cosmetics
Article
Photothermal Radiometry for Skin Research
Perry Xiao
School of Engineering, London South Bank University, 103 Borough Road, London SE1 0AA, UK;
[email protected]; Tel.: +44-207-815-7569; Fax: +44-207-815-7699
Academic Editor: Enzo Berardesca
Received: 3 December 2015; Accepted: 17 February 2016; Published: 29 February 2016
Abstract: Photothermal radiometry is an infrared remote sensing technique that has been used
for skin and skin appendages research, in the areas of skin hydration, hydration gradient, skin
hydration depth profiling, skin thickness measurements, skin pigmentation measurements, effect of
topically applied substances, transdermal drug delivery, moisture content of bio-materials, membrane
permeation, and nail and hair measurements. Compared with other technologies, photothermal
radiometry has the advantages of non-contact, non-destructive, quick to make a measurement (a few
seconds), and being spectroscopic in nature. It is also colour blind, and can work on any arbitrary
sample surfaces. It has a unique depth profiling capability on a sample surface (typically the top
20 µm), which makes it particularly suitable for skin measurements. In this paper, we present a review
of the photothermal radiometry work carried out in our research group. We will first introduce the
theoretical background, then illustrate its applications with experimental results.
Keywords: photothermal radiometry; skin hydration; skin hydration depth profiling; trans-dermal
drug delivery; hair and nail
1. Introduction
Photothermal radiometry (PTR), or opto-thermal radiometry (OTR), is an infrared remote sensing
technique that was independently developed by Tam et al. [1] and Imhof et al. [2] back in the 1980s.
It has since been used for biomedical applications [3–5] and for industrial non-destructive testing
(NDT) [6,7]. Photothermal radiometry uses a modulated (sinusoidal or pulsed) light source (i.e., laser,
etc.) to heat up the sample surface, and uses a fast infrared detector (e.g., Mercury Cadmium Telluride
or MCT) to pick up the sample’s corresponding blackbody radiation due to the temperature increase.
By analyzing the shape of the signal, we can get information on sample’s optical properties, thermal
properties, the thickness of the sample, and its layer structure. The pulsed laser version—Opto-Thermal
Transient Emission Radiometry (OTTER)—developed initially by Imhof et al. [2,8,9], and consequently
by Xiao et al. [9–19], has been intensively used for skin and skin appendage measurements.
2. Theoretical Background
When an incident light beam reaches the skin’s surface, a small fraction of the light will
be reflected due to the change in refractive index between air (nair = 1.0) and stratum corneum
(nsc = 1.55). The rest of the light will pass through the surface and go into the skin, undergoing a
process of multiple scattering and absorption [20]. Absorbed light energy is converted to heat, which
increases the temperature of the skin. Scattered light contributes to a diffuse distribution of light within
the skin. Scattering and absorption determine the light penetration depth and light distribution within
the skin. The degree of scattering and absorption depends on the wavelength of the incident light
and the optical properties of the skin, which can be described by its absorption coefficient, scattering
coefficient, and the anisotropy.
Cosmetics 2016, 3, 10; doi:10.3390/cosmetics3010010
www.mdpi.com/journal/cosmetics
Cosmetics 2016, 3, 10
2 of 14
Some of the light travelling in the skin will also be reflected at the interfaces of the skin layers
and blood vessels. Some of this reflected and back scattered light can travel through the skin and
finally go back into the air. This is called the remitted light. The remittance from epidermis and
dermis, together with the regular reflectance from the skin surface form the total remittance of the
incident radiation, which give the colour of the skin. The measurement of this remittance can give
information on erythema, blanching, melanin, glucose level, oxygen level, haemoglobin, blood flow,
and skin structure.
These light–tissue interactions can normally be divided into three domains: absorption domain,
scattering domain, and equal scattering and absorption domain. Absorption domain is where
absorption events occur more rapidly than effective scattering events. This is true for some UV
(193, 248, 308 nm of ArK, KrF, XeCl excimer lasers) and the IR (2.94 µm Er:YAG, 10.6 µm CO2 lasers)
wavelengths, where tissue absorption can be substantially larger than scattering. Scattering domain is
where multiple scattering events occur before an absorption event occurs. This is true for wavelengths
between 600 and 1300 nm, where haemoglobin and melanin pigments do not absorb strongly [21]. The
band between 600 and 1300 nm is often called the “therapeutic window”, which offers the possibility of
treating large volumes with certain long-wavelength photosensitizers. Equal scattering and absorption
domain is where both absorption and scattering are substantial, and light in the skin has a strongly
collimated component surrounded by a region where light is multiply scattered. This is true for
wavelengths between 450 and 590 nm, which include the argon laser and Nd:YAG laser wavelengths,
the penetration depth is approximately 0.5 to 2.5 mm [22].
OTTER technology mainly works in the absorption domain when 2.94 µm Er:YAG laser is used
and in the equal scattering and absorption domain when Nd:YAG laser (532 nm) or OPO (optical
parametric oscillator) laser (420–590 nm) is used. OTTER is only sensitive to absorbed light, due
to the MCT infrared detector used. The reflected light, scattered light, and remitted light make no
contribution to the OTTER signal.
