Modeling shape memory effect in uncrosslinked amorphous biodegradable polymer Y.S. Wong a , Z.H. Stachurski b , S.S. Venkatraman a, * a School of Materials Science & Engineering, Nanyang Technological University, N4.1-02-06 Nanyang Avenue, Singapore 639798, Singapore b Department of Engineering, The Australian National University, Canberra, ACT 0200, Australia article info Article history: Received 6 October 2010 Received in revised form 1 December 2010 Accepted 1 December 2010 Available online 21 December 2010 Keywords: Shape memory Biodegradable Modeling abstract Shape memory effect (SME) is critical for minimally invasive surgical procedures in medicine. In this paper, the shape memory behavior of amorphous biodegradable polymer, poly(D,L-lactide-co-glycolide), is exper- imentally investigated. Based on the experimental observations and the understanding of the underlying mechanism of SME, a one-dimensional constitutive model is derived to describe the shape memory behavior in the context of (1) the stress-strain behavior in deformation, (2) the isothermal recovery and (3) the recovery at constant heating rate, by using a set of model constants. By fine tuning the model constants, a good agreement between the experimental results and computer predictions was achievable. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Materials displaying shape memory effect have the capability of changing their shape upon application of an external stimulus. A change in shape may be caused by a change in temperature in which case it is called a thermally induced shape memory effect and has been observed in metals, ceramics, and polymers [1,2]. Perhaps the earliest application of the shape memory effect in polymers has been introduced in the 1960s by the Raychem Corporation as heat-shrink tubing. At approximately the same time Buehler and his co-workers at the U.S. Naval Ordnance Laboratory in USA discovered the shape memory effect in a metallic alloy of nickel and titanium. Since the 1960s many other materials have been found to possess the SME capability [3]. A new development in connection with the design of shape memory materials is biocompatible polymers in which macroscopic properties (for example, mechanical properties) can be controlled by a particular variation of molecular parameters. This makes it possible to tailor the specific combination of the properties of the shape memory polymers that are required for specific applications just by a small variation of the chemical composition [4e6]. The shape memory material presented herein belongs to a family of multiphase polymer networks that are both biocompatible and biodegradable. Such materials which combine shape memory with biodegradability/biocompatibility are highly desirable for applica- tions in the field of minimally invasive surgery, where a compacted device could be passed through a smaller incision and deployed to this full shape once inside the body. SME in polymers is a consequence of viscoplastic deformation and network elasticity co-existing in the sample/object, and relies on the associated work of deformation being stored in the material as Helmholtz free energy. At some later stage, upon application of an external stimulus, the free energy released provides the driving force to restore the sample/object to its original shape. In this paper we focus on the thermally induced SME behavior of an amorphous polymer used extensively in biomedical applications [7], namely poly(D,L-lactide-co-glycolide), PDLLGA. PDLLGA has been chosen as the model polymer as it has been long used as implant matrices in the commercially available drug-loaded products for human use (e.g., Zoladex Ò , Eligard Ò , Atridox Ò ). Minimally invasive procedures, such as percutaneous deployment of stents and valves, require the device material to possess some degree of shape memory, in order to deliver the device in “low profile” or with reduced dimensions. Although an ideal shape memory polymer is a crosslinked polymer, use of such a polymer has some limitations, such as non-degrad- ability or delayed degradability; cytotoxicity of residual crosslinker or additive used for the crosslinking. Generally the use of a cross- linked biodegradable polymer also necessitates more safety testing for the device before approval. Thus if PDLLGA can be made to possess shape memory, the patch to clinical trial becomes easier. * Corresponding author. Tel.: þ65 6790 4259; fax: þ65 6790 9081. E-mail address: assubbu@ntu.edu.sg (S.S. Venkatraman). Contents lists available at ScienceDirect Polymer journal homepage: www.elsevier.com/locate/polymer 0032-3861/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2010.12.004 Polymer 52 (2011) 874e880