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W3.27.1

Optimizing the Thermomechanics of Shape-Memory Polymers for Biomedical Applications Christopher M. Yakacki1, Ken Gall1, Robin Shandas1,2, Alicia M. Ortega1, Nick Willett1, and Alan R. Greenberg1 1 2

Department of Mechanical Engineering, University of Colorado at Boulder, 80309 Division of Cardiology, The Children’s Hospital, Denver, CO 80218

Abstract We examine the shape-memory effect in polymer networks intended for biomedical applications. The polymers were photopolymerized from tert-butyl acrylate (tBA) with polyethyleneglycol dimethacrylate (PEGDMA) acting as a crosslinker. Three-point flexural tests were used to systematically investigate the thermomechanics of shape-storage deformation and shape recovery. The glass transition temperature (Tg) of the polymers varied over a range of 100°C and is dependent on the molecular weight and concentration of the crosslinker. The polymers show 100% strain recovery up to maximum strains of approximately 80% at low and high deformation temperatures (Td). Free strain recovery was determined to depend on the temperature during deformation; lower deformation temperatures (Td < Tg) decreased the temperature required for free strain recovery. Constrained stress recovery shows a complex evolution as a function of temperature and also depends on Td. The thermomechanical results are discussed in light of potential biomedical applications and a prototype stent that can be activated at body temperature is presented. 1. Introduction Shape-memory materials are defined by their capacity to recover a predetermined shape after significant mechanical deformation [1]. The shape-memory effect is typically initiated by a change in temperature and has been observed in metals, ceramics, and polymers [1]. One of the first widespread applications of the shape-memory effect in polymers was heat-shrink tubing [2]. However, such early applications did not necessitate a robust understating of the thermomechanical behavior of shape-memory polymers or a solid comprehension of the mechanisms imparting shape memory. On the other hand, the design of emerging shapememory polymer based medical devices [3-9] and microsystem components [8-12] demands thorough characterization of the thermomechanical shape-memory cycle. In addition, recent applications [3-12] require optimized recovery properties achieved through a fundamental understanding of the relationship between polymer structure and ensuing shape-memory characteristics. 2. Materials and Experimental Methods A tert-butyl acrylate (tBA) di-functional monomer (Aldrich), polyethyleneglycol dimethacrylate (PEGDMA) tetra-functional monomer (Aldrich), and 2,2-dimethoxy-2phenylacetophenone photoinitiator (Aldrich) were used in their as-received condition without further purification. In some experiments, diethyleneglycol dimethacrylate (DEGDMA) is used in place of PEGDMA. However, it is important to note that DEGDMA is a form of PEGDMA with an ethyleneglycol (O CH2 CH2)n group with n=2. A monomer solution was then mixed manually in a glass vial and t