Materials Letters 63 (2009) 1656–1658

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Materials Letters

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Novel copolyester for a shape-memory biodegradable material in vivo

Lili Liu, Wei Cai ⁎School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China

⁎ Corresponding author. Tel.: +86-451 86418649; faxE-mail address: weicai@hit.edu.cn (W. Cai).

0167-577X/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.matlet.2009.04.037

a b s t r a c t

a r t i c l e i n f o

Article history:Received 21 March 2009Accepted 28 April 2009Available online 3 May 2009

Keywords:Shape-memory materialsBiomaterialsPolymersPoly (glycol-glycerol-sebacate)

In this paper, we report the excellent shape-memory behavior of a novel Poly(glycol-glycerol-sebacate)(PGGS) terpolymer network for the first time. The polymer, with its crosslinked, three-dimensional networksacting as fixed phase, while its crystalline phase acting as a reversible one, meets the two necessaryconditions to be the material that possesses shape-memory behavior, the response temperature of which isin the neighbourhood of human body temperature. The PGGS terpolymer, with a shape-memory ratio ofabove 99.5% and a recovery temperature of 37.5 °C, shows excellent shape-memory effect and a potential ofbeing used directly in vivo.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

Shape-memory polymers (SMPs), a novel class of smart materials,have been rapidly developed during the past two decades [1]. Ashape-memory polymer can respond to changes in the externalconditions, such as temperature, PH, ionic strength, etc. [2–5]. Thethermally induced shape-memory effect can be achieved by changingthe operation temperature below or above its switching transitiontemperature (Ttrans), which normally can either be a glass transition(Tg) or a melting temperature (Tm) of the polymer. Now, shape-memory polymers have significant applications in the field ofbiomedical engineering, including vascular stents, orthodontic wires,vibration dampers, pipe couplings and actuators [6–8]. As well asresponding to different stimulations, biodegradability would bebeneficial for many medical applications [9]. The combination ofshape-memory capability and biodegradability, as an example ofmultifunctionality in a material, is especially advantageous for medi-cal devices used for minimally invasive surgery. In the applications,removal of the implant in follow-up surgery is not necessary, as theimplant degrades within a predefined time interval [10].

Poly (glycerol-sebacate) (PGS) is a recently synthesized biocompat-ible and biodegradable elastomer [11–14]. In previous research, we havefirstly reported that the polymer shows shape-memory effect and theoriginal shape can be recovered at 10 °C [15]. However, it cannot be useddirectly in human body as a shape-memory material because of its lowswitching temperature. In order to seek a novel shape-memorymaterialwhose switching temperature is around human body temperaturewith inexpensive raw materials as well as easy synthesis method, wedesigned and synthesized the PGGS terpolymer, which shows an

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excellent shape-memory behavior and a potential of being used directlyin vivo.

2. Materials and methods

The synthesis of PGGS terpolymer was carried out using a three-step method. Firstly, stoichiometric amount of sebacic acid and glycolreacted at 160 °C for 24 h under nitrogen, and then the reactiontemperature was lowered to 120 °C over 24 h under a vacuum of0.1 MPa. Secondly, the required quantity of glycerol was added and thereaction was continued for further 24 h resulting in a viscousuncrosslinked PGGS pre-polymer. Finally, the pre-polymer waspoured in a mould and then cured at 120 °C for another 24 h undera vacuum of 0.1 MPa, which produced firm, cross-linked, PGGS layersof approximately 1 mm in thickness. Gel (%) was determined bySoxhlet extractor with tetrahydrofuran. The FTIR spectroscopies werecarried out with a Spectrum One apparatus (Perkin Elmer). The DSCmeasurement was performed with a Diamond DSC (Perkin Elmer)apparatus. The shape-memory effect was examined by a bending testas follows: a straight stripe of the specimenwas folded at Tm, and thencooled to keep the deformation. The deformed sample was thenheated again at a fixed temperature, and the changes in angle θf withtemperature were recorded. The ratio of the recovery was defined asθf/180.

3. Results and discussion

3.1. Characterization of poly(glycol-glycerol-sebacate)

Gel%, defined as the ratio of the remaining mass of insolublematerial (dried to constant weight in vacuum oven) to original mass,

Fig. 1. FT-IR spectra of PGS oligomer (a), PGGS pre-polymer (b), and PGGS terpolymer (c).Fig. 3. Photographs showing the shapememoryeffect of PGGS (recovery temperature 40 °C).

