Stratum corneum compliance enhances tactile sensitivity through increasing skin deformation: A study protocol for a randomized controlled trial

Tactile sensation plays a crucial role in object manipulation, communication, and even emotional well‐being. It has been reported that the deformability of skin (also described as skin compliance) that shows a large mechanical response to stimuli is associated with high tactile sensitivity. However, although the compliance of the stratum corneum, the outermost layer of skin, can change daily due to skin care and environmental factors, few studies have quantified the effect of the stratum corneum on tactile sensation.


| INTRODUC TI ON
Tactile sensation is essential to our interaction with the world and communication with others and can even affect our emotional well-being. 1,2Our ability to perceive touch influences the sense of comfort and relaxation felt when applying skin care products to the cheeks. 3Therefore, maintaining optimal tactile sensation is crucial for our daily lives.
Tactile sensation occurs when tactile stimuli are mechanotransduced through the skin to mechanoreceptors and sensory nerves and transmitted to the brain.5][6] The influence of skin's physical properties on our sense of touch is well established, mainly through hydration interventions; for example, hydration enhances spatial discrimination in individuals with skin with an increased water content, 7 while an increase in the friction coefficient, calculated from the tangential and vertical forces across the finger-texture contact surface, is associated with improved texture discrimination ability. 8In addition to short-term interventions, long-term use of skin care products enhanced tactile discrimination ability. 9These changes are thought to be due to the changes in the mechanical responsiveness of the skin caused by hydration, as greater skin deformation in response to tactile stimuli leads to heightened tactile sensation. 10,11Indeed, the application of different types of water (distilled or surfactant) to human skin can alter the dynamic elastic modulus of skin 12 as well as the tactile sensation in different way. 13Thus, skin deformability, that is, compliance, as determined by the water content of the skin, is an important factor in tactile sensation.The compliance of the stratum corneum, the outermost layer of the skin, can vary daily due to skincare and environmental factors. 14,15However, few studies have quantified the effect of stratum corneum compliance on tactile sensation.It is crucial to examine the specific changes in stratum corneum compliance resulting from hydration interventions to gain deeper insights into tactile sensation.
In this study, we investigated the changes in tactile sensitivity resulting from skin hydration and verified the corresponding alterations in the compliance of the stratum corneum.We performed hydration interventions on the stratum corneum with a cosmetic treatment.The impact of the hydrating effect on this skin layer was evaluated by measuring the water content at different skin depths using two instruments, 16 while compliance was evaluated by isolating the stratum corneum and measuring its dynamic elastic modulus. 12By employing suction as a tactile stimulus, we were able to determine the contribution of the stratum corneum to tactile sensation without the influence of deep muscle or fat. 17,18 this investigation, we accounted for the presence of factors other than compliance that can influence tactile sensation.The effect of hydration on tactile sensation depends on the participant's age and sex, 19 and even within an individual, the tactile sensation itself varies according to the stimulated site due to the complexity of skin tissue that exhibits markedly anisotropic and heterogeneous responses. 20Therefore, we used a device that can measure skin deformation while simultaneously presenting tactile stimuli at the same site on the skin by suction. 11,21,22This approach went beyond observing the compliance of the stratum corneum, allowing us to explore how it influenced the mechanism of tactile sensation.
Measuring the skin displacement caused by stimulation in correspondence with the tactile response is a pioneering approach in the field of tactile research and also a well-established technique for quantifying skin properties. 23

| Participants
Thirty-nine female participants (mean age 45.3 ± 6.0 years) took part in the psychophysical experiments.The participants were required to meet the inclusion criteria shown in Table 1.Individuals were informed about the purpose of this study and gave their written informed consent to participate.The participants were informed that they could quit the experiment at any time if they so wished.The Ethics Committee of Shiseido Research Center approved this study, and all methods were carried out in accordance with the approved guidelines.
perception.The compliance of the thin stratum corneum layer plays a crucial role in tactile experiences that involve skin stretching.

