In the OSTI Collections: Shape-Memory Materials

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Article Acknowledgement: Dr. William N. Watson, Physicist DOE Office of Scientific and Technical Information

Shape-memory alloys

Shape-memory polymers

Transformations between shapes with different intrinsic curvature

References

Journal articles available through DOE PAGES^Beta

Reports available through OSTI's SciTech Connect

Patents available through DOepatents

Additional references

Take a typical bendable or stretchable object and bend or stretch it a little.  When you let go of it, it returns to its original shape, as its atoms relax back into the stable arrangement they had before.  But bend or stretch the same object hard enough (without breaking it), and it will stay bent or stretched, without returning to its original shape.  In this case, your effort has moved the atoms into new stable positions, from which they don’t automatically snap back into their initial arrangement. 

If you then try bending or compressing the object back the other way, it may not end up in quite the same shape that it had originally, though it might get close.  The object’s atoms in that case move out of their new stable arrangement, but they don’t retrace the paths they took earlier.  Instead, at least some of the atoms diffuse along new paths, so they get rearranged somewhat and the object ends up in a different shape than it had before. 

Yet some objects are made of materials whose atoms evidently do retrace their paths under certain conditions instead of randomly diffusing along new paths.  Such an object can be stretched, bent, crushed, or otherwise distorted, and then, under an appropriate stimulus—heat, magnetic fields, or something else—its atoms will backtrack their motions closely enough to return the object to its original shape.  Some materials can even, after proper treatment, shift back and forth between two or more different shapes under appropriate stimuli. 

Materials that can “remember” different shapes in this way were discovered in the mid-20th century.  The first known “shape memory” materials were alloys; ceramics and polymers have also been found to exhibit shape memory.  People have continued to discover more shape-memory materials, invent new devices based on their behavior, and learn in more detail how their atoms move to precisely restore their shapes, and have even begun designing materials that will have shape memory.  Some of the range of recent activity in these fields can be seen from reports of work available through DOE PAGES, DOepatents, SciTech Connect, and E-print Network. 

Shape-memory alloys

One of the first known shape-memory metals is a 50:50 alloy of nickel atoms and titanium atoms^[Wikipedia] named “nitinol” (for its elements’ chemical symbols “Ni” and “Ti” and the initials of the U.S. Naval Ordnance Laboratory^[Wikipedia] where it was discovered^[WOLAA]).  Despite its being the most widely used shape-memory alloy, the precise way its atoms shift when nitinol objects change shape, and the forces driving the shift, have long been unclear.  According to a report in Physical Review Letters by scientists at the Ames Laboratory and Iowa State University, many attempts to deduce this process have relied on intuition but haven’t worked.  The authors had already shown in a recent publication that the arrangement of atoms in the high-temperature phase of nitinol was more complex than the arrangement long assumed—which, as it turns out, was not a stable arrangement anyway.  The Physical Review Letters report, “Shape-Memory Transformations of NiTi: Minimum-Energy Pathways between Austenite, Martensites, and Kinetically Limited Intermediate States”^{[DoE PAGES]}, describes the authors’ demonstration that a different rearrangement of atoms between lower- and higher-temperature states better fits the known shape-memory behavior of nitinol. 

Figure 1.  Schematic diagrams of atomic arrangements relevant for calculating the reversible motions in the shape-memory alloy nitinol.  The smaller nickel atoms are represented as yellow, the larger titanium atoms as blue.  The arrangements at the upper right of both parts of the figure represent the newly discovered stable high-temperature arrangement as viewed from two different directions.  (After “Shape-Memory Transformations of NiTi: Minimum-Energy Pathways between Austenite, Martensites, and Kinetically Limited Intermediate States”^{[DoE PAGES]}.) 

Other aspects of shape-memory materials that weren’t taken into account in the Ames/Iowa State analysis were discovered at the University of Michigan in experiments that resolved details on the order of one nanometer in size—the width of a few atoms.  Among the chief findings:  the transformation between different atomic arrangements, or phases, is not uniform throughout the alloy, but is very heterogeneous within individual crystallites (grains); grains can contain numerous variants of a single phase; and grains whose atoms are similarly aligned don’t transform similarly from one phase to another.  The Michigan group’s work, over the five-year funding period, used new combinations of scanning electron microscopy, diffraction, digital image correlation, and custom testing equipment to understand, at different levels of detail, the mechanics of how shape-memory alloys deform and fail, so that optimal uses of the alloys’ shape memory and other properties could be determined.  As is common with multiyear research funding, the work was not only reported in various journal articles as it progressed, but it was summarized at the end of the funding period in a single report (“Deformation and Failure Mechanisms of Shape Memory Alloys”^{[SciTech Connect]}).  Such final reports can provide a perspective that journal articles, with their focus on one or a few intermediate findings, often lack. 

