Skin wetness detection thresholds and wetness magnitude estimations of the human 1 index fingerpad and their modulation by moisture temperature. 2

9 Humans often experience wet stimuli using their hands, yet we know little on how sensitive our fingers are to 10 wetness and the mechanisms underlying this sensory function. We therefore aimed to quantify the minimum 11 amount of water required to detect wetness on the human index fingerpad, the wetness detection threshold, 12 and assess its modulation by temperature. Eight blinded participants (24.0 ± 5.2 years; 23.3 ± 3.5 body mass 13 index) used their index fingerpad to statically touch stimuli varying in volume (0, 10, 20, 30, 40 or 50 ml) 14 and temperature (25, 29, 33 or 37 °C). During and post contact, participants rated wetness and thermal 15 sensations using a modified yes/no task and a visual analogue scale. The wetness detection threshold at a 16 moisture temperature akin to human skin (33 °C) was 24.7 ± 3.48ml. This threshold shifted depending on 17 moisture temperature (R = 0.746), with cooler temperatures reducing (18.7 ± 3.94ml at 29 °C) and warmer 18 temperatures increasing (27.0 ± 3.04ml at 37 °C) thresholds. When normalised over contact area, the wetness 19 detection threshold at 33 °C corresponded to 1.926x10 -4 ml mm -2 (95% CI: 1.873x10 -4 , 1.979x10 -4 ml mm -2 ). 20 Threshold differences were reflected by magnitude estimation data, which were analysed using linear 21 regression to show that both volume and moisture temperature can predict magnitude estimations of wetness 22 (R = 0.949; R = 0.179). Our results indicate high sensitivity to wetness in the human index fingerpad, which 23 can be modulated by moisture temperature. These findings are relevant for the design of products with wetness management properties. sensations in human wetness perception, the maximum sensitivity of our wetness sensing system remains to be established. This research presents a novel methodology, which for the first time, has quantified the high 31 sensitivity of the human index fingerpad to wetness and its modulation by moisture temperature.

collected for every participant. Larger, more robust thermocouples did not provide sufficient resolution and 147 so were not used. All sessions were conducted in a thermoneutral environment (23.9 ± 0.8 °C, 37 % RH). 148 During sessions participants assumed a seated position and were blinded to the experimental setup using an 149 L-shaped obscuring screen to limit visual cues. Additionally, auditory cues including stirring and pouring of 150 saline were systematically added every 4 minutes during stimulus preparation procedures to counteract any 151 associative learning effects or bias in results. This was preferable to blocking auditory cues entirely, such as 152 via ear defenders, as this would have interfered with the verbal commands used to direct participant 153 interactions. Participants were allowed to take short self-governed breaks during the session, and were only 154 permitted to consume water during this time. 155 156