After the laser pulse, the absorbed laser light energy will decay exponentially into deeper skin
according to the Beer-Lambert law, and the corresponding temperature changes inside skin can be
described as:
E0 α ´αz
e
(1)
θpz, 0q “
ρC
where θpz, 0q is the temperature at time t = 0 of the skin at position z, with z = 0 at the surface and
increasing toward the inside of the skin. α is the absorbance for the excitation laser, C the specific heat,
ρ the density, and E0 the energy density absorbed from the excitation laser. This initial temperature
will then re-distribute following the diffusion law, which, in general, can be expressed as:
„

˙
$ ˆ
B
B
B
’
’
D
ˆ
´
θpz, tq “ 0
’
’
Bz
Bt
& Bz
Initial Condition :
’
’
’
’
%
Boundary Condition :
θpz, tq| t“0 ˇ“ θpz, 0q
Bθpz, tq ˇˇ
´k
z “0 “ 0
Bz ˇ
(2)
where D is the thermal diffusivity of the skin, and k is thermal conductivity of the skin. In skin
measurements, we are mainly interested in optical properties, as thermal properties do not change
very much. Therefore, it is safe to assume that the skin is thermally homogeneous; then, Equation (2)
can be solved by using Green’s function method, to yield a time-dependent transient temperature field
as [10]:
$
,
1
1
’
/
2
2
&
α Dt ´ αz
α Dt ` αz .
E0 α α2 Dt
2 q ` eαz er f cp ?
2 q
θpz, tq “
e
e´αz er f cp ?
(3)
’
/
2Cρ
α2 Dt
α2 Dt
%
-
( z, t ) 
e Dt ez erfc(
2C


0
2
 Dt
2
)  ez erfc(
2
 Dt
2
)


Cosmeticserfc
2016,( 3,
x 10
)  1  erf ( x ) is the complementary error function and erf ( x ) 
where
(3)
2


x
0
e
y2
3d
ofy14
is the error function.
2 şx
2
where
f cpxq “ 1 ´ er f pxq
is thesignal
complementary
function
and temperature
er f pxq “ ? 0(ez´, ty) dy
the
canis be
Theer corresponding
OTTER
generated error
by this
transient
π
calculated
by:
error function.
The corresponding OTTER signal generated by this transient temperature θpz, tqcan be
calculated by:
E  
S (t )  ζ E00β ż 8e z ( z , t )dz
(4)
Sptq “  C 0 e´βz θpz, tqdz
(4)
ρC 0

where β is the skin’s absorption coefficient for the emitted thermal radiation, and the parameter
where  is the skin’s absorption coefficient for the emitted thermal radiation, and the parameter
ζ “ ζpλem qincludes factors that depend on the black body emission curve, detector sensitivity, focusing
and
 alignment,
( em ) includes
that depend
on the black
body
emission
detector sensitivity,
but isfactors
independent
of the properties
of the
sample
per se curve,
[2,8–10].
focusing
and1alignment,
is independent
properties
of the sample
perNd:YAG
se [2,8–10].
Figure
shows the but
schematic
diagram of
forthe
OTTER
measurements.
When
laser (532 nm,
showsor
the
schematic
diagramnm,
for 10
OTTER
measurements.
When
Nd:YAG
laser
(532
nm,
10 nsFigure
pulse1width)
OPO
laser (420–590
ns pulse
width) is used
as the
excitation
heat
source,
10
nsFigure
pulse width)
OPO
laser (420–590
nm,
ns pulse
width)
is used asdeep
the excitation
see
1b, the or
laser
is mainly
absorbed
by10
melanin
and
haemoglobin
inside theheat
skin.source,
In this
see
Figure
1b,measuring
the laser isthe
mainly
absorbed and
by melanin
andthickness
haemoglobin
deep
insidesignal
the skin.
In this
case,
we are
skin pigments
epidermis
[10–14].
OTTER
is typically
case,
we arethermal
measuring
the
skin pigments
and epidermis
thickness
OTTER
a delayed
wave
signal,
and Equation
(4) can be solved
and[10–14].
expressed
as: signal is typically
a delayed thermal wave signal, and Equation (4) can be solved and expressed as:
τ2
t
c
´

t
e t2 t
e
Sptq “ A1t e1τ1 er f c t
(5)
` A2 c
S (t )  A1e erfc
τ1  A2
t
(5)
1
τt 2
2
1
L2 2
where A
the
τ2 “
Equation
(5) (5)
to fit
and AA
are
thesignal
signalamplitudes
amplitudesτ11“ 12 2 and
and
. Using
Equation
to the
fit
where
A11 and
 2  L . Using
2 2are
4D 4 D
α DD
signal we can get the melanin and haemoglobin absorption (α) and epidermis thickness (L).
the signal we can get the melanin and haemoglobin absorption (  ) and epidermis thickness (L).
Figure 1.1. The
Theschematic
schematicdiagram
diagramfor
forOTTER
OTTERhydration
hydrationmeasurements
measurements(a)
(a)and
and skin
skin pigments
pigments
Figure
measurements(b).
(b).
measurements
When the Er:YAG laser (2.94 µm wavelength, 100 ns pulse width, a few mille joules per pulse)
is used as the heat source, see Figure 1a, the laser is absorbed at the skin surface, by the water in the
skin, and the measurements are confined within stratum corneum, the outmost skin layer (~20 µm
in thickness). In this case, OTTER signal is typically a decay signal, and depending on the detection
wavelength, the OTTER signals can either reflect the water concentration information in skin (13.1 µm),
or solvent concentration information within skin (9.5 µm). By applying different mathematical models,
we can get the average water content in stratum corneum [10], the water concentration gradient
Cosmetics 2016, 3, 10
4 of 14
in stratum corneum [10,15], and water concentration depth profiles or solvent concentration depth
profiles in stratum corneum [16].
