NANOINDENTATION OF TOOTH TISSUES - Nanoindentation with Biological Applications - page 253

samples ( Fig. 8-7b) . The displacement rate and limited backcreep of
hydroxyapatite (~0.03 nm/sec) and cast alloy (~0.01 nm/sec) were all
within the limit of system thermal drift. Therefore, only enamel and
amalgam exhibited significant backcreep response. When considered in
more detail, the backcreep curves of enamel and amalgam were totally
different. Enamel showed a progressive recovery while amalgam
recovered very quickly in the first 150 seconds and reached the limit of
nearly -0.05 m in h t ' values. The latter portion of the amalgam curve
only reflects system thermal drift response.
This result implies that enamel and some dental metallic materials all
exhibit creep behaviour but with different characteristics. This indicates
different mechanisms are responsible for this behaviour. The visco-
elastic property of enamel may be considered as follows: First, the
proteins and other organic (and water) components within enamel
may be regarded as a biopolymer. Viscoplastic properties of these
biopolymers stem from the configurational rearrangements, disposition
and interaction among the macromolecules in both their short- and
long-range network interactions. 57 Under indentation load, the local
re-distribution response of the structure is rapid, while the long-range
interaction response is slow due to the time-dependent molecular
rearrangement. Upon unloading, due to the memory effects associated
with the cross-linked network, the molecules have a tendency to return
to their initial form and positions. 57 In the case of indentation, the
surrounding elastically deformed hinterland about the residual
impression provides an additional driving force for such backcreep
deformation. These factors result in the recovery of the indentation
impression being also time-dependent. Second, proteins within enamel
may behave similarly to the recovery domain bond within many proteins,
called a “sacrificial bond”. 58 Rief et al . noted that the longer the delay
before the next application of force, the more reformation of the bond
occurred during relaxation; refolding of protein domains was also
observed during this process. 58 Similarly, proteins within the organic rich
sheath and between the apatite crystallites of enamel may have a similar
function and response as those in bone.
In short, this reversible viscoelastic property of enamel may help
teeth distribute the localized high contact or impact stresses resulting