254
J. Xu et al.
FLAE can be extended to the fabrication of 3D microfluidic devices in polymer
substrates. Hanada et al. recently developed a fabrication technique that involves fs
laser direct writing followed by wet etching with a dilute fluorinated solvent and
annealing to create high-quality 3D microfluidic chips inside a substrate made of the
low refractive index fluoric polymer CYTOP [23]. To ensure clear 3D microscopic
observations of cell motion near the solid–liquid interface, a minimal mismatch
between the refractive indices of the medium and the chip substrate is desirable. The
refractive index of CYTOP is 1.34, which is very close to that of water. Thus, these
CYTOP microfluidic chips enabled precise observation of the flagellar motion of
a Dinoflagellate that typically moves in circles near the fluid surface. The CYTOP
microfluidic chips are expected to provide opportunities to analyze, in detail, the
behavior of various cells near the water–solid interface.
As noted, LAFLD is another method for the manufacture of 3D microfluidic
devices [36–40]. FLAE results in some etching of the unexposed regions around
the laser-exposed areas due to the limited selectivity of wet chemical etching. In
contrast, LAFLD removes only the laser-exposed regions or possibly even smaller
zones, since it relies on ablation. Therefore, LAFLD can be applied to the fabrication of 3D nanofluidic devices [40]. Liao et al. demonstrated the rapid fabrication
of a passive microfluidic mixer consisting of geometrically complex 3D microchannels via LAFLD of porous glass [43]. This 3D mixer was composed of a Y-shape
microchannel embedded 400 µm below the surface of the glass chip in conjunction with a string of mixing units, connected to two opening inlets and one outlet
(Fig. 8.2d). Experimental trials demonstrated that two fluorescent dye solutions (fluorescein sodium and Rhodamine B) were well mixed after passing through three
mixing units (corresponding to a length of 0.9 mm) over a time span of approximately 10 ms. In contrast, efficient mixing was not achieved in a 1D microfluidic
channel over a greater propagation distance of about 1.3 mm. Based on a combination of the threshold effect of fs laser processing using a Gaussian beam and the
formation of a periodic nanograting, LAFLD was also used to successfully manufacture buried nanofluidic channels with transverse widths less than 50 nm in porous
glass [74]. Integrated devices containing nanofluidic-microfluidic systems with 3D
configurations have also been used for the investigation of the stretching of DNA
molecules [75].
8.4 Fabrication of Optofluidic Devices
The fs laser 3D process can also integrate certain microoptic/photonic components
into microfluidic units to create monolithic optofluidic devices for highly sensitive detection of biochemical species and functional manipulation of living cells.
One straightforward strategy involves fs laser direct writing of WGs and WGbased photonic components (such as a Mach-Zehnder interferometer (MZI)) in 3D
microfluidic devices fabricated by FLAE. FLAE itself can simultaneously fabricate
J. Xu et al.
FLAE can be extended to the fabrication of 3D microfluidic devices in polymer
substrates. Hanada et al. recently developed a fabrication technique that involves fs
laser direct writing followed by wet etching with a dilute fluorinated solvent and
annealing to create high-quality 3D microfluidic chips inside a substrate made of the
low refractive index fluoric polymer CYTOP [23]. To ensure clear 3D microscopic
observations of cell motion near the solid–liquid interface, a minimal mismatch
between the refractive indices of the medium and the chip substrate is desirable. The
refractive index of CYTOP is 1.34, which is very close to that of water. Thus, these
CYTOP microfluidic chips enabled precise observation of the flagellar motion of
a Dinoflagellate that typically moves in circles near the fluid surface. The CYTOP
microfluidic chips are expected to provide opportunities to analyze, in detail, the
behavior of various cells near the water–solid interface.
As noted, LAFLD is another method for the manufacture of 3D microfluidic
devices [36–40]. FLAE results in some etching of the unexposed regions around
the laser-exposed areas due to the limited selectivity of wet chemical etching. In
contrast, LAFLD removes only the laser-exposed regions or possibly even smaller
zones, since it relies on ablation. Therefore, LAFLD can be applied to the fabrication of 3D nanofluidic devices [40]. Liao et al. demonstrated the rapid fabrication
of a passive microfluidic mixer consisting of geometrically complex 3D microchannels via LAFLD of porous glass [43]. This 3D mixer was composed of a Y-shape
microchannel embedded 400 µm below the surface of the glass chip in conjunction with a string of mixing units, connected to two opening inlets and one outlet
(Fig. 8.2d). Experimental trials demonstrated that two fluorescent dye solutions (fluorescein sodium and Rhodamine B) were well mixed after passing through three
mixing units (corresponding to a length of 0.9 mm) over a time span of approximately 10 ms. In contrast, efficient mixing was not achieved in a 1D microfluidic
channel over a greater propagation distance of about 1.3 mm. Based on a combination of the threshold effect of fs laser processing using a Gaussian beam and the
formation of a periodic nanograting, LAFLD was also used to successfully manufacture buried nanofluidic channels with transverse widths less than 50 nm in porous
glass [74]. Integrated devices containing nanofluidic-microfluidic systems with 3D
configurations have also been used for the investigation of the stretching of DNA
molecules [75].
8.4 Fabrication of Optofluidic Devices
The fs laser 3D process can also integrate certain microoptic/photonic components
into microfluidic units to create monolithic optofluidic devices for highly sensitive detection of biochemical species and functional manipulation of living cells.
One straightforward strategy involves fs laser direct writing of WGs and WGbased photonic components (such as a Mach-Zehnder interferometer (MZI)) in 3D
microfluidic devices fabricated by FLAE. FLAE itself can simultaneously fabricate