National Federation of the Blind
Research & Development Committee
4424 Brookhaven Avenue
Louisville, KY 40220
October 4, 1993
To: The NFB R&D Committee
It is with pleasure that I forward the following communication from Steven
Leeb. The grant application is filed with the Materials Division of the
National Science Foundation. If it is funded, I will pass the good news on
to you immediately.
Sincerely,
Tim Cranmer
Begin here
Dear Tim,
Please find below a copy of the text of our proposal to the NSF to develop
a touch tablet with polymer gels. I'll keep you posted if and when we
here anything back from the NSF. Please don't hesitate to call if I
can help with anything! (Phone #: 617-253-9360)
Thanks, Steve
__________________________________________________________________________
Gel Polymer Touch Tablet
Proposal Summary
Principal Investigator: Toyoichi Tanaka
Co-Principal Investigator: Steven B. Leeb
Collaborator: John A. Gardner
Proposal Title: Gel Polymer Touch Tablet
Submitting Organization: Massachusetts Institute of Technology
SUMMARY
The purpose of this project is to create a prototype of a practical,
relatively inexpensive, Braille touch tablet for the blind.
Servomechanisms based on gel polymers will be developed to actuate
the individuals dots or ``pixels'' in the tablet. The use of gel
polymers should permit the construction of small actuators which can
be packaged close together in the touch tablet. We anticipate that
gel polymer actuators will permit dot densities which could equal or
exceed the resolving power of the human finger. These gel-based
actuators would, for the first time, make possible the construction
of a high quality, manufacturable touch tablet.
The synthetic polymer gels proposed for use in these novel actuators
are cross-linked networks of polymer chains suspended in a liquid.
Gels are known to exist in two phases, either swollen or collapsed.
Gels can be created which exhibit a discontinuous volume transition
between these two phases. Microscopically, the phase transition is a
reversible folding or unfolding of the polymer chains constituting
the network. The change in volume may be enormous, and a variety of
stimuli may be employed to induce the transition. The discontinuous
phase transition exhibited by certain gels will be exploited to
create high performance actuators which produce direct linear motion
for use in the prototype tablet.
Project Goals
The principal goal of this project is to explore the practicality of
fabricating actuators suitable for use in an inexpensive Braille-type
display that could be used with a personal computer or other
information processing tool. Current technologies for producing
Braille print readable by the blind are either time-consuming in
producing a page, expensive, low in resolution, or a combination of
these and other limitations. This project proposes to explore
novel actuation technologies based on polymer gels, which could, in
theory, be used to construct actuators that provide direct linear
motion quietly, swiftly, and with high force densities. The goals of
this project are to explore the practicality of fabricating actuators
for a Braille touch tablet, including specifically:
The development of light-sensitive gel polymer actuators that
can serve as the prime movers for dots in a Braille display. This
includes chemical design and fine tuning of phase transition
temperature, volume transition size, speed, elastic modulus, and
durability for repetitive phase transitions of gels.
The arrangement of such gels in a two-dimensional tablet, where
individual units can undergo reversible volume phase transition in
response to light.
One or more prototype Braille displays, based on these
actuators, which will operate in concert with a standard personal
computer to provide a tactile display of Braille text and graphics.
Background
A gel consists of a cross-linked network of polymers suspended in a
liquid (\cite{gels}). Under certain conditions, gels have been
observed to undergo abrupt, reversible, discontinuous changes in
volume. Changes in volume may be 1000-fold or more, and can be
triggered by a variety of electrochemical conditions, including
changes in temperature, solvent concentration around the gel, or pH.
Volume change may also be initiated by the application of visible or
invisible light (\cite{gels4}), or by an electric field across a gel
(\cite{gels1}), as illustrated in Fig.~\ref{change}.
Microscopically, a phase transition is a reversible folding or
unfolding of the polymer chains that constitute the cross-linked
network of a gel, as shown in Fig.~\ref{fold}. Gels are unique soft
materials in that they can retain large amounts of fluid and can
exchange this fluid with their surrounding environment. Gels are
promising materials with unusual properties for potential application
as fundamental components in artificial muscles and other types of
servomechanisms, sensors, optical memories, controlled chemical and
drug release systems, and as selective absorbents.
