Mechanisms of tactile sensory deterioration amongst the elderly

Abstract

It is known that roughness-smoothness, hardness-softness, stickiness-slipperiness and warm-cold are predominant perceptual dimensions in macro-, micro- and nano- texture perception. However, it is not clear to what extent active tactile texture discrimination remains intact with age. The general decrease in tactile ability induces physical and emotional dysfunction in elderly, and has increasing significance for an aging population. We report a method to quantify tactile acuity based on blinded active exploration of systematically varying micro-textured surfaces and a same-different paradigm. It reveals that elderly participants show significantly reduced fine texture discrimination ability. The elderly group also displays statistically lower finger friction coefficient, moisture and elasticity, suggesting a link. However, a subpopulation of the elderly retains discrimination ability irrespective of cutaneous condition and this can be related to a higher density of somatosensory receptors on the finger pads. Skin tribology is thus not the primary reason for decline of tactile discrimination with age. The remediation of cutaneous properties through rehydration, however leads to a significantly improved tactile acuity. This indicates unambiguously that neurological tactile loss can be temporarily compensated by restoring the cutaneous contact mechanics. Such mechanical restoration of tactile ability has the potential to increase the quality of life in elderly.

Full text

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Mechanisms of tactile sensory
deterioration amongst the elderly
Lisa Skedung1, Charles El Rawadi2, Martin Arvidsson1, Céline Farcet2, Gustavo S. Luengo
2,
Lionel Breton2 & Mark W. Rutland
1,3
It is known that roughness-smoothness, hardness-softness, stickiness-slipperiness and warm-cold
are predominant perceptual dimensions in macro-, micro- and nano- texture perception. However, it
is not clear to what extent active tactile texture discrimination remains intact with age. The general
decrease in tactile ability induces physical and emotional dysfunction in elderly, and has increasing
signicance for an aging population. We report a method to quantify tactile acuity based on blinded
active exploration of systematically varying micro-textured surfaces and a same-dierent paradigm.
It reveals that elderly participants show signicantly reduced ne texture discrimination ability.
The elderly group also displays statistically lower nger friction coecient, moisture and elasticity,
suggesting a link. However, a subpopulation of the elderly retains discrimination ability irrespective
of cutaneous condition and this can be related to a higher density of somatosensory receptors on the
nger pads. Skin tribology is thus not the primary reason for decline of tactile discrimination with
age. The remediation of cutaneous properties through rehydration, however leads to a signicantly
improved tactile acuity. This indicates unambiguously that neurological tactile loss can be temporarily
compensated by restoring the cutaneous contact mechanics. Such mechanical restoration of tactile
ability has the potential to increase the quality of life in elderly.
In common only with taste, touch is based on the intimate contact of a body surface with a material and as with
all senses, tactile perceptual ability declines with age1–4. Human hands and ngers are used for active exploration
of our surroundings as well as for grasping objects5,6 and are our primary “tactile probes”; ngertip interactions
are thus highly relevant for understanding tactile perception.
e so-called “sensorial” evaluation of touch (the association of sensations with subjective descriptions) is
a well-established eld7 and is paramount when communicating about touch; for example in consumer panel
studies8,9. Although the importance of physical quantities such as roughness, elastic modulus etc. is commonly
accepted, a clear-cut understanding of their individual impact on perception is lacking since it is dicult to vary
one parameter independently10–13. ere is an increased interest recently on methods based on stimuli detec-
tion; such psycho-physical techniques provide a complementary approach and rely on objective tests, such as
whether a dierence can be detected14. In general, such approaches provide quantitative data that can be com-
pared to, and ideally correlated with, individual physical quantities or combinations thereof. Skedung et al.15
observed, for example, how the friction coecient diminished with increased average roughness on printing
papers and explored the perceived similarities of calibrated wrinkled surfaces of dierent wavelengths and ampli-
tudes10. Friction coecient and pattern wavelength were directly linked to the generated tactile space and it
was also found that amplitudes as small as 15 nm could be distinguished from blank surfaces. Furthermore,
although the nger friction coecient is the physical parameter which varies, the applied load is unconsciously
regulated to maintain an optimal friction force10,15. us it is likely that from a perceptual perspective it is the
applied load which is monitored and registered. It is known that roughness-smoothness, hardness-soness and
stickiness-slipperiness are predominant perceptual (as opposed to physical) dimensions in macro texture per-
ception14,16–18. is perceptual dimensionality remains intact when the texture scale is reduced to the micro- and
nanoscale19. It is not clear whether the tactile discrimination ability for these textures remains intact with age.
