Hydroxyapatite and Other Biomedical Coatings by Electrophoretic Deposition - Biological and Biomedical Coatings: Processing and Characterization - page 95

week. The results are interpreted in terms of the appearance of the resultant layers that can
be detected. Figure 3.9 [35] summarizes typical observations for this experimentation.
Aqueous Suspensions
In aqueous media, there are four common ways by which the surface charge of particles
can be altered [30]:
• Dissociation. Partial dissociation of the particle surface provides ions to the sus-
pension. This can be controlled by pH alteration; addition of a solvent, such as
high-valence ions, to the solution; and addition of a polyelectrolyte (cationic or
anionic).
• Nernstian charging. Adsorption or desorption of lattice ions can be altered through
the common ion effect, in which a solution of two solutes with the same ion can
have its equilibrium shifted owing to different levels of ionization. This can be
controlled through the addition of a solute that increases the solubility of the
particle.
• Metal hydrolysis. The surface of a metal oxide or metal hydroxide hydrolyzes by
protonation or deprotonation of acid or base groups. This principle is indicated in
Reactions 1 and 2 . This typically is done by control of the pH.
• Metal hydrolysis and dissolution. The surface of a metal oxide or metal hydroxide
can dissolve incongruently (particle composition ≠ solute composition) into one
or more cationic species. This usually is done with complex metal oxides, where
ions of different valence are leached and then adsorb in the Stern layer. This is
controlled by pH, solute concentration, and solids loading.
It is perceived widely that aqueous suspensions are problematic for electrophoretic deposi-
tion. The main reason for this is that there are several shortcomings that create the follow-
ing risks:
• Electrolysis of the water at low field strengths, which produces gas bubbles that
disrupt the process and may introduce porosity in the deposit
• High current densities, which cause Joule heating
• Potential dissolution of the substrate at unsuitable pH values
• Potential dissolution of the deposit at unsuitable pH values
However, the main advantages are:
• Easy manipulation of the surface charge through pH alteration
• Low processing costs
• Low environmental impact
Table 3.3 [37-85] provides a representative example of reports of the aqueous electropho-
retic deposition of a range of bioceramics.