Shape Memory Alloy Actuated Peristaltic Pump for Use in Microgravity

Shape Memory Alloy Actuated Peristaltic Pump for Use in

Microgravity

PID: 2641S

Toshiba ExploraVision 2013

Maxwell Tucker, Matias Horst, Christopher Zhen, and Catherine Farmer

Mentored by Dr. Myra Halpin

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. This peristaltic pump would function by using a pipe embedded with Nitinol, a

shape memory alloy actuated by electrically derived resistive heating. In addition, the

walls of the tubing would be surfaced with hydrophobic materials, creating a passive

flow system. This mechanism will entirely eliminate the need for moving parts and

would increase the pump reliability, as such a mechanism would only fail if the tubing

itself breaks. In time, this technology can expanded for use in systems on Earth,

increasing the reliability of pumps everywhere.

PID: 2641S Shape Memory Alloy Actuated Peristaltic Pump for Use in Microgravity

Present Technology

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 astronauts 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 [1]. 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 [6].

One of the most essential parts of our pump is Nitinol. Nitinol is a shape memory alloy

(SMA), which has the ability to assume two distinct structures depending on temperature.

This occurs because of a unique change in crystalline structure that occurs when the alloy

is heated past a transition temperature. When at a lower temperature, the alloy is in its

Marstenite crystalline structure, which is an oblong, narrow shape, but when heated, it

transforms to the Austenite structure, which in contrast is a perfect cube. The difference

between these two structures is what causes the unique properties of the alloy. As the

crystalline structure changes, the Nitinol becomes shorter due to the change from the oblong

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to cubic shape [8].

Image showing the various crystalline phases of Nitinol [2].

The Nitinol wire we used in our prototype to use is small in diameter, 0.2mm, and

contracts when heated. Because of its small diameter, the wire has a higher surface-area to

volume ratio and therefore has high resistance. The method we use to actuate our Nitinol

wire is known as Joule heating, or resistive heating, and involves using the resistance of the

wire to generate heat. Because heat is a natural byproduct of electrical current and the

amount of resistance in the wire, the higher the resistance of the wire, the more heat can be

produced.

Another feature that will be implemented in our pump is a hydrophobic tube lining.

This system will passively encourage a degree of flow in the pipe, repelling water from

the internal surface [10]. A wide variety of substances in common use are hydrophobic.

Superhydrophobicity has been created through physical processing of a variety of plastics [9].

Using polyolefin sputter coated in noble metals, researchers have been able to create a

surface of hydrophobic polydimethylsiloxane, which in turn served as a mask for polystyrene,

polycarbonate, and polyethylene. Parallel methods could readily be used to cast frames for

the pump as well as endow superhydrophobicity upon its inner wall. In addition to repulsion

of the water, the surface would function as an antibacterial agent by decreasing adhesion of

bacteria to the walls of the tube and by decreasing the free energy of the system [11]. Given

NASAs closed loop system, minimizing bacterial growth and establishment in the system

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are vital. Current processes exist to modify the inside of tubing for optimal hydrophobicity.

The combination of these technologies, developed through a long history, will continue to

evolve into more efficient future designs.

History

During the millennia since the invention of the first pump, the variety of applications

for this simple tool has widened, necessitating new pump designs. The peristaltic pump,

invented in 1887 by an American doctor, Eugene Allen, was first designed to transfer blood

between patients [4]. Since the internal pump mechanisms did not contact the blood itself,

lysis of the blood cells did not occur. Peristaltic pumps were optimal for applications where

contact between the fluid being pumped and mechanical parts can be either detrimental to

the liquid or the pump mechanism: corrosive chemicals, high viscosity liquids, and biological

substances that are prone to decay [15].

A second invention vital to our project is the shape memory alloy. In 1932, Arne Olan-

der used heat to restore a deformed silver-cadmium alloy to its original shape. A pair of

researchers, Chang and Read, studied the mechanisms behind this transformation nineteen

years later, exploring the transitions between the martensite and austenite crystal struc-

tures through x-ray crystallography. In 1962, the naval ordnance laboratory, led by William

Buehler, focused on designing a number of other alloys, among them an equal molar com-

pound of nickel and titanium. Named Nitinol (Nickel Titanium Naval Ordnance Laboratory),

the substance remains the most commonly used SMA and is used by our prototype [14]. Ac-

tuators employed in peristaltic pumps are often designed out of shape memory alloy, but

rarely employ wave-based motion [16].

