A Conformable Thermo-Actuated Tactile Display

Conformable electronic devices have the capability to adhere to non-flat surfaces without changing their properties and performances. They are therefore able to easily comply with the multiple deformations of the human skin allowing imperceptibility, high comfort level, and continuous use for the user. In addition to several other sensorized devices, conformable electronics could be used to provide stimuli to the human body, for example through the development of tactile displays for visually impaired people or for human-machine interfaces.<br/>In this work, we propose a conformable and disposable thin haptic display capable of providing vibro-tactile sensations. The device consists of a matrix of unit cells that can be selectively activated in order to reproduce Braille characters and/or complex dynamic information. This new device exploits the low thermal capacity of thin conductive films that allow the generation of fast, controlled, and localized heat flow. Each unit cell is characterized by a printed micro serpentine-shaped resistor that can heat up due to the Joule Effect.<br/>Each resistor is inside a small chamber containing air. In particular, the upper wall of this chamber consists of a deformable PDMS membrane that swells - following the applied pulsed voltage - thanks to the thermal expansion of the air. The soft display can be attached to human skin using double-sided medical tape so that this movement elicits a vibro-tactile sensation on the skin.<br/>Finite Element Simulations were first conducted in order to verify that the chosen geometry permitted localized heating with consequent swelling of the membrane. A process of Aerosol Jet Printing technique allowed the fabrication of the very small serpentine-shaped resistors by directly printing silver nanoparticle ink on a 25 µm Kapton substrate. The printing parameters identified were used to achieve a good trade-off between a high resolution of the designs (70 µm line-width) and a suitable level of conductivity. The next steps of the fabrication allowed us to create closed volumes of “trapped” air at the level of the resistors by techniques including plasma treatment and spin coating. After the assessment and optimization of the fabrication process, the device was assessed by functional tests that revealed a fast activation time of each cell (in the order of tens of ms), low working voltages (&lt; 10 V), and an output force above the perception threshold at that working frequency.<br/>This conformable soft display has multiple benefits. Firstly, the printing techniques offer great versatility and fast prototyping for the design of specific applications. Secondly, owing to its low thickness, the display conforms to the skin, increasing the level of perception of tactile sensation. Moreover, the presented working principle allows the fabrication of a light, flexible, and low-voltage device, paving the way for the development of a fully-portable haptic device in the future.