Bacterial Cellulose and Its Polymeric Nanocomposites - The Nanoscience and Technology of Renewable Biomaterials - page 247

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Composite B

Composite A

CAB

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Figure 9.15 Stress-strain curves of tensile tests. (Reprinted from Composites Science and
Technology, 64, W. Gindl and J. Keckes, Tensile properties of cellulose acetate butyrate
composites reinforced with bacterial cellulose, 2407-13. Copyright (2004), with per-
missionfromElsevier.)

the BC fibers beyond the linear elastic range and along the loading direction (26). It was
proposed that yielding occurred in interfacial shear between fibers and matrix allowing
a reorientation of the fibers in the composites and thereby causing a modulus increase
with repeated loading. BC was therefore established as a good reinforcement for CAB,
especially under cyclic loading.

9.4.2 BC/Synthetic Polymer Nanocomposites

Stiffening upon cyclic loading was also observed in physically crosslinked BC/PVA
nanocomposites that were developed for biomedical applications (28, 41). For appli-
cations such as stents and heart valve leaflets, the substitute material must match the
tissue strength and stiffness but also its stress relaxation behavior in order to abide to
the cardiac cycle. After demonstrating the suitability of PVA as hydrogels for such
applications (27), Millon and Wan embarked on finetuning the hydrogel properties by
combining BC and PVA in various proportions and by physically crosslinking PVA
through heat/freeze cycles (28). Note that in this case, the BC nanocomposites were
intended for use in the wet state, i.e. as hydrogels. Cyclic tensile tests up to 75%
strain and tensile stress relaxation experiments indicated that the BC/PVA nanocompos-
ites could be tailored to match the behavior of a specific cardiovascular tissue. Under
tensile loading the PVA/BC nanocomposites displayed a different stress-strain behavior
than that of PVA alone with notably a significant increase in modulus at ca 40% strain
for the nanocomposite and a trend for increasing stiffness with number of cycles. It was
also evident that the higher the BC content in the nanocomposite, the higher the tensile
modulus (Figure 9.16). Increasing the PVA content in the hydrogel nanocomposite also
induced a higher tensile modulus. Consequently by varying composition and crosslink-
ing extent the authors were able to tailor the stress strain properties of the BC/PVA
nanocomposites to match the behavior of specific cardiovascular tissue. For example
the tensile properties of porcine aorta in both the circumferential and axial directions
could be well matched by the nanocomposites (Figure 9.17) whereas the stress relaxation