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Age-related differences in tissue dielectric constant
values of female forearm skin measured noninvasively at
300 MHz
H. N. Mayrovitz, A. Singh and S. Akolkar
College of Medical Sciences, Nova Southeastern University, Ft. Lauderdale, FL, USA
Background/Purpose: We hypothesized that reported age-
related shifts in skin water from less-to-more mobile states
would result in increased skin tissue dielectric constant (TDC)
values as TDC values depend strongly on water content and
state. One aim was to test this hypothesis. Further, as skin-to-
fat TDC values are used as a tool for edema and lymphedema
assessment, a second aim was to establish reference values
suitable for young and older women.
Methods: TDC was measured bilaterally on volar forearm skin
in young (20–40 years) and older (≥60 years) women. There
were four groups with 50, 50, 100, and 50 subjects per age
group measured to depths of 0.5, 1.5, 2.5, and 5.0 mm,
respectively.
Results: For each age group, TDC values decreased with
increasing depth (P<0.001). TDC values at 0.5 and 1.5 mm
were greater for older women (P<0.001). At 2.5 mm, there
was no age-group difference (P=0.108). At 5.0 mm the direc-
tion of the difference reversed with older TDC values less than
the younger (P<0.001).
Conclusion: Results are consistent with age-related shifts in
water state from less-to-more mobile and explain depth-depen-
dence differences between age groups. Data also give age-
related TDC reference values for assessing local edematous
or lymphedematous states.
Key words: skin dielectric constant – skin water – edema –
lymphedema – age-related skin changes
Ó2015 John Wiley & Sons A/S. Published by John
Wiley & Sons Ltd
Accepted for publication 17 May 2015
AN EXTENSIVE set of biophysical measure-
ments aimed at characterizing certain age-
related changes in men and women (1–5)
demonstrated several important features.
Stratum corneum (SC) hydration, assessed via
electrical capacitance measures at forehead,
neck, and forearm, tended to increase with age
in females (3) but in males SC hydration
decreased or remain unchanged (5). In both gen-
ders, reductions in most measures of skin elas-
ticity and distensibility were demonstrated (1,
4). Of particular interest and relevance to the
present study is the finding that SC hydration at
the volar forearm was greater in the oldest
female group (60–80 years) compared to youn-
ger (18–39 years). This was found to be true
with no apparent age-related difference in
female transepidermal water loss (TEWL) (3)
although females had significantly greater
TEWL values than males at all ages (2). The
apparent age-related increase in female SC
hydration has not been explained. Other data
based on depth profile measures of volar fore-
arm SC water suggest a tendency for older
females to have slightly less percentage water
within about 30 lm of the surface, but similar
values deeper (6). We believe that an aspect that
may be involved is the fact that SC and skin
water are largely present as bound water (7)
either tightly or loosely bound to macromole-
cules (8, 9) but shift toward increased percent-
ages of more mobile water with skin aging (10).
As bound water (11) has a lower dielectric con-
stant than mobile water (12–14), such a shift if
present would be associated with an increase in
measured capacitance that could be interpreted
as an increase in age-related water content. A
shift to a greater proportion of non-hydrogen
bound water in photo-aged skin is not restricted
to SC and has been reported to be about 30%
greater in dorsal forearm skin (epidermis and
dermis) (10) with an associated age-related
1
Skin Research and Technology 2015; 0:1–7
Printed in Singapore All rights reserved
doi: 10.1111/srt.12249
©2015 John Wiley & Sons A/S.
Published by John Wiley & Sons Ltd
Skin Research and Technology
thinning of dorsal and ventral forearm skin (15).
Magnetic resonance imaging of forearm skin has
also demonstrated an increase in dermal mobile
water in aged females compared to younger
females (16, 17) although some other data sug-
gest only a small age-related difference (8).
We hypothesized that a further manifestation
of such changes in water state should cause an
age-dependent increase in skin tissue dielectric
constant (TDC) that would be detectible at
depths below the SC in and deeper to the der-
mis. Thus, one aim of this study was to test this
hypothesis by measuring skin TDC values to
several skin depths in groups of younger and
older women. Further, because skin-to-fat TDC
values show promise as a way to characterize
skin water changes in a variety of circumstances
(18–27) and as a tool for lymphedema assess-
ment (28–32), a secondary aim was to establish
reference ranges suitable for use with young
and older women.
