NANOINDENTATION OF BONE - Nanoindentation with Biological Applications - page 187

holds the mineralized collagen fibers together. 32 Additionally, this matrix
may serve as a source of sacrificial bonds, thus increasing the energy to
fracture. 32

1.3.3. Water and pore space in bone

Porosity and bone fluid play an important structural and mechanical role
as fluid-filled pore space makes up approximately 20% of bone's
volume. Pore sizes in bone range from millimeters to nanometers. 5 The
largest pores exist in trabecular bone, vascular porosity includes
Haversian canals ( 20 m), followed by canaliculi and lacunae ( 0.1

m), and finally nanometer-sized pore spaces (or 'matrix micropores' 33 )
that exist between and within the collagen and mineral crystals. Within
these pore spaces, interstitial fluid transports nutrients and wastes
throughout the tissue and likely plays a mechanosensory role, whereby
physiological strains result in a shear fluid flow that is thought to be
detectable by osteocytes. 34
Bone contains two types of water: unbound and bound. Unbound
water, which exists mostly in the interstitial fluid within larger ( m

to millimeter-sized) pore spaces, contributes significantly to the time-
dependent mechanical properties of bone. 5 Water may also exist in
smaller pore spaces between the mineral crystals and collagen fibrils. 35
The collagens and ground substance of the organic matrix contain many
charged sites that facilitate interactions with unbound water. Bound
water may stabilize the mineral crystallites by occupying OH - and Ca 2+
vacancy sites. 17 Overall, the permeability of bone tissue relies on the
degree to which tissue is vascularized and the mineral density of the
tissue.

0.1

1.4. Motivation for nanomechanical analysis of bone

Bone's mechanical properties vary widely at different length scales.
Large hydrated cortical tensile specimens produce modulus values
ranging from 14-20 GPa, 36,37 micrometer-sized specimens yield values of
5.4 GPa, 38 and tensile specimens from dissected osteons measure at
12 GPa. 39 While microhardness testing provides a direct measurement of