| Apparatus
We used a previously developed device that applies quantitative suction stimulation to the skin while simultaneously measuring skin deformation. 21,22A voice-coil linear actuator (H2W Technologies, VMS30-090-LB-1) compressed and expanded the air in an air cylinder (SMC, MQQLL30-100DM).The generated oscillations were transmitted to the contactor through an air tube, and the skin was stimulated by negative pressure through the suction hole of the contactor.The contactor had a 4 mm diameter suction hole in the center.
During the measurements, a spring-loaded contact force adjustment mechanism was used to maintain a constant pressing force on the skin.By applying laser light with a 2D laser displacement sensor (Keyence LJ-V7080) attached to the inside of the contactor, the skin deformation caused by the suction stimulation could be measured.
The deformed shape of the skin on the irradiation line of the laser at the suction hole was observed (Figure 1).A pressure sensor (PISCO, VUS11-AR) was attached to the air tube to quantify the pressure applied to the cheek.

| Experimental location and study design
The study was performed in a room with a constant temperature and humidity; the temperature was 22.0°C, and the relative humidity was 45.0%.A randomized controlled trial was conducted.The cheek was chosen as the experimental site because it provides a valuable opportunity to examine skin physical properties on a tactile sensitive area with hair 24 and considers the significant variations in skin physical properties caused by sun exposure and skin care and massage practices.The participants were randomly assigned to the intervention (n = 20, 45.2 ± 6.0 years) or control (n = 19, 45.5 ± 5.9 years) groups, which differed in terms of skin hydration conditions.6][27] A cream containing 0.2% polyethylene glycol/polypropylene glycol ethers 28 with a skin conditioning agent and emollient function was selected as the common moisturizer for the intervention group, and Milli-Q water was chosen for the control group.The amount of product applied was 600 μL for the cream and 2 mL for the Milli-Q water based on the appropriate doses of cream and lotion for the whole face.Milli-Q water was used in the control group rather than no substance to eliminate changes in skin mechanical properties due to massage and any effects of the application.
The participant group assignment and application were performed by a third party in a separate room in a double-blind approach in which neither the experimenter nor the participant knew what substance was being applied to the cheeks.The participants first washed their faces, and after a 10-min rest period, tactile and skin mechanical properties were measured and recorded as preapplication conditions.Cream or Milli-Q water was then applied to the cheeks; after a 10-min waiting period, the skin was washed with lukewarm water to rinse the applied material off the skin surface.
After another 10-min break, the tactile sensitivity and skin mechanical properties were measured and recorded as postapplication conditions.

| Measurement of tactile sensation in response to suction oscillation stimuli
Both groups of participants completed the psychophysical experiments.The difference threshold of the mechanical stimulus was used as an evaluation index of tactile sensitivity.To reduce evaluation bias, a method of constant stimulus 29 was used as the experimental approach.In this method, the difference threshold is obtained by repeatedly comparing a standard stimulus with a comparison stimulus.Negative pressures were generated as the presented stimuli at five intensity conditions (3.5, 4.8, 6.7, 9.3, and 12.5 kPa) above the stimulus threshold.The comparison stimuli included these five conditions, and the standard stimulus was the third intensity stimulus.To better reflect changes in skin physical properties, we used 10 Hz oscillatory stimuli that produced repetitive skin deformations.
We asked the participants to wear earphones through which white noise was played to eliminate auditory influence during the stimulus presentation.First, the participants were asked to place their faces on the chin rest so that the stimulus point remained constant.The stimulation point was the intersection point of a vertical line drawn from the outer corner of the left eye and a horizontal line drawn from the tip of the nose.In one trial, the standard and comparison stimuli were presented once each with a stimulus duration of 3.5 s, and the interval between the standard and comparison stimuli was 1 s.The participants were asked to choose from three options, namely, whether the present stimulus was "stronger," "the same," or "weaker" than the initial stimulus.When the participants could not judge whether the stimulus was "stronger" or "weaker" or when they felt that the two stimuli had the same pressure, they were instructed to answer "the same".The order of the presentation of the standard F I G U R E 1 Schematic diagram of the suction device.The contactor of the suction device is pressed against the skin to apply negative pressure stimulation to the skin.The skin is pulled by the negative pressure and deforms at the 4 mm diameter suction hole in the center of the contactor.Skin displacement data at center point of suction hole was acquired with laser displacement meter mounted inside the contactor.and comparison stimuli was counterbalanced to eliminate order effects, and the five comparison stimuli were presented in a random order.The difference threshold was derived using least-squares fitting of the response rates for "stronger," "the same," and "weaker". 29e five comparison stimuli were applied with the standard stimuli 10 times each according to the method of constant stimuli.The procedure was repeated five times, with the cheek stimulation site changed by 5 mm each time, and thresholds were obtained from the data acquired during the 50 trials.

