Kenichi Takahata

Batch mode micro-electro-discharge machining

This paper describes a micro-electro-discharge machining (micro-EDM) technique that uses electrode arrays to achieve high parallelism and throughput in the machining. It explores constraints in the fabrication and usage of high aspect ratio LIGA-fabricated electrode arrays, as well as the limits imposed by the pulse discharge circuits on machining rates. An array of 400 Cu electrodes with 20 /spl mu/m diameter was used to machine perforations in 50-/spl mu/m-thick stainless steel. To increase the spatial and temporal multiplicity of discharge pulses, arrays of electrodes with lithographically fabricated interconnect and block-wise independent pulse control resistance-capacitance (RC) circuits are used, resulting in >100/spl times/ improvement in throughput compared to single electrodes. However, it was found to compromise surface smoothness. A modified pulse generation scheme that exploits the parasitic capacitance of the interconnect offers similarly high machining rates and is more amenable to integration. Stainless steel workpieces of 100 /spl mu/m thickness were machined by 100 /spl mu/m/spl times/100 /spl mu/m square cross-section electrodes using in 85 s using an 80-V power supply. Surface smoothness was unaffected by electrode multiplicity. Using electrode arrays with four circuits, batch production of 36 WC-Co gears with 300 /spl mu/m outside diameter and 70 /spl mu/m thickness in 15 min is demonstrated.

Micromachined Antenna Stents and Cuffs for Monitoring Intraluminal Pressure and Flow

This paper describes two stainless steel microstructures that are microelectrodischarge machined from 50-mum-thick planar foil for intraluminal measurements of pressure and flow (with potential for applications ranging from blood vessels to bile ducts). The first structure is an inductive antenna stent (stentenna) with 20-mm length and 3.5-mm expanded diameter. It is coupled with capacitive elements to form resonant LC tanks that can be telemetrically queried. The resulting LC tanks are deployed inside silicone mock arteries using standard angioplasty balloons and used in a passive telemetry scheme to sense changes in pressure and flow. Using water as the test fluid, the resonant peaks shift from about 215 to 208 MHz as the flow is increased from 0 to 370 mL/min. The second structure is a ring-shaped intraluminal cuff with two 400times750-mum2 electrodes that are used to provide a direct transduction of flow velocity in the presence of a magnetic field. It is fabricated in a manner similar to the stentenna, but with an insulating segment. The voltage has a linear dependence on flow rate, changing by 3.1-4.3 muV per cm/s of flow (of saline) over a 180 cm/s dynamic range, with a magnetic field of about 0.25 T

A wireless microsensor for monitoring flow and pressure in a blood vessel utilizing a dual-inductor antenna stent and two pressure sensors

This paper reports a micromachined antenna stent (stentenna) that is integrated with implantable microsensors for wireless sensing of blood flow and pressure with no battery. A device that has 20-mm length and 3.5-mm diameter (after expansion) is fabricated from 50 /spl mu/m thick stainless steel foil by using batch-compatible micro-electro-discharge machining. This is coupled to two micromachined capacitive pressure sensors of approximately 1.4/spl times/1.8/spl times/0.5 mm/sup 3/ dimensions. A 0.5-/spl mu/m thick parylene layer provides electrical insulation. The integrated device is deployed inside a silicone mock artery with a standard angioplasty balloon. The planar structure is plastically deformed to a tubular shape, resulting in dual helical coils with 50-60 nH each. These L-C tanks are used to wirelessly probe pressures at two points along a channel for flow-rate detection. Fluidic experiments that emulate a blockage in the mock artery demonstrate that the resonant impedance and phase provided by the LC-tanks to a separate transmitting coil shift by 5-40 MHz over flow-rate change of 150-300 mL/min. Pressure sensitivity is 273 ppm/Torr, which is >100/spl times/ higher than past results.

Wireless microfluidic control with integrated shape-memory-alloy actuators operated by field frequency modulation

This paper reports wireless microfluidic control enabled by the selective operation of multiple bulk-micromachined shape-memory-alloy actuators using external radiofrequency magnetic fields. Each shape-memory-alloy actuator is driven by a wireless resonant heater which generates heat only when the field frequency is tuned to the resonant frequency of the heater. Multiple actuators coupled with the heater circuits that are designed to have different resonant frequencies in the range of 135–295 MHz are selectively and simultaneously controlled by modulating the field frequency to the resonant frequencies of the corresponding heaters. A wireless microsyringe device that has three actuator–heater components and a flexible parylene reservoir is developed. The 5 µl reservoir is squeezed by the 5 mm long cantilever-type actuators to eject controlled amount of liquid from the reservoir. Using the device with an acidic solution loaded in the reservoir, sequential modifications of the pH level in the liquid are experimentally demonstrated through the selective control of the three actuators. The thermal characterization of the actuator using infrared imaging shows a temperature increase of 50 °C in 4 s and the full activation of the actuator in 8 s with 300 mW field output power.

