John T. W. Yeow

Carbon nanotubes for biomedical applications

Carbon nanotubes (CNTs) have many unique physical, mechanical, and electronic properties. These distinct properties may be exploited such that they can be used for numerous applications ranging from sensors and actuators to composites. As a result, in a very short duration, CNTs appear to have drawn the attention of both the industry and the academia. However, there are certain challenges that need proper attention before the CNT-based devices can be realized on a large scale in the commercial market. In this paper, we report the use of CNTs for biomedical applications. The paper describes the distinct physical, electronic, and mechanical properties of nanotubes. The basics of synthesis and purification of CNTs are also reviewed. The challenges associated with CNTs, which remain to be fully addressed for their maximum utilization for biomedical applications, are discussed.

Polymer-Composite Materials for Radiation Protection

Unwanted exposures to high-energy or ionizing radiation can be hazardous to health. Prolonged or accumulated radiation dosage from either particle-emissions such as alpha/beta, proton, electron, neutron emissions, or high-energy electromagnetic waves such as X-rays/γ rays, may result in carcinogenesis, cell mutations, organ failure, etc. To avoid occupational hazards from these kinds of exposures, researchers have traditionally used heavy metals or their composites to attenuate the radiation. However, protective gear made of heavy metals are not only cumbersome but also are capable of producing more penetrative secondary radiations which requires additional shielding, increasing the cost and the weight factor. Consequently, significant research efforts have been focused toward designing efficient, lightweight, cost-effective, and flexible shielding materials for protection against radiation encountered in various industries (aerospace, hospitals, and nuclear reactors). In this regard, polymer composites have become attractive candidates for developing materials that can be designed to effectively attenuate photon or particle radiation. In this paper, we review the state-of-the-art of polymer composites reinforced with micro/nanomaterials, for their use as radiation shields.

Fabricating capacitive micromachined ultrasonic transducers with a novel silicon-nitride-Based wafer bonding process

We report the fabrication and experimental testing of 1-D 23-element capacitive micromachined ultrasonic transducer (CMUT) arrays that have been fabricated using a novel wafer-bonding process whereby the membrane and the insulation layer are both silicon nitride. The membrane and cell cavities are deposited and patterned on separate wafers and fusion-bonded in a vacuum environment to create CMUT cells. A user-grown silicon-nitride membrane layer avoids the need for expensive silicon-on-insulator (SOI) wafers, reduces parasitic capacitance, and reduces dielectric charging. It allows more freedom in selecting the membrane thickness while also providing the benefits of wafer-bonding fabrication such as excellent fill factor, ease of vacuum sealing, and a simplified fabrication process when compared with the more standard sacrificial release process. The devices fabricated have a cell diameter of 22 mum, a membrane thickness of 400 nm, a gap depth of 150 nm, and an insulation thickness of 250 nm. The resonant frequency of the CMUT in air is 17 MHz and has an attenuation compensated center frequency of ~9 MHz in immersion with a -6 dB fractional bandwidth of 123%. This paper presents the fabrication process and some characterization results.

A 32 x 32 element row-column addressed capacitive micromachined ultrasonic transducer

This paper presents characterization and initial imaging results of a 32 × 32 element two-dimensional capacitive micromachined ultrasonic transducer array. The devices are fabricated using a wafer bonding process in which both the insulation layer and the membrane are user-deposited silicon nitride. The transducers use a row-column addressing scheme to simplify the fabrication process and beamformer. By adjusting the number of rows and columns that are biased, the effective aperture of the transducer can be adjusted. This is significant because it permits imaging in the near-field of the transducer without the use of a lens. The effect on the transmit beam profile is demonstrated. The transducer has a center frequency of 5.9 MHz and a relative bandwidth of 110%. Images of horizontal and vertical wires are taken to demonstrate image resolution. A three-dimensional image of four pin heads is also demonstrated.

Nanotube field electron emission: principles, development, and applications

There is a growing trend to apply field emission (FE) electron sources in vacuum electronic devices due to their fast response, high efficiency and low energy consumption compared to thermionic emission ones. Carbon nanotubes (CNTs) have been regarded as a promising class of electron field emitters since the 1990s and have promoted the development of FE technology greatly because of their high electrical and thermal conductivity, chemical stability, high aspect ratio and small size. Recent studies have shown that FE from CNTs has the potential to replace conventional thermionic emission in many areas and that it exhibits advanced features in practical applications. Consequently, FE from nanotubes and applications thereof have attracted much attention. This paper provides a comprehensive review of both recent advances in CNT field emitters and issues related to applications of CNT based FE. FE theories and principles are introduced, and the early development of field emitters is related. CNT emitter types and their FE performance are discussed. The current situation for applications based on nanotube FE is reviewed. Although challenges remain, the tremendous progress made in CNT FE over the past ten years indicates the fields development potential.

