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

Chapter 1.
Polylactide Based Bio-Materials
Osteoblast secretion does not become entirely fibrous. The secretion also forms an amorphous adhesion
between <strong>the</strong> fibres [Lee and Chuong, 2009].
Many studies can be found in <strong>the</strong> literature pursuing <strong>the</strong> aim to produce biomimetic artificial
bone-like tissue involving hydoxyapatite (HAp) and collagen as fibre, gel or gelatine [Kim et al., 2005;
Kikuchi et al., 2004; Tampieri et al., 2003a; Itoh et al., 2001]. Testing two methods <strong>of</strong> preparation <strong>of</strong>
apatite/collagen composite materials (dispersion <strong>of</strong> HAp in collagen gel or direct nucleation <strong>of</strong> HAp into
collagen fibres), Tampieri et al .[2003b] have shown that <strong>the</strong> bio-inspired method based on <strong>the</strong> direct
nucleation <strong>of</strong> apatite leads to composites analogous to calcified tissue and exhibiting strong interactions
between HAp and collagen.
The replacement and healing <strong>of</strong> damaged hard tissues have always been a concern for human
beings as shown by <strong>the</strong> examination <strong>of</strong> mummies. It is however known that calcium phosphates have been
used for bone substitution and repair [Jarcho et al., 1979]. The first to be used were stoichiometric
hydroxyapatite and tricalcium phosphate (TCP) which are stable calcium phosphates at high temperature
and can be easily sintered into ceramics. They are still <strong>the</strong> major industrial calcium phosphates biomaterials.
TCP was shown to be bio-absorbable and replaced by bone whereas HAp constituted non-degradable
materials. TCP is mainly used as a bio-ceramic whereas HAp is also being processed for o<strong>the</strong>r biomaterials
uses such as <strong>the</strong> coating <strong>of</strong> metallic pros<strong>the</strong>ses where it was found to considerably improve bone repair as an
"osteo-conductive" material or composite ceramic/polymer materials showing strong mechanical analogies
with bone tissues and excellent bone bonding abilities [De Groot et al., 1987]. Biphasic calcium phosphates,
associating <strong>the</strong>se two high-temperature calcium phosphates allow a controlled resorption rate and have been
reported to <strong>of</strong>fer superior biological properties [Daculsi et al., 2003; LeGeros, 2002]. They are progressively
replacing TCP ceramics in Europe. A new technological step was made with <strong>the</strong> development <strong>of</strong> calcium
phosphates cements [Brown and Chow, 1987]. These materials are able to set and harden in a living body
and most can be injected. Despite <strong>the</strong>ir poor mechanical properties <strong>the</strong>y <strong>of</strong>fer a number <strong>of</strong> advantages and
are increasingly used for several applications. More recently biomimetic coatings involving low temperature
nano-crystalline calcium phosphates have been proposed, some have been claimed to exhibit osteo-inductive
properties [Habibovic et al., 2006].
1.2.2 Mechanical Properties <strong>of</strong> 3D Porous Scaffolds
The scaffold for tissue engineering should have a 3D porous structure with a porosity <strong>of</strong> no less
than 70% and a pore size ranging from 50 to 900 μm [Salgado et al., n.d.]. High scaffold porosity facilitates
oxygen, nutrient and metabolic product exchange. The literature results showed that with <strong>the</strong> optimization
design, <strong>the</strong> porosity <strong>of</strong> <strong>the</strong> scaffolds was 82.0 ± 3.8%. It was composed <strong>of</strong> <strong>the</strong> designed interconnectivity
macro-pores and micro-pores. The interconnectivity macro-pores in 3D scaffold would help develop <strong>the</strong>
skeletal network and facilitate <strong>the</strong> internal mineralized bone formation [Cerroni et al., 2002]. The micropores
less than 50 μm on <strong>the</strong> walls <strong>of</strong> <strong>the</strong> macro-pores would help in factors like fluid diffusion and cell
attachment. Additionally, <strong>the</strong> scaffold for bone tissue engineering should also have high mechanical strength
as close as possible to <strong>the</strong> strength <strong>of</strong> natural bone [Hutmacher, 2000]. In this study, <strong>the</strong> compressive
strength and elastic modulus <strong>of</strong> <strong>the</strong> calcium phosphate/PLGA scaffolds were significantly higher than those
<strong>of</strong> <strong>the</strong> pure PLGAs scaffolds. This result was consistent with some studies that improved <strong>the</strong> mechanical
properties <strong>of</strong> biodegradable polymers by adding inorganic materials [Zhang and Zhang, 2001; Thomson et
al., 1998]. However, co-grinding with calcium phosphate increased <strong>the</strong> mechanical properties and structural
quality but <strong>the</strong> porosity and pore diameter <strong>of</strong> <strong>the</strong> scaffolds was decreased.
Compressive modulus values <strong>of</strong> human trabecular bone range from 1 to 5000 MPa, with strength
values ranging from 0.10 to 27.3 MPa reported by Langer and Tirrell [2004]; Porter et al. [2000]; Lang et
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