Skin can be considered a biocomposite material of multiple layers. Consideration of its structure is essential
for understanding the generation and transmission of vibrations across the tissue, which are detected by the
cutaneous receptors underpinning our somatosensory system5,20 and may well be amplied by ngerprints21. e
1RISE Research Institutes of Sweden, Bioscience and Materials, Stockholm, SE-114 28, Sweden. 2L’Oréal Research
and Innovation, Aulnay-sous-Bois, 93600, France. 3KTH Royal Institute of Technology, Surface and Corrosion
Science, Stockholm, SE-100 44, Sweden. Correspondence and requests for materials should be addressed to G.S.L.
(email: gluengo@rd.loreal.com) or M.W.R. (email: mark@kth.se)
Received: 5 December 2017
Accepted: 14 March 2018
Published: xx xx xxxx
OPEN
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inuence of the dermis, epidermis and subcutaneous tissue has been widely studied in relation to the induced
deformation and the friction observed22–24. While less studied, there is increasing evidence of the importance of
the harder stratum corneum layer for friction25. Friction forces depend on the real area of contact, and thus the
local deformability of the stratum corneum properties impacts this parameter19,26. Lévêque et al.3 for example,
investigated how the hydration of this layer inuences its elasticity and demonstrated improved, static, cheek
spatial acuity using the two-point discrimination gap method. Other attributes of the skin surface are also known
to play a role in skin friction27, such as the micro relief12, or the presence of sweat and sebum25.
Life expectancy is increasing28, yet few studies have explored the limits of tactile perception in the elderly. Age
is known to aect both the mechanical and physical properties of the skin as well as neurophysiological capabil-
ities for the detection, transmission or interpretation of touch signals1,3,29–32. e situation is complicated by the
fact that the 2-point threshold approach used in many studies addresses static touch, but there is evidence that
dynamic touch is aected dierently33. e relative extent to which the two potential contributions (mechanical
or neural) impact sensory perception is unknown. In the correction of sensory decline in the senses of sight and
hearing, however, which also deteriorate with age in a well-documented fashion34,35, the mechanical, rather than
the neurological deciency, is systematically addressed: Spectacles correct for the deformation of the cornea and
hearing aids amplify audio signals. us the questions can be posed as to whether i) there is a similar reduc-
tion in tactile sensory discrimination of ne texture with increasing age, ii) this deterioration can be attributed
to age related changes in skin mechanical properties and iii) such mechanical deciencies can be analogously
remediated.
An earlier technical protocol using wrinkled surfaces10,19 has thus been adapted to study in detail the changes
in tactile perception ability that occur at advanced age using active touch on ne textures. e abilities of young
and elderly participants to haptically discriminate between dierent, well dened patterned textures have been
quantied, and compared to physical properties of the skin (nger friction, nger hydration and elasticity).
Results and Discussion
Lower tactile discrimination ability in the elderly group. Six systematic textures varying in surface
pattern wavelength have been fabricated by an established method10. ey were used in a tactile perception test
where the task was to judge whether a presented surface was perceived as the same or dierent to a reference sur-
face. e nominal wavelengths of the test surfaces used in this same-dierent tactile perception test were 20 µm,
40 µm, 60 µm, 80 µm (denoted S20, S40, S60 and S80, respectively), 100 µm (Ref100) as well as a blank, smooth
surface with no systematic texture (S0). Ref 100 is closest to the limit of what can be considered “ne texture”19. A
pilot study with 10 elderly and 10 young female subjects indicated that the young group hit a threshold at 60 µm
i.e., that the diculty of the task increased signicantly for this wavelength and that S60 could not reliably be
distinguished from Ref100. e elderly group had a much lower rate of successful task completion even at 20 µm.
e results below are obtained for a larger study consisting of 30 “young” (19–25 y) and 30 elderly female subjects
(67–85 y), none of whom participated in the pilot. Six repeated tactile perception comparisons to the reference
were performed for each test surface and participant, and were presented and evaluated in a unique randomized
order (in total 36 comparisons in each perception test).