NASA’s unsuccessful use of conventional pumping systems on several space flights moti-

vated research into peristaltic pumps. Consequently, several 1986 projects focused on devel-

oping peristaltic pumps for microgravity environments. These new pumps were hermetically

sealed and, unlike previous pumps, had a mechanism to leave the pipes controlled by the

pumps uncompressed in their off state. An extension of this was a piezoelectrically actuated

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peristaltic pump developed in 2004. Using electricity, metal was expanded and contracted

in a specific wave pattern that induced peristaltic flow [5].

Future Technology

The pump which we have designed mimics peristalsis, a mechanism that is used to great

effect in the digestive systems of many organisms. While a number of pumps exist that

use this mechanism, they are only effective in specific situations. Our pump differs from

previous peristaltic pumps in that it more accurately mimics the process used in the body

through the contraction of Nitinol wires. This manner of activation mimics the contraction

of muscles in the body and removes the need for moving parts.

One of the major pitfalls of current technology is the limited contraction of Nitinol wire.

On average, a contraction of about 5% of the total length of the wire can be expected [8] [2].

In our pump design, this limited contraction does not allow for full occlusion of the tubing,

preventing the pump from operating at full efficiency; it will not be very effective when

under the influence of Earth’s gravity. In the future, better SMAs, with more efficient

contractions, may allow the construction of a pump that fully occludes the tubing and thus

prevents backflow entirely. With this improved technology, this type of pump could see

use in almost any conceivable situation, and its high reliability would make it attractive for

almost any application in which pumping is necessary.

The development of a readily applicable superhydrophobic surface for the interior of our

pipe will have progressed notably over the next twenty years. A multitude of fields exist that

may lead to the development of hydrophobic surfaces. Self-assembling monolayers offer a

relatively high-cost, yet high-value, method for chemically generating hydrophobic surfaces.

Xerogels, a type of solidified silicon-based gels, promise to be more long-lasting as a means of

hydrophobic coating [12]. Fluoropolymers, another readily applicable hydrophobic surface,

are very low-cost, pliable, and flexible in application; however, recent concerns over their

potential toxicity removes them from consideration as a reasonable chemical family [17].

Future developments may include either safe fluoropolymers or another plastic with similar

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properties. Present technologies allow physical modification of some plastics to generate

hydrophobicity; if these technologies are simplified and made applicable to more plastics,

then they can be employed on the peristaltic pump [9].

Another technology that must be improved upon is the embedding of SMA wire into

rubber tubing. Currently, there are very few examples of wire embedded tubing. Those

that do exist are largely used in the medical field and are too rigid to be useful in this

application. Therefore, significant research must be done to create tubing which is both

flexible and embedded with a shape memory alloy. Furthermore, it is important to consider

the placement of SMA wire in the tubing. A pattern will need to be devised which delivers

the best compromise of force and contraction, allowing for the most efficient movement of

fluids through the pump mechanism.

Breakthroughs

Although NASA recognizes that there could be improvements to both the efficiency and

reliability and our product idea addresses both concerns, there are several problems that

must be addressed for peristaltic pumps to become a viable solution in space and perhaps

even Earth. These problems are not simple design problems and will require technological

breakthroughs that we hope will be available in the future. The biggest obstacle in the way

of our pipe is the low efficiency of our SMA wire. As we previously explained, SMAs are

specially made metals which have two crystalline structures that vary based on temperature

and stress. This change from the martensite to the austenite crystalline structure when the

SMA is heated causes the change in shape or, in our case, contraction. The only problem

with this is that the maximum contraction for the most efficient SMA, nickel-titanium, is

only around 8 percent before any deformation is permanent [2]. This 8 percent is definitely

not ideal for use in our peristaltic pump and in the future, scientists may discover more

efficient SMAs that deliver more contraction. Another problem with current SMAs is that

because they deform with heat and need to cool back down to change back to their original

shape, SMAs change shape very slowly. Though the heating process is very quick, the cooling

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process is significantly slower and, for the purposes of the pump, the wire takes too long to

expand. In our pump, this would be a problem because it would limit the amount of water

flowing through the tube and the speed of the water current. Ideally, in the future there will

be an SMA with a lower activation temperature which would take less heat and, as a result,

time to contract and expand. SMAs are a relatively new technology and can be improved

with some more research.

