Front. Chem. Sci. Eng.

DOI 10.1007/s11705-017-1632-4

REVIEW ARTICLE

A mini review: Shape memory polymers for biomedical

applications

Kaojin Wang, Satu Strandman, X. X. Zhu (✉)

Département de Chimie, Université de Montréal, C. P. 6128, Succ. Centre-ville, Montréal, QC, H3C 3J7, Canada

© Higher Education Press and Springer-Verlag Berlin Heidelberg 2017

Abstract Shape memory polymers (SMPs) are smart such as tunability of the actuation temperature, good
materials that can change their shape in a pre-defined mechanical properties and easy processability are also
manner under a stimulus. The shape memory functionality important [13]. To obtain materials with such properties, a
has gained considerable interest for biomedical applica- common approach is to design SMPs based on natural
tions, which require materials that are biocompatible and polymers, such as natural polypeptides, polysaccharides,
sometimes biodegradable. There is a need for SMPs that or synthetic polymers derived from natural compounds
are prepared from renewable sources to be used as present in the body [14,15]. For example, polyesters could
substitutes for conventional SMPs. In this paper, advances be built from compounds abundant in human body, such as
in SMPs based on synthetic monomers and bio-compounds lactic acid, glycerol, and bile acids. Biodegradable SMPs
are discussed. Materials designed for biomedical applica- can also be synthesized from synthetic monomers, such as
tions are highlighted. ε-caprolactone (CL) and p-dioxanone (PDO).
Thermo-induced SMPs are the most investigated poly-
Keywords shape memory polymer, biodegradability, mers, and for biomedical applications, the actuation must
biocompatibility, biomedical application, bile acids be triggered by direct heating at around body temperature
(37 °C). The materials may change back to their permanent
shape before implantation if the triggering temperature is
1 Introduction lower than body temperature, while the shape memory
effect (SME) may not be triggered in the body if the
Shape memory polymers (SMPs) are a promising class of actuation temperature is higher than body temperature.
smart materials which are able to change their shape in a Indirect heating methods have also been developed, where
predefined way under appropriate external stimuli, such as the stimulus can be light, electricity, magnetism, micro-
heating, light, pressure, electrical field, magnetic field, pH wave and ultrasound [16–26], and often the systems
and solvent [1–3]. The history of SMPs may be traced back exploiting indirect heating employ functional fillers
to the 1960s, when cross-linked polyethylene was used to incorporated into SMPs that can convert different energies
design heat-shrinkable tubings [4]. Nevertheless, little to heat. The local heating via indirect methods may
interest from researchers was evinced until the 1990s. damage cells and tissues less than direct heating.
SMPs are mechanically functional smart materials that Shape memory effects (SMEs) can be classified to one-
have recently attracted much interest for biomedical way and two-way SMEs. One-way SMEs imply that the
applications, such as sutures [5], catheters [6], the repair shape recovery is irreversible under a reverse trigger. These
of cardiac valves [7], as well as for drug delivery [8,9] and can be multi-stepwise, such as dual (Fig. 1(a)), triple
cardiovascular [10,11], orthodontic, and surgical applica- (Fig. 1(b)) and quadruple SMEs, etc. Two-way SMEs can
tions [12]. Biocompatibility and biodegradability are reversibly change between two different fixed shapes
desirable for biomedical applications and therefore, it is under two different stimuli (Fig. 1(c)), and can take place
very important to evaluate the potentially harmful effects with or without external stress. Two parameters are used to
and the immune response of implants. Other properties, describe dual SMEs: shape fixity ratio (Rf), which
quantifies the ability of the material to fix the temporary
shape, and shape recovery ratio (Rr), which quantifies the
Received October 18, 2016; accepted January 1, 2017 ability of the material to recover its permanent shape.
E-mail: julian.zhu@umontreal.ca To date, comprehensive review papers on SMPs have
2 Front. Chem. Sci. Eng.

Fig. 1 Basic definitions of SMEs. Top: (a) dual and (b) triple one-way SMEs. Bottom: (c) two-way reversible SMEs