e averaged results are displayed in Fig.1 as percentage of correct responses as box plots, showing the inter-
quartile range IQR (25th and 75th quartile = 50% of the data) with the mean (square), median (line), whiskers
Figure 1. Tactile discrimination ability for the two groups. (a) Proportion of correct answers in identifying
whether the stimulus was dierent to the reference sample Ref100 (100 µm in wrinkle wavelength). e
diculty of the task clearly increases for both groups as the wavelength of the stimulus texture approaches that
of the reference. e “break point” (dened as when the success rate falls below 80%) is seen for S20 (20 µm) for
the elderly group and for S60 (60 µm) for the young group (N = 30 for both groups). (b) Box plot of the means
from each participant in the two groups (N = 2 × 30), showing the tactile discrimination ability of S20 versus
Ref100 as well as the correct responses for Ref100 versus itself (meaning “same” as the correct answer). Since the
young and the elderly groups have a very similar proportion of correct responses for the latter task, the tendency
to guess “dierent” is the same for both groups when the task is perceived as dicult.
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indicating variability within 1.5 IQR, as well as outliers. An answer was held to be correct when the test surface
was judged dierent from the reference or when the reference was judged the same when presented against itself.
e smooth surface has “perfect” scores of 100% and 99% for the young and elderly, respectively, indicating that it
was easily detected as dierent from Ref100. is can be attributed to the greater real contact area between surface
and nger during tactile interaction compared with all the other surfaces and a correspondingly increased fric-
tion coecient22. e young group successfully dierentiated S20 and S40 from Ref100 (average correct response
of 97% and 89%, respectively), and the drop in tactile discrimination ability was seen with S60 (55%). S80 is
even closer to the reference wavelength of 100 µm and not unexpectedly was even harder to distinguish (34%).
e drop in discrimination ability for the elderly group is already observed for S20 where the number of correct
responses is 63% and therefore below the 80% criterion for high performance (see below).
For the comparison of Ref100 with itself, i.e. where “same” is the correct answer, the young and elderly groups
showed a similar correct response level of 79% and 76%, respectively. e fact that this score is not 100% indicates
a tendency to answer “dierent” when the participant is uncertain. is tendency was thus factored into the deci-
sion to set the successful detection criterion to 80% correct responses. It is the same for both groups and should
not represent a bias in the group comparison.
e tactile discrimination ability (S20) was signicantly lower for the elderly group compared to the young.
For the young group, the mean value was 97%, with a standard deviation (s.d.) of 17%. For the elderly group,
the corresponding values were mean = 63%, s.d. = 48% (F(1,58) = 26.65, p < 0.001). Only two individuals in the
young group failed at reliably distinguishing S20 from Ref100, whereas 17 among the elderly failed the same
task. For S60 and S80 both groups showed diculties in perceiving dierences to the reference (<80% correct
responses).
It is thus abundantly clear that dynamic touch on ne textures exhibits an age-related decline in discrimination
ability which may well be related to the decrease in tactile discrimination sensitivity with age reported for static
touch1–4 and object discrimination3,36. e physical dimensions underpinning the active touch discrimination of
ne textures have previously been identied10 as being associated with the friction coecient (and the resulting
nger loading15), and the wavelength of the surfaces. It seems likely that the former depends on responses from
slow adapting receptors (deformation) whereas the latter depends on vibrations detected by the fast adapting
Pacinian receptors37,38. It would thus seem logical to start with frictional, or“biotribological”, studies to identify
the cause of the active touch deterioration. Skin biomechanics aect these properties strongly, and there may be
a direct link to the trends seen for static touch, which is also highly dependent on biomechanical properties. is
has been done under an ensuing heading.
Before leaving the same-difference test however, it remains to identify an additional important finding.
Individual examination of the correct response distribution of all 60 participants (for S20) in Fig.2, clearly reveals
that a signicant fraction of the elderly group displays unimpaired ability. is bimodal distribution needs to be
considered in future studies of age-related perception decline. e retention of an active touch tactile acuity has
earlier been reported in aged, blind individuals39, possibly indicating that retention is associated with use, though
this speculation is beyond the scope of this work. Using the criterion of 80% success as the demarcation between
successful and unsuccessful tactile discrimination ability, the elderly participants could be divided into a high
performing sub-group (13 individuals) and a low performing sub-group (17). us approximately 43% of elderly
group performed at the same level as the young. A direct implication of this observation is that future studies
aimed at identifying means to improve age related texture discrimination should address this distribution and
adopt an appropriate recruitment strategy.
