Sun Woh Lye

Design, fabrication, and characterization of thermoplastic microlenses for fiber-optic probe imaging

Microlens-ended fibers could find great usefulness in future biomedical applications, particularly in endoscopic imaging applications. In this context, this paper focuses on microlens-attached specialty optical fibers such as imaging fiber that can be used for probe imaging applications. Stand-alone self-aligned polymer microlenses have been fabricated by microcompression molding. The fabrication parameters have been optimized for different materials, such as poly(methyl methacrylate) (PMMA), polycarbonate (PC Lexan 123R), Zeonor 1060R (ZNR), and Topas COC. A comparison study of the focusing and spatial resolution of the fabricated lenses is performed prior to employing them for fiber-optic fluorescence imaging applications.

A novel two-degree-of-freedom MEMS electromagnetic vibration energy harvester

In this paper, a vibration-based MEMS electromagnetic energy harvester (EM-EH) device with two-degree-of-freedom (2DOF) configuration has been presented, modeled and characterized. The proposed 2DOF system comprises a primary subsystem for power generation, and an accessory subsystem for frequency tuning. A lumped parametric 2DOF model is built and examined in respect of energy harvesting capabilities. By controlling the mass ratio and frequency ratio, the first two resonances of primary mass can be tuned close to each other while maintaining comparable magnitudes. The 2DOF configuration is expected to be more adaptive and efficient than the conventional 1DOF structure, which could only operate near its sole resonance. The 2DOF EM-EH chip is fabricated on silicon-on-insulator (SOI) wafer through double-sided deep reactive-ion etching (DRIE). Induction coil is only patterned on the primary mass for energy conversion. With current prototype at an acceleration of 0.12 g, two resonances of 326 and 391 Hz with output voltages of 3.6 and 6.5 mV are obtained respectively, providing good validation for the modeling results. This paper offers new insights of implementing a multimodal MEMS EM-EH device.

Out-of-plane electret-based MEMS energy harvester with the combined nonlinear effect from electrostatic force and a mechanical elastic stopper

This paper presents the fabrication, modeling and characterization of an out-of-plane electret-based vibrational energy harvester (e-VEH) that has both positive and negative charged electret plates integrated into a single seismic mass system. Strong electrostatic spring-softening effect is induced due to the electric field provided by the double-charged electret plates. An elastic stopper is introduced for reliability concern by limiting the motion of seismic mass and meanwhile serves as a functional element to realize spring-hardening effect. The device has an overall volume of about 0.14 cm3 and is fabricated based on MEMS compatible silicon micromachining technology. When subject to weak excitations, the device exhibits an approximately linear frequency response but changes to a significantly larger broadband when strongly excited due to the combined nonlinear effect from electrostatic force and a mechanical elastic stopper. At a high excitation level of 0.48 g, the experimental results show that the device has 3 dB bandwidths of 3.7 Hz for frequency-up sweep and 2.8 Hz for frequency-down sweep, respectively, which demonstrate a large enhancement compared to the linear response (1.3 Hz). An optimal output power of 0.95 μW is also achieved with a low resonance of 95 Hz. This corresponds to a normalized power density of 37.4 μW cm−3 g−2.

Sandwich structured electrostatic/electrets parallel-plate power generator for low acceleration and low frequency vibration energy harvesting

This paper reports an improved design, fabrication method, testing and analysis of electrostatic/electrets parallel-plate power generator driven by sinusoidal vibration sources having low acceleration (<;0.05 g) and low frequency vibration (<;50Hz). An electrostatic/electrets power generator is designed with sandwich structure containing two capacitive systems. Apart from the virtue of harvesting small vibration energy source, it also generates higher power output than conventional two-layer parallel-plate structure by reducing damping effect caused by inherent electrostatic force Fe. A localized corona charging method, using charging mask, for ion implantation into electrets has significantly reduced the fast charge decaying problem in micro scale patterned electrets plates and simplified the charging procedure.

