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| Understanding the Need for Multiscaling in Biomaterial Mechanics |
| Vikas Tomar* |
| Associate Professor, School of Aeronautics and Astronautics, Purdue University, West Lafayette, USA |
| *Corresponding author: |
Vikas Tomar
Associate Professor
School of Aeronautics
and Astronautics
Purdue University
West Lafayette, USA
Tel: 765-4943-006
Fax:
765-4940-307
E-mail: tomar@purdue.edu |
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| Received August 22, 2012; Accepted August 22, 2012; Published August 24,
2012 |
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| Citation: Tomar V (2012) Understanding the Need for Multiscaling in Biomaterial
Mechanics. J Nanomed Nanotechol 3:e113. doi:10.4172/2157-7439.1000e113 |
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| Copyright: © 2012 Tomar V. This is an open-access article distributed under the
terms of the Creative Commons Attribution License, which permits unrestricted
use, distribution, and reproduction in any medium, provided the original author and
source are credited. |
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| Biological materials have evolved over millions of years and are
often found as complex composites with superior properties compared
to their relatively weak original constituents. The toughness of spider
silk, the strength and lightweight of bamboos, self-healing of bone, high
toughness of nacre, and the adhesion abilities of the gecko’s feet are a
few of the many examples of high performance natural materials. Hard
biomaterials such as bone, nacre, and dentin have intrigued researchers
for decades for their high stiffness, toughness, multifunctionality, and
self-healing capabilities. For example, Nacre has 3000 times more
toughness compared to its mineral constituent. Tooth enamel is 1000
times stiffer than its constituent protein polymer collagen. The general
mechanical performance of these composites is quite remarkable.
In particular, they combine two properties which are usually quite
contradictory, but essential for the function of these materials. Bones,
for example, need to be stiff to prevent bending and buckling, but
they must also be tough since they should not break catastrophically
even when the load exceeds the normal range. Such hard biological
materials are not only light weight but also possess high toughness and
mechanical strength. |
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| One of the defining features of such biological composites is
that they are highly hierarchical with different structures at different
length scales. Often they are complex nanocomposites of soft fibrous
polymeric phase and hard mineral phase. For instance, bone has up to
seven levels of hierarchy and nacre shows up to six levels of hierarchical
structure. Materials such as bone and nacre have such multi-level
hierarchical structural design that concept of stress concentration at
flaws remains invalid, leading to flaw tolerant structure. In spite of
complex hierarchical structures, the smallest building blocks in such
biological materials are at the nanometer length scale. For example, at
the lowest level in bone, nanometer-sized crystals of carbonate apatite
are embedded in the fibrous protein collagen in a well organized
staggered arrangement. |
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| In nanocomposite materials, the volume fraction of the proteinmineral
interface can be enormous as the mineral bits have nanoscale
size. For example, in a raindrop size volume of a nanocomposite, the
area of interfacial region can be as large as a football field. Interfaces
play crucial role in regulating the overall mechanical properties of
nanocomposites. In case of hard biomaterials such as bone, dentin, and
nacre, they have primarily an organic phase (e.g. tropocollagen (TC) or chitin) and a mineral phase (e.g. hydroxyapatite (HAP) or aragonite)
arranged in a staggered arrangement. In bone, the crystalline mineral
phase is preferentially aligned along the longitudinal axis of the
polypeptide molecules permitting maximum contact area in a staggered
arrangement. The extent of interfacial interaction and the interfacial
arrangement are important determinants of the structure-function
property relationship of biomaterials and influence the mechanical
strength substantially. Such biological materials have been reviewed
in appreciable detail, in the context of their hierarchical structure,
material properties, and failure mechanisms. |
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| An important aspect to focus in biomaterials engineering of
hard biomaterials is the chemo-mechanics of the organic-inorganic
interfaces and its correlation with overall mechanical behavior. This
understanding is vital for selecting appropriate constituents, their size
scales and their relative arrangements, which in turn is governed by
the functional requirements of the composite materials. For example,
a three dimensional (3D) explicit atomistic failure analyses of model
Tropocollagen-Hydroxyapatite interfacial biomaterial (similar to
material found in bone tissues) performed at the nanoscopic length
scale, have pointed out that maximizing the contact area between the
TC and HAP phases result in higher interfacial strength as well as higher
fracture strength. Analyses have also shown that high toughness and
strain hardening behavior of such biomaterial is due to reconstitution of
columbic interactions between TC and HAP surfaces during interfacial
sliding due to mechanical deformation. It has also been shown recently
that changes in the residue sequences of TC molecules at the interface
can affect the material mechanical strength considerably. Further, it
has been shown earlier that moisture can play a major role in affecting
the strength of such hybrid interfaces in biological materials. |
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| Challenges lie in identifying nature’s mechanisms behind imparting
such properties and its pathways in fabricating and optimizing these
composites. The route frequently acquired by nature is by embedding
sub-micron or nano sized mineral particles in protein matrix in a well
organized hierarchical arrangement. The key here is the formation of
large amount of precisely and carefully designed organic-inorganic
interfaces and synergy of mechanisms acting over multiple scales to
distribute loads and damage, dissipate energy, and resist change in
properties owing to damages such as cracking. |
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