Bio-mechanical and bio-tribological dierences in young and aged skin. Figure3 illustrates dif-
ferences in nger elasticity, nger hydration and tactile friction between the young and elderly groups. A one-way
Figure 2. Not all elderly subjects show a decrease in tactile discrimination ability. Average correct responses
for S20 of all 60 subjects, showing that 13/30 from the elderly group perform at the same level as the young. e
criterion of 80% correct responses is used to separate high and low performers. Age is plotted on the abscissa as
a convenient means to separate the two groups.
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ANOVA indicates that the nger hydration level is signicantly lower among the elderly group compared to the
young (young: mean = 66 a.u., s.d. = 23 a.u.; elderly: mean = 36 a.u., s.d. = 14 a.u.) (F(1,59) = 36.14, p < 0.001).
Skin elasticity is determined by extracting various parameters from measurements of skin deformation as a func-
tion of time. e supporting information provides details of the technique (see also Fig.S1 and TableS1) whereas
Fig.S2 shows how the parameters vary between the two groups. e elastic recovery parameter is here used as the
measure of nger elasticity since it has earlier been identied as the relevant parameter for describing age related
changes40–42. Note, that it is not an estimate of elastic modulus, but rather the reversibility of applied deforma-
tions; this (unitless) nger elasticity is signicantly lower in elderly skin (young: mean = 0.54, s.d. = 0.12; elderly:
mean = 0.34, s.d. = 0.08) (F(1,54) = 49.16, p < 0.001). Aged skin is normally reported as drier and less elastic than
young skin42–44.
Friction between a nger and the textured surfaces, (“tactile friction”)10,15,19 was measured in a continuous
reciprocating manner on surfaces S20, S60 and Ref100 (see Fig.3d). e resulting friction coecients (here, the
ratio between friction force and applied load) are shown in Fig.3c. A two-way repeated ANOVA was used to
assess the signicance of dierences between the average tactile friction coecients on the three surfaces and
the two age groups (N = 2*29). A Tukey post hoc test shows that the young group possesses a signicantly higher
tactile friction coecient on all three surfaces (p < 0.001). e large standard deviations reect the well-known
individual spread in tactile friction between individuals15,19,23, and consequently, no signicant dierences are
observed between the three surfaces at group level. However individual dierences indicate that the young vol-
unteers show a slightly greater dierence in tactile friction coecient between surfaces S20 and S60 compared
to the elderly volunteers. In both groups, the majority show highest tactile friction coecient on the smallest
wavelength surface (59% of the elderly and 69% of the young). e higher friction level of the young volunteers
is almost certainly an eect of the higher nger hydration level. Such a relation between skin hydration and skin
friction is well reported in the literature19,23,45.
As statedin the introduction, the friction coecient is unconsciously used to moderate the applied load (how
hard the nger is pressed) and this may well be the perceptual prompt. e elderly do indeed press somewhat
harder as a result of the lower friction coecients experienced though the variation at the individual level is large)
and the data is displayed in the SI. (Note that the load is not recorded during perceptual experiments, only during
tactile friction measurements).
In order to evaluate a possible link between hydration and tactile perception ability, individual correct
responses from all volunteers for S20 are plotted in Fig.4 versus the individual nger hydration value. As can
be seen, all the young participants return above 80% correct responses and the elderly participants are scattered
Figure 3. e groups display signicantly dierent bio-mechanical and bio-tribological values. e elderly
group display (a) lower nger hydration (b) lower nger elasticity and (c) lower tactile friction coecients
obtained by the continuous recording of friction force and applied load upon interaction using a (d)
ForceBoard.
