Biomedical Engineering Reference
In-Depth Information
The key to success lies in tight experimental control of potentially
influencing factors. That is, consistently ensuring the preservation of
relevant properties during tissue collection, attending to conditions
during subsequent storage, and selection of optimal techniques to
preserve the condition of the tissue prior to and during nanomechanical
testing. Because nanoindentation occurs at very small length scales,
consideration must be given to alterations in the surface and near-surface
properties of the sample. Further, the influence of collection, storage, and
sample preparation on the three phases of bone are critical. Each of
bone's components contributes to the mechanical response of the bone,
and thus alteration of any single phase will affect the nanoindentation
response.
2.2.1. Sample collection and postmortem changes
Preservation of properties for a study of tissue-level properties starts with
well-controlled collection methods. Bone cell death and cessation of
fluid movement through pore spaces likely causes rapid alterations in the
biochemical environment. For example, live osteocytes are known to
locally regulate mineralization. 81 Without cellular influence, a
thermodynamically driven mineralization process may run unchecked.
Over hours, catalytic enzymes that are present within the interstitial fluid
or that result from bacterial presence may facilitate degradation of
proteins that make up the organic matrix. Last, dehydration of the bone
tissue may cause permanent changes in the mineral packing, similar to
that seen in dehydration of nano-clays, and reorganization of molecular
bonds in the organic phase through the removal of the polar water
molecules and consequent increase in ionic concentrations in the
remaining interstitial fluid. 47,82
In summary, sample collection from an in vivo condition must be
rapid. Should preservation of specific cells, proteins or genetic material
be desired, immediate tissue processing is mandatory. Even in preparing
for mechanical analysis alone, bone samples must be kept cool and
hydrated. Tissues obtained from an abattoir are acceptable upon
assurance of their history. Irrigation with an isotonic, buffered saline
solution and storage on ice are typically sufficient during dissection,
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