Researchers at Queen Mary University of London, led by Prof. James Busfield, have developed transparent electrodes made of reduced graphene oxide (rGO) for dielectric elastomer actuators (DEAs), enabling a new compact, electrically tunable soft lens. The work addresses a long-standing design constraint in electrostatically actuated lenses, where opaque compliant electrodes have to be placed around the lens periphery rather than on the optical path itself.
A photo of the fabricated DEA actuator (the rGO-coated membrane with circular electrodes). Image from: Advanced Functional Materials
DEAs are soft electromechanical transducers that behave like artificial muscles: a dielectric elastomer membrane sandwiched between compliant electrodes contracts in thickness and expands laterally when an electric field is applied. They're being explored for tunable lenses, adaptive filters, camouflage skins and other electrically reconfigurable optical devices. Most implementations, however, rely on non-transparent electrodes, forcing a separate actuation ring around the lens body and increasing the device's footprint. Transparent compliant electrodes remove that constraint by letting the electrode sit directly on the optical axis.
The team spray-coated a pre-stretched acrylic elastomer membrane (VHB 4905) with an rGO ink dispersed in THF, building electrodes at surface densities between 0.10 and 0.17 µg/mm² (roughly 50-80 nm thick). Raising the rGO density from the low to the high end of that range dropped the sheet resistance from about 600 to 15 kΩ/sq, but also cut optical transmittance at 550 nm from about 42% to 17% - illustrating the familiar trade-off between conductivity and transparency in thin conductive coatings, quantified here with a Haacke figure of merit.
For the lens demonstration, the researchers selected a surface density of 0.12 µg/mm², balancing roughly 32% transmittance against an area strain of about 10% at a driving field of 50 V/µm. The rGO-coated DEA was bonded to a 12.5 mm-wide plano-convex PDMS lens; applying voltage radially stretches the lens and increases its focal length, shifting the focus point across a 30-36 mm range in their prototype. Because the transparent electrode sits directly on the lens rather than around it, the whole assembly keeps a compact form factor.
"This is exciting. It means that in the future, similar technology could find applications in autofocus cameras, wearable displays, virtual and augmented reality headsets, miniature medical imaging devices and scientific instruments where conventional mechanical focusing systems add weight, complexity or cost," said Giacomo Sasso, first author of the study. Alec Lamoreux, second author, added that coupling electrically active polymers with graphene's electrical properties "offer engineers an entirely new approach to designing optical systems" beyond motor- and gear-based focusing mechanisms.
The authors note that electrode uniformity, currently limited by agglomeration and non-planar orientation of the rGO flakes, is the main lever for further gains: smoother, more uniform coatings should improve both transmittance and achievable actuation strain, potentially widening the lens's focusing range.