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over the entire possible interval from 0% to 100% correct responses. An interesting observation is that above a
hydration level of 50 a.u, an almost exclusively high performance is observed, whereas at lower levels of hydra-
tion, both high (>80%) and low (<80%) performers are found. A similar plot is obtained with nger elasticity,
(not shown) but as can be seen in Fig.4b, nger elasticity is clearly related to the nger hydration level. ese
parameters would thus appear to be degenerate and indistinguishable in terms of their intrinsic relevance. From
a contact mechanics perspective, it is the elastic modulus that aects the true area of contact and thus the friction
coecient. Skin is a multilayer structure and the macroscopic deformation of the nger corresponds to an elastic
modulus on the scale of kPa. is is eectively maximised at very low loads, below 1 N. e friction coecient,
and the area of contact are dependent on local deformations of the Stratum Corneum (SC)26 which has a much
higher elastic modulus (ca 0.005–0.1 GPa)46As mentioned before, the elasticity parameter is not a direct measure
of the elastic modulus, but the elastic modulus of the SC also has a strong, inverse, dependence on the moisture
content (as well as the scale of the deformation)46,47, so the parameters are unambiguously linked. At very high
moisture contents the SC also exhibits plastic deformations which lead to even higher contact areas and friction
coecients, and also lower elastic reversibility – though this depends strongly on how the parameters are dened
(see SI). In addition, at higher humidity/moisture content there is the possibility for capillary condensation48 and/
or occlusion45 of liquid water in the contact. is also contributes to increased friction. e role of moisture on
friction is further discussed in the next section.
Clearly, a hypothesis to test is whether an increase or improvement in nger hydration could improve the
dynamic tactile discrimination ability in the elderly group, analogously to observations for static touch, where the
application of a moisturizer containing 5% of glycerol increased both the hydration and static touch acuity3. us,
to evaluate a possible improvement in tactile discrimination and changes in cutaneous properties, the group of 30
elderly subjects were divided into a high performing (N = 13) and a low performing (N = 17) sub-group.
Improvement in tactile ability. Two dierent humectants containing 5% and 7% of the common mois-
turizer glycerol were applied to the same index nger used in the perception test, on two dierent days. In the
treated state, S20 and Ref100 were repeatedly evaluated. As a check, the high performing group (N = 13) were
still high performing aer application of 5% glycerol. As can be seen in Fig.5a, the ability to discriminate S20
signicantly improved for the low performing group (F = 15.911, p = 0.001 for 5% and F = 18:346, p < 0.001 for
7%) aer application of humectant to the nger. e interpretation is further strengthened by the fact that the
ability to correctly identify Ref100 increased as well, indicating a higher degree of condence in the assessment.
is known humectant also improves the nger hydration of the elderly in the low performing group as can be
seen in Fig.5b. ere is a signicant increase in both nger hydration and nger elasticity that could explain the
increased ability to perceptually distinguish S20 and Ref100. Previous work has indicated that two perceptual
mechanisms seem to govern perception, where sticky/slippery has been shown to be the main dimension and
can be related to the friction coecient10,19. It has further been established that nger hydration is a major con-
tributor to the individual variations in the absolute values of the friction coecient19. ese results thus clearly
indicate that hydration and elasticity play a role in the restoration of dynamic tactile discrimination ability of ne
texture analogously to static touch on the arm3, lips49, and index nger50. e sticky/slippery perception has been
addressed by the instrumental measurements of tactile friction. A signicant dierence between the tactile fric-
tion coecients measured in untreated and treated states is observed. In the untreated state, no dierence at the
group level between S20 and Ref100 was noted. However, in the treated state a signicant dierence between the
two surfaces was obtained (F(1,16) = 4.261, p = 0.008 for 5%, and F(1,16) = 4.866, p = 0.003 for 7%), where S20
displays the higher tactile friction. ese observations indicate that the improvement in active tactile ability in
the low performing group could be related to an improvement in both nger hydration and nger elasticity. is
in turn allows the subjects to distinguish the surfaces based on a sticky/slippery perception10,19. e improvement
eect appears to be both immediate and reversible since a new “untreated” perception test performed one day
Figure 4. Biomechanical properties and tactile ability. (a) Scatter plot showing the correct responses on S20
(perceiving S20 as dierent from Ref100). ere is improvement potential for the subjects below 80% correct
responses. (b) Finger elasticity appears to increase with increasing hydration.
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post application shows that both cutaneous parameter levels and tactile discrimination ability return to the same
level as before humectant application.
