Integrated microchips for biological analysis fabricated by femtosecond laser direct writing
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Introduction In the last decade, the demand to reduce the volume of samples and reagents used in chemical reactions, biological analysis, and medical tests has increased significantly due to the need to reduce reagent consumption, waste production, analysis time, and labor costs. To achieve this, the use of microchips such as microreactors, lab-on-a-chip (LOC) devices, micro-total analysis systems (μ-TAS), and optofluidics has been proposed.1,2 Palm-sized microchips into which an entire room of laboratory equipment can be shrunk and packed into a small space are capable of satisfying this demand. In addition, in order to achieve high efficiency, high accuracy, and high sensitivity analysis, increasingly complex microchips are being fabricated in which microfluidic-based systems are integrated with electrical, mechanical, and optical microcomponents in a single chip. Currently, the most common technique for fabricating microfluidics is soft lithography, a technique that uses polydimethylsiloxane (PDMS) molds for replicating microstructures.3
Although soft lithography is fast and cost effective, it cannot be used to directly form three-dimensional (3D) microfluidic structures such as microchannels and microreservoirs in a transparent substrate without stacking and bonding layers. In addition, unlike glass, PDMS is chemically incompatible with many organic solvents, and compositional inhomogeneities frequently cause optical scattering, which is unsuitable for optical sensing in microchips. For microchips with glass substrates, conventional fabrication employs planar microfabrication techniques, such as injection molding or conventional semiconductor processes based on photolithography, which requires stacking and bonding to construct 3D structures. Femtosecond laser direct writing is a promising fabrication technique that can be used to modify the interior of glass in a spatially selective manner. It can be used to directly form 3D microfluidic structures within substrates because it employs a nonlinear interaction (i.e., multiphoton absorption) between a
Koji Sugioka, Riken Advanced Science Institute; [email protected] Ya Cheng, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences; [email protected] DOI: 10.1557/mrs.2011.274
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MRS BULLETIN • VOLUME 36 • DECEMBER 2011 • www.mrs.org/bulletin
© 2011 Materials Research Society
INTEGRATED MICROCHIPS FOR BIOLOGICAL ANALYSIS FABRICATED BY FEMTOSECOND LASER DIRECT WRITING
tightly focused femtosecond laser beam and glass. This interaction is effectively restricted to the vicinity of the focal point where the laser intensity exceeds the threshold for multiphoton absorption.4 The femtosecond laser pulses modify the chemical properties of the substrate in the laser-irradiated regions, which can be selectively removed by successive wet etching using acids such as hydrofluoric (HF) acid, realizing direct fabrication of 3D microfluidics.5–7 This two-step process (i.e., femtosecond laser direct writing followed by wet etching) can also be used to integrat
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