Micrometer-scale poly(N-isopropylacrylamide) (poly-NIPAAm) hydrogel monolith patterns were fabricated on solid surfaces using soft lithography. At sufficiently high aspect ratios, the hydrogel monoliths swell and contract laterally with temperature. The spaces between the monoliths form a series of trenches that catch, hold, and release appropriately sized targets. A series of poly-NIPAAm monoliths were fabricated with dry dimensions of 40 mu m height, 12 mu m diameter, and a spacing of 12 mu m between monoliths. Above the lower critical solution temperature (LCST), the monoliths collapse to their dry dimensions and the spacing between monoliths is 12 mu m. Below the LCST, the monoliths swell by 70% in the lateral direction, reducing the gap size between monoliths to 3 mu m. The potential use of the hydrogel monoliths as size-selective "catch and release" structures was demonstrated with a mixture of 6 and 20 mu m polystyrene microspheres, where the 6 mu m diameter particles were selectively concentrated and separated from the larger particles.