Kenichi Takahata

Professor

Relevant Degree Programs

 

Postdoctoral Fellows

  • Nabil Shalabi (Micro and Nanoelectronics, Biomedical Technologies)

Graduate Student Supervision

Doctoral Student Supervision (Jan 2008 - May 2019)
A study of methodology and technology for wireless monitoring of blood pressure inside a stent (2014)

This thesis presents research on systems for wirelessly monitoring blood pressure inside a vascular stent. Such systems are of interest because changes in the blood pressure gradient across a stent are indicative of a blockage caused by growth of scar tissue (restenosis). No cheap and non-invasive method for detecting the onset of restenosis currently exists, and in-stent wireless blood pressure monitoring may provide a solution.In this work, several monitoring methods are explored. The first utilizes a specially designed stent integrated with a capacitive pressure sensor to form a pressure sensitive inductor-capacitor (LC) resonant circuit (tank) with wireless sensing capability. This approach follows previous work successful in producing a proof-of-principle prototype, but makes several modifications directed at achieving clinical relevance. A custom designed inductive stent and capacitive pressure sensor are developed, and new integration techniques are explored. In vitro resonant frequency responses of integrated devices with applied pressure are measured in the range of 50 - 200 ppm/mmHg.Device characterization reveals reader-device communication range, sensor performance variation, and stent mechanical reliability as areas of concern. Therefore, a dedicated study of the wireless range achievable with inductive stent monitoring and related monitoring approaches is undertaken, finding a maximum read range of 2.75 cm for an inductive stent in air. A surface micromachined capacitive pressure sensor is developed to improve upon the original sensor, and a miniaturized monitoring device formed by integrating this sensor with a micro-inductor is proposed as a means of avoiding wiring problems and expansion non-uniformities encountered when utilizing inductive stents.Finally, as an alternative route to increasing wireless sensing range and resolution, a third system design approach employing a complementary metal-oxide-semiconductor (CMOS) integrated circuit (IC) is explored An IC is designed to mount on a stent to read and transmit pressure information from a micro-electro-mechanical systems (MEMS) pressure sensor. The IC may be driven by using the stent as an antenna to harvest power from an external radio frequency (RF) transmitter. Characteristics of the antenna-IC interface are studied by electromagnetic modeling and circuit simulation.

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Investigation and development of ferrofluid enabled micro-electro-mechanical systems (2014)

Ferrofluids are magnetic fluids that can be manipulated using magnetic field. Ferrofluids have unique properties that have led to various interesting applications. Although, currently they are being used in few commercial products in macro-scale domain, there has been limited success in their applications in micro- devices and microactuators in specific.Literature review shows various efforts to develop ferrofluid-based microactuators however, most of them have utilized non-integrated means (e.g. external magnets or solenoids) to provide the necessary magnetic field for ferrofluid manipulation that inherently limit their application as a micro-device. Moreover, previous ferrofluid-based microactuators with integrated solutions (e. g. microfabricated coils) could only provide unidirectional forces which limited their application range.In the present thesis, development of integrated ferrofluid-based microactuators is investigated. A new actuation method that uses planar spiral coils with bias fields is proposed to enable bidirectional ferrofluid manipulation. To demonstrate the potentials of the proposed actuation method, two proof-of-concept devices were developed. Active mirror cells with variable reflectivity were demonstrated as the first device and then a variable planar inductor with ferrofluid as a moving magnetic core was developed and characterized.Another interesting application of ferrofluids in passive levitation of permanent magnets is also investigated for moving magnet based microactuators. Using this levitation mechanism a structurally simple and reliable microbearing is demonstrated. In order to demonstrate the effectiveness of such microbearing, a linear micromotor is first characterized and demonstrated. Also, frictional force and load carrying capacity of such microbearing is investigated showing very low frictional forces with good load bearing capabilities. Given the promising results in the developed linear micromotor, a rotary micromotor with small axial size is developed for minimally invasive endoscopy applications. The characterization of developed prototype shows its potential to be used for real time medical imaging.

