Characterization of biocompatible parylene-C coating for BioMEMS applications

This thesis characterizes parylene-C films with respect to biological micro-electro-mechanical system (BioMEMS) applications. BioMEMS devices have fueled the growth and research in the area of detecting, analyzing and identifying pathogens rapidly with precision in the bio-medical applications, thereby positively impacting millions of lives and made it extremely popular among researchers. These devices are fabricated using state-of-the-art techniques usually involving more than one material which typically has different biocompatibility and is not acceptable for various BioMEMS and biomedical applications; therefore, a special biocompatible coating is required. The parylene polymer is an example of such a coating as it is known for its biocompatibility (U.S. Pharmacopoeia (USP) Class VI) as well as possessing pinhole free surfaces with low penetrability which provide exceptional barriers to moistures and solvents. The vapor deposition process utilized for depositing parylene coating also provide conformable, uniform thickness throughout targeted sample even with high aspect ratio microstructures, and is compatible with both polymeric (e.g. PMMA, polycarbonate, etc.) and non-polymeric (e.g. nickel, silicon, etc.) substrates, as the samples are kept inside a room temperature (25° C) chamber where the final deposition step occurs. In this study, parylene coatings were characterized with respect to surface roughness, where roughness measurements show no significantly changes when parylene are deposited on “smoother” pristine PMMA (from ~Ra=2.66nm to ~Ra=2.85nm) and polycarbonate (from ~Ra=3.02nm to ~Ra=5.92nm) and reduces roughness of “rougher” surfaces (electroplated nickel from ~Ra=374nm to ~Ra=201nm). Parylene is also characterize with respect to surface energy by measuring contact angles, where pristine parylene surface (contact angle = ~89°) becomes more hydrophilic by treating it with oxygen plasma (contact angle = ~32°). Surface modification was used to control the number of live cells (HeLa) attaching on parylene, where O2 plasma was used to increase this by 2-folds and altering substrate roughness helped in minimizing the cells adhesion to parylene

Micromachined three-dimensional electrode arrays for in-vitro and in-vivo electrogenic cellular networks

This dissertation presents an investigation of micromachined three-dimensional microelectrode arrays (3-D MEAs) targeted toward in-vitro and in-vivo biomedical applications. Current 3-D MEAs are predominantly silicon-based, fabricated in a planar fashion, and are assembled to achieve a true 3-D form: a technique that cannot be extended to micro-manufacturing. The integrated 3-D MEAs developed in this work are polymer-based and thus offer potential for large-scale, high volume manufacturing. Two different techniques are developed for microfabrication of these MEAs - laser micromachining of a conformally deposited polymer on a non-planar surface to create 3-D molds for metal electrodeposition; and metal transfer micromolding, where functional metal layers are transferred from one polymer to another during the process of micromolding thus eliminating the need for complex and non-repeatable 3-D lithography processes. In-vitro and in-vivo 3-D MEAs are microfabricated using these techniques and are packaged utilizing Printed Circuit Boards (PCB) or other low-cost manufacturing techniques. To demonstrate in-vitro applications, growth of 3-D co-cultures of neurons/astrocytes and tissue-slice electrophysiology with brain tissue of rat pups were implemented. To demonstrate in-vivo application, measurements of nerve conduction were implemented. Microelectrode impedance models, noise models and various process models were evaluated. The results confirmed biocompatibility of the polymers involved, acceptable impedance range and noise of the microelectrodes, and potential to improve upon an archaic clinical diagnostic application utilizing these 3-D MEAs.Ph.D.Committee Chair: Mark G. Allen; Committee Member: Elliot L. Chaikof; Committee Member: Ionnis (John) Papapolymerou; Committee Member: Maysam Ghovanloo; Committee Member: Oliver Bran

Microfluidic devices for cell cultivation and proliferation

Microfluidic technology provides precise, controlled-environment, cost-effective, compact, integrated, and high-throughput microsystems that are promising substitutes for conventional biological laboratory methods. In recent years, microfluidic cell culture devices have been used for applications such as tissue engineering, diagnostics, drug screening, immunology, cancer studies, stem cell proliferation and differentiation, and neurite guidance. Microfluidic technology allows dynamic cell culture in microperfusion systems to deliver continuous nutrient supplies for long term cell culture. It offers many opportunities to mimic the cell-cell and cell-extracellular matrix interactions of tissues by creating gradient concentrations of biochemical signals such as growth factors, chemokines, and hormones. Other applications of cell cultivation in microfluidic systems include high resolution cell patterning on a modified substrate with adhesive patterns and the reconstruction of complicated tissue architectures. In this review, recent advances in microfluidic platforms for cell culturing and proliferation, for both simple monolayer (2D) cell seeding processes and 3D configurations as accurate models of in vivo conditions, are examined

