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ABSTRACT
Smart skin is a large-area, flexible array of sensors with data processing
capabilities, which can be used to cover the entire surface of a machine or even a
part of a human body. Depending on the skin electronics, it endows its carrier with
an ability to sense its surroundings via the skin’s proximity, touch, pressure,
temperature, chemical/biological, or other sensors. Sensitive skin devices will
make possible the use of unsupervised machines operating in unstructured,
unpredictable surroundings among people, among many obstacles, outdoors on a
crowded street, undersea, or on faraway planets. Sensitive skin will make machines
“cautious” and thus friendly to their environment. This will allow us to build
machine helpers for the disabled and elderly, bring sensing to human prosthetics,
and widen the scale of machines’ use in service industry. With their ability to
produce and process massive data flow, sensitive skin devices will make yet
another advance in the information revolution. This paper surveys the state of the
art and research issues that need to be resolved in order to make sensitive skin a
reality.
INTRODUCTION
This seminar focuses on the principles, methodology, and prototypes of sensitive
skin-like devices, and the related system intelligence and software that are necessary
to make those devices work. Smart skin represents a new paradigm in sensing and
control. These devices will open doors to a whole class of novel enabling
technologies, with a potentially very wide impact. Far-reaching applications not
feasible today will be realized, ranging from medicine and biology to the machine
industry and defense. They will allow us to fulfill our dream for machines sensitive to
their surroundings and operating in unstructured environment.
Some applications that smart skin devices will make possible are yet hard to foresee.
Flexible semiconductor films and flexible metal interconnects that will result from
this work will allow us to develop new inexpensive consumer electronics products,
new types of displays, printers, new ways to store and share information (like
electronic paper and ―upgradeable‖ books and maps). New device concepts suitable
for large area flexible semiconductor films will lead to new sensors that will find
applications in space exploration and defense, specifically in mine detection and
active camouflage.
An ability of parallel processing of massive amounts of data from millions of
sensors will find applications in environmental control and power industry. These
areas will be further developed because of the highly interdisciplinary nature of the
work on smart skin, which lies at the intersection of information technology,
mechanical engineering, material science, biotechnology, and micro- and nano
electronics. Availability of smart skin hardware is likely to spur theoretical and
experimental work in many other disciplines that are far removed from robotics.
1.1. MACHINES IN UNSTRUCTURED ENVIRONMENTS
Today’s machine automation is almost exclusively limited to the structured
environment of the factory floor. The rest of the world, with perhaps 99% of all tasks
that involve motion and could in principle be automated, goes unautomated. Think of
the unstructured environments in agriculture, construction sites, offices, hospitals, etc.
The majority of tasks that are of interest to us take place in unstructured
environments, to which today’s automation simply cannot be applied.
Automated moving machines can be divided into unattended those that can
operate without continuous supervision by a human operator, and semi-attended,
which are controlled by the operator in a remote (teleoperated) fashion. Today the use
of both types of machines is limited exclusively to highly structured environments - a factory floor, a nuclear reactor, a space telescope. Such machines can operate
successfully with relatively little and fairly localized sensing. Many existing machines
could, in principle, be useful in an unstructured environment, if not for the fact that
they would endanger people, surrounding objects, and themselves.
The same is true for remotely controlled machines. Unless the work cell is ―sanitized‖
into a structured environment, no serious remote operation could be undertaken.
Otherwise, at some instant the operator will overlook a small or occluded object, and
an unfortunate collision will occur. And so the designers take precautions, either by
―sanitizing‖ the environment, or by enforcing maddeningly slow operation with
endless stops and checks. Much of the associated extra expense would not be
necessary if the machines had enough sensing to cope with unpredictable objects
around them.
The Way Out is All-Encompassing Sensing:
To operate in an unstructured environment, every point on the surface of a
moving machine must be protected by this point’s ―own‖ local sensing.
1.2. SOCIETAL NEEDS AND CONCERNS OF SENSITIVE SKIN
1.2.1. HEALTH INDUSTRY
Smart skin will supplant sensing ability of the human skin in limb prosthetics and
as a replacement of damaged human skin. It will augment human sensing in wearable
clothing, by monitoring, processing, and wireless transfer of information about the
well-being of the person wearing sensitive skin. This will advance the post-traumatic
health care, care for disabled and elderly persons, and monitoring of military
personnel on the battlefield.
1.2.2. ENVIRONMENT – FRIENDLY TECHONOLOGY
For the first time in history, machines will be endowed with a capacity to be
careful. By its very nature, sensitive skin will contribute in a dramatic way to the
reversal of the well-known negative impact of machines on our environment, across a
wide spectrum of natural and man-made settings.
