Brie Frame
Sandra Gonzalez
Angela Tong
Chenny Zhu
Abstract
Group 5
This project aimed to create a device that provides heat between 35 and 42 degrees
Celsius over a long time scale for the purpose of pain relief for strained muscles. The
final device is battery-powered, with a high-resistance wire serving as the heating
element, and includes circuitry to regulate temperature and protect against overdischarge of the battery. The heating assembly is housed in a fabric band designed to
be wrapped around the waist and provide heat to the lower back. The warming band as
designed can provide constant, uniform heat in the desired temperature range for
approximately 3 hours.
Advisor: Hao Wang
Background
Recent medical research indicates that the application of prolonged,
low-grade heat to sore or strained muscles provides benefits that rival
aspirin for pain relief. Temperatures near skin temperature are
preferred; between 32 and 45 degrees Celsius are acceptable.
There are several products already on the market that address this
problem. Sodium acetate packs, iron oxidation disks, and electric
bands that plug into a wall socket are the most prevalent.
The chemical packs are convenient and portable, but tend to last for a
short period of time. Electric packs last for as long as you have
electricity, but they tie the user to the wall socket. Our aim was to
explore the possibility of creating a portable electric heating pack
specifically for the lower back.
2500
Energy Density (Wh/kg)
3.082 Spring 2004
2050
2000
1500
1000
500
174.9
68.57
64.03
41.95
22.36
Sodium
Acetate
Tetradecanol
Ethylene
Carbonate
2,6 Di-tertbutylphenol
0
Thermacare
Thunder
Power Double
Chemical vs. Electric
At the outset, our group was interested in the chemical
heating possibilities, specifically using chemicals that
freeze near body temperature. However, our main goal
was to provide long-lasting heat. Our most promising
phase-change chemicals had a low energy density and
would make the band too heavy, as shown on the graph.
Thermacare is a product on the market that produces
heat from an iron oxidation reaction, and Thunder
Power double is the battery we ended up choosing.
Since weight is a concern (aching backs don’t want
heavy packs), the electric option was the best choice for
long heating time.
Insulation
Band Components
A layer of thermal insulator on the air side of
the heating elements helps cut down on heat
loss to the environment. The insulating
material should ideally have a low thermal
conductivity and be thin, lightweight, and
flexible for comfort.
RTV Silicone
The heating element alone would result in a
noticeable heat pattern felt on the back. A
layer of material with a high heat capacity to
retain heat and a middling thermal
conductivity to diffuse the heat a bit would
assure a more even warming across the pad.
Heating Element
The heat is produced by running a current
through small diameter, high-resistance wires.
The chosen material is COHRlastic Solid
Silicone Rubber TC100. It comes in a thin,
flexible, rubbery sheet and has a thermal
conductivity of .0037 W/mK.
The substance chosen is a neutral-cure RTV
silicone. Besides spreading the heat out over a
wider area, the silicone layer also holds the
wires in place and electrically insulates them.
The wires are a Ni80-Cr20 alloy with a
resistance of 6.5 ohms/foot. The wires
produce the desired temperature through a
layer of RTV silicone and a layer of cotton
fabric at a current of 502 amps and 7.4 volts.
A foot and a half of wire was needed to
produce this current.
Outer Band
Battery
The energy source needs to be as lightweight,
compact, and unobtrusive as possible. This
translates into the need for high energydensity batteries.
Lithium polymer batteries ended up having
the best energy density both by weight and by
volume. The chosen battery is a Thunder
Power Double 1300mAh 7.4V battery.
Circuitry
to thermostat
Final Assembly
to thermostat
Voltage Regulator
The purpose of the voltage regulator is to shut
the battery off at a certain voltage. This is
needed so that the lifetime of the battery can be
maintained as long as possible. As the battery
completely discharges, the anode of battery can
begin to corrode, which destroys the battery.
The battery must be shut off before the voltage
through the circuit is less than 6 volts. This is
achieved by a combination of Zener diodes and
transistors.
The final heating area on the pad is 10cm x 16cm. This picture
shows the components just before they were sewn into the band.
The circuits were wrapped in duct tape for comfort and to
protect the pieces. An on/off switch (off picture to left) was
added to the circuit. The battery remains outside the band in a
pocket for easy removal for charging. A special charger must be
used for the lithium polymer battery to prevent overcharging.
The outer band houses the components and
comfortably secures the device around the
user’s waist to provide heat to the muscles of
the lower back. The lightweight cotton
material was chosen for comfort and low cost.
The band is 42” (107 cm) long and 5.5” (14
cm) at its widest. Six inches of velcro fastener
on the arms of the band give nearly a foot of
leeway for fitting varying waist sizes.
to voltage
regulator
heating
element
to voltage
regulator
Thermostat
The thermostat is an integrated circuit that
monitors the ambient temperature and closes or
opens the circuit based on predetermined
temperatures. The set temperature is adjusted
by adding resistors. The thermostat will open
the circuit at 42 °C and close the circuit to
resume heating at 37°C.
Cost Analysis
Product
Cost
Dimensions
Reusable?
One Cycle of
Heat (hrs)
Thermacare
$3.50
4x10
no
8
$69.90
Zap Pac
$44.95
4x10
yes
0.75
$44.95
Heat me
Now
$65.00
4x10
yes
3
$65.00
Cost per Month