covered topics ranging from general concepts of SMPs (PDLA) have been studied as SMPs [50–65]. Lu et al.
[27–33] to more specific aspects, such as SMP composites found that PLLA shows dual SME and its shape recovery
and nanocomposites [34–38], electro-active SMPs [35], decreases and becomes nearly constant with a large
stimuli-responsive SMPs [38], triple SMPs [39], shape number of testing cycles [50]. Zhou’s group prepared
memory epoxy resins [40], and SMPs with multiple poly(D,L-lactide) (PDLLA)/hydroxyapatite composites,
transitions [41]. Various applications of SMPs in textile which showed better SME than pure PDLLA polymer at
[42], aerospace [43], and biomedical [12,44–46] fields a certain range of compositions [51]. The work of Hu and
have also been reviewed. The current paper intends to coworkers on chitosan/PLLA composites showed that
review the progress made in the past few years in shape recovery ratio decreased dramatically with increas-
biodegradable SMPs and SMPs based on bio-compounds, ing the amount of chitosan due to the incompatibility
with highlights on recent examples of SMPs for biome- between PLLA and chitosan [52]. Therefore, compatibility
dical applications and promising future perspectives. is important for the SME of composites. Filion and
coworkers developed a series of biodegradable PLA/POSS
nanocomposites exhibiting stable shape fixity and facile
2 Shape memory polyesters shape recovery within a narrow window of physiological
temperatures. This SMP showed a mild foreign body type
Polyesters are the most common biodegradable SMPs. immune response upon implantation in rats [53].
They include polyhydroxyalkanoates (PHAs), poly(lactic The copolymers of PLA have also been investigated for
acid) (PLA), poly(ε-caprolactone) (PCL), and poly(p- their SMEs. Pan and colleagues designed alternating
dioxanone) (PPDO). Among them, PLA is produced supramolecular multiblock copolymers with hard PLA
from natural compounds, while PCL and PPDO are and soft poly(ethylene-co-butylene) segments, which
made from synthetic monomers. showed good SME. The shape recoveries of supramole-
cular copolymers bearing atactic PDLLA blocks were
2.1 PLA nearly 100% [58]. Shape memory behavior of PLA blends
has also been studied [59–64]. For example, Yuan et al.
PLA is a biodegradable semi-crystalline polyester, which fabricated fully bio-based PLA/natural rubber (NR)
is synthesized from lactic acid derived from natural thermoplastic vulcanizates through peroxide-induced
resources, such as corn [47]. It is one of the most dynamic vulcanization process. The blends showed
promising biodegradable polymers employed in biomedi- excellent shape fixities and recoveries, and fast recovery
cal applications due to its ability to fulfill complex rates ( < 15 s) [62]. Tsujimoto and Uyama synthesized a
requirements on biocompatibility, complete biodegrad- fully bio-based polymeric material from epoxidized
ability, non-toxic degradation products, mechanical soybean oil (ESO) and PLLA that exhibited excellent
strength, and thermoplastic processability [48,49]. There- shape recovery properties (Fig. 2). The shape fixity
fore, both poly(L-lactide) (PLLA) and poly(D-lactide) depended on the feed ratio of epoxidized soybean oil and
 Kaojin Wang et al. Shape memory polymers for biomedical applications 3

Fig. 2 Shape memory-recovery behavior of polyESO/PLLA. Reproduced with permission from Ref. [63]. Copyright (2014) American
Chemical Society

PLLA [63]. Wang et al. reported that PLA-based

polyurethanes display good dual SME under a 150%
deformation or two-fold compression [66].

