Freeze/thawed polyvinyl alcohol hydrogels: Present, past and future

A comprehensive review of polyvinyl alcohol (PVA) hydrogels prepared by the freezing/thawing (F-T) process is presented. We discuss their preparation, gelation mechanisms, handling of physical/mechanical properties, physicochemical characteristics, and various applications. PVA gelation through repeated F-T occurs without an externally added crosslinking agent due to the molecular structure and ability of PVA solutions to crystallize, resulting in ultrapure hydrogels. Gelation factors such as F-T cycle number, maximum and minimum temperature used, cycle duration, as well as molecular characteristics of the employed PVA samples, such as the average molecular weight, and the degree of hydrolysis, enable precise tuning of crystallization, hydrogen bonding as well as the final hydrogel properties. Incorporation of a second polymer and/or nanoparticles, which respectively results in interpenetrating polymer networks and nanocomposites, further facilitates obtaining desirable final properties to customize the gels for variety of applications. Facile customizability of the properties along with biocompatibility of PVA has led to the development of diverse prosthetic materials ranging from vascular stents, cartilages, and even contact lenses. In addition, PVA hydrogels have also been under investigation for drug delivery vehicles, and wound dressings. Furthermore, PVA hydrogels may possess a variety of other interesting features such as autonomous self-healing ability and programmed shape memory characteristic. As such PVA hydrogels can be used as components in actuators. Applications of the F-T PVA hydrogels in ionically conductive- and photonic crystal-based sensors are also discussed.