The increase in incident bone fractures, related to aging or disease, is a major problem of today’s society. According to the International Osteoporosis Foundation (IOF) the costs of osteoporosis, a chronic disease, greatly increasing the risk of a bone fracture, is 37 billion EUR per year in the EU only [58]. The number is projected to rise in coming years. The limited understanding and the complexity of the mechanical properties of bone makes the causes therefore hard to identify.

Bone possesses a hierarchical structure, which builds up from the nanoscale to the macroscale. The different levels of structure hierarchy provide bone with its unique mechanical properties, resembling both strength and toughness. Because of the hierarchical architecture of bone, nanoscale and microscale compositional changes could eventually affect the overall mechanical properties of bone. Bone is composed of an organic, an inorganic phase and water. The organic phase can be subdivided into collagen and non-collagenous proteins (NCPs). Collagen is the most abundant protein in mammals and provides mechanical support in the extracellular matrix of bones. In turn collagen is influenced by water. For example, water provides the ability to confer ductility or plasticity for collagen and leads to a contraction of the collagen triple helix with considerable forces if removed. The NCP’s are a heterogeneous group of matrix proteins. They are dispersed throughout the extracellular matrix of the bone and serve as structural proteins. [4] Proteoglycans are one type of the NCP’s and are present mainly in the extracellular matrix of bone and contain a core protein and glycosaminoglycans. The glycosaminoglycans are polar and highly negatively charged, thus having a great potential of attracting water into the matrix. Therefore, Proteoglycans may be involved in toughening bone through its ability to retain water molecules in the matrix and in water-mediated plastic deformation of bone at ultrastructural levels [4].

The question of how far the removal of water influences the mechanical properties was not completely answered and has remained, to the knowledge of previous authors and scientist, largely open. Luca Bertinetti et al. [1] investigated the influence of water removal from collagen of fibrolamellar bovine bone. These authors described the stress trend acting on the bone during testing, but did not investigate the underlying processes. Understanding the role of water in cortical bone, and therefore in collagen, can provide important insights into mechanisms of how bone structure and mechanics change with age and disease.

As water is an integral part of the bone structure, the influence of water removal on the viscoelastic properties of bone, is a topic of interest for the bone biomechanics community. The aim of this thesis was, to investigate the effects of osmotic pressure induced by a polyethylene glycol solution onto the viscoelastic properties of cortical bovine bone under tension. Therefore, a methodology was developed to test miniaturized differently oriented samples from cortical bone under uniaxial tension, surrounded by aqueous solutions. Further, we could quantify the viscoelastic properties, using the fitting parameters of the 5 parameter generalized Maxwell model. The usage of PEG, should make the bone sample stiffer and tougher which was expected to have an impact on the viscoelastic properties, thus the stress relaxation behavior. The accomplished tests, set-ups and the practical knowledge can be used in further studies to determine the effect of water decrease of bone at lower hierarchical length scale levels. While important discoveries have been made in this thesis regarding the effect of osmotic pressure onto the viscoelastic properties of bone, it is still unclear, at which hierarchical level and which structure the water withdrawal effects. Further, it still has to be investigated which effects cause the stress relaxation in the bone and if this effects exist in bone, suffering from osteoporosis, or in aged bone.