3.3. DEET evaporation and skin permeation — controlled airflow trapping
studies
As calculated from target concentrations, the average mass balance
for microcapsule Formulation A was 163% (42–244%) and the average
mass balance for microcapsule Formulation B was 143% (118–177%).
Subsequent investigation of the high mass balance for these formulations
revealed substantial evaporative loss of the aqueous continuous
phase during the formulation process. Duplicate formulations manufactured
using the same methods showed a total mass loss of between
30 and 40%. The loss of a portion of the aqueous phase led to a higher
concentration of individual microcapsule applied to the skin and therefore
a higher dose of DEET for these formulations. Thus the specific dose
applied in Franz cell experiments was approximately 816 μg/cm2 for
Formulation B and 930 μg/cm2 for Formulation A instead of the target
level of 570 μg/cm2 for both formulations.
The ethanol control formulation showed an average mass balance
of 83% (63–121%). Factors contributing to the low mass balance for
this formulation may include rapid volatilization following application
to the skin and vapor loss from the Franz cell headspace during
the cartridge exchange period.
To investigate the effect of the enrichment of the original Formulation
A and B samples, an additional batch of Formulation B microcapsules
containing exactly 15% w/w DEET was prepared. This formulation,
dosed at 3 μL/cell, yielded a specific dose of 570 μg/cm2
. DEET absorption
from this formulation was compared to that of the original 816 μg/cm2
Formulation B microcapsules. A 15% solution of DEET in ethanol was
used as a control. The results of this experiment are shown in Fig. 3.
There was no statistical difference between the two doses of Formulation
B at 24 h. Total absorption was higher for all formulations than that in
Fig. 2, reflecting the variability of human skin. This difference is reflected
in the ethanol controls.
These results for Formulation B are in agreement with a previously
reported study on the effect of amount of DEET dose on absorption and
evaporation in vitro [14]. Fig. 5 of the earlier study shows that DEET
doses (in ethanol) in the range 500 to 1000 μg/cm2 have similar total percent
absorption. At much larger DEET doses the total percentage absorbed
at a fixed time decreases sharply because much of the dose remains on
the skin.