Article Preview
Top1. Introduction
Finding an ideal biomaterial is one of the major goals in the field of tissue engineering. Many of the native tissues in the human body have elastomeric properties. Thus, the biomaterials selected to repair these tissues should have similar elastic properties in order to sustain and recover from multiple deformations without causing irritation to the surrounding tissues (Wang, 2002). It has been proven that mechanical stimuli can enhance cellular growth, alignment, and extracellular matrix production. Previous research has also shown that the appropriate mechanical constraints can help yield both fibril alignment and the geometry of a native heart valve (Neidert & Tranquillo, 2006; Robinson, 2008). Moreover, the addition of mechanical stimuli has also been shown to have an influence on the stem cell differentiation (Engelmayr, 2006; Zhao, 2009). Mechanical mismatch between host blood vessels and vascular grafts may contribute to the development of myointimal hyperplasia, a major reason for graft failure (Yang, 2005). Therefore, a suitable biomaterial for soft tissue engineering should have the appropriate mechanical properties similar to the target tissue, and be capable of transmitting mechanical stimulus to the seeded cells.
Recently, many researchers have focused their work on the development of biodegradable polyester and polyurethane elastomers for soft tissue engineering. Biodegradable polyurethanes (BPUs) are a family of elastomers, which have been used in a wide variety of biomedical applications due to their good mechanical properties (up to 29 MPa tensile strength) and elasticity (up to 895% elongation) (Guan, 2002, 2004; Guan & Wagner, 2005; Guelcher, 2008; Zhang, 2008). However, due to their aliphatic nature BPUs are susceptible to permanent creep under cyclic mechanical loading. Therefore, the potential long-term success of polyurethanes as scaffold materials for dynamic tissues like blood vessels and ligament is still questionable (Dey, 2008).
Elastomeric polyesters are another family of polymers that have attracted interest due to the appropriate biodegradability, biocompatibility, and elasticity for various biomedical applications (Webb, 2004; Yang, 2009). Thereof, crosslinked polyester elastomers have attracted much attention recently due to their excellent elasticity without permanent deformation under cyclic deformation such as poly (glycerol sebacate) (PGS) and poly(octamethylene citrate) (POC) (Wang, 2002; Yang, 2004, 2006; Chen, 2008). PGS and POC have shown excellent biocompatibility in vitro and in vivo. The crosslinking nature of PGS and POC also confer excellent elasticity to these polymers. However, the mechanical strength of PGS and POC are still relatively weak which range from 0.5±0.2 MPa to 2.9±0.1 MPa, especially when they are made into porous scaffolds. The reported tensile mechanical strength of POC porous scaffolds are only 0.3±0.1 MPa (Yang, 2005). The processability of PGS and POC is also limited with only a salt-leaching method reported for this type of polymer. A commonly used thermally induced phase separation (TIPS) or freeze-drying method for scaffold fabrication cannot be applied to these polymers due to the sticky nature of their low molecular weight pre-polymers, which are the only processable forms for these polymers.