There are a few main different styles of fluid warmers:

In-line: Room temperature (or colder) fluid flows through a device that heats it up as it passes through.

Pros:

• Easiest to use, no preheating of IV bag required.
• Smallest footprint.
• Consistent temperature delivered.

Cons:

• Likely harder to assemble. May have more individual parts to source.
• Most sophisticated and challenging to design and test.

Surface Heater: The IV bag sits on a warming surface that maintains the desired temperature.

Pros:

• Simplest design, likely easiest to 3D print and assemble.
• Easy to verify correct operation.

Cons:

• Once an IV bag is removed the temperature gradually drops as fluid is being administered.
• Larger footprint.

Oven: Fully enclosed unit that heats through ambient air.

Pros:

• Easy to self-monitor
• Easier to scale up for more bags heating at once.

Cons:

• Once an IV bag is removed the temperature gradually drop as fluid is being administered.
• Largest footprint.

In terms of user appeal the In-line heater is likely the most desirable as it is the easiest to use and takes up the least space. The challenge here though isn't to create the best new product, but to make one that is open source, easy and quick to manufacture, cheap, and reliable. The idea I have for my project sacrifices ease of use to maximize ease of production and operation in the scarcest conditions.