Biomedical Engineering Reference
In-Depth Information
[23]. Natural bone is an inorganic-organic composite consisting mainly of
nanohydroxyapatite and collagen fibers. Hybrid materials obtained by the sol-gel route
combine the advantages of both organic and inorganic properties. Several kinds of
organofunctional alkoxysilanes precursors have been studied for the production of silica
nanoparticles. The sol-gel offers advantages such as the possibility of obtaining
homogeneous hybrid materials under low temperature, thereby allowing for the
incorporation of a variety of compounds [23 - 29].
The sol-gel process is based on the hydrolysis and condensation of metal or silicon alkoxides
and is used to obtain a variety of high-purity inorganic oxides or hybrid inorganic-organic
materials that are simple to prepare [30]. This process can be employed for the synthesis of
functionalized silica with controlled particle size and shape [31 - 38].
Apart from the several applications mentioned in the first paragraph of this chapter, more
recently, biomaterials have been utilized as drug delivery systems (DDSs). In this sense,
polymers and biodegradable polymers emerge as potential materials, since they promote
temporal and targeted drug release. Indeed, biomaterials have had an enormous impact on
human health care. Applications include medical devices, diagnosis, sensors, tissue
engineering, besides the aforementioned DDSs [39]. In the latter field, an ideal drug
deliverer should be able to lead a biologically active molecule at the desired rate and for the
desired duration to the desired target, so as to maintain the drug level in the body at
optimum therapeutic concentrations with minimum fluctuation [1, 40]. The use of DDSs
overcomes the problems related to conventional administration routes, such as oral and
intravenous administration.
Several biomaterials have been applied as DDSs. This is because they are biocompatible
and/or biodegradable, which allows for consecutive administrations. Hydroxyapatite-based
materials, natural and synthetic polymers, silica, clays and other layered double hydroxides,
and lipids are some examples of biomaterials that have been employed for the delivery of
active molecules through the body. Liposomes, solid lipid nanoparticles, polymeric nano
and microparticles, micelles, dendrimers, metallic nanoparticles, and nanoemulsion are
currently utilized as DDSs.
Special attention has been given to DDSs comprised of biodegradable polymers and silica.
In polymeric DDSs, the drugs are incorporated into a polymer matrix. Since biodegradable
polymers are degraded to non-toxic substances, they do not have to be removed after
implantation. So they have become attractive candidates for DDS applications. The rate of
drug release from polymeric matrices depends on several parameters such as the nature of
the polymer matrix, matrix geometry, drug properties, initial drug loading, and drug-
matrix interaction. Moreover, the drugs can be effectively released by bioerosion of the
matrices. [40]. Thus, both natural, frequently polysaccharides, and synthetic biodegradable
polymers, usually aliphatic polyesters such as PLA, PGA, and their copolymer (PLGA), are
the most extensively investigated biodegradable materials for drug delivery applications [1].
Inorganic materials, like silica, can offer the necessary properties for a nanoparticle to be
applied as DDS, especially nontoxicity, biocompatibility, high stability, and a hydrophilic
and porous structure. The drug release rate from the silica structures could be controlled by
adjusting particle size and porous structure [41 - 45].
The sol-gel technology is also employed in the preparation of inorganic ceramic and glass
materials. This technique was first used in the mid 1800s, when Ebelman and Graham
carried out studies on silica gels [46]. Initially, the sol-gel process was utilized in the
preparation of silicate from tetraethylorthosilicate (TEOS, Si(OC
2
H
5
)
4
), which is mixed with
