Large Array of Shape Memory Polymer Actuators for Haptics and Microfluidics

My thesis advances the field of shape memory polymer (SMP) actuators by providing a versatile strategy to arbitrarily reconfigure large arrays of densely packed latching soft actuators. It exploits two key intrinsic characteristics of SMPs, which are their multistable nature and their drastic change in Young¿s modulus with temperature, to combine both actuation plus latching in a single actuator. The novel concept consists in individually and selectively addressing arrays of SMP actuators by synchronizing their local Joule heating with a single common air pneumatic supply. Stretchable heaters are integrated and patterned on thin SMP membranes in order to precisely define regions where the stiffness can be changed by over two orders of magnitude. By a timely synchronization of the thermal stimuli with the external air pressure, each actuator can be independently, reversibly, and rapidly latched into any positions. The potential of coupling local Joule heating with global air pneumatic actuation for large arrays of SMP actuators is demonstrated by a 32x24 flexible haptic display and by a 4x4 microfluidic platform. The active layer of the SMP actuator is made of a commercially available SMP material for the SMP membrane and a mixture of carbon black (CB) with soft polydimethylsiloxane (PDMS) for the stretchable heating electrodes. The final SMP actuator geometry corresponds to the best trade-off between displacement and holding force for both haptic and microfluidic applications. The 32x24 flexible haptic display is the first high resolution wearable sleeve capable to vary its surface topology. This device consists of a 40 µm thick SMP membrane, on which a matrix of 25 µm thick stretchable heaters on 4 mm pitch is integrated, interconnected by a 4-layers flexible printed circuit board (PCB) and bonded to a stretchable 3D-printed pneumatic chamber. Each tactile pixel (taxel) can be individually controlled via row/column addressing, requires 250 mW to heat up from 20 °C to 70 °C, and takes 2.5 s to latch to a different state. Each line (row or column) of taxels consumes at most 8 W and the entire haptic display is refreshed in under 1 min 30 s. The haptic display weighs only 55 g and is 2 mm thick. More than 99 % of the 768 taxels are fully functional, with a lifetime in excess of 20000 cycles. The perception tests conducted on the 4x4 tactile tablet with 15 blindfolded sighted users resulted in 98 % correct pattern recognition in less than 10 s exploration, confirming that my SMP actuators are a promising taxel technology. The 4x4 microfluidic platform is the first latching microfluidic array where each valve is directly controlled with a common air pneumatic supply. Its active layer consists of a 50 µm thick SMP membrane, a matrix of 25 µm thick stretchable heaters, and a 37.5 µm thick styrene ethylene butylene styrene (SEBS) membrane. The actuators are electrically interconnected and mechanically bonded to a PCB. On the bottom, a polymethyl methacrylate (PMMA) pneumatic chamber is sealed and, on the top, a micromachined polystyrene (PS) microfluidic chip is bonded. The similarity in design for both normally closed (NC) and normally open (NO) valves enables to implement them in the same chip. These 3 mm in diameter valves remain closed up to 70 mbar of pressure before opening, validating that my SMP actuators are an interesting valve-unit for micropumps, mixers, and multiplexers in microfluidic large scale integration (mLSI) systems.