Other than the actual SMA wire that we use there are several other things that we could

improve with further research. One problem that we ran into while designing a prototype

was that if the wire was wrapped too tightly around the piping, the wire would come in

contact with itself, causing the system to short-circuit and not function correctly. This can

be solved if we find an effective insulator that would prevent the transfer of electricity, but at

the same time, be able to expand and contract with the wire. This electrical insulator would

also have to allow heat to pass through so that it does not interfere with the cooling and

heating of the wire. The final area of improvement is finding suitable piping material. The

actual material for the pipe needs to be both strong and easily compressible. Though there

are materials that satisfy our needs right now to an extent, such as certain types of Tygon

and rubber tubing, as technology advances, there will be new materials that are stronger

and more flexible which would improve our pipe. Similarly, since our final idea involves a

piping that already includes the SMA wire wrapped and embedded inside, in the future, a

pipe like this can be mass-produced to make it more efficient and cost-effective.

Design Process

Before arriving at our current idea of using a SMA-embedded pipe in a peristaltic pump

we considered several other possibilities that would accomplish our task of efficiently trans-

porting water in microgravity. First, we attempted to develop passive control mechanisms

for water. The polar liquid exhibits remarkable properties on earth due to its cohesive and

adhesive nature. By manipulating the hydrophobicity and hydrophilicity of the surfaces,

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we can control the flow of the liquid. Recent research into amphipathic films allowed de-

velopment of surfaces to collect atmospheric moisture. A cylindrical pipe, the interior of

which was coated with a hydrophobic substance, such a self assembled monolayer, would

repel water. If a core composed of another extremely hydrophobic substance were threaded

through this pipe, positive pressure away from the center and towards the wall of the pipe

would develop; countered by the effect of the hydrophobic pipe wall, water would be force

out of the tube. Whenever water was removed from this pump system, water from elsewhere

would be repelled from its location and would diffuse towards the newly opened volume .

However, this design is not optimal for several reasons. First, it is only optimal for water

and only functional for polar liquids. Common coolants, such as liquid nitrogen, as well as

certain substances necessary for atmospheric controls, such as liquid oxygen, are nonpolar;

this system would be useless for controlling their movement. Additionally, this system ex-

hibits exceedingly passive control over the liquid. Pressure is sufficiently minimal the that

backflow becomes a problem; if outside forces oppose the flow of the liquid, this pump design

lacks the force to oppose.

Our next development was the use of shape memory alloys to cause the flow of water

through a segment of tubing via peristalsis. This mechanism was inspired by the natural

movement of earthworms and research on autonomous worm-like robots from MIT [7].

The MIT Meshworm, which uses SMA wire and peristaltic movement to propel itself.

However, the contraction of Nitinol at this time is not sufficient to cause peristaltic

movement when simply wrapped around the a segment of tubing. This lead us to our next

design iteration. Though this design would be very efficient because contraction from Nitinol

wire around the pump would act in all directions, pressing both down along the pump as

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well as contracting it sideways. This type of contraction would be ideal because, taking

advantage of waters adhesion and cohesion, a contraction of the pump along its horizontal

axis would aid contraction in the vertical direction move the water. A major problem that

we ran into is that having the wire wrapped around the outside of the wire would be an

electrical hazard. Since the Nitinol wire is being actuated by an electrical current, having

electricity flowing along an exposed wire on the outside of a pump would be a problem

as it could come in contact with other parts of the spacecraft or the astronauts themselves.

Similarly, if the piping broke, water, or whatever substance that was inside the tubing, would

immediately come in contact with electricity flowing along the Nitinol wire and could cause

even more severe damage. Another problem with wrapping the wire would be that the wire

could possibly slip around and come into contact with itself, causing it to short-circuit. This

would be a large problem because it would cause the whole pump to fail.

A third idea that has been explored was to place the water-filled tubing on a stiff back-

plate. This allows for the use of a segment of Nitinol SMA attached to each end of the plate.

With this innovation, the contraction of a long length of Nitinol can be focused into a very

small region, allowing us to overcome the limitations of the small percentage of contraction

in current SMAs. A prototype of this design has been created, and will be flown on a NASA

microgravity plane this spring as part of the High school students United with NASA to

Create Hardware (HUNCH) program.

The prototype pump, set to fly in microgravity in spring 2013.

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However, this design also possesses many drawbacks. In order to optimize the percent-

age of Nitinol contraction on the tubing, we would have to sacrifice the multi-directional

contraction that was the key factor in the first iteration of our shape-memory alloy pump

and replace it with only lateral contraction, decreasing water flow inside the pump. This

design is also less space-efficient; the greater length of wire necessary to induce contraction

could impede already cramped space-craft operation. By the nature of the design, a wider

back-plate will result in a greater degree of contraction. This also suggests that the back-

plate needs to be at least wide enough to provide significant contraction; we approximated

this width using trigonometry and concluded that the length of the Nitinol wire must be at

least 25 times the inner diameter of the piping used. For example, our prototype was built

with in-diameter piping, so we needed a back-plate with a width of at least 10 inches for the

piping to completely contract. This extra width added to the design adds to the bulkiness

of the system and makes the design as a whole not ideal.

