Polymeric Microsensors for Intraoperative Contact Pressure Measurement

Biocompatible sensors have been demonstrated using traditional microfabrication techniques modified for polymer substrates and utilize only materials suitable for implantation or bodily contact. Sensor arrays for the measurement of the load condition of polyethylene spacers in the total knee arthroplasty (TKA) prosthesis have been developed. Arrays of capacitive sensors are used to determine the three-dimensional strain within the polyethylene prosthesis component. Data from these sensors can be used to give researchers a better understanding of component motion, loading, and wear phenomena for a large range of activities. This dissertation demonstrates both analytically and experimentally the fabrication of these sensor arrays using biocompatible polymer substrates and dielectrics while preserving industry-standard microfabrication processing for micron-level resolution. An array of sensors for real-time measurement of pressure profiles is the long-term goal of this research. A custom design using capacitive-based sensors is an excellent selection for such measurement, giving high spatial resolution across the sensing surface and high load resolution for pressures applied normal to that surface while operating at low power

Doctor of Philosophy

dissertationTactile sensors are a group of sensors that are widely being developed for transduction of touch, force and pressure in the field of robotics, contact sensing and gait analysis. These sensors are employed to measure and register interactions between contact surfaces and the surrounding environment. Since these sensors have gained usage in the field of robotics and gait analysis, there is a need for these sensors to be ultra flexible, highly reliable and capable of measuring pressure and two-axial shear simultaneously. The sensors that are currently available are not capable of achieving all the aforementioned qualities. The goal of this work is to design and develop such a flexible tactile sensor array based on a capacitive sensing scheme and we call it the flexible tactile imager (FTI). The developed design can be easily multiplexed into a high-density array of 676 multi-fingered capacitors that are capable of measuring pressure and two-axial shear simultaneously while maintaining sensor flexibility and reliability. The sensitivity of normal and shear stress for the FTI are 0.74/MPa and 79.5/GPa, respectively, and the resolvable displacement and velocity are as low as 60 µm and 100 µm/s, respectively. The developed FTI demonstrates the ability to detect pressure and shear contours of objects rolling on top of it and capability to measure microdisplacement and microvelocities that are desirable during gait analysis

Incorporating nanomaterials with MEMS devices.

This dissertation demonstrates an elegant method, known as \u27micro-origami\u27 or strain architecture to design and fabricate three-dimensional MEMS structures which are assembled using actuation of a metal-oxide bilayer with conventional planar lithography. Folding allows creating complex, robust, three-dimensional shapes from two-dimensional material simply by choosing folds in the right order and orientation, small disturbances of the initial shape may also be used to produce different final shapes. These are referred to as pop-up structures in this work. The scope of this work presented the deposition of colloidal gold nanoparticles (GNPs) into conformal thin films using a microstenciling technique. Results illustrated that the gold nanoparticle deposition process can easily be integrated into current MEMS microfabrication processes. Thin films of GNPs deposited onto the surfaces of siliconbased bistable MEMS and test devices were shown to have a significant effect on the heating up of microstructures that cause them to fold. The dissertation consists of four chapters, covering details of fabrication methods, theoretical simulations, experimental work, and existing and potential applications. Chapter II illustrates how control of the folding order can generate complex three-dimensional objects from metal-oxide bilayers using this approach. By relying on the fact that narrower structures are released from the substrate first, it is possible to create multiaxis loops and interlinked objects with several sequential release steps, using a single photomask. The structures remain planar until released by dry silicon etching, making it possible to integrate them with other MEMS and microelectronic devices early in the process. Chapter III depicts the fabrication process of different types of bistable structures. It describes the principle of functioning of such structures, and simulations using CoventorWare are used to support the concept. We talk over about advantages and disadvantages of bistable structures, and discuss possible applications. Chapter IV describes fabrication procedure of nanoparticle-MEMS hybrid device. We introduce a convenient synthesis of GNPs with precisely controlled optical absorption in the NIR region by a single step reaction ofHAuCl4 and Na2S203. We take a look at different techniques to pattern gold nanoparticles on the surface of MEMS structures, and also provide a study of their thermal properties under near IR stimulation. We demonstrate the first approach of laser-driven bistable MEMS actuators for bioapplications. Finally, in Conclusion discuss the contributions of this dissertation, existent limitations and plans of the future work

