Milan Shrestha

Electrically tunable and broader-band sound absorption by using micro-perforated dielectric elastomer actuator

Most membrane-type acoustic absorbers for low-frequency sound attenuation are applicable only at a fixed resonant frequency for a narrow bandwidth. Tuning of the acoustic absorption spectrum is desired to match the varying noises. This letter presents a micro-perforated dielectric elastomer actuator (MPDEA) to make a broader-band acoustic absorber electrically tunable. Voltage activation of the MPDEA reduces the membrane tension and hole size and thus enables the active shifting of the acoustic absorption spectrum. Such a membrane tuning method does not require discrete mechanical parts as for the cavity tuning method. Also presented are the analytical models to predict the voltage-induced hole size change and acoustic characteristics of MPDEA.

Microscopically crumpled indium-tin-oxide thin films as compliant electrodes with tunable transmittance

Indium-tin-oxide (ITO) thin films are perceived to be stiff and brittle. This letter reports that crumpled ITO thin films on adhesive poly-acrylate dielectric elastomer can make compliant electrodes, sustaining compression of up to 25% × 25% equi-biaxial strain and unfolding. Its optical transmittance reduces with crumpling, but restored with unfolding. A dielectric elastomer actuator (DEA) using the 14.2% × 14.2% initially crumpled ITO thin-film electrodes is electrically activated to produce a 37% areal strain. Such electric unfolding turns the translucent DEA to be transparent, with transmittance increased from 39.14% to 52.08%. This transmittance tunability promises to make a low-cost smart privacy window.

Ink-Jet Printing of Micro-Electro-Mechanical Systems (MEMS)

Beyond printing text on paper, inkjet printing methods have recently been applied to print passive electrical and optical microparts, such as conductors, resistors, solder bumps and polymeric micro lenses. They are also useful to print micro-electro-mechanical systems (MEMS) as sub-millimeter sensor and actuator arrays, such as multifunctional skins applicable to robotic application and ambient monitoring. This paper presents the latest review of a few successful cases of printable MEMS devices. This review shows that inkjet printing is good for printing two-dimensional or surface MEMS devices from a small unit to an array over a large area. In the future, three-dimensional printing of multi-materials, from metal, plastic, to ceramic, will open the possibility of realizing more variety and function of a large-areal MEMS array, for a mobile electro-mechanical systems.

Dielectric elastomer fingers for versatile grasping and nimble pinching

Boneless soft robotic fingers cannot apply concentrated forces to pinch a delicate object. This letter reports a three-dimensional design of dielectric elastomer fingers with higher flexural stiffness and close to 90° voltage-controllable bending for object gripping and pinching. It makes use of tension arch flexures to elevate a pre-stretched dielectric elastomer actuator (DEA) into a roof shape and thus magnifies the tension-induced moment, 40 times higher than a flat DEA does, to bend a stiff base frame. Such fingers make normally close-grippers to lift a payload 8–9 times their weight. They also make normally open grippers that pinch a highly deformable raw egg yolk.

Stronger multilayer acrylic dielectric elastomer actuators with silicone gel coatings

Multilayer dielectric elastomer actuators (DEA) perform worst off than single-layer DEAs due to higher susceptibility to electro-thermal breakdown. This paper presents a hot-spot model to predict the electro-thermal breakdown field of DEAs and its dependence on thermal insulation. To inhibit the electrothermal breakdown, silicone gel coating was applied as barrier coating to multilayer acrylic DEA. The gel coating helps suppress the electro-thermally induced puncturing of DEA membrane at the hot spot. As a result, the gel-coated DEAs, in either a single layer or a multilayer stack, can produce 30% more isometric stress change as compared to those none-coated. These gel-coated acrylic DEAs show great potential to make stronger artificial muscles.

Electrically tunable window based on microwrinkled ZnO/Ag thin film

Micro-winkling can turn a transparent thin-film of zinc oxide (ZnO) to be ‘opaque’ that can be reversed by unfolding to restore back to the clear state. This principle was previously used to make a mechanically tunable window device. However, ZnO thin film cannot make a compliant electrode to enable electrical unfolding due to its insulator nature. This paper reports the use of multilayer thin films of 10nm silver (Ag) and 30nm thick ZnO to form a compliant electrode with electrically tunable transmittance. A dielectric elastomer actuator (DEA) with a pair of such compliant Ag/ZnO thin films on both sides of a polyacrylate elastomeric membrane (3M VHB 4910) makes an electrically tunable window device. The DEA without radial compression of the elastomer has wrinkle-free electrode. Hence, it is clear with a 47% in-line transmittance (for 550nm wavelength light). In the wrinkled form, under 10% radial compression, it becomes opaque (with less than 1% transmittance). A voltage induced areal expansion of 10% radial strain enables the electrical unfolding of the initial wrinkles. In addition, this device continues to work after 4000 cycles of unfolding and microwrinkling of Ag/ZnO. The performance of electrically tunable window device is comparable to the existing smart window technologies.

