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Introduction
Fibromyalgia is a wide spread musculoskeletal pain and fatigue disorder for
which the cause of the disease is still unknown. It consists of pain in the muscle,
ligaments, and tendons (the soft fibrous tissues in the body). Most patients with
fibromyalgia report an ache all over their body, much the way the average person feels
when they are infected with a bad flu virus. Patients describe the pain of fibromyalgia as
deep muscular aching, throbbing, shooting, stabbing, and burning. Most patients also
have an associated sleep disorder called the alpha-EEG anomaly in which their deep level
sleep is constantly interrupted by bursts of awake-like brain activity. Treatment is trivial
since the cause of the disease is yet unknown. Most treatments consist of improving the
quality of sleep and reducing pain, usually by medication. Fibromyalgia is a chronic
disease, and the symptoms may intensify or fade. Studies have shown that the disease
can be as disabling as rheumatoid arthritis.
Motivation
Our client is studying patients with the disease Fibromyalgia by testing their pain
sensitivity. He has requested an apparatus that elicits the feeling of a painful stimulus
without causing any real pain. In other words, the device tricks the skin into
experiencing the sensation of pain without causing any damage to the tissue. An
apparatus called a thermo-grill is one example of a device that can evoke this sensation.
The client’s ultimate goal of his study is to identify a new therapy for chronic pain.
Background
A thermo-grill uses the idea that pain is a homeostatic emotion, not just a sense
generated by touch. The idea of using a device to create the sensation of pain is based on
recent functional, anatomical, and imaging findings in the brain that indicate pain is
generated by specific sensory channels that ascend in a central homeostatic afferent
pathway. These findings suggest the human feeling of pain is not only a sensation but
also a motivator--rather, pain is an emotion that dictates homeostatic behavioral drive
much like hunger, thirst, and temperature. Therefore, pain should be looked at as not
only a specific sensation but also a variable emotional state. Emotions consist of a
sensation and a motivation with direct autonomic effects, and pain is one of many distinct
homeostatic emotions that directly reflect the condition of the body.
There is physiological evidence that burning pain is a homeostatic feeling. A
class of nociceptive lamina I STT neurons, called HPC cells, is associated with second, or
burning, pain. These HPC cells are responsible for the burning pain sensed by
simultaneous warming and cooling in the thermal grill illusion of pain. HPC cells are
associated with homeostasis by their continuous activity and sensitivity to cold. They
show static responsiveness below 24 degrees Celsius. Cold does not usually produce
pain until 15 degrees Celsius, when HPC activity increases and cooling-specific lamina I
cell activity plateaus. The fact that cold becomes noxious when HPC activity exceeds
cooling-specific activity is confirmed by the observation that an artificial reduction in
cooling activity by simultaneous warming in the thermal grill enables innocuous cool
temperatures to produce burning pain. This indicates the perception of pain depends on
the forebrain integration of thermal and pain sensory channels, implying that pain is a
homeostatic motivation. This physiological evidence verifies that homeostasis is the
essential nature of pain.
It is enlightening to compare the homeostatic function of thermoregulation with
pain. Trigeminal I neurons contain modality-selective, morphologically distinct classes
of neurons that are usually related to pain and temperature. It is known that pain and
temperature are processed together in the mammalian central nervous system, which
shows their underlying commonality as aspects of homeostasis. In the central nervous
system, there is a sensory overlap between the perception of pain and the perception of
temperature. The same nerves are used in each pathway and the message goes to similar
parts of the brain.
Previous Design
Our client, Dr. Coe, became aware of the thermal grill illusion and its application
to pain studies when he attended a lecture by A.D. “Bud” Craig, from the University of
Arizona. Dr. Craig, who also studies pain disorders, talked about a device he had built
which consisted of a thermal grill with alternating warm (40º C) and cool bars (20º C)
(Figure 1). He uses the thermal grill device to test pain sensitivity in patients.
Figure 1: Thermal Grill
When Dr. Coe inquired about the apparatus, Dr. Craig seemed unwilling to
divulge the details of the device construction. During extensive research, we found little
information about the construction of the device since there has been no patent filed on
the device to date. From the photographs of the device that are available, it can be
deduced that it utilizes electric heating and cooling. The device seems to be about 8” x
8” with alternating warm and cool bars laying side by side, each with 0.5” width.
Besides not knowing how to build it himself, the main problem that our client had
with this previous design was that it was quite ominous looking. There were electric
wires and metal rods everywhere, so our client worried that it may cause patients to
become fearful of the pain stimulating device. Dr. Coe added that it may help the
patients feel more comfortable if the wires and rods were enclosed in some sort of
container so that they were not as visible to the patient.
Design Constraints
After the client meeting the requirements of the design were clearly defined.