| Measurement of skin mechanical response to suction oscillation stimuli
A two-dimensional laser displacement meter (Keyence LJ-V7080) mounted inside the contactor was used to measure the skin deformation caused by the suction oscillation stimuli.The skin displacement data at the center of the suction hole were obtained at the laser irradiation line.The displacement at the central point provides informative data for revealing the history-dependent mechanical response characteristics of the skin. 23The distance from the ground plane of the suction hole to the cheek was defined as the skin displacement.The average of the five local maxima of the skin displacement at the center of the section where the 10 Hz suction oscillation stimulation was applied was used as an index of the mechanical response of the skin.

| Measurement of skin mechanical properties
Cheek water content was measured using a SKICON-200EX (IBS Corporation) and Corneometer CM825® (Courage+Khazaka).Each individual moisture value was an average of five repeated measurements on the left cheek.The mechanical properties of the cheek were determined with a Cutometer MPA 580 (Courage+Khazaka) by measuring the vertical displacement of the skin when pulled into a 2 mm diameter probe with an optical sensor.Each measurement consisted of two suction cycles of 2 s each using a constant, negative pressure of 400 mbar, followed by a 1 s period when the pressure was removed (the relaxation phase), allowing the skin to return to its original shape.Each individual parameter value was an average of five repeated measurements on the left cheek.A tape stripping test was performed to evaluate the transdermal absorption of the samples used in the psychological experiments.This test was performed in a room with a constant temperature of 22.0°C and a relative humidity of 45.0%.Four male participants (mean age 29.3 ± 4.0 years old) took part in the experiments.The cream used in the psychophysical experiments was applied (2.0 μL/cm 2 ) to demarcated areas on the left forearm.Ten minutes after the application, the area to which the cream had applied and an area to which the cream had not been applied, serving as the experimental and control conditions, respectively, were washed to remove any residue, similar to the psychophysical experiments.D-Squame® tape with a diameter of 22 mm (CuDerm®) was used.Tape discs were applied and removed by forceps.The first two discs were discarded, and discs three through seven were analyzed.The content of PEG/ PPG-17/4 dimethyl ether in the tape strips was analyzed by liquid chromatography-mass spectrometry (LC-MS) after extraction procedures.The skin-softening effects of the two applications used in the psychophysical experiments were measured using the method reported by Takahashi et al. 12 The measurements were acquired with a specially constructed dynamic measuring instrument at 32°C and 50% relative humidity.A human stratum corneum sample was obtained separately from the above experiments, following the methods of a previous study. 30The stratum corneum strip (20 × 3 mm) was prepared as previously described 12 and held horizontally by two clamps that were spaced 12 mm apart.The left clamp applied fixedamplitude sinusoidal stress (strain 0.2%, 1 Hz) to the left end of the sample.The right clamp had a high-sensitivity sensor that detected the weak stress that was transmitted through the sample.The detected stress signal was calculated and transformed into digital data representing the dynamic elastic modulus (E') by an operation circuit.Before each test sample measurement, the baseline values of the dynamic elastic modulus of each stratum corneum sample were acquired.A test solution (2 μL) was applied to the strip, and measurements were taken for 60 min.The data from 20 to 50 min after the application, when the dynamic modulus (E') had reached equilibrium, were analyzed.The skin-softening effect was evaluated according to the elastic modulus ratio before and after application.

| Analysis
The measured skin displacement and negative pressure data were processed with MATLAB software (MathWorks, R2021b).A logistic regression model was used to examine the relationship between the applied pressure or skin displacement and tactile sensitivity.The statistical software R (version 4.1.3)was used for these analyses, with a 5% significance level.Wilcoxon signed-rank tests were performed to examine the effects of application for both groups.The χ 2 test was employed to examine the relationship between tactile assessments and skin displacement.The statistical analyses were performed using IBM SPSS Statistics V23, and the significance level was set at 5%.

