Wensyang Hsu

An electro-thermally and laterally driven polysilicon microactuator

A novel electro-thermally and laterally driven microactuator made of polysilicon has been designed, fabricated, and tested. The operational principle is based on the asymmetrical thermal expansion of the microstructure with different lengths of two beams, but not based on the variable cross sections of the microstructure. A microgripper to demonstrate one possible application of the microactuator is fabricated and characterized. The input voltage of this design is less than 10 dc to produce 20 m displacement with about 0.6 mJ heat dissipation, and the maximum temperature is less than 600C. A gripping force up to 2.8 N can be generated. Simulation results are compared with the experimental data and show good agreement. Some design parameters strongly influencing the performance of the microactuator are discussed also.

Design and Analysis of an Electro-Thermally Driven Long-Stretch Micro Drive With Cascaded Structure

A single micro actuator usually can provide only limited output force and displacement. Therefore, proper integration of several basic actuators into an arrayed structure becomes an attractive way to magnify the output. Here an electro-thermally driven large-stretch micro drive (LSMD) with cascaded compliant structure and compact size (about 2000μm ×500μm) is designed and analyzed. Two kinds of materials, polysilicon and electroplated nickel, are used in finite element simulations. From simulations, several design parameters are found to have significant influence on the output displacements. The simulated output displacements can achieve over 100μm and 200μm for polysilicon-made and nickel-made LSMDs respectively. Larger out-stretching displacements are feasible by proper choice of design parameters. Preliminary fabrication and testing results of nickel-made LSMDs are also presented.Copyright

Digital Microfluidic Dynamic Culture of Mammalian Embryos on an Electrowetting on Dielectric (EWOD) Chip

Current human fertilization in vitro (IVF) bypasses the female oviduct and manually inseminates, fertilizes and cultivates embryos in a static microdrop containing appropriate chemical compounds. A microfluidic microchannel system for IVF is considered to provide an improved in-vivo-mimicking environment to enhance the development in a culture system for an embryo before implantation. We demonstrate a novel digitalized microfluidic device powered with electrowetting on a dielectric (EWOD) to culture an embryo in vitro in a single droplet in a microfluidic environment to mimic the environment in vivo for development of the embryo and to culture the embryos with good development and live births. Our results show that the dynamic culture powered with EWOD can manipulate a single droplet containing one mouse embryo and culture to the blastocyst stage. The rate of embryo cleavage to a hatching blastocyst with a dynamic culture is significantly greater than that with a traditional static culture (p<0.05). The EWOD chip enhances the culture of mouse embryos in a dynamic environment. To test the reproductive outcome of the embryos collected from an EWOD chip as a culture system, we transferred embryos to pseudo-pregnant female mice and produced live births. These results demonstrate that an EWOD-based microfluidic device is capable of culturing mammalian embryos in a microfluidic biological manner, presaging future clinical application.

Experimental investigation of an embedded root method for stripping SU-8 photoresist in the UV-LIGA process

In much previous research it has been popular to use SU-8 photoresist as a mold for electroplating, facilitating the production of low-cost microelectromechanical systems. However, the thickness of the electroplated structures standing on the substrate could only reach 50 µm or less due to the internal force and deformation of the photoresist in the final stripping process. In order to fabricate thicker structures, an embedded root method has been proposed to consolidate the adhesion of the metal structures to the substrate during the SU-8 removal process. In this paper, an experimental investigation of this method is conducted to characterize the relationship between the root depth, the linewidth and the achievable thickness of the electroplated structures. Some test patterns with embedded roots have been designed and fabricated to estimate the possible size of various structures associated with different depths of niches, which are completely defined through the SiO2 masking and KOH etching processes. Based on the established relationship between the root depth and the geometric sizes, a three-dimensional Ni coil, with a thickness of 200 µm, a width of 80 µm and a root depth of 4 µm, is successfully released by the SU-8 mold, which has a height of 400 µm. This cannot be achieved by the standard SU-8 molding process. The process parameters presented herein may be applied to the fabrication of other thick metal microstructures with similar needs.

