Slippery and magnetically responsive micropillared surfaces for manipulation of droplets and beads

Stimuli-responsive surfaces are of practical importance for applications ranging from enhanced mixing of reagents in lab-on-a-chip systems until probing cellular traction forces. Non-destructive reversible bending of cilia-inspired magnetic pillars can be used for controlled transportation of non-magnetic objects and bio-inspired sensing. Magnetic actuation of micropillars suspended in liquids allows controlled mixing, propelling, and stirring of fluids as well as droplet manipulation, which are important for various applications including generation of cell spheroids and droplet coalescence in microfluidic systems. In order to expand their practical applications, fabrication processes capable of rapid prototyping have to be developed. Inspired by biological cilia and their functionalities, actuating hairy surfaces are herein fabricated and implemented to manipulate both microbeads and droplets. The artificial cilia are based on microscale magnetic pillar arrays made of flexible polydimethylsiloxane functionalized with magnetic microparticles. The arrays are fabricated by a new method using patterned molds that relies on cryogenic separation to produce transparent cilia-inspired arrays without requiring manual interference to clean the templates during the process. Magnetic actuation of the pillar arrays is demonstrated in isopropanol and silicone oil. Filling with oil yields magnetically responsive slippery lubricated surfaces allowing directional motion of droplets by repetitive bending and recovery of the flexible magnetic pillars. The achieved structures allow manipulation of microbeads and droplets which is uncommon even at the sub-mm scale; directional motion is demonstrated for 250 μm–550 μm sized droplets. Droplet transportation is facilitated by extremely low hysteresis and a high degree of omnidirectional bending of the pillar array.