Implants and biodegradable tissue markers

1. A method of making a marker implant in a tissue, comprising filling a iatrogenic site with one or more flowable synthetic precursors in aqueous medium that undergo a covalent cross-linking reaction to make, in the site, a covalently-crosslinked biodegradable hydrogel implant that has a covalently attached radiopaque agent.

2. The method of claim 1 wherein the hydrogel implant has a Hounsfield number of more than about 50.

3. The method of claim 1 wherein the hydrogel implant, as placed in the tissue, has a volume between 1 and 5 ml.

4. The method of claim 1 wherein the hydrogel implant is stable, having dimensions that do not appreciably change following implantation for a predetermined amount of time, and thereafter softens and biodegrades.

5. The method of claim 4 wherein the predetermined amount of time is between 30and 90 days.

6. The method of claim 1 , with the hydrogel implant being biodegradable to produce only degradation products that are absorbed into the circulatory method and cleared from the body via renal filtration.

7. The method of claim 1 with the hydrogel implant being hydrol ytically biodegradable.

8. The method of claim 1 wherein degradation products of the hydrogel implant comprise a polyethylene glycol covalently bound to the radioopaque agent, with the radioopaque agent comprising iodine.

9. The method of claim 1 wherein the hydrogel implant is a product of a covalent crosslinking chemical reaction between at least two precursors, with one of the precursors comprising a branched polyethylene glycol with at least four arms.

10. The method of claim 9 wherein between 25% and 90% of the arms comprise the radioopaque agent.

11. The method of claim 9 wherein the branched polyethylene glycol has a molecular weight from 5,000 to 100,000 Daltons.

12. The method of claim 9 wherein the branched polyethylene glycol has a molecular weight that is more than 20,000 Daltons and less than 100,000 Daltons.

13. The method of claim 1 wherein the hydrogel implant is made from precursors that have no more than three contiguous amino acids.

14. The method of claim 1 wherein the hydrogel implant provides a fiducial marker.

15. The method of claim 1 wherein the hydrogel implant is placed in a tissue cavity.

16. The method of claim 15 wherein the tissue cavity is facial or is located in a breast.

17. The method of claim 15 comprising substantially filling the tissue cavity with the hydrogel implant.

18. The method of claim 15 wherein the hydrogel implant has a volume that is substantially equal to the tissue volume removed to make the tissue cavity.

19. The method of claim 1 further comprising visualizing margins of the hydrogel implant with a machine selected from the group consisting of magnetic resonance imaging, X-ray, and computerized tomography.

20. The method of claim 1 with the one or more flowable synthetic precursors in aqueous medium having a lubricity and maximum diameter suitable for manual passage out of a syringe through a 27 gauge needle.

21. The method of claim 20 further comprising passing the one or more flowable synthetic precursors in aqueous medium through a needle of 27 gauge or a smaller diameter.

22. The method of claim 1 wherein the hydrogel implant is completely biodegradable at a time between about 30 and about 365 days.

23. The method of claim 1 further comprising a therapeutic agent in the hydrogel implant.

24. The method of claim 1 further comprising a radiation source in the hydrogel implant.

25. The method of claim 1 , with the radiopaque agent being present in the hydrogel implant at a concentration of at least about 0.1% w/w.

26. The method of claim 1 wherein the radioopaque agent comprises iodine.