Methods
Subjects
Female volunteers (N=270), divided into two
age groups, young (20–40 years, N=165) and
older (≥60 years, N=105) with Fitzpatrick’s
skin types II–IV, separated into four measure-
ment groups (I, II, III, IV), were evaluated after
the research nature of the study was explained
to them and they had signed an informed con-
sent that was previously approved by the Uni-
versity Institutional Review Board. For each
group, TDC measurements (described subse-
quently) were made at the depths and with the
numbers per group as indicated in Table 1.
Depending on the specific group, mean ages
ranged from 27.3 to 30.3 years for the young
and from 68.9 to 70.5 years for the older group.
For all measurement groups age and body mass
index were significantly greater in the older
group (P<0.01). Prior to participation, subjects
were asked not to apply any lotion or creams to
their forearms on the day of the scheduled pro-
cedure. Subjects with any known skin condition
affecting forearm skin, any injury or open
wound on either arm, or any prior arm trauma
that might have affected tissue water were
excluded from participation.
Tissue dielectric constant measurement method
The dielectric constant, also known as permittiv-
ity, is a dimensionless number equal to the ratio
of the permittivity of a measured tissue to the
permittivity of vacuum. Because TDC values
mainly depend on tissue water content, TDC
values and their change may provide indices of
water content and quantitative estimates of
water content changes. For reference, the dielec-
tric constant of distilled water at 32°C is approx-
imately 76. As the measurement frequency is
300 MHz the TDC values are sensitive to both
free and bound water. Measuring of the bound
water component is important because up to
80–90% of young adult skin water content is
bound (7) although this percentage may
decrease substantially with skin aging (10).
Measurements of TDC have been used in basic
and clinical research studies in which skin tissue
water and its change were of interest at various
anatomical sites including face (33, 34), breast
(25), forearm (21, 35, 36), biceps, axilla and thorax
(37), leg and foot (18, 22), and buttocks (38). TDC
measurements have also been used to characterize
TABLE 1. Group features and forearm TDC values
Effective measurement depth
Group I (0.5 mm) Group II (1.5 mm) Group III (2.5 mm) Group IV (5.0 mm)
Young Older Young Older Young Older Young Older
N50 50 50 50 100 100 50 50
AGE (years) 30.3 7.5 69.4 6.9** 29.9 3.7 69.5 6.8** 27.3 6.2 70.5 6.7** 27.5 4.3 68.9 7.2**
BMI (kg/m
2
) 24.9 5.3 28.5 6.3* 24.2 5.4 27.8 7.6* 23.9 5.3 28.6 5.9** 23.5 4.6 28.4 6.3**
TDC value 31.6 4.3 37.4 5.5** 30.3 3.3 34.2 4.5** 26.6 4.7 27.6 4.0 25.0 4.5 22.3 3.6**
% TDC difference 18.4 12.9 3.8 10.8
BMI, body mass index.
Values are mean SD; Older vs. Younger; *P<0.01, **P<0.001.
TDC values for both groups significantly decrease with increasing depth (P<0.001). Note that the older group TDC values at 0.5 and 1.5 mm depths
are greater than for the young group but at the deepest depth the older group value are significantly less that for the young. Percentage Difference is
calculated as 100 9(older young)/young for each depth.
2
Mayrovitz et al.
and track changes related to lymphedema (29,
30, 39), changes in post-surgical fluid status (27),
and assess skin irradiation effects (25).
In use the device generates and transmits a
very low power 300 MHz signal into a coaxial
probe that is in contact with the skin with the
probe acting as an open-ended coaxial transmis-
sion line (40). Part of the signal is absorbed,
mainly by tissue water, and part is reflected back
to a control unit where the complex reflection
coefficient is calculated (41, 42) from which the
dielectric constant is determined (43, 44). Reflec-
tions from the end of this coaxial transmission
line depend on the complex permittivity of the
tissue which in turn depends on signal fre-
quency and the dielectric constant (the real part
of the complex permittivity) and the conductiv-
ity of the tissue with which the probe is in con-
tact. At 300 MHz the contribution of
conductivity to the overall value of the permit-
tivity is small and the dielectric constant is
mainly determined by water molecules (free and
bound). Consequently, the device includes and
analyzes only the dielectric constant that is
directly proportional to tissue water content in a
manner close to that predicted by Maxwell mix-
ture theory for low water content but a slightly
less good prediction for high water content tis-
sues (45). In all cases, TDC is strongly dependent
on relative water content with TDC values that
decrease with water reductions during hemodi-
alysis (24). Further details including validation
and repeatability data are described in the litera-
ture (18, 24, 46). Each probe is calibrated against
various ethanol–water mixture concentrations
each of known dielectric constant values (46).
