  Heteropolymer gels can be engineered to release specific molecules into or
absorb molecules from a surrounding solution. This remarkable ability is the
basis for developing gel applications in extensive areas such as drug delivery,
waste cleanup, and catalysis. Furthermore, gels are a model system for
proteins, many of whose properties they can be created to mimic. A key aspect
of gels is their volume phase transition, which provides a macroscopic
mechanism for effecting microscopic changes. The phase transition allows one to
control the gel's affinity for target molecules through tiny changes in the
solution temperature, salt concentration, pH, or the like. We summarize recent
experiments that systematically characterize the gel affinity as a function of
adsorbing monomer concentration, solution salt concentration, and cross-linker
concentration, on both sides of the phase transition. We provide a physical
theory that explains the results and discuss enhancements via imprinting.
