Towards realistic haptic organ phantoms for medical training on minimally invasive robotic surgery systems

Kurzfassung

The newly developed minimally invasive robotic surgery (MIRS) platform MiroSurge from the German Aerospace Center (DLR, Oberpfaffenhofen, Germany) promises new surgical possibilities by enhancing the intracorporal manipulability and the surgeon’s immersion. The chance to palpate the intracorporal operation site by re-establishing of the haptic sensation of tool-tissue interaction to the surgeon’s hands, also leads to a shift of focus in medical training systems. Hence, the new platform asks for haptic realistic training phantoms to test and verify newly developed instrumental settings, as well as to present surgeons with a training system. Currently, such training phantoms are discussed very poorly in the research for medical training and only very few approaches have only been conducted recently. As there is no common ground on how to advance such a problem, this thesis presents an approach towards the development of a haptic realistic liver phantom and provides new insights and valuable results for a greater understanding on how such phantoms can be achieved. To portray the complex non-linear viscoelastic behavior of the liver, the synthetic phantom tissue aims to imitate the characteristic mechanical properties (strain-hardening, hysteresis loop in cyclic loading and stress-relaxation at constant strain) of cylindrical porcine liver specimen. Once those characteristics are remodeled correctly, the development can proceed to imitate the whole human liver in successive future working packages. The synthetic phantom tissue is modeled as a viscoelastic Reuss composite with silicone layers of different characteristics, and aims to mimic the properties phenomenologically on a macroscopic level. This guarantees the best combination of tissue mimicking abilities, a feasible production and high longevity of the phantom tissue. Quasi static uniaxial cyclic compression tests were performed on cylindrical porcine liver specimen in-vitro with a diameter of 36 mm and a height of 16 mm and on material specimen of equal dimensions. The tests were made along the lines of standardized norms for rubbers and promised the best compromise of realistic surgical loading conditions and mapping the tissue in a most natural way. Four testing series were performed to study the properties of the biological tissue and to successively adopt the viscoelastic composite to the desired behavior. The characteristic strain-hardening of the liver during the compressive loads was imitated successfully by a composite consisting of a liquid silicone rubber (Shore A = 40) and an upper layer of a very soft silicone gel. Being very diverse in their mechanical properties, this material combination suggests that the behavior of the tissue cannot be portrayed by single homogeneous materials. However, the model failed at mimicking the characteristic hysteresis loop and stress-relaxation. It was concluded that the interactions of the different structural constituents of the liver (vascular, structural, cellular elements) during a deformation need to be studied in more detail. Consequently, remodeling those components on a different hierarchical level might provide a more realistic behavior but automatically increases the complexity of processing such a structure drastically.