Published papers

We developed a set of processes for depositing silver nanoparticles on a glass substrate that involves using a modified silver mirror reaction, whereby dispersants are added to a conventional silver mirror solution. The whole set of processes was completed in 15 minutes in a non-vacuum environment except the cleaning processes with oxygen plasma at 20 Pa and at a low cost. The processes can be applied to the site-selective deposition of silver nanoparticles by modifying the surface characteristics of glass substrates using a self-assembled monolayer (SAM). We used a microcontact printing method that incorporated a non-photolithographic top-down process to pattern a SAM. The proposed processes are readily applicable to promising applications of metal nanoparticles, such as ultrasensitive sensors and catalysts.

Polydimethylsiloxane (PDMS) has been widely used as a material for µTAS devices due to the superior nature of its manufacturability and chemical resistance, etc. PDMS is known to allow the permeation of gas and water-vapor. This characteristic was applied to ensure an efficient oxygen supply to cells cultivated in micro chambers and power-free sample injection into micro fluidic devices. Some applications, such as long-term experiments with aqueous samples, however, prefer to avert the permeation of water-vapor through PDMS. This paper reports on a new method to control the water-vapor permeability of PDMS by dispersing collagen powder (collagen/PDMS polymer). Firstly, we evaluated the manufacturability and material strength of the collagen/PDMS polymer to see whether the polymer could be employed as a material for µTAS devices. Subsequently, we designed and fabricated micro-fluidic devices to directly measure the amount of water-vapor permeating through a PDMS or collagen/PDMS membrane and evaluated the effect of the dispersed collagen. The use of the dispersed collagen powder successfully reduced the water-vapor permeability of PDMS by 54%.

A variety of micro medical devices have been developed to provide more advanced and less invasive medical treatment. An effective power supply is crucial to the operation of these devices. Currently, two types of power supply are used: small batteries or radio power transmission. However, the former limit the operating time of the devices, while radio power transmission affects other medical devices due to the electromagnetic waves. In this paper, we report on a gastric-fluid-utilizing micro battery (GMB) that utilizes the gastric fluid in the stomach as an electrolyte. The GMB is designed to be used in the stomach and to generate electricity based on the principle of the voltaic cell. It consists only of biocompatible materials, including the electrodes, porous ceramic filter, and polydimethylsiloxane casing. Platinum deposited on a thin glass plate and a zinc plate work as positive and negative electrodes, respectively. Though zinc dissolves during the generation of electricity, it is an essential trace metal for humans and, given the small amount released, is not toxic. The porous ceramic filter placed between the electrodes filters out any foreign materials in the stomach fluid and holds the gastric fluid by capillary attraction. In experiments, GMB successfully generated 1.0 mW (0.42 V, 2.41 mA) with a 200-Ω external load. It generated a stable output voltage of 0.6 V for more than 39 minutes with a 5-kΩ external load. We demonstrated the feasibility of the GMB for medical applications by successfully supplying power to a telemetric system and a tiny DC motor that is commercially available.

Detection of the line-of-sight (LOS) has a variety of applications, such as communication tools for elderly or disabled people, assisting drivers, and effective location of traffic signs. To realize such promising applications, there is strong demand for an LOS detection system that does not restrict users’ activities and inflicts no mental stress. The LOS can be obtained by combining the directions of the face and eyes, respectively, where the detection of location and motion of the pupils is a critical technology. In this paper, we propose a novel wearable pupil position detection system, featuring minimum disturbance to users, both physically and mentally, via the use of dye-sensitized photovoltaic devices. These latter are transparent and generate voltage according to the incident light intensity. Arraying the devices on wearable eyeglasses, this system detects the difference in the reflection light from the pupil and the white of the eye and hence determines the position of the pupil. It is wearable as an eyeglass, meaning it does not disturb users’ activities or eyesight. More importantly, it involves minimal physical and psychological stress. We fabricated the photovoltaic device and demonstrated their feasibility to determine the location of the pupil.We found it possible to meet the required reaction time of 100 ms by reducing the size of the device and to improve the accuracy of pupil location detection by arraying the devices.

We present a novel microfilter system used for solutes selective separation by molecular sizes of 1~2nm. The microfilter system consists two layers of micro chambers made of PDMS and one piece of porous membrane made of polyethersulfone (PES) by wet phase immersion method. The PES membrane at the centre of the two micro chambers acts as a barrier of molecules larger than its pores. A novel method of bonding the PES membrane to PDMS surface was developed in the device fabrication. Diffusion in the microfilter was derived theoretically and applied in the filtering evaluation. Filtering capacity of this microfilter for molecules with molecular sizes varying from 0.3nm to 8.4nm was evaluated using solution samples of sodium chloride (NaCl) and Fluorescein Isothiocyanate (FITC) Dextrans with molecular weights of 4k, 20k and 70k. The filtering experiments indicated that the optimal membrane used for the device was P2S2 (Table 1). This microfilter system with the optimal PES membrane was believed to be capable for the selective separation at a molecular size barrier of 1~2nm and have a potential for the use in portable hemodialysis systems.

微小量のサンプルを効率的に反応させるためには、サンプルの微小液滴をいかにハンドリングするかがキーとなる。本論文では微小液滴を、マイクロ流路表面の表面エネルギ制御により、パッシブに位置決め、移動させる方法を論じている。特に表面エネルギの制御をおこなう、自己組織単分子膜（SAM）のマイクロコンタクトプリントによる局所的転写技術を開発した。

Wafer bonding is an emerging and promising technology for the manufacture of complex three-dimensional structures as well as achieving vacuum sealing. Photolithography, which enables the mass production and batch fabrication of MEMS devices, is subject to the limited manufacturability of three-dimensional structures, resulting in a narrow design space and deterioration in the device performance. In wafer bonding, device wafers are patterned individually to create semi-three-dimensional structures by microfabrication and, subsequently, bonded together to form complex three-dimensional structures at the wafer level. In addition, wafer bonding can be conducted in a vacuum ambient and achieve vacuum sealing, which is crucial in many MEMS sensors and actuators since damping by air predominates at the microscale. Wafer bonding techniques utilized in the MEMS field are categorized into four types, namely: direct bonding, anodic bonding, intermediate-layer bonding, and surface activated bonding. This paper gives a brief review of recent work relevant to these bonding techniques and their applications with the emphasis on silicon direct bonding.