Biomedical Engineering Reference
In-Depth Information
membrane. At higher frequencies in the 1-kHz-10-MHz range, DEP dominates
over electroporation and electrofusion. The electric field is switching faster than
the vesicle membrane's time constant, preventing voltage from building up across
it. At even higher frequencies in the 1-GHz range, microwave heating becomes
dominant.
2.3
Integrated Circuit/Microfluidic Chips
Integrated circuit/microfluidic chips contain displays of electrical pixels which
are used to trap and position thousands of cells and droplets. Activating a pixel
produces an electric field which dielectrophoretically attracts and traps cells. By
simultaneously activating thousands of pixels, large numbers of cells can be
precisely trapped and positioned.
Integrated circuit/microfluidic chips have undergone an evolution in the past
decade, progressively increasing in complexity and incorporating more functions.
Tab le
2.2
details the evolution of these chips - beginning with built-in-house DEP
chips containing only 25 pixels and leading to present CMOS devices capable of
producing both electric and magnetic fields and containing up to 32,000 pixels.
The second generation of hybrid IC/microfluidic chips shown in Table
2.3
uses
CMOS integrated circuit technology. The chip is shown in Fig.
2.4
and consists of
a display of 128
256 electrical pixels [
14
]. Each pixel is individually addressable
and contains a single SRAM memory element. The inset in Fig.
2.3
shows two
active pixels radiating electric fields and polarizing and trapping a nearby dielectric
particle. Multiple pixels can be activated simultaneously. The pixels are 11
11m
2
3:3 mm
2
. The pixels
in area, about the size of a living cell, and the entire chip is 2:3
are covered in a 2
m thick layer of polyamide insulator to prevent short circuiting.
Active pixels are charged by an AC square wave running between 0 and 5 V. The
frequency is set between 1 Hz and 1.8 MHz. Inactive pixels are also connected to an
AC square wave, but are run out of phase with the active pixels. This creates electric
field maxima at the
interface
between active and inactive pixels - this is where cells
aretrappedbyDEP.
The third-generation chip, shown in Fig.
2.5
, incorporates larger voltages and the
ability to generate magnetic fields [
6
]. Larger voltages generate stronger DEP forces
which position cells faster and more reliably. Higher voltages also allow cells to be
electroporated which enables cell fusion and the transport of foreign particles across
the cell membrane. Magnetic fields are generated to enable functionalized magnetic
particles - often used in the biomedical community - to be used with this chip.
This chip is a CMOS integrated circuit with an array of 60
61 electric pixels,
as seen in Fig.
2.5
b. Each pixel is individually addressable and can be activated to
0 or 50 V. Similar to the second-generation chips, pixels are activated by sending
an AC electrical signal. Inactive pixels are run out of phase, creating electric field
maxima at the interface between active and inactive pixels. This is where cells are
trapped by DEP. This chip can also generate magnetic fields using a 60
60 array of
