Why would you need access to the sponge? Most people just refill them. Opening the sponge area tends to do more damage and introduces more air bubbles into the printhead causing more trouble.
Damaged cartridge has nothing to do with the sponge. It has to do with the print head itself. When that happens there's nothing to do but replace the cartridge.
Dirty contact points on the cartridge can also cause the printer to reject the cartridge as defective. You could try cleaning the contact points with a folded paper towel/kitchen paper moistened with isopropyl alcohol.
Hey shield4life,
the damaged cartidge message definitely has more to do with a problem on the electronic circuit, or it also could be the temperature sensor. Either way, you can say probably say goodbye to the cartridge.
If you still want to open the cartridge, (I recomend it if only to see the ridiculously small sponge that HP put into these cartridges) then you can just stick the blade of a stanley knife in the seam between the cartridge and the lid and gently tap it with a hammer, starting on a corner and moving around the cartridge. It should come off easy enough. To close it after, a glue gun all around the top , then put the lid back on, leave the glue to dry and trim the extra glue off the cartridge.