respectively. It has often been stated that the availability of water is virtually constant in all soft tissues and varies little from patient to patient. On the other hand, the levels of enzymatic activity may vary widely not only from patient to patient but also between different tissue sites in the same patient. Thus polymers that undergo hydrolytic cleavage tend to have more predictable in vivo erosion rates than polymers whose degradation is mediated predominantly by enzymes. The latter polymers tend to be generally less useful as degradable medical implants.