2.1. Homogeneous Model
If we assume the skin is optically homogeneous, and under the excitation saturation condition,
i.e., α ąą β, (this is true when an Er:YAG laser is used), then Equation (4) can be solved and expressed
as [1,8–10]:
c
t
t
τ
Sptq “ Ae er f c
(6)
τ
1
is the signal decay lifetime. By fitting Equation (6) to
β D
OTTER signals we can the best-fit value for τ, which can then be related to average water or solvent
concentration in the skin, depending on the detection wavelengths used.
where A is the amplitude of the signal, τ “
2
2.2. Gradient Model
If we assume the skin is optically non-homogeneous, but has a linear gradient distribution, e.g.:
βpzq “ β0 ` wz
(7)
where β0 is the absorption coefficient of the surface of the skin, and w is the gradient of the absorption
coefficient. Then, Equation (4) can be solved and expressed as [10,15]:
?
2W tτ
˚ ?πp2Wt ` 1q `
˚
˚
t{τ
a
Sptq “ A ˆ ˚
˚
t{τ
1
2t{τ
`
1
˝ b
?
q
e
er f cp
3
2Wt ` 1
p2Wt ` 1q
¨
˛
‹
‹
‹
‹
‹
‚
(8)
1
is the signal decay lifetime. By fitting Equation (8)
β0 2 D
to OTTER signals we can get information on τ and W, which can then be related to the skin surface water
(or solvent) concentration, and the water (or solvent) concentration gradient within the skin [10,15].
where W = wD is the effective gradient and τ “
2.3. Non-Homogeneous Model
If we assume that skin is optically irregularly non-homogeneous, then Equation (4) will be difficult
to solve. However, we can resolve this non-homogeneous issue using our Segmented Least-Squares
(SLS) fitting technique [16]. Our previous study shows that, for an OTTER decay signal, at different
times after the laser excitation, on average, the signal comes from different depths, which we call the
“mean detection depth”, Zptq. It can be calculated as [9]:
ş8
Zptq “
0
βe´βz ¨ z ¨ θpz, tqdz
ş8
´βz θpz, tqdz
0 βe
(9)
where θpz, tq is the transient temperature field. Again, under the excitation saturation condition
(α ąą β), we have:
c
t
t
2
τ
τ
Zptq “ ? ˆ
(10)
c ´2ˆ β
t
π
t
βe τ er f c
τ
( z , t ) is the transient temperature field. Again, under the excitation saturation condition
(    ), we have:
where
Cosmetics 2016, 3, 10
t
2

Z (t ) 
 t
  e  erfc t
t

 2 

5(10)
of 14
where
theaverage
averagevalue
valueofofβ,β,calculated
calculatedby
byusing
usingEquation
Equation(6)
(6) to
to fit
fit the
the whole
whole time-span
time-span of
of the
the
where βisisthe
signal.
Figure
2
shows
a
typical
relationship
between
the
OTTER
signal
S(t)
and
its
corresponding
mean
signal. Figure 2 shows a typical relationship between the OTTER signal S(t) and its corresponding
detection depth Zptq forZ (at )semi-infinite homogenous sample. Thus, in the case of surface dominant
for a semi-infinite homogenous sample. Thus, in the case of surface
mean detection depth
absorption, the initial signal data points give information about the surface of the sample, and later
dominant absorption, the initial signal data points give information about the surface of the sample,
signal data points give information about deeper parts of the sample. In other cases, it depends on the
and later signal data points give information about deeper parts of the sample. In other cases, it
sample’s actual transient temperature θpz, tq and emission absorption coefficient β.
depends on the sample’s actual transient temperature ( z, t ) and emission absorption coefficient β.
Figure 2.
The OTTER
OTTER signal
signal and its average radiation depth, left Y axis is the normalized
2. The
normalized signal
intensity,
intensity, right
right Y
Y axis
axis is
is the
the mean
mean detection
detection depth,
depth, and
andXX axis
axis isis normalized
normalizedtime
time[10].
[10].
Our Segmented
Segmented Least-Squares
analysis technique
technique is
based on
on the
the
Our
Least-Squares (SLS)
(SLS) fitting
fitting data
data analysis
is based
above-mentioned
principles
[16].
First,
we
divide
the
signal
data
points
into
a
series
of
small
pieces.
above-mentioned principles [16]. First, we divide the signal data points into a series of small pieces.
Second, we
use aa least-squares
least-squares fitting
routine to
to fit
fit each
each piece
piece separately
separately to
to Equation
Equation (6),
get the
the
Second,
we use
fitting routine
(6), to
to get
best-fit value
valueofofthe
thedecay
decay
lifetime
for that
Finally,
we relate
each piece’s
its mean
 that
 to radiation
best-fit
lifetime
τ for
piece.piece.
Finally,
we relate
each piece’s
τ to its mean
radiation
depth
using
Equation
(10),
to
create
a
depth-resolved
profile,
which
can
then
be

depth using Equation (10), to create a depth-resolved τ profile, which can then be related to therelated
water
to the
waterconcentration
(or solvent) concentration
depth
profiles
(or
solvent)
depth profiles
within
skin. within skin.
3. OTTER
OTTERApplications
Applications
3.1. Skin Pigment Measurements
In this case, a Nd:YAG laser (532 nm) or OPO laser (420–590 nm) is used as the heat source in
the measurements, and the measurement signal is typically a delayed thermal wave. Figure 3 shows
the typical signals of skin pigment measurements [10]. The hump in the signal, immediately after the
initial decay, is due to the heat diffusion from deep inside the skin to skin surface.