Fabrication of a stand-alone polymer microlens: design of molding apparatus, simulation and experimental results

Micron size lenses and microlens arrays are new products that are attracting the attention of the scientific community for their possible outstanding applications. Wide applications of microlens arrays, self-standing microlenses for biomedical applications, arrayed device coupling and parallel optical image processing have been reported in the recent past. Though many techniques have been utilized and well established to fabricate microlens arrays, mass production of polymer self-standing micro imaging lenses is still a challenge. This paper in this context discusses the design and development of a precision compression molding apparatus for the fabrication of stand-alone microlenses. Finite element method (FEM) simulation was used to predict the profile of the lens and the residual stresses, which could influence the optical characteristics. The FEM simulation results are in good agreement with the experimental results. Different types of characterization equipment were used to determine both the geometrical and the optical properties of the molded lens. It was observed that the geometrical properties of the molded lens match well with the mold inserts and the optical properties are found to be suitable for the intended applications.

High Q and low resonant frequency micro electret energy harvester for harvesting low amplitude harmonic of vibration

In this paper, we present the design, fabrication and testing of a micro electret power generator harvesting low frequency and low amplitude vibration sources by the aid of its high quality factor of the device. Conventional polymer, LDPE, is used as electret material in the power generator. Silicon inward S-springs with high spring constant ratios are proposed and incorporated into the device design. Resonant frequency of 97 Hz is achieved in the device testing with small volume of 0.26 cm3. High quality factor of 88.2 of the resonant system enables the power generator to output appreciable current when it harvests the low amplitude harmonic of vibration.

Broadband energy harvesting using a nonlinear 2DOF MEMS electret power generator

This paper presents the modeling and experimental results of a novel nonlinear 2-degree-of-freedom (2DOF) MEMS electret power generator for broadband energy harvesting. The proposed 2DOF resonant system that comprises a primary and an accessory mass is constructed on a silicon wafer by double-side DRIE process. Two close resonances with frequency ratio of only 1.24 (590 Hz and 731 Hz) and comparable magnitudes are achieved for the first time for a 2DOF MEMS device. With further increasing the excitation, hardening nonlinearity induced by end-stop effect is engaged, resulting in resonant frequency drift upwards and further bandwidth convergence. The outcome of the work offers a promising wideband approach by combined advantages of both multi-modal and nonlinear mechanisms.

A sandwich-structured MEMS electret power generator for multi-directional vibration energy harvesting

This paper presents the design, modeling and characterization of a compact MEMS electret power generator for multi-directional vibration energy harvesting. The device is built on a symmetrical square-shape resonator that could harvest kinetic energy from multiple directions by using two orthogonal vibration degrees of freedom. Sandwich structure is adopted not only to enhance the output performance, but also to reduce both vertical pull-in and horizontal electrostatic damping force with two separate 180° out-of-phase capacitive circuit. The generator can be a potential candidate towards a practical multi-directional energy harvesting device for low-level ambient vibrational energy harvesting.

A three-dimensional electrostatic/electret micro power generator for low acceleration and low frequency vibration energy harvesting

This paper presents the fabrication and characterization of a novel three-dimensional (3D) electret-based micro power generator, which is capable of converting low acceleration (<;0.05g) and low frequency (<;100Hz) ambient kinetic energy to electrical energy. A localized charging method integrating multiple needles is proposed. The experimental analysis shows that the proposed generator operates an out-of-plane direction at mode I of 66Hz and two in-plane directions at mode II of 75Hz and mode III of 78.5Hz with a phase difference of about 90°. It can be a potential candidate in the development of a 3D vibration energy harvester.

Enhanced performance of electrostatic energy harvester with integrated opposite-charged electrets

This paper presents a novel electrostatic vibration energy harvester with two opposite-charged-electret design for enhanced output performance. The device consists mainly of three parallel silicon plates. The middle plate of spring-mass resonant system is double-side patterned with electrodes. The negatively and positively charged electret plates are located on the bottom and top positions, respectively. Therefore, when the movable mass reaches its highest and lowest positions, maximum charge can be induced at both extremes. These are superior to the traditional two-plate configuration where maximum charge can only be created at lowest position of seismic mass. As a proof of concept, an electret-based energy harvester (e-VEH) has been designed and fabricated. Experimental results demonstrate the output voltage increases by 18.6% and 80.9% compared to the bottom electret alone and top alone configurations, respectively. This research provides a simple and effective method for improving the performance of e-VEHs.