We often hear about the role of computer revolution and office automation in
the growth of economy and improved efficiency, which in turn affects the quality of
life. Note the difference: while unstructured machine automation will have a similar
effect on the economy, its use in service industry will have a direct impact on the
quality of human life. Biology and medical science thrive to prolong human life; the unstructured machine automation will constitute a systematic effort by engineers to
improve the quality of life.
1.2.3. DIFFICULTIES OF ACCEPTANCE
As with any fundamentally new and powerful technology, smart skin technology
may evoke adverse psychological reactions, with a potential of diminishing its
impact. Today we are psychologically unprepared for automatic moving machines
operating in our midst. We are not sure we need them. We are uneasy about the idea
of living side by side with a powerful unattended moving machine. It is difficult to
imagine that one could stand next to a powerful moving machine and trust it enough
to turn one’s back to it, or expect it to step aside when passing. Do we not have more
than enough invasion of machinery in our lives? To need a very new product, one
must first experience it.
SKIN MATERIALS
- Smart Skin material will hold embedded sensors and related signal processing
hardware. It needs to be flexible enough for attaching it to the outer surfaces of
machines with moving parts and flexible joints.
- The skin must stretch, shrink, and wrinkle the way human skin does, or to have
other compensating features. Otherwise, some machine parts may become "exposed"
due to the machine's moving parts, and have no associated sensing.
- Wiring must keep its integrity when Smart Skin is stretched or wrinkled. This
requirement calls for novel wire materials, e.g. conductive elastomers or vessels
carrying conductive liquid, or novel ways of wire design with traditional materials,
such as helical, stretchable wires.
3.1. AERAS OF DISCUSSION
Three areas of potential discussion were considered:
1. What materials might be used for sensors, actuators, and intelligence (transistors)
in such a system?
2. How can we make an interconnection network that can flex and bend?
3. How can we physically combine sensors/intelligence/actuators with the
interconnect substrate?
Fabricating smart skin is based on a new process of depositing polycrystalline
CdSe (1.75 eV), CdS (2.4 eV), PbS (0.4 eV) [13], PbSe (0.24 eV) and CuS
(semiconductor/ metal) films on flexible substrates at temperatures close to room
temperature (eV here are electron-volts). Large area surfaces can be covered. Also,
ternary and quaternary compounds as well as heterostructures can be deposited.
Transparent conductors on flexible substrates (such as CuS), materials for sensors,
with possible combination with higher mobility polycrystalline materials (such as
laser annealed polycrystalline silicon), amorphous (such as a-Si), polycrystalline
(such as CdS or CdSe), and deep submicron crystalline silicon technology (for fast
data processing). We will also need sensors with multiple sensing capabilities,
learning, once again, from the design of human or animal skin. These are new and
exciting challenges for material science and device physics.
3.2. SUBSTRATE / INTERCONNECT ISSUES
3.2.1. STRECHING AND BENDING
A central issue for smart skin is that the skin be able to conform to surfaces of
arbitrary shape, and be able to flex, bend, and stretch. Flexing, bending, and stretching
are important not only for applications (e.g. covering moving arms and joints), but
also for initial installation (like putting on clothes).
When a thin planar foil is deformed into ―developable‖ surface such as a cylinder or a
cone, the average strain in the foil is zero, and there exists a ―neutral plane‖ within its
bulk where the strain locally is zero. The strain on the surfaces scales as the thickness
over the radius of curvature.
Therefore by making the substrate thin and /or placing interconnects at the
neutral plane, bending to thin radii of curvature appears possible. However, deforming
into arbitrary shapes (e.g. spheres), bending in multiple dimensions, and stretching
require a finite strain, and hence may cause failure of the interconnects (e.g. if the
strain is larger than 1%).
Three different models for the substrate/interconnect system evolved. Adding
sensors/actuators/intelligence to the substrate will be discussed in the next major
section.
DEVICES FOR SENSITIVE SKIN
4.1. DEVICE CAPABILITIES SOUGHT FOR SENSITIVE SKIN
From the device point one might wish a Smart Skin to have some of the following
capabilities:
• Flexible or deformable, Can be tiled or cut, This aspect ties in to cost
and repair ability, High detectivity, On-skin switching and signal processing, Fault
tolerances by distributing functions/computing, or protect processor units.
Transmission by wire or optical fiber, or wireless: RF, UHF, free-space optical.
