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PROJECT N°29
Thermal Exoskeleton
Elžbieta Avsiukevičiūtė
European School Alicante
Av Locutor Vicente Hipólito, s/n, 03540 Alicante, España
S7 EN
Abstract
The focus of the following project is to design and create an exoskeleton device that provides the wearer
with external body-heat control. Using heat emitters and/or heat absorbing materials, located in points of
highest heat-sensitivity, desired temperature variation can be achieved. Among other applications this
would be useful in various fields of medicine, especially with patients who do not possess the capacity of
autonomous body-heat control, due to, for instance, hormonal imbalance; or as a localised heat source as
required in healing processes. Additionally, a possible use for the general public could be imagined in
extreme weather conditions, to assist an optimal thermoregulation to an extent unavailable to the
organism itself, in this way preventing hyperthermia or hypothermia, respectively.
1. Introduction
Humans are endothermic beings, meaning they generate body heat through metabolic processes (1). Most
of the heat is generated in the deep organs, more precisely the liver, brain, and heart, as well as in
contraction of skeletal muscles
(2)
. Other important factors determining one's body temperature are
hormonal changes, especially those occurring in the thyroid (3).
However, many external factors may also play a great role in one's internal thermoregulation. When either
the disruption in the normal function of thermoregulation systems in an organism occur, or the conditions
are too severe to prevent complications, a person becomes at risk of hyperthermia or hypothermia. In
those cases, body temperatures that increase beyond 38°C, or drop below 35°C, pose increasing health
risks with every degree (4).
From within, the primary thermoregulatory control centre is located in the hypothalamus, with autonomic
thermoregulatory control on the skin surface, deep abdominal and thoracic tissue, spinal cord, and other
parts of the brain (each contributing approximately 20%). The final contributing factor of thermoregulatory
control is behavioural, involving choice of appropriate dress or seeking shade in summer. These elements
produce the most effective thermoregulatory responses (3).
By simulating behavioural thermoregulatory control in the areas possessing greatest thermoregulatory
impact, those same centres of the body's autonomic thermoregulatory control can be activated on the
surface of the skin, reverting the potentially hazardous effects of drastic body temperature changes as can
occur by internal or external factors.
Alternatively, this process could function for localised heat treatment, which is a common medical
therapeutic application of heat in recovery processes from various conditions (5).
The following design project presents a suit-like exoskeleton device that would regulate body temperature
externally by focusing on the points mentioned above, to optimise the manner in which heat energy is
brought to areas of greatest heat-sensitivity in a simple, unobtrusive way.
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2. Hypothesis
The device is developed in three different stages, as follows:
The first of these involves mapping out the body (or, in the case of a reduced model, general area of
interest) in terms of heat sensitive regions to figure out which specific locations the heat would have to be
applied to.
The second involves understanding the different properties of the materials required for both the
heating process as well as the exact ways they should be used for maximum efficiency.
The last involves the creation of a viable design for the overall suit, and a reduced prototype to test
the functionality of the hypothetical design of this thermal exoskeleton.
The hypothesis is thus that heating up or cooling down specific point-like areas of high heat-sensitivity
would result in an overall temperature increase or decrease in the system, according to needs. To generate
heat, the insides of the suit would contain thin metal wires to allow the flow of electricity and subsequent
heat emission. The heat-sensitive points would have a greater surface relative to the wires, that would
allow heat exchange between the body and the device. All electrical wires will be insulated to prevent any
direct contact with the skin in non-heat-sensitive areas which are not involved in the process. Areas of
high heat-sensitivity would be covered with a material that would allow for heat exchange to occur without
requiring direct contact between the metals and the skin. All remaining areas would remain uncovered to
ensure unobtrusiveness and unrestricted movement of the limbs.
3. Research
Thermoregulation: main focus
As mentioned previously, from an external thermoregulatory perspective, the two areas of focus
accessible for the project are those simulating behavioural thermoregulatory control, and autonomic
thermoregulatory control on the skin, being the only surface that contact can be made with.
Experimentally, the areas where heat sensitivity is greatest on the skin are located on the neck, the lower
back, external laterals of the limbs, sides of the torso and lower abdomen.
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Viable heat emission/absorption methods
For the project in question, a distinction must be made between the heating effect and the cooling effect,
as produced by the device.
To achieve the heating effect, electrical current can be passed through wires inlaid in the suit to produce
heat. The amount of heat output can be approximated to the power input, if there are few other energy
interactions (6). A direct electrical input creating a current in the wires would result in heat energy being
released.
When it comes to power sources, given that a direct current at low voltages is preferable over alternating
current (view next section for more information), a battery is a valid source to generate this type of
current.
