Because the thermal expansion coefficients of polymers are generally one to two orders of
magnitude larger than those of SM alloys and SM ceramics (14), their effect (usually considered
negative) on the SM behavior may become non-negligible. One way to reduce the thermal expansion
is to incorporate inorganic fillers into the SMP matrix. Gunes et al. (14) evaluated the abilities
of four different inorganic fillers in lowering the coefficient of linear thermal expansion (CTLE)
of a chemically cross-linked semicrystalline SMPU. They found that the addition of nanosized
fillers was more efficient at decreasing the CLTE. In addition, high-aspect-ratio fillers such as
organoclay and carbon nanofibers (CNFs) performed better than spherical fillers. However, care
must be taken because inorganic fillers having a strong interaction with the matrix polymer may
disturb its crystallization behavior, hence resulting in reduced SM performance.
Because SMPs exhibit physical properties similar to those of human tissues, they are thought
of as ideal candidates for many medical applications, a topic we discuss in detail below. Although
many SMPUs offer biocompatibility, many groups are attempting to develop new SM materials
that offer a combination of the following characteristics: biocompatibility, bioresorbability,
biodegradability, and low cytotoxicity, among others. In one study, Pierce and coworkers (15)
reported on a thermoplastic poly(ester urethane) that incorporated novel, in-house-synthesized,