Scott R. Green

Wireless Magnetoelastic Monitoring of Biliary Stents

This paper presents a system for wirelessly monitoring the accumulation of sludge in a biliary stent. Two generations of the system are detailed. The first-generation system utilizes a 2 × 37.5-mm ribbon sensor with a mass of 18 mg, along with 0.8-mm-thick × 1.6-mm-diameter neodymium magnets to bias the sensor. Both components are integrated with a 4-mm-diameter stainless steel stent. The second-generation system comprises a sensor and a magnetic layer [consisting of strontium ferrite particles suspended in polydimethylsiloxane (PDMS)] that conform to the meshed topology and tubular curvature of a 5-mm-diameter Elgiloy stent. The second-generation sensors have an active area of 7.5 × 29 mm and a mass of 9.1 mg. The sensors in both generations are fabricated from 28-¿m -thick foils of magnetoelastic 2826 MB Metglas, an amorphous Ni-Fe alloy. Analytical and finite-element models that predict sensor response in the dynamic biological environment are presented. The response of each system to viscosity changes that precede and accompany biliary sludge accumulation is tested, with resonant frequency changes of 2.8% and 6.5% over a 10-cP range for each respective generation. Sludge accumulation is simulated with successive coatings of either paraffin or an acrylate terpolymer. Resonant frequency response to this mass loading effect is similar for both generations of the system, showing a 40% decrease after applying a mass load of 2.5× the mass of the sensor.

Metglas–Elgiloy bi-layer, stent cell resonators for wireless monitoring of viscosity and mass loading

This paper presents the design and evaluation of magnetoelastic sensors intended for wireless monitoring of tissue accumulation in peripheral artery stents. The sensors are fabricated from 28??m thick foils of magnetoelastic 2826MB Metglas?, an amorphous Ni?Fe alloy. The sensor layer consists of a frame and an active resonator portion. The frame consists of 150??m wide struts that are patterned in the same wishbone array pattern as a 12?mm???1.46?mm Elgiloy stent cell. The active portion is a 10?mm long symmetric leaf shape and is anchored to the frame at mid length. The active portion nests within the stent cell, with a uniform gap separating the two. A gold-indium eutectic bonding process is used to bond Metglas? and Elgiloy foils, which are subsequently patterned to form bi-layer resonators. The response of the sensor to viscosity changes and mass loading that precede and accompany artery occlusion is tested in vitro. The typical sensitivity to viscosity of the fundamental, longitudinal resonant frequency at 361?kHz is 427?ppm cP?1?over a 1.1?8.6 cP range. The sensitivity to mass loading is typically between 63000 and 65000?ppm mg?1?with the resonant frequency showing a reduction of 8.1% for an applied mass that is 15% of the unloaded mass of the sensor. This is in good agreement with the theoretical response.

Tailored magnetoelastic sensor geometry for advanced functionality in wireless biliary stent monitoring systems

This paper presents three types of wireless magnetoelastic resonant sensors with specific functionalities for monitoring sludge accumulation within biliary stents. The first design uses a geometry with a repeated cell shape that provides two well-separated resonant mode shapes and associated frequencies to permit spatial localization of mass loading. The second design implements a pattern with specific variation in feature densities to improve sensitivity to mass loading. The third design uses narrow ribbons joined by flexible couplers; this design adopts the advantages in flexibility and expandability of the other designs while maintaining the robust longitudinal mode shapes of a ribbon-shaped sensor. The sensors are batch patterned using photochemical machining from 25 μm thick 2605SA1 Metglas TM , an amorphous Fe‐Si alloy. Accumulation of biliary sludge is simulated with paraffin or gelatin, and the effects of viscous bile are simulated with a range of silicone fluids. Results from the first design show that the location of mass loads can be resolved within ∼5 mm along the length of the sensor. The second design offers twice the sensitivity to mass loads (3000‐36000 ppm mg −1 ) of other designs. The third design provides a wide range of loading (sensitive to at least 10× the mass of the sensor) and survives compression into a 2 mm diameter tube as would be required for catheter-based delivery. (Some figures in this article are in colour only in the electronic version)