Experimental Protocol 157
Prior to the start of each experimental session, a 0.9 % saline solution (8.46 ± 0.02 g NaCl; 1200 ± 10 ml 158 H 2 O) was prepared to mimic the ionic composition of infant urine and therefore be absorbed onto the 159 substrate optimally. The intended application temperatures were 25 °C, 29 °C, 33 °C and 37 °C, with the 160 former two being within and just above the activation range of cold receptors (Filingeri 2016) and the latter 161 two reflecting average skin temperature and average core temperature respectively. To account for heat 162 losses during sample preparation and while the dwell time elapsed, each solution temperature was 163 maintained using a small manually controlled thermal chamber at either 25.1 °C, 29.2 °C, 33.4 °C or 37.7 °C 164 ± 0.1 °C in the four respective sessions. These temperatures were established during initial sample 165 classification studies using the same stimulus preparation protocols and experimental conditions as the full 166 study, but across a wider range of moisture temperatures, with thermocouples embedded to monitor the 167 thermal equilibration patterns towards room temperature. 168 169 Different volumes of saline solution were applied to individual stimuli to moisten them prior to interaction 170 with a participant. The stimuli comprised of the superabsorbent core and associated layers from the centre of 171 a diaper. This 'centre' was cut from the elasticated diaper chassis such that it laid flat, as opposed to curving 172 with the body as designed. Cuts were made such that the superabsorbent core was not ruptured, and no 173 internal material was lost or made prone to leakage. This resulted in a 115 mm x 325 mm rectangular sample.
Each volume was applied to the substrate using a custom-made acquisition plate (Figure 1). This was formed 178 of a plastic frame and foam stage upon which the sample would be placed, followed by a flat plate with 179 aperture tube above it. When the diaper was aligned correctly between the terminal markers, the tube was 180 positioned directly above its centre. The desired volume of solution was then applied via the aperture tube 181 using a graduated plastic syringe (SS+50ES1, Terumo, Leuven, Belgium). After the solution had been 182 absorbed from the aperture tube, the sample was allowed to rest for a period of 20 s, termed the dwell time. 183 This period effectively allowed the solution applied to the topsheet to be absorbed by the acquisition layers 184 and wicked away from application area to ensure a uniform distribution. shown. As the stimuli were combinations that demonstrated the extremes of each condition across the 194 experimental sessions, they provided a frame of reference for the study in addition to acquainting 195 participants with the study protocols. 196 minimising inter and intra-individual variability (Stevens and Choo 1998). When the stimulus had been 204 prepared as aforementioned, the participant was given the command 'contact'. They would then move their 205 hand from the thermoneutral plate and make contact with the stimulus at a static resting pressure. 206 207 Participants had previously been instructed on such commands, and the correct orientation of the finger 208 demonstrated and practiced. Additionally, the stimulus was always positioned correctly below the finger and 209 moved before contact when necessary. At the point of contact participants would immediately complete a 210 digital perceptual form based on the during contact interaction within 3 s. Both a dichotomous response 211 method (dry / wet, cold / warm) and a 100 mm visual analogue scale (very dry to very wet, cold to warm) 212 were employed. The dichotomous response paradigm used a binary scoring system for subsequent analyses, 213 with a 'dry' response designated as 0 and a 'wet' response a 1 for wetness perceptions. Similarly, a 'cold' 214 response was coded as 0 and a 'warm' response as 1 for thermal assessments. 215

216
After a contact period of 3 s the participant was prompted to remove their finger from the stimulus using the 217 command 'lift'. Post contact perceptual assessments identical to those used in the during contact interaction 218 were be completed, again within 3 s. Participants indicated completion using the word 'done', at which point 219 the stimulus would be removed and replaced with a cotton towel. The participant was then instructed 'dry', 220 and would statically press their index finger on to a dry cotton towel to collect residual water for 5 s. This 221 was repeated for all stimuli regardless of wetness to prevent any learning effect or bias. In between this 222 stimulus and the next, the index finger was returned to the thermoneutral plate to maintain T sk at 33 °C. The 223 period in which the finger was on the thermoneutral plate also served as a nervous refractory period lasting a 224 minimum of 20 s, during which time the next stimulus was prepared before repeating the protocol. 225 226

Threshold Determination 227
Individual thresholds were determined using a modified dichotomous response method. There are two 228 methods typically employed in threshold determination, which are considered to be equally effective in 229 different physiological measures. A classic two-alternative forced choice method allows participants to 230 choose which of two stimuli correspond best to a single descriptor, whereas the associated yes/no task 231 involves only a single stimulus to which either a positive or negative response must be assigned (Green Coding the dichotomous responses as 0 or 1 allowed an average response ratio to be generated at each 236 applied volume. These ratios were plotted across all applied volumes and fitted with a logistic sigmoidal 237 curve; an s-shaped fit typically used to establish thresholds. The point at which the curve crossed 0.5 was 238 decided as the detection threshold, on the basis that approximately half of values would feel dry, subceeding 239 the threshold. and half would feel wet, exceeding it. However, sigmoidal curves generated across the test 240 temperatures in the pilot studies indicated that the wetness detection thresholds for all temperatures lay 241 between 15 ml and 35 ml, which falls at the lower end of the range of tested volumes of 0 ml -125 ml. 242 Beyond this the curves peaked and plateaued, resulting in a poor overall fit. Despite this, there was still a 243 notable difference in overall perception across temperatures, with lower temperatures being associated with 244 lower wetness detection thresholds. This data allowed the secondary volume range of 0 ml -50 ml to be 245 established, centring the range around the proposed 0.5 threshold by providing a roughly equal quantity of 246 stimuli below and above threshold (Figure 2), hence providing a balanced design and validating the use of 247 the chosen threshold value (Filingeri et al. 2013). Additionally, a this informed setup promotes a superior fit, 248 allowing a higher resolution surrounding the detection threshold to be achieved in the full experiment, and 249 also reduces anticipatory bias that could be expected by participation in a 'wetness perception' study, which 250 inherently implies the presence of moisture in stimuli. (n = 1), as determined via the dichotomous response method using sigmoidal curves. Note the equal numbers 254 of data points above and below threshold. Each data point represents x̄ ± 95% CIs wetness response from 255 twelve repeat stimuli presented to a participant at a specific applied volume and temperature. 256 257

