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The interplay of forces

Biomechanics not only deals with forces and movements generated by an organism, but also with the effects of external forces acting on cells and organs: Mechanical forces can be specifically used to boost changes in the body. Scientists in the Continuum Mechanics Department at the University of Stuttgart are investigating the biomechanical basis of such forces and their effects.

“Continuum mechanics is the mother of mechanics,” said Prof. Dr.-Ing. Wolfgang Ehlers from the Institute of Applied Mechanics (Continuum Mechanics) referring to the core of his research area. Or to put it a different way, continuum mechanics deals with the deformation of any kind of bodies. Bodies that consist of several components, including solid substances, fluids and gases, are more difficult to assess. In order to be able to calculate the forces acting on and in more complex bodies, researchers simplify the bodies, a process referred to by scientists as homogenisation: The engineers define calculational limits within which they see a body as an homogenous substance, i.e. as a uniform continuum.

Biomechanics is one of Prof. Dr.-Ing Wolfgang Ehlers’ research priorities. © SimTech, University of Stuttgart

"We homogenise by means of representative volume elements whose components are able to interact with each other by way of suitable coupling terms. This requires us to have a good command of abstraction," said Ehlers indicating that continuum mechanics is anything but simple. "Mechanics and mathematics are great obstacles to be hurdled in engineering studies," said the professor.

Ehlers is a construction engineer, but it is not unusual for him to deal with continuum biomechanics. To go from one to the other is a logical development. "As a construction engineer I had to calculate the positioning of buildings as well as swelling and shrinking processes. Over time, I also came in contact with biological media. It is worth pointing out that biomechanics was previously more part of rigid body mechanics, something that usually came under sports science," said Ehlers. Continuum mechanics came into existence around 15 years ago; Prof. Dr. Gerhard Holzapfel from the University of Graz was one of the pioneers in this area. Nowadays, continuum biomechanics is attracting growing interest, partly because one of the discipline's major visions is close to becoming reality: a comprehensive human model. Ehlers and his team at Stuttgart University are making a considerable contribution to this work.

Vision: a comprehensive human model

“Nowadays calculating all the parts of an organism is within the realms of possibility – it is not as unthinkable as it once was,” said Ehlers. Ehlers and his team have already made great progress; they have succeeded in developing a computational representation of the human intervertebral disc. In simulations, the intervertebral discs can be exposed to realistic forces and strain and the researchers can see on the screen how they deform when pathological situations occur. But before doctors are able to use the calculations and models for their diagnoses and therapy decisions, the models need to be transferred into suitable medical tools. But huge progress has already been made and the use of the simulation by doctors is not so far off. Another important step is the holistic examination of the entire musculoskeletal system of the spinal cord. Junior professor Dr. Oliver Röhrle is part of Ehlers’ team and he is intending to develop one- and three-dimensional muscle models. The two professors are part of the Simulation Technology (SimTech) excellence cluster, in which they work closely with other top researchers on an interdisciplinary basis with the aim of developing the human model.

Biomechanics also investigates the strain on the spinal cord. © Institute of Applied Mechanics

Although the work done in Ehlers’ group focuses on basic research, the medical value of the biomechanical projects is obvious. Ideas for research projects sometimes come directly from practice. For example, one such idea was the result of a contact the Stuttgart researchers had with a heart specialist who had seen abnormal heart muscle alterations on an X-ray image, but who could not correlate these alterations with any kind of strain. Ehlers’ team used a computer model to simulate strain. “The model showed that particular strain occurred exactly in the areas where the heart specialist had seen abnormalities,” said Ehlers. The results of this project also contribute to the development of the human model. “We are going to further develop the human model and in the long term also include the entire metabolism,” said Ehlers highlighting their future goals.

Mechanical stimuli in the development of stem cells

The model shows the irregular distribution of the collagen fibres in the fibre cartilage ring (anulus fibrosis) of the intervertebral disc. © Dr. Nils Karajan, Institute of Applied Mechanics

Several months ago, Ehlers and his group started a project on the continuum mechanical calculation of mesenchymal stem cells (MSC). This project is part of a BMBF-funded cooperative project that is investigating the mechanisms that enable MSCs to differentiate into bone cells. Ehlers' group investigates mechanical stimuli in terms of bone formation. "The investigation is being carried out using a multicomponent approach. We have the cell membrane, which contains fluid and solid particles, and use this as the basis for developing mechanical abstraction models," said Ehlers. In addition, one of Ehlers' doctoral students is focusing on the continuum mechanical calculation of cell growth on the bone level. This is another project that might provide new insights for tumour research.

Another project Ehlers' team hopes to undertake in the future relates to brain tumour therapy. Since the majority of pharmaceutical substances are unable to penetrate the blood-brain barrier, they are directly injected into the brain of patients suffering from specific tumours. The biomechanics now hope to calculate the expansive alterations that occur during this type of treatment. "Besides the effect a certain substance has on the tumour cells, we also need to take into account the mechanical pressure on the tumour cells exerted by the injected fluid. This has a considerable effect on the distribution of the drug in the brain. Although previous simulations take this distribution into account, they do not take look at it in terms of spatial alterations," said Ehlers.

Since Ehlers and his team focus on such a broad range of medically relevant research, it is no surprise that he also teaches biomechanics in his lectures and seminars. Biomechanics is to become part of the curriculum of the new interuniversity course on medical technology offered by the universities of Stuttgart and Tübingen from the winter term 2010/2011. "Biomechanics will be offered as an optional subject and we are currently working on the course content," said Ehlers.

 

Further information:
University of Stuttgart
Institute of Applied Mechanics (Building Construction)
Prof. Dr.-Ing. Wolfgang Ehlers
Pfaffenwaldring 7
70569 Stuttgart
Tel.: +49 (0)711 685-66346
E-mail: ehlers[at]mechbau.uni-stuttgart.de

Website address: https://www.gesundheitsindustrie-bw.de/en/article/news/the-interplay-of-forces