Biomedical Engineering Reference
In-Depth Information
Table 3.1
Comparison of magnetic energy sources.
Magnetic energy source
Advantages
Disadvantages
Electromagnet
On - chip integration
Magnetic fi eld switching
Reliable
Joule heating
Energy consumption
Limited fi eld strength
Nonuniform magnetic fi eld
Complicated fabrication
Permanent magnet
Inexpensive
No power consumption
Confi ned fringe fi eld
Easy accessibility
Limited fi eld strength
Limited fi eld uniformity
Temperature sensitivity
Weight
with distance from the magnetic energy source. Therefore, when designing
the microfl uidic device it is critical that the microchannel is adjacent either to
the magnetic energy source or to the magnetic fi eld - applied ferromagnetic micro-
structure. Because the magnetic fl ux lines diverge from, and converge into, the
corner of the permanent magnet (which results in a high magnetic fl ux density
gradient in the region of the corner), it is advantageous to use the edge of the
magnet adjacent to the microchannels in order to obtain an enhanced magnetic
force. Unfortunately, a potential problem may arise when using an enhanced
magnetic fi eld at the edge that provides nonuniform directions of the magnetic
fl ux lines, as this may result in magnetic forces acting on the particle, as shown
in Figure 3.2 .
In the past, nickel has often been used as a ferromagnetic material to enhance
the magnetic fl ux density gradient, mainly because nickel microstructures can be
obtained relatively easily by using conventional electroplating processes. Details
of the microfabrication and of the microfl uidic PDMS device are shown in Figures
3.3 and 3.4. Although not described in detail, the remaining PDMS fi lm (ca.
<
m) at the thermal compression method (stage A5 in Figure 3.3) plays a crucial
role in the prevention of fl uid leakage at the interface between the microchannel
and nickel microstructures. Because the deposition rate in electroplating is depen-
dent on the current density within the electroplating seed layer, the thickness of
the nickel microstructures is not uniform; consequently, a chemical mechanical
polishing (CMP) process should be followed to obtain an even height of the micro-
structure. An alternative approach would be to use a permanent magnet to deposit
a thick fi lm of the permanent magnet (e.g., NdFeB), using triode sputtering (see
Figure 3.5); this method is often used for integrated microscaled rather than mac-
roscaled permanent magnets [43]. The same method is also useful for producing
integrated microfl uidic devices for magnetophoresis, although it does not allow
the magnetic fi eld to be switched on and off.
10
μ