Figure 2.  Example of the progressive, heterogeneous transformation of a shape-memory alloy in one microscopic region.  In the pictures on the right, white lines represent grain boundaries.  Graphs at the left of each picture tell how many pixels represent portions of the alloy that have been strained (deformed) along one axis by varying amounts.  (After “Deformation and Failure Mechanisms of Shape Memory Alloys”^{[SciTech Connect]}.) 

Figure 3.  An illustration that similarly oriented grains of a shape-memory alloy do not transform similarly during a shape-changing transformation of the material’s phase.  Represented here are the grains observed within an approximately 100-micrometer  100-micrometer area that were misoriented less than 5° from the direction in which the alloy sample was stressed.  Even grains whose misorientations are similar (represented by points near each other along the graph’s horizontal axis) had quite different changes in dimensions (different strains), as shown by the great variation in the points’ positions with respect to the vertical axis.  (After “Deformation and Failure Mechanisms of Shape Memory Alloys”^{[SciTech Connect]}.) 

While the reasons that some alloys can retain a “memory” of their shapes under various conditions continue to be explored, the shape memory itself continues to be put to work in new ways.  The “Actively controlled vibration welding system and method”^[DOepatents] assigned by patent to GM Global Technology Operations LLC describes a mechanism for welding a workpiece with ultrasound or other mechanical vibrations.  The workpiece is clamped between an anvil and a vibrating welding horn.  If the welding horn causes the workpiece and anvil to vibrate in unwanted ways, the workpiece’s weld can be degraded, so the invention counteracts this with an actuator in the anvil assembly that receives compensating feedback or feedforward signals.  The actuator can be made of a shape-memory alloy, or a piezoelectric material that stretches or shrinks in response to an electric field, or some other “smart” material whose shape change in response to the compensating signal steadies the anvil, which results in a better weld. 

Two other patents, both assigned jointly to GM Global Technology Operations and to Dynalloy, Inc. with the second also assigned to the Regents of the University of Michigan, describe “Shape memory alloy heat engines and energy harvesting systems” ^{[DOepatents, DOepatents]}.  Both patents describe heat engines that consist of at least two pulleys connected by a loop of prestretched shape-memory alloy, different parts of which are at different enough temperatures to be in different phases.  Each of these pulleys is also attached to a timing pulley, both of which are connected by a timing loop.  The diameter ratio of the two timing pulleys is different from the diameter ratio of the other two pulleys.  In this condition, the shape-memory alloy loop applies a torque to the pulleys, so that thermal energy absorbed by the loop at the warmer temperature is turned into mechanical energy to drive the pulleys.  As the pulleys and loops rotate, previously cooler portions of the shape-memory loop approach the warmer temperature and absorb heat, while previously warmer portions of the loop cool down as they move away.  Thus the system is driven as long as the warmer and cooler temperatures are maintained, harvesting thermal energy and turning it into mechanical energy of the loop and pulleys, which can be used to drive other machinery. 

Figure 4.  From “Shape memory alloy heat engines and energy harvesting systems” ^[DOepatents].  38 and 40 are drive pulleys in the heat engine’s warm region 18, 39 and 41 are timing pulleys connected by a timing loop 43, 42 is an optional idler pulley in the engine’s cooler region 20, and 22 is a drive loop made of shape-memory alloy.  With the drive pulleys’ size ratio differing from that of the two timing pulleys, the temperature difference between the heat engine’s warmer and cooler regions produces a varying tension along the shape-memory loop that results in a torque on the pulleys. 

