Biomedical Engineering Reference
In-Depth Information
3.1 Conventional Scaffold Materials and Fabrication Methods
According to Williams Dictionary, scaffolds for tissue engineering for regenerative
medicine are defined as porous substrate materials (metals, ceramics, polymers)
guiding cell adhesion, differentiation, proliferation, and growth [
83
]. Scaffolds are
designed to promote new tissue formation by providing adequate 3D architecture
for tailored porosity and appropriate surfaces for cellular adhesion and migration.
Pioneer studies in scaffold development for tissue engineering were performed by
Langer and Griffith [
84
-
86
]. An comprehensive overview of the variety of
appropriate scaffold materials is given by Park [
87
] with specific focus on scaffold-
based bone engineering by Hutmacher [
88
]. Concerning their chemical nature,
scaffolds are prepared from metals, ceramics and natural or synthetic polymers.
In addition to pure ceramic or polymer compounds, a broad variety of corre-
sponding composites, i.e., ceramic-polymer composites, have been developed, in
particular for bone tissue engineering. Those composites are designed to gain
synergetic effects by combining required mechanical properties with osteocon-
ductive characteristics.
Ceramics. Biocompatible ceramics include hydroxyapatite (HA), tricalcium
phosphate, calcium phosphates (TP) and their composites, especially combined
with natural or synthetic polymers. Pure ceramic materials are brittle and lack
interconnected pores required for cell proliferation and angiogenesis, limiting the
use of ceramic scaffolds to rather small defects. Thus, ceramic phosphate com-
posites containing natural or synthetic polymers are more attractive for large bone
tissue engineering, combining improved mechanics with osteoconductive proper-
ties [
89
]. Those ceramic-polymer composites are synthesized with ceramic par-
ticles embedded into the polymer matrix and exposed on the surface to improve
osteoconductive effects. Mechanical properties are mainly influenced by calcium
phosphate particle size and its distribution within the polymer matrix. The com-
posite stiffness increases with decreasing particle size. HA bioceramics have been
examined with mesenchymal stem and progenitor cells to study SC attachment,
migration and differentiation into osteoblasts [
90
]. A recent review summarized
the biological response, in particular the cell attachment influenced by ionic dis-
solution products from bioactive glasses and glass-ceramic composites [
91
].
Natural polymers. Naturally derived proteins or carbohydrate polymers are
widely used as scaffolds for tissue engineering. Natural polymeric materials used
for bone regeneration mainly include polysaccharides such as cellulose and cor-
responding derivatives [
92
], alginates (e.g., polyanionic co-polysaccharides [
93
],
agar, and agarose derivatives), chitosan [
94
], hyaluronates [
95
], fibrin, fibronectin,
collagen and gelatine and corresponding derivatives [
96
]. Since natural scaffold
materials are often used in a gel-like phase, biological agents can be incorporated
via gel formulation [
88
].
Collagens are the principal structural proteins in mammals, widely distributed
in the body and a major component of the extracellular matrix (ECM): they occur
in skin, bone, cartilage, tendons, ligaments, and blood vessels. Fibrillar collagens
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