Page 229 - Bio Engineering Approaches to Cancer Diagnosis and Treatment

228 CHAPTER 9 Application of microfluidics in cancer treatment

FIGURE 9.7 The schematic of the syringe pump connected to a PDMS microchannel.

electroosmotic pump is a more appropriate device which applies an electric field to
liquid movement. If the walls of a microchannel have an electric charge, an electric
double layer of counter ions will form at the walls [27]. Fig. 9.7 shows the schematic
of the syringe pump connected to a PDMS microchannel. The ions in the double
layer move toward the opposite polarity electrode by applying an electric field across
the channel. This causes the movement of fluid near the walls and convective motion
of the bulk fluid occurs because of viscous forces. The fluid flow can be controlled
by adjusting the voltage. Usually high voltages are required in electroosmotic pump.
If a specific protein or molecule adds to the walls of the microchannel, the fluid flow
can alter due to their charges [27].
9.2.1.3 Micromixers
Micromixers are necessary for a variety of applications particularly in the field
of biology and chemistry. The basic concept of mixing involves diffusion, which
changes the distribution of particles as well as the concentration gradient in an aque-
ous solution. The laws of Brownian motion explain that random motion occurs when
the forces are imposed on spherical beads in a water solution and pushes them to the
left and right creating an imbalance [28].
In macroscopic scales, turbulence causes the random variation in fluid flow and
dispersion of the molecules resulting in the mixing of particles along with the fluid
flow. In microscopic scales, creating turbulence is a daunting task, thus micromixers
specifically designed to cause disturbances or alterations to fluid flow are incorpo-
rated into microchips. In microfluidic systems, two categories of micromixing meth-
ods are employed to induce a more rapid mixing either by passive or active mixing.
In passive mixing, obstacles are created to multiple fluid streams to fold the liquid
layers that enhance diffusion in a defined area of the channel. Indeed, passive mix-
ing relies on diffusion. Some designed micromixers are included parallel lamina-
tion, where a basic design of a microchannel with a geometry consisting of a T or Y
shaped mixer (Fig. 9.8).
Active micromixers utilize different external energy sources to achieve dynamic
mixing. These micromixers are categorized based on the types of external sources as
pressure field driven, electrical field driven, sound field driven, magnetic field driven,
and thermal field driven. In electrohydrodynamic mixing, two platinum wires are
implemented and placed perpendicular to the mixing channel that induces mixing
by changing the voltage and frequency. In magnetohydrodynamic disturbance, an