Cosmetics 2016, 3, 10
6 of 14
In this case, a Nd:YAG laser (532 nm) or OPO laser (420–590 nm) is used as the heat source in
the measurements, and the measurement signal is typically a delayed thermal wave. Figure 3 shows
Cosmetics 2016, 3, 10
6 of 14
the typical signals of skin pigment measurements [10]. The hump in the signal, immediately after the
initial decay, is due to the heat diffusion from deep inside the skin to skin surface.
Figure 3. The OTTER signals of different types of skin (a); different position on forearm (b); hot water
effect (c);
(c); and
and bruised
bruised skin
skin (d)
(d) [10].
[10].
effect
Figure 3a
3a shows
shows the
the signals
signals from
from different
different types
types of
of skin;
skin; changes
changes in
in signals
signals are
are due
due to
to melanin
melanin
Figure
differences. Figure
different positions
positions of
of the
the volar
volar forearm,
forearm, changes
changes in
in
differences.
Figure 3b
3b shows
shows the
the signals
signals from
from different
signals
are
due
to
epidermis
thicknesses
changes.
Figure
3c
shows
the
signals
before
and
after
signals are due to epidermis thicknesses changes. Figure 3c shows the signals before and after forearm
forearm
was by
washed
by hot
water, in
changes
signals
due to haemoglogbin
blood
was
washed
hot water,
changes
signalsinare
due toare
haemoglogbin
changes, changes,
i.e., bloodi.e.,
flows
to
flows
to
surface.
Figure
3d
shows
the
signals
of
normal
skin
and
bruised
skin
(sport
injury,
caused
surface. Figure 3d shows the signals of normal skin and bruised skin (sport injury, caused by a bump,
by a bump,
blood
leaksunder
into tissues
the skin
causes the black-and-blue
colour),
where
blood where
leaks into
tissues
the skinunder
and causes
the and
black-and-blue
colour), again, changes
again,
changes
in
signals
are
due
to
blood
flowing
to
skin
surface.
By
fitting
the
Figure
3
signals
in signals are due to blood flowing to skin surface. By fitting the Figure 3 signals using Equation
(5),
using
(5), we about
can getthe
information
about
thehaemoglobin
mean melanin
and haemoglobin
concentration
we
canEquation
get information
mean melanin
and
concentration
and mean
epidermis
and meanBy
epidermis
By analysis
applyingtechniques,
to the inverse
analysis
techniques,
such as singular
thickness.
applying thickness.
to the inverse
such as
singular
value decomposition
(SVD),
value
decomposition
(SVD),
Conjugate
Gradient
(CG),
Maximum
Entropy
Method
(MEM),
etc.,
we
Conjugate Gradient (CG), Maximum Entropy Method (MEM), etc., we can get more information about
can
get
more
information
about
the
skin
pigments’
(melanin
and
haemoglobin)
depth
profiles
the skin pigments’ (melanin and haemoglobin) depth profiles [12,13]. By using different wavelengths
with the OPO laser, we can also differentiate melanin from haemoglobin [14,15].
Cosmetics 2016, 3, 10
7 of 14
[12,13]. By using different wavelengths with the OPO laser, we can also differentiate melanin from
haemoglobin
Cosmetics
2016, 3, [14,15].
10
7 of 14
3.2. Skin Water Content Measurements
3.2. Skin Water Content Measurements
In this case, an Er:YAG laser (2.94 µm) is used as the heat source in the measurements, and the
In this
case, an Er:YAG
(2.94measurement
µm) is used as
the heat
source aindecay
the measurements,
the
detection
wavelength
is 13.1 laser
µm. The
signal
is typically
curve. Figure 4and
shows
detection
13.1 µm.skin
Thesite
measurement
signal is typically
decay curve. Figure
shows
the
typicalwavelength
signals of is
different
(a) and corresponding
watera concentration
depth 4profiles
the typical
of different
skin site(b).
(a) The
and results
corresponding
water
concentration
profiles
within
skin signals
using SLS
fitting technique
show that
forehead,
face, anddepth
forearm
have
within
skin water
using SLS
fitting
(b). The
results
showhave
thatthe
forehead,
forearm
the highest
content
andtechnique
high gradient.
Nail
and hair
lowest face,
waterand
content
andhave
low
the highestThe
water
content
and high
gradient.
Naildepth
and hair
have is
thelikely
lowest
water
content
and hair
low
gradient.
curved
feature
of hair
hydration
profile
due
to the
layered
gradient. The curved feature of hair hydration depth profile is likely due to the layered hair structure.
structure.
OTTER Signals
1
35
0.8
30
0.6
25
0.4
15
0
10
0
0.5
1
1.5
2
2.5
Time [ms]
(a)
3
3.5
4
4.5
Forehead
Face
Arm
Hand
Finger
Nail
Hair
20
0.2
-0.2
Hydration Depth Profiles
40
Hydration [%]
Intensity [a.u.]
1.2
5
0
2
4
6
8
10
12
14
Depth [um]
(b)
Figure
4. The
the
Figure 4.
The typical
typical OTTER
OTTER signals
signals of
of different
different skin
skin sites,
sites, including
including hair
hair and
and nail
nail (a),
(a), and
and the
corresponding
water
concentration
depth
profiles
using
SLS
fitting
technique
(b).
corresponding water concentration depth profiles using SLS fitting technique (b).
By scanning along the surface of volar forearm, we can also produce a 3D water concentration
By scanning along the surface of volar forearm, we can also produce a 3D water concentration
depth profile within skin, see Figure 5. The result shows that water distribution on the surface is
depth profile within skin, see Figure 5. The result shows that water distribution on the surface is
relatively uniform, but the depth profiles are quite different along the arm from wrist to elbow.
relatively uniform, but the depth profiles are quite different along the arm from wrist to elbow. Across
Across the arm, near the wrist, water distributions are also different from one side to another. This
the arm, near the wrist, water distributions are also different from one side to another. This is likely to
is likely to reflect the stratum corneum thickness changes at different positions of the arm.
reflect the stratum corneum thickness changes at different positions of the arm.
