3D Printed Stretchable Electronics: Fabrication using Aerosol Jet, Fundamental Mechanics, and Applications

Electrical signals form the basis for information transfer in several biological systems and devices that capture these signals are of significant interest to the healthcare industry. Such devices are currently made from Si based components and metal interconnects and typically have an elastic modulus about 3-4 orders of magnitude higher than that of biological tissue. This gap has spurred the development of stretchable electronic devices where a soft platform ‘hosts’ the mechanically stiff electrical elements such as chips and interconnects, while the low elastic modulus substrate achieves a seamless interface with the biological tissue of interest. Such systems are playing an increasingly important role in emerging areas such as soft robotics and smart clothing. In this thesis, aerosol jet nanoparticle 3D printing is used to create a stretchable electronic system consisting of three-dimensional interconnects enabling multilayered connections embedded within a thin (<100 µm in thickness) biocompatible elastomer membrane where elastic modulus gradients are used to mitigate the effects of the system stress. 

The stretchable interconnect system chosen for this work is fabricated via a wafercompatible process consisting of silver conductor encapsulated by polyimide printed directly on a polydimethylsiloxane (PDMS) elastomer substrate, followed by thermal sintering/curing. The different ideal processing conditions for each material result in several problems including delamination between polyimide layers, low electrical conductivity, and cracking of the 3D interconnects during fabrication. The delamination problem is solved by understanding the underlying mechanisms via a hypothesis-driven experimental and theoretical investigation to identify processing conditions that can eliminate delamination. The cracking problem for silver interconnects is solved by first identifying that the cracks originate due to capillary pressure prior to sintering, followed by changes to the process to prevent their formation. Fully embedded stretchable interconnects are enabled by the development of a compatible process that retains the stretchability of the PDMS and electrical conductivity of the silver. A method to print 3D pillars that extend through the top PDMS layer is developed to create multi-layered interconnection and external connection points, and Electrocardiogram (ECG) signals are captured by a multi-layered skin-wearable device fabricated with the developed process. The research presented in this thesis aims to develop an additive fabrication method for stretchable electronics, understand the fundamental mechanics of such systems, and use this knowledge to create a high-fidelity stretchable system that can capture bioelectronic signals