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J. Xu et al.
systems [4–6] and the controllable manipulation of bio-species [7–12]. The use of
conventional microfabrication methods based on two-dimensional photolithography
to produce microfluidic biochips limits the possible 3D geometric designs and also
requires highly complex processes. In contrast, femtosecond (fs) laser microfabrication enables the direct fabrication of 3D microstructures inside transparent materials via a maskless, resistless process based on nonlinear multiphoton absorption,
and has been shown to represent a superior method of fabricating 3D microfluidic
biochips [13–18]. Irradiation of the interior of transparent materials with a focused
fs laser beam modifies the physical, chemical, and/or optical properties of the material only within the focal volume of the beam. With translation of the focused laser
beam (or the sample, using a high-precision stage), the aforementioned modifications can fabricate a variety of functional microcomponents inside glass substrates,
including microoptic/photonic (optical waveguides, optical gratings, etc.), microfluidic (microchannels, microchambers, etc.), and microelectronic (microelectrodes,
etc.) elements. In addition, these individual functional elements can be monolithically and seamlessly integrated within a single substrate with high flexibility and
compatibility. Consequently, 3D microfluidic biochips fabricated by fs laser direct
writing have been applied to diverse applications, including single-cell detection [19]
and manipulation [20], functional observation of microorganisms and cells [21–23],
cell sorting [24, 25] and cell counting [19, 26, 27], in recent years. In this chapter,
we present a comprehensive review of the principles and practical techniques associated with fs laser direct writing, as well as recent advances in the fabrication of
3D microfluidic biochips. Section 8.2 consists of a general introduction of fs laser
direct 3D processing, classified according to three different schemes based on volumetric changes at the laser exposed regions. Section 8.3 discusses the fabrication of
microfluidic devices by subtractive processing along with the applications of these
units. Sections 8.4 and 8.5 detail the monolithic fabrication of optofluidic and electrofluidic biochips, respectively, using various fs laser 3D processing techniques.
Section 8.6 presents a new hybrid technique termed “ship-in-a-bottle” integration,
by which 3D functionalized polymeric micro/nanocomponents are integrated into 3D
glass microfluidic devices. Finally, Sect. 8.7 provides a summary and a discussion
of future directions for this technology.
8.2 Femtosecond Laser 3D Processing
Selecting the appropriate processing parameters (such as pulse energy and exposure
time) during fs laser irradiation allows localized modification of the interior of transparent materials such as glass, crystals, and polymers, as a result of multiphoton
absorption. Coupled with the translation of the focused laser beam (or the sample)
in 3D space, these modifications can form 3D functional microstructures, such as
optical waveguides (WGs), along the trajectory of the laser beam. In some materials,
the chemical or physical properties at the laser-exposed regions can be also modified. Subsequent processing to selectively remove the modified regions by chemical
J. Xu et al.
systems [4–6] and the controllable manipulation of bio-species [7–12]. The use of
conventional microfabrication methods based on two-dimensional photolithography
to produce microfluidic biochips limits the possible 3D geometric designs and also
requires highly complex processes. In contrast, femtosecond (fs) laser microfabrication enables the direct fabrication of 3D microstructures inside transparent materials via a maskless, resistless process based on nonlinear multiphoton absorption,
and has been shown to represent a superior method of fabricating 3D microfluidic
biochips [13–18]. Irradiation of the interior of transparent materials with a focused
fs laser beam modifies the physical, chemical, and/or optical properties of the material only within the focal volume of the beam. With translation of the focused laser
beam (or the sample, using a high-precision stage), the aforementioned modifications can fabricate a variety of functional microcomponents inside glass substrates,
including microoptic/photonic (optical waveguides, optical gratings, etc.), microfluidic (microchannels, microchambers, etc.), and microelectronic (microelectrodes,
etc.) elements. In addition, these individual functional elements can be monolithically and seamlessly integrated within a single substrate with high flexibility and
compatibility. Consequently, 3D microfluidic biochips fabricated by fs laser direct
writing have been applied to diverse applications, including single-cell detection [19]
and manipulation [20], functional observation of microorganisms and cells [21–23],
cell sorting [24, 25] and cell counting [19, 26, 27], in recent years. In this chapter,
we present a comprehensive review of the principles and practical techniques associated with fs laser direct writing, as well as recent advances in the fabrication of
3D microfluidic biochips. Section 8.2 consists of a general introduction of fs laser
direct 3D processing, classified according to three different schemes based on volumetric changes at the laser exposed regions. Section 8.3 discusses the fabrication of
microfluidic devices by subtractive processing along with the applications of these
units. Sections 8.4 and 8.5 detail the monolithic fabrication of optofluidic and electrofluidic biochips, respectively, using various fs laser 3D processing techniques.
Section 8.6 presents a new hybrid technique termed “ship-in-a-bottle” integration,
by which 3D functionalized polymeric micro/nanocomponents are integrated into 3D
glass microfluidic devices. Finally, Sect. 8.7 provides a summary and a discussion
of future directions for this technology.
8.2 Femtosecond Laser 3D Processing
Selecting the appropriate processing parameters (such as pulse energy and exposure
time) during fs laser irradiation allows localized modification of the interior of transparent materials such as glass, crystals, and polymers, as a result of multiphoton
absorption. Coupled with the translation of the focused laser beam (or the sample)
in 3D space, these modifications can form 3D functional microstructures, such as
optical waveguides (WGs), along the trajectory of the laser beam. In some materials,
the chemical or physical properties at the laser-exposed regions can be also modified. Subsequent processing to selectively remove the modified regions by chemical