Understanding the Mechanical Behavior of Costal Cartilage at Their Curved Exterior Surface Via a Tactile Sensor with a Built-In Probe for Distributed-Deflection Detection

UNDERSTANDING THE MECHANICAL BEHAVIOR OF COSTAL CARTILAGE AT THEIR CURVED EXTERIOR SURFACE VIA A TACTILE SENSOR WITH A BUILT-IN PROBE FOR DISTRIBUTED-DEFLECTION DETECTION Jiayue Shen Old Dominion University, 2017 Director: Dr. Zhili Hao This dissertation is aimed to determine the mechanical properties at the exterior surface of costal cartilages (CC) and examine how they vary with the cartilage length and the anatomical sites of CC in the ribcage via conformal indentation testing which is built upon a tactile sensor for distributed-deflection detection. The sensor entails a rectangular Polydimethylsiloxane (PDMS) microstructure sensing-plate integrated with a 5 ×1 transducer array with 0.75mm spatial resolution underneath and a built-in probe of 0.5mm×5mm×3mm above. By pressing the sensor against the exterior surface of a CC tissue with a pre-defined indentation pattern, the sensor conforms to the curved tissue surface via the built-in probe first, and then the mechanical properties of the tissue translate to the spatially distributed deflection in the sensor and register as resistance changes by the transducer array. As a load-bearing and nonstop deforming tissue from respiration, the mechanical properties of CC are critical for maintaining their structural health and delivering their function. CC have been used as a viable source of graft tissue for many autologous therapies and as a cell source for engineered articular cartilage (AC) due to its abundance and surgical accessibility. However, the mechanical properties of CC are not well understood yet. Chest wall deformities, such as Pectus Carinatum (PC), are known to arise from the disorder of CC, but their pathogenesis remains unknown and their surgical outcomes are unpredictable. The mechanical properties of the CC exterior surface influence diffusion of oxygen and nutrients and thus are intrinsic to maintaining their structural characteristics. However, very limited knowledge exists on the mechanical properties of peripheral CC due to their highly irregular geometries. In this dissertation, a novel testing method, conformal indentation, was used to measure the mechanical properties at the CC curved exterior surface, where the structural integrity of CC is retained. Conformal indentation was conducted at the anterior/posterior surfaces of whole porcine 5 th -12 th CC segments and the anterior/posterior surfaces and the superior/inferior borders of five human PC CC segments from the 7 th ~10 th ribs along the cartilage length to record their time-dependent response to a multi-step indentation-relaxation testing protocol. The instant indentation modulus and normalized relaxation of the CC segments were derived from the recorded data to quantify their elasticity and viscosity, respectively. The instant indentation modulus at the porcine CC and PC CC exterior surface are in the range of 130kPa ~500kPa and 98kPa~1173kPa, respectively, which are well below their counterpart at the CC transverse crosssections. The normalized relaxation at the CC exterior surface is relatively high with low applied stress but becomes constant with high applied stress. The constant normalized relaxation at the porcine and PC CC exterior surfaces are in the range of 25%~40% and 5%~25%, respectively. The human CC have higher elasticity and lower viscosity than the porcine CC. Overall, the measured mechanical properties of CC vary with their anatomical sites and thus indicate the adaptation of CC to their local biomechanical environment in the ribcage.