2.2 PCL

PCL is a common biodegradable aliphatic polyester. Due

to its low in vivo degradation rate, high drug permeability,
good solubility and low melting point (55–60 °C), PCL has
been used in biomedical field for many years, especially in
long-term implants and delivery devices [67,68]. An
example of such a device is Capronor®, a commercial
subdermal contraceptive product based on PCL, that can
release levonorgestrel in vivo for 12 to 18 months and
which has been on the market for almost 30 years [69].
Dual SME of PCL polymer systems has been extensively
studied. For example, Lendlein et al. prepared CLEG
copolymer networks consisting of PCL and PEG. Swelling
of the CLEG copolymer networks in water resulted in Fig. 3 Photographs exhibiting the self-expansion of the stent
hydrogels with dual shape-memory capability [70]. Li et made from PCL and P(3HB-co-3HV). Reproduced with permis-
al. developed block copolymers consisting of hyper- sion from Ref. [10]. Copyright (2010) Elsevier Science Ltd.
branched three-arm PCL and a microbial polyester, P
(3HB-co-3HV). A stent made from this copolymer showed arising from recrystallization. The results suggested that
nearly complete self-expansion at 37 °C within only 25 s the recovery ratio increased under a high pressure
(Fig. 3) [10]. Chen et al. prepared cross-linked PCL/natural environment because of the formation of imperfect crystals
rubber blends by melt blending process using dicumyl with smaller sizes during the recrystallization in the
peroxide as a cross-linking agent. The results showed that nanocomposite [75].
the shape fixity decreased slightly with increasing peroxide PCL polymer systems also showed triple and multiple
content, while the ratios were close to 100% [71]. SMEs, memorizing three or more shapes (one permanent
Mya et al. reported that star-shaped polyhedral oligo- shape and two or more temporary shapes). Zhou et al.
meric silsesquioxane (SPOSS)/PCL polyurethanes exhib- synthesized a triple shape memory polyurethane based on
ited remarkable shape fixities and strain recoveries of 98% PCL-diols, diphenyl methane diisocyanate and N,N-bis(2-
[72]. Weng et al. synthesized PCL-based polyurethanes via hydroxyethyl) cinnamamide. This polymer was photo-
reactive extrusion and the materials showed dual SME cross-linked and has one Tm of PCL and one glass
with fixity ratios of around 100% and recovery ratios of transition temperature (Tg) [76]. In another example, Wei
80%–98% [73]. PCL composites have also exhibited dual and coworkers prepared a dynamic network via self-
SME. For example, Kashif et al. prepared PCL/trisilanol- complementary quadruple hydrogen bonding based on
phenyl polyhedral oligomeric silsequioxane (TspPOSS) poly(tetramethylene ether) glycol (PTMEG) and four-arm
nanocomposite films via solvent casting method and the star-shaped PCL precursors. This polymer network
films exhibited dual SME with shape fixity ratio of 81% showed two separate Tms, resulting in a triple SME
and recovery ratio of 85% [74]. Zhou et al. investigated the (Fig. 4) [77]. Xiao and colleagues also developed PCL/
effects of high temperature and high pressure on the dual PPDO interpenetrating polymer networks by self-comple-
SME of PCL/nano-crystalline cellulose nanocomposites mentary quadruple hydrogen bonding and covalent
4 Front. Chem. Sci. Eng.

Fig. 4 Triple SME of a strip sample of PCL/PTMEG polymer network. Reproduced with permission from Ref. [77]. Copyright (2015)
American Chemical Society

bonding. These networks exhibited triple SMEs due to two material switched between three distinct “permanent”
separate Tms [78]. Bai et al. prepared PCL/ethyl cellulose shapes under four external stimuli under external stress.
polymer networks with one broad transition temperature In this system, the polymer network with two distinct Tms
and the materials exhibited triple, quadruple, and even could alternate reversibly among three shapes with a
quintuple SMEs [79]. constant external loading [88]. Afterwards, they found that
In addition to dual, triple, and multiple SMEs (one-way this polymer network can show two-way reversible SME
SMEs), PCL polymer systems have been shown to exhibit even without any external stress [89]. Hu’s group designed
two-way SMEs, where the materials are able to vary an interpenetrating network with PCL crystalline phase
between two distinct “permanent” shapes upon the and PTMEG elastomeric component, which may also
exposure to two external stimuli under external stress. display two-way reversible SME without stress [90]. In
For example, PCL/POSS chemical/physical double net- another example, Zhou’s group designed cross-linked
works [80], PCL polyurethanes [81], cross-linked PCL-co- PEG-PCL block copolymer network, exhibiting two way
PEG foams [82], and chemically cross-linked PCL [83–86] SME under stress-free conditions with the cyclic heating
could display two-way SMEs with suitable constant stress. and cooling between 43 and 0 °C (Fig. 5) [91].
Bai and coworkers extended a two-way SME with external
stress to a tough PCL polymer network [87].
Lendlein et al. fabricated a copolymer network with PCL 3 SMPs based on bile acids
and poly(ω-pentadecalactone) (PPDL) segments, which
exhibited two-way reversible triple SME, meaning the Bile acids are naturally-occurring compounds present in

Fig. 5 Two-way reversible SME of cross-linked PEG-PCL block copolymer network. Reproduced with permission from Ref. [91].
Copyright (2014) The Royal Society of Chemistry
 Kaojin Wang et al. Shape memory polymers for biomedical applications 5

