The shaft of the cOFM guide is surrounded by brain tissue after implantation and the healing dummy inside the cOFM guide prevents in-growth of brain tissue during blood-brain barrier re-establishment and trauma healing. The healing dummy is replaced by the sampling inlet, which is the tubing inside the cOFM guide shaft. The tissue interface for cOFM is already implanted with the cOFM guide, unlike microdialysis, where the interface (membrane) to the tissue is introduced by microdialysis probe.