In this paper a semi-active fluid-mechanism is presented, which offers a variable stiffness and damping by utilizing two magnetorheological fluid valves and two springs. The study incorporates the attributes of variable damping and stiffness into one compact device. A model for the magnetical, rheological, fluidical and mechanical behaviour of the whole system is derived. An experimental setup of the proposed system and an appropriate test bench are built in order to study the variable mechanical impedance behaviour with the corresponding simulations. The results proof that the stiffness of the system can be varied among three different values, while its damping is continuously variable.

For planar tubular continuum structures based on precurved shape memory alloy tubes a beam model with respect to the pseudoelastic material behaviour of NiTi is derived. Thereunto a constitutive material law respecting tension-compression asymmetry as well as hysteresis is used. The beam model is then employed to calculate equilibrium curvatures of concentric tube assemblies without clearance between the tubes. In a second step, the influence of clearance is approximated to account for non-concentric tube assemblies. These elastokinematic results are integrated into a purely kinematic model to describe the cannula path under the presence of material hysteresis and clearance. Finally a photogrammetric measurement system is used to track the path of an exemplary two-tube continuum structure to examine the accuracy of the proposed model. It is shown that material hysteresis leads to a hysteresis phenomena in the path of the tubular continuum structure.

Introduction: For functional and structural restoration of bone deficiencies, various resorbable and nonresorbable alloplastic materials have been introduced, including metals, polymers and ceramics. However, an "optimal" artificial replacement for craniofacial bone has not been found yet and the search for improved reconstruction methods and alternative materials is going on. To assess and compare biocompatibility and osseointegration of these materials, adequate animal test models are indispensable. Methods: In a rabbit cranial defect model, biocompatibility and osseointegration of polymeric and composite bone replacement materials were evaluated at different time points after implantation. Calvaria including implants and surrounding tissue were explanted and embedded in methacrylate resin. The samples were scanned with a nanotom® (phoenix|x-ray) µCT scanner and proceeded for histological examination by sawing-grinding technique. Avizo® Fire (vsg) software was used for visualisation and processing of µCT data. Qualitative and morphometric evaluation of osseointegration and fibrous encapsulation was performed on undecalcified histologic preparations of the explants, and on 3D reconstructions plus virtual slices derived from corresponding 3D µCT datasets. Results: The obtained 3D µCT data enabled a comprehensive qualitative and quantitative assessment of osseointegration and biodegradation of radioopaque composite implants. Prerequisite for visualization and discrimination of materials by µCT is a significant difference of their hounsfield values. Due to this limitation, radiolucent polymeric implant materials and soft tissue could not be distinguished from embedding resin. In contrast, histologic preparations of undecalcified hard tissue and implant materials enabled detailed visualization and examination of all tissues and implant materials. The substantial disadvantage of hard tissue histology was the inevitable loss of information due to small number of slices and large gaps between specimens yielded by this method. Conclusion: To obtain comprehensive and quantifiable information about biodegradation, biocompatibility and osseointegration of alloplastic bone replacement materials, µCT scans as well as histologic evaluation should be performed.

We consider the design and control problem of concentric tubes used in stereotactic neurosurgery. The goal is to optimally reach a configuration of the cannula linking an entry point on the skullcap to a pre-specified region inside the brain. Key issues related to this task are the mechanical behaviour of the cannula and the topography of the brain. We formulate an optimal control problem in order to determine a feasible path while minimizing brain damage caused by missing follow-the-leader behaviour. Numerical results show the potential of the proposed approach.

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