1
Biomimetic t
emperature
sensing
layer for
artificial
skins
Raffaele Di Giacomo
1
, Luca Bonanomi
1
, Vincenzo Costanza
1
, Bruno Maresca
2
and
Chiara Daraio
1
,3
1
Department of Mechanical and Process Engineering (D
-
MAVT), Swiss Federal Institute
of
Technology
(ETH), Zurich,
Switzerland.
2
Department
of Pharmacy, Division of Biomedicine,
University of Salerno, Fisciano, Italy.
3
Division of Engineering and Applied Science, California Institute of Technology, Pasadena,
CA, USA.
A
rtificial membranes
that are
sensitive to temperature
are needed
in
robotics
to
augment interactions with humans and the
environment
and
in
bio
engineering
to
improve prosthetic limbs. Existing flexible sensors achieved sensitivities
of
<
100
mK
and
large responsivity
albeit
within
narrow (
<
5 K) temperature ranges.
Other flexible
devices
,
worki
ng
i
n wider temperature ranges,
exhibit
orders of magnitude
poorer
responses
.
However,
much more versatile and temperature sensitive membranes are
present
in animals such as pit vipers, whose pit membranes have the highest sensitivit
y
and responsivity
in nature and are used to locate warm
-
blooded preys at distance.
Here,
we show that
pectin films mimic the sensing mechanism of pit membranes and parallel
the
ir
record
performance
s
.
These films map temperature on surfaces
with
a sensitivity
of at least 1
0 mK in a wide temperature range (4
5
K)
,
have very high responsivity,
and
detect warm bodies at distance.
The produced material
can be integrated as a layer in
artificial skins platforms and boost their
temperature
sensitivity to reach the best
biological
performance
.
Introduction
Artificial skins
(
1
,
2
)
are essential to augment robotics
(
3
)
and improve prosthetic limbs
(
4
)
.
Existing platforms are designed to emulate properties of the human skin by incorporating
sensitive functions
(
4
–
8
)
that respond
to different external stimuli
, e.
g., to variations of
temperature
(
4
,
8
–
10
)
. A
vailable a
rtificial skins that sense temperature variations use either
passive, flexible resistors
(
8
–
11
)
or active electronic devices
(
4
,
12
)
.
Their functionality is
limited by the choice of temperature sensitive materials incorporated in
the electronics
(
2
)
.
For example
p
-
n
junctions have small responsivity
,
require a
complex architecture and
demand for a
non
-
trivial
fabrication
procedures
(
4
,
12
)
.
Flexible sensors made of m
onolayer
-
capped nanoparticles
are at the same time as sensitive to temperature as
they are
to pressure
and humidity making impossible to deconvolve the t
hree
variable
s
in practical applications
(
8
)
.
C
omposites
based on a polymer matrix and
electrically
conducti
ve
filler
s
operate in
a
too
narrow
temperature
range and
have 2 orders of magnitude uncertainty on the current value
corresponding to the same temperature
(
9
,
13
)
.
Significant advanc
es on
artificial skins
require
the use of new flexible materials with higher temperature sensitivity
, responsivity, range of
operation and stability
.
Recently,
it has been shown that
materials composed of plant cells and carbon nanotubes have
very high r
esponsivity over large temperature ranges
(
14
)
. However, these materials are not
suitable for artificial skins,
since
they have mechanical properties similar to wood, are not
flexible and require cumbersome fabrication approaches.
In this work, we focus
on the
active
2
molecule responsible for the large temperature responsivity in plant cells (
pectin
)
(
14
)
and
engineer films suitable for flexible electronic devices.
Pectin, a component of all higher plant cell walls, is made of structurally and functionally
very complex, acid
-
rich polysaccharides
(
15
)
. Pectin plays several roles in plants, for
example,
it
is an essential structural component of cell walls
that
binds ions and enzymes
(
16
)
. In high
-
ester pectins, at acidic pH, individual chains are
linked together by hydrogen
bonds and hydrophobic interactions. In contrast, in low
-
ester pectins, ionic bridges are
formed, at near neutral pH, between Ca
2+
ions and the ionized carboxyl groups of the
galacturonic acid, forming an “egg box” in which cati
ons are stored
(
17
)
. Since the
crosslinkings between pectin molecules decrease
exponentially with temperature
(
18
)
,
increasing the temperature of a Ca
2+
-
crosslinked pectin increases ionic conduction
(
14
)
.