There appeared to be two main objectives of the design. The first objective is to create a
device that increases from room temperature to 40ºC within one second. The second
objective is to create a device which decreases from room temperature to 20ºC within one
second. The elevated and reduced temperature bands need to alternate in bands which
are adequately spaced so the sensory receptors in the hand can detect both the hot and
cold sensations. The client specified various other requirements which are stated in the
attached PDS.
Design 1: Water and Copper Tubing
The first design consists of a hot water bath and copper tubing (Figure 2). The
hot water bath would contain water at a temperature of 45°C. The temperature would be
maintained with the use of a hot plate. The hot water bath would include a valve that
could be opened to allow the water to flow into alternating tubes in the grid. A return
tube would allow the water to be returned to the bath. The copper tubing would have an
outside diameter of 0.5 inches, a length of 8 inches, and a wall thickness of 0.04 inches.
The tubing would form a grid of alternating hot and room temperature tubes.
Figure 2: Water and Copper Tubing Design
The first strength of this design is it would be relatively easy to construct. The
copper tubing is inexpensive, and finding a suitable container for the water bath would
not be difficult. The next strength of this design is the thin copper tubing conducts heat
well, so the tubing would quickly become the same temperature as the water flowing
through it. The final strength of this design is the 40° C water temperature is not difficult
to maintain, so the tubing the hand would be in contact with would remain at the correct
temperature for the duration of the experiment.
Although there are strengths incorporated in this design, there are also
weaknesses. The vital weakness of this design is the setup of the device. Our client
would need to handle the hot plate with the container of water on top, regulate the flow of
the water out of the water bath, and record and monitor the results of the experiment
simultaneously. This would make the job of accurately observing and recording all
aspects of the experiment a rather daunting task. Another weakness of this design is the
ominous aesthetic appearance. The patients our client will be studying are very
susceptible and cautious of pain, and to have a container of water at near scalding
temperature will make the patient uncomfortable and likely alter the results of the
experiment. Because of these crucial weaknesses another design had to be created.
Design 2: Peltier Device
A Peltier device (Figure 3), also know as a thermoelectric module, operates by
applying the Peltier effect, which occurs when a current flows through two dissimilar
conductors. The junction between the two conductors will either absorb or release heat
depending on the direction of current flow, thus creating one hot and one cold surface.
This effect is then applied to a system of semiconductors to supply one surface with an
increased temperature and one surface with a cooled temperature.
Figure 3: Peltier Device
A Peltier device consists of Bismuth Telluride pellets soldered to copper on
opposite sides of the pellet. The junction between the Bismuth and the copper is where
heat absorption and heat release take place. The Bismuth Telluride pellets used to
construct the device can be adopted to be either N or P-type. In an N type pellet the
carriers are electrons whereas in a P-type the carriers are holes, or spaces where there are
no electrons. As illustrated in Figure 4 below, the electrons in the N-type pellet are
repelled by the negative charge of the battery and are attracted to the positive, so the
electrons in the pellet move up along with the heat. (Thermoelectic Modules)
Figure 4: Diagram of the Peltier Effect
In order to create more heat than what a single pellet would provide, multiple P
and N-type pellets are used. The multiple pellets join together in the following fashion.
Figure 5: Thermoelectric Module
When combined together in the above fashion, the electrons flow in the same
direction. The electrons/carriers in the N-type pellet are repelled by the negative charge
of the battery and thus flow towards the positive side, whereas the P-type holes/carriers
are repulsed by the positive charge of the battery and flow toward the negative end. The
carriers in each type of pellet are moving in the same direction and allow for maximum
heat absorption and release. Due to the fact that the pellets are not connected to each
other and are only connected by the copper, the pellets do not form diodes with a
depletion zone. This allows the pellets to be connected to a power supply in either
direction, and by simply changing the power supply polarity; the hot and cold sides can
be switched.
The entire internal pellet construction is sandwiched between two ceramic plates
for mechanical support and a protective barrier. Peltier devices are manufactured for a
range of heating and cooling temperatures. Peltier devices are also very small, on the
order of mm or cm. The temperatures of a Peltier device can be controlled by
temperature sensor feedback device such as a thermistor, or a commercially available
sensor.
In order to create the alternating sensations of hot and cold, numerous Peltier
devices could be used. For the hot regions, one side of a surface would be in contact with
the subject and for the cold surface the opposite side of the surface would be in contact
with the subject’s hand. Eight to ten different Peltier devices aligned parallel could be
used to cover the length of the hand. Each Peltier device would need to be six inches
long, in order to span the entire width of the subject’s hand. The individual Peltier
devices could be contained in one holding container which would enhance the aesthetics
of the design as well as improve overall organization of the design. The device would be
turned on by plugging in the voltage sources for each Peltier device used.
The Peltier device is able to provide both hot and cold temperatures. The Peltier
device is capable of obtaining the required temperatures of 40º C and 20º C. The Peltier
device is also an unobtrusive device which could be used without causing a lot of mental
stress on the patient. In addition the Peltier device would be simple to operate because
only a power source is needed and the temperature control could be automatically set.
