Humans often experience wet stimuli using their hands, yet we know little on how sensitive our fingers are to wetness and the mechanisms underlying this sensory function. We therefore aimed to quantify the minimum amount of water required to detect wetness on the human index fingerpad, the wetness detection threshold, and assess its modulation by temperature. Eight blinded participants (24.0 ± 5.2 y; 23.3 ± 3.5 BMI) used their index fingerpad to statically touch stimuli varying in volume (0, 10, 20, 30, 40 or 50 ml) and temperature (25, 29, 33 or 37 °C). During and post contact, participants rated wetness and thermal sensations using a modified yes/no task and a visual analogue scale. The wetness detection threshold at a moisture temperature akin to human skin (33 °C) was 24.7 ± 3.2ml. This threshold shifted depending on moisture temperature (P = 0.002), with cooler temperatures reducing (18.7 ± 3.9ml at 29 °C) and warmer temperatures increasing (27.0 ± 3.0ml at 37 °C) thresholds. When normalised over contact area, the wetness detection threshold at 33 °C corresponded to 1.926x10-4 ml mm-2 (95% CI: 1.873x10-4, 1.979x10-4 ml mm-2). Threshold differences were reflected by magnitude estimation data, which were analysed using linear regression to show that both volume and moisture temperature can predict magnitude estimations of wetness (P < 0.001). Our results indicate high sensitivity to wetness in the human index fingerpad, which can be modulated by moisture temperature. These findings are relevant for the design of products with wetness management properties.

Hygroreceptors are a type of humidity sensor that have been identified in several invertebrate classes including Insecta and Arachnida. While their structure has been well researched, the nature of the mechanisms behind their function is debated as being either mechanical, evaporative, or psychrometric in insects and potentially also olfactory in arachnids. There is evidence that can be used to support or oppose each of these concepts, which also invites the possibility of multiple unified mechanisms occurring together. The integration of multiple sensory modalities has also formed the foundation of wetness perception in humans, led by thermal and tactile cues with supplementary information from vision and sound. These inputs are integrated by a vast neural network in the brain, which also occurs on a smaller scale in insects and arachnids. It is possible that as cerebral capacity increased throughout human evolution, this facilitated a preferable system of wetness perception via multisensory integration and rendered hygroreceptors obsolete. While this cerebral development hypothesis is only speculative, it gives a framework for further investigation. Additional research needs to be conducted to correctly classify hygroreceptor types in invertebrates and their relative prevalence before evolutionary associations can be made with vertebrate species. This integratory premise also applies to the human system, as knowing the relative contribution and compounding effects of each sensory modality on wetness perception will aid the overall understanding of the system and help to uncover the evolutionary development pathways underpinning each sense.

Mechanosensory inputs arising from dynamic interactions between the skin and moisture, such as when sliding a finger over a wet substrate, contribute to the perception of skin wetness. Yet, the exact relationship between the mechanical properties of a wet substrate, such as friction, and the resulting wetness perception, remains to be established under naturalistic haptic interactions. We modelled the relationship between mechanical and thermal properties of substrates varying in moisture levels (0.49x10-4; 1.10x10-4; 2.67x10-4 ml mm-2), coefficient of friction (0.783, 0.848, 1.033, 0.839, 0.876, 0.763), and maximum thermal transfer rate (Qmax, ranging from 511 to 1260 W m-2 K-1), and wetness perception arising from the index finger pad's contact with such substrates. Forty young participants (20M/20F) performed dynamic interactions with 21 different stimuli using their index finger pad at a controlled angle, pressure, and speed. Participants rated their wetness perception using a 100 mm visual analogue scale (very dry to very wet). Partial least squares regression analysis indicated that coefficient of friction explained only ~11% of the variance in wetness perception, while Qmax and moisture content accounted for ~22% and 18% of the variance, respectively. These parameters shared positive relationships with wetness perception, such that the greater the Qmax, moisture content, and coefficient of friction, the wetter the perception experienced. We found no differences in wetness perception between males and females. Our findings indicate that while the friction of a wet substrate modulates wetness perception, it is still secondary to thermal parameters such as Qmax.

Carrying babies in a sling, that is, babywearing, is a popular practice among new parents. Babies are thermally vulnerable and public health bodies advise to dress them in one extra layer than the adult. However, these guidelines do not consider babywearing and it is unclear whether babies’ clothing insulation should be modified during babywearing. Here we quantified the effects of babies’ clothing layers on the thermoregulatory responses to short duration babywearing in babies under 12 months old. Nine babies (4F/5M; 7.3 ± 3.1 months; 9 ± 2.5 kg) and 9 mothers (34 ± 3.0 years) performed two trials in a thermoneutral environment (23°C; 50%RH). During trials, babies wore either 1 (sleepsuit) or 2 (vest + sleepsuit) clothing layers, and mothers performed 15‐min stepping exercise while babywearing. We recorded mothers and babies’ tympanic temperature (Tty), babies’ local skin temperatures (Tsk; on the carotid artery area, arm, abdomen, lower back, and thigh), and mothers’ perception of babies’ thermal state. Babies’ Tty did not change after 15‐min babywearing (mean change: −0.13°C [−0.30, +0.05]; p = .141), in either clothing trial (difference between trials: +0.05°C [−0.15, +0.25]; p = .591). On the contrary, local Tskin increased across all sites tested (mean increase = +0.71°C [+0.41, +1.01]; p = .038) and similarly between clothing trials, with the abdomen showing the largest change (+1.10°C [+0.32, +1.85]). Mothers did not perceive any change in babies’ thermal state. We show that 15‐min babywearing increase babies’ skin, but not tympanic, temperature by up to 1.1°C on certain body regions, and that this effect is not exacerbated by adding 1 layer of light clothing to the baby.