The aforementioned shape-memory materials undergo non-diffusive rearrangements of their atoms when their temperatures change, but other shape-memory materials change their phase in response to stimuli other than heat.  The patent “Actuation method and apparatus, micropump, and PCR enhancement method”^[DOepatents] assigned to Boise State University describes a use of materials that change to “memorized” shapes when actuated by magnetic fields.  The specific shape change here is to provide a pumping action in a microfluidic^[OSTI] device, particularly one that pumps a DNA sample through a channel toward a reagent that will produce multiple copies of the DNA to facilitate its analysis.  While other microfluidic devices use large external pumps to drive fluids through them, pumps that use an alloy’s shape-memory cycle to pump microliters of fluid can be small enough to be part of the microfluidic device itself.  For some pump designs, the magnets whose rotation stimulates cyclic changes in the pump’s shape can also be as small as the microfluidic device. 

Figure 5.  Drawings from the patent “Actuation method and apparatus, micropump, and PCR enhancement method”^[DOepatents].  In one embodiment^[Wikipedia] of the micropump (top left), small amounts of fluid are pumped between openings (97) along a channel (94) as pumping elements (95) made of magnetically-activated shape-memory material (MSM) expand into the channel and contract out of it in turn (top middle) in response to magnetic fields applied to them by magnets carried on a rotor that rotates below the micropump (top right).  In another embodiment (bottom left), a single pumping element (210) pumps fluid between openings (206) as a small magnetized rotor (208) induces cyclic changes in its shape (bottom right). 

A team of researchers in the United States, China, and Australia report a quite different use for shape-memory alloy in “A biopolymer-like metal enabled hybrid material with exceptional mechanical prowess”^{[DoE PAGES]}.  Inspired by the “unusual combination of strength and toughness” of the natural substance nacre^[Wikipedia] (mother-of-pearl) to design a stronger substitute, these researchers considered nacre’s structure of brittle inorganic platelets sandwiched with soft organic layers of biopolymer.  The way nacre’s biopolymer layers deform enable the nacre to dissipate energy and distribute stresses.  The biopolymer elongates more and more, requiring less and less extra stress to stretch further and further, until it stiffens and elongates little more despite a great deal of additional stress, until the stress is large enough to break the biopolymer.  Synthetic polymers are generally not strong enough to duplicate nacre’s pre-fracture behavior.  Conventional metals don’t duplicate it either:  they distort very little under even large stresses until they reach a yield point, then distort a great deal under little additional stress (but by non-uniform atomic dislocations instead of uniform stretching like a biopolymer), and then they break before reaching a point of stiffening.  But shape-memory alloys distort under stress in a pattern that resembles the stress-distortion behavior of nacre’s biopolymer, and their distortions are relatively uniform on an atomic scale. 

Figure 6.  From “A biopolymer-like metal enabled hybrid material with exceptional mechanical prowess”^{[DoE PAGES]}.  Different materials’ variation in strain for given stresses.  Organic biopolymers, whose small-scale shapes at various values of stress and strain are schematically represented in the upper left inset, have a response to stress more similar to that of shape-memory alloys than to ordinary metals.  Inset illustrations for the conventional metals and shape-memory alloys schematically represent atomic arrangements of each at various states of stress and strain. 

The researchers thus experimented with a composite material in which a titanium-nickel shape-memory alloy filled the role of the biopolymer, while the place of nacre’s inorganic mineral aragonite^[Wikipedia] was taken by a titanium-tin alloy.  This composite’s behavior under stress was found superior in compressive strength and fracture strain to similar composites whose soft components are conventional metals; the composite also damped low-frequency mechanical vibrations heavily and attenuated ultrasound.  The researchers considered their experiments to demonstrate a feasible design strategy for developing high-performance bulk materials inspired by biological ones. 

Figure 7.  From “A biopolymer-like metal enabled hybrid material with exceptional mechanical prowess”^{[DoE PAGES]}.  Left:  Scanning electron microscope images of a composite of titanium nickel and titanium tin (TiNi-Ti₃Sn), with Ti₃Sn appearing as light bands and TiNi as dark bands.  Right:  Transmission electron microscope image of the same composite, with inset showing a button ingot of the composite. 

