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A D V A N C E D M A T E R I A L S & P R O C E S S E S | J A N U A R Y 2 0 1 8 9 of PUU displays greater dynamic stiff- ening at those strain rates than glassy polycarbonate. In addition, PUUs bounce back after impact, and their re- sistance to micro-particle penetration is easily optimized—a 50% reduction in the average maximum depth of pen- etration is achieved simply by varying molecular composition. PUU microstructures contain mol- ecules that exhibit a broad range of re- laxation times, from the microsecond scale—enabling dynamic stiffening—to the nanosecond range, which contrib- utes to dynamic strengthening. This blend of relaxation times could allow PUUs to deform differently depending on impact speed. The team hypoth- esized that a cooperative molecular relaxation mechanism—resembling a resonance phenomenon—facilitated by intermolecular hydrogen bonding in the PUU could be at the root of this behavior. Notably, polycarbonate does not exhibit microsecond relaxation or experience hydrogen bonding. The dis- covery could lead to development of composite matrix materials for advan- ced military protective gear as well as athletic helmets. army.mil . SYNTHETIC SKIN GETS A GRIP Engineers from the University of Washington, Seattle, and University of California, Los Angeles invented a flexi- ble sensor skin for robots and prosthet- ics that can detect shear forces and vi- brations, enabling an artificial hand to feel when an object is sliding out of its Bio-inspired sensor skin can be wrapped around a finger or any other part of a robot or prosthetic device to help convey a sense of touch. Courtesy of UCLA Engineering. grasp, which was impossible until now. The bio-inspired skin is made of silicone rubber embedded with microscale channels filled with electrically conduc- tive liquid metal. The channels are stra- tegically positioned on a robot finger on either side of where a fingernail would be. When a human finger slides over a surface, one side of the nailbed bulg- es while the other becomes taut. The sensor skin mimics this, which shifts its channel geometry and alters the electricity flow. When these changes in electrical resistance are correlated with shear forces and vibrations, the skin can sense the sliding motion. The new skin detects normal forces, shear forces, and vibration at once, something past sen- sor skins could not do, and distinguish- es shear forces from normal forces, also a challenge in the past. Experiments also demonstrate that the synthetic skin can detect vibrations at 800 Hz— better than human fingers. washington. edu, ucla.edu .