| The magnitude of the skin displacement difference is directly related to the tactile strength assessment
To explore the relationship between tactile sensitivity and the compliance of the stratum corneum, our study initially focused on quantifying the skin's contribution to tactile sensitivity.This was achieved by simultaneously measuring the skin displacement elicited in response to a carefully selected range of stimuli.An example of the presented negative pressure and resulting skin displacement, measured continuously during mechanical stimulation, is shown in Figure 2. The skin began to stretch when the suction stimulus was applied and repeatedly stretched and contracted in response to the 10 Hz negative pressure stimulation.For the preapplication condition in both groups, the negative pressure difference and the measured skin displacement difference for the stimulus pairs presented during the psychophysical experiment were compared with the participant's intensity assessments in each trial, as shown in Figure 3. Compared to the stepwise presented negative pressure difference (Figure 3A), the measured skin displacement difference varied (Figure 3B), and the participants did not always show the same skin displacement difference when the same pressure stimulus difference was presented.
Logistic regression models revealed that the participants' stronger or weaker responses to the intensity of the suction stimulus were related to the presented pressure difference and the amount of skin displacement difference (each, OR = 1.42, 95% CI: 1.32-1.52,p < 0.001, OR = 7.08, 95% CI: 4.79-10.5,p < 0.001).Furthermore, the relatively small Akaike information criterion (AIC) and large loglikelihood values indicated that skin displacement differences fit the regression model better than the presented pressure differences and better explained the relationship with participant responses.In brief, simultaneous measurements showed that the actual amount of skin displacement induced by tactile stimuli and the corresponding tactile assessments of the participants were significantly related.This indicates that the employed stimulation method allows for the quantification of tactile sensitivity based on skin extensibility.

| Hydration improved the discrimination of tactile stimulus intensity with an increase in skin displacement differences
Here, we examined the effect of the application intervention on tac- to the changes in tactile sensitivity.First, we investigated whether tactile sensitivity was altered by the application interventions on the skin surface layer.We calculated the difference threshold from the participant's response to suction stimuli measured by the method of constant stimuli before and after applying cream or Milli-Q water to the cheek.Figure 4 shows the difference threshold before and after application.The vertical axis represents the difference threshold, which is the minimum amount of stimulus required to perceive the difference between two stimulus intensities.Therefore, a lower difference threshold indicates higher tactile sensitivity.Wilcoxon signed-rank tests revealed that the difference thresholds of the intervention group were significantly lower after application than before application (Z = −2.80,p < 0.01).A significant intervention effect was observed among the majority of participants, regardless of their difference thresholds.However, in the control group, the change in the difference threshold was not significant (Z = 0.75, p = 0.46).
In brief, the participants perceived slight differences in the intensity of the mechanical stimulus after cream was applied to the skin, while the application of Milli-Q water did not significantly change the sensations felt.Second, we examined the relationship between the change in tactile sensitivity due to the application intervention and the corresponding skin displacement.We compared intraparticipant changes in the accuracy of participants' assessments of tactile intensity to the presented suction stimulus and the change in skin displacement difference (Figure 3B) after the application intervention for both groups.As shown in Table 2, there was a significant relationship between improvement in the participant's assessment and increase in the skin displacement difference (χ 2 = 5.39, p < 0.05).
In short, the accuracy of tactile intensity assessments for each trial increased as the actual skin displacement difference during the tactile stimulation increased.The intervention group exhibited a greater number of trials with an "increased" skin displacement difference after the application, with a median increase of 1.9 μm.Conversely, the control group had more trials with a "decreased" skin displacement difference after application, resulting in a median decrease of 2.3 μm.Taken together, these results suggest that cream application increased tactile sensitivity to skin stretching in a limited area, and these perceptual changes corresponded directionally to the change in skin displacement during tactile stimulation.