The stylized waveform in Fig.~\ref{phase}, which has been adapted
from measured data presented in \cite{gels3}, shows the swelling or
volume phase transition curve for a bulk cylindrical gel created with
N-isopropylacrylamide, henceforth referred to as a NIPA gel. In the
experiments schematically illustrated by Fig.~\ref{phase}, the
temperature of the gel and its surrounding solvent regulate the
volume of the gel. Around approximately $34$ degrees Celsius, the
gel undergoes an abrupt change in volume. At lower temperatures, the
gel swells. At higher temperatures, the gel sinks.
When a NIPA gel is constructed with the appropriate constituents,
other stimuli besides temperature change can be used to initiate a
volume change, and a variety of useful properties may be observed in
the gel. A gel which is carefully synthesized and manipulated can
exhibit a hysteresis or ``memory'' in which a region remains
collapsed even after the initiating event is removed. This behavior
suggests that a gel could serve as a prime mover in a latching linear
actuator or gel ``solenoid.'' We have begun to consider the process
of fabricating actuators with NIPA gel fibers and beads that can
create linear or radial motion.
Our preliminary experiments with gel fibers, for example, are
outlined schematically in Fig.~\ref{proc}. In Step~1 in
Fig.~\ref{proc}, a syringe loaded with NIPA gel is used to spray a
viscous stream of gel at a rotating copper bar. The stream catches
on the bar and creates a thin fiber. As the stream is swept across
the bar, many fibers are created. In Step~2, these fibers are
cross-linked by exposure to ultraviolet radiation. The cross-linking
in the fibers after Step~2 is anisotropic. When the gel fibers are
suspended in a solvent such as water and subjected to heat, they will
contract longitudinally. This next phase in our experimentation is
illustrated in Step~3 in Fig.~\ref{proc}. A bundle of fibers is
bound at each end and suspended in a solvent. When the solvent is
heated, the fibers contract, and when the temperature is lowered, the
fibers relax and expand. This process could be carried out
repeatedly, suggesting a technique for creating a linear actuator.
We have already succeeded in creating gel fibers with diameters as
small as 9 microns which contract in times under 0.1 seconds.
Preliminary experiments with gel beads are outlined in \cite{bead}.
Beads have been prepared by inverse emulsion polymerization and
have been fabricated with radii as small as $0.2$ micrometers. Small
beads can contract and expand very quickly and can exhibit volume
changes of two or three orders of magnitude.
Experiments with test cells like the one illustrated in Step~3 of
Fig.~\ref{proc} and described in \cite{bead} will permit the precise
determination of the phase transition characteristics of basic
``building block'' gel components like anisotropic fibers and
miniature gel beads. These experiments are an important advance in
the understanding of the behavior of gel polymers. They will provide
an understanding essential for constructing useful, rugged actuators
from polymer gels.
Project Approach
We have a number of ideas on how gel actuators might be constructed,
how they might operate, and we can postulate the benefits of
servomechanisms based on such actuators. The first stages of this
project will involve vigorous experimentation to fully understand the
material properties, especially the phase transition characteristics,
in the context of practical applications. Gel fibers and beads
composed from a variety of constituents which permit phase transition
under different forms of stimulation (light, heat, electric field,
etc.) will be examined. Some of the fundamental materials science
and engineering concerns which will have to be addressed include:
Triggering Mechanisms
We anticipate that light induced phase transition will be the
triggering mechanism of choice. Light is safe and relatively easy to
control. Depending on the chemical structure of the constituent
monomers, gels can be crafted which either swell or shrink in the
presence of light \cite{Illman}. We will consider the applicability
of both types of gel to the practical problem of constructing a touch
tablet.
There are several candidate polymers for gels which swell in the
presence of light. Choices include, but are not limited to:
An interpenetrating polymer network of acrylic acid and
acrylamide to which chlorophyllin is copolymerized. We have some
preliminary results that demonstrate that such gels undergo a
swelling phase transition in the presence of light. The required
triggering intensity of light is in the range of milliwatts, and can
be controlled by the degree of ionization of the acrylic acid. This
is achieved by replacing some portion of the acrylic acid with sodium
acrylate.
Copolymer gels of dimethylacrylamide and methacrylic acid
polymerized with dimethylsulfoxide \cite{Xiaohong}. This gel was
shown to undergo phase transition in water at pH=7, and is expected
to respond to light when an appropriate chromophore is incorporated
into the gel. Our first candidate for the chromophore is again
cholorphyllin.
