The first iteration of the system architecture assumes two 100W silicone pad heaters inside a 3in diameter HVAC air duct. PID control on an Arduino Nano will be used to maintain constant temperature. Four thermistor temperature sensors will be used for temperature feedback. Heater power is controlled with two mosfet boards that are typically used for 3d printer heated beds. IV tubing is wrapped around the 3in air duct multiple times. Heat is generated by the silicone heater pads, transferred to the air duct, and then transferred through the IV tubing to the fluid.

 The system will maintain temperature at 39 degC. If temperature exceeds 42 degC during normal operation, the heaters will turn off and the user will be informed through a visual and audible alarm. If the device temperature drops below 36 degC during normal operation, the user will be informed through a visual and audible alarm. Upon startup, the system will inform the user when the device has warmed up to 39 degC through a visual indicator. I may include input voltage sense for battery powered systems. When battery voltage drops below the voltage threshold a visual indicator would turn on to show low battery power. The voltage threshold is dependent on which battery system is used so would have to be configurable in the microcontroller code.

The latest cost breakdown shows a total system cost of $60 (excluding the required 12V, 20A power source). This cost estimate is not conservative. I expect the final design to be closer to $75.