Fabrication of a polymer-based torsional vertical comb drive using a double-side partial exposure method

A novel approach which uses a double-side partial exposure method to fabricate a polymer-based torsional vertical comb drive (VCD) with thick photoresist AZ9260 R � as the structural material is proposed in this paper. Front-side partial exposure defines the height of the fixed lower fingers, and back-side partial exposure creates the suspending space of the upper fingers, where the overlap and self-alignment between fingers can easily be achieved in this way. It does not need any sacrificial layer and etching process. A metal layer is finally deposited on the structural surface by a sputtering system for suitable electric conductivity to activate the polymer torsional VCD. The finite element method is used here to simulate the capacitance at different finger positions, and twelfth-order polynomial curve fitting is performed to obtain capacitance as a function of finger displacement. Then the rotation angle can be calculated analytically with the capacitance function. Also, the static deflection and dynamic response of polymer torsional VCDs are characterized experimentally, where the dimensions of the torsion plate are 300 µm wide and 360 µm long with movable fingers of length 100 µm; the torsion spring is 60 µm long and 4 µm wide. Both have a thickness of 31 µm, and the initial overlap is 11 µm in depth and 80 µm in length between the lower and upper fingers. By comparing the simulated and experimental results, the feasibility of the proposed fabrication method of polymer torsional VCDs is verified with a measured rotation angle of 2.31 ◦ under a driving voltage of 158.3 V. (Some figures in this article are in colour only in the electronic version)

A highly efficient bead extraction technique with low bead number for digital microfluidic immunoassay

Here, we describe a technique to manipulate a low number of beads to achieve high washing efficiency with zero bead loss in the washing process of a digital microfluidic (DMF) immunoassay. Previously, two magnetic bead extraction methods were reported in the DMF platform: (1) single-side electrowetting method and (2) double-side electrowetting method. The first approach could provide high washing efficiency, but it required a large number of beads. The second approach could reduce the required number of beads, but it was inefficient where multiple washes were required. More importantly, bead loss during the washing process was unavoidable in both methods. Here, an improved double-side electrowetting method is proposed for bead extraction by utilizing a series of unequal electrodes. It is shown that, with proper electrode size ratio, only one wash step is required to achieve 98% washing rate without any bead loss at bead number less than 100 in a droplet. It allows using only about 25 magnetic beads in DMF immunoassay to increase the number of captured analytes on each bead effectively. In our human soluble tumor necrosis factor receptor I (sTNF-RI) model immunoassay, the experimental results show that, comparing to our previous results without using the proposed bead extraction technique, the immunoassay with low bead number significantly enhances the fluorescence signal to provide a better limit of detection (3.14 pg/ml) with smaller reagent volumes (200 nl) and shorter analysis time (<1 h). This improved bead extraction technique not only can be used in the DMF immunoassay but also has great potential to be used in any other bead-based DMF systems for different applications.

Instability in micromachined curved thermal bimorph structures

For the micromachined thermal bimorph structure, there is often initial deflection, or so-called geometrical imperfection, which may affect the stability of the structure. Here, finite element simulations and experiments are conducted to study the influences of initial deflection and actuating region on mechanical behaviors of a curved bimorph structure with clamped boundary condition. Devices are fabricated by adjusting the internal stresses of polysilicon layers on bimorph structures to achieve various initial deflections. Various actuation regions are achieved by designing different sizes of top layers covering the bottom layers of the bimorph structures. Stable and unstable regions in terms of two design factors, initial deflection ratio and bimorph ratio, are characterized by simulations and experiments. It is found that the curved bimorph structure is stable when the bottom layer is fully covered with the top layer or the initial deflection is much smaller than the structure thickness. The stable device is found to deflect in one direction only. The bimorph structure becomes unstable while the initial deflection is close to or larger than structure thickness. For unstable curved bimorph structures, we find the snap buckling effect with two-way deflections and a hysteresis loop.

