Based on the results in Fig. 3 we determined it was appropriate to
normalize the mass balance in each individual Franz cell to 100% before
averaging them together to give further results. This procedure yielded
a conservative estimate of the DEET evaporation rates achieved from
the microcapsule formulations as compared to the ethanol control,
since the measured recoveries were decreased for the former and increased
for the latter by the normalization procedure.
The normalized results of the controlled air flow trapping studies
are shown in Table 3 and Figs. 4 and 5. The results differ from those
shown in Fig. 2 because of the different airflow conditions [19] and
the different skin donors. Nevertheless the relative performance of
formulations can still be directly compared.
The cumulative percentage of the DEET extracted from the Tenax TA
cartridges, corresponding to DEET evaporation, is shown in Fig. 4a.
Evaporation rates calculated from these data are shown in Fig. 5. The
ethanol control formulation showed the most rapid initial DEET evaporation
rate, but the lowest total evaporation after 48 h. Evaporation
from the control formulation ceased after about 12 h when presumably
the full dose had been consumed by evaporation and absorption processes.
The two microencapsulated DEET formulations yielded 36–40%
higher cumulative evaporation versus control, and while both had similar
total evaporation after 48 h, rate (Fig. 5) and mass balance (Table 3)
data show that evaporation from Formulation B continued through the
48 h timepoint. More than 24% of the Formulation B DEET dose was still
on or in the skin after 48 h versus 8% for Formulation A.
Skin absorption of DEET after 48 h was lowest for Formulation B. The
ethanol control formulation showed the most rapid absorption, which
reached a plateau at about 20 h. Total absorption at 48 h was similar
for Formulation A and control (Fig. 4b), while Formulation B yielded a
30% reduction in DEET absorption versus control after 48 h. However,
because absorption from Formulation B had not reached zero after
48 h, further absorption would have occurred had the experiment
been continued (Fig. 5a).
The evaporation rates shown in Fig. 5a can be compared to the
reported minimum effective evaporation rate of 1.2± 0.3 μg/cm2
/h
for repelling A. aegypti mosquito [7]. When converted to percentageof dose per h based on the ethanol control, this rate is 0.21% dose/h,
shown as the dotted line in Fig. 5a. Based on this comparison, Formulation
B would provide >48 h of effective evaporation rate for
repellency, Formulation A b35 h and the ethanol control b15 h. Theevaporation rate from Formulation B was still significantly higher than
that of Formulation A (pb0.001) and ethanol control (pb0.001) after
48 h.
Based on the results in Fig. 3 we determined it was appropriate tonormalize the mass balance in each individual Franz cell to 100% beforeaveraging them together to give further results. This procedure yieldeda conservative estimate of the DEET evaporation rates achieved fromthe microcapsule formulations as compared to the ethanol control,since the measured recoveries were decreased for the former and increasedfor the latter by the normalization procedure.The normalized results of the controlled air flow trapping studiesare shown in Table 3 and Figs. 4 and 5. The results differ from thoseshown in Fig. 2 because of the different airflow conditions [19] andthe different skin donors. Nevertheless the relative performance offormulations can still be directly compared.The cumulative percentage of the DEET extracted from the Tenax TAcartridges, corresponding to DEET evaporation, is shown in Fig. 4a.Evaporation rates calculated from these data are shown in Fig. 5. Theethanol control formulation showed the most rapid initial DEET evaporationrate, but the lowest total evaporation after 48 h. Evaporationfrom the control formulation ceased after about 12 h when presumablythe full dose had been consumed by evaporation and absorption processes.The two microencapsulated DEET formulations yielded 36–40%higher cumulative evaporation versus control, and while both had similartotal evaporation after 48 h, rate (Fig. 5) and mass balance (Table 3)data show that evaporation from Formulation B continued through the48 h timepoint. More than 24% of the Formulation B DEET dose was stillon or in the skin after 48 h versus 8% for Formulation A.Skin absorption of DEET after 48 h was lowest for Formulation B. Theethanol control formulation showed the most rapid absorption, whichreached a plateau at about 20 h. Total absorption at 48 h was similarfor Formulation A and control (Fig. 4b), while Formulation B yielded a30% reduction in DEET absorption versus control after 48 h. However,because absorption from Formulation B had not reached zero after48 h, further absorption would have occurred had the experimentbeen continued (Fig. 5a).The evaporation rates shown in Fig. 5a can be compared to thereported minimum effective evaporation rate of 1.2± 0.3 μg/cm2/hfor repelling A. aegypti mosquito [7]. When converted to percentageof dose per h based on the ethanol control, this rate is 0.21% dose/h,shown as the dotted line in Fig. 5a. Based on this comparison, FormulationB would provide >48 h of effective evaporation rate forrepellency, Formulation A b35 h and the ethanol control b15 h. Theevaporation rate from Formulation B was still significantly higher thanthat of Formulation A (pb0.001) and ethanol control (pb0.001) after48 h.
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