Techniques are provided to independently control 3D shape and chemistry of rapidly produced colloids. A pre-polymer mixture including a monomer is made to flow into a channel with insular relief in a wall at a known location of the channel. A stimulus that polymerizes the pre-polymer mixture is directed onto the known location to form a structure locked in place at the known location by the insular relief. A pressure is applied to the channel that is sufficient to deflect the wall having the insular relief sufficiently to release a hydrogel particle comprising the structure.