Assessing the quality of biological tissues and gelatine phantoms using multi-scale structure-property relationship
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The main aim of this work is to develop methods of assessing soft biological tissue quality using structure-property relationships. “Tissue quality” is here taken to mean the condition of the biological tissue structure which can be quantified in terms of features at a range of scales, including histological features. The purpose is to serve as a diagnostic aid in clinics and provide quantitative information that might replace the more traditional palpation. It is recognised that any structure-property relationship may be local to particular areas and may be differently expressed at a range of scales of probe size. In order to arrive at some generic outcomes, two model systems were chosen, one a biological mimic (gelatin-oil mixtures) which had the advantage that certain aspects of the structure could be varied. The biological system was chosen to be ovine connective tissue (collagen-lipid mixture) because it was expected to be similar in its components to the mimic, although with a more natural variation in structure. Static and dynamic indentation tests were carried out on all of the material and the static and dynamic elastic and viscoelastic properties were determined. Two experimental rigs were developed, a macro-scale one with a 0.5mm radius indenter and micro-scale one with a, pyramidal indenter of base side 4µm, height 6µm and opening angle . The macro-scale rig was purpose-built with indentation distance being controlled using software specifically configured for the work. The macro-scale rig was a modified atomic force microscope with a piezo-ceramic actuator being used to vibrate the specimen stage. Experiments were carried out under displacement control, with the mean indentation distance in the range from 1-4% of the specimen height, a frequency range of 1-10Hz and an indentation displacement amplitude of 30 and 90µm. Nine different oil in gelatine formulations were used and the ovine biological tissue was classified as; collagen (from the ligamentum nuchae), fat (from the tail) and a collagen-fat mixture (from the abdomen). The dynamic tests were carried out over a range of frequency and the variation of the loss and storage modulus with frequency was used to arrive at an appropriate visco-elastic model for each of gelatine-oil and biological tissue systems. Finally, the elastic and viscous moduli were correlated with tissue quality (structure) to obtain the structure-property relationship. Of the various three-parameter visco-elastic models tested, a Maxwell model in parallel with a spring was found to be the most suitable for the biological tissues whereas a Kelvin model in series with a spring was best for the gelatine-oil mixtures. Using the resulting visco-elastic moduli, it was found that a structure-property relationship exists for biological tissues and gelatine phantoms at both scales. Furthermore, the results suggest that the scale of the probe affects the dynamic mechanical properties for the biological tissues but not for the gelatine phantoms.