Biomedical Engineering Reference
In-Depth Information
3.5
3.0
2.5
2.0
1.5
(a)
TPS
(b)
G-TPS
1.0
(c)
A-TPS
0.5
0.0
0
5
10
15
20
25
Soaking time (day)
FIGURE 1.11
Dissolution behavior of sol-gel derived HA coatings in different testing conditions: (a) Tris-buffered physiologi-
cal saline solution (TPS), (b) glucose-modified TPS (G-TPS), and (c) albumin-containing TPS (A-TPS).
is applied, such as a residual, the system is no longer in equilibrium because the chemical
potential of the coatings is altered by an amount proportional to the stress, namely,
σ
ss
Ω
/3
per unit volume. (
Ω
is the unit volume of the species under consideration.) As a result,
according to the thermodynamic analysis, with the presence of residual stress, additional
free energy for the dissolution equilibrium becomes available in the form of elastic strain
energy stored in the coatings, and the new equilibrium condition is represented by an
altered value of the solubility product,
K
s
σ
:
σ
0
ss
∆
G
−
= −
RT K
ln
σ
sp
3
The negative sign implies that a compressive residual stress will hinder, whereas a tensile
residual stress will enhance the dissolution. Therefore, the ratio of the solubility product
under different stresses (e.g.,
σ
1
and
σ
2
) is:
K
K
σ
1
(
)
sp
=
exp
−
σ
σ
1
2
3
RT
σ
2
sp
This expression can be used to estimate the change of solubility product owing to the pres-
ence of different residual stresses. Therefore, if the residual stresses in the sol-gel derived
HA coatings are tensile stresses, a higher dissolution rate would be observed than the
stress-free coatings correspondingly.
All in all, even though the mass loss caused by dissolution is somehow detrimental
to the adhesion, it is still believed that the initial dissolution behaviors are quite benefi-
cial to the healing process as well as the long-term successful osteointegration (Martini et
al. 2003). First, the initial fast dissolution of the coating will result in a local super satu-
rated environment (e.g., Ca
2+
), which is more favorable for the nucleation and growth of
bonelike apatite, speeding up the formation of chemical bonds between the implant and
