For folks wondering

The black stuff is foam. It is packed into the wound to create a pathway for all the stuff the vacuum pump (where vac in wound vac comes from) sucks up from living tissue to flow through on its way to that tube coming out his elbow.

On top of the foam, and spanning on to his skin is a clear plastic film (strong stuff with a strong glue on it) that creates an airtight seal across the wound.

Then the tube is attached to a break in the clear plastic. When the vacuum is started the previously springy foam condenses down as air pressure outside the seal pushes it all into place.

On the far end of the tube (not pictured) is a vacuum, which houses a pump, sensors, batteries (when portable) and a chamber to catch all the crap that it sucks out of the wound.

The gentle suction of the vac both cleans the wound site out and encourages blood flow to the area to aid healing.

(Source - My wife lived with one of these for about 7 months)