Sub-Torr Chip-Scale Sputter-Ion Pump Based on a Penning Cell Array Architecture

This paper investigates a miniaturized, chip-scale Penning cell array for sputter-ion pumping. In a 2.5 cm3 package, a 0.2 cm3 pump architecture with 1.5 mm diameter cells in a 4 × 2 array reduces pressure from 1 Torr (133.3 Pa) to <; 200 mTorr (26.6 Pa) in about 10 h, and from 115 mTorr (15.3 Pa) to <; 10 mTorr (1.33 Pa) in 4 h. The rate of molecular removal in this range of pressures is 0.09 ×1013 to 1.17 ×1013 molecules/s. Experiments show that the architecture is capable of igniting and sustaining the plasma required for operating at a pressure at least as low as 1.5 μTorr (200 nPa). The advantage of the Penning cell architecture reported here in comparison to other chip-scale ion pump architectures is the drastically reduced operating pressures that can be achieved and maintained. The microdischarge requires 450-600 V applied across the device and consumes 100-250 mW. The Penning cell array architecture shows significant promise for power efficient high-vacuum pumping in chip-scale and smaller systems.

Passive Wireless Strain Sensors Using Microfabricated Magnetoelastic Beam Elements

This paper describes resonant wireless strain sensors fabricated from magnetoelastic alloys. The transduction mechanism is the ΔE effect-the change in stiffness of magnetoelastic materials with applied strain or magnetic field. This is measured as a shift in the resonant frequency and is detected wirelessly using pick-up coils utilizing the magnetoelastic coupling of these materials. The sensors are fabricated from a 28-μm-thick foil of Metglas 2826 MB (Fe40Ni38Mo4B18), a ferromagnetic magnetoelastic alloy, using microelectrodischarge machining. Two sensor types are described-single and differential. The single sensor has an active area of 7 × 2 mm2, excluding the anchors. At 23°C, it operates at a resonant frequency of 230.8 kHz and has a sensitivity of 13 × 103 ppm/mstrain; the dynamic range is 0.05-1.05 mstrain. The differential sensor includes a strain independent reference resonator of area 2 × 0.5 mm2 in addition to a sensing element of area 2.5 × 0.5 mm2 that is divided into two segments. The sensor resonance is at 266.4 kHz and reference resonance is at 492.75 kHz. The differential sensor provides a dynamic range for 0-1.85 mstrain with a sensitivity of 12.5 × 103 ppm/mstrain at 23°C. The reference resonator of the differential sensor is used to compensate for the temperature dependence of the Youngs modulus of Metglas 2826 MB, which is experimentally estimated to be -524 ppm/°C. For an increment of 35°C, uncompensated sensors exhibit a resonant frequency shift of up to 42% of the dynamic range for the single sensor and 30% of the dynamic range of the differential sensor, underscoring the necessity of temperature compensation. The geometry of both types of sensors can be modified to accommodate a variety of sensitivity and dynamic range requirements.

Miniature Wireless Magnetoelastic Resonant Motor With Frequency Selectable Bidirectional Rotation

This paper presents the analysis, fabrication, and experimental results of wirelessly actuated chip-scale rotary motors. Two designs are described. Design M is actuated by a φ8 -mm magnetoelastic stator lithographically micromachined from Metglas 2826MB-bulk-foil with 25 μm thickness. It operates at a resonant frequency of 11.35 kHz while 3-Oe dc and 2-Oe amplitude ac magnetic fields are applied. The measured rotation speed, start torque, calculated driving step size, and payload are 44 r/min, 2 nN ·m, ≈23 mdeg, and 9 mg, respectively. Design S uses a stator that is a sandwich of Si (φ8 mm diameter and 65 μm thickness) and magnetoelastic foil (φ8 mm diameter and 25 μm thickness) to tailor the stiffness. The typical resonant frequencies of clockwise (CW) mode and counterclockwise (CCW) mode are 6.1 and 7.9 kHz, respectively. The CCW mode provides a rotation rate of about 100 r/min, start torque of 30 nN ·m, and driving step size of 74 mdeg while 8-Oe dc and 6-Oe amplitude ac magnetic fields are applied. Bidirectional rotation is realized by switching the applied frequency, thereby exciting the stator in a slightly different mode shape. Design S shows at least 43-mg payload capability.

