Characteristics and young's modulus of collagen fibrils from expanded skin using anisotropic controlled rate self-inflating tissue expander

Mechanical properties of expanded skin tissue are different from normal skin, which is dependent mainly on the structural and functional integrity of dermal collagen fibrils. In the present study, mechanical properties and surface topography of both expanded and nonexpanded skin collagen fibrils were evaluated. Anisotropic controlled rate self-inflating tissue expanders were placed beneath the skin of sheep's forelimbs. The tissue expanders gradually increased in height and reached equilibrium in 2 weeks. They were left in situ for another 2 weeks before explantation. Expanded and normal skin samples were surgically harvested from the sheep (n = 5). Young's modulus and surface topography of collagen fibrils were measured using an atomic force microscope. A surface topographic scan showed organized hierarchical structural levels: collagen molecules, fibrils and fibers. No significant difference was detected for the D-banding pattern: 63.5 ± 2.6 nm (normal skin) and 63.7 ± 2.7 nm (expanded skin). Fibrils from expanded tissues consisted of loosely packed collagen fibrils and the width of the fibrils was significantly narrower compared to those from normal skin: 153.9 ± 25.3 and 106.7 ± 28.5 nm, respectively. Young's modulus of the collagen fibrils in the expanded and normal skin was not statistically significant: 46.5 ± 19.4 and 35.2 ± 27.0 MPa, respectively. In conclusion, the anisotropic controlled rate self-inflating tissue expander produced a loosely packed collagen network and the fibrils exhibited similar D-banding characteristics as the control group in a sheep model. However, the fibrils from the expanded skin were significantly narrower. The stiffness of the fibrils from the expanded skin was higher but it was not statistically different

Investigation on the swelling mechanics of hydrogel tissue expanders using a skin-mimicking apparatus

The concept of self-expanding hydrogel tissue expanders has opened a new pathway to the treatment of soft tissue defects. Traditionally designed to expand the skin, modifications to the swelling behaviour of these devices may lead to various new-found applications. Properties of poly(vinylpyrrolidone) /poly(methyl-methacrylate) (VP/MMA) hydrogel system such as the degree, rate, force generation, and anisotropic behaviour of swelling were investigated in this study as part of a preliminary understanding of the hydrogel material. The swelling of hydrogel network is the balance between two main driving forces: 1) the relaxation of the gel network and 2) the osmotic diffusion of solvent via gel hydrophilicity. A direct relationship between the elastic modulus and the swelling pressure was found by measuring the swelling force over the hydration period. Despite reduction in the overall swelling ratio, gels with a larger modulus were observed to have a larger corresponding force generation. This is due to the straining of gels with higher cross-linking density. The results showed a maximum stress of 64.0 ± 3.2 kPa and 30.8 ± 1.5 kPa for 90:10 wt% and 99:1 wt% VP/MMA, respectively. Specially modified anisotropic swelling gels were shown to have larger stress generation along the axis of directional swelling (~70-80 kPa). Limitation factors to the anisotropic modification are materialistic and geometrical. Shape-memory characteristics were found to be reduced at high compression ratio as a result of plastic deformation. Gels with slenderness ratio above 2.5 were evaluated to be susceptible to device slippage/buckling. These results provide useful design limitations for self-expanding tissue expanders. The overall goal in creating a new testing system for self-swelling tissue expanders was achieved.A skin-mimicking apparatus was specially built to highlight the amount of skin expansion and simulate surface topography. Strain contours along the expanded skin were also plotted to represent skin tension. Prior to this study,these measurements were only possible under in vivo animal testing. The advantage in having this device is to support the prevention of related animal testing.This thesis is not currently available in ORA

In vivo adaptation of tendon material properties in healthy and diseased tendons with application to rotator cuff disease