Hydration is known to modify the properties of the stratum corneum51–53 and in part is responsible for a sof-
tening thereof. Such deformations in the stratum corneum facilitate pressure stress transmission to the subsurface
somatosensory system (touch corpuscles). Friction is also known to increase with the moisture level mainly due
to the contribution of skin deformation, in particular at higher pressures. Note that there is no information as to
how the topography of the nger print ridges varies with age and between the two elderly groups, but there are
no publications indicating bimodal distributions of topography amongst the elderly. Even if topography were to
vary between any of the groups, it would manifest via the nger friction parameter and would thus eectively be
degenerate with the tribological properties.
e simultaneous improvement in the tactile perception discrimination ability and cutaneous parameters
upon treatment with glycerol together with the observed dierences between young and elderly groups in the dry
state indicate that the skin state is important for active tactile perception ability. A comparison of the high and low
performing elderly sub-groups in the untreated state however, shows no signicant dierences in biomechanical
or biotribological properties, raising interesting questions as to the mechanisms of the decline in tactile acuity,
or rather its retention by the high performers. One possibility is that the high performing elderly sub-group has
developed a greater (compensating) sensitivity towards another perceptual prompt – for example the vibrations
detected in the Pacinian corpuscles37,38. e vibration occurs as the nger traverses the periodic structures and
its value depends on the wavelength. e sensitivity to these vibrations decreases as the vibration frequency
diverges from the optimal sensing frequency of the Pacinian corpuscles37,38. e fact that there is no dierence in
the biomechanical properties of the skin indicates that vibration transmission is unlikely to be the dierence. e
question however remains as to why two populations exist and this implicates an additional, neural, contribution
to age-related loss of tactile perception. is could, for example, be related to a decrease in density of peripheral
nerves and mechanoreceptors in the ngertip, or their ability to transmit signals. Based on the data above it is
Figure 5. Improvement in tactile discrimination ability with increased nger hydration. Data is shown only for
the low performing elderly group (a) Improvement in correct responses measured 30 min aer application of
humectants containing 5% and 7% glycerol (applied on two dierent days). (b) As expected, both humectants
increase the nger hydration level. (c) Finger elasticity is analogously increased. (d) Greater dierences in
tactile friction between S20 and Ref100 in the treated nger state that could explain the increase in tactile
discrimination ability based on greater dierences in sticky/slippery feel. Note that when humectant was
applied, the physical parameters were measured both before and aer. us there are “untreated” data which are
nonetheless labelled as (5%) and (7%) according to the respective, subsequent treatment.
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dicult to draw more conclusions. us, a follow up experiment was performed. A new, larger population of
elderly participants were screened using the same perceptual discrimination experiment. Two elderly groups
were selected (30 each) based on identication as poor performing (in this case less than 60% success) or high
performing (>80% success). e density of Meissner Corpuscles (MC) was measured using a microscope as a
simple, non-invasive measurement of the somatosensory condition. (Meissner Corpuscles are generally held to
be the most important of the mechanoreceptors used in touch and pressure on glabrous skin54). It was found that
the higher performing group had a statistically higher MC density (see Fig.S8). e average was 2.67 mm−2 with
a standard deviation of 0.89 mm−2. For the lower performing elderly group the average was 1.35 mm−2 with SD
0.70 mm−2. us the lower performance of this group can be condently linked to a neural decline, though we
stress that we do not link the reduced acuity directly to the lower MC density, since there is no data on the other
tactile receptors.
Concluding remarks. ere is a clear, statistically signicant reduction in the ne texture discrimination
with active touch amongst the elderly. Identifying such decline quantitatively is not trivial, but the test presented
here, provides a straightforward approach. Not only can the deterioration be measured, but its remediation by
training or treatment can also easily be assessed. e reduction in acuity amongst the elderly is matched by a
reduction in the biomechanical properties of the skin – moisture, elasticity and nger friction coecient are all
signicantly reduced. e nger friction is now known as an important perceptual identier for ne texture dis-
crimination so this provides a clear pathway for amelioration of the reduced tactile perception.
e observation of that a large proportion of the elderly retain their active touch ne texture discrimination
acuity has several important ramications. At the simplest level, it means that future studies aimed at remediating
tactile discrimination need to take this into account. More importantly, this observation casts signicant light on
the mechanism of the decline in acuity. Since the biomechanical decline of the skin properties is indistinguishable
for these two populations, the reduced performance cannot be associated directly with the age related changes
in elasticity, moisture and friction. is almost certainly rules out the sticky-slippery perceptual dimension as
the prompt for the performance. Another dimension is associated with vibrations detected by Pacinian and/
or Meissner Corpuscles. Since the mechanical skin properties in the two groups are comparable, the vibration
transmission itself can be cautiously ruled out, which strongly implies that the dierence between the two groups
has a neural decline associated with receptor sensitivity, density, or signal transmission. A reduced density of
receptors has clearly been shown for the poor performers. us, age-related decline in neural properties is the
primary explanation for the reduction in active touch acuity. Nonetheless, an improvement in the biomechanical
properties of the skin through rehydration, for example as performed here, means that the loss in discrimination
in the one dimension, can be signicantly ameliorated by an improvement in tactile discrimination ability in the
other, friction based, dimension.