Dynamic Braille Display Utilizing Phase-Change Microactuators

In this paper a phase-change microactuator is presented for use in a dynamic Braille display. The state of the art for phase-change actuators is briefly discussed. Then, key design parameters are specified which lead to the formation of a concept. The concept is characterized in terms of the design parameters, and key performance metrics such as actuation time (135-285 ms), and average power consumption (30-40 mW) have been simulated. In comparison to recent literature, the response time is expected to be improved by over two orders of magnitude, whereas average power consumption is also reduced.

Magnetoelastic wireless sensing of tissue growth for self-expanding biliary stents

Tissue growth in biliary stents causes a loss of patency after only 2-12 months in 50% of cases, leading to complications such as jaundice or cholangitis. We report micro-machined resonant magnetoelastic sensors for monitoring tissue growth on biliary stents and an appropriate transmit/receive coil configuration for these sensors. An 8 mm diameter stainless steel mesh that was used as a self-expanding stent had a 33% recovery expansion in the absence of loading. Paraffin mass loads up to 251 mg simulated tissue growth on 37.5x2 mm2, 28 mum thick sensors. A resonant frequency shift from 57.85 kHz to 22.35 kHz was observed. Varying the local viscosity over the range of healthy and diseased bile gave a shift of 1 kHz. The sensor response without mounting in the stent was negligibly different.

Scalable, high-performance magnetoelastic tags using frame-suspended hexagonal resonators

This paper presents the analysis, design and experimental evaluation of miniaturized magnetoelastic tags using frame-suspended hexagonal resonators. Magnetoelastic tags?also known as acousto-magnetic or magnetomechanical tags-?are used in wireless detection systems?for example, electronic article surveillance and location mapping systems?that electromagnetically query the resonant response of the tags. In order to obtain a strong resonant response for miniaturized tags, a frame-suspended configuration is utilized to diminish the interaction between the vibrating portion of the tag and the substrate. The signal strength can be boosted by utilizing signal superposition with arrayed or clustered magnetoelastic tags. The hexagonal tags with a diameter of 1.3?mm are batch fabricated by photochemical machining from 27??m thick Metglas? 2826 MB, which is an amorphous NiFeMoB alloy. A preferred dc magnetic field bias for these tags is experimentally determined to be ?31.5 Oe. A single frame-suspended magnetoelastic tag shows quality factors of 100?200. This design provides ?75X improvement in signal amplitude compared to the non-suspended disc tag with similar size and resonant frequency. Across ten individual frame-suspended tags, the average resonant frequency is 2.13?MHz with a standard deviation of 0.44%, illustrating that this fabrication method provides repeatability. Linear signal superposition of the response has been experimentally measured for sets of frame-suspended tags that include as many as 500 units.

Photochemically Patterned Biliary Stents with Integrated Permanent Magnets and Deformable Assembly Features for Wireless Magnetoelastic Tissue Growth Sensing

Sludge accumulation in biliary stents causes a loss of patency, leading to complications such as jaundice. We report batch-fabricated biliary stents with integrated magnetoelastic sensors for monitoring tissue growth. Integrated neodymium magnets bias the sensor and remove a source of measurement error. Paraffin mass loads up to 44.2 mg simulated tissue growth on 37.5times2 mm2, 28 mum thick sensors. A resonant frequency shift from 58.81 kHz to 34.42 kHz was observed. The sensor provides a usable signal to the interrogation system if present within at least a 320 cm3 volume adjacent to the interrogation coils.