With regard to the wiring used in the device, it is known that magnetic materials are more easily heated
than non-magnetic ones
(7)
, meaning wires that possess this quality, may prove to be more efficient
choices. Small or thin pieces generally heat more quickly than large thick pieces, which is necessary to note
when creating a well-distributed layout of the suit. Additionally, an important consideration to be made
is the nature of the materials used, when perceived from an electrical perspective. In order to remain at
workable voltages and currents, the resistivity of the metal used in the wiring must be high enough to
provide heating effects, and avoid excessive current flow.
For the cooling effect, phase change materials (best known as PCMs) can be used. These materials are
latent heat storage materials that absorb and release heat without rising in temperature themselves
(8)
.
These materials could be used in the form of patches to be distributed in similar regions to the localised
point of great heat sensitivity, and to be activated when a cooling effect is required.
[IMPORTANT NOTE: while this idea would be of great importance in a full version of the device developed,
the experimentation that follows does not revolve around this particular aspect of the device, due to the
complexity of the creation process thereof, as well as limited resources. The concept, however, remains
one to be developed in a future for this project]
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Safety measures/insulation
Considering the use of a current in sensitive regions of a human body, great care must be taken when
selecting both the levels of power as well as the materials used, to avoid any potential harm to the
organism that this could bring upon.
Assuming the use of low voltages (which is the case, as only slight increases in temperature are desired),
DC power is far less harmful to the human body than is AC power (9). Voltages below the established SELV
(safety extra-low voltage) level, at 42.4V, are non-hazardous to the human body (10). Considering the sort
of power ranges being dealt with in the overall process of the suit, nothing exceeding this value will be
required, thus the power source is not expected to create any safety risks.
Applying knowledge of the relationships between the usable voltages, as well as the dimensions and
resistivities of materials, safe currents can be calculated (seen in next section).
Having established that the power output is at safe levels in the project, having metals at the points of
heat emission in close proximity to the skin (shielded, in fact) can also be counted as non-hazardous.
Lastly, for electrical insulation in the rest of the suit, most plastics work, at the sort of temperatures that
are being targeted with the device (not exceeding 50°C). The choice in this case was silicone due to it
containing no free flowing charges, property for which it is a common material used in applications in
contact with heat (in the following section, the tests carried out to prove the efficiency of this material are
described also).
Testing
The original idea consisted in using small metal plates connected by the wires located in the areas of
highest sensitivity, which, having a greater surface area, would heat up most efficiently. Thus the first step
of testing consisted in connecting plates of four different metals (Aluminium, Iron, Zinc and Copper) to a
power source and testing the heat that was created in this way. Details about the chosen plates can be
viewed in Annex 1.
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Prior to carrying out the experiment, calculations were made in order to determine the current generated
in each case, as a relation of the voltages that could be used by the power generator, and the individual
resistances of the plates to ensure that the current created by the voltage does not exceed the limits of
the equipment, at 6A. Applying Ohm's Law:
𝑉
𝐼=
𝑅
I: Current (A)
V: Voltage (V)
R: Resistance (Ω)
The resulting values of the currents turned out to be far too high (view Annex 2 for the specific values),
exceeding the maximum current that could be passed through them without burning the generator,
following which a redesign was made.
To increase the resistance of the metals, either the length of the piece would have to be increased or the
cross-section area decreased. The resistance, as defined by the metal's geometry and resistivity, is
expressed as the following:
R: Resistance (Ω)
𝑅=
ρL
𝐴
ρ: Resistivity of the metal (Ω/m)
L: Length (m)
A: Cross-section area (m2)
The new design involved wires, given that their geometry favours the required resistance far more.
With that in mind, several different wires (varying in the nature of the metal and thickness, but of the
same length, at 2 metres) were selected for testing. The specifications of these wires can be seen in Annex
3. The same calculations as done before for the plates were carried out to determine if the currents
generated would not exceed the aforementioned current-limit. These calculations can be found in Annexes
4 – 8. The wires were connected to a power generator at safe voltages (ones that were within the 20V
limit of the power generator and which the calculations above had proved to not generate excessive
currents) to see if, and to what extent, the heating effect would be produced, to achieve the required
temperatures.
An additional test was then carried out using Constantan (Cu55Ni45), as it is the one of greatest internal
resistivity, thus with the greatest margin in the unlikely (yet, possible) event of an overcharge of current,
making it the safest wire. Wire lengths of 1 metre and 0.5 metres were used, to view how the results
compare in those cases.
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Wire was also integrated into a sheet of silicone to test whether this material would provide sufficient
insulation. A 1 metre length of Constantan wire was used, for the same reasons as mentioned above.