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Shape-memory polymers

While shape memory effects from magnetism or heat are displayed by a number of alloys, any polymer whose structure is a set of crosslinked chains of chemically bound molecules (or monomers^[Wikipedia]) exhibits some shape memory when its temperature changes, as is noted by the authors of the Smart Materials and Structures paper “Microscopic mechanisms of the shape memory effect in crosslinked polymers”^[IOP].  “The term ‘shape memory polymer’”, they point out, “is therefore reserved for materials that display desirable shape holding and recovery properties, since all crosslinked polymers display the shape memory effect to some degree.”  But the mechanisms of polymer shape memory are even harder to determine than those of alloy shape memory, since polymers’ atomic structures are more complicated than alloys’.  Computer-simulating the motions of all the atoms in even a few crosslinked polymer chains requires a lot of computer time, so mathematically demonstrating the causes of shape memory in larger polymers require simplified mathematical models, not unlike the simplifications used in some analyses of protein folding^[OSTI].  The authors of the Smart Materials and Structures paper demonstrate how polymer shape memory results from temperature-dependent mobility of the polymer chains, and the importance for this mobility of attraction between monomers. 

Figure 8.  Demonstrationof the shape-memory behavior of the material PFOEA-95.  1 and 2:  Converting a rigid film to a soft, flexible film by heating with a heat gun; the rigidity change is accompanied by the film’s becoming transparent.  3 and 4:  Twisted soft film “remembers” its shape when it cools down.  5 and 6:  Original shape is recovered upon heating.  (From “Fluorogel elastomers with tunable transparency, elasticity, shape-memory, and antifouling properties” ^{[DoE PAGES]}.) 

As many recent reports indicate, shape-memory polymers appear particularly attractive for medical purposes.  For example, in a paper published in Angewandte Chemie, researchers at Harvard University note that “biological implants, such as catheters and artificial blood vessels, need materials with controllable mechanics while exhibiting sustained biocompatibility and anti-biofouling properties to prevent infection.”  The kind of shape-memory polymers that they’ve developed to meet these criteria are described by the paper’s title, “Fluorogel elastomers with tunable transparency, elasticity, shape-memory, and antifouling properties”^{[DoE PAGES]}, and in more detail in the paper itself. 

Other investigations to improve shape-memory polymers for medical uses have been made elsewhere, particularly by researchers at Texas A&M University and Lawrence Livermore National Laboratory.   One of their projects resulted in “Controlling the Actuation Rate of Low Density Shape Memory Polymer Foams in Water”^{[SciTech Connect]}, with the purpose, according to the 2012 report of that title, of enhancing the foams’ use in a device for aneurysm^[Wikipedia] treatment.  The device would be delivered by catheterization in a compressed rodlike shape to an aneurysm site and carefully positioned to not protrude into the parent artery.  Only then should the device start expanding to fill the aneurysm, but it should fill it quickly.  The device should thus be able to withstand fluids at body temperature without actuating its expansion during the few to several minutes needed for delivery and positioning.  The researchers found that polymers with different composition and varying tendency to repel water would have different actuation rates, from within 2 minutes to over 24 hours—foundational results for a “polymer synthesis toolbox” that they expected would be biomedically useful.  Their report also discusses advantages of longer versus shorter actuation times—the device will work longer if it takes longer to actuate, but short actuation times may be preferable in some cases.  This tradeoff might be avoided if the polymer used is hydrophobic enough to make it easy to deliver and position, and then activated by extra heat at the aneurysm site.  The amount of heat required also depends on the polymer composition, which may be chosen to minimize tissue damage from heating. 

Figure 9.  Top:  Cell structure of shape-memory polymer foams with different compositions.  Scale bars at the lower right of each micrograph represents 500 micrometers (0.5 millimeter).  Bottom:  Foam samples of different compositions (a, b, and c) have different actuation rates (5 minutes to over 50 minutes).  (From “Controlling the Actuation Rate of Low Density Shape Memory Polymer Foams in Water”^{[SciTech Connect]}.) 

A 2013 Texas A&M/Lawrence Livermore report, “Low density biodegradable shape memory polyurethane foams for embolic biomedical applications”^{[SciTech Connect]}, investigated how one might treat aneurysms to even better effect by preparing materials with an additional property:  a definite rate of biodegradability.  “It was hypothesized,” says the report’ introduction, “that if the implant degraded over time after the initial endothelialization^[Wikipedia] of the aneurysm neck, the isolated aneurysmal bulge would eventually atrophy leading to a complete cure of the vasculature.”  While the researchers did find polymer foams that had the desired biodegradability and shape-memory properties, they pointed out that since the degradation rate is directly affected by materials’ water uptake, high degradation rate and low plasticization may not be possible in one material, so they suggested that a different strategy like surface modification might be used to limit plasticization during device delivery.  The materials investigated were also expected to degrade into substances that don’t produce adverse effects, though the authors noted that “animal studies or cell culture tests will need to be performed to fully ascertain the material biocompatibility.” 

