Biomedical Engineering Reference
In-Depth Information
Fig. 6.6.
MPM Images of collagen at various stretches: (i) 1.10, (ii) 1.33, (iii) 1.44, (iv) 1.55, (v)
1.66, and (vi) 1.77 (Bars = 50
μ
m). Sample 1
Recruitment stretch
In Sample 01, all visible fibres appear to be wavy at stretch of 1.1. Between, stretch
values of 1.44 and 1.55, the collagen crimp appears to be nearly if not completely
eliminated. The slope of the stress stretch curve visibly increases in this range, con-
sistent with recruitment of collagen Fig. 6.5. These results suggest medial collagen
fibres are recruited at a finite stretch over a relatively small stretch range [3]. Further
analysis is being performed to evaluate this hypothesis [50].
We have recently developed techniques to quantify the degree of crimp and these
methods are being used to quantify the fibre recruitment process, including an as-
sessment of the validity of the discrete recruitment approximation [50]. Briefly, a
threshold level for each fibre is selected manually. Then skeletonization and edge
detecting algorithms are used to trace a curve passing along a fibre segment, Fig. 6.7,
MATLAB (The Mathworks, Inc.). A commonly used measure of tortuosity is used
to quantify crimp [10, 45], defined as the ratio of fibre arc length
(
L
)
to the linear
distance of the fibre
, Fig. 6.7. The linear distance was obtained as the inter-
section of the curve and the line with minimal distance to the curve (obtained using
linear least squares), (Fig. 6.7). Tortuosity was computed at six different stretches
for Sample 02 (Fig. 6.8a). A decrease in average tortuosity can be seen between
data at stretches of 1.7 and 1.8. The corresponding stretch-stress curve as well as the
slope of this curve is shown in (Fig. 6.8b). The slope of the stress stretch curve be-
gins to increase rapidly in this stretch range, Fig. 6.8a, consistent with the decreased
tortuosity during collagen recruitment.
(
L
0
)
