Hydrogels: Soft, wet, elastic materials with unusual properties


Ronald A. Siegel, siege017@tc.umn.edu, Departments of Pharmaceutics and Biomedical Engineering, University of Minnesota, 308 Harvard St. SE, Minneapolis, MN 55455
Hydrogels are crosslinked polymeric networks that are capable of imbibing water and swelling. These materials are best known for their use in soft contact lenses, but they are also primary components of biological tissues such as cartilage, cornea, and extracellular matrix. A remarkable property of hydrogels is their dramatic change in swelling in response to external stimuli such as temperature, pH, electricity, light, and concentration of specific molecules such as electrolytes or glucose. When confined in a cavity or adhered to a surface, hydrogel swelling exerts forces on the adjacent solid structure. Swelling is affected by forces due to polymer elasticity, which is determined by crosslink density, and the affinity of the hydrogel's chains to water. When the hydrogel contains ionic sidechains, an electrostatic/osmotic force is also important. The relative magnitudes of these forces can be altered by the external stimuli just described. The sensitivity of hydrogels to external stimuli permits their use in drug delivery, biosensing, chemical separations, and cell culture engineering. Recently, methods for incorporating hydrogels into microstructures have been investigated. In this tutorial, we will discuss the basics of hydrogel science and describe these applications in more detail.