Trapping of 20-μm-diameter polystyrene microbeads in an optically patterned hydrogel. The scale bar is 200 μm. (a) A PNIPAAm solution containing 10- and 20-μm-diameter microbeads was introduced into the microfluidic chamber. The solution is flowing from the right to the left at a linear flow rate of 100 μm/s. A clump of beads was targeted for trapping, as indicated by the arrow. (b) An optical pattern is used to heat the substrate, causing a hydrogel to form in the illuminated area. (c) The hydrogel with the optical pattern removed, showing the trapped target particles. (d) Substrate cooling in the absence of optical heating reverses the trap, and the beads begin to flow away.
