Time: 11:00 AM – 12:00 PM

Location: MBB 1.210

Abstract:

In many biological tissues, structural integrity comes from the extracellular

matrix (ECM). The ECM is a complicated network of proteins and polysaccharides,

but the main protein is fibrillar collagen, the most abundant protein in mammals. The

structure and mechanical parameters of collagen networks determine the properties

of tissues like skin, tendons, and myocardium, influence the differentiation and gene

expression of cells, and can affect the migration of cancer cells. But some aspects

of collagen networks’ mechanical properties, like the origin of their strain stiffening

behavior, are still poorly understood, partly because of the lack of experimental

techniques which bridge the gap between the mechanics of individual filaments and

the mechanics of the entire filament network. In this dissertation, we address this

need by continuing the development of the novel technique called activity microscopy.

Activity microscopy is a scanning-laser technique which we use both to image collagen

networks and to study local network mechanics by measuring the dynamics of

individual filaments. For imaging, we study how image brightness depends on an

object’s orientation, size, and position in the beam-propagation direction. We develop

a method for correcting the orientation dependence, a necessary step on the

way to developing an imaging technique where brightness indicates the diameter of

a filament, even though the filament is smaller than the diffraction limit. To study

filament mechanics, activity microscopy measures a filament’s thermal fluctuations

with megahertz bandwidth, even for fluctuations on a scale of ≲ 10 nm. In controlled

experiments, we determine that our fluctuation measurements are accurate to within

a nanometer, with a standard deviation that is ≈ 8% of the measured value. By

fitting fluctuation data to predictions made by the worm-like chain (WLC) model, we

measure the bending stiffness of individual filaments in a network and obtain values

which are consistent with previously-measured values for filaments in isolation.