The heart protector is constructed of liquid gavantium and an activating agent, that once combined, form a malleable, and yet near-indestructible skin around the heart. It is formed when the Heart Protector Deployment Device is used to inject both of these substance in sequence near to the heart. The gavantium finds the heart, and then the activating agent solidifies it, providing protection. 

They aren’t permanent however. During the healing process, the human body automatically breaks the protector down, allowing it to be expelled naturally from the body. Even without the healing process, the body will naturally break down and expel the gavantium, which is totally non-toxic and leaves no residue after a few days.