
and HCI knowledge help us explore
applications in everyday settings. Using
a combination of self-sustaining sensors
and backscatter technique, there is an
opportunity for creating thin wireless
sensing solutions for many dierent
phenomena. Developing them will
require researchers who can adopt an
aggressively multi-disciplinary mindset
to collaborate and learn the language of
many dierent elds.
CONCLUSION
SATURN is an example of a self-sustaining
wireless mechanical vibration and sound
sensing computational material. Its simple
multilayer construction results in a compu-
tational device that resembles everyday
materials. It looks and feels like paper, yet it
behaves like a wireless microphone. Using
power (requiring no external power source),
cost (large-scale manufacturing with simple
materials), and form factor (looking more like
everyday objects) as driving factors in the
design of computational devices can lead
to a whole new generation of interesting
computational materials. ese materials may
nally allow us to create technologies that, in
the words of Mark Weiser, “weave themselves
into the fabric of our everyday lives until they
are indistinguishable from it.” [7] n
FIGURE 5. Self-sustaining computational systems for SATURN as a sound sensor.
FIGURE 6. Self-sustaining computational systems for SATURN as a loud sound energy harvester.
FIGURE 4. Spectrogram of speech signal recorded simultaneously by iPhone and SATURN
microphone. The acoustic sensitivity of a 16 cm SATURN patch is comparable to an active
microphone in acoustic sensitivity till 5000Hz.