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

SILICON ON INSULATOR BIPOLAR JUNCTION TRANSISTORS FOR FLEXIBLE MICROWAVE APPLICATIONS

Microwave frequency flexible electronic devices require a high quality semiconducting material and a set of fabrication techniques that are compatible with device integration onto flexible polymer substrates. Over the past ten years, monocrystalline silicon nanomembranes (SiNMs) have been studied as a flexible semiconducting material that is compatible with industrial Si processing. Fabricated from commercial silicon on insulator (SOI) wafers, SiNMs can be transferred to flexible substrates using a variety of techniques. Due to their high carrier mobilities, SiNMs are a promising candidate for flexible microwave frequency devices. This dissertation presents fabrication techniques for flexible SiNM devices in general, as well as the progress made towards the development of a microwave frequency SiNM bipolar junction transistor (BJT). In order to overcome previous limitations associated with adhesion, novel methods for transfer printing of metal films and SiNMs are presented. These techniques enable transfer printing of a range of metal films and improve the alignment of small transfer printed SiNM devices. Work towards the development of a microwave frequency BJT on SOI for SiNM devices is also described. Utilizing a self-aligned polysilicon sidewall spacer technique, a BJT with an ultra-narrow base region is fabricated and tested. Two regimes of operation are identified and characterized under DC conditions. At low base currents, devices exhibited forward current gain as high as βF = 900. At higher base current values, a transconductance of 59 mS was observed. Microwave scattering parameters were obtained for the BJTs under both biasing conditions and compared to unbiased measurements. Microwave frequency gain was not observed. Instead, bias-dependent non-reciprocal behavior was observed and examined. Limitations associated with the microwave impedance-matched electrode configuration are presented. High current densities in the narrow electrodes cause localized heating, which leads to electrode material damage and ultimately dopant diffusion in the BJT. Finally, device design improvements are proposed to address the problem of localized heating and increase device lifetime under testing conditions. High values for DC current gain suggest that future modifications should improve microwave frequency performance and measurement reproducibility

Doctor of Philosophy

dissertationBiomedical implantable devices have been developed for both research and clinical applications, to stimulate and record physiological signals in vivo. Chronic use of biomedical devices with thin-film-based encapsulation in large scale is impeded by their lack of long-term functionality and stability. Biostable, biocompatible, conformal, and electrically insulating coatings that sustain chronic implantation are essential for chip-scale implantable electronic systems. Even though many materials have been studied to for this purpose, to date, no encapsulation method has been thoroughly characterized or qualified as a broadly applicable long-term hermetic encapsulation for biomedical implantable devices. In this work, atomic layer deposited Al2O3 and Parylene C bi-layer was investigated as encapsulation for biomedical devices. The combination of ALD Al2O3 and CVD Parylene C encapsulation extended the lifetime of coated interdigitated electrodes (IDEs) to up to 72 months (to date) with low leakage current of ~ 15 pA. The long lifetime was achieved by significantly reducing moisture permeation due to the ALD Al2O3 layer. Moreover, the bi-layer encapsulation separates the permeated moisture (mostly at the Al2O3 and Parylene interface) from the surface contaminants (mostly at the device and Al2O3 interface), preventing the formation of localized electrolyte through condensation. Al2O3 works as an inner moisture barrier and Parylene works as an external ion barrier, preventing contact of AI2O3 with liquid water, and slowing the kinetics of alumina corrosion. Selective removal of encapsulation materials is required to expose the active sites for interacting with physiological environment. A self-aligned mask process with three steps was developed to expose active sites, composed of laser ablation, oxygen plasma etching, and BOE etching. Al2O3 layer was found to prevent the formation of microcracks in the iridium oxide film during laser ablation. Bi-layer encapsulated iridium oxide had higher charge injection capacity and similar electrochemical impedance compared with Parylene C coated iridium oxide film after deinsulation. The Al2O3 and Parylene C bi-layer encapsulation was applied to Utah electrode array (UEA)-based neural interfaces to study its long-term performance. The median tip impedance of the bi-layer encapsulated wired Utah electrode array increased slowly during the 960 days of equivalent soak testing at 37 °C. Impedance for Parylene coated UEA dropped 50% to 75% within 6 months. In addition, bi-layer coated fully integrated Utah array-based wireless neural interfaces had stable power-up frequencies at ~910 MHz and constant RF signal strength of -50 dBm during the 1044 days of equivalent soaking time at 37 °C. This is much longer than lifetime achieved with Parylene C coating, which was about one year at room temperature

Active Matrix Flexible Sensory Systems: Materials, Design, Fabrication, and Integration

