| Summary: | This report described the work carried out in 2007 using the equipment purchased from the supplementary equipment purchase (SEP) grant RG123/06. The aim of this work is to investigate the mechanisms of stress transfer at the collagen fibril and proteoglycan (PG) gel interface in extra-cellular matrices. To carry out this work, we have designed an experiment to investigate how freezing of soft tissues will alter the extra-cellular structure of the tissue and how this could implicate the mechanisms of stress transfer from the hydrated PG-rich gel to the fibrils. Freezing of biological tissues leads to the formation of ice crystallites. These crystallites disrupt cells, increase the electrolytic concentration around proteoglycans (PG) in the extra-cellular matrix (ECM), which comprises collagen fibrils embedded in and reinforcing a PG-rich gel, and consequently alter the organisation of the ECM. Here, we analyse the influence of freezing temperature (T) on the tensile properties of tendons and the underlying mechanisms of failure resulting from changes in the ECM structure. Fresh (unfrozen) and thawed samples of tail tendons (from a C57BL6 mouse) preserved at –20 oC and –80 oC were subjected to an axial-tensile load to rupture. The maximum stress (σ), stiffness (E), strain at the maximum stress (ε), strain energy density to the maximum stress (u) of tendons, critical strain (εc) at the elastic limit, the corresponding stress (σc) and elastic strain energy density (uc), and strain energy density during fibril pull-out and PG gel rupture (up), were determined. Freezing at –20 oC revealed no significant effect on the mechanical properties. However, apart from ε, σc, uc and up, freezing at –80 oC demonstrated significantly higher σ, E and u and lower εc. Analysis of these results implicates two key factors influencing the mechanical properties. These are: (1) the long-range order of radial (i.e. side-to-side) packing of collagen molecules in fibrils, the mechanics of fibril-fibril sliding during the elastic loading stage, and (2) the underlying (van Der Waals forces of) interactions between the glycosaminoglycans (GAGs), associated with collagen-bound PGs,from adjacent fibrils. However, the toughening mechanism resulting in fibril pull-out and PG gel rupture is not affected by freezing.
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