A hydrogel is a three-dimensional network of hydrophilic polymers, which can hold a large amount of water while maintaining the structure. Due to its "soft" and "wet" characteristics, the resemblance to living tissue opens up many opportunities for applications in biomedical areas. However, compared with soft tissues, traditional hydrogels have rather weak mechanical properties due to the uneven cross-linking structures during polymerizations. Also, the mechanical properties of the hydrogels are isotropic, as oppose to most of the soft tissues, which are always anisotropic. Many soft tissues in nature are composed of multi-level anisotropic structures, which give animals the abilities to adapt changes in the harsh environment. Inspired by the anisotropic structure in nature, how to introduce highly ordered structures into hydrogel networks has become a hot research topic in recent years. Among the thermos-responsive polymers, poly (N-isopropylacrylamide) (PNIPAM) is the most important one because of transition across the lower critical solubility temperature(LCST). This unique property endows PNIPAM the ability to be used as smart actuators. In this review, several strategies of fabricating anisotropic PNIPAM-based smart actuators are summarized, and the impact of anisotropic structures on their actuating abilities are discussed in detail.
