Shape Memory Alloy Peristaltic
Pump Mechanism
North Carolina School of Science and Mathematics
HUNCH Project
April 2013
Flight Date: April 9th
Durham, North Carolina
Teacher: Dr. Myra Halpin
Students: Max Tucker, Catherine Farmer, Matias Horst
Introduction
Critical problems stand between the human race and long-term deep space flight; one of
these is the need for an easy to control, low-noise, and reliable pump mechanism. Pumps
currently used on the space station do not meet these criteria; a number of prominent, recent
media events highlight this. The goal of this project was to construct a peristaltic pump
prototype that would address these issues in microgravity pump engineering. We sought to
observe the effects of microgravity on the pump mechanism and determine whether it could be a
functional alternative to current devices. Conclusions based on the research will aid in designing
a more effective pump mechanism next year.
Abstract
In any spacecraft, plumbing is of vital importance. Pump mechanisms used today in the
International Space Station are prone to frequent malfunction due to their large number of
moving parts. We propose an alternate mechanism, a biomimetic pump that employs peristalsis,
the sequenced contraction of segments used by the human digestive tract. To this end, we
constructed a pump prototype that employs Nitinol wire, a shape memory alloy, to cause
contraction in a segment of tubing. The wire is actuated using resistive heating through the
application of electrical current to the wire. Four wires are contracted in sequence using an
Arduino microcontroller. These wires press upon a length of natural rubber tubing looped
around a Lexan plate, to which the entire mechanism is attached. After flying the project in the
NASA Reduced Gravity Program Microgravity Plane, we obtained mixed results. Though we
were unable to obtain quantitative data, we demonstrated the safety of the project and the
viability of resistive heating-actuated shape memory alloy as a means of contracting the pump.
We plan to continue development and miniaturization of the pump for use in orbital and deep
space applications.
Research Question
Plumbing in space is extremely different from plumbing on Earth, chiefly in that the lack
of gravity renders ineffective conventional pumps designed for use in environments with gravity.
To provide running water in space, NASA employs a system of pumps and fans which coerce
liquid to flow in the right direction. While the aforementioned system works well, the pumps
often break or malfunction due to the sheer number of pieces and systems within the pump
system. NASA is forced to take multiple pump systems on each spaceflight, which takes up
valuable space that could be used for other purposes. In addition, fixing a plumbing problem in
space is very difficult. The only way to repair pumps in space is to haul up spare equipment and
perform repairs that take up an astronaut’s valuable time. Furthermore, a pipe malfunction in
spaceflight can be deadly to the astronauts on board. Pumps on spacecraft handle a variety of
jobs, including heating, cooling, and recycling of water. Any failures in the system could lead to
insufficient water to maintain life support on the spacecraft. Current small scale pumps, with
dozens of moving parts, are often prone to failure because they tend to wear easily. Even when
the pumps are functioning perfectly, they are inefficient because energy used to actuate the pump
is wasted because of friction and heat in between moving parts. Much of the energy used in
current mechanical systems, especially fans like the ones employed by the pump system, is
converted to noise pollution, a major aggravation and health hazard for astronauts. The goal of
this project then is to design a better, more efficient pump that will be very reliable and less
prone to breakage in microgravity.
Several concepts that are employed in design of the pump are vital to an understanding of
our work. Peristalsis is a sequenced contraction of muscle rings to induce relative flow between
a surface and the contents of that surface. Shape memory alloys (SMAs) are metals that change
crystal phase when they cross a temperature transition. This change in crystal phase in
longitudinally aligned SMAs induces a change in length, resulting in contraction when heat is
applied. Integration of these concepts has allowed engineering of peristaltic pump.
Method
How did your research begin?
In considering a variety of applications for biomimetic engineering principles, peristalsis came to
the forefront after an MIT robot employed it as a highly-stable means of movement. The human
esophagus uses peristalsis in a different manner, to push a combination of liquids and solids from
the throat to deeper in the digestive system where they are processed. A pump therefore seemed
to be one potential application of peristalsis.
Upon brainstorming a list of the problems at the forefront of aeronautics development, we saw
that pump failures were a regrettably frequent occurrence in microgravity. Reasoning that a
biomimetic pump mechanism would use fewer parts and consequently demonstrate less potential
for failure, we began to design a pump utilizing peristaltic motion.
Describe your experiment setup.