Cosmetics
Cosmetics 2016,
2016, 3,
3, 10
10
88of
of14
14
Figure
The 3D
3D water
water concentration
concentration depth
depth profiles
profiles along
along the
the volar
volar forearm.
forearm.
Figure 5.
5.The
3.3. Solvent Penetration Measurements
By selecting detection wavelength at 9.5 µm,
µm, where water has relatively low absorption and
chemical solvents have relatively high absorption, we can also measure the solvent
solvent penetration
penetration
through the skin [16,17,23,24]. Figure
Figure 66 shows
shows the OTTER results of Dimethyl sulfoxide (DMSO)
penetration
a small
amount
of undiluted
DMSO
solvent
(~0.1(~0.1
mL)
penetration through
throughskin.
skin.InInthis
thismeasurement,
measurement,
a small
amount
of undiluted
DMSO
solvent
is
applied
on a test
for site
5 min.
theAfter
skin surface
wiped dry,
tape stripping
performed,
mL)
is applied
on skin
a testsite
skin
for After
5 min.
the skinissurface
is wiped
dry, tapeisstripping
is
in
order to see
how
deepand
thehow
DMSO
penetrates.
OTTER
measurements
are performed,
performed,
in how
ordermuch
to seeand
how
much
deep
the DMSO
penetrates.
OTTER measurements
both
before andboth
after before
the DMSO
solvent
and application,
after each tape
stripping,
with
a total
of 10
are performed,
and after
theapplication,
DMSO solvent
and
after each
tape
stripping,
tape
Figure
6 shows
the Figure
OTTER6signals
the corresponding
DMSO
concentration
with strips
a totalapplied.
of 10 tape
strips
applied.
shows(a),
theand
OTTER
signals (a), and
the corresponding
depth
using SLS
fitting
technique
at different
tape stripping
(b). The
results
show(b).
a high
DMSOprofiles
concentration
depth
profiles
using SLS
fitting technique
at different
tape
stripping
The
DMSO
after its application,
andafter
thenits
as tape
stripping
continued,
results concentration
show a high immediately
DMSO concentration
immediately
application,
and
then as deep
tape
into
the skin,
DMSO concentration
gradually
reduced.
Other solvents
that
have been
studied
include
stripping
continued,
deep into the skin,
DMSO
concentration
gradually
reduced.
Other
solvents
that
propylene
glycol,
ethylene
glycol,
glycerol,
alcohol,
decanol,
butyl
acetate,
etc.
[23,24].
OTTER
can
have been studied include propylene glycol, ethylene glycol, glycerol, alcohol, decanol, butyl
work
well
both inOTTER
vivo and
vitro skin
acetate,
etc.on
[23,24].
caninwork
well samples.
on both in vivo and in vitro skin samples.
Cosmetics 2016, 3, 10
Cosmetics 2016, 3, 10
9 of 14
9 of 14
OTTER Signals
1.2
1
12
Before
After
Tape 2
Tape 3
Tape 5
Tape 10
10
DMSO Concentrations [a.u.]
Intensity [a.u.]
0.8
0.6
0.4
8
6
0.2
4
0
2
-0.2
DMSO Depth Profiles
14
0
0.5
1
1.5
2
2.5
Time [ms]
(a)
3
3.5
4
4.5
0
0
1
2
3
4
5
6
7
8
Depth [um]
(b)
Figure
6. The
the corresponding
corresponding DMSO
Figure 6.
The OTTER
OTTER signals
signals of
of DMSO
DMSO penetrating
penetrating through
through skin
skin (a)
(a) and
and the
DMSO
concentration
depth
profiles
using
SLS
fitting
technique
after
different
tape
stripping
(b).
concentration depth profiles using SLS fitting technique after different tape stripping (b).
3.4. Nail Measurements
3.4. Nail Measurements
Apart from in vivo and in vitro skin samples, OTTER can also work on nails, for both water
Apart from in vivo and in vitro skin samples, OTTER can also work on nails, for both water content
content measurements and solvent penetration measurements [24,25]. Figure 7 shows the OTTER
measurements and solvent penetration measurements [24,25]. Figure 7 shows the OTTER signals of
signals of human fingernail (a) and corresponding water concentration depth profiles before and
human fingernail (a) and corresponding water concentration depth profiles before and after a 15 min
after a 15 min soaking in water (b). In this experiment, a left hand nail was soaked in room
soaking in water (b). In this experiment, a left hand nail was soaked in room temperature water
temperature water for 15 min, then carefully patted dry. The OTTER measurements were performed
for 15 min, then carefully patted dry. The OTTER measurements were performed both before and
both before and after soaking. The nail water concentration has increased after the soaking,
after soaking. The nail water concentration has increased after the soaking, indicating absorbing of
indicating absorbing of water through soaking, and then as nail recovered in ambient condition, the
water through soaking, and then as nail recovered in ambient condition, the nail water concentration
nail water concentration gradually reduced to the normal level.
gradually reduced to the normal level.
Figure 8 shows the solvent penetration through nail measurements, data from Reference [24].
Figure 8 shows the solvent penetration through nail measurements, data from Reference [24].
