Hydrogels can play both passive and active roles in chemical oscillations and pattern formations. Passively, they can provide a nonconvective environment in which reactions occur, or they can respond by swelling or shrinking according changes in the chemical environment, which is time and/or space varying. In recent years, more active feedback couplings between hydrogels and chemical reactions have been investigated. Hydrogel systems that are coupled to the oscillating Belousov-Zhabotinsky reaction have been well studied. In a second system, a hydrogel separates two volumes, one containing the substrate for an enzyme catalzed reaction, and the other containing the enzyme. The swelling of the hydrogel, and hence transport of the substrate, are controlled by product concentration. With sufficient nonlinear delayed feedback between membrane swelling and enzyme reaction, the system is driven into an oscillatory mode, which can be harnessed to achieve rhythmic hormone release. Experimental evidence and a sequence of increasingly complex mathematical models pertaining to this second system will be presented.