Muhamed Niklaus

Large-Stroke Dielectric Elastomer Actuators With Ion-Implanted Electrodes

In this paper, we present miniaturized polydimethylsiloxane (PDMS)-based diaphragm dielectric elastomer actuators capable of out-of-plane displacement up to 25% of their diameter. This very large percentage displacement is made possible by the use of compliant electrodes fabricated by low-energy gold ion implantation. This technique forms nanometer-scale metallic clusters up to 50 nm below the PDMS surface, creating an electrode that can sustain up to 175% strain while remaining conductive yet having only a minimal impact on the elastomers mechanical properties. We present a vastly improved chip-scale process flow for fabricating suspended-membrane actuators with low-resistance contacts to implanted electrodes on both sides of the membrane. This process leads to a factor of two increase in breakdown voltage and to RC time constant shorter than mechanical time constants. For circular diaphragm actuator of 1.5-3-mm diameter, voltage-controlled static out-of-plane deflections of up to 25% of their diameter is observed, which is a factor of four higher than our previous published results. Dynamic characterization shows a mechanically limited behavior, with a resonance frequency near 1 kHz and a quality factor of 7.5 in air. Lifetime tests have shown no degradation after more than 4 million cycles at 1.5 kV. Conductive stretchable electrodes photolithographically defined on PDMS were demonstrated as a key step to further miniaturization, enabling large arrays of independent diaphragm actuators on a chip, for instance for tunable microlens arrays or arrays of micropumps and microvalves.

Voltage Control of the Resonance Frequency of Dielectric Electroactive Polymer (DEAP) Membranes

We report on the characterization, active tuning, and modeling of the first mode resonance frequency of dielectric electroactive polymer (DEAP) membranes. Unlike other resonance frequency tuning techniques, the tuning procedure presented here requires no external actuators or variable elements. Compliant electrodes were sputtered or implanted on both sides of 20-35-mum-thick and 2-4-mm-diameter polydimethylsiloxane membranes. The electrostatic force from an applied voltage adds compressive stress to the membrane, effectively softening the device and reducing its resonance frequency, in principle to zero at the buckling threshold. A reduction in resonance frequency up to 77% (limited by dielectric breakdown) from the initial value of 1620 Hz was observed at 1800 V for ion-implanted membranes. Excellent agreement was found between our measurements and an analytical model we developed based on the Rayleigh-Ritz theory. This model is more accurate in the tensile domain than the existing model for thick plates applied to DEAPs. By varying the resonance frequency of the membranes (and, hence, their compliance), they can be used as frequency-tunable attenuators. The same technology could also allow the fine-tuning of the resonance frequencies in the megahertz range of devices made from much stiffer polymers.

3-dimensional electrode patterning within a microfluidic channel using metal ion implantation

The application of electrical fields within a microfluidic channel enables many forms of manipulation necessary for lab-on-a-chip devices. Patterning electrodes inside the microfluidic channel generally requires multi-step optical lithography. Here, we utilize an ion-implantation process to pattern 3D electrodes within a fluidic channel made of polydimethylsiloxane (PDMS). Electrode structuring within the channel is achieved by ion implantation at a 40 degrees angle with a metal shadow mask. The advantages of three-dimensional structuring of electrodes within a fluidic channel over traditional planar electrode designs are discussed. Two possible applications are presented: asymmetric particles can be aligned in any of the three axial dimensions with electro-orientation; colloidal focusing and concentration within a fluidic channel can be achieved through dielectrophoresis. Demonstrations are shown with E. coli, a rod shaped bacteria, and indicate the potential that ion-implanted microfluidic channels have for manipulations in the context of lab-on-a-chip devices.

Array of lenses with individually tunable focal-length based on transparent ion-implanted EAPs

We report on the fabrication and characterization of 2x2 arrays of mm-diameter PDMS lenses whose focal length can be electrically tuned. Dielectric elastomer actuators generally rely on carbon powder or carbon grease electrodes, which are not transparent, precluding the polymer actuator from also being a lens. However compliant electrodes fabricated by low-energy ion implantation are over 50% transparent in the visible, enabling the polymer lens to simultaneously be an actuator. We have developed a chip-scale process to microfabricate lens arrays, consisting of a molded socket bonded to a Pyrex chip supporting 4 membrane actuators. The actuators are interconnected via an incompressible fluid. The Pyrex chip has four through-holes, 1 to 3 mm in diameter, on which a 30 μm thick Polydimethysiloxane (PDMS) layer is bonded. The PDMS layer is implanted on both sides with 5 keV gold ions to define the transparent electrodes for EAP actuation. Applying a voltage to one of the lens/actuators leads to an area expansion and hence to a change in radius of curvature, varying the focal length. We report tuning the focal length from 4 mm to 8 mm at 1.7 kV, and present changes in optical transmission and membrane stiffness following gamma and proton irradiation.

