Fabrication and Analysis of Stretchable Films on Rough Ultra-low Modulus Polydimethylsiloxane Using Non-vacuum Techniques

Abstract

Fascinating ideas like electronic skin, stretchable and flexible energy harvesting devices, biomedical sensors, stretchable and foldable displays, artificial muscles and many more have made stretchable electronics an interesting and challenging engineering domain which also amalgamates different sciences like electronics, materials, fabrication, surface sciences, numerical computations and mechanics. In stretchable electronics the presence of a thin film on a compliant substrate as an electrode or any other functional layer is inevitable and therefore, plays a significant role in the device functionality. Conventional fabrication technique such as chemical vapor deposition (CVD), physical vapor deposition (PVD) and photolithography that has long been used for the realization of such intricate devices are costly and requires complex accessories and procedures that usually operates under high vacuum and temperatures. Unlike these conventional techniques the advent of printed electronics has revolutionized the industry. Printed electronics is actually a collection of simple, cost effective, non-vacuum based fabrication techniques. Printed electronics has an added advantage of being acquiescent towards the material usage. For instance, use of solution based direct writable inks that can even consist of nano or micro structures of different materials like micro and nanoparticles, nano wires, flakes and whiskers are common in printed electronics, making it a promising alternative over existing technologies. Stretchable behavior of thin films of both metallic and nonmetallic materials on compliant substrates, for instance polydimethylsiloxane (PDMS), polyimide (PI) and polyethylene terephthalate (PET) have been extensively studied in the past. However, these films were mostly fabricated using vacuum based techniques that require intermediate metallic adhesive layers. On the other hand, a majority of the research work regarding thin films on PDMS has utilized high or the conventional value for the Young's modulus which is approximately 1 MPa and having a thickness that is 1 or 2 mm. It is to be noted that even though the stiffer substrates can better delocalize the strains within the films, the usage of ultra-low modulus PDMS (120-140 kPa) and sub millimeter thicknesses can be of vital importance in applications like bio medical sensing, artificial skin and epidermal electronics. Therefore, the main focus of this thesis is to investigate the stretchable and flexible resistive behaviors of silver (Ag) nanoparticles and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) based films on rough ultra-low modulus PDMS substrates. However, in certain cases films on the conventional modulus ( 1MPa) PDMS has also been studied. All the films studied in this thesis were fabricated by simple, cost effective and non-vacuum based techniques like spin coating, blade coating, rod coating and electrospray deposition (ESD). The intentionally generated roughness patterns were introduced for the purpose of enhancing the adhesion between the film and the hydrophobic PDMS surface. New models that relate the change in resistance as a function of applied strain on these rough PDMS substrates have been presented. The research results forms an important database on the stretchable behavior of Ag and PEDOT:PSS films on rough PDMS substrates fabricated by simple, cost effective and non-vacuum based techniques. Different laminates studied in this research work have shown intriguing results that can be used in applications like stretchable and flexible electronics and energy harvesting devices, microfluidics and microelectromechanical systems.