Biomedical Engineering Reference
In-Depth Information
particles are nucleated and grown into these
chemically patterned SWNTs in a well-aligned
platelet form. The thickness of these hydroxyapa-
tite layers is controlled by the coating time and
can reach values as high as 3 μ m.
(2) The bonding between mineral and polymer
phases is not as efficient as it is in natural
composites. In nacre, the sacrificial ionic
bonds at the interface, which can reform,
are one of the keys to its superior mechani-
cal properties. This encourages fundamen-
tal research to incorporate such bonding in
biomimetic materials. The presence of these
bonds prevents catastrophic failure as the
broken bonds at the interfaces can reform
so that the integrity of the structure is
maintained, even at high levels of strains.
The challenging question of whether this
break-reform fashion can be engaged in
other bonds such as covalent bonds remains
open [87] .
3.5 CONCLUSION AND OUTL OOK
Despite significant efforts to duplicate the
structural and functional properties of biologi-
cal hard and stiff materials, only a few success-
ful implementations of these mechanisms in
biomimetic materials have been reported [48,
54] . Also, none of these studies have been able
to develop micro-/nanocomposites with the
high level of structural organization observed
in natural composites. Finally, the high level
of mineral concentration in hard biological
materials (e.g., 95 %w/w for nacre, 99 %w/w
for enamel) has not been achieved in their syn-
thetic counterparts.
Experimental investigations show that high
mineral concentrations result in poor mechani-
cal properties, particularly toughness, in biomi-
metic materials [46, 86, 87] whereas models for
fracture toughness of biological composites pre-
dict the opposite trend [29] . This moderate suc-
cess of biomimetic materials can be explained by
the following limitations:
Meanwhile, composites with well-organized
structures made of macroscale inclusions have
been developed relatively easily at the expense
of losing the advantages of using inclusions of
small size. Therefore, although reducing the
size of the inclusions is beneficial according to
the design guidelines for staggered composites,
it may not result in the expected high perfor-
mance due to the limitations of the fabrication
processes at small scales, i.e., poor structural
organization. There is therefore a trade-off
between the size of inclusions and scalability of
well-designed structure. State-of-the-art biomi-
metic studies, however, aim to explore innova-
tive and promising fabrication methods to
develop structures with high levels of struc-
tural organization made of micro/nano
inclusions.
(1) In biomimetic materials, the mineral tablets
are not organized in a controlled fashion in
the polymer matrix [88] , whereas the min-
eral tablets in natural composites like bone
and nacre are arranged in a well-organized,
staggered structure. This lack of configura-
tion in biomimetic materials necessitates the
use of highly directed fabrication methods
rather than simple mixing of the phases [89] .
This also restricts fabrication of these materi-
als to higher-length scales so that controlled
arrangements of tablets can be more easily
achieved.
References
[1] M.F. Ashby, Hybrids to fill holes in material property
space, Phil Mag 85 (2005), 3235-3257.
[2] F. Barthelat, Biomimetics for next-generation materials,
Phil Trans R Soc Lond A 365 (2007), 2907-2919.
[3] M.F. Ashby and Y.J.M. Bréchet, Designing hybrid mate-
rials, Acta Mater 51 (2003), 5801-5821.
[4] R. Lakes, Advances in negative Poisson's ratio materi-
als, Adv Mater 5 (1993), 293-296.
 
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