Biomedical Engineering Reference
In-Depth Information
focused on the impregnation of natural bioactive substances into wound dressings
for wound infections as is the case of honey [
37
], tea tree oil (antimicrobial and
antiinflammatory activities) being a promising adjunctive wound treatment [
38
],
extract of cinnamon leaves [
39
], extract of grapefruit flesh or seeds (bactericidal,
fungicidal, antiviral, antiparasitic and antiinflammatory agent) [
40
]and
Bletilla
striata
herbal extract [
41
].
Studies of the topical application of compounds with free radical scavenging
properties have shown to significantly improve healing and protect tissue from
oxidative damage [
42
]. However, a too high concentrated application of antioxidants
may result in toxic response in the wound. To obtain good results on wound healing,
a slow release of the antioxidant is required. Moreover, and since wound dressings
are directly in contact with damaged skin, there are strict toxicity/dose requirements
for the type of applicable antioxidant/antimicrobial agents.
The incorporation of drugs or other bioactive species into polymeric matrices is
usually done by physical mixing of these substances during polymeric synthesis,
or by immersing and soaking the previously prepared polymeric materials into a
solution which contains the bioactive substances to be impregnated. These methods
are relatively simple to implement, but they also present some disadvantages
like the use of organic solvents (which have to be removed to acceptable limits
both for health/safety reasons and for product integrity maintenance), possible
occurrence of undesired substances reactions and/or degradation, low incorporation
yields and heterogeneous dispersion [
43
-
45
]. To overcome these issues, the use of
supercritical fluid methodologies is being presented as a valuable alternative to
prepare specialized materials for biomedical applications, for which restrictive
toxicity/biocompatibility limits are imposed and need to be accomplished.
3
Supercritical Fluid Impregnation/Deposition
The use of supercritical fluids for chemical and physical polymer processing is an
actual and growing research field supported by its broad range of possible applica-
tions which includes polymerization, fractionation, foaming, dyeing/impregnation,
encapsulation and micronization (by different methods including supercritical anti-
solvent (SAS), rapid expansion of supercritical solutions (RESS), particles from gas
saturated solution (PGSS), supercritical assisted atomization (SAA)), sterilization,
crystal growth, and as mixing/blending aids for crystalline or viscous materials
[
46
,
47
].
A supercritical fluid or a dense gas is a substance that exists at or near its critical
point, which is located at the end of the phase boundary between the gas and liquid
phases of the substance (Fig.
1
). When pressure and temperature increase beyond
the critical point the phases become indistinguishable as either a gas or a liquid,
and become a supercritical fluid (SCF) [
48
]. At this condition, the fluid can have
density and solvent properties similar to those of the corresponding liquid but with
lower viscosity and higher diffusion rate much closer to a gas, as listed in Table
1
.