Relative Threshold Determination 258
Following the collection of perceptual and physiological participant data the established absolute wetness 259 detection threshold values were converted into relative values, which relied on several principles. The first of 260 these was the isolation of the topsheet of the diaper, which is the uppermost layer in contact with the participant's finger and therefore the acting interface between skin and moisture. Leading on from this, the 262 level of moisture contained within the topsheet itself was critical. By establishing the surface area which it 263 occupied and the corresponding change in weight of the topsheet, the relative water retention could be 264 calculated within a given area. corresponds to an applied volume. As the absolute wetness detection threshold was calculated in terms of an 289 applied volume it can be inputted as X. This generates a Y value for surface wetness in the topsheets, which 290 can be considered as the calibrated relative detection threshold.

Relative surface wetness (ml mm -2 ) = gradient x absolute threshold (ml) + intercept 295
Equation 2: Calculation of surface wetness in the topsheet using a previously generated linear regression 296 equation. 297 298

Statistical Analyses 299
In this study, the independent variables were the temperature of the stimuli (25 °C, 29 °C, 33 °C, 37 °C) and 300 the applied volume (0 ml, 10 ml, 20 ml, 30 ml, 40 ml, 50 ml). The dependent variables were absolute 301 wetness detection thresholds, wetness perception (mm), thermal sensation (mm) and fingerpad T sk (°C). 302 Absolute wetness detection thresholds are specified in ml, relative wetness detection thresholds are specified 303 in ml mm -2 . All data was tested for normality of distribution and homogeneity of variances using Shapiro-304 Wilk and Levene's tests respectively. In cases where the assumptions of these tests were violated, parametric 305 means-based tests were nonetheless applied as they best fit the required analyses of the datasets. All 306 statistical data reported in text are means (x̄) ± standard deviation (SD), with means and 95% confidence 307 intervals (95% CIs) given in figures unless otherwise stated; α = 0.05. All statistical analyses were conducted 308 During contact absolute wetness detection thresholds were established using dichotomous data, by plotting 313 mean binary perceptual scores from each participant against applied volume. For example, 5 dry responses 314 and 7 wet responses would generate a value of 0.42. This was analysed as part of a logistic sigmoidal curve 315 fit using a lapse rate of zero, assumed given the responses from pilot studies. The point at which the response 316 rate exceeds 0.5 (50 %) indicates that the data is no longer due to chance, and so can be considered threshold. Threshold values were established for each of the eight participants at each of the test temperatures and 318 converted into relative values. The mean of these individual thresholds was subsequently calculated to give 319 an overall mean relative wetness detection threshold for each test temperature. To investigate the influence of applied temperature on during contact absolute wetness detection thresholds, a parametric one-way Magnitude estimation data was used to assess the influence of applied volume and applied temperature 324 during and post contact wetness perception. These were processed using linear regression in conjunction 325 with a two-way ANOVA. The difference between during and post contact wetness perceptions was then 326 compared using paired t-tests. Further to this, magnitude estimation data was also used to assess the 327 influence of applied volume and applied moisture temperature on during and post contact thermal 328 perceptions. The data was initially analysed using a two-way ANOVA with post-hoc pairwise Tukey tests to 329 establish whether both during and post contact thermal perceptions significantly differed at different applied 330 volumes and moisture temperatures respectively. A linear regression analysis was then conducted to assess 331 the overall contribution of applied volume and applied moisture temperature on during and post contact 332 thermal perception. The post contact thermal perceptual data was then compared to during contact, and the 333 relationship analysed using a series of paired t-tests. 334 335 T sk data from thermocouples was plotted to validate applied temperatures and time intervals. Additionally, 336 the average T sk at each temperature was compared during and post contact with a paired t-test. Linear 337 regression analyses were used to assess the relationship between during and post contact T sk in relation to 338 during and post contact thermal perceptions. Finally, a linear regression analysis was conducted to assess the 339 independent and interactive influence of applied volume and moisture temperature on the magnitude 340 estimation of wetness perception.  relationship between applied moisture temperatures and wetness detection thresholds was analysed in 371 absolute terms as larger temperatures are more stable. Additionally, any uncertainties in the surface area of 372 topsheet liquid content used to determine relative thresholds will be magnified in this form of analysis. 23.0 ± 3.17 1.918 x 10 -4 ± 6.67 x 10 -5 33 24.7 ± 3.48 1.926 x 10 -4 ± 6.30 x 10 -5 37 27.0 ± 3.04 1.933 x 10 -4 ± 6.14 x 10 -5