• Power by wire photovoltaics, RF, fuel cells, micro engines, or from
energy harvesting - (skin-integrated mechanical power generators). Power storage
in batteries. Or as fuel for fuel cells and micro engines.
• Smart Skin sensor components will be deployed in two dimensional
arrays of sufficiently high density
• Smaller arrays may be of use as well: the key feature is that the skin
should allow, by itself or with appropriate data processing, to identify with
reasonable accuracy the points of the machine's body where the corresponding
sensor readings take place.
• ―Self-sensing‖ ability of the skin is highly desirable; this may include
sensing of contamination, dust, chemical substances, temperature, radiation, as
well as detection of failure of individual or multiple skin sensors and the ability to
work around failed areas.
• The ability to measure distance to objects would be a great advantage
for enabling dexterous motion of the machine that carries the skin.
• Ideally, sensors and their signal processing hardware would be spread
within the array so as to allow cutting it to any shape (disc, rectangle, an arbitrary
figure) without losing its sensing and control functionality. This suggests
interesting studies in hardware architecture.
• Sensor arrays with special or unique properties are of much interest, for
example a cleanable/washable skin for "dirty" tasks in nuclear / chemical waste site
applications; radiation-hardened skin for nuclear reactor and space applications;
and skins that can smell, taste, react to, or disregard ambient light.
4.2. LARGE-AREA ELECTRONICS IS COMING OF AGE
Smart Skin will be a form of large-area electronics, and a large-area electronics
industry already does exist. Flat panel displays, including active matrix liquid crystal displays and plasma panel displays, are products of this industry. The
medical X-ray sensor panels that are in pilot use likewise are large-area electronic
products. These flat panel products use glass plates for substrate and encapsulation,
and are rigid. Flexible, active circuit technology is just coming out of the research
laboratory, like OLEDs on plastic foil, laser crystallized polysilicon on polyester,
TFTs on polyamide, and OLEDs integrated with TFTs on steel foil. In other words,
the basic technology for flexible skin electronics is coming together.
ORGANIC ELECTRONICS AND OPTOELECTRONICS ON
FLEXIBLE SUBSTRATES
Organic thin film transistors (OTFT) are based on a new class of materials
called conjugated polymers. Organic thin film transistors are considered as a
competitive alternative to the traditional inorganic semiconductor based thin film
transistors. In terms of performance, organic materials are not likely to catch the
inorganic semiconductor based transistors, however, low cost, large area, and reel-to
reel manufacturing can bring new opportunities where inorganic electronics cannot
obtain.
The capability of plastic-based displays provides broad applications for
industrial and product designers. The technical venture plans to create flexible
organic-TFT technology, which has the potential to dramatically reduce the cost of
display back planes while enabling the fabrication of lower cost flexible display
devices.
Organic materials are poised as never before to transform the world of circuit
and display technology. The future holds tremendous opportunity for the low cost and
sometimes surprisingly high performance offered by organic electronic and
optoelectronic devices. Using organic light-emitting devices (OLEDs), organic fullcolor
displays may eventually replace liquid-crystal displays (LCDs). Such displays
can be deposited on flexible plastic foils, eliminating the fragile and heavy glass
substrates used in LCDs, and can emit bright light without the pronounced
directionality inherent in LCD viewing, all with efficiencies higher than can be
obtained with incandescent light bulbs.
THIN FILM MEMS ON FLEXIBLE SUBSTRATES
The fabrication of silicon electronics into sensitive skin backplanes can be
integrated with silicon based sensor devices. Among these, silicon photodetectors are
the most prominent. Silicon transistor/photosensor cells would follow the structure of
amorphous silicon based photosensor arrays. An important recent development is thin
film micro electromechanical (MEMS) devices on plastic substrates. These devices
demonstrate that mechanical sensors (and actuators) can be built on the type of
flexible substrate that sensitive skin requires.
4.2.3. NANOSTRUCTURES ON FLEXIBLE SUBSTRATES
The progress in microelectronics has been associated with scaling of the minimum
feature size of integrated circuits. This trend described by the famous Moore's law is
now running out of steam as this minimum feature size approaches the values where
limitations related to non-ideal effects become important or even dominant. At the
same time, the opposite trend of increasing the overall size of integrated circuits has
emerged stimulated primarily by the development of flat panel displays. Emerging
technology of nanostructures on flexible substrates promises to merge these opposing
trends and lead to the development of ultra large area integrated circuits embedded
into electrotextiles or into stretchable and flexible ''sensitive skin''.