Biomedical Engineering Reference
In-Depth Information
Bifunctional silanes with a functional group at the opposite end to
the Si(OR) group can be grafted onto certain substrates by forming self-
assembled monolayers (SAMs, Figure 10.3). SAMs are probably beyond
the common concept of hybrids, but they can control adsorption of
proteins or other physiological substances. A typical bifunctional silane
for functionalization of implants or for SAM applications is amino-
propyl trimethoxysilane (APTS). Figure 10.3 schematically represents
their microstructure. The substrate could be a silicate glass or a silicon
wafer. Hydroxyl groups on the substrate surface are condensed with
the methoxy groups to form siloxane (-Si-O-Si-) bonds and an NH
2
overlayer is formed. The layer is positively charged even under neutral
pH conditions, and hence the amino groups would attract negatively
charged ions like phosphate ions, which means that they could induce
apatite nucleation. However, the NH
2
overlayer is scarcely bioactive.
Bio-inert polymers can be grafted with species (for example, a layer of
3-methacryloxypropyl trimethoxysilane) via emulsion polymerization.
Substrates such as high-density polyethylene (HDPE), poly(vinyl chlo-
ride) (PVC), and polyamide (PA, nylon-6) deposit apatite in simulated
body fluid (SBF), but it takes more than 14 days for HDPE while PVC
and PA deposit apatite in at least 7 days. The rate of apatite formation
is much lower than Bioglass (8 hours) and they cannot be said to be
really bioactive. However, surface functionalization does allow control
of surface chemistry of materials. This chapter will now concentrate on
hybrids that have organic-inorganic interaction throughout the bulk.
10.4 SOL-GEL HYBRIDS
Synthesis of organic-inorganic hybrids was launched in the early 1980s,
first among synthetic organic components and inorganic components,
and then natural polymers were employed to be hybridized with inor-
ganic counterparts involving Si-O and Ti-O bonds. Ormosils denote
a series of the former type hybrids, whose name is originated from
''organically modified silicates.'' They involve skeletons composed by
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bonds. Polydimethylsilane
(PDMS) and tetraethoxysilane (or tetraethyl orthosilicate, TEOS) are
commonly used starting materials. Chapter 3 describes how silica glass
can be synthesized by the sol-gel process. The most common procedure
is to react a silicon alkoxide precursor (e.g. TEOS) with water under
acidic or basic catalysis, which produces Si-OH groups. Condensation
of these bonds results in oligomers and clusters. Various sizes of clusters
are produced in the solution, and when those clusters grow to around
Si-O-Si
bonds as well as by
C-C
