Reversible Actuation of Microstructures by Surface-Chemical Modification of Thin-Film Bilayers By Jatinder S. Randhawa, Michael D. Keung, Pawan Tyagi, and David H. Gracias* The dominant paradigm for enabling mechanical motion in microstructures is based on tethered actuation with electro- magnetic [1–4] or pneumatic signals. [5] There are several advan- tages of this paradigm such as precise control, reversible operation, and fast response times. However, this concept also has limitations since these devices need wires, tethers, or on-board batteries. Here, we demonstrate a conceptually new strategy to actuate microstructures by chemical reactions. Specifically, a spontaneous and reversible curling motion was generated within chromium (Cr)/copper (Cu) bilayers on exposure to oxidative or reductive environments. These environ- ments caused the surface of Cu to be oxidized or reduced, which dramatically altered the curvature of the bilayer. Since these films can be easily deposited and patterned, we also used them as hinges to reversibly close and open microstructures such as bidirectional microgrippers and containers. An emerging engineering paradigm seeks to enable mechan- ical motion of microstructures in response to thermal or chemical signals, thereby obviating the need for tethers, wires or batteries. For example, devices constructed with composite shape memory materials [6,7] and bimetallic strips [8] can be reversibly reconfi- gured based on temperature. Reversible reconfiguration of functional microstructures based on chemical cues is relatively unexplored. [9,10] Apart from enabling tetherless actuation, chemical signals are attractive since the high versatility and specificity of chemical interactions can be utilized to couple chemical sensing to mechanical actuation and, hence, develop systems that respond autonomously when exposed to specific chemicals. Additionally, chemomechanical actuation is intellec- tually intriguing since it is widely observed in living systems. It is well known that differentially stressed bilayers will curl spontaneously on release from the substrate and this concept has been widely utilized to construct micro- and nanoscale struc- tures. [11–18] However, one significant limitation of this paradigm is the lack of reversibility, especially over multiple cycles. Once the bilayer curls up or down, it retains its shape. In order to reverse the curvature of the bilayer, it would be necessary to dramatically alter the differential stress within the bilayer and this is the focus of our current investigation. We have discovered that this alteration can be achieved by chemically modifying the surface of only one of the layers, thereby altering the differential stress of the bilayer. This strategy was demonstrated by modifying the surface of Cu within a Cr/Cu bilayer on exposure to oxidative or reductive environments. We observed that the bilayers then curled reversibly in both directions (concave and convex or up and down) based on a stress reversal due to surface oxidation or reduction of Cu. COMMUNICATION www.advmat.de [*] Prof. D. H. Gracias Department of Chemistry Johns Hopkins University Baltimore, MD 21218 (USA) E-mail: dgracias@jhu.edu Prof. D. H. Gracias, J. S. Randhawa, M. D. Keung, P. Tyagi Department of Chemical and Biomolecular Engineering Johns Hopkins University Baltimore, MD 21218 (USA) DOI: 10.1002/adma.200902337 Figure 1. Schematic diagram of the growth and reversible curling of the Cr/Cu bilayer. a) Cr and Cu thin films were sequentially deposited on a polymeric sacrificial layer by thermal evaporation. b,c) A layer of photoresist was patterned atop the Cr/Cu bilayer allowing the bilayer to be released without any curling by dissolution of the sacrificial layer. On d) dissolution of the photoresist, the bilayer curves with the Cr layer on the inner side. e) On oxidation of the Cr/Cu bilayer, the curvature is reversed, whereas on reduction, the curvature is reversed again, to its initial state (d). Adv. Mater. 2010, 22, 407–410 ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 407