Page 240 - Biodegradable Polyesters

218 9 Environment-Friendly Methods for Converting Biodegradable Polyesters

within the body and eventually it will break down, leaving the newly formed tissue
which will take over the mechanical load.
The following are the basic requirements of materials used for scaffolds in
regenerative medicine and their manufacturing conditions: (i) biocompatibility
and biodegradability, (ii) degradation products that are harmless to the body, (iii)
high interconnectivity of pores (large specific surface) including larger pores, and
(iv) manufacturing conditions that are free of contact with any toxic substance.
A number of methods have been described in literature for preparing porous
structures to be employed as tissue engineering scaffolds. Each of these techniques
presents its own advantages, but none is devoid of drawbacks [10]:

• Nanofiber self-assembly: Molecular self-assembly is one of the few methods to
create biomaterials with properties similar in scale and chemistry to that of the
natural in vivo extracellular matrix.
• Textile technologies: These techniques include all the approaches that have been
successfully employed for the preparation of nonwoven meshes of different
polymers. The principal drawbacks are related to the difficulties of obtaining
high porosity and regular pore size.
• Solvent casting and particulate leaching: This approach allows the preparation
of porous structures with regular porosity. Other than the small thickness
range that can be obtained, another drawback of such scaffolds lies in the use
of organic solvents, which must be fully removed to avoid any possible damage
to the cells seeded on the scaffold.
• Gas foaming: To overcome the necessity to use organic solvents and solid poro-
gens, a technique using gas as a porogen has been developed. The excessive
heat used during compression molding (which prohibits the incorporation of
any temperature-labile material into the polymer matrix) and the fact that the
pores do not form an interconnected structure represent the main drawbacks
of this technique.
• Emulsification/freeze-drying: This technique does not require the use of a solid
porogen. While emulsification and freeze-drying allows a faster preparation and
although the technique does not require a time-consuming leaching step, it still
requires the use of solvents; moreover, pore size is relatively small and porosity
is often irregular.
• Thermally induced phase separation: Similar to the previous technique, this
phase separation procedure requires the use of a solvent with a low melting
point that is easy to sublime. Liquid–liquid phase separation presents the same
drawbacks of emulsification/freeze-drying.
• CAD/CAM technologies: Since most of the above described approaches are lim-
ited when it comes to the control of porosity and pore size, computer-assisted
design and manufacturing techniques have been introduced in tissue engineer-
ing. First, a three-dimensional structure is designed using CAD software and
then the scaffold is realized by using ink-jet printing of polymer powders or
through fused deposition modeling of a polymer melt.