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Shape-Memory and Shape-ChangingPolymersChristopher M. Yakacki aa Department of Mechanical Engineering, The University of ColoradoDenver, Denver, CO, USA

To cite this article: Christopher M. Yakacki (2013): Shape-Memory and Shape-Changing Polymers,Polymer Reviews, 53:1, 1-5

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Polymer Reviews, 53:1–5, 2013Copyright © Taylor & Francis Group, LLCISSN: 1558-3724 print / 1558-3716 onlineDOI: 10.1080/15583724.2012.752745

Perspective

Shape-Memory and Shape-Changing Polymers

CHRISTOPHER M. YAKACKI

Department of Mechanical Engineering, The University of Colorado Denver,Denver, CO, USA

This article is a perspective that introduces several review articles in the field of shape-memory and shape-changing polymers. It is intended to emphasize the versatility ofthese materials as active, smart polymers as well as highlight the potential impact ofthese materials. The biomedical device industry commonly attracts these materials;however, examples for non-biomedical applications, such as origami structures andtheir recent attention are also discussed.

Keywords shape-memory polymers, self-folding polymers, polymer origami

The field of active, smart polymers has grown dramatically over the past 20 years. Thesematerials by definition are capable of performing a predetermined function in response to astimulus or their environment. For example, the function-stimuli couplings of polymers cannow range from biodegradation or drug elution in response to implantation to changes inmechanical properties and shape in response to heat, light, or electric currents.1,2 Polymershave gained more attention over their metallic and ceramic counterparts partly due to theirsuperior tailorability along with their low fabrication costs. A wide range of synthesistechniques is available such that these materials can be made in a majority of researchlaboratories. The field of smart polymer research is no longer confined to polymer chemistsbut includes an interdisciplinary consortium of biomedical, chemical, and mechanicalengineers as well as materials scientists. Each one of these disciplines brings a new approachto the design and application of these materials. Another reason for increased attention forpolymeric smart materials is their ability for multi-functionality. Combining the previousexamples, a minimally invasive biocompatible polymeric implant can be designed to beimplanted into the body, expanded into place when heated to body temperature,3 andexperience a designed increase in stiffness as biodegradation occurs.4 The combination offunctionalities makes smart polymers extremely versatile and effective as an active material.

This special issue of Polymer Reviews focuses on shape-memory and shape-changingpolymers. Shape-memory polymers (SMPs) are characterized by their ability to transform

Received November 20, 2012; accepted November 21, 2012.Address correspondence to C.M. Yakacki, Department of Mechanical Engineering, The Univer-

sity of Colorado Denver, 1200 Larimer Street, PO Box 173364, Denver, CO, 80217-3364. E-mail:chris.yakacki@ucdenver.edu

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Figure 1. Demonstration of the shape-memory cycle for triple shape-memory in polymers. Thecycle consists of three stages: Programming, Storage, and Recovery. During the Programming Stage,the polymer is deformed in two different configurations at two different temperatures. The samplewill remain in the temporary, programmed state indefinitely when stored below the polymer’s glasstransition temperature. Recovery, or the shape-memory effect, occurs when the polymer sample isreheated. Triple-shape-memory behavior is exhibited when the polymer exhibits three distinct shapesthroughout the shape-memory cycle. Reproduced from Xie29 with permission from Nature PublishingGroup. (Color figure available online).

between two or more shapes on command (Fig. 1). From a simplistic standpoint, the shape-memory effect is enabled by a reversible change in chain mobility associated around athermal transition, such as a melting or glass transition, and driven by entropy elasticity.As a result there are a great deal of polymer chemistries and structures that can satisfythe necessary conditions to promote shape memory.5 If an SMP comes into contact withan object or external constraint during shape change, the SMP will generate a recoveryforce and be capable of performing work. As a result, SMPs can be utilized for bothshape-changing structures as well as actuators.

