Mechanics and Electromagnetic Modeling of Injectable, Implantable, and Skin Integrated Electronics


Bio-integrated electronics with advanced monitoring capabilities have garnered considerable interest as a means of expanding patient care beyond traditional hospital and clinic settings. These mechanically soft microsystems, many of which are bioresorbable and wireless, have the potential to replace bulky, rigid, and wired medical technologies by matching or exceeding their performance. Combined advances in material science, mechanical engineering, and electrical engineering form the foundations of thin, soft electronic/optoelectronic platforms that have unique capabilities in wireless monitoring and control of various biological processes in cells, tissues, and organs through different characterization and/or stimulation platforms (e.g., light/drugs). Biomedical applications typically involve a demanding set of considerations in material selection, performance, size, electromagnetic efficiency, and stretchability to enable compliant systems for control, power delivery, actuation, data processing, and/or communication. In this dissertation, I examine the design and optimization of bioelectronics platforms through systematic numerical and analytical modeling. The goal is to identify scaling relationships and construct models from physical parameters related to mechanical and electromagnetic performance parameters to inform the development of skin-integrated and implantable bioelectronic platforms that can administer drugs in a programable fashion, wirelessly pace the heart, induce vibrotactile sensations at different tissue depths, and monitor physiological signals through mechanically compliant device architectures. First, I examine injectable drug delivery systems with electrochemical actuation, which provide programmable volume and flow rates in miniaturized form factors. This technology is particularly useful for in vivo pharmacological experiments in freely moving animals where flow rate control and delivery time are important. However, for programmable drug delivery, the available flowrate and drug delivery time modeling strategies fail to consider key variables of the bioelectronics system – microfluidic resistance and flexible membrane stiffness. I present an analytical model that accounts for all the variables that influence drug delivery in non-dimensional parameters related to pressure, volume, and microfluidic channels. This approach does not require numerical simulations and allows for a faster system optimization based on a scalable understanding of the non-dimensional parameters for different in vivo experiments involving programmable drug delivery systems for neuroscience and clinical research. Next, I consider a leadless, battery-free, fully implantable cardiac pacemaker for postoperative control of cardiac rate and rhythm. This device completely dissolves and is cleared by natural biological processes after a defined operating timeframe where the bioresorbable aspect is governed by a reaction-diffusion process between the biofluids and the pacemaker materials and geometries. I discuss the design and performance of this temporary device to effectively pace hearts of various sizes in mouse, rat, rabbit, canine, and human cardiac models, with tailored geometries and operating timeframes, powered by a wireless energy transfer link that relies on inductive coupling between an external transmission coil and the internal pacemaker. The safety and reliability of the device is examined in the context of thermal and electromagnetic exposure with biological tissues to ensure that the maximum temperature increase and specific absorption rate from the electromagnetic fields is below the safety limits. This bioresorbable approach overcomes the key disadvantages of traditional temporary pacing devices and may serve as the foundation for the next generation of postoperative temporary and wireless pacing technology. Then, I consider skin-integrated electronic platforms using vibro-tactile actuators for sensory perceptions in virtual or augmented reality applications. Through finite element modeling, and comparison with three-dimensional digital image correlation measurements, I investigate the vibrational dynamics induced by the three main classes of vibro-tactile actuators in a bilayer elastomer structure that captures essential mechanical properties of human skin to provide a fundamental understanding of the mechanics associated with coupling between vibro-tactile actuators and the skin. These studies examine the effects of key parameters relevant to the mechanics and resulting sensations, such as those related to contact area, actuation amplitude and spatiotemporal distributions of displacements in terms of both surface and body waves. Lastly, I examine materials strategies and design concepts for enhanced mechanical performance and water diffusion in skin-integrated electronics used for continuous monitoring of vital signs in critically ill patients in neonatal and pediatric intensive care units (NICUs and PICUs). Current approaches require multiple sensors taped to the skin and connected via hard-wired interfaces to external data acquisition electronics. Through numerical modeling, I identify materials strategies and design concepts that significantly improve these skin-integrated platforms using optimized materials, open (i.e., “holey”) layouts and pre-curved designs to reduce the stresses at the skin interface, facilitate release of interfacial moisture from trans epidermal water loss and allow visual inspection of the skin for rashes or other forms of irritation.

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