Echolucent implants

A pharmaceutically acceptable implant composition comprising:

• a hydrogel precursor composition, a therapeutic agent, and a pharmaceutically acceptable fluid, wherein the hydrogel precursor composition comprises a first precursor compound with a plurality of first functional groups and a second precursor compound with a plurality of second functional groups, wherein the first functional groups are capable of forming covalent bonds with the second functional groups at physiological conditions, and wherein the pharmaceutically acceptable implant composition is echolucent following reaction of the hydrogel precursor composition to form a collection of covalently-cross-linked, hydrolytically biodegradable hydrogel particles.