Results and Discussion
W
e
produced pectin films
and
compare
d
the
ir
temperature responsivity (i.e., the signal
variation in a given temperature increment) with the best
flexible temperature sensing film
s
(
4
,
8
,
10
)
, in a
biologically relevant 45
K temperature interval (
Fig. 1
A
). The pectin films
(Fig. 1B)
signal variation is
ca.
2
order of m
agnitude greater than the others
(
4
,
8
,
10
)
. A
higher responsivity has been reported only for a very narrow temperature range (<5 K) for a
two
-
state dev
ice
(
9
)
(i.e., a temperature switch).
To find a closer match to the pectin film’s
responsivity,
sensitivity
and range of operation
it
must
be
compa
re
d
directly
to
biological
membranes
.
Human skin
, for
example,
senses
temperature
with a sensitivity of 20 mK
(
19
)
through ion channels
(
20
)
that belong to the
family of the TRP s
ensors and include the snake TRPA1 orthologous
(
20
)
,
which is the most
sensitive temperature sensor in nature
.
Snakes
are cold
-
blooded animals and their body
temperature corresponds to that of the environment. Snakes’ pit membranes
(
21
)
distinguish
minute temperature variations.
The extraordinary sensitivity of p
it membranes
is due to the
presence of
voltage
-
gate
d ion channels
orthologues
of
the
wasabi receptor
(
21
)
in humans
.
At
night, thermal emission from a mammalian prey
at a maximum distance of
1 m away cause
s
a
small, local temperature increase on the membranes
(
21
)
. The small temperature i
ncrease
leads to an increased opening of TRPA1 ion channels
(
20
–
22
)
and to an increased current
carried by Ca
2+
ions
(
21
)
(Fig. 1
C
) through the cell membrane.
Interestingly, the mechanism
of detection
(
23
)
of the pit membrane i
s not photochemical since the incident thermal
radiation is not converted directly into electrical current
1
.
For this
reason,
the
pit membrane
response to temperature has been characterized
measuring the
current variation when placed
in contact
with a warm
surface
,
i.e.
,
as
a thermometer film
rather than an optical receiver in
the far infrared range
(
21
)
.
So far, no engineered material with similar
thermal
sensitivity
or
responsivity in a comparable range of temperatures
has been reported.
Here we show that
pectin films mimic
the
mechanism of
the
TRP receptors by using a similar Ca
2+
current
regulation (Fig. 1
D
)
and achieve the same sensing performance
of snakes’ pit membranes
(Fig. 1
E
)
.
We fabricated films (~200 μm thick) by casting a pectin solution
in a mold (see Methods)
,
thinner films can be produced by spin coating
.
The pectin was crosslinked in a CaCl
2
solution
and
dehydrated
in vacuum to obtain a transparent film.
After dehydration
,
the
conductivity of
the hydrogel is
in
the order of 0.1 mSm
-
1
.
T
he current
-
voltage characteristic of a typical
film
is linear
(Supplementary Fig. S1)
.
T
o
characterize
the response
of the
material
to
temperature
,
samples
’
current was
measured
between 10
and 55 °C
on a Peltier element
. The temperature
was
monitored
with
an independent calibrated Pt100 sensor
.
The thermal responsivity
achieved was comparable with that of rat and rattle snakes’ pit membranes
Fig 1
E
. The
3
responsivity was
also
within
the same
order of magnitude
of
the plant
cells
-
carbon nanotubes
composi
tes
(
14
)
.
However, t
he
produced pectin films
are
transparent
,
flexible
and
conformable to any surface
,
thus ideal as sensitive layer in syn
thetic skins
.
To prove the
ir
sensing mechanism, we made
three
control experiment
s
measuring
the
temperature responsivity of
(i)
pure
water,
(ii)
pectin film
s
with
pure
water
and
no
crosslinking
ions
,
and
(iii)
a
CaCl
2
solution
. The temperature
respons
e
in th
e
three
case
s
was
much lower than
that of the
Ca
2+
-
crosslinked
pectin
(see Supplementary Fig. S2)
.
T
his proved
that
the large responsivity
of the crosslinked pectin films
is
due to interaction
s
between pectin
chains and the
Ca
2+
-
ions
as shown in
Fig
.
