A novel concept for assembling various chemical functions onto a single microfluidic device is proposed. The concept, called a capillary-assembled microchip, involves embedding chemically functionalized capillaries into a lattice microchannel network fabricated on poly(dimethylsiloxane) (PDMS). The network has the same channel dimensions as the outer dimensions of the capillaries. In this paper, we focus on square capillaries to be embedded into a PDMS microchannel network having a square cross section. The combination of hard glass square capillary and soft square PDMS channel allows successful fabrication of a microfluidic device without any solution leakage, and which can use diffusion-based two-solution mixing. Two different types of chemically modified capillaries, an ion-sensing capillary and a pH-sensing capillary, are prepared by coating a hydrophobic plasticized poly(vinyl chloride) membrane and a hydrophilic poly(ethyleneglycol) membrane containing functional molecules onto the inner surface of capillaries. Then, they are cut into appropriate lengths and arranged on a single microchip to prepare a dual-analyte sensing system. The concept proposed here offers advantages inherent to using a planar microfluidic device and of chemical functionality of immobilized molecules. Therefore, we expect to fabricate various types of chemically functionalized microfluidic devices soon.

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A simple integration of both flow control valves and a reaction-based sensing function on a single microchip was performed by using capillary-assembled microchip (CAs-CHIP: Hisamoto, H.; Nakashima, Y.; Kitamura, C.; Funano, S.-i.; Yasuoka, M.; Morishima, K.; Kikutani, Y.; Kitamori, T.; Terabe, S. Anal. Chem. 2004, 76, 3222-3228.). In contrast to the previously reported on-chip valving systems, where the simple valving functions were integrated, our system can integrate not only valving function but also many other chemical functions to perform a complex chemical operation on a single microchip. Here, an enzymatic reaction-based readout system is employed as an example. A square capillary immobilizing N-isopropylacrylamide polymer monolith (referred to as "valving capillary") is used as a thermoresponsive "valving part" and the immobilizing enzyme-modified glycidyl methacrylate polymer monolith (referred to as "sensing capillary") is used as a "sensing part" of the CAs-CHIP. These capillaries are embedded into a lattice microchannel network fabricated on poly(dimethylsiloxane), which has the same channel dimensions as the outer dimensions of the square capillaries. After bonding, a small Peltier device (2 mm x 2 mm) for temperature control is placed on the embedded valving capillaries to control fluid flow. Using this for heating or cooling, fast operation times of 1.4 and 3.2 s for opening and closing valves, respectively, are successfully achieved. Finally, two valving capillaries are independently controlled to trap sample solution within a bypass channel, where the enzyme-immobilized capillary is embedded, and then enzymatic reaction-based sensing of chemical species is performed as an example. The fundamental characteristics of the valve-integrated microchip are fully investigated, and an application to the analysis of an enzyme substrate by using two independent valving capillaries and a sensing capillary is demonstrated.

A single-step, easy-to-use enzyme immunoassay capillary sensor, composed of functional multilayer coatings, was developed in this study. The coatings were composed of substrate-immobilized hydrophobic coating, hydrogel coating, and soluble coating containing an enzyme-labeled antibody. The response mechanism involved a spontaneous immunoreaction triggered by capillary action-mediated introduction of a sample antigen solution and subsequent separation of unreacted enzyme-labeled antibodies and antigen-enzyme-labeled antibody complexes by the molecular sieving effect of the hydrogel. An enzyme reaction at the substrate-immobilized hydrophobic coating/hydrogel coating interface resulted in a protein-selective fluorescence response. An antigen concentration-dependent response was obtained for diagnostic marker protein samples (hemoglobin A1c (HbA1c), 7.14-16.7 mg mL(-1); alpha-fetoprotein (AFP), 1.4-140 ng mL(-1); C-reactive protein (CRP), 0.5-10 μg mL(-1)) that cover a clinically important concentration range. The successful measurement of CRP in diluted serum samples demonstrated the application of this capillary sensor.