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Micro-electro-discharge machining of carbon-nanotube forests and its application (2014)

Carbon nanotubes (CNTs) are relatively new materials with exceptional properties which have attracted significant interest in the past two decades. The ability to grow arrays of vertically aligned carbon nanotubes, so called CNT forest, opened up opportunities to develop different types of novel devices enabled by the material. A key in facilitating micro-electro-mechanical systems (MEMS) applications of the material is the ability to pattern the material in a batch mode with high precision and high reproducibility. Patterning CNT forests prior to, during or after the growth is reported. The mentioned techniques are, however, primarily for the formation of two-dimensional types of patterns (with uniform heights). Laser micromachining is reported to shape CNT forests for different applications while exhibiting its inherent limitations including tapered sidewalls, lack of high-precision depth control, and thermal damages. Hence, there is a need to develop machining techniques to fabricate CNT forests in any shape for MEMS and other applications. This thesis is based on the idea that a powerful micromachining technique is a path that should be taken to reach a successful integration of smart materials such as nanotubes and MEMS (and other) devices to achieve more complex and improved devices. This work develops an effective micromachining technique based on dry micro-electro-discharge machining (µEDM) to produce free-form, three-dimensional (3D) patterns out of CNT forests with high precision (~2-µm machining tolerance), high-aspect-ratios (of about 20), high reproducibility, and at very small machining voltages (~10 V) which corresponds to several orders of magnitude smaller discharge energy (0.5 nJ compared to 15 µJ). The machining mechanism has been found to be different from the one in typical µEDM. Furthermore, techniques to achieve high removal precision with tighter tolerance are investigated. Also, elemental and molecular analysis of the machined structures is carried out to observe the level of cross-contamination of the process. To demonstrate an application of the processed nanotubes, high-power MEMS switches that integrate micropatterned CNT forests as electrical contact have been developed. Micropatterned CNT forests as field emitters and atomic force microscopy (AFM) probe tips are also demonstrated.

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Integration and wireless control methods for micromachined shape-memory-alloy actuators and their MEMS applications (2012)

Bulk-shape memory alloy actuators have great potential to be used in various microdevices. Previous studies show that this material is very attractive due to its very large force, high mechanical robustness with a simple structure and biocompatibility. These properties have resulted in its commercialization for various applications including those in biomedical field. Yet their advantages have not been fully utilized. For example, the commonly used actuation mechanism using Joule heating which requires wired interfaces limits their application especially in those instances where access and space are very limited. In addition, their incompatibility with the standard MEMS fabrication process further limits their potential for use in microscale devices.This thesis presents a novel technique for the wireless control of shape-memory alloy microactuators and the method for integrating bulk-micromachined shape memory alloy material into the MEMS fabrication process. The wireless control of shape memory alloy actuators using radiofrequency magnetic field wireless heating through resonant planar coils to directly drive the actuator without the use of conditioning circuits is demonstrated. An electroplating bonding technique is developed to integrate the bulk-micromachined shape-memory alloy to the planar heater and the bonding strength is evaluated. A shape-memory alloy microgripper is fabricated and reported using developed actuation and the integration technique. Multiple actuator control is demonstrated using frequency modulated signals and its application in a form a microsyringe employing three actuators is reported. To improve the temporal response of the actuator, the wireless resonant heater circuit is fabricated using a shape-memory alloy to form an out-of-plane spiral coil which acts as the receiver coil as well as the actuator. Wireless displacement control and monitoring is also demonstrated using the fabricated device.The presented radiofrequency wireless control method also provides a platform to investigate the wireless actuation of the thermal based actuators other than the shape-memory alloy. The reported integration method also provides a means to exploit bulk materials into the MEMS fabrication process.

