There is a common misconception that you can patent an idea or a concept. I’m sure you’ve heard the barbecue conversations: “I think it would be great if we all had shoes with lasers in them that kill bugs as we walk toward them. I’m going to patent that!”
Your patent application must show and tell someone how to make or use your invention. Does this mean you have to prototype, build, or test the invention? Not necessarily, but you have to at least consider how to build it.
For example, say you want to patent a new robotic arm. Here is your concept:
Here is what you have after giving some thought to how to build and use it.
That's come a long way from concept phase, right?
Why are you required to show someone how to make your invention? The concept behind the patent system is to give innovators temporary exclusive rights to their invention in exchange for a disclosure on how to make it. After the temporary rights expire, anyone has the right to make or use the patented device. In this way, there is an incentive to invest the money and time required to innovate (the temporary exclusivity) but the public benefits as well (from the knowledge of how to make the device) when the exclusive rights expire.
So while you don't technically need a fully functional prototype before applying for a patent, it's extremely beneficial to have done at least some proof of concept demonstration.