基礎講座 電気泳動(Electrophoresis)

Two-photon excitation provides a means of activating chemical or physical processes with high spatial resolution in three dimensions and has made possible the development of three-dimensional fluorescence imaging, optical data storage, and lithographic microfabrication. These applications take advantage of the fact that the two-photon absorption probability depends quadratically on intensity, so under tight-focusing conditions, the absorption is confined at the focus to a volume of order λ3 (where λ is the laser wavelength). Any subsequent process, such as fluorescence or a photoinduced chemical reaction, is also localized in this small volume. Although three-dimensional data storage and microfabrication have been illustrated using two-photon-initiated polymerization of resins incorporating conventional ultraviolet-absorbing initiators, such photopolymer systems exhibit low photosensitivity as the initiators have small two-photon absorption cross-sections (δ). Consequently, this approach requires high laser power, and its widespread use remains impractical. Here we report on a class of π;-conjugated compounds that exhibit large δ (as high as 1, 250 × 10−50 cm4 s per photon) and enhanced two-photon sensitivity relative to ultraviolet initiators. Two-photon excitable resins based on these new initiators have been developed and used to demonstrate a scheme for three-dimensional data storage which permits fluorescent and refractive read-out, and the fabrication of three-dimensional micro-optical and micromechanical structures, including photonic-bandgap-type structures.

Two-photon polymerization initiators for three-dimensional optical data storage and microfabrication

Micromachining technology was used to prepare chemical analysis systems on glass chips (1 centimeter by 2 centimeters or larger) that utilize electroosmotic pumping to drive fluid flow and electrophoretic separation to distinguish sample components. Capillaries 1 to 10 centimeters long etched in the glass (cross section, 10 micrometers by 30 micrometers) allow for capillary electrophoresis-based separations of amino acids with up to 75,000 theoretical plates in about 15 seconds, and separations of about 600 plates can be effected within 4 seconds. Sample treatment steps within a manifold of intersecting capillaries were demonstrated for a simple sample dilution process. Manipulation of the applied voltages controlled the directions of fluid flow within the manifold. The principles demonstrated in this study can be used to develop a miniaturized system for sample handling and separation with no moving parts.

Micromachining a miniaturized capillary electrophoresis-based chemical analysis system on a chip

Micro total analysis systems. Recent developments.

Polymers have assumed the leading role as substrate materials for microfluidic devices in recent years. They offer a broad range of material parameters as well as material and surface chemical properties which enable microscopic design features that cannot be realised by any other class of materials. A similar range of fabrication technologies exist to generate microfluidic devices from these materials. This review will introduce the currently relevant microfabrication technologies such as replication methods like hot embossing, injection molding, microthermoforming and casting as well as photodefining methods like lithography and laser ablation for microfluidic systems and discuss academic and industrial considerations for their use. A section on back-end processing completes the overview.

Polymer microfabrication technologies for microfluidic systems

Free-standing SU-8 chips with enclosed microchannels and high density of fluidic inlets have been made in a three-layer process which involves SU-8 to SU-8 adhesive bonding and sacrificial etching. With this process we can fabricate microchannels with depths ranging from 10 to 500 μm, channel widths from 10 to 2000 μm and lengths up to 6 cm. The process is optimized with respect to SU-8 glass transition temperature. Thermal stresses and thickness non-uniformities of SU-8 are compensated by novel mask design features, the auxiliary moats. With these process innovations filling of microchannels can be prevented, non-bonded area is minimized and bonding yields are 90% for large-area microfluidic chips. We have released up to 100 mm in diameter sized microfluidic chips completely from carrier wafers. These free-standing SU-8 chips are mechanically strong and show consistent wetting and capillary filling with aqueous fluids. Fluidic inlets were made in SU-8 chips by adding one lithography step, eliminating through-wafer etching or drilling. In our process the inlet size and density is limited by lithography only.

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Free-standing SU-8 microfluidic chips by adhesive bonding and release etching

The characterization of SU-8 microchannels for electrokinetic microfluidic applications is reported. The electroosmotic (EO) mobility in SU-8 microchannels was determined with respect to pH and ionic strength by the current monitoring method. Extensive electroosmotic flow (EOF), equal to that for glass microchannels, was observed at pH ≥4. The highest EO mobility was detected at pH ≥7 and was of the order of 5.8 × 10−4 cm2 V−1 s−1 in 10 mM phosphate buffer. At pH ≤3 the electroosmotic flow was shown to reverse towards the anode and to reach a magnitude of 1.8 × 10−4 cm2 V−1 s−1 in 10 mM phosphate buffer (pH 2). Also the zeta-potential on the SU-8 surface was determined, employing lithographically defined SU-8 microparticles for which a similar pH dependence was observed. SU-8 microchannels were shown to perform repeatably from day to day and no aging effects were observed in long-term use.

