Bioactive Glass-Based Scaffolds for Bone Tissue Engineering - Tissue Engineering: Cell-Surface Interactions for Tissue Culture - page 197

Surface of bioactive glass

1 Exchange of alkali ions with hydrogen ions from body fluids

2 Network dissolution and formation of silanol (SiOH) bonds

3 Silica-gel polymerization: SiOH + SiOH Si-O-Si

4 Chemisorption of amorphous Ca + PO 4 + CO 3

5 Crystallization of the HCA layer

Biochemical adsorption of growth factors on HCA layer 6

7 Action of macrophages

8 Attachment of stem cells

9 Differentiation of stem cells

10 Generation of matrix

11 Crystallization of matrix

12 Proliferation and growth of bone

Fig. 1 Sequence of interfacial reactions involved in forming a bond between bone and a
bioactive glass (modified from Ref. [ 5 ])

Since the late 1990s and the beginning of the new millennium, great
potential has been attributed to the application of bioactive glasses in TE and
regenerative medicine [ 1 , 7 , 9 , 13 , 34 - 39 ]. The application involves both
micron-sized and nanoscale bioactive glass particles of different compositions
[ 24 , 40 , 41 ] as well as the fabrication of composite materials which are
developed by combining biodegradable polymers and bioactive glass particles
or fibres [ 34 , 42 - 47 ].
Based on the attractive osteogenic and angiogenic properties of bioactive
glasses, bone TE is one of the most exciting future clinical applications of these
materials. Both micron-sized and nanoscale particles [ 40 , 43 ] are considered in this
application field. Bioactive silicate glasses exhibit three major advantages for bone
TE applications over other conventional non-degradable (insoluble) bioceramics
such as TiO 2 ,Al 2 O 3 , ZrO 2 , or sintered hydroxyapatite (Fig. 2 ). Firstly, chemical
reactions on the material surface lead to a strong bond to bone by means of a
hydroxyl carbonate apatite (HCA) layer [ 5 ]. Secondly, ion release and dissolution
products from the bioactive glass activate and up-regulate gene expression in
osteoprogenitor cells that give rise to rapid bone regeneration, which explains the
higher rate of bone formation in comparison to other inorganic ceramics such as
hydroxyapatite [ 7 , 9 - 11 , 13 , 48 ]. Thirdly, recent studies (reviewed in Ref. [ 49 ])
have demonstrated angiogenic effects of 45S5 Bioglass , i.e., increased secretion
of vascular endothelial growth factor (VEGF) and VEGF gene expression in
fibroblasts, the proliferation of endothelial cells and formation of endothelial
tubules in vitro, as well as enhancement of vascularization in vivo [ 49 - 53 ].
Figure 2 summarizes schematically these three effects of bioactive glasses in the
context of tissue engineering.