Hydrogels, i.e., water-swollen polymer networks, have been studied and utilized for
decades. These materials can either passively support mass transport, or can actively
respond in their swelling properties, enabling modulation of mass and fluid transport,
and chemomechanical actuation. Response rates increase with decreasing hydrogel dimension.
In this paper, we present three examples where incorporation of hydrogels into solid
microstructures permits acceleration of their response, and also provides novel functional
capabilities. In the first example, a hydrogel is immobilized inside microfabricated
pores within a thin silicon membrane. This hydrogel does not have a swelling response
under the conditions investigated, but under proper conditions it can be utilized
as a part of an electrolytic diode. In the second example, hydrogels are polymerized
under microcantilever beams, and their swelling response to pH or glucose concentration
causes variable deflection of the beam, observable under a microscope. In the third
example, swelling and shrinking of a hydrogel embedded in a microfabricated valve
structure leads to chemical gating of fluid motion through that valve. In all cases,
the small size of the system enhances its response rate.
