The skin of the third trimester foetus and early newborn exhibits a complex, multifunctional, highly hydrated but viscous skin-surface biofilm called vernix caseosa. During birth, vernix caseosa undergoes a substantial change from an aqueous and warm surrounding into a gaseous and colder environment postnatally. The aim of this study was to investigate the structural and physicochemical changes in vernix caseosa which accompany physiologically relevant variations in environment parameters such

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... s temperature and humidity. A remarkable difference was observed in water release and uptake properties: dehydration and rehydration processes take place 2-4 times faster at 37°C than at room temperature. The dehydration was irreversible; rehydration was only possible to a final weight of 55 % (37°C) and 46 % (room temperature) of the pre-desiccation weight. Differential scanning calorimetry showed two different overlapping phase transitions within physiological temperature range. Investigation of the lipid organization by Fourier transform infrared spectroscopy and small-angle X-ray diffraction revealed a more disordered state of lipids at 37°C than at room temperature, which might explain the faster dehydration and rehydration process at 37°C as well as the changes in thermotropic rheological behaviour. In conclusion, we demonstrated that vernix caseosa properties adjust to the fundamental change from the intrauterine to the postnatal environment.