These electronic controls require minimal effort from the client in order to perform the
experiment repeatedly.
One requirement that the Peltier device did not satisfy was the time constraint of
heating and cooling. The heating and cooling speed can change very quickly, however to
avoid damage from thermal expansion the rate of temperature control should be about 1
degree C per second. This does not comply with the design constraint of obtaining the
desired temperature in 1 second.
Design 3: Electric Heater and Cooled Copper Rods
Warm Component
This design utilizes an electric heating component because it allows for quick
achievement of the desired temperature as well as temperature control. Electric
resistance heaters are coils of wire through which current is run, creating energy in the
form of heat. These electric resistance heaters come in many shapes, sizes, and time
specifications, so it is an ideal choice for our design.
In order to make sure that the optimal temperature (40º C) is achieved, a
temperature controller will need to be connected to the electric resistance heater. Several
types of controllers are commercially available. Lower order controllers tend to
overshoot the desired temperature and then oscillate until the correct temperature is
achieved several seconds to minutes later. Our specifications show that we need to
achieve asymptotic heating, meaning that the temperature gradually rises, eventually
reaching the desired temperature and plateauing at that level. Higher order controllers,
which provide assymptotic temperature regulation would make certain that the
temperature does not overshoot 40º C, ensuring that the patient would be safe from burns.
Cool Component
Instead of using an electric cooling apparatus or cold water through plumbing,
this design utilizes a much more simplified cooling method. Copper is an excellent
conductor of both electricity and thermal energy, which causes the material to feel cool to
the touch. So, for this design, solid copper rods will be used as the cooling component
because they can retain a relatively constant temperature for a long period of time. The
thicker the copper, the less its temperature will vary. At room temperature, the copper
rods should be about the desired temperature, 20 C. Temperature of the cool rods will be
verified by a thermocouple before they are placed in the apparatus. This design of the
cooling component is advantageous because it does not require electric input or messy
water—it is by far the easiest to use.
Overview
Scaffolding for the grid will be constructed with 12 cradles for the warm and and
cool rods. The six cold rods will then be placed in their respective cradles (alternating
between the warm rods) once their temperature is verified. The cold rods may need to be
refrigerated for a short time to achieve the correct temperature, so they will be removable.
Six warm rods, each consisting of a copper tube with an electric resistance heater
inside, will be a permanent part of the apparatus. Circuitry must be developed to connect
all of the heating elements and temperature controllers to an AC power supply and an
on/off switch.
In the end, the apparatus will consist of 12 exposed parallel rods laying flat. The
circuitry will be enclosed in an outer casing to make the device less ominous looking for
the patient. The on/off switch will be connected to the device via a long cord so that the
researcher may control the thermal grill a few feet from the patient so that the patient is
not aware of when the stimulus is turned on.
Ethics
The ethical issues that arise when designing the thermal grill are minimal. The
design merely needs to have the integrity that it will comply with the safety requirements
decided upon by the design team. The safety requirements of this design involve being
able to control the increase in temperature so that the subject is not burned. The design
needs to be compliant with the heat restriction and be tested numerous times to assure
that it will not burn the subjects. If due diligence is taken in designing a well regulated
heating strip and repeated testing is performed, the design satisfies any ethical
requirements.
Ethical issues may arise from the application of the design. The design elicits the
sensation of pain using thermal variation. Since pain is normally a sensation which is
avoided and is unpleasant, is it morally correct to expose subjects to something
unpleasant? In this particular situation, the subjects are allowed to remove their hand at
their own will at any point and only have to tolerate as much pain as they desire. It is a
self regulated experiment. Additionally, the ultimate goal of the project is to learn more
about what is causing chronic pain. The goal of the project is honest and forthright which
makes the slight discomfort experienced during the experiment tolerable.
Future Work
Several experiments will need to be done to find the optimal configuration and
construction of this system. First, the optimal thickness of the cool copper tubing will
need to be determined so that the rods maintain the correct temperature for a long period
of time without being affected by the heat from the patient’s hand. Air is a very effective
insulator, so there should be little effect form the neighboring warm bars. To do this,
heat capacity calculations will be performed to find the optimal thickness and mass of the
bar in order to make sure that the temperature does not change too quickly. Hands-on
experiments would also be beneficial in this case.
Further research also needs to be done to find electric resistance heaters and
temperature controllers that fit our specifications. We will use heat transfer equations to
find out how big the electric heater needs to be. By using the heat capacity of the
material (copper) to find the energy flux and knowing that we want to heat the material
within one second, we can find the power required.
Supply companies provide
information on their heaters in terms of power, so we will be able to find the right heater
for our project. Some research on higher order temperature controllers will also need to
be done.