Alexandre Poulin

Printing low-voltage dielectric elastomer actuators

We demonstrate the fabrication of fully printed thin dielectric elastomer actuators (DEAs), reducing the operation voltage below 300 V while keeping good actuation strain. DEAs are soft actuators capable of strains greater than 100% and response times below 1 ms, but they require driving voltage in the kV range, limiting the possible applications. One way to reduce the driving voltage of DEAs is to decrease the dielectric membrane thickness, which is typically in the 20–100 μm range, as reliable fabrication becomes challenging below this thickness. We report here the use of pad-printing to produce μm thick silicone membranes, on which we pad-print μm thick compliant electrodes to create DEAs. We achieve a lateral actuation strain of 7.5% at only 245 V on a 3 μm thick pad-printed membrane. This corresponds to a ratio of 125%/kV2, by far the highest reported value for DEAs. To quantify the increasing stiffening impact of the electrodes on DEA performance as the membrane thickness decreases, we compare two c...

Flexible Zinc-Tin Oxide Thin Film Transistors Operating at 1 kV for Integrated Switching of Dielectric Elastomer Actuators Arrays

Flexible high-voltage thin-film transistors (HVTFTs) operating at more than 1 kV are integrated with compliant dielectric elastomer actuators (DEA) to create a flexible array of 16 independent actuators. To allow for high-voltage operation, the HVTFT implements a zinc-tin oxide channel, a thick dielectric stack, and an offset gate. At a source-drain bias of 1 kV, the HVTFT has a 20 µA on-current at a gate voltage bias of 30 V. Their electrical characteristics enable the switching of DEAs which require drive voltages of over 1 kV, making control of an array simpler in comparison to the use of external high-voltage switching. These HVTFTs are integrated in a flexible haptic display consisting of a 4 × 4 matrix of DEAs and HVTFTs. Using a single 1.4 kV supply, each DEA is independently switched by its associated HVTFT, requiring only a 30 V gate voltage for full DEA deflection. The 4 × 4 display operates well even when bent to a 5 mm radius of curvature. By enabling DEA switching at low voltages, flexible metal-oxide HVTFTs enable complex flexible systems with dozens to hundreds of independent DEAs for applications in haptics, Braille displays, and soft robotics.

Dielectric elastomer actuator for mechanical loading of 2D cell cultures

We demonstrate the use of dielectric elastomer actuators (DEAs) for mechanical stimulation of cells in vitro. The development of living tissues is regulated by their mechanical environment through the modification of fundamental cellular functions such as proliferation, differentiation and gene expression. Mechanical cues have been linked to numerous pathological conditions, and progress in cellular mechanobiology could lead to better diagnosis and treatments of diseases such as atherosclerosis and cancers. Research in this field heavily relies on in vitro models due to the high complexity of the in vivo environment. Current in vitro models however build on bulky and often complex sets of mechanical motors or pneumatic systems. In this work we present an alternative approach based on DEAs, a class of soft actuators capable of large deformation (>100%) and fast response time (<1 ms). The key advantage of DEAs is that they can be integrated within the culture substrate, therefore providing a very compact solution. Here we present a DEA-based deformable bioreactor which can generate up to 35% uniaxial tensile strain, and is compatible with standard cell culture protocols. Our transparent device also includes a static control area, and enables real-time optical monitoring of both the stimulated and control cell populations. As a proof of concept we cycled a population of lymphatic endothelial cells (LECs) between 0% and 10% strain at a 0.1 Hz frequency for 24 h. We observe stretch-induced alignment and elongation of LECs, providing the first demonstration that DEAs can be interfaced with living cells and used to control their mechanical environment.

Thin-film dielectric elastomer sensors to measure the contraction force of smooth muscle cells

The development of thin-film dielectric elastomer strain sensors for the characterization of smooth muscle cell (SMC) contraction is presented here. Smooth muscle disorders are an integral part of diseases such as asthma and emphysema. Analytical tools enabling the characterization of SMC function i.e. contractile force and strain, in a low-cost and highly parallelized manner are necessary for toxicology screening and for the development of new and more effective drugs. The main challenge with the design of such tools is the accurate measurement of the extremely low contractile cell forces expected as a result of SMC monolayer contraction (as low as ~ 100 μN). Our approach utilizes ultrathin (~5 μm) and soft elastomer membranes patterned with elastomer-carbon composite electrodes, onto which the SMCs are cultured. The cell contraction induces an in-plane strain in the elastomer membrane, predicted to be in the order 1 %, which can be measured via the change in the membrane capacitance. The cell force can subsequently be deduced knowing the mechanical properties of the elastomer membrane. We discuss the materials and fabrication methods selected for our system and present preliminary results indicating their biocompatibility. We fabricate functional capacitive senor prototypes with good signal stability over the several hours (~ 0.5% variation). We succeed in measuring in-plane strains of 1 % with our fabricated devices with good repeatability and signal to noise ratio.

Interfacing dielectric elastomer actuators with liquids

Methods and materials for liquid encapsulation in thin (19 μm) silicone membranes are presented in this work. A set of 12 liquids including solvents, oils, silicone pre-polymers and one ionic liquid are experimentally tested. We show that all selected liquids are chemically inert to silicone and that vapor pressure is the key parameter for stable encapsulation. It is demonstrated that encapsulated volume of silicone pre-polymers and ionic liquids can stay stable for more than 1 month. The actuation of dielectric elastomer actuators (DEAs) in conductive liquids is also investigated. An analysis of the equivalent electrical circuits of immersed DEAs shows that non-overlapping regions of the electrodes should be minimized. It also provides guidelines to determine when the electrodes should be passivated. The effects of immersion in a conductive liquid are assessed by measuring the actuation strain and capacitance over periodic actuation. The experimental results show no sign of liquid-induced degradation over more than 45k actuation cycles.