The first challenge in designing a peristaltic motion-based pump is that of developing a means of
sequentially contracting the tube composing the pump body. Shape memory alloys provide a way
to do this. To induce temperature change necessary for SMA contraction, resistive heating is
employed. When electrical currents are passed through the material, electrical energy converted
into heat, inducing contraction. We found an SMA ideal for our purpose based on its electrical
properties, contraction ratio, and price: nitinol, a nickel titanium alloy developed in the sixties at
the naval ordnance lab.
The pump itself consists of a length of 3/8-in diameter natural rubber tubing attached to a Lexan
baseplate. The tubing comes into contact with four pieces of nitinol wire stretched taut over the
tubing on one side. To increase compression, we threaded small ceramic cylinders onto the wire
to rest on top of the tubing. The wire was connected to a power source and an Android
Development Kit (ADK) 2011, which was programmed to send electrical impulses to each wire
at regular time intervals. The core electronics system of the ADK was a Arduino Mega board,
but the more advanced electronic set-up was selected to demonstrate scalability and facilitate
integration with more modern systems.
For flight, the pump was filled with water and placed into a Lexan box to meet fluid containment
safety standards. To test whether or not the pump worked, we deliberately left an air bubble in
the pump after it had been filled and placed a camera on the outside of the box to track the
progress of the bubble.
What was your hypothesis?
We hypothesized that our shape memory alloy actuated peristaltic pump would be able to
successfully transport water in microgravity.
What research did you do prior to your flight?
Prior to flight we read a body of research papers relating to the topics of microgravity pump
mechanisms, peristalsis, and shape memory alloys. We used these sources to create a paper
about the pump, the concept of which we submitted to the 2012 Exploravision research
competition for an honorable mention.
What tests did you do to prepare?
We first ran tests of the ADK output to confirm that appropriate electrical inputs were being
applied to the Nitinol wire. After these tests proved successful, we attempted dry ground testing
of the product. The nitinol wire was observed to contract when current was applied to it.
Ground testing of the filled pump mechanism did not demonstrate discernible water flow. We
reasoned that terrestrial resistance was sufficient to prohibit flow and consequentially felt that the
unique environment of microgravity would be an appropriate testing ground for the product in
this state.
Results
We recorded approximately one hour of footage of the pump in microgravity. Movement
of the water/air interface could not be detected in this footage. Several other observations
resulted: safety procedures allowed the circuitry of the pump to function throughout the flight,
effectively containing water and separating it from electrical components. After the flight,
greater tension in the Nitinol wire was detected, showing that some contraction of the SMA
occurred during flight.
Discussion
Based on the lack of detectable flow during the experiment, we concluded that the pump
in its current state was ineffective in pumping water in microgravity. Results provided several
conclusions about potentials for future work. The same basic design for the pump can be
maintained because it is effective in maintaining safe containment of liquids. Based on the
change in Nitinol tension, consideration should be given to implementation of an elastic force
acting against the Nitinol to restore it to original length. As contraction of the Nitinol did not
result in flow, several modifications should be made. Computational models of water flow in the
pump can be constructed and optimized to calculate ideal sequential contraction of the pumps. A
one-way valve, placed at one side of the line of pumps, could aid in one-directional flow.
Another future change to facilitate quantification of data is replacement of the gas with another
liquid that is insoluble in water to prevent potential expansion of the liquid. This would
guarantee that movement in the correct direction was readily measurable and would make any
flow easier to detect. Miniaturization of the project can be readily advanced, as the pump
components may be coiled to adapt to smaller dimensions. The primary mass of the pump, a
power supply, can be replaced with a smaller, more specialized model. By continuing testing of
this project aboard the microgravity plane, we hope to construct a final project suitable for
spaceflight.
Conclusions
What did we learn?
We learned that while a peristaltic pump may still be a viable mechanism for microgravity
pumping, our current design is insufficient to induce flow. Continuing to engineer the pump will
be vital if progress is to be made in the field.
Now that you have tested your experiment, what would you change if you were to re-test
the experiment again?
Re-testing of the project would occur with an elastic piece to re-stretch the Nitinol wire, with
an optimized contraction pattern, and with a liquid to be pumped composed of two immiscible
similar molar mass parts.
How would the research you conducted contribute to NASA’s goal of heading back to the
moon, on to Mars and beyond?
In order to effectively return to the moon or continue deep space exploration, long-lived and
stable pump systems are necessary. Current pumps lack the stability to provide safe and efficient
transport for astronauts aboard length flights. As the distance between missions and supply
bases increases, the need for stable and reliable parts increases correspondingly. Long-term
exposure to noise pollution from pumps may be negative, giving this peristaltic pump another
advantage over conventional mechanisms. In conclusion, this project could forward NASA’s
goal of developing safe support systems to expand the bounds of human exploration.