Measurement procedure
Tissue dielectric constant measurements were
made with the MoistureMeterD (MMD, Delfin
Technologies, Kuopio, Finland). This device
measures skin and the skin-to-fat TDC at a fre-
quency of 300 MHz by touching skin with a
small hand held probe for about 10 s. In prac-
tice, one of four different probes can be used
having outer diameters from 10 mm, for a
0.5 mm effective measurement, depth to
55 mm, for a 5 mm measurement depth. Effec-
tive measurement depth is defined as the depth
at which the 300 MHz electric field decreases to
1/e of its surface field. In this study, all probes
were used allowing for TDC measurements to
effective depths of 0.5, 1.5, 2.5, and 5.0 mm.
Measurements were done with subjects supine
on a padded and insulated examination table
with arms resting palms up and were started
after they had been supine for a minimum of
5 min. Measurement sites were both volar fore-
arms 6 cm distal to the antecubital fossa with
each site measured in triplicate. Measurements
between right and left arm were alternated until
three values per arm were obtained. The aver-
age of the three measurements was used to
characterize the TDC value of each arm and
reported as the average of the two arms. This
procedure was done for each of the four effec-
tive measurement depths.
Analysis
All statistical tests were done with SPSS (V 13;
IBM, Armonk, NY, USA). TDC values at all
depths proved to be normally distributed as
tested using the Shapiro-Wilks test. Differences
in TDC values among depths within age groups
were tested using ANOVA with post hoc evalua-
tions of between depth differences done with
Bonferroni adjustments. Differences between
age groups were evaluated using independent
t-tests for each depth separately. A P-value less
than or equal to 0.01 was accepted as significant.
This significance level was adopted to reduce
type II errors that might arise because of using
four independent t-tests, one for each depth.
Results
The main numerical experimental results
(mean SD) are summarized in Table 1 and
the age pattern visualized graphically in Fig. 1
for each effective measurement depth. Results
show that within each age group TDC values
monotonically decrease with increasing depth
(P<0.001). For the young group, TDC values
at 0.5 and 1.5 mm depths were not significantly
different from each other (P=0.441) but were
both greater than TDC values measured at 2.5
and 5.0 mm depths (P<0.001). The 2.5 and
5.0 mm TDC values were not significantly dif-
ferent from each other (P=0.113). For the older
group, TDC values at all depths differed from
all other depths (P<0.001).
Comparisons of TDC values between age
groups at each depth showed a pattern (Fig. 1)
in which older values at 0.5 and 1.5 mm depths
3
Age-related difference in skin dielectric constant
were both significantly greater than the younger
group TDC values at corresponding depths
(P<0.001). At a depth of 2.5 mm there was no
significant difference between age groups
(P=0.108), and at a depth of 5.0 mm the direc-
tion of the difference reversed with the older
group now having TDC values that were less
than the younger group (P<0.001). TDC values
were significantly correlated with body mass
index only for the deepest measurement
(5.0 mm) yielding negative correlation coeffi-
cients of 0.335 (P=0.01) and 0.301 (P=0.05)
for the younger and older groups, respectively.
Discussion
Investigations of potential age-related differ-
ences in skin properties have previously
focused on a variety of skin parameters includ-
ing skin thickness (15, 47–51), TEWL (2, 3, 52,
53), mechanical properties (1, 4, 54–59), pH and
sebum content (60–62) and SC properties, and
water content (6, 8, 61, 63–66). To our knowl-
edge the present is the first systematic investi-
gation and report of age-related differences in
skin-to-fat TDC values between young and
older females. A major new finding based on
these TDC measurements of volar forearm skin
in a large number of females is that the magni-
tude and direction of differences between age
groups depends on the depth of the tissue
included in the measurement.
For shallower depths, that included tissue vol-
umes to 0.5 and 1.5 mm below the skin surface,
TDC values were greater for older females. Dif-
ferences between age groups disappeared when
measurements were to depths of 2.5 mm. At an
effective measurement depth of 5.0 mm, TDC
values were now greater for the younger
females. The greater TDC values of the older
females measured at the shallower depths is
consistent with the previously noted age-related
shift in water state from mostly bound water to
more mobile water with increasing age. This
would be predicted as mobile water has a
greater dielectric constant than bound water.
