A microfluidic actuator based on thermoresponsive hydrogels

We have evaluated the potential use of thermoresponsive hydrogels based on N-isopropylacrylamide as actuators in microfluidic and lab-on-a-chip devices. This required fabrication of hydrogel actuators on the μm length scale, anisotropic swelling of the resulting materials, and control over the kinetics of the hydrogel volume phase transition. The fabrication procedure combined gel polymerization and casting techniques from the life sciences with more traditional semiconductor fabrication protocols for spin-coating, patterning, and etching. The actuator design used a PDMS membrane to separate the hydrogel actuator from the microfluidic channel and a separate reservoir for fluid to swell the actuator. As a result, the actuator could control flow for organic as well as aqueous solutions over a wide range of pH and ionic strength. The presence of a fixed substrate causes the gel swelling to be highly anisotropic, and the actuating motion is perpendicular to the substrate. The anisotropic swelling also limits the degree of swelling for the responsive hydrogel, and the total volume change is lower than the corresponding bulk materials by as much as an order of magnitude. The resulting actuators conform easily to the shape of the microfluidic channel, and the rate of the hydrogel response could be increased by using a series of semi-interpenetrating hydrogel networks. The microfluidic channels ranged in diameter from 180 to 380 μm, and the typical actuator was between 100 to 500 μm in diameter. The time scale of the actuator response was approximated by fitting with a single exponential (∼exp[−t/τ]). The time scale (τ) varied as a function of the actuator size and composition, ranging from 10 min to less than 10 s.