Portable Heat Therapy

3.082 Final PresentationMay 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

tissue surrounding spine

Restricts proper circulation

Pain signals sent to brain

Increases Circulation

Decreases pain signal transmissions

Reduces tension

Portable Heat Therapy

Design Priorities1. Long-term reusability2. Portability3. Heat distribution4. Flexibility and comfort5. Long-lasting heat6. 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

22.3041.9564.01

174.91

2050

0

500

1000

1500

2000

2500

Thermacare Thunder PowerDouble

Tetradecanol EthyleneCarbonate

2,6-Di-tert-butylphenol

Energy Density (Wh/kg)

Portable Heat Therapy

Electrical Superiority

ElectricalChemical (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

Overall Rating 1.3 2.3 3.3 1.8

1 = Best 2 = Good 3 = Satisfactory 4 = Bad

Best Worst

Portable Heat Therapy

Final Design SchematicFront View

Heating Pack

InsulationC

ircui

t

Side View

Towards Body

Towards Outside

Wire

On/OffSwitchBattery

RTV SiliconeRTV SiliconeInsulation

Cotton

Cotton

Portable Heat Therapy

Voltage Cut-off Circuit

Prevents over-discharging of battery• 7.4V > V > 6 V On• V < 6 V 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

RTV

Sili

conc

cce

Cot

ton

Envi

ronm

ent

Insu

lato

in

Hea

ting

Wire

W

AkL

AkL

AkL

AkL

TTqPower

env

env

ton

ton

insultion

insulation

RTV

RTV

tenvironmenwire

3

cot

cot

1046.4 −×=

+++

−==

Portable Heat Therapy

Heat Flux to Body

RTV

Sili

cone

Cot

ton

Bod

y

Hea

ting

Wire

WAk

LAk

LTTqPower

ton

ton

wire

wire

skinwire

754.0cot

cot

=×

+×

−==

T = Temperature (°C)L = Length (m)k = Thermal Conductivity (W/mK)A = Surface Area

RTV

RTV

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

(rigid, rubbery silicone)

H2O

(liquid polymer compound)

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 minutesHeat Time = 130 minutesWeight = 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 $44.95Heat 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