3.082 Final Presentation
May 13, 2004
Brie Frame • Sandra Gonzalez • Angela Tong • Chenny Zhu
Portable Heat Therapy
Objective
 Design and fabricate a heat therapy device
for lower back pain.
Portable Heat Therapy
Heat Therapy
Strain or
over-exert
back
Creates tension in
muscles and soft
Reduces tension
tissue surrounding
spine
Restricts
Increases
proper
Circulation
circulation
Pain
Decreases
signals sent
pain to
signal transmissions
brain
Portable Heat Therapy
Design Priorities
1.
2.
3.
4.
5.
6.
Long-term reusability
Portability
Heat distribution
Flexibility and comfort
Long-lasting heat
Light weight
Portable Heat Therapy
Potential Portable Heat Therapy
Designs
 Chemical
– Employ Phase Change Material (PCM)
– Freezes at room temperature  releases heat
 Electric
– Uses battery only
 Hybrid
– PCM melted by battery
http://www.hotsnapz.com/
Portable Heat Therapy
Energy Densities
2500
2050
2000
1500
Energy Density
(Wh/kg)
1000
500
174.91
64.01
41.95
22.30
0
Thermacare
Thunder Power
Double
Tetradecanol
Ethylene
Carbonate
2,6-Di-tertbutylphenol
Portable Heat Therapy
Electrical Superiority
Electrical
Chemical (Sodium
Acetate)
Hybrid
Thermacare ©
1. Long-term Reusability
1
2
3
4
2. Portable
1
3
4
1
3. Heat Distribution
1
1
1
2
4. Flexibility/Comfort
1
3
4
2
5. Long Lasting
2
3
4
1
6. Weight
2
2
4
1
1.3
2.3
3.3
1.8
Overall Rating
1 = Best
Best
2 = Good
3 = Satisfactory
4 = Bad
Worst
Portable Heat Therapy
Final Design Schematic
Circuit
Front View
Heating
Pack
Insulation
Side View
Towards Body
Battery
On/Off
Switch
Cotton
Cotton
RTV Silicone
Wire
RTV Silicone
Insulation
Towards Outside
Portable Heat Therapy
Voltage Cut-off Circuit
 Prevents over-discharging of battery
• 7.4V > V > 6 VOn
• V<6V
Off
Portable Heat Therapy
Thermostat Circuit
 Switches heating pad
on and off based on
temperature
 Hysteresis
– Turns on 38°C
– Turns off 42°C
Portable Heat Therapy
Constructing the Circuit
 Design circuit
 Assemble components
on breadboard
 Test circuit
 Solder circuit
Portable Heat Therapy
Testing the Circuit
 Voltage Cut-off
Circuit
– Ensure circuit is
opened at < 6V
 Thermostat Circuit
– Fine tune Set-point
temperature
 Connect circuits
together
Portable Heat Therapy
Battery
 Design Limitations
–
–
–
–
Voltage cut-off circuit needed
7.4V
1300 mAh – limits heating cycle
Need special charger
Portable Heat Therapy
Battery Charger
 Purpose
– Prevents over-charging
 Requirements
– 12V DC power supply
– Charge time > 1 hour
 Charge Time
– 12V, 800mA power supply
– 1.6 hours
Portable Heat Therapy
Heat Lost to Environment
Twire  Tenvironment
Power  q 
LRTV
Linsulation Lcot ton
Lenv



k RTV A kinsultionA kcot ton A kenv A
Heating Wire
RTV Siliconccce
Insulatoin
Cotton
Environment
 4.46 10 3W
Portable Heat Therapy
Heat Flux to Body
Twire  Tskin
Power  q 
Lwire
Lcot ton
RTV

k wire
k cot ton  A
RTV  A
Body
Cotton
RTV Silicone
T = Temperature (°C)
L = Length (m)
k = Thermal
Conductivity (W/mK)
A = Surface Area
Heating Wire
 0.754W
Portable Heat Therapy
Heating Element
 High Resistance Wire
– NiCr 80/20
– 30 Gauge (0.010 in.
diameter)
– Resistance = 6.5 Ω/ft
 Heat  Power (P = V2/R )
–
–
–
–
Length = 26 inches
Resistance = 14 Ω
Current = 500 mA
Power = 3.5W
Portable Heat Therapy
RTV Silicone
 RTV = Room Temperature Vulcanization (cures in
absence of heat)
 Flexible, thermally conductive (0.83 W/mK), and flame
retardant
 Property retention at temperatures of -55°C to 200°C
 Moisture from the air activates cross-linking agents
 Reactions produce non-acidic byproducts
CH3OH
H2O
(liquid polymer compound)
(rigid, rubbery silicone)
Portable Heat Therapy
Insulation and Outer Layer
 Solid Silicone Rubber (Insulation)
• Produced by COHRlastic
• Low Thermal Conductivity 0.0037 W/mK
• Flexible
 Cotton (Outer Layer)
• Comfort
• Low cost
http://www.tapecase.com/tc/rubber.asp
Portable Heat Therapy
Final Design
Charge Time = 75 minutes
Heat Time = 130 minutes
Weight = 260g
Portable Heat Therapy
Safety Concerns
 Must use in dry environment
 Battery
–
–
–
–
Only use charger provided
Watch battery while charging
Charge Battery in non-flammable area
Don’t short the battery
Portable Heat Therapy
Cost Analysis
Product
Cost
Dimensions
Reusable?
One Cycle of
Heat (hrs)
Cost per Month
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
Portable Heat Therapy
Future Work
 Longer lasting
– Use battery with larger Ahr
 Decrease size of chip
– Surface mount chip
 Convenient battery charger
– Build simple to use charger
– Attach car adapter
Portable Heat Therapy
Acknowledgements






3.082 Teaching Staff
David Bono
Hao Wang
Yet-Ming Chiang
Course 6 lab
Course 6 friends
Portable Heat Therapy