Nano Lett. 14, 4164–4170 (2014)

Because cell cultures, tumours and tissues are heterogeneous, tools that can capture and analyse individual cells are important for diagnosis, therapy and surgery. There are many techniques to analyse single cells and some robotic devices can trap and manipulate particles and cells quite precisely. However, few tools can do this in narrow conduits such as blood vessels. Now, David Gracias and colleagues at Johns Hopkins University and the United States Army Research Laboratory report a self-folding 'gripper' that can capture single cells without the need for any external batteries to move the parts.

Inspired by previous work on stress-based folding of thin films, the researchers used photolithography to create arrays of grippers on silicon wafers that could be used either as analytical assays or released as free-floating tools for capturing cells. The grippers consist of four flexible arms made of thin films of SiO and SiO2, which are connected to rigid segments formed from thicker films of SiO. The thickness of the films can be varied to control the desired angle of folding. Differential compressive stress between the SiO and SiO2 films cause the grippers to fold on their own. When fibroblast cells were pipetted on a substrate consisting of attached grippers, live single cells could be captured within the arms of the grippers. The gap at the intersection of the arms allowed nutrients, waste and biochemicals to flow in and out. Free-floating grippers could capture red blood cells from a sample of beagle blood.