Nice game! I think the core gameplay loop works really well: It's a cool cycle of switching between killing the enemies and shielding the scientists.
I like the interactions of different mechanics: The overheating and friendly-fire forces the player to aim their bullets well. The upgrades affect both the protecting and the killing phase. It was a good idea to make the player invincible, it eliminates the extra cognitive/visual "noise" of a health bar and leads to interesting shielding gameplay.
I think you could add more juice to the gameplay (check out jw's talk if you don't know it yet, it's great https://www.youtube.com/watch?v=AJdEqssNZ-U). One thing in particuar, I think, would be to make the hitboxes of the scientists smaller, and maybe the players hitbox bigger, to facilitate shielding. In general the game would profit from a bit more action-y gameplay.
But other than that the core game-concept is really well executed, and quite elegant!

Fun and interesting idea, that lead to some cool game-play. I liked how you'd have to plan where your gun would fall, and how you could act as a shield for the scientists. Sometimes it seemed too hard in a frustrating way. If you scrambled to finish one level and then had to drop your gun immediately for the next one it felt a bit unfair. But overall, nice work!