large quantities in the gallbladder of humans and most molecular weights (Mn above 100 kg/mol), typical rubber-
animals and may serve as an ideal starting material for the like elasticity with tunable mechanical behavior, glass
synthesis of biomaterials because of its multiple functional transition temperatures varying from 21 to 63 °C, and
groups and biological origin [92–95]. Bile acids may be excellent thermo-induced SMEs combined with biode-
incorporated into polymer backbone or pendant group gradability (Fig. 7) [97]. These polymers were not
providing a certain rigidity to the polymer. chemically cross-linked and they did not manifest any
crystallinity although they showed orientation as a result of
3.1 Bile acids in the main chain: degradable polyesters stretching. Hence, the rigidity of bile acid moieties and
chain entanglements may be the origin of the SME [98].
A series of polyesters were synthesized with bile acids in More recently, ring-opening polymerization of the macro-
the backbone using entropy-driven ring opening metath- cyclic monomers was performed using an immobilized
esis polymerization (ED-ROMPs). Large amounts of lipase, which eliminates the need of a transition metal
coupling agents are usually required in condensation catalyst used in the ROMP reactions [99].
polymerization, but are not needed in ED-ROMP, which
simplifies the synthesis and lowers the toxicity of the 3.2 Bile acids as pendant groups
material [93]. Bile acid-based monomers can be copoly-
merized with ricinoleic acid, which is a natural compound Norbornene-based copolymers bearing cholic acid and
derived from castor oil (Fig. 6(a)). The copolymers were triethylene glycol monomethyl ether pendant groups were
degradable and showed 20% relative weight loss after 5 also prepared using ROMP method. The copolymers were
months of degradation in vitro. The degradation pro- non-degradable and displayed broad and tunable Tgs from
ceeded via heterogeneous mechanism, because the mole- –58 to 176 °C, which allowed for multiple SMEs. The
cular weight of the bulk polymer decreased slowly over copolymer selected for detailed studies showed very high-
time (Figs. 6(b) and 6(c)) and the surface of the films performance dual SMEs over several cycles and also
remained smooth with no apparent porosity (Figs. 6(d) and exhibited triple and quadruple SMEs [100]. To avoid the
6(e)) [96]. The obtained polyesters showed relatively high use of heavy metal-based catalyst in the synthesis of

Fig. 6 (a) Structure of bile acid-based copolyester; (b) relative mass loss; and (c) Mn decrease upon degradation; SEM images (d) before
and (e) after 5-month degradation of polymer film in PBS at 37 °C. Adapted with permission from Ref. [96]. Copyright (2008) The Royal
Society of Chemistry
6 Front. Chem. Sci. Eng.

Fig. 7 (a) Structure of selected bile acid-based polymers and (b) an example of shape recovery behavior. Adapted with permission from
Ref. [97]. Copyright (2009) American Chemical Society

biomaterials, polymers with similar side pendants but with and remote actuation. In addition, the functional fillers
methacrylate backbone were prepared via simple free used that can convert different energies to heat may also
radical polymerization, and the resulting materials showed improve the mechanical properties of SMPs. In addition,
both dual and triple SMEs. Cinnamic acid-based metha- the fixity ratio, recovery ratio and mechanical properties
crylate monomer was incorporated into the copolymers to are undoubtedly important for biomedical applications.
further improve the SMEs by photo-cross-linking, and the They vary a great deal depending on the materials. The
resulting terpolymer showed improved dual and triple requirements on this vary depending on the intended
SMEs and even very good quadruple SME (Fig. 8) [101]. applications.
One of the most important applications of SMPs is in
drug delivery. Ameer et al. synthesized a series of novel
4 Biomedical applications biodegradable citric acid-based elastomers with shape
memory effect at clinically relevant temperatures for drug-
Biodegradable SMPs or SMPs based on bio-compounds eluting devices. When small hydrophobic compounds
are interesting for biomedical applications, including drug were loaded in these elastomers, temperature-dependent
delivery carriers, self-tightening sutures, fasteners, self- slow release was observed [102]. Recently, Wang and
expansion stents, endovascular clot removal, and ortho- coworkers developed biocompatible and biodegradable
dontic appliances. For biodegradable SMPs, the switching cross-linked poly(ethylene glycol) (PEG)-PCL copolymer
temperature (Tg or Tm) should be close to or slightly above networks with melting temperature (Tm) close to body
body temperature. Direct heating of SMPs has been tested temperature, displaying an excellent SME. Mitomycin C
for most biomedical applications due to the polymer design was conjugated with the polymer and curcumin was coated
and the need for simple equipment and fast response. to a stent prepared from this material, which could
However, indirect heating of SMPs has many advantages sustainably release curcumin over 14 d and mitomycin C
over the direct method, such as uniform and local heating over 70 d [103]. In another example, biodegradable PCL/

Fig. 8 Synthesis of photo-cross-linked cinnamic acid-based terpolymer and its triple shape memory behavior. Reproduced with
permission from Ref. [101]. Copyright (2015) American Chemical Society
 Kaojin Wang et al. Shape memory polymers for biomedical applications 7

