You can kill the heart by injecting the biomechanical cocktail into the heart.
Three ingredients you find from the lab.
From the greenhouse showcase.
Paint the right part of the greenhouse room to reveal them.
Great! Now, mix them.
With the liquid mixer. Add all three ingredients in there.
There are the drawers left of the tube. Search the top one.
Put the liguid into the liquid mixed. Also add all the three ingredients (the space fruit, the red plant and the mushroom).
There is a syringe from the bottom drawer. Use it to extract the cocktail out. Go inject the cocktail into the giant heart at the tech depot.