In this experiment, three solvents were studied, glycerol, decanol, and butyl acetate. Solvents were
In this experiment, three solvents were studied, glycerol, decanol, and butyl acetate. Solvents were
applied to the fingernail of a volunteer using a filter pad (Cat. No. 1001 110, Whatman, Kent, UK)
applied to the fingernail of a volunteer using a filter pad (Cat. No. 1001 110, Whatman, Kent, UK) for
for a period of 5 min. The pad was then removed, the nail wiped ,and the solvent decay measured
a period of 5 min. The pad was then removed, the nail wiped ,and the solvent decay measured on
on removal and then at 8 and 10 min after the initial application. Glycerol exhibits the fastest
removal and then at 8 and 10 min after the initial application. Glycerol exhibits the fastest diffusion
diffusion through nail tissue of the three solvents tested, as after 5 min the concentration profile
through nail tissue of the three solvents tested, as after 5 min the concentration profile immediately
immediately starts to decrease.
starts to decrease.
Cosmetics 2016, 3, 10
10 of 14
Cosmetics 2016, 3, 10
10 of 14
Cosmetics 2016, 3, 10
OTTER Signals
10 of 14
Nail Hydration Depth Profiles After 15min Soaking
1.2
26
OTTER Signals
Nail Hydration Depth Profiles After 15min Soaking
1.2
26
24
1
24
1
22
0.8
22
20
20
0.6
Hydration
[%]
Hydration
[%]
IntensityIntensity
[a.u.] [a.u.]
0.8
0.6
0.4
0.4
18
18
16
16
14
0.2
14
12
0.2
Before
After
2 min
4 min
Before
13 min
After
min
236
min
12
0
10
0
10
-0.2
0
0.5
1
1.5
2
2.5
3
3.5
4
8
4.5
0
1
2
3
4
Time [ms]
-0.2
0
0.5
1
1.5
2
(a)
2.5
5
6
7
8
Depth [um]
3
3.5
4
8
4.5
0
Time [ms]
1
2
3
4
5
(b)6
7
8
4 min
9
10
13 min
36 min
9
10
Depth [um]
Figure 7. The OTTER signals
of in vivo human fingernail (a) and corresponding
water concentration
(a) of
(b) water
Figure 7. The OTTER signals
in vivo human fingernail (a) and corresponding
concentration
depth profiles using SLS fitting technique before and after a 15-min soaking (b).
depth
profiles
using
SLS
fitting
technique
before
and
after
a
15-min
soaking
(b).
Figure 7. The OTTER signals of in vivo human fingernail (a) and corresponding water concentration
depth profiles using SLS fitting technique before and after a 15-min soaking (b).
(a)
Figure
(a)8.Cont.
Figure 8.
8.Cont.
Figure
Cont.
Cosmetics 2016, 3, 10
11 of 14
Cosmetics 2016, 3, 10
11 of 14
(b)
(c)
Figure 8. Solvents penetration through in vivo human fingernail, (a) decanol, (b) glyceral, and (c)
Figure 8. Solvents penetration through in vivo human fingernail, (a) decanol, (b) glyceral, and (c) butyl
butyl acetate. For each solvent, the left plot shows the solvent concentration depth profiles and the
acetate. For each solvent, the left plot shows the solvent concentration depth profiles and the right plot
right plot shows the corresponding solvent concentration in nail at 5 min, 8 min and 10 min after the
shows the corresponding solvent concentration in nail at 5 min, 8 min and 10 min after the time of
time
of solvent
application.
Reproduced
with permission
from
[24], published
by Elsevier
B.V., 2011.
solvent
application.
Reproduced
with permission
from [24],
published
by Elsevier
B.V., 2011.
3.5. Hair Measurements
3.5. Hair Measurements
Our latest study shows that OTTER can also be used for hair measurements, despite of low
Our latest study shows that OTTER can also be used for hair measurements, despite of low water
water content in the hair. Figure 9 shows the hair water content measurements. In this experiment,
content in the hair. Figure 9 shows the hair water content measurements. In this experiment, an ex vivo
an ex vivo hair sample, freshly cut from a healthy volunteer, was hydrated by soaking in room
hair sample, freshly cut from a healthy volunteer, was hydrated by soaking in room temperature water
temperature water for 15 min, then patted dry afterwards. The OTTER measurements were
performed both before and periodically after the soaking. As explained earlier, in Figure 4, the
curved feature of hair hydration depth profile is likely due to the layered hair structure. It is
Cosmetics 2016, 3, 10
12 of 14
for
15 min,
Cosmetics
2016,then
3, 10 patted dry afterwards. The OTTER measurements were performed both before
12 and
of 14
periodically after the soaking. As explained earlier, in Figure 4, the curved feature of hair hydration
interesting
toispoint
soaking,
curvedtofeature
seems
disappear,
depth
profile
likelyout
duethat,
to theimmediately
layered hairafter
structure.
It is this
interesting
point out
that,toimmediately
whichsoaking,
might suggest
excess
water seems
absorbed
during soaking
gonesuggest
into different
layers absorbed
in hair. It
after
this curved
feature
to disappear,
whichhas
might
excess water
is also soaking
interesting
to point
that layers
it takes
40interesting
min for hair
to recover
its normal
during
has gone
into out
different
in more
hair. Itthan
is also
to point
out thattoit takes
more
hydration
level,
this to
is slower
skin and
the naillevel,
in ourthis
previous
studies
than
40 min
for hair
recoverthan
to itsthe
normal
hydration
is slower
than[10,26].
the skinAdditionally,
and the nail
as our
hairprevious
gradually
loses its
excess
water to theasambient
environment,
theexcess
curved
feature
of ambient
the hair
in
studies
[10,26].
Additionally,
hair gradually
loses its
water
to the
hydration
depth
profile
started
to
re-appear.
environment, the curved feature of the hair hydration depth profile started to re-appear.
