dc.description.abstract
Realistic anatomical models are an important tool in research and education, used for aiding the development of new medical products and procedures, for teaching anatomy, and for surgical training. However, producing anatomical models, exhibiting accurate mechanical properties, is challenging due to the inherently complex mechanical behaviour of soft tissues.In this dissertation, it was assessed which properties need be be considered, in the context of describing what soft tissues actually feel like. Focussing especially on liver parenchyma, different experimental and data analysis methods were developed, considering non-linear and viscoelastic properties of fresh human liver, fresh animal liver, Thiel embalmed human liver, and different tissue-mimicking artificial materials.On the one hand, macroindentation, mimicking soft tissue palpation, was applied for objectively assessing what materials feel like, allowing easy comparison between different materials. For examining the non-linear elastic behaviour, the materials were slowly indented and stiffness was defined for different levels of indentation. Considering viscoelasticity, force relaxation of the material was measured during a period of constant indentation. The decline of force over time was described with rheological modelling and frequency-dependent storage stiffness, loss stiffness, and loss tangent were calculated. By comparing resulting mechanical properties of different artificialmaterials with liver tissue, using the newly introduced “tactile similarity error”, a soft silicone elastomer was found that resembled liver best in terms of the tested properties.On the other hand, tensile tests were conducted with similar testing protocols (ramp loading-unloading and stress relaxation) to define properties on a material level. The ramp loading-unloading stress-stretch data was analysed with a pseudo-hyperelastic Veronda-Westmann model and strain-specific tensile moduli were found.Viscoelastic properties were expressed in terms of storage modulus, loss modulus, and loss tangent, based on the stress relaxation data. Additionally, equivalent viscoelastic properties were measured with dynamic cyclic testing, in order to compare relaxation and cyclic testing. The results showed that stress relaxation and cyclic testing yielded comparable viscoelastic results, as long as non-linear behaviour was considered and tests were conducted at the same level of strain.Based on these experimental methods, novel insight into tensile viscoelastic properties of fresh human, animal, and Thiel preserved liver was gained. Thus, differences between human and animal liver could be analysed and explained by their characteristic histological morphologies. Furthermore, Thiel preservation was found to be associated with tissue stiffening and decreased viscous behaviour. Results from testing composite materials in macroindentation, revealed that combining different materials on a structural level is a promising strategy for fine-tuning mechanical properties to better match liver tissue. In order to design such composites, a viscoelastic Mori-Tanaka model (vMTM) for homogenising effective properties of materials, exhibiting matrix-inclusion morphologies with soft viscoelastic phases, was developed. The vMTM was validated experimentally by testing samples, consisting of two soft silicones with varying inclusion volume fractions, for comparison with the vMTM predictions. Applying the developed vMTM, frequency dependent viscoelastic material properties of various soft silicone elastomers were then used as input quantities. The thereby resulting effective properties were compared to those of human and animal fresh liver from the tensile tests. Furthermore, it was assessed how well the effective properties matched other soft biological tissues, whose properties were previously reported in literature.Finally, suitable microstructures of matrix-inclusion-type were identified, which exhibited very similar viscoelastic properties to certain soft biological tissues. These microstructures can hopefully be produced via additive manufacturing in future, thanks to recent developments in the field of soft material 3D printing.The work, presented in this dissertation, provides new insight into the mechanical properties of liver parenchyma, based on rigorous experimental evaluation and computational analysis. Furthermore, microstructural material design for realistic anatomical models was presented, based on the viscoelastic Mori-Tanaka model. Therefore, the results of this dissertation can contribute to the production of more realistic soft tissue models, for instance for applications in medical education.
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