MICROFABRICATION OF BULK PZT TRANSDUCERS AND DEVELOPMENT OF A MINIATURIZED TRAVELING WAVE MOTOR

Diverse applications including consumer electronics, robotic systems, and medical devices require compact, high-torque motors capable of operating at speeds in the range of 10s to a 1000 rpm. Traveling wave ultrasonic motors are a perfect fit for these specifications as they generate higher torques for a given size-scale compared to electrostatic and electromagnetic motors. Furthermore, the electrostatic and electromagnetic motors require an additional gearing mechanism to operate at low speeds, which adds more complexity to the system. The miniaturization of ultrasonic rotary traveling wave motor has had limited success due to lack of high-resolution, high-precision fabrication techniques. This dissertation describes the development of a novel microfabrication technique for the manufacture of bulk lead zirconate titanate (PZT) microsystems involving only two lithography steps that enables the realization of bending-mode piezoelectric microsystems from a single homogeneous layer of bulk piezoceramic, requiring a few hours to fabricate. This novel fabrication process and device design concept is applied to the development of a new class of bulk PZT rotary traveling wave micromotor fabricated using a single sheet of commercially available bulk PZT. For the microfabrication of bulk PZT microsystems, relationships between micro powder blasting process parameters and PZT etching characteristics are presented, including key process parameters such as particle size, nozzle pressure and nozzle-to-substrate distance, with etch rate and etch anisotropy evaluated as a function of these parameters and space resolution. Furthermore, the photolithographic masking of bulk PZT using dry film photoresist, yielding a facile method for achieving precise and high-resolution features in PZT is presented. The work on the development of a new class of homogeneous bulk PZT unimorphs, which eliminates the need of additional elastic layers found in traditional piezoelectric bimorphs, is also reported. The developed fabrication and actuation process are key parameters to developing miniaturized bulk PZT traveling wave motor. The challenges of generating traveling waves are described in detail, followed by the successful demonstration of bi-directional traveling waves and rotor motion. The stator and rotor performance under varying stator/rotor preload forces and actuation conditions have been characterized

MEMS Capacitive Strain Sensing Elements for Integrated Total Knee Arthroplasty Prosthesis Monitoring

Measuring the in vivo load state of Total Knee Arthroplasty (TKA) components is required to understand the structural environment and wear characteristics of the devices. The ability to acquire this information gives tremendous insight into the mechanics of the joint replacement prosthesis. Data corresponding to normal loads, in-plane loads, shear loads, load center, contact area, and the rate of loading is needed to fully understand the kinematics and kinetics of the orthopedic implant. In this research, a novel sensing system has been developed which is capable of fully characterizing three-dimensional strain and stress at a single location. Capacitance-based sensors were chosen to avoid the power loss and drift characteristics typical of resistive elements due to resistive heating effects. A design and optimization methodology has been developed by combining conformal mapping electrostatic analysis techniques with methods from micromechanics of composite materials. Results of the design and optimization technique are used to understand the behavior of the sensing system. Simulation of these systems was performed using multiphysics finite element analysis, and novel methods for fabricating the sensors were adapted from techniques for fabricating microelectromechanical systems (MEMS) using biocompatible materials. An array of six sensors was fabricated with a critical dimension of 2.25 micrometers. This array consisted of a parallel plate capacitor for measuring normal strain, two differential elements for sensing shear strain normal to the plane of the array, and three interdigitated transducer (IDT) elements for characterizing strain in the plane of the sensor. The normal strain sensor exhibited a sensitivity of 1.54×10-3 picofarads per megapascal, and the shear sensor had a sensitivity of 4.77×10-5 picofarads per megapascal. Testing results showed that all sensors had linear response to loading and insignificant drift. Multiaxial testing results illustrated the ability of the differential sensors to determine loading direction. A multiaxial, MEMS sensor array has been developed for use in orthopedic, load-measuring conditions. This system has been optimized for use in soft materials such as ultra-high molecular weight polyethylene (UHMWPE). In the future, arrays of sensors will be embedded in orthopedic components to determine the total state of stress at local positions within the component

Next-Generation Diamond Electrodes for Neurochemical Sensing: Challenges and Opportunities