1657L.L. Liu, W. Cai / Materials Letters 63 (2009) 1656–1658

shows the cross-link density of the cross-linked polymer. The Gel% ofPGGS terpolymer is 68.35%.

Fig. 1 shows the FTIR spectra of oligomer, pre-polymer andterpolymer. In the spectra, the peaks at 2924 cm−1 and 2853 cm−1

are for the stretching vibration of −CH2. In the spectrum of oligomer,the peak at 1165 cm−1, for the stretching vibration C\O bond of esterbonds, and the intense CfO stretches at 1732 cm−1 confirm theformation of ester bonds. However, the peak for the absorption bandsof aliphatic acid at 1690 cm−1 is stronger because sebacic acid isexcessive in reactants. The peaks for the absorption bands ofcarboxylic acid group and the intense −OH stretches are shown inthe spectrum of pre-polymer, but the absorption bands of ester bondsbecomemarkedly strong. In the spectrum of terpolymer, the peaks forthe absorption bands of carboxylic acid group and those for hydroxylgroups disappear, instead, the peaks at 1165 cm−1 and 1736 cm−1 forester bonds become strong. These changes result from formation ofthe crosslinked, three-dimensional PGGS network.

Differential scanning calorimetry (DSC) results for PGGS terpolymerare showed in Fig. 2. The glass transition temperature (Tg), meltingtemperature(Tm) and crystallization temperature (Tc) of PGGS terpoly-mer is −37.9 °C, 39.3 °C and −34.5 °C, respectively The DSC resultsindicate that PGGS exhibits crystalline at room temperature. The cyclic

Fig. 2. The DSC results is a cycle (a heating curve and a cooling curve).

DSC results show that the thermal properties of PGGS are very stable. Asshown by opticalmicroscopy, the PGGS is a semicrystalline polymer, thecrystalline structure of which is a kind of spherulites.

The shape-memory behavior is found to occur when the operationtemperature is changed below and above the shape transitiontemperature (Ttran) of PGGS terpolymer. The polymer can easily bedeformed to screw under external force above Tm.When the deformedsample is cooled down to 0 °C, its temporary shape is fixed withoutloading. The deformed state is very stable, even after unloading.However, if the operation temperature is raised to Tm again, thepolymer will quickly return to its initial shape in 10 s with a shaperecovery ratio of above 99.5%. Fig. 3 presents the recovery process ofthe deformed sample at 40 °C.

Shape-memory behavior is detected in the temperature study, asshown in Fig. 4. The data curve, based on the recovery ratio andtemperature, is S-shape. Under the influence of hot water, thepermanent shape is recovered with a precision of almost 100% assoon as the switching temperature (Ttrans) is reached. This precisionmakes these materials suitable for highly demanding applications.PGGS terpolymer shows higher deformation recovery temperatures(RT=37.5 °C) and smaller temperature range between the startingtemperature of shape recovery (RTS) and end temperature of shape

Fig. 4. Relationship between shape recovery rate and temperature of PGGS terpolymer(deformation strain 15%).

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recovery (RTe). It indicates that the shape-recovery of PGGS can berealized at human body temperature. In this sense, the terpolymermay find clinical applications as shape-memory implant.

A shape-memory polymer basically contains a fixing phase and areversible phase. The fixing phase imparts a level of dimensionalstability and thermal resistance, whereas the reversible one providesthe elastic properties, primarily recovery and energy absorption. ForPGGS terpolymer, the crosslinked PGGS network is designed as thefixed phase, while the crystalline PGGS phase acts as the reversiblephase. With a shape-memory ratio of almost 100%, it shows excellentshape memory effect.

4. Conclusions

Poly (sebacate-glycol-glycerol) (PGGS), a biocompatible and bio-degradable elastomer, is synthesized, and its structure, thermalproperties and shape-memory effect are researched. PGGS, as a novelshape-memory material with a recovery ratio of above 99.5% and arecovery temperature of 37.5 °C, showsexcellent shape-memoryeffect.The terpolymer may not only broaden the list of shape-memorypolymer, but also can be designed as potential biomaterials for prac-tical use in medicine and for other applications interfacing withbiological system.

Acknowledgement

This work was supported by the scientific and technologicalproject of Heilongjiang province (No. 2006G1793-00).

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.matlet.2009.04.037.

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