Arrays of EAP micro-actuators for single-cell stretching applications

Mechanical stimuli are critical for the development and maintenance of most tissues such as muscles, cartilage, bones and blood vessels. The commercially available cell culture systems replicating the in vivo environment are typically based on simple membrane cell-stretching equipment, which can only measure the average response of large colonies of cells over areas of greater than one cm2. We present here the conceptual design and the complete fabrication process of an array of 128 Electro-Active Polymer (EAP) micro-actuators which are uni-axially stretched and hence used to impose unidirectional strain on single cells, make it feasible to do experiments on the cytomechanics of individual cells. The Finite Element Method is employed to study the effect of different design parameters on achievable strain, leading to the optimized design. Compliant gold electrodes are deposited by low-energy ion implantation on both sides of a PDMS membrane, as this technique allows making electrodes that support large strain with minimal stiffening of the elastomer. The membrane is bonded to a rigid support, leading to an array of 100×100 μm2 EAP actuators.

Performance characterization of miniaturized dielectric elastomer actuators fabricated using metal ion implantation

We report measurements of displacement and mechanical work for miniaturized dielectric elastomer actuators (DEAs) whose compliant electrodes were fabricated using metal ion implantation. 20 to 30 mum thick polydimethylsiloxane (PDMS) membranes were bonded to silicon chips with through holes of diameter 2 to 3 mm and were implanted on both sides with gold ions. Out-of-plane deflection recorded as a function of voltage and applied mechanical distributed load was in very good agreement with an analytical model. Unloaded vertical displacements up to 7% of the membranes diameter were recorded and mechanical work up to 0.3 muJ was obtained with an applied pressure of 1 kPa. This performance data and associated model allow such miniaturized polymer actuators to be efficiently dimensioned for different applications, for instance in micropumps and active optical devices.

Large stroke miniaturized dielectric elastomer actuators

We report on miniaturized diaphragm dielectric elastomer actuators (DEAs) capable of vertical displacement up to 25% of their diameter. Low-energy metal ion implantation was used to create patternable and compliant gold electrodes on both sides of suspended 30 µm thick silicone (PDMS) membranes. This technique enables the microfabrication of bucking-mode actuators capable of out-of-plane displacement of up to 500 µm for 2 mm diameter devices at high frequency (≫1 kHz). Device speed is limited by the resonant frequency of the device, not by visco-elastic effects or electrical considerations. This represents the largest percentage and fastest displacement of miniaturized DEAs reported to date.

Metal Ion Implanted Compliant Electrodes in Dielectric Electroactive Polymer (EAP) Membranes

One of the key factors to obtain large displacements and high efficiency with dielectric electroactive polymer (DEAPs) actuators is to have compliant electrodes. Attempts to scale DEAPs down to the mm or micrometer range have encountered major difficulties, mostly due to the challenge of micropatterning sufficiently compliant electrodes. Simply evaporating or sputtering thin metallic films on elastomer membranes produces DEAPs whose stiffness is dominated by the metallic film. Low energy metal ion implantation for fabricating compliant electrodes in DEAPs presents several advantages: a) it is clean to work with, b) it does not add thick passive layers, and c) it can be easily patterned. We use this technology to fabricate DEAPs micro-actuators whose relative displacement is the same as for macro-scale DEAPs. With transmission electron microscope (TEM) we observed the formation of metallic clusters within the elastomer (PDMS) matrix, forming a nano-composite. We focus our studies on relating the properties of this nano-composite to the implantation parameters. We identified the optimal implantation parameters for which an implanted electrode presents an exceptional combination of high electrical conductivity and low compliance.

Electro and pressure tunable cholesteric liquid crystal devices based on ion-implanted flexible substrates

We report an electro-responsive and pressure sensitive device based on conductive polydimethylsiloxane (PDMS) combined with a short pitch Cholesteric Liquid Crystal (CLC). Ion-implantation and surface chemistry in PDMS enable both the induction of conducting properties in this elastomer as well as long range organization of the CLC. Sample electro-optical and pressure dependent optical properties are explored by applying a modulated electric field through the conducting PDMS and a uniform pressure on the top cover substrate. We show that both an electric field of a few V μm−1 and an external pressure of up to 128 kPa can tune the reflection band by about 100 nm.

Ion-implanted compliant electrodes for mm-size dielectric elastomer actuators

The miniaturization of dielectric elastomer actuators requires compliant electrodes that are clean, reliable, and that can be easily patterned on a mm or μm scale. Carbon-based electrodes, which are commonly used to make large-scale actuators, are not well suited for this application. Metal ion implantation at low energies has, on the other hand, the ability to create compliant and patternable electrode through the creation of nanometer scale clusters in the first tens of nanometers below the elastomer surface. We present the mechanical and electrical properties of metal (Au, Pd, Ti, Cu) implanted electrodes on polydimethylsiloxane, as well as the application of Au-implanted electrode to the fabrication of small-size (∅1.5-3 mm) diaphragm actuators that exhibit vertical displacements up to 25% of their diameter.