TspPOSS shape memory nanocomposite films [74] were

loaded with theophylline using a solution casting method.
Such films with 10 wt-% TspPOSS showed drug release of
~25% after 2 d. A two-way reversible SMP based on six-
arm PEG-PCL polymer network was designed by Gong
et al. and this polymer may be employed as a reversibly
convertible micrometer-sized carrier for drug delivery [91].
Biodegradable SMPs may also be processed to sutures
which self-tighten upon heating. Lendlein and Langer
reported that biodegradable PCL/PPDO multiblock copo-
lymers could be used in sutures that self-tighten upon
heating to around body temperature [5]. Zhang et al.
prepared a novel blend SMP system for the same
application, based on styrene-butadiene-styrene tri-block
copolymer and PCL. These materials were able to tighten
automatically a knot within 10 s after the immersion in
water at 70 °C [104].
Other applications of SMPs include endovascular stents
and clot removal devices. Chen et al. developed a
biodegradable shape memory stent, made of blended
chitosan films with glycerol and poly(ethylene oxide)
cross-linked with an epoxy compound. The stent could
rapidly (~150 s) extend from its crimped (temporary) to
fully expanded (permanent) state upon hydration [105].
The design of expandable or shape changing micro-
devices to remove blood clots has drawn much attention in
minimally invasive surgery as an alternative to traditional
clot-dissolving drug treatments. For example, Small et al.
designed an intravascular laser-activated therapeutic
device using commercially available SMPs to mechani-
cally remove the blood clot and recover blood flow to the
brain [106]. Lendlein reported cross-linked polymer net-
works consisting of PCL and PEG (Fig. 9), which may be
implanted into the body as a removable stent with a
compressed shape (a). Such a stent may be extended to
shape (b) at a required position, and finally contract to Fig. 9 The recovery process of triple SMP for removable stent,
shape (c) at 60 °C for easy removal [107]. shapes (b) and (c) were recovered by heating to 40 and 60 °C,
In addition, other biomedical applications were reported respectively, beginning from shape (a). Reprinted with permission
recently. Gong and coworkers reported a micropatterned from Ref. [107]. Copyright (2006) National Academy of Sciences,
surface which was made of a biodegradable and biocom- USA
patible PEG-PCL polymer. The surface micro-patterns can
regulate the specific distribution of vascular endothelial replace the conventional SMPs. In this review, we
and smooth muscle cells via thermally switched shape summarized the recent progress on cross-linked polymer
memory [108]. One suggested use for two-way reversible networks, polymer composites, and block copolymers,
SMPs is artificial tendons (Fig. 10), although this which showed dual, triple, multiple, and two-way
application has not been tested yet [90]. Furthermore, reversible SMEs. Some SMPs were tested for use in drug
shape memory hydrogels were synthesized by Kabir et al. delivery systems and devices for minimally invasive
from N,N-dimethyl acrylamide and stearyl acrylate to be surgery.
used as artificial bandages and lenses [109]. It is obvious that such polymers have drawn increasing
interest from researchers for new designs and preparation
methods. Some potential future directions in this field have
5 Conclusions and perspectives emerged. Firstly, the heat-activated shape memory, indirect
heating of the bio-based SMPs through the use of infrared
Biocompatibility and biodegradability are desirable prop- or ultrasound should help to reduce the damage of
erties of SMPs for biomedical applications. Biodegradable surrounding cells and tissues. This may also allow for
and bio-based polymers can meet this need and may remote control of drug-eluting SMP carriers and devices
8 Front. Chem. Sci. Eng.

Fig. 10 Proposed artificial tendon of two-way reversible SMPs. Reproduced with permission from Ref. [90]. Copyright (2014) The
Royal Society of Chemistry

for minimally invasive surgery. Secondly, in certain delivery. Advanced Drug Delivery Reviews, 2006, 58(15): 1655–
biomedical applications, such as artificial muscles, fast- 1670
response SMPs are necessary. In addition, tunable 10. Xue L, Dai S, Li Z. Biodegradable shape-memory block co-
biodegradation rate and complex SMPs (multiple and polymers for fast self-expandable stents. Biomaterials, 2010, 31
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Unconstrained recovery characterization of shape-memory poly-
Acknowledgements Financial support from NSERC of Canada and mer networks for cardiovascular applications. Biomaterials, 2007,
FQRNT of Quebec is gratefully acknowledged. K. Wang is grateful to the 28(14): 2255–2263
China Scholarship Council for a scholarship. The authors are members of
12. Small W IV, Singhal P, Wilson T S, Maitland D J. Biomedical
CSACS funded by FQRNT and GRSTB funded by FRSQ.
applications of thermally activated shape memory polymers.
Journal of Materials Chemistry, 2010, 20(17): 3356–3366
13. Yahia L. Shape Memory Polymers for Biomedical Applications.
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