An investigation into the role of skin biophysics and fluid thermal properties in underarm wetness perception with applications to antiperspirant deodorants

Abstract

Sensing wetness on the skin plays a fundamental role in supporting human homeostasis by influencing thermal discomfort, behavioural responses, and autonomic thermoregulation. However, the skin lacks dedicated hygroreceptors; instead, wetness is thought to be a learnt sensory experience, arising from repeated exposure to wet stimuli. When moisture is present on the skin, a complex integration of multisensory inputs occurs, primarily involving thermal cues generated by heat transfer and evaporation, combined with tactile cues such as pressure and friction. Together, these inputs create the subjective experience of wetness.
Beyond its evolutionary significance, wetness perception is also important for product comfort and acceptability, particularly during the application of antiperspirant deodorants. The underarm has unique biophysical characteristics, including hair coverage, high sweat gland density, and variations in stratum corneum hydration, which may impact sensory response. During application, users may experience transient, unwanted sensations of wetness arising from product movement across the skin and thermal exchange between the formulation and the skin surface. Despite the industrial and sensory relevance of this interaction, neither the underarm region nor antiperspirant formulations have been systematically investigated within the field of wetness perception.
Following a comprehensive literature review, a series of controlled experimental studies were conducted using modulated external wet stimuli via a temperature-controlled probe to the underarm. These studies were designed to isolate and replicate key components of antiperspirant application (i.e. time-dependent changes, Chapter 6 - 8), user hygiene routines (i.e. the presence and removal of hair [Chapter 5] and skin hydration [Chapter 4 and Chapter 8]) and fluid formulation (i.e. thermal conductivity, Chapter 6 - 8) to understand the mechanisms of the skin-moisture interactions.
Collectively, this PhD demonstrates that wetness perception at the underarm is governed by an interaction between skin biophysical properties (hydration state and hair coverage) and the thermal properties of applied fluids, mediated primarily by changes in skin cooling. Established wetness mechanisms were confirmed at the underarm, with cold-wet stimuli perceived as wetter and overhydration amplifying wetness during cold contact. Underarm hair was shown to influence wetness, acting as a barrier during static contact while enhancing tactile contributions during dynamic interactions. Methodologically, the development of a continuous wetness perception test enabled quantification of the temporal dynamics of wetness with integrated skin temperature responses, revealing that the rate and magnitude of skin cooling—rather than fluid thermal properties alone—drive wetness perception differences. Together, these findings established and refined a time-dependent model of wetness perception, in which temperature-driven experience is shaped by local biophysical characteristics and fluid thermal properties. Collectively, these results can inform the design of antiperspirant deodorants for improved comfort upon application.

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Identifiers

ORCID for Jade Ward: orcid.org/0000-0003-2304-1342

ORCID for Davide Filingeri: orcid.org/0000-0001-5652-395X

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