CELLS OF THE NERVOUS SYSTEM AND ELECTRICAL STIMULATION - Advanced Biomaterials: Fundamentals, Processing, and Applications - page 615

constant voltage) (Bawin et al. 1986; Bikson et al. 2004), alternating current
(Bawin et al. 1984; Bawin et al. 1986), biphasic, or monophasic (pulses with only
either positive or negative components) (Zeck and Fromherz 2001; Fromherz
2003). Furthermore, the signal can be modifi ed in terms of frequency, amplitude,
impulse duration, impulse delay, and waveform. Typical waveforms used are
square or rectangular (Zeck and Fromherz 2001; Fromherz 2003; Mitchell et al.
1992), triangular and sinusoidal (Grumet 1994; Grumet et al. 2000). Most function
generators now allow unique waveforms to be created so the signal possibilities
are endless. The biggest constraints when applying electrical stimulation to live
cells or tissue is that the current should not cause cell death.
18.5.6.3 Electrode Material. When considering the material to be used as
electrodes, one must consider cost, reusability, reversibility, and how the material
may affect the cells. Some common electrode materials used in electrophysiology
are Ag/AgCl, Pt, graphite and gold. These electrodes are used to establish an EF
directly in the media, converting the fl ow of electrons into the fl ow of ions and
vice versa, allowing current to be passed in a consistent manner.
Ag/AgCl electrodes are economical and reversible, yet they are exhaustible
and brittle. They can also become imbalanced when using two half-cells to drive
create an EF. In other words, differences in the concentration of AgCl can build
up on the electrodes. This decreases or increases the amount of current that is
being applied (depending on direction of applied current) from one electrode to
the other. To equilibrate, electrodes can be connected and placed in the same
saline solution (typically that used for experiments). To increase the lifetime of
the electrodes, when using a DC current, electrodes should alternated as cathode
and anode, so that the AgCl that is removed (anode) can then be regenerated
(cathode) and vice versa. In other words, the amount of current and time that
passes through the electrode as a cathode should be approximately the same
when using that same electrode as an anode to limit electrode exhaustion and
keep electrodes balanced.
Graphite electrodes are not easily exhaustible and do not accumulate ionic
species. Graphite is a good electrode material because of high electrical conduc-
tivity; acceptable corrosion resistance; high purity; inertness with ionic species;
low cost; and ease of fabrication into composite structures. However, graphite is
brittle and will easily crumble when handled.
Platinum is inexhaustible and does not accumulate ionic species, but is expen-
sive. Pt does not produce material byproducts; however, H 2 and O 2 gas are pro-
duced by electrolysis of water. To minimize water electrolysis, it is best to maintain
a low current, no larger than 1 mA (Katzberg 1974). If electrolysis of water occurs,
the pH of the media will change, harming cells residing in the media.
18.5.6.4 Changes in Hydrogen Concentration. With the complexity of
choosing a signal, it is logical to attempt a constant DC signal that eliminates
variables (for example, frequency, waveform, and so on) and allows any effect to
be correlated with the strength and time of stimulation. The caveat is that the pH