Biomedical Engineering Reference
In-Depth Information
on the tissue composition, structure, and load carrying mechanisms of individual
components. In doing so, they provide more insight into the function and mechanics
of tissue components, at the expense of requiring constitutive data for each of the el-
ements. For example, recent experimental and modelling studies have addressed the
mechanical behaviour of isolated arterial elastin [42, 43, 112]. In between these two
extremes are so called structurally motivated models, which bring in an intermediate
level of structural information such as the fibrous nature of the tissue without directly
incorporating the response of individual components. For example, the mechanical
behaviour of individual fibres is not prescribed. Mixture theories have been devel-
oped to describe the growth and mechanical response of individual wall constituents
[59]. These models have shown great promise in studies of aneurysm development,
vasospasm, and remodelling under altered loads [37, 38, 58, 60].
Lanir appears to be the first to develop a three dimensional microstructural model
for fibre reinforced soft tissues that directly includes fibre orientation [67]. Though
Lanir formulated this structural model more than 25 years ago [66, 67], these models
have recently received increasing attention. This surge in interest arises partially due
to improvements in imaging and computational technology as well as the desire to
model more complex behaviour of the artery such as subfailure damage [74, 87, 88],
growth, and remodelling [59, 60, 94].
One of the most significant challenges in applying microstructural models is ac-
quiring experimental values for the structural components. With few exceptions,
most structural information about the orientation and distribution of wall compo-
nents are obtained using destructive techniques such as fixation, followed by section-
ing and staining for the particular constituent of interest. As a result, multiple tissue
samples and tissue fixation steps are typically needed to explore the wall structure
under different loading conditions. For example, in the most detailed study available
on collagen fibre alignment in cerebral vessels during loading, Finlay, McCullough
and Canham evaluated vessels that were embedded in paraffin and sectioned [28].
Sacks developed an approach using small angle light scattering (SALS) to non-
destructively measure collagen fibre orientation [98] which he then directly incorpo-
rated into Lanir's structural model [97]. While this approach was used effectively in
a number of applications including bovine pericardium, it is not suitable for thicker
tissues samples such as the intact arterial wall. Further, this approach does not distin-
guish between fibre types or provide information about the distribution of orientation
through the thickness of the tissue.
Recent advances in multi-photon microscopy (MPM) provide an opportunity to
nondestructively acquire quantitative structural information during the loading pro-
cess of interest for thicker tissues such as cerebral vessels. Using MPM, elastin and
collagen can be imaged without staining or fixation at greater depths than previ-
ously possibly [123]. We recently developed an MPM compatible uniaxial system
(UA-MPM system) which enables quantitative assessment of the microstructure,
simultaneous with mechanical loading experiments [49, 50]. This makes it possi-
ble to correlate microstructure (including some changes due to damage) and me-
chanical response in a single sample over a range of loading conditions and time
points. A major advantage of using a MPM is the ability to obtain images of arte-
