Biomechanics & Mechanobiology
This research area comprises theoretical modelling, numerical simulation and experimental validation of biomechanical processes at macro and micro level. The scientific investigations carried out here are characterized by the fact that clinically relevant problems from biomechanics and mechanobiology are investigated on human organs as well as on the cellular level. The interaction between mechanical stress and cellular remodelling processes plays a decisive role. The research topics are not only modelled using mechanics, but also verified experimentally.
Development of cartilage replacement material
In cellular soft tissue material, a change in the material properties can occur depending on the mechanical load (remodeling). This effect can be investigated experimentally in a bioreactor as shown in Fig. 1. A cellular sample is stimulated under cyclic mechanical stress while both the number of cycles and the force acting on the sample are measured.
Figure 2 shows the preparation, which is performed in the biological laboratory of the University Hospital Aachen. The sample is surrounded by a culture medium during the experiment. The entire bioreactor with gas exchange is stored in an incubator at 37°C. The biological evaluation of the sample material is carried out by means of histological sections, as shown in Figure 3, in which, compared to the unstimulated control sample, an increased activity of the cartilage cells can be observed through protein ejection. This increased protein formation can be even more pronounced during longer cultivation, e.g. in an animal experiment.
The remodelling observed in experiments will be modelled theoretically and numerically in further work. Further bioreactor experiments will be conducted to validate the model, which will be combined with the viscoelastic diffusion model. For this purpose, a first approach using a remodeling law , which was originally developed for bones, has already been chosen .
 Weinans, H., Huiskes, R., Grootenboer, H.J.: The bahvior of adaptive bone-remodeling simulation models. J. Biomechanics 25 (12), 1425-1441, 1992.
 Stoffel, M., Yi, J.H., Weichert, D., Gavénis, K., Müller-Rath, R.: A biomechanical model for cartilage replacement material, BIOmaterialien, in press.
Bioreactors are used to determine the increased cell activity due to mechanical stress. The cell activity in mechanically stimulated tissue is expressed by additional protein formation, which in turn results in fibre growth. This effect leads to an adaptation to the external mechanical load and thus to an increase in stiffness of the biological material. The type and magnitude of fibre formation is of great importance for clinical application. Furthermore, the experimental investigations in the bioreactor lay the foundation for theoretical and numerical modelling of the cartilage replacement material.
The mechanical stimulation is performed by a punch, which is cyclically moved by an eccentrically mounted disc. Since the bioreactor has to be operated under sterile conditions, the samples are cultivated in a cylindrical chamber (see Figure 1 and 2). Above the chamber, the passage of the rod that moves the plunger is additionally sealed against germs. During bioreactor operation, the number of cycles is recorded by a forked light barrier. In addition, there is a flexible membrane underneath the sample storage, which in turn is mounted on a load cell, so that the force is also recorded during cultivation. To investigate different influencing variables, such as cell count, cultivation time, and to record statistical scatter, four of these bioreactors with four samples each are operated simultaneously in an incubator. These experiments are carried out in cooperation with the Department of Orthopaedics of the University Hospital Aachen. For testing the cartilage replacement material under physiological conditions in the human knee, see Figure 3, the development of a bioreactor with a knee test bench is planned.
A new development of the bioreactor with a Raspberry Pi control is shown in Figure 4.
Investigation of biomechanical joint properties
As part of the BMBF-funded workHEALTH project, an interdisciplinary investigation of the etiology and development of work-related musculoskeletal disorders (MSD) will be conducted to enable better prevention and treatment, which will effectively reduce the prevalence of work-related MSD.
In this regard, indicators of work-related MSD, particularly at the spine and knee, will be investigated at both the macro and microscopic levels. For this purpose, in-vitro experiments are performed on human cadaver specimens in a self-developed test rig with magnetic tracking sensors in a humidity- and temperature-controlled environment. The further development of the bioreactors mentioned above provides insight into the cellular response to physiological stresses. The results of the experimental studies will be used, among other things, to validate material models and finite element (FE) models of the lumbar spine and knee joint created and calibrated in previous studies. This allows additional mechanical parameters to be analyzed.
This multi-method approach allows for new insights into the biomechanical processes within the joints. Interrelationships between the different levels will be detected by artificial neural networks. This enables an optimization of working conditions with regard to the reduction of work-related MSD-induced, economic losses and health restrictions for the affected persons.