4. Analysis
Due to the calculations made prior to the testing phase, at the selected voltages, all testing done on wires
worked as expected. The results of the tests can be seen in Annexes 9 – 15.
Several factors were taken into account when selecting which wires would be most useful for the final
product, making the selection process one of discarding those with unfavourable qualities.
Both Fe [0.5mm] and Cu55Ni45 [0.4mm] were discarded due to their thickness which was excessive, as it
would encumber their integration into the silicone.
Of the remaining three, at 2m of length, due to its high internal resistivity, Cu55Ni45 [0.2mm] proved to
require too much power to reach a significant change in temperature, which is an inconvenience
considering the required portable quality of the power source (batteries, having a standard voltage output
of 4.5V would be insufficient). Shorter lengths (tested: 1 metre and 0.5 metres) demonstrated exceptional
efficiency.
Cu [0.2mm] gave efficient results, but due to its low resistivity, at shorter lengths this may begin to cause
a risk as the currents would be considerably higher than in the cases of the other metals. Nonetheless, at
2 metres, results were satisfactory.
Fe [0.2mm] gave slightly lower results in terms of temperature compared to Cu [0.2mm], but would allow
shorter lengths of wire to be used also.
The original design involved pieces of a single, select type of wire to be used in the entire suit, but
experimentally, assuming only one power source, it appears using several types of wires varying in length
and nature yields more realistic results for the model. The same level of power output can create a uniform
temperature rise if the lengths of individual coiled wires and their individual resistivities are taken into
consideration. In this way, points closer to the power source can use wires of higher resistivity and shorter
length (such as Cu55Ni45), whereas points further away from the source can use wires of greater length and
lower resistivity (such as Cu). All wires would be of the same thickness, for easier manageability.
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For the rest of the suit, silicone proved to be a good insulator due to the extremely low variation in
temperature resulting from the contact with the wires. Integrating the Constantan (Cu55Ni45) wire (which
rose to 47°C on its own, i.e. 25°C above room temperature), resulted in the surrounding silicone to only
heat to 26°C (4°C above room temperature) which confirmed the claim with regard to the thermal
insulating properties of the material. Silicone in itself has numerous benefits for the specific role it carries
in the device. While other materials may also work similarly for the required function, silicone is also
known to be safe for the skin and extremely unlikely to result in any negative reactions. The costeffectiveness of this material also makes it a favourable choice. Additionally, due to the nature of this
material, it would not restrict the wearer's movement, which is another important point, considering the
function of the suit, which is intended to be comfortable to the wearer.
5. Design
Having tested the materials in question, the original concept of the suit proved itself as plausible, with the
exception of the concentrated heat points, which were redesigned as coiled wire instead of metal plates.
The (full) device would thus be described as silicone-coated wires varying in length and nature, of a
uniform 0.2mm thickness. These would lead outward from a 4.5V power supply (standard battery) located
on the back, towards the points in the body of highest heat sensitivity, where this wire would be coiled
more densely to create a closely-packed heat-emitting surface area. The coated wires would then be
secured by additional straps where necessary.
In the areas of highest wire density, a fabric layer is to be placed in order to properly integrate the wire
into the silicone, to prevent movement of the wires and to prevent any hazardous situations (since a short
circuit, though unlikely based on the design, may be possible nonetheless).
The overall layout sketch of the exoskeleton can be viewed in the Annex 16. However, due to limitations
of the project itself, the entirety of it could not be recreated with neither the full details (as mentioned
throughout the research) nor in the full extent. The prototype only concentrates around the device at a
reduced, local scale.
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6. Conclusions
In conclusion, the originally proposed design appears to be one that functions in a way that may be useful,
and thus has potential (with additional future improvements) to be used in various fields, including
medicine. Many adaptations were made compared to the original idea, based on experimentally observed
specifications of the materials used, which ultimately improve the potential functionality of the
exoskeleton.
7. Further work
Considering the time constraints, as well as limited access to required materials to create a more complete
prototype of the device in question, many further developments can still be made in the future to turn
this initial idea into a more widely applicable device. Following an order, these points could be considered:
-
Completing the full suit and testing its efficiency in real situations, as mentioned at the start.
-
Implementing a Bluetooth module to allow for the control of temperature variation in the suit,
subsequently allowing users to manage the power output in the suit.
-
Adding counteracting cooling modules, as described earlier (due to time constraints this was not
possible to achieve, despite it being a key element in the full functionality of the suit).
-
Developing the connecting materials, to make the suit more manageable in ordinary contexts/less
obtrusive for use.
-
Further test the efficiency of the alternating heat/cold function of the suit.
8. Acknowledgements
-
Cinzia di Fede, Chemistry teacher at the European School of Alicante, and mentor of the project.
