Babak Assadsangabi

Ferrofluid Sacrificial Microfabrication of Capacitive Pressure Sensors

A novel production approach to the fabrication of capacitive micropressure sensors is reported. A magnetic fluid known as ferrofluid is used as the liquid-phase sacrificial layer in the microfabrication process, enabling extremely simple, fast, and low-cost production of the sensors while eliminating the need for photolithographic, bonding, and/or chemical processes. The entire sensor fabrication is performed at/near room temperature. The sensors are designed to be constructed on the 1.5 × 1.5-mm2 stainless-steel chip, being micromachined to have capacitive cavities with 10-30 μm depths. A Parylene-C membrane with a titanium electrode is formed to seal the cavity by depositing it directly on top of the ferrofluid filled in the cavity. The ferrofluid is magnetically extracted from the cavity after the formation of the membrane, suspending it to establish the sensing capacitor. A highly linear response of 12.4 fF/KPa is obtained with the fabricated device. The temperature dependence of the sensor capacitance is experimentally characterized and reported as well.

Planar Variable Inductor Controlled by Ferrofluid Actuation

This paper reports a micropatterned variable inductor that utilizes ferrofluid as the movable magnetic core. Ferrofluid is displaced over the planar inductor using the magnetic field provided by another “actuation” coil aligned to the inductor, varying permeability distribution on the inductor to control its inductance. Superimposing a bias field on the varying magnetic field uniquely enables repelling of the fluid from the inductor, as well as switching between repelling and attracting motions simply by changing the direction of the driving current to the actuation coil. Microfabricated devices are tested to reveal that the repelling-mode operation provides 6.1 × larger inductive changes than those available with the attracting mode. The continuous modulation of inductance is demonstrated with a tuning range of 16% using input currents up to 1.2 A. Frequency tuning using a resonant tank formed with the device is also demonstrated to show a 76-ppm/mA frequency sensitivity to the input current.

Electromagnetic Microactuator Realized by Ferrofluid-Assisted Levitation Mechanism

This paper reports a highly simple and robust levitation method realized by ferrofluid for micromotor applications. A layer of ferrofluid is self-sustained on a permanent magnet to serve as liquid bearing that lifts the magnet up on the substrate, enabling low-friction movements of the magnet. A levitation height of ~500 μm is observed with a 1.6-mm-sized NdFeB magnet. The load carrying capacity and friction force of the ferrofluid-levitated magnet are measured to be ~2.9 g and 14 μN, respectively. The levitated magnet is electromagnetically driven by microfabricated planar coils to demonstrate a linear micromotor. The dynamic responses of the magnet slider are characterized in detail and shown to match well with the results from electromagnetic simulations of the driving coil. The actuation force of 386 μN or greater is obtained. Using the electromagnetic rail track that contains a micropatterned array of planar coils, continuous actuation of the slider along the array is achieved with the average velocity of 19 mm/s. The feasibility of stepping displacements between two adjacent coils is demonstrated by controlling power distribution to the two coils.

Wirelessly addressable heater array for centrifugal microfluidics and escherichia coli sterilization

Localized temperature control and heater interface remain challenges in centrifugal microfluidics and integrated lab-on-a-chip devices. This paper presents a new wireless heating method that enables selective activation of micropatterned resonant heaters using external radiofrequency (RF) fields and its applications. The wireless heaters in an array are individually activated by modulating the frequency of the external field. Temperature of 93°C is achieved in the heater when resonated with a 0.49-W RF output power. The wireless method is demonstrated to be fully effective for heating samples under spinning at high speeds, showing its applicability to centrifugal systems. Selective sterilization of Escherichia coli through the wireless heating is also demonstrated. Healthcare applications with a focus on wound sterilization are discussed along with preliminary experiments, showing promising results.

Ferrofluid-assisted micro rotary motor for minimally invasive endoscopy applications

This paper reports a micro rotary motor that is enabled with magnetic fluid called ferrofluid used as an extremely simple, miniaturized bearing material for microendoscopy applications. The ferrofluid bearing is magnetically sustained on the permanent magnet rotor that is levitated by the bearing layer inside a tubular substrate, an endoscope catheter. The levitated rotor is electromagnetically driven by two photo-defined meander-type coils formed around the outer walls of the catheter that enables 90°-step angular actuation of the rotor. The fabricated prototype with the rotor coupled with a 1-mm-sized prism mirror is revealed to provide both step-wise and continuous rotations with revolution rates up to 1875 rpm, verifying the effectiveness of the bearing and motor mechanism. The prototype device is operated to demonstrate its ability of endoscopic imaging in an experimental model.

