Multiscale Modeling of Ligaments and Tendons - Multiscale Computer Modeling in Biomechanics and Biomedical Engineering - page 112

3.4 Microscale Mechanical Characterization
The collagen fiber is the primary load bearing tissue constituent in ligaments and
tendons at the microscale. Located between collagen fibrils are fibroblasts in
ligaments and tenocytes in tendons, which are responsible for secreting collagen
and other ECM materials in order to maintain the mechanical integrity of the tissue
[ 230 ]. It is well established that fibroblasts respond to local strain fields via
mechanotransduction, making the study of microscale force transfer particularly
important [ 27 ]. Both direct (e.g. isolating individual fibers) and indirect (e.g.
confocal imaging of loaded tissue) studies have been performed.
Only one study has directly examined the stress-strain response of individual
fibers [ 162 ]. In this study, individual fibers (*1 lm in diameter) were isolated
from rabbit patellar tendon and subjected to uniaxial tensile loading. The reported
stiffness was compared to the fascicle and whole tendon data from a previous study
[ 250 ]. The individual fibers were less stiff than both individual fascicles and whole
tendons. This result implies that the macrostructures are stiffer than their con-
stituents. As with the mesoscale fascicle test data discussed in Sect. 3.3 , this result
awaits a satisfactory explanation.
Confocal imaging of rat tail tendon fascicles has yielded considerable insight
into the microscale strain environment of collagen fibers and tenocytes. In these
studies single rat tail tendon fascicles were stained for collagen and cell nuclei,
subjected to tensile loading and imaged using confocal microscopy [ 199 , 201 ,
202 ]. These studies have revealed that the local strain field within fascicles is
highly inhomogeneous. In response to uniaxial tensile loading, the predominant
mode of microscale deformation is shearing, whereby individual fibers slide rel-
ative to adjacent fibers. As a result, local fiber strains are much smaller in mag-
nitude than applied tensile strains. In one study, the local fiber strain was *1%in
response to an applied strain of 6 % [ 201 ]. Resulting tenocyte strains were also
inhomogeneous, with both tensile strains and large shearing strains being induced
by tensile loading.
These experiments have also yielded insights into the microscale viscoelastic
response. Two microscale mechanisms of viscoelasticity have been observed: a
time dependent shearing of adjacent fascicles, and a time dependent stretching of
individual fibers. The inter-fiber sliding response displayed a much larger mag-
nitude of stress relaxation than the individual fibers, suggesting that microscale
shearing may play an important role in the solid phase viscoelastic component of
tendon. Although the source of the microscale strain inhomogeneity and large
inter-fiber shear is still under investigation, it has been suggested that this may
result from the uncrimping of the ubiquitous collagen crimping pattern [ 201 , 202 ].