artificial chemical means. The most common agent used today is formaldehyde, which is infused to replace body fluids.
The second sentence can best be replaced by:
1) Body fluids are replaced by formaldehyde, the most widely used agent today.
2) The most common agent "body fluids" are used to replace formaldehyde.
3) Formaldehyde is most widely used today along with the body fluids.
4) To replace body fluids the most common agent is to be used.