If we choose to incorporate gels which shrink in response to light,
we will begin with NIPA gels dyed with chlorophyllin, which have been
studied extensively in Tanaka's laboratory.
Response Time
The response time of a gel upon phase transition depends on the size
and shape of the gel, and the distance of the triggering condition
from the phase transition threshold value in the phase diagram. In
order to obtain a quick response, the gel has to be fashioned into
small units, e.g., spheres, cylinders, or thin fibers. Since the
characteristic time we are aiming to achieve is on the order of 1.0
to 0.1 seconds, the gel units should be made with diameters less than
100 microns.
Swelling Degree and Modulus
We have begun discussions with Professor Gardner to help determine
the required degree of swelling and modulus (particularly when the
gel is swollen). The swelling degree can be controlled by the extent
of the ionization incorporated into the polymer network. The modulus
of the gel depends strongly on the extent of cross-linking, which is
adjusted by varying the concentration of a cross-linker (for example,
N,N' methylenebisacrylamide). Since the degree of swelling is
reduced when cross-linking density increases, an optimization will be
conducted to determine the best relative concentrations of monomer,
ionizable groups, and cross-linker in the context of actuators for a
touch tablet.
Assembly and Control of Gels
We will design, in collaboration with Professor Gardner, appropriate
techniques for encapsulating and aligning gels in soft, elastic
membranes. Individual gels could be triggered practically, for
example, by one or a small number of low power solid state lasers
which scanned the many gels in a touch tablet via a rotating mirror
or disk.
Significance
Gel actuators will be quite different from traditional
electromagnetic actuators. Since gel actuators could be layered and
flexed or routed conformably with the underlying structure of a
manipulator, gel actuators may lead to servomechanisms with ranges of
motion that are very difficult to obtain with bulk electromagnetic
actuators. For the case of a Braille tablet, consider that the
spatial resolution of the human finger is approximately 1 mm, and the
height of standard Braille is 0.5 mm. Gel actuators will be
essential for constructing a high quality Braille tablet with a pixel
density of 20 to 30 ``dots'' per inch with a travel or height of 0.25
to 0.5 millimeters per pixel. These packing densities would be very
difficult to achieve with conventional electromagnetic actuators, but
relatively easy to achieve with gel actuators.
Gel beads or cylinders, for example, could be imbedded in an inert,
flexible gel matrix to create a lightweight, rugged Braille touch
pad. When appropriately stimulated, expanded gels would deform the
surface of the inert gel to create Braille ``pixels.'' The
stimulation to induce phase transition in these gel beads could be
supplied by a light beam which scanned across and down the tablet in
a manner similar to the raster scan of a cathode ray tube. A
microprocessor interface could be used to control the scanning of the
beam and to interface with a personal computer.
These preliminary considerations indicate the directions which our
work may take, but other possibilities will arise as we study the
fundamental behavior of gels more closely in the context of the touch
tablet. Any successful design must closely couple fundamental
materials research on ``building block'' gel actuator components
(fibers, beads, etc.) with engineering design so that an optimal
tablet in terms of reliability, cost, tactile performance, and speed
may be developed. Conclusions and theoretical results will be
verified with results from at least one prototype which will suggest
a manufacturable design for a commercial tablet.
Project Team
The principal investigators for the project are Toyoichi Tanaka,
Professor of Physics, and Steven B. Leeb, Assistant Professor of
Electrical Engineering. The principal investigators will collaborate
with Professor John A. Gardner of Oregon State University to
determine precise peformance requirements and to test experimental
implementations of prototype Braille tablets. There will be two
doctoral level students on the project team, drawn from the School of
Engineering and the School of Science.
Leeb was appointed in 1993 as an Assistant Professor
in the Laboratory for Electromagnetic and Electronic Systems in the
Department of Electrical Engineering and Computer Science at MIT. He
is a specialist in the design and control of servomechanical systems
and power electronics drives. His work, widely published in several
IEEE journals and conferences, includes the development of analog and
digital control and signal processing systems. His knowledge and
practical experience will be instrumental in constructing precision
machinery to employ gel actuators in a Braille tablet. His
experience will also be critical in developing useful actuators from
gels, and in developing kinematic models of these actuators for
closed loop control.