1
D
and reported in
(
14
)
.
To measure the stability
of
the Ca
2+
crosslinked
pectin films
,
we
cycled
them
in a 30 K interval
Fig. 2
A
.
The films are
very stable
over
the
215
cycl
es
tested (
F
ig
.
2
A
), showing no
significant change
of
responsivity
(
Fig. 2
B
)
n
or
of
absolute
current
values
at each temperature
(Fig. 2
C
)
.
The
activation energy for
pectin film
s
is 81.9 kJ/mol (
Supplementary
Fig. S3
a,b
and
Supplementary
Materials
)
.
A
similar
value
of
the
pectin
activation energy
was
reported in
rheological measurements
(
18
)
.
We
test
ed
the sensitivity of the
pectin
films
,
monitoring the
local
temperature
of
a sample
(
Fig. 2
D
)
with
a
thermal camera
,
while
an independent s
ource
-
meter
measured
its
electrical
current.
The
film
respond
s
with fidelity
to
small
changes of the environmental temperature
(
Fig
.s
2
D
,
E
)
.
The detailed data
for a 2
-
sec time interval
(
Fig
.
2
E
)
rev
eal that the film
senses
temperature variations
of at least
1
0
mK.
To
compare the performance of our
material to
that
of the
viper’s pit membrane
,
we
characterized the sensitivity of pectin films
when facing
small, warm bodies at a distance.
A microwavable
teddy bear
was heated up to 37 °C
.
A
thermal camera was used to
determine
its temperature
and ensure
it remained
constant during
the
measurement
s
. We placed
the teddy bear
1
m
from the membrane
for ca. 2
0 sec and then
remove
d
it. We
repeated
th
is
procedure
also
at
distances of 0.6 m and 0.4 m. The results show
that the membrane
detects
warm bodies
,
about
the size of a
rat or a small rabbit,
at
a
distance
of
1 m
(Fig. 2
F
)
.
We
also
performed
experiment
s
with large
r
films (
21
x
29.7 cm
)
connected to
two carbon
electro
des
and operating at 20 V
(see Methods
,
Supplementary Video 1
and discussion
).
Th
ese samples have
the same sensitivity as measured in
small
er
film
s
(Fig
.
1
E
)
.
To test the
response
of
the
films
to bending
,
we monitored
their
current
at constant
temperature
in
different bending positions
(Fig 3
A
and Supplementary Fig. S4). The current variations due to
bending are negligible, compared to the variations induced by small temperature changes (Fig
3
A
). We also tested the response of a bent sample to temperature v
ariations (
Fig.
3B
)
and
found no change in responsivity.
The experiments in Fig 3B were performed on a copper bent
substrate covered with and insulation layer.
To verify that pectin films can be integrated in a synthetic skin as a temperature sensitive
ma
terial
,
w
e fabricated
samples
(
52 mm
52 mm
)
with multiple
electrodes (
8 or 16 contacts)
deposited
on the external frame
(
Supplementary Fig. S
5
A
,
B
)
.
These samples
we
re
made
of
pectin films
with
chromium/gold
electrical contacts
sandwiched between two
insulating
layers
, to protect them from the effect of direct contact with external conductors and/or
humidity/water
(Fig
.
3
C
).
We
monitored the
signal
between electrodes of each
row and
column
,
wh
ile
increasing
the temperature in selected areas
of the skins
.
Based on
the number
of contacts on the outer frame, we divided the area of the sample in four (or sixteen) blocks,
corresponding to the
number of
“
pixels
”
.
E
ach pixel is
addressed
as the intersection between
each r
o
w and column, according to
the electrodes
’
position
.
This arrangement allow
ed us to
reconstruct
the temperature map on the material
without cumbersome
or
pixel addressed
4
electronics
(
3
,
9
)
.
M
apping o
f complex temperature profiles
can be further e
nhanced by
algorithmic analysis
(
24
,
25
)
.
The measurements obtained in the
4
-
pixel
sample
were performed using the circuit shown in
Supplementary
Fig
.
S6
operating at 18 V
and
explained in
the
SM
file.
T
he
position of a
finger touching 4 different pixels of a skin for ~2 sec
is
clearly distinguishable
from the
electrical response of the materials
(Fig.
3
D
)
.
