Biomedical Engineering Reference
In-Depth Information
Another direct observation made by Yin et al. [3] was the significant
recovery of indentation deformation as shown in Figure 9.7. In particular,
at lower applied loads than 0.45 N, the bone exhibited more viscoelasticity
than at high loads. Bone recovery following microindentation has also been
observed in wet bovine femur [26]. The time dependence of viscoelasticity
in bone has been observed in R-curve testing of human cortical bone as well
[13]. Time-dependent crack growth occurs in bone under sustained (noncyc-
lic) in vitro loads at stress intensities lower than the nominal crack-initiation
toughness [13]. Effects of viscoelasticity and time-dependent plasticity of
human cortical bone have been further investigated using nanoindenta-
tion [27]. However, the exact nature of such behavior is as yet unclear [13].
Hardness has been found to increase with load, as shown in Figure 9.5. This
phenomenon has also been observed in microindentation of dry embalmed
human rib [28]. However, in microindentation of wet bovine metacarpus,
hardness decreased with the applied load [16].
9.4 Stretching-Relaxation Properties of Bone Piezovoltage
In Sections 9.2 and 9.3, experiments on the mechanical behavior of bones
were discussed. Experiments on the coupling behaviors of mechanical and
electric fields (e.g., piezoelectric behavior) in bone are described in this and
the subsequent section. Experiments presented in Hou et al. [4] on stretch-
ing-relaxation properties of bone piezovoltage are described in this section.
These researchers measured piezovoltages between the two opposing sur-
faces of bovine tibia bone samples under three-point bending deformation
using an ultrahigh-input impedance bioamplifier. The experimental results
presented there indicated that the piezovoltages of bone showed different
relaxation behaviors during loading and unloading processes. Hou et al.
found that the piezovoltage decay followed a stretched exponential law
when the load increased from zero to its maximum value, whereas it fol-
lowed a typical relaxation exponential law when the load was maintained at
its maximum value. The stretching-exponential behavior was independent
of loading amplitude and rate.
9.4.1 Sample Preparation
As indicated in Hou et al. [4], the cortical bone samples used were harvested
from mid-diaphysis of dry bovine tibias (age 2-3 years) and machined into
rectangular beams (Figure 9.9a) with the dimension range shown in Table 9.2.
Six samples were prepared and dried in air for at least 2 weeks until the
resistance or impedance between the two lateral surfaces was over 10
9
Ω,
and the maximum resistance of complete dry bone is usually in the order