Shape memory polymers (SMPs) are a class of environment stimulated intelligent materials that developed in recent years. Under extra stimulation, physical and chemical changes will take place within or between molecules, and the deformed material can recover to its original shape due to the change of molecular structure and morphology. Various common SMPs such as poly(ethylene), poly(urethane), poly(caprolactone) have been developed, however, the shape memory effect of poly(vinyl alcohol) (PVA) has not attracted the attention until the thermal induced shape memory behavior of PVA gel was reported. Because there are abundant hydroxyl groups with high chemical activity in PVA side chains, it is easy to modify PVA with other functional groups. Different stimuli induced molecular structures can be designed according to the environmental requirement. Up to now, different methods including freeze-thaw cycle, chemical or irradiation cross-linking, graft modification and blending have been used to prepared various multi-stimuli (e.g. temperature, solvent, light, electricity, microwave and ultrasound) induced shape memory poly(vinyl alcohol) (SM-PVA), PVA derivatives and composites. The present article summarizes the research development of different stimuli induced SM-PVA of recent years. The structures and properties of different materials, the recovery mechanism, and the problems are described. The future development and applications of PVA in this field are also prospected.

Contents
1 Introduction
2 Multi-stimuli induced shape memory polymers based on PVA
2.1 Temperature induced shape memory gel and polymer
2.2 Light induced shape fix of PVA derivative
2.3 Solvent induced shape memory PVA and composites
2.4 Electroactive shape memory PVA composites
2.5 Ultrasound-triggered shape memory PVA
2.6 Microwave induced shape memory PVA and composites
3 Conclusion