Degenerative disorders of the rotator cuff tendons account for nearly 75% of all shoulder pain, causing considerable pain and morbidity. Given the strong correlation between age and tendinopathy, and unprecedented population aging, these disorders will become increasingly prevalent. Improved understanding of tendon degeneration will guide the development of future diagnostic and treatments, and is therefore urgently needed. However, the aetiology and pathology of rotator cuff tendinopathy remain unclear. The complicated mechanical environment of the rotator cuff is hypothesised to influence the susceptibility of the tendons to degeneration and tearing. Studies have reported biological adaptations in torn cuff tendons indicative of increased compressive loading within the tendon. The material adaptations of healthy and degenerative cuff tendons are largely unreported but will provide further insight into the role of the mechanical environment in rotator cuff aetiology and pathology. This thesis examined the material adaptations of healthy and diseased tendons to explore the role of mechanical loading in rotator cuff pathology. The material adaptations of healthy animal tendons, and healthy and delaminated human cadaveric rotator cuff tendons, in response to different loading environments were characterised. The effects of age, tears, steroid injection and subacromial decompression surgery on the structural adaptations of human cuff tendons were also studied, as was the effect of tendon cell proliferation on the mechanical properties and degradation behaviour of collagen scaffolds. Loading environment significantly affected the structural adaptations of healthy tendons. Regions exposed to compressive and shear strains exhibited thinner fibres, shorter crimp lengths and thinner, less aligned fibrils compared with regions exposed to tensile strains alone. In healthy rotator cuff tendons, the inhomogeneous loading environment produced topographically inhomogeneous structural adaptations. The tendons of a delaminated rotator cuff exhibited less topographical variation in properties and thinner, less aligned fibrils compared with healthy cuff tendons. Torn cuff tendons exhibited thinner fibrils and shorter crimp lengths compared with control samples. These adaptations were identifiable early in the disease progression, and neither steroid injection nor subacromial decompression surgery significantly influenced these adaptations at seven weeks post‐treatment. Significant correlations between decreasing dimensions and increasing tear size were found when age was included as a confounding factor, reflecting the importance of age and tear size in determining the material properties of tendons. Tendon cell proliferation influenced the mechanical properties and degradation behaviour of the collagen scaffolds, emphasising the integral role of cells in the functional adaptation of biological materials. These results demonstrate the effect of mechanical environment on the material adaptations of tendons. They also indicate the importance of the complicated mechanical environment experienced by the rotator cuff tendons in predisposing the tendons to degeneration and tearing. The observed material adaptations of degenerative and torn tendons suggest that rotator cuff pathology is associated with increased levels of compressive and/or shear strains within the tendon. These changes begin early in the disease progression and neither steroid injection nor sub‐acromial decompression surgery are capable of reversing the changes in the timeframe investigated. These findings highlight the urgent clinical need for pre‐rupture diagnostic techniques for the detection of early pathological changes in the rotator cuff. They also emphasize the requirement for new intervention strategies that restore the healthy mechanical environment and reverse early pathological adaptations in order to prevent catastrophic failure of the tendons.This thesis is not currently available in ORA

Nanoscale origins of spider dragline mechanical properties

Several mechanical models, in which a material is treated as a composite of crystalline and amorphous and/or interphase material, were used to predict the tensile modulus of spider dragline along the fiber direction. The models included the Voigt average (which assumes that the fibers/crystals are continuous, and that the strain is the same in all components of the composite); a modified Halpin-Tsai model (which is suitable for predicting the longitudinal elastic modulus for short aligned fiber composites, and is thus more appropriate for silk); and the shear-lag or Cox model (which is a modification of the Voigt average that takes into account a discontinuous nature of stiff fibers/crystals and the resulting shear stress in the amorphous matrix). The latter two models yielded close approximations of an experimentally measured elastic modulus of Latrodectus hesperus (black widow spider) dragline under conditions of controlled temperature and humidity, given realistic inputs for the moduli of the individual components and the percent crystallinity. A literature model for the stress-strain behavior of silk [1] was also considered, in the context of our experimental results from transmission electron microscopy (TEM) and X-ray diffraction (XRD) studies of L. hesperus dragline. TEM and XRD results indicated a bimodal size distribution of ordered regions; one population of crystals has a mean size of 2 nm, and another spans the size range 40-120 nm. The average elastic modulus measured from L. hesperus dragline is 23 GPa - close to the 25 GPa theoretical modulus for the case of large crystals in Termonia's model. The tensile strength of L. hesperus dragline is ca. 1.7 GPa, close to the case predicted for small crystals in Termonia's model. A combination of the small and large crystals could explain the forced elongation behavior of L hesperus dragline