Magnitude estimation of wetness perception 378
The effects of applied volume and applied temperature on during contact wetness perception were reflected 379 in magnitude estimation data. Applied volume had a proportional relationship with perceived wetness 380 perception, such that greater volumes resulted in higher wetness perceptions. Conversely, applied 381 temperature was inversely proportional to wetness perception such that lower temperatures resulted in 382 greater wetness perceptions ( Figure 6). The data showed a statistically significant difference between 383 perceived wetnesses at different applied volumes (F 5, 35 = 3010; P < 0.001) and applied temperatures (F 3, 21 = 384 177; P < 0.001). Applied volume accounts for 84.0 % of variance in during contact wetness perceptions 385 whereas applied temperature only accounts for 2.96 %. Even so, there is a significant interaction between the 386 two factors such that they act synergistically to produce a compounding effect (F 15, 105 = 3.73; P < 0.001) 387 which accounts for 0.31 % of variance. 388 Thermal perceptions also varied in post contact interactions, with overall differences between during and 457 post found to be significant at all temperatures (25 °C, t 7 = -5.79, P < 0.001; 29 °C, t 7 = -11.4, P < 0.001; 33 458 °C, t 7 = 9.26, P < 0.001; 37 °C, t 7 = 13.6, P < 0.001). In post contact interactions, perceptions tended back 459 towards neutrality once the finger was lifted. The difference in during and post contact perception was 460 proportional to the difference of the applied temperature from neutrality, with changes of -2.0 mm (CIs = - When Tsk was associated with wetness perception during contact, similar trends could be identified in terms 489 of responses at 25 °C and 29 °C sharing similar perceptual magnitudes despite changes in T sk (Figure 10). 490 The aforementioned influence of applied volume can also be highlighted in relation to T sk .

Wetness detection thresholds 510
Use of a dichotomous response method enabled the determination of the relative wetness detection threshold 511 of the human index finger at a moisture temperature akin to human T sk (33°C). Application of moisture at 512 different temperatures modulated the threshold such that it was lower at cooler temperatures and higher at 513 warmer temperatures, effectively showing that a greater quantity of moisture is required to illicit the 514 sensation of wetness as temperature increases. From a physiological perspective, this implies that humans are 515 more sensitive to wetness at cooler moisture temperatures. In this case wetness was experienced at higher 516 temperatures in addition to those below skin temperature, albeit at a lower intensity. This shows that there 517 must be other factors involved in wetness perception and supports previous mechanistic work. For example, 518 cold cues have been associated with wetness in the absence of physical wetness, when using a nerve block 519 (Filingeri et al. 2014) and in the absence of physical skin cooling (Typolt and Filingeri 2020). As visual and 520 auditory cues were removed or controlled in the present study, this brings a focus back on to the roles of 521 haptics. Although interactions were static in this case, there is potential for characteristics such as adhesive 522 forces and textural changes to play a role in wetness perception, which requires further investigation. 523 524