A planar approach for manufacturing cardiac stents: design, fabrication, and mechanical evaluation

A new approach that uses planar batch manufacturing technologies is presented for the design and fabrication of coronary artery stents. Stent samples with different wall patterns have been fabricated from 50-/spl mu/m-thick stainless steel foil using microelectrodischarge machining. Stents have been expanded to tubular shapes by using angioplasty balloons, both inside mock arteries and without external confinement (i.e., free-standing). Free-standing stents exhibit diameter variations of </spl plusmn/4%, almost zero radial recoil after deflation of the balloon, and longitudinal shrinkage of <3% upon expansion. A variation that uses breakable links to provide additional customization in electrical and mechanical properties is also presented. Loading tests reveal that the radial stiffness of some patterns is comparable to that of commercially available stents with greater wall thickness, while bending compliance, at 0.02 m/N for a 4-mm-long section of the stent, is also favorably large.

Stentenna: a micromachined antenna stent for wireless monitoring of implantable microsensors

This paper reports on a micromachined stent that has been developed to serve as an antenna for wireless monitoring of implantable microsensors. A 4 mm long, 3.5 mm diameter design is fabricated from 50 /spl mu/m thick stainless steel foil using a batch-compatible micro electro-discharge machining process. As it is expanded during deployment, the stent transforms from a mesh that fits snugly around the angioplasty balloon into an inductive coil. This is accomplished by strategically located breakable links which change its electrical characteristics during the plastic deformation into its final shape. This 20 nH coil is coupled to a capacitive pressure microsensor that is approximately 1.2/spl times/4/spl times/0.5 mm/sup 3/ in dimensions. Wireless monitoring is demonstrated by showing that the resonant electrical loading provided by this LC tank to a separate transmitting coil shifts by 400 KHz over a pressure change of 800 Torr. The tests are performed in a liquid environment.

Magnetoelastic microsensors for environmental monitoring

This paper reports on the operational characteristics, and application, of a new passive wireless micro-sensor platform based on magnetoelastic materials. We have used micro-electro-discharge machining (micro-EDM) to fabricate magnetoelastic micro-sensor arrays. In response to a time varying magnetic field, amorphous ferromagnetic magnetoelastic thin films efficiently convert magnetic energy into elastic energy. The elastic waves mechanically deform the sensor which, for ribbon shaped elements, has a characteristic mechanical resonant frequency that is a function of its length, elasticity, and material density. Since the magnetoelastic material is also magnetostrictive, as the sensor mechanically deforms it generates magnetic flux that extends remotely about the device, which can be detected by a pickup coil. The remote query capability of this sensor technology enables a host of new monitoring applications including in-situ and in-vivo experiments. The bare sensor is capable of measuring ambient temperature and pressure, liquid density, liquid viscosity, and fluid flow velocity. The sensor platform can be used for chemical sensing when used in combination with mass-changing chemically responsive layers. Using an array of the magnetoelastic sensors one can determine multiple environmental conditions simultaneously.

Transforming carbon nanotube forest from darkest absorber to reflective mirror

Carbon nanotube (CNT) forests are known to be among the darkest materials on earth. They can absorb the entire visible range of electromagnetic wave more efficiently than any other known black material. We have attempted controlled mechanical processing of the CNTs and, surprisingly, observed mirror-like reflection from the processed area with 10%–15% reflectivity, a level higher than typical reflectivity of pure forests by over two orders of magnitude, for a wide range of the spectrum (570–1100 nm). Patterning of micro mirrors in the forest is demonstrated to show its potential application for producing monolithically integrated reflector-absorber arrays in the material.

A novel micro electro-discharge machining method using electrodes fabricated by the LIGA process

This paper describes a new micromachining method which can produce ultrafine patterned high-aspect-ratio micro components from various kinds of materials that include those which cannot be used by silicon or the LIGA processes. This method, based on micro electro-discharge machining employs patterned high-aspect-ratio machining electrodes fabricated by the LIGA process. With this method patterned structures of several metals such as stainless steel and tungsten carbide super hard alloy have been successfully machined with short machining times. In this machining, copper electrodes electroplated via the LIGA process show good performance as far as wear resistance in the fine discharge energy region is concerned. Multiple structures have been also produced in parallel by using an electrode array. These results show that this method can be used to produce high-aspect-ratio micro components. It also shows that structures from materials which cannot be used by other micromachining techniques have become possible.

Radio-Controlled Microactuator Based on Shape-Memory-Alloy Spiral-Coil Inductor

This paper reports a bulk-micromachined shape-memory-alloy (SMA) actuator in the form of a spiral coil that constitutes an inductor-capacitor resonant circuit. The out-of-plane actuation of the SMA spiral-coil inductor is wirelessly controlled using external radio frequency (RF) magnetic fields. The resonant circuit is used as a frequency-selective wireless heater in which the SMA inductor produces heat to activate its own actuation when resonated with the RF magnetic field. The direct integration of bulk-micromachined nitinol SMA with a threshold temperature of 65 °C into a planar microfabrication process is enabled to build the 3-D spiral-coil SMA actuator in a self-assembled manner using a SiO2 reset layer patterned on the SMA coil. The fabricated SMA structure yields an out-of-plane displacement of 466 μm in the cold state. The full actuation to the flat state is reached at 70°C upon tuning the field frequency to ~ 230 MHz with an RF output power of 0.7 W. The developed actuator is demonstrated to provide a maximum force of 30 mN. The temporal response of the actuator is revealed to be two to three times faster than that of previously reported wireless SMA actuators with separate heat sources.