The fact that the greatest difference between
older and younger females occurs at the shal-
lowest depth of 0.5 mm (18.4%, Table 1) and
decreases to 12.9% at 1.5 mm depth is explain-
able by considering the tissue type included in
the two measurement depths. TDC measure-
ments to a depth of 0.5 mm would include all of
the epidermis and a portion of the dermis,
whereas measurements to a depth of 1.5 mm
would include the dermis and also include a
portion of hypodermis with its low water con-
tent fat. Because the shift in water state occurs in
epidermis and dermis, the expected increased
TDC value that would be associated with more
mobile water would be blunted by including
greater amounts of fat-rich hypodermis. This
process could also explain the non-significant
difference in TDC values between age groups at
a measurement depth of 2.5 mm as at this depth
there is a large fraction of the total tissue volume
attributable to hypodermis thereby diminishing
the net effect of the increase in dermal TDC
value of the older group.
The reversal of the TDC difference at the
deepest depth (5.0 mm) is not fully explainable
based on a shift in water state. However, other
factors may be involved. From volar forearm
dermal thickness values reported (50) for the
age range of female subjects evaluated in the
present study, a reduction in average thickness
of 0.95 mm (age 20–40 years) to an average of
0.85 mm (age 60–90 years) occurs with similar
thickness reductions reported by others (15). By
itself this change would favor a lesser TDC
value in older females as percentage wise, less
high content dermal water would be included
in a fixed-depth measurement volume. Thus,
the cross-over herein observed at the deepest
measurement depth (5.0 mm), characterized by
20
22
24
26
28
30
32
34
36
38
40
0.5 1.5 2.5 5.0
Young
Older
Depth (mm)
TDC
**
**
**
Fig. 1. Age and depth dependence of TDC values. TDC values for
young and older groups decrease with increasing depth (P <0.001)
with the older group having greater TDC values at the two shal-
lower depths (0.5 and 1.5 mm) but a lesser value at the deepest
depth; **P<0.001 older vs. young groups with no significant dif-
ference at an effective depth of 2.5 mm. Error bars are one SEM.
4
Mayrovitz et al.
a lesser TDC value in the older group, might be
explained by this process because at this depth
the large percentage of low TDC value hypo-
dermis might overwhelm the impact of the
change in water state within a now thinner
dermis.
In addition to providing insights into factors
affecting TDC measurements and differences
between age groups, the present data also pro-
vide a substantial reference set of TDC values
for each effective measurement depth. As
already noted, forearm TDC measurements
have shown potential for characterizing and
possibly detecting early onset lymphedema (28,
32, 67) because of the strong dependence of skin
TDC on water content. The present data pro-
vide a set of non-edematous forearm skin refer-
ence values that could be used to aid in
detecting edema or lymphedema presenting in
forearms. This could be done by determining
the departure of measured values from the ref-
erence set herein contained specific to the age-
range group of the patient being evaluated and
the depth of the measurement. For that purpose
a multiple of the SD presented in Table 1
would be added to the corresponding TDC
mean value to arrive at a threshold value above
which the presence of edema or lymphedema is
likely to be present. For example, if TDC mea-
surements were made to a depth of 2.5 mm, a
value exceeding a 2.5 SD threshold (99.5% of
cases would be less than this value) would
likely represent the presence of edema or lym-
phedema. For the data herein obtained, these
thresholds are 38.3 and 37.7 for the younger
and older age groups, respectively. Other
thresholds can be computed and used in a simi-
lar manner depending on the degree of
diagnostic conservatism desired.
In summary, the present TDC measurement
results; (i) are consistent with the hypothesis of
an age-related shift in the water state from less-
to-more mobile, (ii) help explain the depth-
dependence differences between ages, and (iii)
provide for the first time age-related TDC refer-
ence values for use in assessing local edematous
or lymphedematous states that manifest in fore-
arms.
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Address:
H. N. Mayrovitz
Professor of Physiology
College of Medical Sciences
Nova Southeastern University
3200 S. University Drive
Fr. Lauderdale
FL 33328
USA
Tel: +1 954-262-1313
Fax: +1 954-262-1802
e-mail: mayrovit@nova.edu
7
Age-related difference in skin dielectric constant