Magnitude estimation of wetness perception 525
In relation to magnitude estimation data, similar trends were seen in terms of greater volumes being 526 perceived as wetter. Additionally, the effect of applied temperature on wetness perception seems similar to 527 that of the dichotomous response method, with wetness sensation being inversely proportional to applied 528 moisture temperature. Although dichotomous response and visual analogue scale methods show that the 529 magnitude of difference in wetness perceptions is proportional to the magnitude of difference in volume, the 530 trend is more prominent in the latter method. This is perhaps because of the higher resolution of visual 531 analogue scales, which effectively allow variability to be more prominent. 532

533
The relationship between applied volume, moisture temperature and wetness perception is likely a product of 534 multimodal transduction in A -type somatosensory afferents, which account for cold cutaneous sensations 535 and are strongly linked to the perception of wetness (Filingeri et al. 2014). Their collective response may be 536 exacerbated by application of larger volumes in the preparation of stimuli. This may be caused by several 537 factors, such as there being a larger concentration of moisture retained in the stimulus such that a higher 538 proportion of receptors in a given area can be triggered in a form of spatial summation (Stevens et al. 1974). 539 It was also considered that changed thermodynamics at larger applied volumes may result in more stable 540 temperatures and hence alter the physical temperature at the point of interaction, but this difference between 541 prepared temperature and temperature at the point of contact was accounted for using data from initial 542 sample classification studies, as noted in the methodology.
Interestingly, magnitude data has also shown positive wetness responses in dry conditions, with the neutral 545 and warm values of 33 °C and 37 °C predicting low perceived wetnesses and the cooler values of 29 °C and 546 25 °C resulting in slightly higher wetness perceptions. While this variation in wetness perception across 547 different temperatures is largely consistent with data in which liquid is physically applied, it is interesting 548 that the same principles can be applied in the absence of physical wetness such that cooler temperatures still 549 illicit greater wetness sensations. While existing studies have shown this influence of cold thermal inputs on 550 wetness perception (Filingeri et al. 2014), the fact that there is still a positive response at the warmer 551 temperatures despite the absence of physical wetness affirms that there are more factors involved in wetness 552 perception. The importance of thermal cues is also reflected in the slightly different linear regression 553 gradient associated with perceived wetness and applied volume at 33 °C. As neutrality effectively lacks any 554 form of thermal cues, neither cooling nor warming, this is likely to have caused an uncertainty in judgement 555 which resulted in a steeper gradient compared to other temperatures. 556 Overall, at all applied moisture temperatures, stimuli were perceived as drier post contact. As colder 558 sensations are associated with an increase in wetness perception, it could be expected that wetness 559 perceptions would actually have increased as a result of the finger being lifted and temperatures reducing 560 post contact. This would have been more prominent at the neutral and warm temperatures of 33 °C and 37 °C 561 respectively, as the higher temperatures would result in the participant experiencing a greater thermal 562 gradient towards ambient, in addition to evaporative cooling. The temperatures below neutral, 25 °C and 563 29 °C, would have also been subject to some evaporative cooling, but the skin would also have increased in 564 temperature towards neutral when in the ambient air and no longer in contact with the stimulus, effectively 565 negating the change. Despite this, each applied moisture temperature was perceived to be drier post contact. 566 While this interpretation of dryness post contact is factually correct, it was thought that the associated 567 evaporative cooling cues would effectively counteract or even override the dryness experienced, especially at 568 temperatures above neutral. The fact that it does not implies that additional cues beyond thermal inputs are 569 also involved in the sensory feedback mechanism. 570 The effect of cooling cues on wetness perception can be further investigated by observing the corresponding 573 thermal perceptions and physical temperature data surrounding interactions. In during and post contact 574 interactions, applied volume and applied moisture temperature were found to have a significant effect on 575 thermal perceptions. As can be expected this effect is mainly attributed to the applied temperature, but 576 interestingly the interactive effect of applied temperature and volume was greater than the effect of volume 577 alone. This may result from changed thermodynamics at larger applied volumes, coupled with the greater 578 proportion of receptors being triggered in a given skin surface area as stimuli concentration increases, 579 resulting in spatial summation and a heightened collective response. At a neutral temperature there is very 580 little change in thermal perception across the volumes, which can be expected as there is effectively no 581 directional change in thermal sensation that can be exacerbated by volume. This gives further evidence 582 towards aforementioned linear regression gradients between wetness perception and applied volume, at 583 which 33 °C was steeper than other temperatures, as this is resulting from thermal sensations. Interestingly, 584 the limited difference in thermal perception between 25 °C and 29 °C may account for the lack of significant 585 differences between the absolute wetness detection thresholds at those temperatures, which is also reflected 586 in wetness perception magnitude data at all volumes between these temperatures. This gives further evidence 587 to the inherent link between thermal sensation and wetness perception, which is exacerbated by volume. 588

