My feedback for Vein Invader:
What your game does well:
Camera navigation and player movement felt like the right pace. I didn't find my character to be moving at an unreasonable speed.
Enemies are fair to fight against. I enjoyed performing the attack combo against them.
What your game doesn't do well:
The power-ups didn't affect the gameplay to a noticeable degree. When the game starts the player is presented with one, but has 0 past experience to base their choice off of. The player has to choose 1 of 4 different power-ups before they've fought their first enemy.
While on power-ups, the health recover power-up states 'by a specific amount'. Not really sure why it doesn't just state the amount it heals the player by. This could be a whole number or a percentage.
Your game nails the 'inside' part of the theme with its setting, but doesn't do anything with the 'out' part. At least, it doesn't offer anything for that in the gameplay.
Areas of Opportunity:
What happens before/after the micro-biotic entity enters/exits the patient's body? If the setting of the game is the part that incorporates the theme, a great opportunity is to build its narrative of the outside world.
You could also tie the gameplay to your theme, as well. If you're in a patient's body, then the health of the patient should be something the player has to consider, too.
With power-ups, you can make them appear less frequently but more potent. With more combat and less opportunities to strengthen themselves you give the player's choice more weight.
I hope this feedback helps!