The preliminary design review was very productive. In all, three mechanical engineers and one electrical engineer were in attendance. 

A big focus of the design review discussion was on how to significantly improve (i.e. reduce thermal resistance of) the heat transfer interface between the silicone heaters and the IV fluid. The thermal performance design requirement for the OpenFluidWarmer device is to warm 4°C fluid to 38°C at 20mL/min flow rate. Based on testing, the current prototype design can only warm the fluid from 4°C to 24°C at 20mL/min. A lower thermal network between the heaters and IV fluid is required to close that 14°C gap. There are three approaches that have the potential to reduce the thermal resistance and still satisfy the cost target for this device:

1. increase the length of tubing that is in contact with the hot plates. Heater set point and contact area staying the same, it could likely take just as much if not more tubing length to close the 14°C gap to 38°C than it does to achieve the 20°C temperature rise from 4°C to 24°C. The reason is Netwon's Law of Cooling -> the rate of heat transfer between two bodies is proportional to the temperature difference between them. The closer the fluid temperature is to the heater set point the longer the tube must be to achieve each successive 1°C fluid temperature rise because the reduced rate of heat transfer. At the same time, the heater set point is 105 degC; far from the 38°C target outlet temperature. An extra 150mm tube length (20% increase) in contact with the hot plates may result in something close to an additional 5°C temperature rise.

A couple other factors at play here are the maximum length of IV tubing that can be used by the device without requiring an IV tubing extension (a cost add) and the minimum allowable bending radius of the IV tubing before it kinks. It is not well known what the acceptable length of IV tubing can be used by the device. Obviously, if it uses up too much of the IV tubing, the IV tubing will not be able to reach the patient. More research and feedback from potential users is required here. IV tubing can be made out of different types of materials. In general, the cheaper IV tubing option is more likely to be the less flexible and more prone to kinking. More research is required to identify the IV tubing option most likely to be used by users in medical settings with few resources and determine the minimum allowable bending radius of that tubing.

2. increase the surface area contact between the IV tubing and hot plates per unit length of the IV tubing. The current prototype design sandwiches the IV tubing on two "sides" between the hot plates. So the thinking is that by better form fitting to the IV tubing outside surface, the thermal resistance contribution of the IV tubing can be reduced without increasing the length of tubing used by the device. Form fitting the IV tubing achieves more efficient heat transfer per unit length of IV tubing. Of the three approaches, this one arguably has the most potential for closing the 14°C gap. But it is dependent on finding a low-cost, thermally conductive, material that can form fit the IV tubing. Initial thoughts on this material are thermal gap pad, gel/gel-like material, or a fluid. 

3. increase the heater temperature set point. The temperature rating of insulative materials currently used at the heaters is just above 100°C. So the 105°C heater set point being used is pushing the limit of the current materials. These insulation materials could be replaced with higher temperature rated materials and the heater set point increased. The increased heater set point would create a larger temperature differential between the heaters and the IV fluid and drive up rate of heat transfer. The only downside is that higher heater set points potentially result in increased risk of hemolysis. Though this risk has not been evaluated yet.

An interested test proposed during this meeting was to create a constant temperature bath using a sous vide cooker and perform several tests to identify what bath set point and submerged tubing length is required to achieve the 4°C to 36°C at 20mL/min performance target. This test serves as an ideal upper bounds to the thermal performance that can be achieved when warming IV fluids through standard IV tubing. There is nothing more form fitting than a fluid and the sous vide cooker has an agitator to achieve a decent fluid flow rate over the outside of the IV tube. The $50 sous vide cooker I ordered yesterday just arrived today. I plan to perform the testing sometime tomorrow.