Several research groups have reported light-induced fluid control based on change of surface wettability.(1, 2) Fluids on microchips were addressed by the external ultraviolet (UV) light or 785 nm laser light irradiation. The method is simple method to control fluids on microchips. However, once the channel has been wetted with the fluid, it is difficult to stop the fluid. Sershern et al. proposed optically controlled microvalve based on the volume change of nanocomposit hydrogel composed of thermoresoponsive polymer poly(N-isopropylacrylamide) (pNIPAAm) gel containing nanoparticles that have strong optical-adsorption.(3) The optical energy as strong as 1600 to 2700 mW/cm2 was transformed into heat by nanoparticles, and the heat induced shrinkage of thermoresponsive pNIPAAm gel and resulted in the opening of microvalves. In their system, the two microvalves were controlled independently by irradiating light with different wavelength. However, we think heat transfer between adjacent valves should be a problem for integrated microvalve operation.
Recently, our research group developed a photoresponsive polymer, pNIPAAm functionalized with spirobenzopyran chromophore (pSPNIPAAm), which shows solubility change in aqueous solution triggered by light irradiation.(4) The pSPNIPAAm gel has also been introduced to the surface of the porous membrane and applied to photoresponsive gate membrane.(5) The driving force for physical property change of pSPNIPAAm is based on the photoisomerization of chromophore, which is induced by the light intensity as small as 30 mW/cm2.(4) Therefore, expensive laser apparatus is unnecessary to trigger photoresponsive physical property change.
In this study, pSPNIPAAm gels were applied to the photoresponsive microvalve. The pSPNIPAAm gels were fabricated in a polydimethylsiloxane (PDMS) microchchannel by in situ photo-polymerization. By means of local light irradiation, independent control of multiple pSPNIPAAm gel microvalves were demonstrated.
Polydimethylsiloxane (PDMS) microchip for modeling multiple sample distribution was fabricated. PDMS microchip was fabricated using the standard soft lithographic techniques.(6-8) Three photoresponsive polymer, pSPNIPAAm, gel microvalves were fabricated by in situ photo-polymeriztion on a single PDMS microchip. Fluid microchannel made from PDMS was filled with the reaction mixture solution containing NIPAAm monomer, the acrylated spirobenzopyran monomer, MBAAm, and DMPA. Focused UV light was irradiated to 450 µm circular areas at the desired positions in microchannel for 10 seconds. The gels were swelled to larger size than the width of microchannels after washing and swelling.
Microvalve control by local light irradiation was demonstrated. A solution containing blue die was introduced into main microchannel at 3.4 kPa. Blue light with a wavelength range from 420 to 440 nm was irradiated to each gel at 29 °C. Each valve was opened by blue light irradiation from 18 to 30 seconds. Local light irradiation enabled the independent control of multiple microvalves. Blue light irradiation induced the colour fade out of the pSPNIPAAm gels, which indicates isomerization of spirobenzopyran chromophore, and resulted in the shrinkage of the gels. It caused the opening of microvalve.
The typical intensity of the irradiated blue light was 20 mW/cm2 in our system. Since the gel shrinkage is induced by photoisomerization of spirobenzopyran chromophore, the intensity to control pSPNIPAAm gel is very weak compared to the previously reported optically controlled microvalve using nanocomposit hydrogel (more than 1600 mW/cm2).(3) Therefore, our system does not require expensive lasor equipment. Low light intensity leads the small increase in temperature. Therefore, in the present study, the heat transfer between adjacent valves does not affect the microvalve control. Threfore, pSPNIPAAm gel microvalve is suitable for integrated fluid control system compared to the previously reported microvalves composed of thermoresponsive hydrogel.
The flow control by light irradiation provides a non-contact and independent flow control on microchip. Micro-patterned light irradaiation enables the pallarel control of multiple microvalves. Therefore, optical control of microfluids will create a novel prospect for highly integrated microfluidic devices. We think photoresponsive polymer gel microvalve will be an advantageous technique for multifunctional microfluidic chip.
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