The concept of using stimuli-responsive hydrogels to actuate fluids in microfluidic devices is particularly attractive, but limitations, in terms of spatial resolution, speed, reliability and integration, have hindered its development during the past two decades. By patterning and grafting poly(N-isopropylacrylamide) PNIPAM hydrogel films on plane substrates with a 2 μm horizontal resolution and closing the system afterward, we have succeeded in unblocking bottlenecks that thermo-sensitive hydrogel technology has been challenged with until now. In this paper, we demonstrate, for the first time with this technology, devices with up to 7800 actuated micro-cages that sequester and release solutes, along with valves actuated individually with closing and opening switching times of 0.6 ± 0.1 and 0.25 ± 0.15 s, respectively. Two applications of this technology are illustrated in the domain of single cell handling and the nuclear acid amplification test (NAAT) for the Human Synaptojanin 1 gene, which is suspected to be involved in several neurodegenerative diseases such as Parkinson's disease. The performance of the temperature-responsive hydrogels we demonstrate here suggests that in association with their moderate costs, hydrogels may represent an alternative to the actuation or handling techniques currently used in microfluidics, that are, pressure actuated polydimethylsiloxane (PDMS) valves and droplets.

Spatial control of wettability is key to many applications of microfluidic devices, ranging from double emulsion generation to localized cell adhesion. A number of techniques, often based on masking, have been developed to produce spatially-resolved wettability patterns at the surface of poly(dimethylsiloxane) (PDMS) elastomers. A major impediment they face is the natural hydrophobic recovery of PDMS: hydrophilized PDMS surfaces tend to return to hydrophobicity with time, mainly because of diffusion of low molecular weight silicone species to the surface. Instead of trying to avoid this phenomenon, we propose in this work to take advantage of hydrophobic recovery to modulate spatially the surface wettability of PDMS. Because temperature speeds up the rate of hydrophobic recovery, we show that space-resolved hydrophobic patterns can be produced by locally heating a plasma-hydrophilized PDMS surface with microresistors. Importantly, local wettability is quantified in microchannels using a fluorescent probe. This "thermo-patterning" technique provides a simple route to in situ wettability patterning in closed PDMS chips, without requiring further surface chemistry.