Integration of hydrogels with hard and soft microstructures

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.