Ultimately, we decided that the best solution would be to draw from all of the above

ideas and create an integrated system that would be simple to use and install, as well as

more efficient than any of the above ideas alone. Drawing from our first rejected concept, the

interior of our pump tubing can be coated with materials of varying levels of hydrophobicity.

This will allow for the passive flow of water through the tubing. To create an active pressure

gradient, shape memory alloys will be embedded in the wall of the tubing and contract in a

peristaltic motion. This design eliminates the need for a hard back-plate, and and allows for

much simpler installation and use. Wire-embedded tubing would also alleviate the electrical

hazard present in wire-wrapped tubing because the pipe wall would serve as an insulator

for the wire. This design results in a pump which is extremely efficient, requires no moving

parts, and which can transport a large variety of fluids in microgravity.

Consequences

The potential benefits from adoption of our pump both by NASA and by others would

be innumerable. The simple design and low number of parts needed make the pump easy

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to repair in spaceflight, allowing the space normally taken up by redundant systems needed

in case a traditional pump broke to be used for other purposes. In a shuttle with a highly

specific weight capacity, the extra space could be used to maximize mission productivity by

carrying different experiments or holding new equipment. In time, the SMA pump could

also be used on Earth in both private homes and larger endeavors. Repair and maintenance

of such a pump would be much simpler than that of a traditional pump, as any leaks or

breaks would be easily detectable and quickly rectified.

While we believe that the SMA pump would have a largely positive influence on society,

there are also some potential drawbacks to incorporating this technology into homes. One of

the main problems would be reduced water flow, as an SMA pump cannot rival the power of

a traditional pump in a gravity environment. The slow contraction of the SMA means that

water is not forced quickly through the pump system, resulting in reduced water pressure

and flow. However, this would probably be less of a problem in space, as the water would

not be inhibited by gravity and would continue to move with little resistance. Another

problem with the SMA pump technology is the cost of the pump. At its current price, the

amount of SMA necessary to create a full-size working pump is far more expensive than the

cost of an ordinary pump of equal size. Unless the cost of the SMA decreased dramatically,

the SMA pump technology might be inaccessible to many who would otherwise benefit by

it. After prolonged use, the heat applied to the SMA permanently deforms the wire. While

replacement of the alloy would not be difficult, it could be expensive and also environmentally

unsustainable, especially if the pump is adopted for widespread use.

Overall, the biggest consequence of adoption of a peristaltic pump would be that piping

on space and Earth would be simpler, and eventually, more efficient. As was previously

mentioned, the simple design would allow fewer repair materials to be taken on the flight,

leaving more room for scientific experiments and innovation, which forms the core of the

space program. As space colonization becomes more prevalent in the distant future, the

need for pumps that are efficient in microgravity will increase. If the peristaltic pump we

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designed is used, pumping would become more efficient and easier to assemble and fix. Our

final design of a Nitinol wire embedded in a length of tubing would be much simpler than

the extremely complicated system used in current NASA ships. A simpler design is useful

because it can be transported pre-assembled and would be extremely easy to install and

maintain. Installation would be as simple as hooking up the pipe and setting up a power

system that would provide a stable source of electricity and prevent against spikes which

could short the Nitinol wire. If a section of piping fails due to a shortage of the wire or

breakage of the pipe, it can be replaced by removing that section of piping and adding a new

piece of piping and rewiring the Nitinol wire inside. Our pump is also more energy efficient

because the amount of electricity needed to power the length of piping is significantly less

because the electrical energy is used to directly power the pipe by generating heat instead

of being converted into mechanical energy.

The pump could also be very beneficial in many applications on Earth. For example, such

a pump could dramatically change medical systems, including the IV pump, which already

uses a form of peristalsis. It would be a simple matter to replace current IV tubing with

tubing embedded with SMA wire, and such a change would decrease the machines chance of

failure . There are also many other medical applications in which a micro-section of a wire-

embedded tube could be beneficial, such as stents and major artery replacements. The pump

could also be used on an industrial scale as a closed-loop cooling system. The pump could

take in water from any source, pump it through a system to cool it, then return the water to

its original source without contaminating the water or the system it is cooling. The potential

applications for the pump are numerous; we believe that our design of a Nitinol-actuated

peristaltic pump has the potential to greatly benefit society as a whole.

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Education

Shape Memory Alloy Actuated Peristaltic Pump for Use in Microgravity