The Transport of Insulin-Like Growth Factor through Cartilage - Porous Media: Applications in Biological Systems and Biotechnology - page 420

25

Experimental data (Bhakta et al. 2000)
Numerical solution with binding
C BP0 = 70 nM

20

15 C BP0 = 45 nM

C BP0 = 20 nM

10

5

Numerical solution without binding

0
0.01

0.1 1 10

Concentration of IGF-I in bath solution (nM)

100 1000

FIGURE 11.6
Comparison of numerical predictions with experimental results of Bhakta
et al. [29].

that binding results in the total solute concentration throughout the carti-
lage exceeding the solute bath concentration, that is, R u > 1 (as shown in
Figure 11.7a). On the other hand, if binding is not included, the curves of
diffusion profile are constrained by c I 0 at every stage of solute transport (see
Figure 11.7b). On the other hand, binding was seen to inhibit the rate of
free IGF-I uptake as shown in Figure 11.8. This inhibition is due to removal
of free IGF-I from the diffusion process by binding to the IGFBP. However,
once the binding sites are saturated the free IGF-I concentration achieves a
steady-state level equal to the surrounding bath concentration.

11.3.3.2 Diffusion with Cyclic Deformation and IGF-I,

IGFBP Interaction

From the results and discussion presented in Section 11.3.1, we would expect
that the optimal loading regime to enhance IGF-I transport should corre-
spond to high-strain magnitudes and frequencies, so as to obtain large advec-
tive Darcy velocities. Indeed this expectation is confirmed by the numerical
predictions shown in Figures 11.9 and 11.10 of the average percent increase
in total IGF-I uptake ratio into a cyclically loaded cartilage in comparison to
free diffusion. Specifically it can be seen that a loading regime with a combi-
nation of high-strain amplitudes (e.g., 6% strain) and high frequencies (e.g.,
0.1 Hz) produces the most dramatic total IGF-I enhancement, especially at
early time (i.e., time < 2 h). More critically to the current discussion is the