| Enhanced compliance and skin extensibility due to hydration in the stratum corneum
We have shown that cream application increased tactile sensitivity.
Here, we identify the actual changes that occurred in the stratum corneum due to the application intervention in terms of skin mechanics and skin physiology.The amount of skin displacement before and after application was compared when the same stimulus intensity (in this case, a standard stimulus) was presented in a psychophysical experiment (Figure 5).Wilcoxon signed-rank tests showed that the intervention group exhibited significantly increased peak skin displacement during the presentation of the 10 Hz dynamic mechanical stimulus (Z = 3.17, p < 0.01).In contrast, no significant changes were observed in the control group (Z = −0.43,p = 0.67).Figure 6 shows the skin mechanical properties as measured by the Cutometer before and after the application of cream or Milli-Q water.As shown in Figure 6A, only the intervention group showed significantly greater skin distensibility (R0; Z = −2.11,p < 0.05).For net elasticity (R5), gross elasticity (R2, data not shown), and the ratio of elastic recovery to distensibility (R7, data not shown), which indicates elasticity, no significant differences were observed between the groups (Figure 6B).Thus, the application of cream resulted in increased skin extensibility, which was confirmed through the tactile stimulation experiment.
Figure 7 shows the skin water content before and after the application of cream or Milli-Q water.Regarding the conductance values measured by SKICON (Figure 7A), only the values of the intervention group were significantly greater after application (Z = −3.12,p < 0.01).The capacitance values were not significantly changed by application in either group (Figure 7B).The tape stripping experiment showed that PEG/PPG-17/4 dimethyl ether, a component of the cream that is expected to have a softening effect, penetrated into the superficial skin layer at the sites where the cream was applied.
In the control condition, the amount was below the detection limit (Figure 8).According to the dynamic elastic modulus measurements, skin softening was observed in the cream-applied stratum corneum samples, while no skin softening was observed in the Milli-Q waterapplied stratum corneum samples (Figure 9).To summarize the effects of the application interventions, the stratum corneum was F I G U R E 4 Applying skin cream increased tactile sensitivity.Comparison of the tactile threshold before and after application between the two groups.The vertical axis represents the difference threshold, which is the minimum amount of stimulus required to perceive the difference between the two stimulus intensities.Therefore, the lower this value is, the higher the tactile sensitivity.Wilcoxon signed-rank tests revealed that the difference thresholds of the intervention group were significantly lower after application than before application (Z = −2.80,p < 0.01).On the other hand, in the control group, the change in the difference threshold was not significant (Z = 0.75, p = 0.46).**p < 0.01.characterized by higher water content and compliance after cream application, contributing to greater skin extensibility against mechanical stimuli.On the other hand, the application of Milli-Q water did not significantly change any of the evaluation indices.

Note:
The table shows how the change in skin displacement difference affects participants' intensity assessment accuracy.The trials with correct-to-incorrect assessment changes after application are labeled as "Deterioration", and those with incorrect-to-correct changes as "Improvement".The trials with larger postapplication displacement differences are labeled as "Increased", and those with smaller differences as "Decreased".To account for measurement errors, trials with skin displacement differences of 1 μm or less (n = 19) were excluded from analyses.The numbers in parentheses indicate the corresponding number of trials for the intervention and control groups, respectively.There was a significant relationship between the improvement in assessment accuracy and increase in the skin displacement (χ 2 = 5.39, p < 0.05).
TA B L E 2 Increased intraparticipant skin displacement differential change led to accurate stimulus intensity assessments.