Toward compression of small cell population: Harnessing stress in passive regions of dielectric elastomer actuators

We present a dielectric elastomer actuator (DEA) for in vitro analysis of mm2 biological samples under periodic compressive stress. Understanding how mechanical stimuli affect cell functions could lead to significant advances in diseases diagnosis and drugs development. We previously reported an array of 72 micro-DEAs on a chip to apply a periodic stretch to cells. To diversify our cell mechanotransduction toolkit we have developed an actuator for periodic compression of small cell populations. The device is based on a novel design which exploits the effects of non-equibiaxial pre-stretch and takes advantage of the stress induced in passive regions of DEAs. The device consists of two active regions separated by a 2mm x 2mm passive area. When connected to an AC high-voltage source, the two active regions periodically compress the passive region. Due to the non-equibiaxial pre-stretch it induces uniaxial compressive strain greater than 10%. Cells adsorbed on top of this passive gap would experience the same uniaxial compressive stain. The electrodes configuration confines the electric field and prevents it from reaching the biological sample. A thin layer of silicone is casted on top of the device to ensure a biocompatible environment. This design provides several advantages over alternative technologies such as high optical transparency of the area of interest (passive region under compression) and its potential for miniaturization and parallelization.

Fabrication and characterization of silicone-based dielectric elastomer actuators for mechanical stimulation of living cells

Cellular biology is a promising field of application for dielectric elastomer actuators (DEAs) given that large strains are needed but only low forces are required. The development of devices compatible with standard cell culture protocols and equipment is however challenging. We recently demonstrated that DEAs can be interfaced with living cells and used to control their mechanical environment. Here we detail the fabrication process of our DEA-based cell stretcher and present a holder which was designed to provide a simple and safe experience for our biologist partners. We also evaluate the actuation performance of the device in terms of strain amplitude, spatial distribution and stability during periodic actuation. Results show that the device can generate more than 30% uniaxial tensile strain, and achieve more than 12 h of stable actuation performance when cycled between 0% and 12% strain at a 1 Hz frequency.

Assessing the degradation of compliant electrodes for soft actuators

We present an automated system to measure the degradation of compliant electrodes used in dielectric elastomer actuators (DEAs) over millions of cycles. Electrodes for DEAs generally experience biaxial linear strains of more than 10%. The decrease in electrode conductivity induced by this repeated fast mechanical deformation impacts the bandwidth of the actuator and its strain homogeneity. Changes in the electrode mechanical properties lead to reduced actuation strain. Rather than using an external actuator to periodically deform the electrodes, our measurement method consists of measuring the properties of an electrode in an expanding circle DEA. A programmable high voltage power supply drives the actuator with a square signal up to 1 kHz, periodically actuating the DEA, and thus stretching the electrodes. The DEA strain is monitored with a universal serial bus camera, while the resistance of the ground electrode is measured with a multimeter. The system can be used for any type of electrode. We validated the test setup by characterising a carbon black/silicone composite that we commonly use as compliant electrode. Although the composite is well-suited for tens of millions of cycles of actuation below 5%, we observe important degradation for higher deformations. When activated at a 20% radial strain, the electrodes suffer from important damage after a few thousand cycles, and an inhomogeneous actuation is observed, with the strain localised in a sub-region of the actuator only.

An ultra-fast mechanically active cell culture substrate

We present a mechanically active cell culture substrate that produces complex strain patterns and generates extremely high strain rates. The transparent miniaturized cell stretcher is compatible with live cell microscopy and provides a very compact and portable alternative to other systems. A cell monolayer is cultured on a dielectric elastomer actuator (DEA) made of a 30 μm thick silicone membrane sandwiched between stretchable electrodes. A potential difference of several kV’s is applied across the electrodes to generate electrostatic forces and induce mechanical deformation of the silicone membrane. The DEA cell stretcher we present here applies up to 38% tensile and 12% compressive strain, while allowing real-time live cell imaging. It reaches the set strain in well under 1 ms and generates strain rates as high as 870 s−1, or 87%/ms. With the unique capability to stretch and compress cells, our ultra-fast device can reproduce the rich mechanical environment experienced by cells in normal physiological conditions, as well as in extreme conditions such as blunt force trauma. This new tool will help solving lingering questions in the field of mechanobiology, including the strain-rate dependence of axonal injury and the role of mechanics in actin stress fiber kinetics.

The NERD setup: assessing the life time of electrodes for dielectric elastomer transducers

DEAs used in applications such as tunable lenses, soft robotics, etc. are expected to survive many thousands to millions of stretching cycles without degradation of their performance. Here, we present a measurement technique to characterise the evolution of the resistance of compliant electrodes submitted to cyclic biaxial strain, which represents the stretching configuration to which DEAs are usually submitted. We apply the novel electrode resistance degradation (NERD) method to the characterisation of compliant electrodes obtained by inkjet printing a carbon black suspension. We show that although the electrodes can sustain 1 million cycles of stretching at 5%, a 10% cyclic strain causes a much faster degradation, leading to a reduced actuation strain over time. We show that increasing the thickness of the electrodes leads to cracking and accelerated degradation; two layer electrodes degrade more rapidly than single layer electrodes. The NERD setup represents an efficient tool to quickly evaluate the suitability of different electrode formulations for use as compliant electrodes for DEAs.