Biomedical Engineering Reference
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higher pH values, the drug loading efficacy was decreased. One problem in these equilib-
rium experiments was the flocculation of microspheres at high drug concentrations.
Probably due to binding of the charged drug, the positive charge of the microspheres was
neutralized. This in turn decreased the repulsive forces between the microspheres. In the
experiments displayed in
Figure 3.13,
only those drug concentrations that did not lead to
flocculation were used. The adsorbed amount of drug depended on the MW of the chito-
san. Adsorption increased with decreasing MWs. Probably due to the lower amount of
sodium sulfate required for microsphere preparation with LMWC, more amino groups in
LMWC were available for drug binding.
Differential scanning calorimetry (DSC) is a possible method to determine the modifica-
tion of a drug. Figure 3.14 shows the DSC thermoanalysis of blank microspheres, drug-
loaded microspheres, physical mixture, and PSP. PSP crystals show an endothermic peak
at 341.5°C due to decomposition of PSP. Blank microspheres showed two peaks, one at
233.4°C and the other at 271.9°C corresponding to the α- and β-form of glucosamine. The
DSC curve of a physical mixture of microspheres with PSP showed three peaks at 235.9°C,
275.4°C, and 341.9°C, demonstrating no interactions of the drug with chitosan. In contrast,
in the case of microspheres with adsorbed PSP, only two peaks at 235.2°C and 263.9°C
appeared. No PSP peak was observable. This leads to the assumption that PSP is present
on the microsphere surface in amorphous form. Furthermore, one peak of chitosan sulfate
was shifted from 271.9°C to 263.9°C. These observations indicate an interaction between
the amino group of chitosan and the phosphate group of the drug such as the formation of
an ion pair.
The model antiinflammatory drug prednisolone (PS) was retained in chitosan gel beads,
which were prepared in a 10% aqueous amino acid solution (pH 9.0). Sustained release of
PS from the chitosan gel beads was observed. Carrageenan solution was injected into air
a
b
c
d
180.00
210.00
240.00
270.00 300.00
Te mperature (°C)
330.00
360.00
390.00
420.00
Figure 3.14
(a) DSC thermoanalysis of blank microspheres, (b) drug-loaded microspheres, (c) physical mixture, and (d) PSP.
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