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.

Keywords: Bone; Solvents; Toughness; Ultrastructural spaces; Water.

Figure 1

Schematic illustration of the progressive loading scheme and determination of instantaneous mechanical properties of bone in each load cycle.

Figure 2

Stress-strain relationship obtained using the progressive loading protocol. The H₂O, D₂O and base line control specimens indicated the initial elastic, yielding, and post-yield deformation, whereas EG, DMF, CCl₄, and Dehydrated specimens exhibited a brittle behavior, with an increased stiffness, strength, and no yielding.

Figure 3

Elastic modulus as a function of applied strain obtained using the progressive loading protocol. The H₂O, D₂O and base line control specimens indicated an exponential decay (E_i=E₀e^-mεi); EG and DMF soaked specimens showed a slight linear decrease; and CCl₄, and Dehydrated specimens exhibited little changes in the elastic modulus with the increasing applied strain.

Figure 4

Plastic strain ε_p vs. applied strain ε_i. The H₂O, D₂O and base line control specimens indicated a linear increase of plastic strain; EG and DMF soaked specimens show an onset of yielding; and CCl₄, and Dehydrated specimens exhibited no plastic deformation with the increasing applied strain.

Figure 5

Stress relaxation as a function of applied strain obtained using the progressive loading protocol. The H₂O, D₂O and base line control specimens behave similarly, indicating a linear increase prior to yielding and leveled off with a slight decrease in stress relaxation with increasing strain; EG and DMF, CCl₄, and Dehydrated specimens show a slight increase with the increasing applied strain.

Figure 6

Released elastic strain energy (a), plastic flow (b), and hysteresis (c) energy dissipation in bone specimens soaked in different solvents. The H₂O, D₂O and base line control specimens show similar capacity of dissipating energy in all three mechanisms aforementioned, while EG and DMF, CCl₄, and Dehydrated specimens exhibit very limited capacity in energy dissipation.

Figure 6

Released elastic strain energy (a), plastic flow (b), and hysteresis (c) energy dissipation in bone specimens soaked in different solvents. The H₂O, D₂O and base line control specimens show similar capacity of dissipating energy in all three mechanisms aforementioned, while EG and DMF, CCl₄, and Dehydrated specimens exhibit very limited capacity in energy dissipation.