Light Directs Microfluid Flow
Researchers at the University of Tokyo (Tokyo, Japan) and the Kanagawa Academy of Science and Technology (Kawasaki, Japan) have increased the utility of biochemical lab-on-a-chip designs by creating a new way to control microfluid flows using light.
"Our method takes advantage of changes in surface wettability of substrates coated with spirobenzopyran [SP] that occur in response to photochromism [a reversible change in a compound's absorption spectrum induced by excitation of the compound by a specific wavelength or waveband of light]," explains Haeng-Boo Kim, researcher at the University of Tokyo. "We found that the water contact angle of SP-coated glass was 60° after irradiation with ultraviolet light and 75° after irradiation with visible light [VIS], and we assumed it would be possible to photoinduce separations of phase flows in Y-shaped SP-coated microchannels." Water contact angle refers to the angle between the tangent plane of the liquid surface and the tangent plane of the solid surface at any point along the line of contact. High contact angles relate to increased liquid flow.
Microfluidics and microarrays are indispensable to the production of lab-on-a-chip microchemical systems, Kim says. Until now, such control has been effected with micropumps and microvalves integrated with microfluidics. "It is quite difficult to fabricate tiny physical pumps and valves," Kim remarks. "And we knew that the fluidic characteristics in microchannels were strongly affected by the surface properties of the channel walls because of the relatively large surface area."
The research team synthesized an SP derivative containing N-alkyl carboxylic acid, then fabricated microchannels 200-mm wide and 100-mm deep on Pyrex glass with photolithography and wet-etching techniques. They treated the inner surfaces of the channels with (3-amonopropyl) triethoxysilane modified with SP derivatives. "The average density of SP on the channel surfaces, according to the absorption measurement, was one SP unit per 39Å2," says Kim.
To test the wettability, or microfluid flow tendency, the researchers irradiated the SP derivative on a flat glass substrate with UV light, leading to the absorption of light at 362 nm by the merocyanine form of the spiropyran molecule, resulting in a water contact angle of 60°, and then light absorption of VIS at 562 nm by the neutral-spiro form, which returned the water contact angle to 75° (see figure).
The photochromism, or changes in molecular structure associated with changes in spectral absorption, and associated changes in liquid contact angles make spirobenzopyran's neutral spiro form and merocyanine form interesting candidates for microfluidics control systems.
The team prepared two test fluids, one water/air and the other water/n-hexane. Prior to introducing the fluids to the microchannels, the upper leg of the Y-channel was irradiated with UV and the lower leg with VIS, which made the upper channel more hydrophobic than the lower one. The two-phase fluids were then introduced with a syringe. With the initial two-phase fluid, water flowed up the UV-irradiated channel and air flowed down the VIS-irradiated one. And reverse irradiation resulted in a reverse of the flow, water flowing down and air up. The second two-phase fluid performed the same as the first. Water flowed to the channel irradiated with UV and n-hexane to the channel irradiated with VIS.
"Our results clearly show that the wettability difference between channels controls the flow path," says Kim, "and that means we can use this method as a photocontrolled separation device."
Eivind Hovig, a senior scientist of the department of tumor biology at the Institute for Cancer Research in The Norwegian Radium Hospital (Oslo, Norway), says there is a definite need for valve-like mechanisms such as those Kim's team demonstrated for a number of chip applications. While Hovig thinks the work described could function in a valve-like manner, "it seems that the question of time remains," he says, "as it seems to take several hundred seconds to make the shift described."
Hovig says he has not examined the compatibility of SP molecules and other chemicals and biologicals relevant for molecular biology, or the systems' ability to control precise amounts of fluid. "That may be another hurdle," he says. "If the hurdles I have mentioned can be overcome, then I believe the technology could find use in a host of on-chip liquid handling systems, primarily within time-controlled serial reactions."
