On the Rheology of Soft Cellular Materials, An Experimental Investigation of the Microstructure and Rheology of a Novel Tissue-Mimic Material

Abstract

Taking inspiration from the mechanics of biological development, in which cellular adhesion and membrane tension have emerged as key determinants of the mechanical properties of tissues, this thesis is a study of the rheology of a synthetic tissue-mimic material made from functionalised giant unilamellar lipid vesicles, in which inter-particle adhesion and membrane tension can be varied experimentally by controlling the functionalisation and sample temperature. This thesis reports on contributions made in three areas. The first describes how 3D samples of a novel tissue mimic material can be made from functionalised lipid vesicles, using a protocol developed to produce a large quantity of vesicles. Alongside the sample production protocol, the first contribution covers the development of instrumentation and techniques for visualising and quantifying the samples’ microstructure. The second contribution focusses on the measurement of samples’ rheology. It describes a rheometer developed for the purpose; magnetic tweezers with an integrated thermostatic water bath, capable of conducting creep tests on samples of vesicle aggregate material over a range of temperatures. This strand also presents discussions on the calibration of magnetic tweezers, and on the interpretation of data from probe-based rheometry. An experimental section presents data from two experiments, which examine the effect of using different functionalisation parameters, and of varying the sample temperature, on the samples’ rheology as measured in creep tests. These experiments demonstrate that the com- pliance of samples varies widely with the affinity and avidity of functionalisation molecules on the vesicles’ surface, and with the sample temperature. A fractional Maxwell model is used to model the creep response, revealing a correlation between the model’s parameters that persists across a wide range of sample preparation and measurement parameters. The final contribution addresses the question of how the sample’s preparation, described in terms of its functionalisation, microstructure and temperature, can be linked to its rheology, as measured and characterised using the parameters of the fractional Maxwell model. The relationship between preparation and rheological parameters is captured using a kernelised re- gression trained on the preparation and rheological parameters, which predicts the rheological parameters more accurately than a null model, at the 1% significance level.