Hydrodynamics in Porous Media with Applications to Tissue Engineering - Porous Media: Applications in Biological Systems and Biotechnology - page 94

be interesting because this material has high water content and can offer an
internal structure close to ECM structure of several biological tissues (Drury
and Mooney 2003). It is also of importance to mention that some ceramics
(synthetic: hydroxyapatite [Rose et al . 2004], tricalcic phosphate [Pothuaud
et al . 2005; Janssen et al . 2006] or natural: coral [Petite et al . 2000] and nacre
[Atlan et al . 1999]) show a chemical composition rich in calcium, which makes
them specific to the culture of bone implants.
Furthermore, the interaction between biomaterials and the biological envi-
ronment has to be carefully considered. Some ceramics (hydroxyapatite and
other bioactive calcium phosphates) are known for their bioactivity because
they interact positively with the body by accelerating the tissue-regeneration
processes. Other biomaterials are referred to as “bioresorbable” because their
resorption takes place after implantation within a period of time varying
from a few weeks to several years. Bioresorbable materials may be natural
(alginate, hyaluronic acid, collagen, fibrin gels, or coral) or synthetic (PCL,
bioglass, tricalcium phosphate, etc.). As regards the interaction between the
biomaterials and the biological environment, another key factor resides in the
materials surface because its nature may or may not favor the adsorption of
biological molecules acting on the cell behavior. For example, an experimen-
tal study has shown that by coating a biomaterial's surface with hydroxyap-
atite, the behavior of bone cells can be modified (El-Ghannam et al . 1997).
In that way, numerous biological molecules (ligands increasing the cell adhe-
sion, growth factors, hormones, enzymes, etc.) can be grafted on the bio-
material's surface (Jagur-Grodzinski 2006) to control or to adjust the cell
behavior.
At last, but not at least, the microstructure of scaffolds plays a key role.
As regards the porous biomaterials with interconnected pores, it is generally
considered that a pore size smaller than 100-150

m does not allow a satisfying
tissue in-growth within the implant (Shors 1999; Rose et al . 2004; Rezwan
et al . 2006) and that a mean pore size between 200 and 400

µ

µ

m is optimal to
favor not only the cell proliferation and differentiation but also the generation
of a vascular network after implantation (Brekke and Toth 1998).
Since a careful design of the scaffold microstructure is essential to the cul-
ture success, various techniques such as “rapid prototyping” have been used to
obtain well-defined and controllable microarchitectural properties (Hutmacher
et al . 2001; Zein et al . 2002). Among the rapid prototyping techniques, the
“Fused Deposition Modeling” based on the principle of extrusion allows the
manufacturing of polymeric scaffolds (Hutmacher et al . 2004). Scaffolds are
first virtually designed using a computer and then manufactured in three
dimensions by superposing flat fibrous layers. With such a technique, pores
are fully interconnected and the scaffold geometry is controllable and repro-
ducible (for more details see Zein et al . 2002). Other techniques such as “jet
based methods” are also developed to “print” directly the cells within the
scaffold (Ringeisen et al . 2006).