-
Antti Torvikoski, Physics teacher at the European School of Alicante, for Physics-related
contributions.
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9. References
1. Ducoff, N. Homeostasis: Thermoregulation. Boundless.com. [Online] October 16, 2013.
https://www.boundless.com/biology/textbooks/boundless-biology-textbook/the-animal-body-basicform-and-function-33/homeostasis-194/homeostasis-thermoregulation-743-11974/.
2. Textbook of Medical Physiology (11th ed.) p.890. [book auth.] A.C., & Hall, J.E. Guyton. Textbook of
Medical Physiology (11th ed.). Philadelphia : Elsevier Saunders, 2006.
3. MD, Júlia Szakács. Pathophysiology - Thermoregulation. u-szeged.hu. [Online] [Cited: August 1,
2016.] http://web.med.u-szeged.hu/patph/Thermoregulation.
4. Thermoregulations in Humans (PDF). [Online] [Cited: December 29, 2016.]
http://faculty.ung.edu/jhamilton/2120/docs/webnotes/human-thermoregulation.pdf.
5. Pfiedler Enterprises. pfiedler.com. 1107 Caridnal Online . [Online] 2017. [Cited: 02 12, 2017.]
http://www.pfiedler.com/ce/1107/index.html#9.
6. Cengel, Yunus and Ghajar, Afshin. Chapter 15: Cooling of Electronic Equipment. Heat and Mass
Transfer: Fundamentals and Applications; ISBN 0073398187. New York : McGraw Hill, 2015.
7. What Is Induction Heating? GH Induction Atmospheres. [Online] [Cited: December 1, 2016.]
http://www.gh-ia.com/induction_heating.html .
8. Are There Materials That Can Absorb Heat Without Becoming Hot? Ask An Engineer. [Online] May
14, 2013. [Cited: December 29, 2016.] http://engineering.mit.edu/ask/are-there-materials-can-absorbheat-without-becoming-hot.
9. Krishnan, M. Which is more dangerous: AC or DC power? Quora. [Online] February 5, 2014. [Cited:
December 29, 2016.] https://www.quora.com/Which-is-more-dangerous-AC-or-DC-power.
10. Unknown. What is the maximum DC voltage, that is not lethal? Edaboard. [Online] September 5,
2006. [Cited: December 29, 2016.] http://www.edaboard.com/thread74087.html.
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10.Annexes
Annex 1: Information table on initially selected plates
Metal
Al (Aluminium)
Fe (Iron)
Zn (Zinc)
Cu (Copper)
Mass (g)
3.0
5.7
9.6
6.3
Thickness (mm)
0.4
0.2
0.7
0.2
Length (cm)
7.5
7.5
7.5
7.5
Width (cm)
2.0
2.0
2.0
2.0
Resistance (Ω)
2.48438E-7
1.820625E-6
3.17143E-7
3.15E-7
Annex 2: Table of calculated currents from the initially selected plates
Voltage
5
10
15
20
Al
2.013E7
4.025E7
6.038E7
8.050E7
Current (A)
Fe
Zn
2.746E6 1.577E7
5.493E6 3.153E7
8.239E6 4.730E7
1.099E7 6.306E7
Cu
1.586E7
3.175E7
4.762E7
6.349E7
Annex 3: Information table on selected wires
Metal
Fe (Iron)
Cu55Ni45 (Constantan)
Cu (Copper)
Diameter (mm)
0.5
0.2
0.4
0.2
0.2
Section area (m²)
1.9635E-7
3.1416E-8
1.2567E-7
3.1416E-8
3.1416E-8
Resistivity (Ω/m)
9.71E-8
4.9E-7
1.68E-8
Annexes 4 – 8: Calculations of current by length, metal, and voltage.
-
Annex 4: Copper (0.2)
Annex 5: Iron (0.5)
Annex 6: Iron (0.2)
Annex 7: Constantan (0.4)
Annex 8: Constantan (0.2)
All are labelled below.
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Annexes 9 – 15: Measurements of heat produced by passing a current in different wires
-
Annex 9: Copper (0.2) – 2m
Annex 10: Iron (0.5) – 2m
Annex 11: Iron (0.2) – 2m
Annex 12: Constantan (0.4) – 2m
Annex 13: Constantan (0.2) – 2m
Annex 13: Constantan (0.2) – 1m
Annex 13: Constantan (0.2) – 0.5m
All are labelled below.
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Annex 16: Design sketch (heating only)
Legend:
Battery: control centre, with 4.5V power generator (battery) and (eventually) Bluetooth module
Cell: more densely coiled wire, with fabric screen inside, integrated in a silicone patch.
Wired connector: silicone coated wire
Attachment: non-wired strap holding device together
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