A variety of modern applications including soft robotics, prosthetics, and health monitoring devices that cover electronic skins (e-skins), wearables as well as implants have been developed within the last two decades to bridge the gap between artificial and biological systems. During this development, high-density integration of various sensing modalities into flexible electronic devices becomes vitally important to improve the perception and interaction of the human bodies and robotic appliances with external environment. As a key component in flexible electronics, the flexible thin-film transistors (TFTs) have seen significant advances, allowing for building flexible active matrices. The flexible active matrices have been integrated with distributed arrays of sensing elements, enabling the detection of signals over a large area. The integration of sensors within pixels of flexible active matrices has brought the application scenarios to a higher level of sophistication with many advanced functionalities. Herein, recent progress in the active matrix flexible sensory systems is reviewed. The materials used to construct the semiconductor channels, the dielectric layers, and the flexible substrates for the active matrices are summarized. The pixel designs and fabrication strategies for the active matrix flexible sensory systems are briefly discussed. The applications of the flexible sensory systems are exemplified by reviewing pressure sensors, temperature sensors, photodetectors, magnetic sensors, and biosignal sensors. At the end, the recent development is summarized and the vision on the further advances of flexible active matrix sensory systems is provided

The Application of a Piezoelectric MEMS Cantilever Array as a Completely Implantable Cochlear Implant.

Aluminum nitride (AlN) is an excellent material for MEMS sensors because of its low dissipation factor, high resistance, and relatively high piezoelectric coefficients. We present an array of AlN bimorph cantilevers, fabricated using MEMS batch fabrication, which has applications as a completely implantable cochlear implant (CI). Unlike traditional CIs, this probe is designed to locally transduce mechanical vibrations of the cochlear fluid into electrical signals that stimulate the auditory nerves. A silicon backbone supports an array of five cantilevers that have a range of resonances spanning 20-40kHz in water. Fabricated cabling extends from the probe for external monitoring during in vitro and in vivo studies. We present characterization of the effect of deposition power on the growth of 1.5μm thick AlN films grown on 565nm of thermal oxide. X-ray diffraction analysis of the films indicates well-ordered, c-axis oriented growth. High resolution transmission electron microscope imaging was used to identify atomic-level characteristics of process induced faults and dislocations occurring at the start of deposition and when vacuum is broken between two consecutive AlN depositions. Initial acute in vivo testing of an implanted fabricated device produced a 2μV device response when a 110dBSPL sound source was played at the entrance to the ear canal, demonstrating that this device has the potential to restore hearing with sufficient amplification. Benefits of this design include lower power, smaller size, and lower latency when compared with current commercial CIs.PHDMechanical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/110503/1/kknisely_1.pd

Design, manufacturing and characterisation of a wireless flexible pressure sensor system for the monitoring of the gastro-intestinal tract

Ingestible motility capsule (IMC) endoscopy holds a strong potential in providing advanced diagnostic capabilities within the small intestine with higher patient tolerance for pathologies such as irritable bowel syndrome, gastroparesis and chronic abdominal amongst others. Currently state-of-the art IMCs are limited by the use of obstructive off-the-shelf sensing modules that are unable to provide multi-site tactile monitoring of the Gastro-Intestinal tract. In this work a novel 12 mm in diameter by 30 mm in length IMC is presented that utilises custom-built flexible, thin-film, biocompatible, wireless and highly sensitive tactile pressure sensors arrays functionalising the capsule shell. The 150 μm thick, microstructured, PDMS flexible passive pressure sensors are wirelessly powered and interrogated, and are capable of detecting pressure values ranging from 0.1 kPa up to 30 kPa with a 0.1 kPa resolution. A novel bottom-up wafer-scale microfabrication process is presented which enables the development of these ultra-dense, self-aligned, scalable and uniquely addressable flexible wireless sensors with high yield (>80%). This thesis also presents an innovative metallisation microfabrication process on soft-elastomeric substrates capable to withstand without failure of the tracks 180o bending, folding and iterative deformation such as to allow conformable mapping of these sensors. A custom-built and low-cost reflectometer system was also designed, built and tested within the capsule that can provide a fast (100 ms) and accurate extraction (±0.1 kPa) of their response. In vitro and in vivo characterisation of the developed IMC device is also presented, facilitated respectively via the use of a biomimetic phantom gut and via live porcine subjects. The capsule device was found to successfully capture respiration, low-amplitude and peristaltic motility of the GI tract from multiple sites of the capsule.UK Engineering & Physical Sciences Research Council (EPSRC) through the Programme Grant Sonopill (EP/K034537/2)James Watt Scholarshi