8 Femtosecond Laser Direct Writing for 3D Microfluidic Biochip …
259
water containing the seedling root (the yellow line) is comparable to that obtained
from 50 ml water mixed with 15 ml CO 2 (the black line). This result implies that
the CO 2 concentration generated by root respiration was comparable to that of the
carbonic water used in this experiment (15 ml CO 2 in 50 ml H 2 O). Interestingly, this
CO 2 concentration equaled the critical concentration at which the Phormidium did
not glide toward the seedling root or the carbonic water in the T-shaped microfluidic
channel, confirming that CO 2 is the sole attractant for the Phormidium.
8.5 Fabrication of Electrofluidic Devices
Electrical control of biological samples in microfluidic systems is of importance for
many biochip applications, such as cell sorting, dielectric measurements of cell properties, and cell manipulation [80, 81]. To this end, the integration of microelectric
components into 3D microfluidic devices is highly desirable as a means of fabricating electrofluidic devices. Considering that microfluidic substrates are typically
not conductive, one key technology for microelectronic integration is spatially selective metallization of microfluidic structures. Selective metallization of the internal
walls of microfluidic structures is possible via fs laser-assisted electroless metal
plating. This process consists of two main steps: 3D spatially selective modification
by fs laser direct writing and selective metal deposition on the laser-modified regions.
Catalytic patterning with metals such as palladium or silver via fs laser irradiation
allows selective metallization of the surfaces of many insulators (including fused
silica and crystalline lithium niobate) as a result of selective metal deposition [82–
86]. It has also been demonstrated that areas in Foturan glass irradiated with a fs laser
can be selectively metallized using a commercially-available electroless plating solution without prior catalytic patterning (Fig. 8.5). The associated mechanism may rely
on the increased roughness obtained by fs laser-ablation, which induces an anchor
effect that selectively adheres metal atoms from the plating solutions [54, 56, 87].
Electrofluidic devices have been produced by first preparing 3D microfluidic structures in Foturan glass using FLAE (Fig. 8.5a). The internal walls of these structures
are made highly smooth by an additional thermal treatment following wet chemical etching. Spatially selective metallization of the 3D microfluidic structures is
subsequently performed via a two-step process [56, 88–90]. The first step involves
fs laser direct writing ablation to create modified patterns at desired positions within
the structures, which may include glass chip surfaces and the interior or sidewalls
of the microchannels (Fig. 8.5b). This ablation generates the necessary roughness
in the laser-exposed regions, enabling the selective deposition of metal structures in
these regions by subsequent electroless metal plating due to the anchor effect. The
main role of the laser ablation is to homogeneously roughen the glass surface in a
highly selective, well-controlled manner at various positions within the 3D structure. However, even careful optimization of laser ablation parameters such as the
pulse energy, writing speed, and writing scheme, cannot prevent the generation of
259
water containing the seedling root (the yellow line) is comparable to that obtained
from 50 ml water mixed with 15 ml CO 2 (the black line). This result implies that
the CO 2 concentration generated by root respiration was comparable to that of the
carbonic water used in this experiment (15 ml CO 2 in 50 ml H 2 O). Interestingly, this
CO 2 concentration equaled the critical concentration at which the Phormidium did
not glide toward the seedling root or the carbonic water in the T-shaped microfluidic
channel, confirming that CO 2 is the sole attractant for the Phormidium.
8.5 Fabrication of Electrofluidic Devices
Electrical control of biological samples in microfluidic systems is of importance for
many biochip applications, such as cell sorting, dielectric measurements of cell properties, and cell manipulation [80, 81]. To this end, the integration of microelectric
components into 3D microfluidic devices is highly desirable as a means of fabricating electrofluidic devices. Considering that microfluidic substrates are typically
not conductive, one key technology for microelectronic integration is spatially selective metallization of microfluidic structures. Selective metallization of the internal
walls of microfluidic structures is possible via fs laser-assisted electroless metal
plating. This process consists of two main steps: 3D spatially selective modification
by fs laser direct writing and selective metal deposition on the laser-modified regions.
Catalytic patterning with metals such as palladium or silver via fs laser irradiation
allows selective metallization of the surfaces of many insulators (including fused
silica and crystalline lithium niobate) as a result of selective metal deposition [82–
86]. It has also been demonstrated that areas in Foturan glass irradiated with a fs laser
can be selectively metallized using a commercially-available electroless plating solution without prior catalytic patterning (Fig. 8.5). The associated mechanism may rely
on the increased roughness obtained by fs laser-ablation, which induces an anchor
effect that selectively adheres metal atoms from the plating solutions [54, 56, 87].
Electrofluidic devices have been produced by first preparing 3D microfluidic structures in Foturan glass using FLAE (Fig. 8.5a). The internal walls of these structures
are made highly smooth by an additional thermal treatment following wet chemical etching. Spatially selective metallization of the 3D microfluidic structures is
subsequently performed via a two-step process [56, 88–90]. The first step involves
fs laser direct writing ablation to create modified patterns at desired positions within
the structures, which may include glass chip surfaces and the interior or sidewalls
of the microchannels (Fig. 8.5b). This ablation generates the necessary roughness
in the laser-exposed regions, enabling the selective deposition of metal structures in
these regions by subsequent electroless metal plating due to the anchor effect. The
main role of the laser ablation is to homogeneously roughen the glass surface in a
highly selective, well-controlled manner at various positions within the 3D structure. However, even careful optimization of laser ablation parameters such as the
pulse energy, writing speed, and writing scheme, cannot prevent the generation of