Silicone as a Multifunctional Material: Ultrafast Tuneable Lenses and Energy Harvesting Based on Silicone Membranes

In recent years, the development of soft and stretchable electronics and sensors has seen the wide use of Polydimethylsiloxane (PDMS) as a convenient soft substrate on which such circuits are fabricated. When coupled with compliant electrodes, silicone membranes can be used however not simply as a passive elastic substrate, but also as an active material, for sensing, actuation, and for optics, allowing for completely flexible objects with a complex and intelligent behaviour. We illustrate this potential for integration in this papers with two devices: a) a bio-inspired Dielectric Elastomer Actuator (DEA) -based electrically tuneable lens capable of changing its focal length by up to 28% in less than 500 μs, and b) a compliant DEA energy harvesting system with 128 active silicone layers. Our tunable lens consists of two circular PDMS membranes bonded together by oxygen plasma. At the centre, a small amount of liquid (un-crosslinked PDMS) is encapsulated between the two membranes to form a Ø5 mm bi-convex lens with an initial curvature radius between 8 and 16mm. Annulus-shaped compliant electrodes are patterned on both sides of the device around the lens. When a voltage is applied between the electrodes, the charges squeeze the membrane in the electroded zone, which expands in plane. The lens is therefore compressed, and its diameter decreases, causing an increase of the lens curvature and decrease of the focal lens, similar to what happens in the crystalline lens of our eyes. Here, PDMS plays the multifaceted task of constitutive material for the whole device, active material providing actuation, stretchable optical material defining the lens, and high refractive index liquid inside the lens. Unlike traditional optical systems, which achieve tuning by moving rigid fixed-focal lenses relative to each other, our bio-inspired lens minimizes the displaced mass allowing very fast tuning. It also minimizes the number of parts needed to build a lens, leading to a fast and cheap fabrication process. The amount of liquid displaced upon activation is minimized, and thanks to the low viscoelasticity of PDMS, the lens presents sub ms response time, and no long-term viscoelastic drift, as often observed with DEAs based on acrylic elastomer membranes We also show how PDMS in a dielectric elastomer structure can be used as a soft and deformable capacitor for energy harvesting. We present a 1cm3 generator formed by a stack of 128 active layers connected in parallel for an initial capacitance of 2.2nF. The cylindrical structure is compressed until its capacitance doubles, and then relaxed to the initial position. We demonstrate an energy generation of 4mJ/cycle when activated at 1Hz (ie. 4 mW power output) Combining such actuators or generators with soft electronics allows using PDMS for new smart and soft machines integrating intelligence and actuation and capable of harvesting the energy they need from their environment.