Retrievability for implantable medical devices

1. An implantable medical device (IMD), comprising:

an outer housing having a titanium outer surface;

the titanium outer surface comprising a plurality of titanium atoms;

a tissue growth-inhibiting layer extending over the titanium outer surface;

the tissue growth-inhibiting layer comprising a plurality of polyethylene glycol molecules, at least some of the plurality of polyethylene glycol molecules covalently bonded via an ether bond to one of the plurality of titanium atoms; and

wherein the tissue growth-inhibiting layer is formed by hydroxylating at least some of the plurality of titanium atoms and covalently bonding a plurality of hydroxyl-terminated polyethylene glycol molecules to the hydroxylated titanium atoms with a concomitant loss of water molecules.

2. The IMD of claim 1 , wherein the titanium outer surface comprises an electrically active surface.

3. The IMD of claim 2 , further comprising an electrically insulating layer disposed over a first portion of the outer housing and the electrically active surface comprises a second portion of the outer housing without the electrically insulating layer.

4. The IMD of claim 2 , wherein the tissue growth-inhibiting layer extends over at least a portion of the electrically active surface.

5. The IMD of claim 1 , wherein the titanium outer surface comprises a retrieval feature.

6. The IMD of claim 5 , wherein the tissue growth-inhibiting layer extends over at least a portion of the retrieval feature.

7. The IMD of claim 1 , wherein the tissue growth-inhibiting layer exhibits hydrolytic stability in the presence of water.

8. The IMD of claim 1 , wherein the tissue growth-inhibiting layer is water-swellable.

9. A leadless cardiac pacemaker (LCP) configured for removal after having been chronically implanted, the LCP comprising:

an outer housing extending from a proximal end to a distal end;

a fixation element extending distally from the distal end;

a retrieval feature extending proximally from the proximal end;

at least a portion of the LCP including a titanium outer surface; and

a tissue growth-inhibiting layer extending over at least a portion of the titanium outer surface, the tissue growth-inhibiting layer comprising a plurality of polyethylene glycol molecules each covalently bonded to titanium atoms within the titanium outer surface.

10. The LCP of claim 9 , further comprising a titanium electrode disposed relative to the outer housing.

11. The LCP of claim 9 , wherein the tissue growth-inhibiting layer extends over at least a portion of the titanium electrode.

12. The LCP of claim 9 , wherein the retrieval feature comprises the titanium outer surface, and the tissue growth-inhibiting layer extends over at least a portion of the retrieval feature.

13. The LCP of claim 9 , wherein the outer housing comprises the titanium outer surface, and the tissue growth-inhibiting layer extends over at least a portion of the outer housing.

14. A method of instilling tissue growth resistance to at least a portion of an implantable medical device (IMD), the IMD having a titanium outer surface, the method comprising:

subjecting the titanium outer surface to a plasma treatment to provide hydroxyl groups on the titanium outer surface, the hydroxyl groups covalently bonded to titanium atoms within the titanium outer surface;

contacting the hydroxyl groups on the titanium outer surface with hydroxyl-terminated polyethylene glycol molecules; and

wherein the hydroxyl-terminated polyethylene glycol molecules covalently bond with the hydroxyl groups on the titanium outer surface with a concomitant loss of water molecules.

15. The method of claim 14 , wherein contacting the hydroxyl groups on the titanium outer surface with hydroxyl-terminated polyethylene glycol molecules takes place within about one hour or less of subjecting the titanium outer surface to a plasma treatment to provide hydroxyl groups on the titanium outer surface.

16. The method of claim 14 , wherein subjecting the titanium outer surface to a plasma treatment to provide hydroxyl groups on the titanium outer surface comprises a water plasma treatment.

17. The method of claim 14 , wherein subjecting the titanium outer surface to a plasma treatment to provide hydroxyl groups on the titanium outer surface comprises an O 2 plasma treatment.

18. The method of claim 14 , wherein the IMD is a leadless cardiac pacemaker (LCP).

19. The method of claim 14 , wherein the covalently bonded polyethylene glycol molecules inhibit tissue growth.