Shape Memory PolyUrethanes are the most common type of thermo-responsive shape memory polymers available on the market. A key SMP feature is their ability to memorize an initial shape, to be deformed and held in a temporary shape under specific conditions of temperature and stress, and to then relax to the initial shape upon thermal command. During this thermal cycle, the SMP material we use has a 100 fold change in Young’s modulus.
To make a haptic displays, we take advantage of this enormous change in Young’s modulus with temperature (up to 100 times) to intrinsically implement latching and to selectively actuate individual taxels in the array. Our unique and novel concept, which enables large scale manufacturing and faster refresh rate, is to integrate a stretchable microheater on each taxel and to use a common pneumatic source to move all taxels (i.e. one pump drives all the taxels, but only those that were heated move). As illustrated below, there are two stable taxel states when the membrane is cold: “UP” or “DOWN”; with taxel motion obtained uniquely when the global pneumatic actuation and the local Joule heating are synchronized.
Movie showing the flexible version of our 24×32 element active skin: i) displaying different patterns, ii) device operating principle, and iii) dynamic thermal imaging showing selective taxel heating. Published in Advanced Materials Technologies 2017.
The fully latching 4×4 SMP-based haptic display illustrated below has 4 mm pitch, 5 s refresh time, 400 mN holding force and 500 µm displacement per taxel. Each taxel is individually electrically addressed. The intrinsic latching property of SMP enables holding any array configuration with zero power consumption. More than 98 % of displayed symbols were correctly recognized in tests with 15 blindfolded users.
3. 24×32 Shape Memory Polymer (SMP) based haptic display
Our current device size (both rigid and flexible) is 768 taxels, with yields of over 99%. Taxels have been tested for over 10’000 cycles with no degradation.
24×32 haptic display with 768 independent SMP actuators, showing the BlindPad logo. The SMP actuators were made at EPFL, the control electronics and firmware by BlindPad partner IIT, and control software by BlindPad partner Geomobile.
Video showing the 768 SMP taxels being refreshed to generate a map of Switzerland, in response to a single pressure source and a matrix of micro-heaters. The inset video (top left) is taken with a thermal camera to show which taxels are heated. Note how only the heated taxels move. The taxels latch into position once the heating is removed.