Contents lists available at ScienceDirect Journal of the Mechanical Behavior of Biomedical Materials journal homepage: www.elsevier.com/locate/jmbbm Interrelationships between electrical, mechanical and hydration properties of cortical bone Mustafa Unal a , Fatih Cingoz b , Cevat Bagcioglu c , Yilmaz Sozer b, ⁎ , Ozan Akkus a,d,e, ⁎⁎ a Department of Mechanical and Aerospace Engineering, Case Western Reserve University, Cleveland, OH 44106, USA b Department of Electrical and Computer Engineering, The University of Akron, Akron, OH 44325, USA c Department of Mechanical Engineering, The University of Akron, Akron, OH 44325, USA d Department of Orthopaedics, Case Western Reserve University, Cleveland, OH 44106, USA e Department of Biomedical Engineering, Case Western Reserve University, Cleveland, OH 44106, USA ARTICLE INFO Keywords: Bone Electrical properties Mechanical properties Water Dielectric constant Electrical impedance ABSTRACT Interrelationship between electrical and mechanical properties of cortical bone and the role of bone composition in this interrelationship are not comprehensively investigated to date. This study aimed to investigate associa- tions of electrical properties (i.e., specific impedance, dielectric constant, and conductivity) with mechanical properties (i.e., toughness, strength and elastic modulus) of wet and sequentially dehydrated cortical bone. Bovine cortical bone samples (N = 24) were subjected to three-point bending test. A sequential heat treatment protocol ensued to tease out contributions of unbound water and bound water. Demineralization was performed to understand contributions of organic matrix and the mineral phase to the electrical properties of cortical bone. Raman-spectroscopy based water measurement was used to investigate involvement of collagen- and mineral- bound water in the electrical properties. Our results showed statistically significant correlations between elec- trical and mechanical properties of cortical bone. Toughness and ultimate strength were negatively correlated with impedance and positively correlated with conductivity and dielectric constant. The highest correlations between electrical and mechanical properties of cortical bone were typically found at the frequencies of 0.2, 0.5 and 1 MHz. The electrical properties of bone changed significantly as a result of sequential dehydration, in- dicating that unbound and bound water compartments are the key determinants of the electrical properties. Comparison of porosity matched bone samples with high and low amount of bound water showed that bound water compartments may have an independent role in determining electrical properties of cortical bone. Furthermore, the results indicated that collagen and mineral-bound water may contribute differentially to the electrical properties of a bone. In the overall, our results suggest that electrical properties of cortical bone may be used to assess bone toughness and strength, and also underline the necessity for developing techniques to measure these electrical properties in vivo. 1. Introduction The fracture resistance of bone is associated with several factors including bone mass, bone morphology and architecture, degree of mineralization as well as the quality of bone's constituents (e.g., col- lagen, collagen crosslinks, water, non-collagenous proteins and crys- tallinity) (Currey, 1988; McCalden et al., 1993; Akkus et al., 2004; Vashishth, 2007; Unal and Akkus, 2015a, 2015b; Unal et al., 2016). Therefore, bone mineral density (BMD)-based diagnosis alone cannot assess bone fragility with high accuracy (Schuit et al., 2004; Kanis et al., 2005), calling for information on other measures that affect bone's fracture resistance. The clinical interest in bone bioelectricity dates back to 1970s during when electrical stimulation was used in treating nonunions and congenital pseudoarthrosis (Isaacson and Bloebaum, 2010), although the first experimental measurement of electrical properties of bone was reported in 1937 (Osswald, 1937; Geddes and Baker, 1967). After dis- covery of bone piezoelectric properties in 1957 (Fukada and Yasuda, 1957), several investigators reported electrical properties of bone in 1960s–1970s (Cochran et al., 1968; Swanson and Lafferty, 1972; Behari et al., 1974; Liboff et al., 1975; Reinish and Nowick, 1976; Durand et al., 1978; Sansen et al., 1978). Considerable progress has been achieved in 1980s in understanding the association of electrical prop- erties of bone with frequency, moisture/hydration levels and http://dx.doi.org/10.1016/j.jmbbm.2017.08.033 Received 23 February 2016; Received in revised form 14 July 2017; Accepted 28 August 2017 ⁎ Correspondence to: The University of Akron, 302 E Buchtel Ave, Akron, OH 44325, USA. ⁎⁎ Corresponding author at: Case Western Reserve University, 10900 Euclid Ave, Cleveland, OH 44106, USA. E-mail addresses: mustafa.unal@case.edu (M. Unal), ys@uakron.edu (Y. Sozer), ozan.akkus@case.edu (O. Akkus). Journal of the Mechanical Behavior of Biomedical Materials 77 (2018) 12–23 Available online 30 August 2017 1751-6161/ Published by Elsevier Ltd. MARK