OTTER Signals
Hair Hydration Depth Profiles
1.2
22
20
1
18
0.8
0.6
Hydrytion [%]
Intensity [a.u.]
16
0.4
14
12
10
0.2
8
Normal
After Soaking
5min
10min
15min
20min
30min
40min
0
6
-0.2
0
0.5
1
1.5
2
2.5
3
Time [ms]
(a)
3.5
4
4.5
4
0
2
4
6
8
10
12
Depth [um]
(b)
Figure9.9. OTTER
and corresponding
corresponding water
water concentration
concentration depth
depth
Figure
OTTERsignals
signals of
of ex
ex vivo
vivo human
human hair
hair (a),
(a), and
profiles in
in hair
hair before
before and
and after
after soaking
soaking (b).
(b).
profiles
3.6. Future Developments
3.6. Future Developments
By combining photothermal radiometry and Fourier Transform InfraRed spectroscopy (FTIR)
By combining photothermal radiometry and Fourier Transform InfraRed spectroscopy
[9,26,27] or diffraction grating [28], it is possible to achieve photothermal spectroscopy. Different
(FTIR) [9,26,27] or diffraction grating [28], it is possible to achieve photothermal spectroscopy. Different
from traditional FTIR, photothermal spectroscopy is depth-resolved, which means we can get the
from traditional FTIR, photothermal spectroscopy is depth-resolved, which means we can get the
spectra information from both the skin surface and at different depth underneath the skin surface.
spectra information from both the skin surface and at different depth underneath the skin surface.
This could be very useful for the study of skin properties, trans-dermal drug delivery, and skin
This could be very useful for the study of skin properties, trans-dermal drug delivery, and skin active
active ingredients. By using 2D matrix mercury cadmium telluride (MCT) detector, or the X–Y
ingredients. By using 2D matrix mercury cadmium telluride (MCT) detector, or the X–Y scanning
scanning stage, it is also possible to achieve photothermal microscopy. The combination of
stage, it is also possible to achieve photothermal microscopy. The combination of depth-resolved
depth-resolved photothermal spectroscopy and photothermal microscopy will create a whole new
photothermal spectroscopy and photothermal microscopy will create a whole new measurement
measurement technology—Photothermal Hyperspectral Imaging, which is imaging based,
technology—Photothermal Hyperspectral Imaging, which is imaging based, non-contact, non-invasive,
non-contact, non-invasive, depth-resolved, and spectroscopic.
depth-resolved, and spectroscopic.
4. Conclusions
4.
Conclusionsand
andFuture
FutureWorks
Works
This paper
paper presents
presents aa review
review of
of the
the photothermal
photothermal radiometry
radiometry technology
technology and
and its
its latest
latest
This
developments
for
skin
and
skin
appendages
research
and
reports
its
latest
developments.
Our
developments for skin and skin appendages research and reports its latest developments. Our studies
studies show that photothermal radiometry can be used for a range of skin measurements, including
skin pigments, skin water content, solvent penetrations, etc. It can work on skin, nail, and hair, both
in vivo and in vitro. Compared with other technologies, photothermal radiometry is non-contact,
non-destructive, and spectroscopic in nature. Additionally, it is quick to make a measurement (a few
Cosmetics 2016, 3, 10
13 of 14
show that photothermal radiometry can be used for a range of skin measurements, including skin
pigments, skin water content, solvent penetrations, etc. It can work on skin, nail, and hair, both
in vivo and in vitro. Compared with other technologies, photothermal radiometry is non-contact,
non-destructive, and spectroscopic in nature. Additionally, it is quick to make a measurement (a few
seconds). It is colour blind, and can work on any arbitrary sample surface. Photothermal radiometry
also has a unique depth profiling capability, around the top 20 µm of skin surface. For future works,
our studies show that is possible to create depth-resolved Photothermal Hyperspectral Imaging, which
is capable of generating skin images at different infrared wavelengths and at different skin depths.
This opens more potential opportunities, such as skin disease detection, skin cancer diagnosis, etc.
Acknowledgments: We thank London South Bank University for the finance support of this project. We also like
to thank the Engineering and Physical Sciences Research Council (EPSRC), Biotechnology and Biological Sciences
Research Council (BBSRC), Royal Society, and Unilever Port Sunlight Research for the previous support.
Conflicts of Interest: The author declares no conflict of interest.
References
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
Tam, A.C.; Sullivan, B. Remote sensing applications of pulsed photothermal radiometry. Appl. Phys. Lett.
1983, 43, 333–335. [CrossRef]
Imhof, R.E.; Birch, D.J.S.; Thornley, F.R.; Gilchrist, J.R.; Strivens, T.A. Opto-thermal transient emission
radiometry. J. Phys. E Sci. Instrum. 1984, 17, 521–525. [CrossRef]
Long, F.H.; Anderson, R.R.; Deutsch, T.F. Pulsed photothermal radiometry for depth profiling of layered
media. Appl. Phys. Lett. 1987, 51, 2076–2078. [CrossRef]
Jacques, S.L.; Nelson, J.S.; Wright, W.H.; Milner, T.E. Pulsed photothermal radiometry of port-wine-stain
lesions. Appl. Opt. 1993, 32, 2439–2446. [CrossRef] [PubMed]
Vitkin, I.A.; Wilson, B.C.; Anderson, R.R.; Prahl, S.A. Pulsed photothermal radiometry in optically transparent
media containing discrete optical absorbers. Phys. Med. Biol. 1994, 39, 1721–1744. [CrossRef] [PubMed]
Almond, D.P.; Patel, P.M. Photothermal Science and Techniques; ASIN: B017PNMQS8; Chapman and Hall:
London, UK, 1996.
