A microstructured silicon membrane with entrapped hydrogels for environmentally sensitive fluid gating

In this paper, we report on the fabrication and characterization of a new hydrogel-based microvalve. The basic structure is a silicon membrane having an array of orifices with an internal structure designed to anchor the hydrogel while allowing it to gate the flow across the membrane. Each orifice (140 μm diameter) has a central post suspended by four tethers on each side of the membrane. A stimuli-sensitive hydrogel is polymerized inside each orifice. In the swollen state, the hydrogel completely occupies the void space of the orifice, completely blocking pressure-driven fluid flow. In the shrunken state, the hydrogel contracts around the post, allowing fluid to flow through an opened annular gap. Fabrication of the microstructured silicon membrane requires only two masking steps and involves a combination of deep trench and KOH etch. Two different hydrogels, based on N-isopropylacrylamide (temperature-sensitive) and phenylboronic acid (pH and glucose-sensitive) were trapped and tested in this microvalve. The measured response times were 10 s (temperature), 4 min (pH), and 10 min (glucose). The maximum pressure drop the microvalve can sustain before breakage of the hydrogel is 21 kPa and 16 kPa for temperature-sensitive and (pH/glucose)-sensitive hydrogels, respectively.