Recent shape-memory polymer patents assigned to Lawrence Livermore's managing and operating contractor show that these last two reports describe results of an extensive research program that involves medical devices as well as materials whose functions make the devices possible.  This research has led to several patents for shape-memory polymers and biomedical devices.  A patent entitled “Shape memory polymers”^[DOepatents] describes “new shape memory polymer compositions, methods for synthesizing new shape memory polymers, and apparatus comprising an actuator and a shape memory polymer wherein the shape memory polymer comprises at least a portion of the actuator.”  The uniform placement of crosslinks in shape-memory polymers, so that their shape-transition conditions are sharply defined, is addressed in the patent “Post polymerization cure shape memory polymers”^[DOepatents].  Three patents, two entitled “Stent with expandable foam” ^{[DOepatents, DOepatents]} and the other named “Shape memory polymer foams for endovascular therapies”^[DOepatents], describe stents with skeletal support structure for expanding in an aneurysm or other physical anomaly coupled to a shape-memory foam, and a heating system to trigger the foam’s expansion from the compressed shape it has during insertion to a “remembered” shape for treating the anomaly.  The patent “Shape memory system with integrated actuation using embedded particles”^[DOepatents] describes a device that expands when heated by embedded magnetic particles, which warm up when subjected to alternating magnetic fields—a within-device heating method that could avoid heating body tissue between the device and some outside heat source.  And the patent “System for closure of a physical anomaly”^[DOepatents] describes devices for closing defects, ducts, or punctures in vascular or nonvascular walls whose shape changes are activated by radiofrequency waves, heat, light, or physical stress.  

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Transformations between shapes with different intrinsic curvature

The aforementioned reports either deal with details of how the atomic arrangements change in shape-memory materials, or with ways that such materials can be used.  Many uses don’t require more drastic shape changes than stretching, contracting, or curving along a single direction at any point of the material.  But there are other ways to change the shape of something that require curving the material along more than one axis at the same point.  For example, a spheroidal dent in a horizontal surface is curved at every point along both the left-right axis and the forward-backward axis, as well as other directions in between.  Shape-memory materials that could curve reversibly, along multiple axes through the same points, could be used in new ways corresponding to the greater range of shape changes that such curvature would afford. 

A 2011 paper available through E-print Network, entitled “Blueprinting Nematic Glass: Systematically Constructing and Combining Active Points of Curvature for Emergent Morphology”^[arXiv], describes an approach many researchers are now taking to achieve this.  The authors point out that, while many materials would need to undergo stress to change their curvature along multiple axes at each point, materials that exhibit a nematic or threadlike phase^[Wikipedia], in which their molecules line up along a single axis—possibly different single axes at different points throughout the material—may undergo mulitiaxis curvature changes in a stress-free way.  The authors show how shape-memory devices made of such a material can be designed to change their curvature at every point in specific ways by choosing which direction the material’s molecules tend to align with in the nematic phase.  The same authors discuss progress since 2011 in designing shape-memory devices with such curvature changes, with references to recent publications, in their article “Shape-programmable materials”^[AIP] published in the January 2016 issue of Physics Today. 

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References

Wikipedia

• Nickel titanium (nitinol)
• Naval Ordnance Laboratory, formerly located in White Oak, Maryland
• Patent claim:  Basic types and categories (Explains use of the term “embodiment” in patents.) 
• Nacre
• Aragonite
• Monomer
• Aneurysm
• Endothelium
• Liquid crystal:  Nematic phase

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Journal articles available through DoE PAGES^Beta

• “Shape-Memory Transformations of NiTi: Minimum-Energy Pathways between Austenite, Martensites, and Kinetically Limited Intermediate States” [Metadata]

Ames Laboratory, Ames, IA (United States)

Iowa State University, Ames, IA (United States)

2014-12-24

Physical Review Letters

• “A biopolymer-like metal enabled hybrid material with exceptional mechanical prowess” [Metadata]

China University of Petroleum, Beijing, (China)

University of Western Australia, Crawley, WA (Australia)