The potential applications of the shape-memory effect are what make SMPs exciting.SMP biomedical devices have gained the most notoriety over the past two decades.3,6–8

SMPs offer a way to place large, bulky devices into a patient in a minimally invasiveprocedure, in an effort to decrease recovery times (Fig. 2). Shape-memory alloys havebeen used for cardiovascular stents for nearly two decades, but are susceptible to fatiguewithin the strut joints. Polymer-coated, drug-eluting metallic stents have been proposedbut have limited drug-loading properties due associated with the requirements of having athin polymer layer.9 SMP stents offer the potential for increased drug-loading capabilitiesas well as bioresorption to help mitigate these problems.10,11 Another potential area foradvancement is in the field of soft-tissue fixation. Polymeric interference screws have beenthe standard to fixation tendons and ligaments within bone tunnels; however, are proneto complications during insertion. The inherent nature of a screw can induce twisting ofthe graft during insertion, which can result in improper placement, while screw threadscan lacerate and damage the graft if the screw if oversized.12,13 SMPs offer the ability tobe inserted without the potential hazards of twisting or thread damage.14 Furthermore, bycontrolling the polymer structure, the amount of fixation force applied to the graft can betailored. Manufacturing and programming conditions can be controlled such that the surfaceof the implant can be dynamic to provide texture to either enhance frictional properties orcell proliferation and attachment.15

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Shape-Memory and Shape-Changing Polymers 3

Figure 2. Examples of shape-memory polymer (SMP) devices. (Top) An SMP stent is deployedfrom an 18 Fr. Catheter into a 22 mm glass tube at body temperature. Reprinted with permissionfrom 10 from Elsevier. (Bottom) An SMP interference device expands to fixate a bovine tendon.Reproduced from Yakacki et al.14 with permission from John Wiley and Sons, Inc.

These biomedical applications are examples from my own personal research, in whichI have pursued commercialization. This has become an increasing trend within the SMPfield, as there are a number of university-based companies aimed at commercializingbiomedical products based on SMP technology. Many of the authors within this specialissue are involved with ventures to develop novel SMP technologies. The existence ofthese companies helps validate the potential commercial impact of SMPs. As examples,the US market size for medical devices related to knee reconstructions is estimated to beapproximately $500 million in 2012, while the spinal implant market was estimated to be$5.4 billion in 2010,16,17 Shape change in polymers has the potential to significantly impactfields other than biomedical devices. For example, researchers have recently proposed usingactive polymers to induce surface instabilities, such as cratering and crumpling, to serveas a mechanical removal of biofouling.18–20 Biofouling on ship hauls increases drag andsubsequently fuel efficiency. As a result, it was estimated that for a single class of navalships (the DDG51), fuel and maintenance costs associated with biofouling were $56 millionannually.21 The worldwide costs of biofouling increase significantly when these numbersare applied to the entire shipping industry.

There has also been an increase in research attention given to self-folding and self-bending polymers.22–26 The majority of manufacturing starting materials comes in two-dimensional planar geometries. The ability for these materials to actively form three-dimensional structures may lead to a new paradigm of manufacturing design. For example,Ryu et al. recently introduced a method to use SMPs capable of photo-stress relaxation tocreate bends and folds into polymer sheets (Fig. 3).27 Their results showed that the foldingangles could be tailored such that a 6-sided box could be formed from a stretched SMP.Stoychev et al. recently demonstrated thermoresponive folding of microcapsules capableof capturing and releasing cells.28 Interest in these processes has increased to the point thatthe Emerging Frontiers in Research and Innovation (EFRI) office at US National ScienceFoundation (NSF) has recently introduced a new program on origami designs with focuson responsive structures and self-folding.

This special edition is aimed to serve more as a guide to the application of these uniquematerial phenomena rather than serve as an exhaustive review of the underlying mechanismsof shape change in polymers. Sauter et al. introduce how to quantify the shape-memory

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Figure 3. Photo-origami hinges are created by irradiating a pre-strained strip of SMP through aphoto-mask. The exposed portion of the SMP experiences stress-relaxation and thus creates bendingduring shape recovery. Reproduced from Ryu et al.27 with permission from American Institute ofPhysics. (Color figure available online).

effect in polymers as standard characterization techniques do not apply to fully describe apolymer’s ability for shape change, whereas Safranski et al. discuss which requirements arenecessary for biomedical device design. Hearon et al. discusses the applications of porousSMP foams, while Ware et al. discuss how SMPs can be used for smart neural interfaces.Different approaches to modeling the shape-memory effect are covered by Nguyen. Finally,three-dimensional microfabrication using self-folding polymer films is covered by Ionov.

The field of shape-memory and shape-changing polymers has a strong foundation offundamental understanding. While not complete, researchers and engineers have the toolsnecessary to apply this foundation towards novel applications and designs. We are currentlyon the precipice of seeing a boom in these smart-polymer devices; especially in the field ofbiomedical devices. The multifunctional nature of these polymers gives way to an extremelywide range of possibilities, which will not be exhausted for some time.

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