8 Femtosecond Laser Direct Writing for 3D Microfluidic Biochip …
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metals, such as the line width and thickness, can be preciously controlled by tuning
the ablation and electroless plating parameters to fit the designs of the desired microelectric elements. This flexible metallization of 3D microchannels may provide a new
means for producing precision electric field patterns in microfluidics and thus could
have potential uses in many biochip applications. Figure 8.5f, g present some examples of electrode-integrated glass channels fabricated by this technique, in which
uniform, continuous metal pads have been selectively deposited from the interior
and the sidewalls to the exterior of the channels.
Using the technique described above, electrofluidic devices with different configurations consisting of microfluidic channels integrated with pairs of electrically
isolated electrodes were successfully fabricated. These units were subsequently
applied to orient the movement direction of Euglena cells. To test the electroorientation performance, Euglena cells were introduced into the devices as shown in
Fig. 8.6. Figure 8.6a, c, g demonstrate the random motion of the Euglena cells in the
channels in the absence of an electric field between the integrated microelectrodes. It
is also evident that the application of a proper AC electric field dramatically changed
the movement of the cells to a bidirectional orientation along the field direction
(Fig. 8.6b). This controllable alignment resulted from the interaction between the
dipole moments induced in the cells by the electric field and the field itself. As soon
as the electric field was turned off, movement of the microorganisms became random
again. Electro-orientation in channels using this type of device has been shown to be
both reproducible and nondestructive [56, 89]. The single pair of opposing electrodes
shown in Fig. 8.6b allowed the 1D orientation of cells along the x-axis. In addition,
a four-electrode arrangement with the electrodes at right angles on the base of the
channel successfully demonstrated 2D electro-orientation of cell movement in the
x–y plane upon varying the direction of the field generated in the microscale space
(Fig. 8.6d–f). Furthermore, the swimming direction of the cells could be oriented
along the z-direction by using electrodes with square outlines formed at the top and
bottom of the channel (Fig. 8.6h). This electrofluidic device allowed the continuous
observation of the motion of 45 Euglena cells swimming along the z-direction over
a span of one minute within an imaging area of approximately 160 × 120 µm.
Due to the electro-orientation effect, the average time required for the continuous
observation of five cells swimming along the z-direction was reduced by a factor of
approximately 43 as compared with the necessary time interval with no electric field
[89]. Although the transparent window at the center of the electrodes was not coated
with metal, the surrounding electrode patterns were able to generate sufficiently
high electric field intensities for z-directional orientation. This type of manipulation
enabled the ready observation of the Euglena cells from the front side, potentially
providing new insights into the functions of such microorganisms.
Electrotaxis (electro-tactic control) is another interesting phenomenon that can
be applied to the electromanipulation of cells. In this process, the locomotion of
biological samples towards the cathode or anode is induced under an applied DC
electric field. In-situ control of electrotaxis in a chip allows precise positioning and
manipulation of biological samples. To achieve this, a new type of electrofluidic
devices with integrated vertical electrodes on the sidewalls having aspect ratios as
261
metals, such as the line width and thickness, can be preciously controlled by tuning
the ablation and electroless plating parameters to fit the designs of the desired microelectric elements. This flexible metallization of 3D microchannels may provide a new
means for producing precision electric field patterns in microfluidics and thus could
have potential uses in many biochip applications. Figure 8.5f, g present some examples of electrode-integrated glass channels fabricated by this technique, in which
uniform, continuous metal pads have been selectively deposited from the interior
and the sidewalls to the exterior of the channels.
Using the technique described above, electrofluidic devices with different configurations consisting of microfluidic channels integrated with pairs of electrically
isolated electrodes were successfully fabricated. These units were subsequently
applied to orient the movement direction of Euglena cells. To test the electroorientation performance, Euglena cells were introduced into the devices as shown in
Fig. 8.6. Figure 8.6a, c, g demonstrate the random motion of the Euglena cells in the
channels in the absence of an electric field between the integrated microelectrodes. It
is also evident that the application of a proper AC electric field dramatically changed
the movement of the cells to a bidirectional orientation along the field direction
(Fig. 8.6b). This controllable alignment resulted from the interaction between the
dipole moments induced in the cells by the electric field and the field itself. As soon
as the electric field was turned off, movement of the microorganisms became random
again. Electro-orientation in channels using this type of device has been shown to be
both reproducible and nondestructive [56, 89]. The single pair of opposing electrodes
shown in Fig. 8.6b allowed the 1D orientation of cells along the x-axis. In addition,
a four-electrode arrangement with the electrodes at right angles on the base of the
channel successfully demonstrated 2D electro-orientation of cell movement in the
x–y plane upon varying the direction of the field generated in the microscale space
(Fig. 8.6d–f). Furthermore, the swimming direction of the cells could be oriented
along the z-direction by using electrodes with square outlines formed at the top and
bottom of the channel (Fig. 8.6h). This electrofluidic device allowed the continuous
observation of the motion of 45 Euglena cells swimming along the z-direction over
a span of one minute within an imaging area of approximately 160 × 120 µm.
Due to the electro-orientation effect, the average time required for the continuous
observation of five cells swimming along the z-direction was reduced by a factor of
approximately 43 as compared with the necessary time interval with no electric field
[89]. Although the transparent window at the center of the electrodes was not coated
with metal, the surrounding electrode patterns were able to generate sufficiently
high electric field intensities for z-directional orientation. This type of manipulation
enabled the ready observation of the Euglena cells from the front side, potentially
providing new insights into the functions of such microorganisms.
Electrotaxis (electro-tactic control) is another interesting phenomenon that can
be applied to the electromanipulation of cells. In this process, the locomotion of
biological samples towards the cathode or anode is induced under an applied DC
electric field. In-situ control of electrotaxis in a chip allows precise positioning and
manipulation of biological samples. To achieve this, a new type of electrofluidic
devices with integrated vertical electrodes on the sidewalls having aspect ratios as