Abstract:
Cardiac disease is the leading cause of death worldwide and thus there is a great need
to develop a better understanding of the heart in both healthy and diseased states. Cardiac
muscle cells are fundamental to the correct functioning of the heart, yet there is much to
be discovered about their mechanical properties in health and disease. Cardiac trabeculae
are small multicellular samples of heart muscle tissue that contain many hundreds of axially
aligned myocytes, and are therefore often studied in vitro to better understand the underlying
physiology of cardiac muscle. However, an order of magnitude difference between the stress
production of myocytes and of cardiac trabeculae has been reported. A deeper knowledge of
the properties of these tissues would assist in developing treatments for disease, as well as
clarify the underlying causes of variability at different tissue levels.
While both individual myocytes and cardiac trabeculae have been extensively studied to
get a better understanding of cardiac mechanics, the instruments available for experimentation
on these tissues often involve delicate mounting of the muscle and are limited to examining at
one cell/muscle at a time. Devices and methods for high-throughput testing of cardiac tissues
would provide the potential to gain deeper insight into cardiac mechanics. Experimentation
under the influence of a variety of pharmacological interventions could improve the rate at
which treatments for cardiac disease are developed.
In this thesis, methods that are useful for high-throughput testing of both myocytes
and trabeculae are explored. Robust image registration techniques are used to estimate 2D
displacements within a myocyte during contraction, and to quantify internal displacement
and strain fields with subpixel accuracy. Using these tracking techniques, localised sarcomere
length estimation is performed using a tracked window. The internal strain field and localised
sarcomere length measurements demonstrate that contraction across the cell is non-uniform.
Next, these algorithms are implemented in real-time for cell length measurements, producing
more robust results than currently used techniques. Methods for easy manipulation of cells
and trabeculae using visible-light cured hydrogels are then developed, and experiments on
cell-gel constructs are performed, using a finite element model to estimate force production
from tracked displacements of the cell. Stresses reported in this thesis were similar to those
found in literature.
Real-time tracking is next applied to trabeculae that are secured at one end with hydrogel
while the other end is manipulated via a force sensor. The muscles are stimulated while being
imaged and real-time feedback control based on cantilever and trabecula movement during
contraction is performed. Additionally, both ends of a trabecula are fixed and sarcomere
variation along the length of the muscle is examined. Isometric contractions were performed
on trabeculae based on muscle length defined using internal points in the muscle, as well as
using the end tissue attachment points.
The techniques developed in this thesis for experimentation of cardiac samples at both
the tissue and cell level have the potential to be used in high-throughput systems to explore
cardiac muscle function. Various interventions can now be applied to cardiac muscle using
these techniques to determine their effects on the mechanical properties of the muscle. The
devices developed in this thesis have the potential to provide new information for medical and
scientific fields. The function of muscle in healthy and diseased states could be assessed, in
order to develop therapies for cardiac disease.