Abstract
The physical properties of collagen, and in particular individual collagen fibrils have been extensively studied in the past. These physical properties include the mechanical behaviour, and to a lesser extent the electrostatic behaviour of single fibrils. The influence of different loads, strains or the influence of the chemical environment on the stiffness or surface potential of collagen fibrils is experiencing an increase in interest in the community due to the recent advancements in the field of tissue engineering. Collagen, due to its inherent bio-compatibility is deemed as a good scaffold material for tissue engineering. Since these scaffolds are then transferred into the human body it is important to understand how the scaffolds, and hence single collagen fibrils, behave when subjected to different stress. First, I developed a novel technique to stretch and compress individual collagen fibrils which consists in depositing fibrils on a highly stretchable polymer-based substrate. By stretching the substrate, or depositing the fibrils on a pre-strained substrate and releasing the substrate from strain, the fibrils are either strained along with the polymer or compressed. The stretching technique presented in this thesis significantly expands the strain range of tensile tests of individual collagen fibrils. With this technique many fibrils are deposited on the substrate which means that a large number of fibrils can be studied in parallel. Since the fibrils are lying flat on the substrate, the fibrils are still accessible to AFM imaging and probing, and the nanometric resolution is preserved during AFM imaging. This technique also does not require any specialised hardware and can be implemented by most labs at a low cost. By compressing individual collagen fibrils, I was able to show that simply imaging the resulting buckling patterns with an Atomic Force Microscope (AFM) is sufficient to calculate the Young’s modulus of the individual fibrils. I then showed that axial loading of individual fibrils, with an AFM tip on a soft substrate does not only result in indentation of the fibril but also in bending of the fibril inside the polymer-based substrate, which can make the determination of the Young’s modulus more accurate as bending is a more defined mechanical test compared to indenting. Contrary to AFM nanoindentation, models for the bending of collagen fibrils do not require the user to determine the exact contact area between the tip and the sample. In this thesis, I studied the stiffness of individual collagen fibrils under strain. To do this, single fibrils were strained up to roughly 25% and I used an AFM tip to apply an axial load to single fibrils. I found that through a possible remodelling of the inter-molecular crosslink network, fibrils undergo strain-stiffening followed by strain-softening. Finally, I studied the change of the surface potential of single fibrils as a function of strain and found that fibrils first become more positively charged when strained up to roughly 12% strains and then become less positively charged with greater strains. I also discovered that when fibrils are released from strain their surface potential does not return to the native state surface potential which is indicative, once again, of a remodelling of the inter-molecular crosslinknetwork.
| Date of Award | 1 Dec 2021 |
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| Original language | English |
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| Supervisor | Patrick Mesquida (Supervisor) & Lucy Di-Silvio (Supervisor) |