Biomedical Engineering Reference
In-Depth Information
Also, the replacement of unbound water by ethanol has been shown
to increase the modulus of bone by more than 20%. 43 This substantial
increase is more than expected of a simple fluid substitution and
emphasizes the importance of water as a key component of bone.
Unbound water may fill fine pore spaces and contribute to the stability of
the collagen, while dehydration can cause stiffening of the fiber. 102
Histological fixation with formalin (typically a 10% buffered solution
of formaldehyde in water) or glutaraldehyde will also preserve bone
tissue. If not buffered properly, formalin can become acidic, through the
generation of formic acid, and dissolve bone mineral. 83 Formalin fixation
has been shown to produce a significant decrease in hardness 83 with no
concomitant change in stiffness. 103 Storage of dental tissues in ethanol
resulted in no significant change in indentation modulus or hardness. 90
Overall, chemical fixatives alter bone's tissue-level properties through
precipitation (ethanol) of proteins and the formation of cross-links
between proteins (aldehydes). While the ability of bone tissue to fully
recover from ethanol dehydration is unclear, chemical fixation in
formalin or glutaraldehyde is irreversible.
Freezing is one of the most common approaches to preservation of
bone tissue and has the potential to preserve water content, bone
structure, cells, proteins, and even genetic material. However, freezing
and thawing is known to cause significant damage when done
improperly. A rapid transition to the frozen state minimizes the damage
to the tissue by ice crystals. Use of a cryoprotectant, to replace water and
reduce the amount of ice formed, also prevents destruction by the
formation of large ice crystals. 104 However the cryoprotectant is only
as effective as its ability to penetrate into the tissue—a process that is
likely inhibited by small pore spaces and complicated microstructural
geometries within the bone tissue.
Maintaining water content during freezing and thawing may prevent
inadvertent damage from even slight dehydration. A common practice is
to wrap bones in saline-soaked gauze and freeze in water-tight
containers. Alternately, one can wrap a bone sample in cling film (to
prevent escape of water) prior to freezing. Use of excess water is not
ideal, and the study of cryoprotectants for preserving bone allografts
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