Ink-jet printing of transparent and stretchable electrodes for dielectric elastomer actuator

Dielectric elastomer actuators have recently been used to drive loudspeakers and acoustic absorbers. So far, these acoustic devices are opaque due to use of metallic or carbongrease compliant electrodes. A transparent device of acoustic-absorber is desirable for large-areal installation to glass window or roof. There were reports of transparent compliant electrode based on ionic hydrogel, which however does not last long when water evaporates. This paper investigates the use of transparent conductive polymer and its printing to make a transparent acoustic absorber. We formulated the aqueous ink with improved ink’s wettability to the elastomeric substrate. In addition, we optimized the droplet spacing to form a continuous electrode coating. The ink-jet printing enables the hassle-free patterning of transparent compliant electrodes to make a micro-perforated dielectric elastomer actuator. Testing shows that this transparent membrane DEA can produce a maximum voltage-induced radial expansion of 20%; whereas, a transparent perforated-membrane DEA can size down the holes by 15%, for tuning the acoustic resonant frequency.

Transparent tunable acoustic absorber membrane using inkjet printed PEDOT:PSS thin-film compliant electrodes

Window glasses can block noise from outdoor, but they reverberate sound within a large indoor space. Microperforated glass absorbers have been developed to absorb sound over a fixed but narrow bandwidth. To tune the frequency spectrum of acoustic absorption, we developed a transparent tunable acoustic absorber based on microperforated dielectric elastomer actuator (MPDEA) and transparent compliant electrodes. Such transparent compliant electrodes were inkjet printed from Triton X-plasticized poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) ink, which shows improved wettability on the acrylate dielectric elastomer substrate. These transparent polymeric electrodes are softer with uptake of moisture while being self-clearable and durable. A single layer of MPDEA using two inkjet-printed electrodes is 78.64% clear, but the clarity of a two-layer MPDEA decreases to 61.8%. Among the two designs, the two-layer MPDEA exhibits a broader acoustic absorption bandwidth of 444 Hz for absorbing more than 80% of the sound energy. Inactivated resonant frequency of this MPDEA is 1170 Hz, whereas the 6 kV activation can reduce the resonant frequency for 15.2% by causing 9% hole-diameter contraction. This transparent tunable acoustic absorber can be fitted to window glass; its acoustic performance is better than that of translucent curtains.

Strong dielectric-elastomer grippers with tension arch flexures

Soft grippers based on dielectric elastomer actuator (DEA) are usually too flimsy to perform the task of pick and place on a heavier object given their low payload capacity. This work developed a new design of DEA unimorph consists of a flexible frame holding at a DEA on the discrete support by a stiffer spine-like flexure of 380μm thick Polyvinyl chloride (PVC) sheet. It finds an equilibrium of curling up when the DEAs pre-stretch is partially released; it can electrically unfolds upon a voltage application. This dielectric elastomer unimorph of 3 grams produced a maximum voltage induced bending of close to 90° and a maximum voltage-induced blocked force of up to 168mN. Given their higher stiffness and large actuation, these 3-D shaped and strengthened DEA unimorphs can make stronger grippers for passive grasping and active pinching.

Controlled micro-wrinkling of ultrathin indium-tin-oxide films for transparency tuning

Smart windows can electrically switch between clear and opaque states. Current smart windows based on polymer dispersed liquid crystal are expensive and they have moderate range of transparency tuning. Elastomeric tunable window devices are being researched as the low-cost alternates. They consist of a transparent elastomer substrate with surface electrodes that provide electrically controlled micro-wrinkling. They diffusely scatter the transmitted light and thus appear opaque when the surfaces are micro-wrinkled. On electrical activation the wrinkles are flattened, thus making the windows transparent like window blinds. However, the initial prototypes of these electrically tunable window devices showed limited transparency tuning because their transparent electrodes cannot be completely flattened. For example, the brownish e-beam evaporated indium-tin-oxide thin films (50 nm thick) remains mildly wrinkled (with 52.08% transmittance) even when subjected to 37% areal expansion, while its opaque state allows 39.14% transmittance. There is a need for more transparent thin-film electrode with better controllability of surface micro-wrinkling. This work reports a greatly improved tunable window device with enlarged range of transmittance tuning: a clear state of 71.5% transmittance and an opaque state of 2% transmittance. This new device made use of ultra-thin (6 nm) ITO thin films as the transparent compliant electrodes, which were initially wrinkled and can be flatten by 12.2% voltage-induced areal expansion. These ultra-thin ITO thin films are clearer with fewer thermally-induced wrinkles on the flat elastomer substrate (VHB 4905) as they were deposited at a lower surface growth temperature using the RF magnetron sputtering technique. In addition, they make compliant electrodes of higher electrical conductivity and can electrically unfold the mechanically induced micro-wrinkles by a small voltage-induced areal expansion (~12.2%). With the greatly enhanced performance, this electrically tunable window device is promising approach for low-cost smart windows.