Influence of the Processes Parameters on the Properties of The ...

Chapter 3.
Analytical Methods and Designs <strong>of</strong> Experiments
There is <strong>of</strong>ten a compromise between porosity and scaffold mechanical characteristics. Therefore,
<strong>the</strong> biomechanical challenge in designing a scaffold is to achieve sufficient stiffness and strength in a highly
porous structure to provide mechanical integrity [Zhang and Ma, 1999a]. The biostability <strong>of</strong> many implants
depends on factors such as strength, stiffness, absorption at <strong>the</strong> material interface and chemical degradation
[Hutmacher et al., 2008]. The review by Gibson and Ashby [1999], concluded <strong>the</strong> mechanical characteristics
<strong>of</strong> a porous solid depended mainly on its relative density, <strong>the</strong> properties <strong>of</strong> <strong>the</strong> material that made up <strong>the</strong> pore
edges or walls and anisotropic nature cause <strong>of</strong> processing technique.
The type <strong>of</strong> deformation and failure <strong>of</strong> foam depends on <strong>the</strong> structure and physical characteristics
<strong>of</strong> <strong>the</strong> used materials and on <strong>the</strong> macrostructure behaviour under compression. The macrostructure <strong>of</strong> glassy
foam consists <strong>of</strong> closed cells. The main part <strong>of</strong> its mass is concentrated at <strong>the</strong> nodes and junctions <strong>of</strong> <strong>the</strong>
cells. The deformations and <strong>the</strong> failure <strong>of</strong> <strong>the</strong> material under compression occur according to <strong>the</strong> stress-strain
diagram. Three mechanical states are marked by points A, B, C on <strong>the</strong> graph representing <strong>the</strong> compression <strong>of</strong>
foam sample (cf. Figure 3.19). A corresponds to <strong>the</strong> end point <strong>of</strong> <strong>the</strong> elastic region; B marked <strong>the</strong> end <strong>of</strong> <strong>the</strong>
plastic region and C <strong>the</strong> failure <strong>of</strong> <strong>the</strong> foam. The compressive elastic (plastic) modulus can be deduced from
<strong>the</strong> slope <strong>of</strong> <strong>the</strong> corresponding curve.
Figure 3.19: Compression testing result output for foams
[Gnip et al., 2004]
Compression is one <strong>of</strong> <strong>the</strong> main stressed states <strong>of</strong> foam used in a number <strong>of</strong> medical applications.
The yielding point A describes compressive strength. For <strong>the</strong> foams with a stress peak, A is
defined as <strong>the</strong> peak value. In <strong>the</strong> case <strong>of</strong> no stress peak, at highly porous foams, A can be obtained from <strong>the</strong>
intersection point <strong>of</strong> two tangent lines besides <strong>the</strong> flexure region. Two alternative parameters were also used
to characterize compressive strength. For instance, S is defined as <strong>the</strong> intersection <strong>of</strong> <strong>the</strong> stress–strain curve
with <strong>the</strong> modulus slope at an <strong>of</strong>fset <strong>of</strong> 1% strain adopting <strong>the</strong> guidelines for compression testing <strong>of</strong> bone
cement set in ASTM F451-99a, and 10 is defined as <strong>the</strong> stress at 10 % strain according to ISO 844-2004 for
determination <strong>of</strong> compressive properties <strong>of</strong> rigid cellular foams. The compression stress B corresponds to
<strong>the</strong> attenuation <strong>of</strong> flexural deformations <strong>of</strong> cell walls when <strong>the</strong>ir stability is lost [Gnip et al., 2004]. It can
also be denoted as stress corresponding to maximum possible compaction σ comp. <strong>of</strong> <strong>the</strong> damaged elements <strong>of</strong>
foam macrostructure.
6.2 Surface Energy Experiments
Surface tension <strong>of</strong> a liquid is <strong>the</strong> force required per unit length to stretch a pre-existing surface
(N/m) while <strong>the</strong> surface energy <strong>of</strong> a solid is <strong>the</strong> work required per unit area to create a new surface (J/m 2 ).
The surface tension is an intensely sensitive indicator that provides a lot <strong>of</strong> information about <strong>the</strong>
characteristics (e.g. wetting, foaming, emulsification…) <strong>of</strong> a liquid. It is obvious that a high liquid surface
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