Methods
Wrinkle-patterned model surfaces. Six wrinkled surfaces were used in the initial perceptual test and
were manufactured in-house based on a surface wrinkling technique presented earlier10,55. They consist of
UV-curable adhesive polymer NOA81 (Norland Optical Adhesive 81, Norland Products Inc., Cranbury, USA)
and were templated from sinusoidal textures imposed on a PDMS surface (polydimethoxysiloxane, Sylgard 184
Dow Corning, USA), with nominal wavelengths according to previous recipes19 ~20 µm (S20), ~40 µm (S40),
~60 µm (S60), ~80 µm (S80) and ~100 µm (Ref100). A blank reference surface was replicated against smooth,
unwrinkled PDMS. See FigsS3 and S4 prolometry results.
Tactile discrimination test. A same-dierent paradigm was used, i.e. the task was to judge whether two
surfaces were perceived identical or not. e method of constant stimuli was employed for the presentation
of stimuli, i.e. the participants were always presented with Ref100 rst, followed by the test stimuli and were
instructed to judge whether the second surface was perceived as the same as or dierent from the reference. Each
surface was compared with the reference surface six times in a randomized order, resulting in 36 comparisons in
total for each participant in the untreated state. e instruction was to stroke the surface back and forth with the
index nger of the dominant hand. e perception test was performed blind-folded. All subjects were allowed to
practice the test procedure on the same pairs of surfaces prior to the test. e surfaces were cleaned with acetone
both during and aer each test series.
Subjects and experimental conditions. 30 elderly women (mean: 73 ± 4.5 years; range 67–85) and 30
young women (mean: 22 ± 1.5 years; range 19–25) participated in the study. Each participant had a unique ran-
domized order of surface presentation for perception tests and tactile friction measurements. e study was per-
formed in ambient indoor air, where the mean temperature was 23 ± 1.3°C and the relative humidity 53 ± 8.7%.
Inclusion criteria, ethics and guidelines. e inclusion criteria were addressed during the recruitment
process, and were also checked at the start of the experimental session. None of the participants had any skin
disease, neurological disease, diabetes or were allergic to cosmetic products. None of the participants were preg-
nant or had been breastfeeding the last six months. Signed, informed consent was given and the subjects were
informed that they could quit the experiment at any time if they so wished. All procedures were performed in
accordance with the ethical standards of the 1964 Helsinki declaration and its later amendments of ethical stand-
ards regarding studies with human participants. e protocols are as for reference10, and were approved by the
Research Committee of the Institute for Surface Chemistry and the Ethics Committee of Stockholm University.
Cutaneous measurements. Cutaneous status or biomechanical properties were measured by means of dif-
ferent probes from Courage & Khazaka Electronic GmbH (Cologne, Germany). Finger hydration was measured
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with a Corneometer CM 825 in arbitrary units (a.u.), based on electrical capacitance. Each individual moisture
value is an average (mean ± standard deviation) of ve repeated measurements on the index ngertip of the
dominant hand. Finger elasticity was measured on the index nger of the dominant hand with a Cutometer MPA
580 by measuring the vertical deformation of the skin when pulled into a 2 mm diameter probe with an optical
sensor. Each measurement consisted of three suction cycles of 2 s using a constant negative pressure of 450 mbar,
followed by a 2 s period when the pressure was switched o (relaxation phase) allowing the skin to return to its
original shape. e elastic recovery parameter (also called net elasticity), R5 (Ur/Ue) is used to represent nger
elasticity in this work obtained, as it has been identied as a suitable parameter for comparing dierence between
young and aged skin41. e dierent parameters from the time/strain curves (elasticity curves) are described in
supplementary information, the parameters Ue and Ur refer to the linear elastic response on deformation and
relaxation respectively.