T
he
voltage signals acquired are
reported in
Fig.
s
3
D
,
Supplementary
S7
and
Tab. S1
.
The
noise in
Supplementary
F
ig.
S7
derives from
the electronic
readout
circuit
and not from the sensor
as
confirmed performing
similar
measurement
s
with a pico
-
amperometer
(
Supplementary
Fig
.
S
8
).
The temperature variation
on
each
pixel
was
c. a.
1
K
,
as shown in the thermal image in
Supplementary
F
ig
.
S
9
.
To
exclude pie
z
o
-
resistive effects, we performed the same measurements
pressing
the sample
with a
metal object at
the
same
temperature of the pixel
(
Supplementary
Fig
.
S
10
)
.
To
test
the
response of pectin skins to an increased sensing spatial density, we
fabricated a
16
-
pixel
device
in the same skin area
and
evaluated
its temperature mapping ability
.
We p
laced
near
the
lower
right corner of
the skin an aluminum
parallelepiped
(
12
m
m
x
12
mm
x
3mm
)
at 26
°C
(
with
an
ambient temperature
of
20 °C). As shown in Fig. 3
E
,
we measured
the
signal
on
the skin
for each of the 16 pixels,
0.8 sec after the aluminum square was laid
in contact
,
(
Supplementary
Tab
.
S2)
.
The thermal camera map
(
Supplementary Fig. S11
and pixelated in
Fig. 3
F
)
and the temperature map obtained with our
skin show
an excellent
match
.
The exquisite temperature sensitivity and mapping ability of pectin
skins reveal opportunities
in robotic sensing and haptics, where biomimetic sensors are important
(
2
)
.
For ex
ample,
pectin skins could be embodied in robotic prosthetics, which are limited today by the need of
improved sensory feedback
(
26
)
.
Feedback from prosthetics is essential to restore the
complete functionality of a limb and is especially critical for achieving proper control of
robotic devices, attaining much
better results than with the single use of vision
(
27
)
.
Pectin
skins can be used as a high
-
performance layer in flexible electronic devices, for example,
when sandwiched in the architecture of an artificial skin or a prosthetic limb.
The record
-
high sensitivity of pectin skins make them suitable to record fin
ely distributed
temperature maps on surfaces. Their
ease of fabrication and minimal requirements for
electronic circuitry make them compatible with most existing flexible technologies.
Some
limitations arise from the need of an insulating layer against exc
essive humidity. The insertion
of a polymeric insulation layer in synthetic skins is a common practice and can offer a direct
solution to the problem. Another limitation is the need for accurate initial calibration.
Improving the uniformity of the pectin l
ayers is expected to reduce the c
urrent calibration
complexity.
Conclusions
The present
work
demonstrate
s
that a material composed exclusively of purified plant pectin
and crosslinking ions
, engineered into a film
,
has
a performance
equivalent
to that of
the
snake’s pit membrane
and superior to other
flexible materials
.
The
pectin films are
u
ltra
-
low
cost and scalable
, insensitive to pressure and bending
and
can be used to
augment
temperature
sensing
when integrated
in
synthetic skin
platforms
.
Material
s
and Methods
To produce the
materials,
w
e used c
ommercially available citrus low
-
methoxylated pectin
(LMP) with a degree of methylation of 34% and a content of galacturonic acid of 84%
5
(Herbstreith&Fox
©
).
P
ectin powder (2% w/vol) was dissolved
at
8
0
°C
in
de
ionized
water
and
stirred
at
1
,
400
rpm
until
a uniform solution
was obtained
. To
jellify the
film
s
,
a 32 mM
CaCl
2
solution
was
prepared
(corresponding to a stoichiometric ratio R = [Ca
2+
]/2[COO
-
]
=
1
)
.
T
he pectin solution was
poured
in
to
a
mold
and
the
CaCl
2
solution
then adde
d
.
After
gelation
,
the
highly hydrated films were
transferred to a vacuum chamber and dehydrated at
12 mbar overnight.
S
amples were then detached from the Petri dish using a razor blade. The
large samples shown in Supplementary
Video 1 were produced
pouring the gel
on a glass
substrate (28 cm
30 cm
0.5 cm) as the
lower
insulating layer. The electrical contacts were
made of
carbon tape, and a clear
insulating
acetate sheet (A4 paper format)
was layered on
top
.
