Water residing in small ultrastructural spaces plays a critical role in the mechanical behavior of bone

Abstract

Water may affect the mechanical behavior of bone by interacting with the mineral and organic phases through two major pathways: i.e. hydrogen bonding and polar interactions. In this study, dehydrated bone was soaked in several solvents (i.e. water, heavy water (D2O), ethylene glycol (EG), dimethylformamide (DMF), and carbon tetrachloride(CCl4)) that are chemically harmless to bone and different in polarity, hydrogen bonding capability and molecular size. The objective was to examine how replacing the original matrix water with the solvents would affect the mechanical behavior of bone. The mechanical properties of bone specimens soaked in these solvents were measured in tension in a progressive loading scheme. In addition, bone specimens without any treatments were tested as the baseline control whereas the dehydrated bone specimens served as the negative control. The experimental results indicated that 22.3±5.17vol% of original matrix water in bone could be replaced by CCl4, 71.8±3.77vol% by DMF, 85.5±5.15vol% by EG, and nearly 100% by D2O and H2O, respectively. CCl4 soaked specimens showed similar mechanical properties with the dehydrated ones. Despite of great differences in replacing water, only slight differences were observed in the mechanical behavior of EG and DMF soaked specimens compared with dehydrated bone samples. In contrast, D2O preserved the mechanical properties of bone comparable to water. The results of this study suggest that a limited portion of water (<15vol% of the original matrix water) plays a pivotal role in the mechanical behavior of bone and it most likely resides in small matrix spaces, into which the solvent molecules larger than 4.0Å cannot infiltrate.

Effect of water on nanomechanics of bone is different between tension and compression

“…It has been well known that the toughness of bone is considerably influenced by its hydration state, with the absence of water making bone behave in a brittle manner (Nyman, et al, 2006, Samuel, et al, 2014). Among the three forms of water in bone, removal of bound water was shown to have a dominant effect on bone toughness and strength (Nyman, et al, 2008, Wilson, et al, 2006).…”

“…Among the three forms of water in bone, removal of bound water was shown to have a dominant effect on bone toughness and strength (Nyman, et al, 2008, Wilson, et al, 2006). A recent study reveals that water molecules in very small gap regions (<4Å in size) impose the greatest influence on bone toughness (Samuel, et al, 2014), thus suggesting that the effect of hydration status on bone toughness is most likely originated from the ultrastructural level. However, little is known about how water interacts with bone constituents at the ultrastructural level and how those interactions would consequently influence the bulk mechanical properties of bulk bone tissues.…”

“…The specimens in dry groups were dehydrated at 70°C temperature and 25 in Hg vacuum for 8 hours. Dehydration at 70°C was found sufficient to remove mobile and bound water from the tissue (Nyman et al, 2006), without causing significant loss to the bulk mechanical properties (Samuel, Sinha, Zhao, & Wang, 2014). The structural water was not considered in this study since the water is part of the mineral lattice and can be removed only at 200-400°C (LeGeros, Bonel, & Legros, 1978), which may completely denature the collagen phase.…”

“…Acting as a plasticizer, water makes bone tougher, but more compliant and weaker (Broz, et al, 1993, Nyman, et al, 2006). A recent study revealed that water molecules that are present in small gap regions (presumably < 0.4 nm) play a dominant role in the mechanical behavior of bone (Samuel, et al, 2014). Nonetheless, a clear understanding of the mechanistic role of water is yet to emerge.…”

The Role of Water Compartments in the Material Properties of Cortical Bone

“…The extent to which a solvent can replace water depends on its polarity (potential difference between positive and negative electrical charges across the molecule that arises because two covalently bonded atoms do not share electrons equally), molecular size (dependent on the number of atoms and their size), and hydrogen bonding capability (the ability of an electronegative atom like oxygen to share a proton with another electronegative atom in which the strength of the hydrogen bond increases as the electronegativity of sharing atoms increases) [72]. When dehydrated bone (70 °C for ~4 hours) was soaked in ethylene glycol (similar polarity as water but larger molecular size), dimethylformamide (acceptor of proton), or carbon tetrachloride (non-polar), a significant volume fraction of the original matrix water was not replaced (at least 15% not replaced depending on the solvent).…”