Indirect optical gripping with triplet traps

We describe an indirect optical gripper for noninvasive micromanipulation of sensitive objects, such as cells. Our optical gripper, driven by dynamic holographic optical tweezers, consists of an arrangement of six silica beads, each held in place by an optical trap. The beads are moved so as to grip an object, which itself is not significantly exposed to laser light. Six beads are sufficient to grip an object reliably, consistent with robotics design rules. We find that improved gripping is achieved when beads are arranged into groups of triplets, since each triplet of trapped beads forms a weakly attractive pocket into which other objects are drawn.