Microfluidics - Introduction to BioMEMS - page 116

limits on voltage in real devices. In microchannels, it is possible to use higher voltages because
the removal of the heat produced by current low is faster in narrower channels. he limiting
material problem is that if the channel is made of a conductor, there must be a very good insu-
lator on the channel surface to keep the current conined to the luid channel itself. Silicon is
rarely used, but glass and plastics are excellent materials.
Because the velocity of the species in a given electric ield depends strongly on the charge
of the particle, it stands to reason that any change in the charge of the particle can either slow
down or speed up the particle. he strongest inluence on the charge of biomolecules (which
contain multiple copies of carboxyl, amine, and other titratable groups as their charged spe-
cies) is the pH. here is always a pH for such macromolecules in which the net charge on the
molecule as a whole is zero—the isoelectric point. As a consequence, if one can establish a pH
gradient, biomolecules exposed to an electrostatic ield along the same direction as that gradient
can be made to migrate to their isoelectric points, where they will no longer be afected by the
ield. Molecules can thereby be concentrated and sorted by their isoelectric points in a technique
known as isoelectric focusing (IEF, see Section 4.4.3). (Note that the positive electrode must be
at the low-pH side of the gradient to prevent all the molecules from simply lying of the gradi-
ent in one direction.) he IEF technique has gained wide acceptance in the macro-world as one
of the two dimensions in so-called two-dimensional (2-D) gel electrophoresis, which allows the
identiication of thousands of diferent proteins from one sample.
It is also possible to use IEF in microdevices, with some trade-ofs. he unique advantage of
microdevices is that one can create the pH gradient using the microdevice itself. Because at suf-
iciently high voltages to perform electrophoresis, one inevitably produces acid at the anode and
base at the cathode by electrolysis of water, the two electrodes always create their own pH gradi-
ent. his gradient reaches a steady state in a short time by interdifusion of the two sets of extreme
pH regions, which allows IEF to occur. If the gap between the accelerating voltages is on the order
of a few millimeters or less, and the solution is not heavily bufered, an IEF system is automati-
cally created in the microdevice. he downside is that very high accelerating voltages are needed
to allow signiicant separations in small distances, which are hard to establish in a microchannel
without the generation of bubbles, which disrupt the orderly alignment of the separated proteins.
3.3.2 Electro-Osmosis
Electrophoresis has a lesser-known cousin— electro-osmosis —that is very important in micro-
luidics. It occurs because of electrophoresis of the charges that reside near channel surfaces. he
chemistry of the walls of microchannels is usually such that the wetted channel is charged. For
example, the surface of silica contains silanol (-Si-OH) groups, whose hydroxyl group will, at
neutral pH, lose a proton to become -Si-O - . As the pH is lowered, the walls of a silica channel
can be neutral or even positively charged but, for the moment, we can consider that at reason-
able pH values in aqueous solvents, the surfaces of silica and most partially oxidized polymer
surfaces are negatively charged in water.
his observation becomes interesting because of what a “charged surface” really means in
water. he nature of water is such that there is no total charge imbalance within the channel-
solution system. However, the distribution of charges can be very interesting (see Figure 3.6 ).
here are immobilized charges covalently attached to the surface, and a balanced set of other
co- and counterions that are distributed near the surface at positions determined by the surface
charge, the dielectric constant of the medium, and the temperature. he temperature is crucial
because at zero temperature, all the counterions would collapse onto the charged surface and the
surface charge would be neutral. Because there is energy available (according to the Boltzmann
distribution ), those ions not covalently attached try to escape to ininity, only to be pulled back
by the electrostatic attraction of the surface. his charged sheath of counterions, which resides
within a few nanometers or tens of nanometers from the ixed charged surface of the chan-
nel (the so-called Debye layer , or electrical double layer , whose characteristic thickness is the