Recently, studies have been actively made on microminiaturization of chemical reactions and a separation systems making use of a microprocessing technique, which are called a microreactor and a micrototal analysis system (μTAS). They are expected for applications to analysis and synthesis of nucleic acids, proteins, sugar chains and the like; quick analysis of microchemical substances; and high throughput screening of drugs and medicines; which are performed on a microchip having a microchannel.
Microminiaturization merits of such a system include: (1) being able to reduce a used amount and a disposed amount of a sample and a reagent which are used in chemical reactions and antigen-antibody reactions; (2) being able to reduce a power source required for a process; (3) being able to realize thermal transport and substance transport at high rates by improvement in ratio of a surface area to a volume, resulting in that accurately control of reaction and separation, high rate and high efficiency, and side reaction suppression are expected; (4) being able to simultaneously treat a large number of samples on the same one substrate; and (5) being able to perform on the same substrate from sampling to detection, and realization of a small-footprint, portable and low-priced system is considered.
On the other hand, demerits include: (1) that there are many cases where detection sensitivity deteriorates due to a decrease in a detected area; (2) that it is difficult to generate a turbulence in a fluid flow of a microscale, and it takes time to mix a reagent or the like due to occurring diffused mixing; (3) that a method for highly accurately transport a liquid in minute amount is required for transport the reagent and the like to a fine channel; and (4) that in the case of occurrence of bubbles or the like it is difficult to remove them because an effect of a surface tension is significant, and a large adverse effect is often exerted upon a measurement system. While there are such merits and demerits, a microfluidics technique has been studied, and has been seen in our daily life, in such forms as: an acceleration sensor, a pressure sensor, a position sensor (gyroscope) and the like in the automobile industry field, a light guide, a light switch, a mirror, a lens and the like in the telecommunications industry field; and applications of a blood analysis, a DNA analysis, scientific criminal investigation in the life science industrial field. Besides, the applications are further expanded to the food field, the environmental test field, and the military field.
As microfluidics techniques currently under development, reports have been made on an enzyme or antigen-antibody reaction, an ion-sensitive field-effect transistor (ISFET), a microelectrode, a microcantilever, and a microsensor making use of an acoustic wave and resonance, many of which are sensor applications. As applications, a number of reports have been made on a microelectrophoresis chip, a micro-PCR (Polymerase Chain Reaction) chip, a microgas chromatography chip, a microliquid chromatography chip, a DNA separation chip, and the like. Further, a report has been made on development of a lab-on-a-chip that makes implementation from sampling to analysis on the same chip, and examples thereof include: a multifunctional biochip using a peculiar nucleic acid or antibody for anthrax or bacillus; a portable measurement apparatus that monitors glucose, lactose and the like; a clinical test chip using the antigen-antibody reaction.
In the microfluidic device as described above, a back-pressure type pump is often used as a liquid transport mechanism, and a plunger pump, a peristaltic pump, a syringe pump or the like is used. Further, in a system of performing capillary electrophoresis, an electroosmotic flow is chiefly used. Moreover, by making full use of microprocessing, there have been developed and presented a pump formed by combination of a piezoelectric element and a diaphragm, and further, a diffuser type pump making use of asymmetry of channels. Furthermore, there are methods such as a liquid transport method making use of centrifugal force and an ink-jet liquid transport method. However, since the method for highly accurately transporting a liquid in minute amount is required for transporting a reagent or the like to a fine channel, there has been a problem in that the liquid transport mechanism is large in size or high-priced as compared with a microchannel chip. In point-of-care applications where portability is required, applications of environment or food analysis, and disposable applications often seen in the biological and biochemical fields, where a portion touched by a sample is not reused for avoiding contamination, the liquid transport mechanism is also required to be simple and low-priced, and hence there have been used a liquid transport method making use of capillarity phenomenon or surface tension, or a liquid transport method by means of a fall method making use of gravity.
As the liquid transport method using capillarity phenomenon, paper immuno chromatography and immuno chromatography have already been utilized in many fields. However, since it is the method using a membrane, there are limitations in such fields of use as control of a liquid transport rate and taking-out of a transported liquid. As the liquid transport method making use of surface tension, there has been devised a microliquid droplet transport device provided with a channel made up of a hydrophilic surface and a hydrophobic surface, where a value obtained by dividing the hydrophilic surface by the hydrophobic surface is continuously increased from the upstream toward the downstream so as to transport the liquid in one direction (see Patent Document 1). According to the microliquid droplet transport device, the liquid is transported along the channel due to the gradual increase in ratio of the hydrophilic property from the upstream toward the downstream.
Similarly, there have been devised a microchannel system and a liquid transport device where a channel is formed by a hydrophilic area and a hydrophobic area, and a liquid is transported along the channel (see Patent Documents 2 and 3). The liquid transport method making use of surface tension requires minute and accurate processing of the hydrophilic area and the hydrophobic area, which is industrially problematic method. As the fall method making use of gravity, there have been devised methods making use of pressure attributed to a height difference in a gravity direction between liquid surfaces of an inlet reservoir and an outlet reservoir which are connected through the channel (see Patent Documents 4 and 5). However, stable control of a flow rate is difficult under the influence of surface tension, and it has been a challenge to provide a microfluidic device capable of allowing a fluid to stably flow in a microchannel without using an external source such as a pump or a suction device.