Biomedical Engineering Reference
In-Depth Information
Poly(esters) such as polylactide (PLA) and polyglycolide (PGA) and their copolymers,
poly(lactide-
co
-glycolide) (PLGA), are some of the best defi ned and most extensively employed
synthetic biodegradable polymers in clinical applications of controlled drug delivery.
14,16,17
Origi-
nally, PLA and PLGA were used as resorbable suture materials (the fi rst patent for the use of PLA
as a resorbable suture material was fi led in 1967
18
) and, not surprisingly, were later applied in
controlled-drug delivery systems. Miscellaneous polyester-based biodegradable delivery systems,
for example, injectable drug carriers and implant devices, have been developed.
8,17
Although the
primary advantage of biodegradable polymers is that these polymers can be removed from the
body through normal metabolic pathways by breaking down into biologically acceptable molecules,
biodegradable materials produce degradation by-products and the biodegradable polymers must be
tolerated with little or no adverse reaction.
A variety of copolymers consisting of various biodegradable, nonbiodegradable, natural,
seminatural, and synthetic components are often designed to fi t different drug delivery applica-
tions. In an effort to achieve sustained intravesical drug delivery, thermo-sensitive hydrogel formed
by poly(ethylene glycol-
b
-[dl-lactic acid-
co
glycolic acid]-
b
-ethylene glycol) (PEG-PLGA-PEG)
triblock copolymers has been used for
in situ
gel formation for a depot of hydrophobic and hydro-
philic drugs in rats. The triblock copolymer turns from an aqueous solution at room temperature to
a viscous gel at body temperature, sustaining the residence time of hydrophobic drugs in rat bladder
after its instillation. Nontoxic PEG, glycolic acid, and lactic acid are the products from bioerosion
of the triblock copolymer.
19
8.2 NANOSTRUCTURED POROUS MATERIALS
Nanotechnology is in essence an extension of existing sciences into the nanoscale. Nanotechnol-
ogy studies phenomena, synthesis, manipulation, and application of materials and devices at the
nanoscale by using technology in colloidal science, biology, physics, chemistry, and other scientifi c
fi elds. Since nanotechnology operates at the same size domain as biology, the tools of nanotechnol-
ogy are naturally applied to serve the purpose of biomedical development, for example, the effort
to design and apply the nanostructured pore-containing biomaterials to offer a degree of control in
both the rate and the location of drug delivery.
Although IUPAC defi nes porous materials into three classes, microporous (
<
2 nm), mesoporous
50 nm),
20
according to their pore sizes, terms such as porous nano-
materials, nanoporous materials, and nanostructured porous materials have been widely used to
cover a variety of porous materials studied under nanotechnology. In this chapter, nanostructured
pore-containing materials or nanostructured porous materials refer to those materials with pore size
ranging from a few nanometers to several microns.
Nanostructured pore-containing materials can be made of various components, including soft
materials
21
and inorganic materials. Common soft materials (liposomes, micelles, microemulsions,
nanoemulsions, solid lipid nanoparticles, and microgels) used for drug delivery systems are fabri-
cated, polymers,
22
and lipids.
23,24
These nanostructured pore-containing materials can be tailored to
provide fl exibility in drug loading and release control. By controlling morphological and chemical
parameters such as the size and the shape of the material, the number and the volume of the “reser-
voirs,” the wall thickness and the surface chemistry, and the permeability and the resorption rate,
we can design more sophisticated systems based on nanostructured pore-containing materials for
precise control of drug delivery.
(2
-
50 nm), and macroporous (
>
8.2.1 S
OFT
N
ANOSTRUCTURED
P
OROUS
M
ATERIALS
Despite the great promise of numerous nanotechnologies, the existing commercial products of
nanotechnology have mainly utilized the advantages of nanoparticle additives for applications such
as suntan lotion, cosmetics, protective coatings, and stain-resistant textiles. Some other examples