AMPFLUID: Aggregation Magnified Post-Assay Fluorescence for Ultrasensitive Immunodetection on Digital Microfluidics

Several strategies are currently employed to enhance the detection limit of bead-based assays, but all these approaches improve the sensitivity by varying the assay procedures chemically or biologically. In previous digital microfluidic setups for bead-based immunoassay, the magnetic beads were suspended for detection. We investigated the effect of bead aggregation in such an immunoassay system and propose the aggregation magnified post-assay fluorescence for ultrasensitive immunodetection on digital microfluidics (AMPFLUID). The detection signal and sensitivity are further enhanced even at the post-assay stage without altering the original assay protocol on employing magnetically triggered post-assay aggregation of beads in a digital microfluidic setup followed by processing of the fluorescent signal. This method is shown to enhance the fluorescent signal with increased consistency and sensitivity after appropriate charge-coupled device (CCD) calibration. This method of on-chip detection allows the fulfilment of consumption of a volume at the nanoliter level and a limit of detection in the range picogram/mL. In our sTNF-RI model immunoassay, only 2.5 nL of sample is required; a detection limit 15 pg/mL is achieved. The decreased uncertainty of the measure is indicated by the error bars and coefficient of variation.

Laser micromachining of the miniature functional mechanisms

The actual performance of a miniature mechanism significantly depends on the geometric quality of the machined part and specific features therein. To fabricate functional parts and features with accuracy and precision within +/- 1 μm or less, the laser micromachining system requires the capabilities of following the desired toolpath trajectories with minimum dynamic errors, high positional repeatability, and synchronization of laser firing events at precise time-and-location to ablate the material. The major objectives of this study are to fabricate miniature functional mechanisms using precision laser micromachining method, explore the machining challenges and evaluate the geometrical quality of the machined parts in terms of accuracy, precision and surface quality. Two functional mechanisms based on electro-thermal actuation have been studied. Several machining challenges related to the corner accuracy, the asynchronization of motions and, the laser-on/off events in space and time with respect to the part geometry have been addressed. The source of inaccuracies primarily stems from the geometric complexity of the mechanism that consists of several features, such as, arcs, radii, lines, curvatures, segments and pockets, along with their dimensional aspect ratio. Such a complex design requires a large number of inconsecutive trajectories to avoid thermal deformations. Copper and nickel foils with a thickness of 25 and 12.5 µm respectively were used in the fabrication of the prototypes. The machining challenges were successfully tackled and the geometrical performance of the fabricated prototypes was evaluated. Local feature accuracies within 0.1 - 0.2 µm have been recorded.

Electromagnetic/Magnetic-Coupled Targeting System for Screw-Hole Locating in Intramedullary Interlocking-Nail Surgery

At present, intramedullary interlocking nails are widely used for bone-fracture fixation in orthopedic surgeries. Surgeons often use X-ray imaging to find the actual location of the distal screw-holes of the nail after the nail is inserted into the medullary canal of a bone for fixation. Thus, the patients and medical team are inevitably exposed to radioactivity. In this paper, we report a radiation-free electromagnetic/magnetic-coupled targeting system to locate the distal screw-holes of the nail used in interlocking-nail surgery. The targeting system consists of a c-shaped electromagnet with a pick-up coil, a highly permeable curved silicon-steel strip embedded on the nail, a guiding mechanism, and electronic measuring instruments. An alternative current is applied to the electromagnet to generate a uniform magnetic field/flux in the electromagnets air gap. When the nail inserted into the medullary canal of a bone is scanned through or rotated in the air gap of the electromagnet, the magnetic flux in the air gap is influenced by the silicon-steel strip embedded on the nail. The variation of the magnetic flux induces a voltage response in the pick-up coil due to electromagnetic induction. The pattern of the voltage response is analyzed to establish a criterion for screw-hole targeting. The results obtained using this criterion reveal that the maximum targeting error of the location and orientation targeting for a screw-hole with a diameter of 5 mm is <;2 mm and 10°, respectively. Thus, the system/approach is sufficiently simple and accurate to be used by surgeons in clinical surgery.