F I G U R E 5
Applying skin cream modulated skin deformation in response to 10 Hz periodic tactile stimulus.Comparison of skin displacement before and after application between the two groups.
The vertical axis represents the skin deformation when standard stimuli were presented.The values correspond to trials with the same participant at the same stimulus location.Wilcoxon signedrank tests showed that the intervention group had significantly increased peak skin deformation during the presentation of a 10 Hz dynamic mechanical stimulus (Z = 3.17, p < 0.01).In contrast, no significant changes were observed in the control group (Z = −0.43,p = 0.67).The data are from 19 subjects for whom skin displacement could be measured (intervention group: 10 subjects, control group: 9 subjects).*p < 0.05.
in Figure 3B, we were able to show that the stimulus intensity can be discriminated approximately 50% of the time when skin displacement induced by the comparison stimulus differs by approximately 50 μm from the skin displacement induced by the standard stimulus (median value of approximately 700 μm) (Figure 5).Although a very small skin displacement of 5-10 μm has been used as the stimulus threshold for detecting a 10 Hz vibration amplitude in the human finger, [32][33][34] we were also able to infer a discriminable tactile skin displacement difference on the cheek.Moreover, our findings from the application intervention revealed that the accuracy of were expected to receive richer mechanical information from tactile stimuli that propagate into the skin due to increased skin displacement.Specifically, the mechanoreceptors that predominantly respond to the applied 10 Hz stimulus are thought to be Merkel cells and Meissner corpuscles. 348][39][40]   and softened.This finding is consistent with the fact that the top layer of the stratum corneum is known to experience hydration even after short periods. 14The R0 value measured with the Cutometer increased significantly after cream application.Similar to the Cutometer data, the skin displacement in response to 10 Hz periodic tactile stimuli during the tactile threshold measurements also increased, and the suction stimulation induced skin mechanical properties that were consistent with those typically measured using the Cutometer.Skin hydration results in greater strain in response to mechanical stimuli 41 and more elastic properties in the stratum corneum. 42The modifications in the viscoelastic characteristics of the skin's outer layer likely contributed to a more rapid and pronounced response of the skin to oscillation stimuli, leading to significant skin displacements.By employing a controlled hydration intervention and applying suction stimulation to stretch the skin, we were able to isolate and assess the specific alterations in the properties of the outermost layer of the skin (stratum corneum) that influence tactile sensitivity.
In the present study, we were able to evaluate the effects on tactile sensitivity that were limited to the contribution of the stratum corneum.In fact, while the compliance of the skin surface layer changed after cream application, the elasticity parameters did not change significantly according to the Cutometer data.This may be because the cheek application process and short penetration time did not significantly affect the composition of elastin and collagen in the dermal layer, which contribute to skin viscoelastic properties. 43e influence of skin's physical properties on tactile sensitivity is not limited to the superficial layer of the skin.5][46][47] Thus, considerable work should be conducted to determine what physical properties in skin layers other than the superficial layer influence tactile sensitivity.
The limitations of this study and future research directions can be summarized as follows.First, the present study focused on skin deformation at a single point inside a limited 4-mm stimulus area.However, we know that tactile sensation is a function of the population of many afferents over the entire body surface and their recruitment patterns.The contributing effects of the stratum corneum can be further clarified by examining the effects of the stimulation area 48 and the effects of skin anisotropy and heterogeneity 20 in detail.In addition, we used the peak skin displacement of 10 Hz stimulation as an index of skin mechanical responsiveness, but it could be useful to investigate the history-dependent response of the skin to periodic stimulation.It could also be valuable to focus on time-series changes in skin deformation behavior to clarify the contribution of viscoelastic properties, which change significantly with aging, 47 to sensory perception.Moreover, although the present study measured tactile perception of local skin deformations using suction stimulation to clarify the constraint conditions, the universality of the present findings in stimulation methods other than suction needs to be confirmed.Some previous papers have suggested that the ability of the skin to conform to the shape of the stimulus is important for the spatial perception of touch, 10 while others have indicated that thin or soft skin does not necessarily result in a lower vibration threshold. 49To comprehensively understand these factors, it is important to consider not only externally observable physical phenomena