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Master's Student Supervision (2010 - 2018)
Direct writing of metal films via microplasma scanning and applications to printed sensors (2017)

Direct writing of precise micro-patterns of metallic films is an emerging fabrication technique applicable for a vast range of devices. Techniques for printing various materials are being developed for some time owing to the escalating demand, and have seen substantial progress over the years. The objective of this research is to develop a simple direct writing technique that can accurately form desired patterns with high film quality, low cost, and rapid time. Micro glow discharge manipulation may be used to print metal structures with high precision. The proposed micro-scale process for local deposition and direct writing of metal films through sputtering of a micro-machined target electrode is achieved via a highly confined micro glow plasma generated between the electrode’s end and the substrate without the need of processing under vacuum. Through the use of micro-machined cylindrical-shaped target electrodes of the desired metal or metal alloy, the microplasma is steadily sustained to confirm the local deposition of the electrode material on the conductive substrate. Film characterization, performed by thickness profilometer reveals the patterning of target material with thicknesses ranging from the 100-nm order to several micrometers, dependent on the discharge current and feed rate. The viability of the process to pattern films over non-planar surfaces is achieved with high quality. The applicability of the devised technique to micro-pattern T-type thermocouple junctions is demonstrated and the thermoelectric performance of the printed sensors is measured to verify their thermoelectric function, with a sensitivity of 39 µV/oC that matches well the Seebeck voltage of a typical T-type device. The process scheme is promising for rapid, and low-cost production of thin film patterns and applicable to print of temperature sensors, potentially on a variety of objects including three-dimensional components and flexible substrates.

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Selective and regulated RF heating of stent toward endohyperthermia treatment of in-stent restenosis (2015)

This thesis presents a novel active stent system with a selective radio frequency (RF)heating and temperature regulation capabilities. Such system is targeted at the application to endohyperthermia treatment for in-stent restenosis problems, providing a cheap and non-invasive long-term solution to the blood vessel blockage caused by a growth of scar tissue across the stent structure after implantation. The research work consists of two major portions. Firstly, a novel active stent device with ability of selective RF heating has been custom designed and explored. The device isformed by integrating a stainless-steel based stent with a flexible capacitor strip, whichserves as a frequency-selective wireless heater controlled using a tuned RF electromagnetic field applied externally. The proof-of-concept prototype device has been developed based on micro-electromechanical systems (MEMS) fabrication processes; its electrical and thermal characteristics are studied thoroughly. The finalized device is tested and evaluated within an artificial artery for validating its potential feasibility of wireless stent hyperthermia.Secondly, a MEMS-based, thermally sensitive circuit breaker chip has been designed and fabricated for the active stent temperature regulation. The temperature of an active stentdevice can be managed within a certain range after integrated with the chip, offering the controllability of RF heating of the device. Customized design and packaging methods are used in the chip fabrication; the chip-stent integration technique is also explored. The finalized device is evaluated with in-vitro tests, showing its temporal capability and wireless reliability. The experiment result verifies device working principles and suggests a direction of future research on non-invasive endohyperthermia treatments for long-term restenosis management.

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Wireless MEMS drug delivery device enabled by a micromachined Nitinol actuator as a pumping mechanism (2015)

Traditional drug delivery methods utilize systemic administration where the medication is circulated through the entire body. These methods require a high dosage at the point of entry in order to reach the therapeutic level at the targeted location and can result in serious side effects. Implantable drug delivery devices can be used to increase efficacy by targeting specific regions in the body and by safely using higher drug concentrations. Microfabrication allows for the creation of these minimally invasive devices to treat conditions not previously possible due to the limited amount of space surrounding the target area. Devices with passive releasing mechanisms have been commercialized but ones with active mechanisms are still in the works.In this thesis, a shape memory alloy (SMA) actuator is micromachined into a rectangular, planar coil to perform cantilever-like actuation. The SMA-coil actuator forms a passive resonant circuit that functions as a wireless heat source activated using external radio-frequency (RF) electromagnetic fields. SiO₂ stress layers are selectively patterned on the Nitinol SMA structure to manipulate the cantilever profile at the nominal cold state. RF radiation with varying field frequencies showed strong frequency dependence of wireless heating, actuation displacement, and force generation by several actuators with resonant frequencies of 170-245 MHz. When excited at resonance, these actuators exhibited maximum out-of-plane displacement and force of 215 µm and 71 mN, respectively. The actuator was integrated into a 10.0×10.5×2.1 mm³ polyimide-packaged chip containing a micromachined Parylene-C pump chamber to force the release of the drug from the reservoir by wirelessly activating the actuator. Experimental operation of the prototypes showed successful release of the test agents from devices placed in liquid and excited by radiating tuned RF fields with an output power of 1.1 W. These tests revealed a single release volume of 219 nL, suggesting that the device’s capacity of 76 µL is equivalent to ~350 individual ejections. Thermal behavior of the activated device is also reported in detail. This proof-of-concept prototype validates the effectiveness of wireless RF pumping for fully controlled, long-lasting drug delivery, a key step towards enabling patient-tailored, targeted local drug delivery through highly miniaturized implants.

