Biomedical Engineering Reference
In-Depth Information
After 12 h of immersion in SBF, the fibers were coated in undense, porous calcium phos-
phate. Densification of the calcium phosphate coating was not observed even after 2 days
of immersion in SBF solution.
Biological Approach
In addition to providing a structural support, scaffolds must also provide biological sig-
nals. They are therefore usually loaded with growth factors, antibiotics, and bone morpho-
genic proteins or used as delivery vehicles for the release of drugs/proteins or genes [66].
Biological modifications are aimed at mimicking the molecular biological mechanisms
of the ECM morphology (e.g., structure and function) by immobilizing bimolecular cues
on the surface of biomaterials. Techniques used include adsorption, entrapment, and cova-
lent attachment.
Nonspecific adsorption of proteins can lead to their partial or complete denaturation
resulting in losses in their functionality. Also, protein absorbed coatings are susceptible to
loss or replacement of their components due to desorption or competitive binding. Thus,
covalent attachment of the proteins to substrates is a preferred approach for surface engi-
neering. Little is stated in the literature regarding physiochemical modifications of bioac-
tive glass surfaces via grafting biomolecules onto their surface. At the time of writing, only
two techniques have been reported. These include: direct protein absorption [67, 68] and
silanization methods.
Silanization (Silane Modification) Methods
Functional groups (e.g., methyl (-CH
3
), hydroxyl (-OH), carboxyl (-COOH), and amino
(-NH
2
)) are present in many biological molecules and have specific physical and chemical
properties that influence cellular processes. Silanization offers the possibility of immobi-
lization of biological molecules on biomaterial surfaces via -OH groups activated on the
biomaterials surface that facilitate grafting of biomolecules due to free unbound, amino ter-
minal groups [69, 70, 94]. The surface reaction stages responsible for bioactivity of bioactive
glass make them ideal substrates for protein immobilization due to their inherent surface
hydroxylation [71]. However, the reaction layer kinetics, including cation migration from
the glass network, occurs, quickly making it difficult to preserve free hydroxyl groups.
Recently, Verne and coworkers [94] investigated, using three bioactive glass compositions
substrates (see Table 9.25), and different cleaning and silanization methods (Figure 9.23)
in order to covalently bond bone morphogenetic proteins (BMP-2) to the bioactive glass
surfaces.
All the cleaning methods produced surfaces with increased hydrophilicity nature due
to the presence of hydroxyl groups on their surfaces as confirmed by x-ray photoelectron
(XPS) analyses. Washing in acids, bases, and SBF resulted in hydroxyl condensation coupled
TABLE 9.25
Bioactive Glass Compositions Used by Verne and Coworkers to Covalently Bond Bone
Morphogenetic Proteins
Material
SiO
2
P
2
O
5
Al
2
O
3
MgO
CaO
Na
2
O
K
2
O
SCNA
57
-
9
-
34
6
-
4.5 A
47.5
2.5
-
-
30
10
10
CEL2
45
3
-
7
26
15
4
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