K
E Y W O R D S skin deformation, skin hydration, skin softness, stratum corneum, tactile sensitivity TA B L E 1 Criteria for inclusion of participants in psychophysical experiments.No acne, atopy, or skin disease on the face No excessive sunburn or other noticeable skin damage on the face No piercings other than ears No psychiatric disorders No pregnancy or lactation tile sensitivity.Additionally, we observed the corresponding actual skin displacement to investigate the underlying factors contributing F I G U R E 2 Skin response to periodic tactile suction stimuli that fluctuated at 10 Hz.Time-varying data of the presented negative pressure (kPa) versus the skin deformation (mm).The left vertical axis shows the presented negative pressure, and the right vertical axis shows the amount of skin deformation in response to the suction.The graph shows a representative single stimulus presentation from a single trial (n = 1).F I G U R E 3Participants assessed stimulus intensity based on actual skin displacement differences.The difference in intensity between the comparison stimulus and the standard stimulus is shown on the x-axis, and the participant's assessment of the intensity is shown on the y-axis.Each figure shows the 453 trials in which the participant responded "stronger" or "weaker" in both groups before the application, except for trials in which the comparison stimulus was equivalent to the standard stimulus.Logistic regression of participants' reactions and the presented pressure (A) or skin displacement (B).The presented negative pressure and skin displacement were associated with the participants' assessments of the stimulus intensity, with adjusted ORs of 1.42 for the presented negative pressure (95% CI: 1.32-1.52,p < 0.001, log likelihood = −259.7,AIC = 523.3)and 7.08 for the skin displacement (95% CI: 4.79-10.5,p < 0.001, log likelihood = −245.3,AIC = 494.6).
Tactile sensation plays a crucial role in communication, emotion, and object manipulation, and it is well established that skin conditions can significantly influence tactile sensation.Thorough investigation of the contribution of skin's physical properties to mechanotransduction to elicit appropriate tactile sensation is imperative.Therefore, we identified the contribution of the stratum corneum to tactile sensitivity through hydration interventions on the skin.First, we observed skin displacement behavior during tactile stimulation to quantify changes in tactile phenomena and psychophysically evaluated the impact of hydration on tactile sensation during skin stretching.Our findings revealed a clear association between tactile stimulus intensity perception and the extent of skin deformation.Second, experimental investigations into the contribution of the stratum corneum to tactile sensitivity, from the perspective of both skin mechanics and skin physiology, confirmed that the immediate mechanical response due to the compliance of the stratum corneum plays a notably important role in shaping the perception of tactile stimulus intensity associated with skin stretching.These phenomena were brought about by hydration interventions in the stratum corneum, a thin layer of the skin.This concrete evidence is highly promising for advancing our comprehension of the intricate relationship between the stratum corneum and the tactile experience.These findings will be pivotal in guiding the development of interventions that can effectively modulate skin physical properties, thus facilitating the attainment of the desired tactile experience.The tactile sensation is influenced by the skin's mechanical response characteristics, specifically observed as the phenomenon of skin displacement in response to stimulation.By observing the physical phenomena occurring during tactile sensation, we focused not on skin hydration per se but on how any skin mechanical response affects tactile sensitivity.There are precedents of reduced light pressure sensitivity induced by monofilaments with the addition of petrolatum31 despite its presumed ability to hydrate the skin, which is thought to be due to reduced skin displacement in response to mechanical stimuli.Hence, it is crucial to measure the actual skin displacement to gain insights into the mechanism of tactile sensation.In our study, we employed simultaneous measurement of skin displacement during tactile stimulation, allowing us to directly observe the relationship between perceived tactile intensity and skin displacement magnitude.By measuring the skin displacement occurring at the time of each touch intensity assessment, as shown

F I G U R E 6 F I G U R E 7
Applying skin cream increased skin extensibility but did not change elasticity.Comparison of skin extensibility before and after application between the two groups.(A) R0 is an index of skin extensibility; a higher value indicates that the skin is easier to stretch.(B) R5 is an index of elasticity; a higher value indicates more elastic skin.The values correspond to trials with the same participant.(A) Wilcoxon signed-rank tests showed a significant increase in the R0 value before and after application in the intervention group (Z = −2.11,p < 0.05), while no significant difference was observed in the control group (Z = −0.74,p = 0.46).(B) R5 values were not significantly different before and after application in either group (Z = 0.11, p = 0.91, Z = 1.25, p = 0.21, respectively).*p < 0.05.Applying skin cream hydrated the superficial layer of the skin.Comparison of skin water content before and after application between the two groups.The y-axis represents the conductance and capacitance values, with higher values indicating higher water content.The values correspond to trials with the same participant.(A) Wilcoxon signed-rank tests showed a significant increase in the conductance values measured by SKICON before and after application in the intervention group (Z = −3.12,p < 0.01), while no significant difference was observed in the control group (Z = 1.31, p = 0.19).(B) Capacitance values measured with the Corneometer, which reflect the water content in deeper layers than those detected by SKICON, were not significantly different in the two groups before and after application (Z = −1.55,p = 0.12, Z = −0.99,p = 0.32, respectively).Data for one participant in the control group with a conductance value of 1332 before and 288 after application are not shown in the graph.**p < 0.01.
tactile intensity assessments was contingent upon the magnitude of the skin displacement difference.This indicates the importance of capturing the actual physical phenomenon of skin displacement occurring during tactile stimulation on the skin, rather than solely focusing on the applied intervention.This is crucial for advancing our understanding of tactile sensation and facilitating the development of effective interventions.Hydration-induced alterations in skin structural mechanics are believed to influence mechanotransduction processes in the deeper layers of the skin, where mechanoreceptors are situated.Previous simulation studies have evaluated the extent to which changes in skin stiffness35 and structure36 influence the tactile stimuli received by mechanoreceptors.The tactile stimuli received by mechanoreceptors were greater in conditions where the skin was softer or the structure was more easily deformed.Thus, mechanoreceptors According to the hydration experiments, larger displacement of the skin surface may have increased the skin tissue strain near these receptors, and the softening of the relatively stiff stratum corneum may have caused stress to be generated in the deeper areas where mechanoreceptors are located, rather than in the superficial layers.To understand the role of skin mechanical properties in the transmission of mechanical information within the skin, further investigation through simulation studies is warranted.These findings can then be interpreted in conjunction with actual tactile sensitivity data.These studies may discover the skin mechanical properties and stimulus presentation methods that facilitate the transmission of tactile stimuli to mechanoreceptors.The stratum corneum with heightened compliance increased tactile sensitivity while increasing skin displacement to periodic stimulation.The application of skin care creams increased tactile sensitivity, and the associated changes in skin properties included an increase in skin water content, heightened compliance, and amplified skin displacement due to mechanical stimuli.Concerning skin water content, the conductance values measured by SKICON increased after cream application, while the capacitance values measured by a Corneometer did not change significantly.Since the cream remaining in the superficial layers of the stratum corneum was washed off before skin measurements, these results suggest a change in the amount of moisture inside the skin due to cream application.The penetration depth of the SKICON probe is very superficial (15 μm), while that of the Corneometer probe is 45 μm. 16Our results suggest that while short-term moisturization may have changed the physical properties of the superficial skin layer, this moisturization did not significantly change the physical properties of the deeper skin layer.The results of the transdermal absorption tests of the stratum corneum samples and the dynamic elastic modulus ratio indicated that the skin-hydrating component penetrated into the stratum corneum by passive diffusion F I G U R E 8 Hydrating ingredients penetrate the stratum corneum after 10 min of the application.The vertical axis shows penetration profiles for human stratum corneum of PEG/PPG-17/4 dimethylether formulated in the cream used in the psychophysical experiments (mean ± SE; n = 4).The LOD value (2.6 ng) or lower is not shown.The bars represent the penetration profiles after cream application (experimental condition).PEG/PPG-17/4 dimethylether was not detected in the control condition.

