Scaffolds for Tissue Engineering - Biodegradable Polymer-Based Scaffolds for Bone Tissue Engineering - page 3

Two levels of composite structures are considered when developing bone sub-
stitutes. First of which is the bone apatite reinforced collagen forming individual
lamella (nanometer to micrometer scale) and secondly the osteon reinforced inter-
stitial bone (on the micrometer to millimeter scale). The apatite-collagen compos-
ite at the microscopic level provides the basis for producing bioceramic-polymer
composites for bone replacement.

1.2.2 Mechanical Properties of Bone

By assessing whole bones in vivo, the mechanical behavior of bones can be inves-
tigated. The mechanical properties of cortical or cancellous bones are determined
in vitro using standard or miniature specimens that match up to various standards
originally designed for testing conventional materials such as metals and plas-
tics (Wang 2004 ). It is very important to maintain the water content of bone for
mechanical assessment as the behavior of bone in the “wet” condition can be sig-
nificantly different from that bone in a “dry” condition (Fung 1993 ). Cortical bone
has a range of associated properties rather than a unique set of values (Table 1.1 )
with respect to orientation, location and age (Wang 2004 ).
The mechanical behavior of bone can be explained using a simple composite
model by treating bone as a nanometer-scale composite (Fig. 1.1 ). In bone, brittle
apatite acts as a stiffening phase whereas ductile collagen provides a tough matrix.
Therefore the tensile behavior of bone reveals the combinational effect of these
two major constituents. A good understanding of the structure and properties of
bone yields a good insight into the structural features of bones as well as provides
the property range for approximating mechanical compatibility that is required of
a bone analogue material for structural replacement with a stabilized bone-implant
interface (Wang 2004 ). It is also important to take into account that, bone can alter
its properties and configuration in response to changes in mechanical demand
which is unlike any engineering material.

Table 1.1 Mechanical properties of bone and current implant materials (Wang 2004 )
Material

E (GPA) σ (MPa) ε (%)

Cortical bone 7-30 50-150 1-3

Cancellous bone 0.05-0.5 10-20 5-7

Co-Cr alloys 230 900-1540 10-30

Stainless steel 200 540-1000 6-70

Ti-6Al-4 V 106 900 12.5

Alumina 400 450 ~0.5

Hydroxyapatite 30-100 60-190
Polyethylene 1 30 >300
E Young's modulus, σ tensile strength (flexural strength for alumina), ε elongation at fracture