Miniaturized Dielectric Elastomer Actuator for Mechanical Stimulation of Monolayer Cell Cultures

Abstract

This thesis advances the field of dielectric elastomer actuators (DEAs) through the development of device designs, fabrication processes, strain characterization technique and modelling tool. It provides the first demonstration that DEAs can be interfaced with living cells, opening the door to real-world applications in mechanobiology, an important step for the development of this emerging soft-actuator technology. It also provides a practical approach towards low voltage DEAs, demonstrating a fully-printed actuator that works below 300 V, a range compatible with commercially available CMOS circuitry, hence enabling a variety of new applications for DEA-based technologies. The mechanisms by which cells can sense and react to their mechanical environment are still partly unknown, and advances in this field will contribute to better diagnosis and treatment of serious diseases like cancer. Research heavily relies on in vitro models, and there is therefore great interests in systems capable of applying precise mechanical strain on cell cultures. This thesis overcomes the many challenges of interfacing DEAs with living cells, and presents a biocompatible device which can sustain standard cell culture protocols like sterilization, incubation, and immersion in growth medium. The device can apply from -10% to 35% uniaxial strain on a small cell population ( 100 cells), located in a transparent area (0.5mm x 1.5mm) of a larger biocompatible membrane. It can be mounted on an inverted microscope, where its novel design enables real-time high-resolution optical imaging of cells during stretching. With strain rates in the excess of 700 %/s, the in vivo environment can be reproduced with unprecedented accuracy. As a demonstration of the technology, in collaboration with the Vascular and Tumor Biology Laboratory at UNIL in Switzerland, a population of lymphatic endothelial cells (LECs) was cycled from 0% to 10% strain at 1 Hz for 24 h. The results show stretch-induced alignment of cells perpendicular to strain, and confirm that the device fringing electric field has no effect on LECs morphology. This is the first demonstration that DEAs can be interfaced with living cells, and the first time they are used to observe cell mechanosensitivity. The driving voltage of DEAs is typically in the kV range, which limits their possible applications. One approach to reduce the actuation voltage is to decrease the membrane thickness, which is typically in the 20-100 microns range, as reliable fabrication becomes challenging below this thickness. This thesis presents a pad-printed 3 microns thick DEA, and demonstrates that decreasing the membrane thickness to only a few microns significantly reduces the driving voltage, while maintaining good actuation performance. A radial strain of 7.5% was achieved at only 245 V, which corresponds to a strain-to-voltage-squared ratio of 125%/kV^2, the highest reported value to date. This thesis also investigates the electrodes stiffening impact, often overlooked in the design and development DEAs. It presents an analytical model which accounts for the electrodes stiffness, and presents a strain-mapping algorithm to compares the strain uniformity of 3 microns- and 30 microns-thick DEAs. The simulation results and the strain mapping measurements identify the electrodes as an important parameter that should not be neglected in the design and optimization of thin-DEAs.