9
Cream application decreases the dynamic elastic modulus of the stratum corneum.Smaller values indicate greater stratum corneum compliance.Each point represents the dynamic elastic modulus after application compared to the preapplication baseline value for each test sample.The dynamic elastic modulus of the cream-applied sample decreased, while that of the Milli-Q water applied sample increased compared to the preapplication condition.

(
such as skin displacement) but also what external mechanical information is ultimately sensed by the mechanoreceptors inside the skin.Quantification of the tactile phenomena affected by stratum corneum compliance in this study is an important step for promoting research to visualize mechanotransduction within the skin.5 | CON CLUS IONBy employing tactile stimulation using a suction device and conducting limited hydration interventions in the stratum corneum, we were able to evaluate sensory thresholds while observing skin displacement during tactile sensation.Our findings revealed that hydrating the stratum corneum significantly enhances tactile sensitivity and is accompanied by changes in skin deformability.These empirical findings provide valuable insights for advancing our ability to effectively modulate stratum corneum compliance and elicit appropriate tactile sensations.This thin layer is likely to have a significant impact on tactile experiences involving skin stretching, such as interpersonal touch gestures, gentle massage, and product application.Impaired mechanical responsiveness of the skin, such as extreme dryness of the stratum corneum, can have negative consequences on tactile comfort and the accurate interpretation of interpersonal tactile cues and intentions.Skin care formulations that selectively modulate skin mechanical responses by regulating moisture levels can enhance their effectiveness in achieving the desired tactile experience.In addition, tactile sensation varies depending on the skin condition even with the same stimulus.To eliminate individual differences in sensation and provide or estimate the desired tactile experience, it may be effective to control the skin deformation that occurs during tactile stimulation instead of the stimulus intensity or adjust the tactile stimuli according to the skin condition.These considerations could contribute to the advancement of tactile presentation techniques.AUTH O R CO NTR I B UTI O N SSaito Sakaguchi contributed to conceptualization, study design, data collection, data analysis, original draft preparation and final manuscript writing.Kaoru Saito contributed to data collection and data analysis.Naomi Arakawa contributed to study design, data collection, interpretation of data and supervision.Masashi Konyo contributed to data collection, interpretation of data and revise the manuscript.