NANO- AND MICROMECHANICAL PROPERTIES OF HIERARCHICAL BIOLOGICAL MATERIALS Mechanics of molecular collagen is influenced by hydroxyapatite in natural bone Rahul Bhowmik Æ Kalpana S. Katti Æ Dinesh R. Katti Received: 17 January 2007 / Accepted: 4 June 2007 / Published online: 10 July 2007 Ó Springer Science+Business Media, LLC 2007 Abstract As often seen in biological structural mate- rials, bone exhibits complex hierarchical structure. The primary constituents of bone are collagen and hydroxy- apatite (HAP). HAP mineralizes at specific locations at collagen, in such a way that the c-axis of HAP aligns parallel to collagen molecule. The collagen molecule is helical overall with non-helical ends that are N- or C-telopeptides. The collagen molecule with telopeptides interacts with specific surfaces of mineralized HAP. When subjected to load, the interactions at the interface between HAP and collagen may significantly affect the overall mechanics of the collagen molecule. Here, we have performed molecular dynamics (MD) and steered MD (SMD) simulations in order to understand the load carrying behavior of collagen in the proximity of HAP. Our simulations indicate that the load-deformation re- sponse of collagen is different when it interacts with HAP as compared to its response in the absence of HAP. The interface between HAP and collagen affects the overall load-deformation response of collagen. Further, bone also has considerable amount of water and we have observed that water significantly influences the load- deformation response of collagen due to collagen-water- HAP interactions. Introduction Bone is an important structural component of the human body and is a part of the skeletal system. It performs var- ious mechanical, biological, and chemical functions which include: supporting the body structure, protecting internal organs, producing red and white blood cells, and storing various ions [1, 2]. It has a unique capability of self- regeneration under appropriate conditions [3]. Bone exhibits a distinct set of mechanical properties depending on its location in the skeletal system [4]. This is due to the fact that the different p’arts (bone) of the skeletal system undergo different loading paths during normal daily activities [5, 6]. These unique sets of mechanical properties are a result of structural hierarchy in bone [7–9] which encompasses molecular to macroscopic level spanning over several orders of magnitude of length scale as shown in Fig. 1. However, the role of mechanics of various structures at different length scales on overall mechanical response is not clearly understood. This understanding is vital for designing novel implant materials with mechanical properties similar to natural bone [10–14]. Several attempts have been made to understand the mechanical properties of bone and its relation to the hierarchal structural organiza- tion [15–31]. Ji et al. have used finite element modeling (FEM) methods to calculate the fundamental mechanical properties of bone [21]. Buehler et al have explained the reasons for the specific length of collagen molecules which is observed in the nanostructure of bone and its mechanical properties by multiscale modeling techniques [22, 23]. Hellmich et al. have examined the mechanical behavior of bone through micromechanics formulations [24, 25]. It has been observed that the young’s modulus of cortical bone is in the range of 14–20 GPa [26], whereas that of the osteon lamellar structure is about 22 GPa [27]. However, the R. Bhowmik Á K. S. Katti (&) Á D. R. Katti Department of Civil Engineering, North Dakota State University, Fargo, ND 58105, USA e-mail: Kalpana.Katti@ndsu.edu R. Bhowmik e-mail: Rahul.Bhowmik@ndsu.edu D. R. Katti e-mail: Dinesh.Katti@ndsu.edu 123 J Mater Sci (2007) 42:8795–8803 DOI 10.1007/s10853-007-1914-1