Characterization of SU-8 for electrokinetic microfluidic applications.

This study presents coupling of a poly(dimethylsiloxane) (PDMS) micro-chip with electrospray ionization-mass spectrometry (ESI-MS). Stable electrospray is generated directly from a PDMS micro-channel without pressure assistance. Hydrophobic PDMS aids the formation of a small Taylor cone in the ESI process and facilitates straightforward and low-cost batch production of the ESI-MS chips. PDMS chips were replicated with masters fabricated from SU-8 negative photoresist. A novel coating, an amorphous diamond-like carbon–poly(dimethylsiloxane) hybrid, deposited on the masters by the filtered pulsed plasma arc discharge technique, improved significantly the lifetime of the masters in PDMS replications. PDMS chip fabrication conditions were observed to affect the amount of background peaks in the MS spectra. With an optimized fabrication process (PDMS curing agent/silicone elastomer base ratio of 1/8 (w/w), curing at 70 °C for 48 h) low background spectra were recorded for the analytes. The performance of PDMS devices was examined in the ESI-MS analysis of some pharmaceutical compounds and amino acids.

Poly(dimethylsiloxane) electrospray devices fabricated with diamond-like carbon–poly(dimethylsiloxane) coated SU-8 masters

We present a fully microfabricated and monolithically integrated capillary electrophoresis (CE)-electrospray ionization (ESI) chip for coupling with high-throughput mass spectrometric (MS) analysis. The chips are fabricated fully of a negative photoresist SU-8 by a standard lithographic process which enables straightforward batch fabrication of multiple chips with precisely controlled dimensions and, thus, reproducible analytical performance from chip to chip. As the coaxial sheath flow interface is patterned as an integral part of the SU-8 chip, the fluidic design is dead-volume-free. No significant peak broadening occurs so that very narrow peak widths (down to 2−3 s) are obtained. The sheath flow interface also enables comprehensive optimization of both the CE and the ESI conditions separately so that the same chip design is adaptable to diverse analytical conditions. Plate numbers of the order of 105 m-1 and good resolution are routinely reached for small molecules and peptides within a 2 cm separatio...

Fully Microfabricated and Integrated SU-8-Based Capillary Electrophoresis-Electrospray Ionization Microchips for Mass Spectrometry

A microfabricated re-usable multi-inlet fluidic connector chip in PDMS (polydimethylsiloxane) is introduced. A connector takes up ca. 1 mm 2 area and inlet density is limited principally by capillary tubing dimensions. Anisotropically etched silicon mold, with patterns matching those of the microfluidic chip, and dummy fibers have been used in PDMS casting. The connector chip material is self-sealing PDMS, which eliminates the need for gluing. This feature enables tubing to be attached and removed repeatedly, as long as the PDMS surfaces in contact with the microfluidic chip remain clean. Both imide-coated glass capillaries and polymeric PEEK (polyetheretherketone) tubes have been successfully connected to a microfluidic nebulizer chip which has one liquid and two gas ports. The connections were measured to withstand overpressures up to 220 and 20 kPa with and without additional compression, respectively. The fact that the connector chip is fabricated independently of the microfluidic chip improves significantly assembly yield of the fluidic system, because the microfluidic chip is not subject to gluing or other assembly operations. We tested the PDMS chip successfully with the nebulizer chip in mass spectrometry application. Even though room temperature performance of the connector chip was excellent, at elevated temperatures (above 50 °C) significant background signal originating from PDMS can be seen in mass spectra. This limits the usability of PDMS parts in sensitive mass spectrometry applications.

Santeri Tuomikoski

Papers

Free-standing SU-8 microfluidic chips by adhesive bonding and release etching

Characterization of SU-8 for electrokinetic microfluidic applications.

Poly(dimethylsiloxane) electrospray devices fabricated with diamond-like carbon–poly(dimethylsiloxane) coated SU-8 masters

Fully Microfabricated and Integrated SU-8-Based Capillary Electrophoresis-Electrospray Ionization Microchips for Mass Spectrometry

Re-usable multi-inlet PDMS fluidic connector