The effects of gold nanoparticles with different sizes on polymerase chain reaction efficiency

We report the effect of 5, 10 and 20 nm gold nanoparticles (AuNPs) on polymerase chain reaction (PCR) efficiency and a proposed mechanism for AuNPs affecting the PCR. It is observed that AuNPs can cause PCR inhibition, the degree of which is affected by the concentration of the AuNPs. AuNPs of larger sizes can cause complete PCR inhibition at a lower particle concentration than those of smaller sizes. Evidence from different experiments suggests that a probable mechanism is through polymerase-AuNP binding. The collected data show that the product yield is modulated by the total surface area of the AuNPs regardless of the size, which further supports the hypothesis.

2-D CMUT wafer bonded imaging arrays with a row-column addressing scheme

This paper presents fabrication and characterization results of two-dimensional capacitive micromachined ultrasonic transducer arrays which use a row-column addressing scheme. The devices are fabricated using a wafer bonding process where both the insulation layer and the membrane are user deposited silicon nitride. Two types of arrays with different resonant frequencies are presented. One is a 32×32 element array which has a resonant frequency of 15 MHz. In immersion, when transmitting and receiving with columns a centre frequency of 5.45 MHz is measured with a relative -6 dB bandwidth of 119%. Using rows, a centre frequency of 5.75 MHz is measured with a −6 dB bandwidth of 135%. The other device is a 32×32 element array with a resonant frequency of 28 MHz and in immersion has a centre frequency of ∼12 MHz. Membrane resonant frequency uniformity across the 15 MHz device has a measured standard deviation of 0.3%.

Design, Fabrication, and Characteristics of a MEMS Micromirror With Sidewall Electrodes

This paper presents a 2-DOF silicon-on-insulator (SOI) microelectromechanical systems (MEMS) mirror with sidewall (SW) electrodes. The biaxial MEMS mirror with SW is actuated by electrostatic actuators. The dimension of mirror plate is 1000¿m × 1000¿m, with a thickness of 35 ¿m. The analytical modeling, fabrication process, and performance characteristics are described. This paper analyzes the effects of the three-end single crystal serpentine torsion bar width and the bottom and SW electrodes on the performance of the mirror. A new fabrication process based on SOI wafer, hybrid bulk/surface micromachined technology, and a high-aspect-ratio shadow mask is presented. In comparison to previous fabrication processes and the Optical iMEMS process, the process is novel, easily understood, and simple to realize. The measured maximum angular deflection achieved is ±11°(mechanical angle) at a static operating voltage and is ± 21°(mechanical angle) at resonance frequency driving. This mirror is well suited for applications where these characteristics are critical, such as in confocal or endoscopic scanning elements

Nanotechnology-Enabled Wireless Sensor Networks: From a Device Perspective

The advancement of wireless communications and integrated circuit technology has enabled the development of low-cost sensor networks. The sensor networks can be used for various application areas (disaster recovery, health, military, homeland security, environment, home, etc.). For each application area, there are different technical issues that researchers are currently resolving. However, many of them are trying to tackle the limitations of this field from a network perspective. Sometimes, the effectiveness of some proposed approaches must be complemented by the supports of hardware design. This article points out the possibilities of overcoming the same problem set from a device perspective by taking advantage of the merits of nanotechnologies. At the same time, open research issues and challenges are identified to spark new interests and developments in this field

Second-order sliding mode control of a 2D torsional MEMS micromirror with sidewall electrodes

A second-order sliding mode control (2-SMC) scheme with a proportional integral derivative (PID) sliding surface, to achieve enhanced transient response, accurate positioning and precise tracking performance of a 2-degree-of-freedom (2D) torsional MEMS micromirror with sidewall electrodes, is developed in this paper. The PID sliding surface is chosen to achieve a zero steady-state error of the closed-loop system. The 2-SMC is able to reduce the chattering phenomena, which comprises of an equivalent control and switching control to dominate model uncertainty and external disturbances leading to an enhanced performance of the controlled system. Finite-time convergence of the closed-loop system in the presence of bounded parameter uncertainties and external disturbances is guaranteed through Lyapunov stability analysis. The proposed 2-SMC is programmed in a LABVIEW environment and implemented based on National Instrument (NI) field-programmable gate array hardware to verify the effectiveness and robustness. The experimental results of set-point regulation and sinusoidal trajectory tacking demonstrate that the closed-loop system with the proposed control scheme significantly improves the transient performance, accurate positioning and trajectory tracking with robustness against external disturbance. The 95% settling time is shortened from 70 to 3 ms for the X-axis and from 60 to 3 ms for the Y-axis respectively, the overshoots and steady-state errors are eliminated in both axes, and less than 5% maximum positioning error is achieved in the presence of external disturbance.