Stimuli-Responsive Soft Microactuators For Dynamic Microfluidics

<br/>Microfluidic tools spark interest as drug delivery systems, miniaturized cell cultures and lab-on-a-chip devices. The main advantages of microfluidic systems arises from the ability to reduce the sample size and decrease the reagents volume by miniaturizing large scale systems, facilitating the development of rapid diagnostic devices. Still, there is a limit in the flexibility of microfluidics and the devices that are commonly adapted to a single static application. By combining responsive hydrogels with microfluidic systems we provide a compelling option to improve functionality. Stimuli responsive hydrogels are gaining traction for applications in soft microactuators. One of the most prominent thermoresponsive hydrogels is poly(<i>N</i>-isopropylacrylamide) (pNIPAM). The actuation of pNIPAM is based on a globule-to-coil transition at the lower critical solution temperature (LCST). Additionally, pNIPAM offers a LCST of around T~32°C close to physiological conditions. At this temperature pNIPAM undergoes a reversible volumetric change due to a transformation in its hydrophilicity. We observed a significantly higher volume change when using pNIPAM hydrogels engineered with 3D microchannels compared to the bulk pNIPAM hydrogel. Besides using such microengineering approaches, the modification of pNIPAM hydrogels with light-, pH-, or magneto-responsive molecules and particles led to multi-stimuli responsive soft actuators with new characteristics. Further, a variety of pNIPAM-based soft actuators have been developed. Recently, we fabricated a dynamic microfluidic system based on pNIPAM microactuators with a broad range of applications in analytical chemistry as lab-on-a-chip devices, biosciences, and diagnostic devices. Ultimately, the combination of microfluidic devices with stimuli-responsive hydrogels simplifies the fabrication of complex microfluidic systems, and additive manufacturing opens doors towards dynamic microfluidics.