© 2021 by the authors. Licensee MDPI, Basel, Switzerland. Carbon-based electrodes combined with fast-scan cyclic voltammetry (FSCV) enable neurochemical sensing with high spatiotemporal resolution and sensitivity. While their attractive electrochemical and conductive properties have established a long history of use in the detection of neurotransmitters both in vitro and in vivo, carbon fiber microelectrodes (CFMEs) also have limitations in their fabrication, flexibility, and chronic stability. Diamond is a form of carbon with a more rigid bonding structure (sp3-hybridized) which can become conductive when boron-doped. Boron-doped diamond (BDD) is characterized by an extremely wide potential window, low background current, and good biocompatibility. Additionally, methods for processing and patterning diamond allow for high-throughput batch fabrication and customization of electrode arrays with unique architectures. While tradeoffs in sensitivity can undermine the advantages of BDD as a neurochemical sensor, there are numerous untapped opportunities to further improve performance, including anodic pretreatment, or optimization of the FSCV waveform, instrumentation, sp2 /sp3 character, doping, surface characteristics, and signal processing. Here, we review the state-of-the-art in diamond electrodes for neurochemical sensing and discuss potential opportunities for future advancements of the technology. We highlight our team’s progress with the development of an all-diamond fiber ultramicroelectrode as a novel approach to advance the performance and applications of diamond-based neurochemical sensors

Doctor of Philosophy

dissertationBy enabling neuroprosthetic technologies, neural microelectrodes can greatly improve diagnostic and treatment options for millions of individuals living with limb loss, paralysis, and sensory and autonomic neural disorders. However, clinical use of these devices is restricted by the limited functional lifetimes of implanted electrodes, which are commonly less than a few years. One cause is the evolution of damage to dielectric encapsulation that insulates microelectrodes from the physiological environment. Fluid penetration and exposure to an aggressive immunological response over time may weaken encapsulating films and cause electrical shunting. This reduces electrode impedance, diverts electrical signal away from target tissue, and causes multi-channel crosstalk. To date, no neural microelectrode encapsulating material or design approach has reliably resolved this issue. We employ the parylene C-encapsulated Utah Electrode Array (UEA), a silicon-micromachined neural interface FDA-cleared for human use, to execute three aims that address this challenge through investigations of new materials, electrode designs, and testing methods. We first evaluate a novel bilayer encapsulating film comprised of atomic layer deposited Al2O3 and parylene C, testing this film using UEAs and devices with UEA-relevant topography. Contrasting with previous work employing simplified planar structures, the incorporation of neural electrode features on test structures revealed failure modes pointing to the dissolution of Al2O3 over time. Our results emphasize the need for dielectric coatings resistant to water degradation as well as test methods that take electrode features into account. In our second aim, we show through finite element modeling and aggressive in vitro testing that use of degenerately doped silicon as a conductive neural electrode material can mitigate the consequences of encapsulation damage, owing to the high electrochemical impedance of silicon. Our final aim compares oxidative in vitro aging to long-term in vivo material damages and finds clear evidence that such in vitro testbeds may help predict certain in vivo damage modes. By pairing this testing with absorption and emission spectroscopic characterization modalities, we identify contributors to material damage and future design solutions. Our results will inform future material and testing choices, to improve the resilience of neural electrode dielectric encapsulation and enhance the longevity of neuroprostheses

Inkjet printing of organic transistor devices

In the last two decades inkjet printing passed from the field of graphic art and newspaper industry to that of organic and flexible electronics, as a manufacturing tool, becoming a major topic in scientific research. The appeal of this kind of technology is mainly due to its low cost, non-contact and additive approach, which makes it surely the most promising technique over the other technologies of Printed Electronics. The focus of this thesis is the optimization of the printing process, employing a piezo- electric Drop-on-Demand inkjet printer, for the realization of organic transistors on highly flexible plastic substrates, and their development in more complex systems for sensing applications. Indeed, all the devices realized have been investigated by means of electrical measures and spectroscopic techniques, in order to assess their performances and, consequently, to evaluate the reliability of inkjet printing as fabrication technique for such devices. In the first chapter a general introduction to the field of Printed Electronics, with particular focus on inkjet printing technique, is given. The second chapter provides informations concerning the fabrication characterization procedure followed, including a detailed description of the inkjet printing technology used, a report about the main physical and chemical properties of the materials employed, the explanation of the inkjet printing procedure for each material used in this thesis (as the printing parameters optimization and the approach for the resolution of some technical issues); finally also a brief description of the experimental techniques employed in order to characterize the devices is given. The third chapter is fully dedicated to the results concerning the fabrication and the characterization of all-Organic ElectroChemical Transistors (OECTs), while in the fourth chapter the results about inkjet printed Organic Field Effect Transistors (OFETs) are discussed. Finally, a brief chapter reports a summary of the main results achieved