Argonne National Laboratory (ANL), Argonne, IL (United States)

Beijing University of Technology (China) [English]

Xi’an Jiaotong University (China)

The Ohio State University, Columbus, OH (United States)

Chinese Academy of Sciences, Beijing (China) [English]

Beihang University, Beijing (China) [English]

University of Virginia, Charlottesville, VA (United States)

2015-02-10

Scientific Reports

• “Fluorogel elastomers with tunable transparency, elasticity, shape-memory, and antifouling properties” [Metadata]

Harvard University, Cambridge, MA (United States)

2014-03-18

Angewandte Chemie (International Edition)

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Reports available through OSTI’s SciTech Connect

• “Deformation and Failure Mechanisms of Shape Memory Alloys” [Metadata]

University of Michigan, Ann Arbor, MI (United States)

2015-04-15

• “Controlling the Actuation Rate of Low Density Shape Memory Polymer Foams in Water” [Metadata]

Texas A&M University, College Station, TX (United States)

Lawrence Livermore National Laboratory, Livermore, CA (United States)

2012-07-06

Macromolecular Chemistry and Physics, vol. 214, October 9, 2012, pp. 1204-1214

• “Low density biodegradable shape memory polyurethane foams for embolic biomedical applications” [Metadata]

Texas A&M University, College Station, TX (United States)

Lawrence Livermore National Laboratory, Livermore, CA (United States)

2013-02-27

Acta Biomaterialia, vol. 10, October 1, 2013, pp. 67-76

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Patents available through DOepatents

• “Actively controlled vibration welding system and method” [Metadata]

GM Global Technology Operations LLC, Detroit, MI (United States)

2013-04-02

• “Shape memory alloy heat engines and energy harvesting systems” [Metadata]

GM Global Technology Operations LLC, Detroit, MI (United States)

Dynalloy, Inc., Tustin, CA (United States)

2013-12-17

• “Shape memory alloy heat engines and energy harvesting systems” [Metadata]

GM Global Technology Operations LLC, Detroit, MI (United States)

Dynalloy, Inc., Tustin, CA (United States)

The Regents of the University of Michigan, Ann Arbor, MI (United States)

2014-09-30

• “Actuation method and apparatus, micropump, and PCR enhancement method” [Metadata]

Boise State University, Boise, ID (United States)

2015-07-28

• “Shape memory polymers” [Metadata]

Lawrence Livermore National Security, LLC, Livermore, CA (United States)

2015-06-09

• “Post polymerization cure shape memory polymers” [Metadata]

Lawrence Livermore National Security, LLC, Livermore, CA (United States)

2014-11-11

• “Stent with expandable foam” [Metadata]

Regents of the University of California, Oakland, CA (United States)
Lawrence Livermore National Security, LLC, Livermore, CA (United States)

2013-05-28

• “Stent with expandable foam” [Metadata]

Lawrence Livermore National Security, LLC, Livermore, CA (United States)

2015-07-14

• “Shape memory system with integrated actuation using embedded particles” [Metadata]

Lawrence Livermore National Security, LLC, Livermore, CA (United States)

2014-04-01

• “System for closure of a physical anomaly” [Metadata]

Lawrence Livermore National Security, LLC, Livermore, CA (United States)

2014-11-11

• “Shape memory polymer foams for endovascular therapies” [Metadata]

Lawrence Livermore National Security, LLC, Livermore, CA (United States)

2015-05-26

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Additional references

• “NITINOL Re-Examination” by William J. Buehler^[WOLAA].  Account of the discovery of nitinol. 
• White Oak Laboratory Alumni Association
• “Microscopic mechanisms of the shape memory effect in crosslinked polymers”

University of Michigan, Ann Arbor, MI (United States)

Smart Materials and Structures

• “In the OSTI Collections:  Determining How Proteins Fold”, section “Investigations Focused on Computation”^[OSTI]
• “Blueprinting Nematic Glass: Systematically Constructing and Combining Active Points of Curvature for Emergent Morphology”^[arXiv]

University of Cambridge (Cambridge, UK)

Available by searching Titles for “blueprinting nematic glass” in OSTI’s E-print Network. 

• “Shape-programmable materials”^[AIP]

Physics Today, Vol. 69, January 2016, pp. 32-38

• American Institute of Physics

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