Ferrofluid-based variable inductor

This paper reports a planar variable inductor that utilizes ferrofluid as the movable magnetic core. Ferrofluid is displaced over the planar inductor using the magnetic field provided by another coil aligned to the inductor, varying permeability distribution on the inductor to control its inductance. The repelling motion of ferrofluid from the inductor is uniquely enabled by superimposing a bias field on the varying magnetic field. The continuous modulation of inductance is experimentally demonstrated with the fabricated device. A 16% inductance variation is obtained at the full removal of the fluid from the inductor with a driving current of 1.2 A provided to the actuation coil. Frequency tuning is also demonstrated using a passive resonant tank constructed with the developed device.

Microfabrication of capacitive pressure sensors using ferrofluid sacrificial layers

A novel micromachined capacitive pressure sensor that is fabricated using a liquid-phase sacrificial layer is reported. Ferrofluid serves as the sacrificial layer in the microfabrication process, enabling extremely simple, fast, and low-cost production of the sensor, while eliminating the need for photolithographic, bonding, and/or chemical processes. The entire sensor fabrication is performed at/near room temperature. The sensor is based on the 1.5×1.5-mm2 stainless-steel chip with a 10-μm-deep capacitive cavity. A Parylene-C membrane with a Ti electrode is formed to seal the cavity by depositing it directly on top of the ferrofluid filled in the cavity. The ferrofluid is magnetically extracted from the cavity, suspending the membrane to form the sensing capacitor. A highly linear response with a sensitivity of 12.4 fF/KPa is obtained with the fabricated device. The temperature dependence of the sensor capacitance is experimentally characterized and reported as well.

Enabling Angioplasty-Ready “Smart” Stents to Detect In-Stent Restenosis and Occlusion

Abstract Despite the multitude of stents implanted annually worldwide, the most common complication called in‐stent restenosis still poses a significant risk to patients. Here, a “smart” stent equipped with microscale sensors and wireless interface is developed to enable continuous monitoring of restenosis through the implanted stent. This electrically active stent functions as a radiofrequency wireless pressure transducer to track local hemodynamic changes upon a renarrowing condition. The smart stent is devised and constructed to fulfill both engineering and clinical requirements while proving its compatibility with the standard angioplasty procedure. Prototypes pass testing through assembly on balloon catheters withstanding crimping forces of >100 N and balloon expansion pressure up to 16 atm, and show wireless sensing with a resolution of 12.4 mmHg. In a swine model, this device demonstrates wireless detection of blood clot formation, as well as real‐time tracking of local blood pressure change over a range of 108 mmHg that well covers the range involved in human. The demonstrated results are expected to greatly advance smart stent technology toward its clinical practice.

Catheter-Based Microrotary Motor Enabled by Ferrofluid for Microendoscope Applications

This paper reports the first microrotary motor enabled with a novel ferrofluid-based levitation mechanism used as an extremely simple miniaturized bearing material for microendoscopy applications. The ferrofluid bearing is magnetically sustained on the permanent magnet rotor that is levitated by the bearing layer inside a tubular substrate, an endoscope catheter. The levitated rotor is electromagnetically driven by two photo-defined meander-type coils formed around the outer walls of the catheter that enables 90°-step angular actuation of the rotor. Two types of micromotors with 1.6-mm and 500-μm-sized rotors are designed, fabricated, and tested. The fabricated prototypes of the motors are successfully operated to rotate the prism mirrors integrated with the motors, with revolution rates as high as 1875 and 1500 r/min for the 1.6-mm and 500-μm rotor types, respectively. Thermal behaviors of the devices are also characterized and reported. The experimental results demonstrate the effectiveness of the motor design and indicate high potential for side-viewing microendoscopic applications.

Micro rotary-linear actuator assisted by ferrofluid levitation for 3-dimensional endoscopic imaging

This paper reports the first microactuator that selectively and simultaneously produces rotational and linear motions of its rotor/slider targeted at applications for minimally invasive active catheters. The magnet rotor/slider component is levitated by ferrofluid accumulated around it inside a catheter tube, achieving low-friction actuation for electromagnetic rotation of the component and/or its pneumatic sliding along the tubes axis. A microfabricated prototype of the actuator is coupled with a prism mirror to demonstrate full 3-dimentional (3D) scanning of an energy beam, verifying the designed functionality of the actuator. Testing of the prototype reveals a superior axial stability of rotation over the preceding device with no linear actuation function. The results indicate a promising potential for circumferential 3D imaging usable for different types of endoscopic imaging including ultrasound and optical coherence tomography.