Gardner is a materials physicist. He lost his
sight in 1988 and has become an expert in assistive technologies for
persons with visual disabilities. Under sponsorship of the National
Science Foundation he has developed the ``Dotplus'' method for
printing math, science, and technological literature for blind
readers -- who presently can obtain nothing better than tape
recordings. Gardner is encouraging other scientists to seek
solutions to the most pressing problems impeding access. A tactile
computer screen may be the most important of these.
Tanaka has been involved in understanding the
physics of gels. He uncovered many of the physical principles of
polymer gels through finding of the fundamental phenomena, including
phase transition, critical phenomena, various patterns in gels, and
principles of motions of gels. These findings helped the
understanding and analysis of polymer gels, and also opened a door to
a wide variety of technological applications of gels in the chemical,
agricultural, and medical industries. Various experiments have now
been conducted world-wide demonstrating rudimentary artificial
muscles, mechano-chemical engines, controlled drug delivery systems,
super-absorbents, molecular sensors, chemical memories, and toys.
Tanaka has been the Editor-in-Chief of a newly launched journal, {\sl
Polymer Gels and Networks,} published by Elsevier Science Publishers.
Facilities
MIT has the resources to carry out the prototyping work involved in
this project. The Center for Materials Science and Engineering has
specialized facilities for determining the characteristics and
experimenting with the fabrication of polymer gels. In addition to
standard chemistry facilities (water purification, fume hoods,
chemical benches), standard and microscope light scattering
spectroscopy, NMR, fluorescence photo-bleaching recovery, HPLC, and a
range of light and electron microscopy facilities are available. The
Laboratory for Electromagnetic and Electronic Systems has machining
and electronic prototyping facilities available, including a range of
standard and specialized electronic test equipment such as fast
analog and digital storage oscilloscopes, signal generators, and power
supplies and power amplifiers.
Schedule
We have already begun studying different designs for gel polymer
actuators based on fibers, beads, and cylinders of gels. If funding
were available immediately, a tentative schedule would be as
follows:
Task Definition and Program Organization & November - December 1993
Materials Experimentation, Information Gathering & January -
December, 1994
Experiments with Fiber, Bead, and Cylinder Bundles - Thermal
Activation & February - December, 1993
Experiments with Fiber, Bead and Cylinder Bundles - Light and Electrical
Activation & June, 1993 - June, 1995
Conceptualization of Prototype Tablet Designs & June, 1993 - June, 1994
Initial Prototype Construction & September, 1993 - January, 1995
Prototype Design Iteration and Construction & January, 1995 - December,
1996
\begin{thebibliography}{100}
\bibitem{gels} T. Tanaka, ``Gels,'' {\sl Scientific American}, Vol.
244, No. 1, January 1981, pp. 124 -- 138.
\bibitem{gels4} A. Suzuki and T. Tanaka, ``Phase Transition in
Polymer Gels Induced by Visible Light,'' {\sl Nature}, Vol. 346, No.
6282, July 1990, pp. 345 -- 347.
\bibitem{gels1} T. Tanaka, I. Nishio, S. Sun, S. Ueno-Nisho,
``Collapse of Gels in an Electric Field,'' {\sl Science}, Vol. 218,
No. 29, October 1982, pp. 467 -- 469.
\bibitem{gels3} S. Hirotsu, Y. Hirokawa, and T. Tanaka,
``Volume-Phase Transitions of Ionized N-isopropylacrylamide Gels,''
{\sl J. Chem. Phys.}, Vol. 87, No. 2, July 1987, pp. 1392 -- 1395.
\bibitem{bead} Y. Hirose, T. Amiya, Y. Hirokawa, and T. Tanaka,
``Phase Transition of Submicron Gel Beads,'' {\sl Macromolecules,}
Vol. 20, 1987, pp. 1342 -- 1344.
\bibitem{Illman} F. Ilmain, T. Tanaka, and E. Kokufuta, ``Volume
Transition in a Gel Driven by Hydrogen Bonding," {\sl Nature}, Vol.
349, No. 6308, January 1991, pp. 400-401.
\bibitem{Xiaohong} Xiaohong Yu, ``Polymer Interactions and the Phase
Transition of Gels,'' Doctoral Thesis, submitted to the Physics
Department of the Massachusetts Institute of Technology, August 1993.
\end{thebibliography}
\end{document}
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