Temperature-Sensitive Hydrogels

Abstract

Synonyms Intelligent gels; Thermally sensitive hydrogels; Thermosensitive hydrogels Definition Temperature-sensitive hydrogels, which are part of the family of stimulus-sensitive hydrogels, consist of water-filled polymer networks (i.e., gels) that display a temperature-dependent degree of swelling. Temperature-sensitive hydrogels are of interest to microactuation purposes for a number of reasons [1]: their volumetric size changes are large compared to other microactuation principles, hydrogels are soft and resilient, temperature-sensitive hydrogel actuators are operated at conveniently low temperatures, and the hydrogel polymer networks can be microfabricated using standard photolithography. Temperature-sensitive hydrogels can display either a positive temperature response, meaning that the gel material swells with increasing temperature, or a negative temperature response, meaning that the gel material shrinks with increasing temperature. Chemical and Physical Principles The amount of swelling of hydrogels is a function of two opposing forces. On one side water molecules moving into the hydrogel cause a swelling force, due to molecular interactions with the polymer molecules in the hydrogel. On the other side, the elastic elongation of the coiled-up polymer molecules in the polymer network results in an elastic retractive force opposing the swelling force in the hydrogel. When water is added to a dry, hydrophilic polymer network (xerogel), a swelling hydrogel is formed. During the swelling process, the swelling forces dominate and the elastic retractive force gradually increases with progressive swelling. When the hydrogel is swollen to a degree that the elastic retractive forces have the same magnitude as the swelling forces, the hydrogel reaches equilibrium swelling. If the conditions of the hydrogel are kept stable, the volume of the hydrogel will remain constant indefinitely. Stimulus-sensitive hydrogels have the unique property that an external physical parameter, e.g., temperature, pH, or ionic strength, has an effect on the swelling forces inside the hydrogel. When the magnitude of the physical parameter is changed, the swelling force increases or decreases. As a result, the hydrogel changes its degree of swelling to equilibrate the new swelling force with the elastic retractive force. *