A device for fixing a reference element with respect to a field of surgery includes a first fixing portion operable to provide a mechanical resistance against pulling andor pushing forces, a second fixing portion provided with a surface structure operable to secure the device against an angular force, and a coupling structure. The first and second groups of tubes are in contact with a heat-conductive medium. The brush holder has a thick-wall portion integral with a bottom wall of the brush-housing chamber, the thick-wall portion being formed with fitting grooves at lateral sides thereof. The first sealing member includes a first aperture extending therethrough. More specifically the invention relates to the inhibition of proliferation andor differentiation of keratinocytes.