Biomaterials - Introduction to Biomedical Engineering - page 242

the life of the implant, except in bone, where there is direct bone apposition on calcium
phosphate surfaces without an intervening fibrous tissue layer.

The rapidly formed neotissue will be remodeled by cells into functional
tissue more similar to the original tissue, although typically a scar remains.

4. Remodeling:

5.4.3 Metallic Corrosion
There are a number of mechanisms by which metals can corrode, and corrosion resis-
tance is one of the most important properties of metals used for biomedical implants. The
mechanisms of most significance to implant applications in the aqueous saline solutions
of the human body are galvanic (or mixed metal) corrosion, crevice corrosion, and fretting
corrosion.
Galvanic (mixed metal) corrosion results when two dissimilar metals in electrical contact
are immersed in an electrolyte. There are four essential components that must exist for a
galvanic reaction to occur: an anode, a cathode, an electrolyte, and an external electrical con-
ductor. The in vivo environment contains electrolytes. A patient with two total hip replace-
ments made of different alloys is not subject to mixed metal corrosion, since there is no
electrical connection. However, a hip replacement made of two alloys or a fracture plate of
one metal fixed with screws of another metal may be susceptible to mixed metal corrosion.
When two dissimilar metals are connected in an electrochemical cell, one will act as an
anode, and the other will be the cathode. Metal oxidation will occur at the anode, as shown
in Eq. (5.1). Metal oxidation may produce metal ions, and these ions can migrate away from
the metal surface as free ions in solution or form metal oxide or other chemical compounds
with other species. Various elements (e.g., nickel, chromium, etc.) are not well tolerated by
tissue and have possible adverse reactions, including allergic and inflammatory reactions.
The reaction at the cathode will depend on the pH of the environment (Eqs. (5.2) and
(5.3)) and will lead to immunity from corrosion, passivation, or active corrosion. The direc-
tion of the reaction can be determined by examining the electromotive force (EMF) series,
a short listing of which is shown in Table 5.3. These potentials represent half-cell potentials
of metals in equilibrium with 1 molar solution of their ionic species. The potential for
hydrogen is defined as zero. As shown, the standard potential for iron is 0.44 V. If iron

is connected to copper with an EMF of 0.34 V, the potential difference is 0.78 V. Since iron

is the anode, iron oxidation will occur according to the reaction shown in Eq. (5.1). The
reaction at the copper cathode will depend on the pH of the solutions, as shown in
Eqs. (5.2) and (5.3).

þ

Fe þ2 2e

anodic reaction Fe ! þ ð 5 : 1 Þ

2H þ þ 2e !

cathodic reaction ð acidic solution Þ H 2 ð 5 : 2 Þ

4e ! 4OH

þ 2H 2 O þ ð 5 : 3 Þ

cathodic reaction ð neutral or basic Þ O 2

Because the free energy per mole of any dissolved species depends on its concentration,
the free energy change and electrode potential of any cell depends on the composition of
the electrolyte. Thus, the direction and rate of the reactions also depends on the concentra-
tion of the solutions. Increasing the concentration of Fe þ2 in the environment will shift the