Implants and biodegradable tissue markers

1. A hydrogel implant, said hydrogel implant formed by a method that comprises combining one or more precursors that undergo a covalent crosslinking reaction to make a covalently-crosslinked biodegradable hydrogel implant that has a covalently attached radiopaque agent that comprises iodine, wherein the one or more precursors comprise a water soluble branched polyethylene glycol (PEG) with at least four arms wherein a portion of the arms comprise the radiopaque agent linked to the arm via a linkage group and wherein the remaining arms comprise an electrophilic functional group linked to the arm by a hydrolytically labile linkage.

2. The hydrogel implant according to claim 1 , wherein between 25% and 90% of the arms comprise the radiopaque agent linked to the arm via the linkage group.

3. The hydrogel implant according to claim 1 , wherein the electrophilic functional group is selected from carbodiimidazole, sulfonyl chloride, chlorocarbonates, n-hydroxysuccinimidyl ester, succinimidyl ester or sulfasuccinimidyl esters.

4. The hydrogel implant according to claim 1 , wherein the one or more precursors further comprise a multifunctional precursor that comprises two or more nucleophilic functional groups.

5. The hydrogel implant according to claim 4 , wherein the nucleophilic functional groups are amine groups.

6. The hydrogel implant according to claim 4 , wherein the multifunctional precursor comprises trilysine.

7. The hydrogel implant according to claim 1 , wherein the branched polyethylene glycol has a molecular weight ranging from 10,000 to 100,000 Daltons.

8. The hydrogel implant according to claim 1 , further comprising a therapeutic agent or a radiation source.

9. The hydrogel implant according to claim 1 , wherein the hydrogel implant produces degradation products that are absorbed into the circulatory system and cleared from the body via renal filtration.

10. The hydrogel implant according to claim 1 , wherein the hydrogel implant is biodegradable at a time between about 30 and about 365 days.

11. A hydrogel implant, said hydrogel implant formed by a method comprising combining first and second precursors that undergo a covalent crosslinking reaction to make a covalently-crosslinked biodegradable hydrogel implant that has a covalently attached radiopaque agent, the first precursor comprising a water soluble branched polyethylene glycol (PEG) having a plurality of arms wherein a portion of the arms comprise the radiopaque agent linked to the arm via a linkage group and wherein the remaining arms comprise an electrophilic functional group linked to the arm by a hydrolytically labile linkage.

12. The hydrogel implant according to claim 11 , wherein between 25% and 90% of the arms comprise the radiopaque agent linked to the arm via the linkage group.

13. The hydrogel implant according to claim 11 , wherein the electrophilic functional group is selected from carbodiimidazole, sulfonyl chloride, chlorocarbonates, n-hydroxysuccinimidyl ester, succinimidyl ester or sulfasuccinimidyl esters.

14. The hydrogel implant according to claim 11 , wherein second precursor comprises a multifunctional precursor that comprises two or more nucleophilic functional groups.

15. The hydrogel implant according to claim 14 , wherein the nucleophilic functional groups are amine groups.

16. A hydrogel implant, said hydrogel implant formed by a method comprising combining first and second precursors that undergo a covalent crosslinking reaction to make a covalently-crosslinked hydrogel implant that has a covalently attached radiopaque agent, the first precursor comprising a branched component having a plurality of arms wherein a portion of the arms comprise the radiopaque agent linked to the arm via a linkage group and wherein the remaining arms comprise an electrophilic functional group linked to the arm by a hydrolytically labile linkage, and the second precursor comprising a multifunctional precursor that comprises two or more nucleophilic functional groups reactive with the electrophilic functional groups.

17. The hydrogel implant according to claim 16 , wherein between 25% and 90% of the arms comprise the radiopaque agent linked to the arm via the linkage group.

18. The hydrogel implant according to claim 16 , wherein the electrophilic functional group is selected from carbodiimidazole, sulfonyl chloride, chlorocarbonates, n-hydroxysuccinimidyl ester, succinimidyl ester or sulfasuccinimidyl esters.

19. The hydrogel implant according to claim 16 , wherein the nucleophilic functional groups are amine groups.

20. The hydrogel implant according to claim 16 , wherein the first precursor is a water soluble branched polymer comprising at least four arms.