To
produce
the
skins,
the pectin solution was deposited directly on different substrates
(PDMS
,
cellophane
or SiO
2
)
with pre
-
deposited electrical contacts
made by
sputtering
chromium/gold
or using carbon tape
.
In the
experiments
described
in Fig.
s
1 and 2
a d.c.
polarizing voltage of 20
V was applied to the samples and the current allowed to decrease for
ca. 2 hours. After
the initial discharge,
the current
remained
stable for several hours
(during
which experiments where performed).
The current was measured with
a Keithley 2336B
source meter.
For the experiments in
Fig
.
3
t
he applied voltage was a square wave with an
amplitude of 20 V and a frequency of 5 Hz. Sampling rate was 10 samples per second.
Temperature on the film was actuated by a Peltier element QC
-
31
-
1
.4
-
8.5M. Independent
temperature measurements on the film were measured with a Pt100 platinum thermometer.
W
e
also
performed
measurements
at different frequencies (see Fig. S1
2
)
up to
50
°C
. We
found no difference in the temperature response
of the pectin
films under a.c. or d.c.
conditions (see Supplementary
Materials
)
.
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Acknowledgements
:
The authors thank U
e
li M
arti
(
ETH
Zürich)
for the technical support
and useful discussions
.
R.D.G., B.M., C.D., L.B.,
and
V.C.
are inventors on patent
application
s EP15161042.5,
EP15195729.7, PCT/EP2016/056642
submitted by
ETH Zurich
that cover
Gel based thermal
sensors.
Funding:
This work was supported by
the
Swiss National Science Foundation
,
Grant
#
157162.
7
Author contributions
:
R.D.G., L.B.
, V.C.
,
B.M
.
,
and
C.
D.
conceived the system
and designed the research
. R.D.G
.
,
L.B and V.C. designed
and performed
the experiments.
R.D.G. and V.C. designed the readout
circuit
for the electrical measurements
.
All authors contributed to the analysis of
the data
and
discussions
.
L.B. and V.C.
prepared
the figures.
R.D.G, V.C
.,
L.B.
and C.D.
designed the
supplementary
video.
L.B.
edited
the video.
R.D.G
.
and C.D. wrote the paper
.
Additional information
:
Supplementary
Material
is available in the online version of the paper.
Competing financial interests
:
The authors declare no competing financial interests.
8
Fig
ure
1
│
Compa
rison between
artificial skins,
snakes’
pit membrane and pectin
film
s
.
A
,
Respons
e
of different artificial skins.
Normalized signal variation as a function of relative
temperature change. Red dots: pectin film resistance.
Black Squares: resistance replotted from
Park
et al.
(
10
)
. Blue crosses:
resistance replotted from Segev
-
Bar
et al.
(8
). Orange
diamonds: resistance replotted from Trung
et al. (12
)
.
Green triangles:
voltage replotted from
Kim
et al.
(4
)
. Violet triangles: resistance replotted from Webb
et al. (1
1
)
.
B
,
Digital image of
a sample of the produced pectin films.
C
,
Molecular
mechanism governing pit membrane
sensitivity.
Dark orange:
TRPA1 channels
. Light orange: cell membrane. Red dots:
Ca
2
+
ions.
D
,
Molecular mechanism governing
crosslinked
pectin film
s’
sensitivity.
B
lack lines
:
galacturonic acid
.
R
ed dots
:
Ca
2
+
ions
. G
rey
dots
:
water molecules.
E
,
Comparison with pit
membranes:
R
ed dots
: nor
malized current in a
pectin
film
.
The points with error bars and
dotted lines are plotted from
Gracheva
et al.
(2
1
)
.
Dark grey dots: rattle snake
.
L
ight grey
dots: rat snake.
Inset: Digital image of a rattle snake and
schematic of the pit organ and pit
membrane.
9
Fig
ure
2
│
Charac
terization of the pectin film
s
. A
,
Red dots: f
orced
temperature,
215
cycles
superimposed
.
Blue dots: corresponding electrical current in a pectin film,
2
15
cycles
superimposed.
B
,
Responsivity:
electrical
current ratio between 40 and 10
°C
during the 2
15
cycles
displayed in (A
)
.
