Hydroxyapatite and Other Biomedical Coatings by Electrophoretic Deposition - Biological and Biomedical Coatings: Processing and Characterization - page 106

OtherFactorsAffectingElectrophoreticDeposition
Overview

Although the preceding text provides general coverage of a range of features relevant to
the success or failure of electrophoretic deposition, there are some other specific param-
eters that are critical to the process [21]. These are more of a practical nature than those
discussed to this point and these points are made only briefly. These features can be sepa-
rated into two categories, as shown in Table 3.4.

Suspension Parameters

Particle Size

Particle sizes in the range 1 to 20 µm have been suggested to ensure good deposition [20].
Larger particles have a tendency to settle rapidly, so their suspension requires either a
stronger surface charge or a thicker double layer [21]. If large particles are subject to rapid
settling during the process, then nonuniform and inhomogeneous films result.

Electrical Conductivity of Suspension

It has been shown that the dielectric constant and bulk electrical conductivity for solid-
liquid mixtures show a linear proportionality [87]. Hence, the electrical conductivity must
be sufficiently high to facilitate ionization and thus charging but sufficiently low so that
excessive ionization does not compress the double layer [21]. This is why deposition occurs
only over a limited range of dielectric constants (12-25) for aprotic media.
An alternative approach to overcome this limitation is the addition of polyelectrolytes
or temperature regulation to control a suspension's electrical conductivity. It is known
that the electrical conductivity of a suspension is proportional to the ionic strength of
the suspension in acidic or basic conditions [88]. Hence, when the pH is close to the pH iep ,
the electrical conductivity is low. Conversely, when the electrical conductivity is high, the
main current carriers are the free ions in the suspension rather than the particles, so par-
ticle motion is reduced.

Dielectric Constant of Medium

As mentioned, the dielectric constant of aprotic organic media should be neither too low
nor too high [20]. A range of 12 to 25 has been observed to be advantageous. More gener-
ally, for aqueous and amphiprotic media, high dielectric constants are advantageous. The

TABLE 3.4
Summary of Parameters Relevant to Electrophoretic Deposition
SuspensionParameters ProcessingParameters

Particle size Applied voltage and deposition time

Electrical conductivity of suspension Solids concentration

Dielectric constant of medium Electrical conductivity of substrate

Solids loading Surface area of substrate

Viscosity of medium Electrode separation