Controllable elastocapillary folding of three-dimensional micro-objects by through-wafer filling

We present a technique for the controllable capillary folding of planar silicon nitride templates into 3D micro-structures by means of through-wafer liquid application. We demonstrate for the first time hydro-mechanical, repeatable, actuation of capillary folded structures via the addition or retraction of water on demand. Silicon nitride objects with a central through-wafer tube are connected to a dedicated pumping system to enable assembly. When remaining wetted, structures can be assembled and reopened up to several dozens of times and still reach the same final folding angle. Objects were actuated up to 60 times without signs of wear. Extracted curves from our self-folding experiments are in agreement with our two-dimensional elastocapillary folding model. When structures are allowed to dry in between foldings, we observe an increase in the bending stiffness of the hinges, by a factor 50% after first folding and subsequent drying. This stiffening causes a decrease of the finally achieved angle. Residue from the fabrication process found on the structures after folding is suspected to be the cause of the stiffening.