589
After expressing all thermal perception data as a deviation from the midpoint of the scale, the proportional 590 influence of applied temperature, applied volume and their interaction shifted. As the basis for this data 591 transformation was to align the direction of perceptual magnitudes changes and negate cancelling from their 592 opposing directions, it could be easily predicted that the influence of volume would increase in relation to 593 other variables. However, there is little to justify such a dramatic decrease in the influence of applied 594 The magnitude estimation data also showed a significant difference in thermal perception between during 601 and post contact interactions. In each case the difference in thermal perceptions between during and post 602 contact interactions was proportional to the magnitude of the applied temperature from neutrality. From a 603 thermal perspective, this is because after the finger is lifted to perform post contact assessments its 604 temperature will change as it equilibrates with the ambient air. This rate of change varies greatly depending 605 on the initial temperature, as was demonstrated in thermal profiles. For example, after interaction with a cool 606 stimulus the finger temperature would instead increase back to neutrality, and as such the two cooler 607 temperatures of 25 °C and 29 °C were perceived as warmer post contact, effectively overcoming any 608 influence of evaporative cooling. Conversely, after interaction with a warm stimulus the finger temperature 609 begins to decrease back to the point of neutrality, which was shown in the two warmer stimuli of 33 °C and 610 37 °C being perceived as cooler. 611

612
It should be noted that all stimuli were perceived as drier post contact despite thermal perceptions being 613 varied in directionality. As thermal inputs have been shown in a range of research to have a large influence 614 on wetness perceptions, a concept that was reflected in the wetness detection threshold data, it could be 615 expected that the associated thermal perceptions would have had a greater influence in this case. While the 616 stimuli were correctly perceived as being drier post contact, the evidence from thermal perceptions again 617 implies that additional cues are involved in the sensory feedback mechanism and add further complexity to 618 the processing and interpretation of wetness perception. 619 620

Multiple regression analysis 621
Physical and perceptual factors both contributed to a statistically significant relationship within a linear 622 regression model, accounting for 85.5 % of total variance in during contact wetness perceptions. This is to be 623 expected as temperature and volume form an integral part of the physical sensations associated with wetness 624 perceptions, as previously discussed. From the linear regression equation, it can be seen that volume has a 625 greater effect than temperature. This can be expected on a fundamental level, as larger applied volumes 626 typically result in a greater surface area such that a larger quantity of mechanoreceptors and thermoreceptors 627 in the skin will be activated. However, in this case all surface areas resulting from the application of moisture were sufficient to cover the average index fingerpad in females, which typically ranges from 78 mm 2 at rest Instead, the main variation caused by the application of different volumes will be the quantity of applied 632 liquid retained in the topsheet. This is effectively the concentration of moisture contained within a given 633 area, termed the surface wetness, and increases with higher volumes. Those stimuli with higher surface 634 wetnesses will have a greater number of contact points with the skin and therefore there is a higher 635 likelihood of activating thermoreceptors or even mechanoreceptors in the skin, resulting in spatial 636 summation (Raccuglia et al. 2017). Therefore, the increase in liquid volume giving rise to increased thermal 637 stimulation may in turn effect the perceived level of wetness, which highlights an interactive effect between 638 them. This greater level of liquid will also result in a slower thermal loss due to a lower surface area to 639 volume ratio, and can alter thermodynamics such that there is decreased thermal conductance at larger 640 applied volumes. Conversely, if only a relatively small quantity of liquid is transferred to the finger from the 641 stimuli such that a thin liquid layer is present on the skin, it is more susceptible to evaporative cooling 642             23.0 ± 3.17 1.918 x 10 -4 ± 6.67 x 10 -5 33 24.7 ± 3.48 1.926 x 10 -4 ± 6.30 x 10 -5 37 27.0 ± 3.04 1.933 x 10 -4 ± 6.14 x 10 -5