Biomedical Engineering Reference
In-Depth Information
dispersed particles with nanometric diameter [26] . In general, the hydrolysis reaction of TEOS can
be expressed by the following simple equation [27] :
Si
ð
OR
Þ 4 1
2H 2 O
SiO 2 1
4ROH
(15.1)
-
In fact, the controlled hydrolysis process produces the singly hydrolyzed monomer [(OR) 3 Si(OH)]:
Si
NH 4
ð
OR
Þ 4 1
xH 2 O
NH 3 - ð 2
0
Þ x Si
ð
OR
Þ 4 2 x 1
xROH
(15.2)
1
1
These intermediate reaction products participate in the condensation reactions, Eqs (15.3) and
(15.4) , in turn forming silica nanoparticles.
Si
ð
OR
Þ 4 1 ð
OH
Þ
Si
ð
OR
Þ 3 - ð
OR
Þ 3 Si
O
Si
ð
OR
Þ 3 1
ROH
(15.3)
2
2
ð
OR
Þ 3 Si
ð
OH
Þ 1 ð
OH
Þ
Si
ð
OR
Þ 3 - ð
OR
Þ 3 Si
O
Si
ð
OR
Þ 3 1
H 2 O
(15.4)
2
2
15.3 Bioactivity of glass nanoparticles
Numerous responses can be observed when a biomaterial comes in contact with living cells and tis-
sues, and these responses depend on the type of material used. If this material is biologically active
and induces a specific response, leading to formation of a continuous interface between the material
and living tissue, it is called bioactive. Bioactive glasses have this unique feature and when they
come in contact with the body, form a layer of carbonated HA on the surface of the material,
which permits the interaction of biomaterials especially with mineralized tissue ( Figure 15.3 ).
The complex process of HA formation has been extensively studied in the literature [27] .
A sequence of interfacial reactions, which begin immediately after bioactive glass is soaked in
simulated body fluid, is interpreted in terms of the electrostatic interaction of the functional groups
with the ions in the fluid. The reaction is divided into four steps [9,27,28] :
1. Initially, the surface becomes negative, attributed to the silanols (Si
OH) formed on bioactive
glass particles upon being soaked in simulated body fluid, by two mechanisms: (i) The
OH
formation occurs due to release of calcium ions (Ca 2 1 ) from their surfaces by means of exchange
with H 3 O 1 ,Na 1 ,andK 1 ions in simulated body fluid and (ii) a loss of soluble silica to solution, in
Si(OH) 4 form, causing the breakdown of Si
O
Si bonds and silanols formation. As a result, many
OH groups are formed on their surfaces and a local supersaturation of Ca 2 1 ions is established.
2. As time passes, there occurs a selective combination of OH charged surface with the Ca 2 1
ions from the simulated body fluid solution. As the calcium ions accumulate on the surface, the
surface gradually gains an overall positive charge.
3. The pH of simulated body fluid solution, where the concentration of HPO 2 2 is much larger
than that of PO 3 2 , favors the incorporation of HPO 2 2 ions in the bioactive glass surface and
the migration of PO 3 2 ions from the bulk to the surface of the glass. The result leads to
calcium deficiency, and the surface once again becomes negatively charged.
4. The calcium ions combine with the phosphate ions (PO 3 2 and HPO 2 2 ) and form amorphous
calcium phosphates (Ca 3 (PO 4 ) 2 and CaHPO 4 ). These calcium phosphates spontaneously
transform into the apatite through the incorporation of OH and CO 2 2 anions from the solution
Si
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