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A study of piezoresistive sensing based on carbon-nanotube forests (2011)

Carbon nanotubes have attracted considerable attention since their discovery due to their exceptional electrical, mechanical, and optical properties. Piezoresistance of carbon nanotubes is promising, and can be utilized to enable various types of devices. This work investigates devices functionalized with vertically aligned multi-walled carbon-nanotube forests, with a focus on pressure and strain sensors. A fabrication process based on Si-micromachining techniques that overcomes the challenges associated with using carbon-nanotube forests was developed for the devices construction.A pressure sensor is fabricated to have a multi-walled carbon-nanotube forest supported by a deflectable 8-µm-thick Parylene-C membrane suspended by a silicon frame. The responses of the fabricated sensors are experimentally characterized. The sensitivities to positive and negative gauge pressures are found to be comparable in magnitude with the average values of -986 ppm/KPa and +816 ppm/KPa, respectively. The measurement also reveals that the temperature coefficient of the resistance for a forest suspended with a Parylene membrane is -515 ppm/ºC, ~3x smaller than that for a forest fixed onto a silicon substrate.A strain gauge is also fabricated to have a multi-walled carbon-nanotube forest supported by an 8-µm-thick Parylene-C membrane that is supported by two silicon substrates at both ends. The response of the fabricated strain gauge is experimentally characterized. The experiments show that the fabricated device has two sensitivity regions: a sensitive region with a gauge factor of 4.52, about 3.76x more than that for a previously reported carbon-nanotube forest/PDMS based strain gauge, and a less sensitive region with a gauge factor of 0.87. Moreover, the response to gradual strain decreases is very similar to that for gradual strain increases, and the measured gauge factors are 4.4 and 0.77 for both sensitivity regions. The results are analyzed and the source of piezoresistance is explained. Finite element analysis is performed for the strain gauge. The results show that the change in lateral separations between the carbons nanotubes, which are transversal to the direction of the applied force, are not equal in the center region, whereas the change in longitudinal separations between the carbon nanotubes, which are parallel to the applied force, are more equal.

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Investigation for improvement and application of MEMS-based micro-electro-discharge machining (M3EDM) (2011)

MEMS-based micro-electro-discharge machining (M3EDM) is a batch microfabrication technique utilizing planar electrode actuators to machine conductive materials. The low contrast pattern transfer issue in electroplating mold fabrication is firstly analyzed and improved by eliminating the contact gap and adding a rehydration step. The new method gives a better structure profile with near vertical sidewalls. The causes and mechanisms of spin coating non-uniformity and bonding voids are discussed, as well. The deformation on the foil electrode area where discharges occur is explained by abnormal heat shock, tool wear, material softening and discharge-brought reactive force. A feedback control circuit with pulse discrimination is developed to detect the harmful short pulse and prevent thermal shock. Nickel is proposed and tested as the new material for actuators owing to its higher mechanical and thermal resistance. The optimized nickel based electrodes together with the affiliation circuit are applied to cantilever MEMS contact switch fabrication. The photoresist melting in the photoresist sandwich structure is observed. A new reverse fabrication process is proposed and processed in order to minimize the photoresist melting. The method partially addresses the issue. The further directions for improvements and the potential application of the reverse process to reusable M3EDM devices are discussed.

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