Biomedical Engineering Reference
In-Depth Information
the fluid is kept constant by the progressive dissolution of other amounts of drug.
The second mechanism is a simple physical entrapment of the drug molecules inside
the polymeric matrix when the solvent is removed. The first mechanism, obviously,
leads to higher impregnation amounts and usually prevents re-crystallization of the
drug [
61
]. When polymer-SCF phase interactions are present and are favorable,
high pressures usually facilitate the diffusion process mostly because they will allow
more fluid absorption which will generate a higher swelling degree. This is the case
when higher operational pressure determines higher drug loading in the polymeric
matrix. On the other hand, when drug-SCF phase interactions are stronger than
drug-polymer interactions, pressure usually will be an unfavorable factor because
higher pressures will originate an increase in SCF phase density, thus leading to
an increased solvating power of the mobile phase. At the same time, and if the
polymer-SCF phase interactions are still appreciable, this increased density will
also originate an increase in polymer swelling. As a result of these two combined
factors, more drug will “choose” to diffuse out the polymeric matrix and stay in the
mobile phase, originating a low polymeric loading [
62
-
65
].
A compilation of some of the works published over the last decade concerning
the supercritical impregnation/loading of bioactive compounds into different poly-
meric matrices is resumed in Table
2
. These works intended to develop optimized
drug delivery systems (DDS) for different ophthalmic, topical, dental applications
aiming to obtain sustained releases able to deliver the bioactive compound for
longer periods of time at a constant rate, avoiding dose fluctuation, and excessive
dosage. In some cases it was also attempted to reduce the cristallinity of the solute
in order to improve its solubility in aqueous media and consequent bioavailability.
The bioactive compounds impregnated include synthetic and natural based materials
with different hydrophilicities. In the case of the more hydroplylic compounds, the
effects of the use of safe co-solvents like water and ethanol was addressed proving
to be an effective way to overcome the hydrophobic character of scCO
2
.
4
Case Study
The incorporation of bioactive compounds like anti-inflammatories, anti-microbials
and antiseptics into wound dressing materials can play an active role in the
wound healing process to prevent inflammation and bacterial infection and to
accelerate tissue regeneration, by stimulating body's healing responses with the
minimum final scar formation [
74
,
75
]. With this idea in mind, a recent work
was carried out to study the possibility of developing potential hydrogel-type
delivery systems based on natural-origin bioactive species envisaged for wound
healing applications [
45
]. The impregnated wound dressing models were prepared
using biodegradable and biocompatible natural-based polymeric matrices, namely
N
-carboxybutylchitosan (CBC) and agarose (AGA), that were impregnated/loaded
with two natural-origin bioactive compounds, quercetin and thymol, separately
or as mixture, by using a supercritical solvent impregnation (SSI) methodology.