Vavilov, V.P.; Burleigh, D.D. Review of pulsed thermal NDT: Physical principles, theory and data processing.
NDT & E Int. 2015, 73, 28–52.
Imhof, R.E.; Zhang, B.; Birch, D.J.S. Photothermal Radiometry for NDE. In Progress in Photothermal and
Photoacoustic Science and Technology; Mandelis, A., Ed.; PTR Prentice Hall: Englewood Cliffs, NJ, USA, 1994;
Volume II, pp. 185–236.
Imhof, R.E.; McKendrick, A.D.; Xiao, P. Thermal emission decay Fourier transform infrared spectroscopy.
Rev. Sci. Instrum. 1995, 66, 5203–5213. [CrossRef]
Xiao, P. Opto-Thermal Mathematical Modeling and Data Analysis in Skin Measurements. Ph.D. Thesis,
London South Bank University, London, UK, 1997.
Xiao, P.; Imhof, R.E. Inverse Method Analysis in Opto-Thermal Skin Measurements. SPIE Proc. 1999, 3601,
340–347.
Xiao, P.; Gull, S.F.; Imhof, R.E. Opto-Thermal Inverse Modelling Using a Maximum Entropy Approach. Anal.
Sci. 2001, 17, s394–s397.
Xiao, P.; Guo, X.; Notingher, I.; Cowen, J.A.; O’Driscoll, D.; Imhof, R.E. Optothermal Skin Pigment Spectral
Depth Profiling using an OPO Laser. SPIE Proc. 1999, 3601, 348–354.
Xiao, P.; Cowen, J.A.; Guo, X.; O’Driscoll, D.; Imhof, R.E. Photothermal In-vivo Characterization of Human
Skin using Tuneable OPO laser Excitation. In Photoacoustic and Photothermal Phenomena: 10th International
Conference, Rome, Italy, 23–27 August 1998; Scudieri, F., Bertolotti, M., Eds.; American Institute of Physics:
College Park, MD, USA, 1999; pp. 621–623.
Xiao, P.; Imhof, R.E. Opto-thermal measurement of water distribution within the stratum corneum. In Skin
Bioengineering Techniques and Applications in Dermatology and Cosmetology; Karger: Basel, Switzerland, 1998;
Volume 26, pp. 48–60.
Xiao, P.; Cowen, J.A.; Imhof, R.E. In-Vivo Transdermal Drug Diffusion Depth Profiling—A New Approach to
Opto-Thermal Signal Analysis. Anal. Sci. 2001, 17, s349–s352.
Cosmetics 2016, 3, 10
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
27.
28.
14 of 14
Xiao, P.; Ou, X.; Ciortea, L.I.; Berg, E.P.; Imhof, R.E. In Vivo Skin Solvent Penetration Measurements Using
Opto-thermal Radiometry and Fingerprint Sensor. Int. J. Thermophys. 2012, 33, 1787–1794. [CrossRef]
Xiao, P.; Imhof, R.E. Data Analysis Technique for Pulsed Opto-Thermal Measurements. UK Patent
Application 0004374.5, 25 February 2000.
Xiao, P.; Imhof, R.E. Apparatus for in vivo Skin Characterization. UK Patent Application GB1014212.3, 26
August 2010.
Scheuplein, R.J. A survey of some fundamental aspects of the absorption and reflection of light by tissue.
J. Soc. Cosmet. Chem. 1964, 15, 111–122.
Jacques, S.L. Laser Tissue Interactions. Las. Gener. Surg. 1992, 72, 531–557.
Yoon, G.; Welch, A.J.; Motamedi, M.; van Gement, M.C.J. Development and application of Three-Dimentional
Light Distribution Model for Laser Irradiated Tissue. IEEE J. Quan. Elec. 1987, 23, 1721–1733. [CrossRef]
Cowen, J.A. In-vivo Opto-thermal Transdermal Diffusion Measurement. Ph.D. Thesis, London South Bank
University, London, UK, 1999.
Xiao, P.; Zheng, X.; Imhof, R.E.; Hirata, K.; McAuley, W.J.; Mateus, R.; Hadgraft, J.; Lane, M.E. Opto-Thermal
Transient Emission Radiometry (OTTER) to image diffusion in nails in vivo. Int. J. Pharm. 2011, 406, 111–113.
[CrossRef] [PubMed]
Xiao, P.; Ciortea, L.I.; Singh, H.; Berg, E.P.; Imhof, R.E. Opto-thermal Radiometry for In-Vivo Nail
Measurements. J. Phys. Conf. Ser. 2009, 214, 012008. [CrossRef]
Notingher, I.; Imhof, R.E.; Xiao, P.; Pascut, F.C. Spectral Depth Profiling of Arbitrary Surfaces by Thermal
Emission Decay-Fourier Transform Infrared Spectroscopy. Appl. Spectrosc. 2003, 57, 1494–1501. [CrossRef]
[PubMed]
Notingher, I.; Imhof, R.E.; Xiao, P.; Pascut, F.C. Near-surface depth-resolved midinfrared emission
spectroscopy. Rev. Sci. Instrum. 2003, 74, 346–348. [CrossRef]
Pascut, F.C. Fiber-Optic Optothermal Emission Radiometry for In-vivo Skin Measurements. Ph.D. Thesis,
London South Bank University, London, UK, 2004.
© 2016 by the author; licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons by Attribution
(CC-BY) license (http://creativecommons.org/licenses/by/4.0/).