Tactile friction measurements. Finger friction (or tactile friction) was measured with a ForceBoard
(Industrial Dynamics AB, Sweden), a universal friction and force tester equipped with one horizontal and one
tangential load cell, individually connected to the same plate of assembly. Upon interaction, a mechanical load is
converted into voltage signals that are amplied and proportional to the applied load in N. e tangential force
(friction force), and vertical force (applied load) were continuously recorded using DAQFactory soware at a rate
of 100 Hz as a nger was moved over the surfaces. Friction coecients were calculated at each data point as the
ratio of the friction force and applied load the average dynamic friction coecients of ten stroking cycles were
calculated and compared19. (Note that here “friction coecient” refers to the macroscopic interaction of the entire
nger-textured surface contact, rather than the local, “asperity contact” shear stress which should be invariant
since the surfaces are all the same material). e surfaces S20, S60 and Ref100 were measure in triplicate in each
experimental session (randomized order). e participants were instructed to use their preferred load, speed and
angle of the index nger the same for all the measurements. e surfaces were measured in randomized order.
Tests after application of a humectant. To assess the eects of skin remediation on perception, the 30
elderly participants performed a second perception test aer application of a humectant. Two dierent humectant
systems, were used in which the active moisturizing ingredient in both cases was glycerol at concentrations of 5%
and 7%, respectively. Two humectant systems, with dierent carrier compositions were employed to limit poten-
tial artefacts from the non-humectant components. e two dierent systems were tested on two dierent days,
and their order randomized. e humectant was applied with three ngers (including the index nger) onto the
cheek (as a counter surface to distribute the material evenly and in a manner perceived as “natural” by the partic-
ipants). e cutaneous parameters and tactile friction were also measured in the treated state on the index nger.
Meissner Corpuscle density. Meissner corpuscles (MC) densities were obtained according to an estab-
lished protocol described and referenced in the ESI. Optical images were obtained for each subject by sampling a
2,5 × 2 mm area over the midpoint of the volar aspect of the distal phalanx of Digit I, on the dominant hand. An in
vivo reectance Confocal Microscope (RCM) (Vivascope 1500, Lucid Inc., NY) was used to obtain all the images
at a specic depth. A fuller description is provided in the Supplementary Information.
Statistical analysis. One-way ANOVAs or two-way repeated ANOVAS have been done with Origin (phys-
ical data) and SPSS (perception data) to test signicance between variables, i.e. the eect of age, eect of humec-
tant and eect of dierent concentrations of glycerol. e Alpha level for statistically signicance was set to
p < 0.05. Due to a wide spread in absolute values obtained when measuring tactile perception ability with human
subjects as well as properties on the skin in vivo, we show all results as box plots, showing the interquartile range
IQR (25th and 75th quartile = 50% of the data) with the mean (square), median (line), whiskers indicating varia-
bility within 1.5 IQR, as well as outliers. e age dierences in the ability to correctly identify S20 and the ability
to correctly detect S40 were analysed in separate one-way ANOVAs. Tukey post hoc analyses were used to identify
which of the conditions were signicantly dierent.
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Acknowledgements
Marie Lise Chiron and Julien Laboureau are acknowledged for providing the humectants and Olivia Dufour
for statistical advice. Johan Andersson is acknowledged for making the wrinkled surfaces. Annika Bergström
is thanked for recruiting the participants. We acknowledge “PhD trials, Lisbon” for performing the Meissner
corpuscle measurements.
Author Contributions
L.S., M.A., G.L., C.F., M.W.R. and C.E.R. designed and planned the research. (L.B. and C.E.R designed the
neurobiologicalresearch onMeissner Corpuscles) L.S. and M.A. performed the research and conceived the data
collection with subjects and the data analyses. (with the exception of Meissner Corpuscle results) L.S., G.L. and
M.W.R. wrote the dras of the manuscript and L.B., C.F. and C.E.R. reviewed the dras. L.S. and C.E.R. made
the gures. All authors discussed and interpreted the results and contributed to dras of this nal paper. L.S.
organized the work.
Additional Information
Supplementary information accompanies this paper at https://doi.org/10.1038/s41598-018-23688-6.
Competing Interests: C.F., C.E.R., G.L. and L.B. are employees of L’Oréal. M.A. and L.S. were full time
employees of RISE Research Institutes of Sweden at time of completion of the study which is a govt. owned
consulting organization having received nancing from L’Oréal to perform the majority of the research leading
to this study. M.R. currently receives funding from L’Oréal via KTH to perform research on an unrelated topic
and is a senior advisor at RISE on a part time basis.
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