C
, Electrical current in the pectin film at different temperatures
durin
g the 215
cycles displayed in (A
)
D
,
Electrical c
urrent value
in the pectin film (blue
dots
,
left axis)
plotted as a function of time and compared to the sample’s
temperature
measured by
the thermal camera
(red
dots
, right axis)
.
The temperature
oscillations are caused by
variations of the ambient temperature
during the measurements
.
E
,
M
agnification of the
data
in the
pink
box
in panel
(
D
)
,
t
he dots: measurement points,
lines are included as a guiding
reference.
F
,
S
ensing
heat from
a warm object (37
°
C)
at a distance
. Blue
dots
:
electrical
current in the pectin
film
, blue line is included as a guiding reference
. Black line, position of
the object with respect to the membrane positioned in 0.
10
Fig
ure
3
│
Char
acterization of the
pectin films as materials for artificial skins
.
A,
Current
and temperature as a function of
time while bending. At t = 75 sec the temperature was
increased and then decreased by ca. 4 K. On the right, c
artoon
s
of the bending position
s
tested
,
see Supplemen
tary Fig. S4
for pictures.
B
,
Current response when the sample is bent.
Cartoon of the bending position
of the film on a copper/insulator substrate
.
C
,
Schematic
view
of
the pectin skins in
cross section
.
D
,
Electrical response
and temperature maps obtained
with
a
4
-
pixel skin
,
when a finger touched it in different positions
(refer to the finger print
location in each panel)
.
The voltage
-
time panels show the signal readout for the
corresponding rows and columns
.
The colors (and h
eights of the blocks) correspond to the
product between the maximum signal variations (in %) detected in each row and column (see
Supplementary
Materials
, Tab.
S1), normalized to 1. Supplementary
Figure S7
shows the
percentage increase of the signal in time, for each of the 4
-
pixels when individually touched.
E
,
Electrical response of a 16
-
pixel skin when a warm
object is placed
on its
bottom right
corner
.
F
,
Pixelated thermal
camera
image
of the skin corre
sponding to
(
E
)
.
1
Supplementary
Material
Biomimetic temperature sensing layer for artificial skins
Raffaele Di Giacomo
1
, Luca Bonanomi
1
, Vincenzo Costanza
1
, Bruno Maresca
2
and
Chiara Daraio
1,3
1
Department of Mechanical and Process Engineering (D
-
MAVT), Swiss Federal
Institute of
Technology (ETH), Zurich, Switzerland.
2
Department of Pharmacy, Division of Biomedicine, University of Salerno, Fisciano, Italy.
3
Division of Engineering and Applied Science, California Institute of Technology, Pasadena,
CA, USA.
Supplementar
y Material
s
and Methods
Polymeric insulation
We utilized polymeric insulation layers such as acetate (polyvinyl acetate) to protect the
sensing layer from humidity and pH variations. This is a common practice in artificial skins.
No chemical interaction
between pectin and polyvinyl acetate or PDMS is expected due to
their stable polymerized state. No change in responsivity or sensitivity was found with respect
to pectin films without insulating layer when acetate or PDMS were used. Any other
insulating ma
terial already in use for synthetic skins would serve for the scope.
Measurements
The electrical measurements reported in Fig
.
1
, 2
,
3A,B,
S1, S2,
S3, S8, S10
were performed
in a two
-
point contact geometry using a source measurement unit (Keithley model 2635), also
referred to as amperometer or pico
-
amperometer in the main text of the paper.
The electrical
measurements in Fig
.
S12
were acquired with a
lock
-
in a
mplifier model SR830 Stanford
research systems.
For the electrical measurements reported in Fig
.
3
D,E
and S
7
, we applied
sequencially a
square wave
voltage
having an amplitude
of 18V to the electrical contacts in
each row and column. We measured the signal
output with the readout circuit (in Fig
.
S
6
),
connected to a DAQ board (National Instruments
®
BNC
-
2110).
The thermal c
amera used in
the experiments was
a FLIR
®
A655sc.
Sensor
’s
response
and comparrison
“he metrological quantity of choice for the
comparison in figure 1A is the
response/responsivity defined as the amount of change in the output (readout signal) for a
given change in the input (in this case temperature). The scale in the plot is the same for all
the sensors. Each value on the plot ca
n be calculated as (Output
T2
)/(Output
T1
) for each T2
-
T1
and with T1 fixed. The values were taken from the references cited, as reported in the legends