1. 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 over- discharge 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. Abstract Chemical vs. Electric Heating Element Band Components Circuitry RTV Silicone Battery Insulation Outer Band Thermostat Voltage Regulator Final Assembly 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. Background 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. 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. 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 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. 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 energy source needs to be as lightweight, compact, and unobtrusive as possible. This translates into the need for high energy- density 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. The heat is produced by running a current through small diameter, high-resistance wires. 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. 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 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. 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. 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. heating element to voltage regulator to thermostat Brie Frame Sandra Gonzalez Angela Tong Chenny Zhu Group 5 3.082 Spring 2004 Advisor: Hao Wang Cost Analysis