The causes, mechanisms, and biology of bone adaptation have been under intense investigation ever since Julius Wolff proposed that bone architecture is determined by mathematical laws as a result of mechanical loading. How bone responds to mechanical loads by converting the mechanical signals into chemical signals is known as mechanotransduction. The in vivo environment of bone is complex, and most studies of cell-level phenomena have relied on the use of in vitro experiments using mechanical bioreactors. The main types of bioreactors are fluid flow shear stress, tensile and/or compressive strain, and hydrostatic pressure [1–2]. Of these bioreactors, the most intuitive mechanical stimulus for bone would be the tensile and compressive strain bioreactors. However, many researchers now claim that shear stress via interstitial fluid flow in the lacunar-canalicular porosity is the primary mechanosensory stimulus [3]. A handful of studies have attempted to compare the effects of both of these mechanical stimuli on osteoblasts, but these studies are lacking in two respects [4–6]. First, if both fluid flow and strain are performed in the same bioreactor, the magnitude of one loading mode is explicitly determined through constitutive equations, while the other is only estimated. Second, if the magnitudes of the loading modes are able to be explicitly determined they are performed in different bioreactors, providing the cells different extracellular environments. Therefore, a highly controllable dual-loading mode mechanical bioreactor, as described and characterized in this study, is a necessary tool to further understand the mechanotransduction of bone.

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