A molecule selective sensor has an enzymatically active membrane which is prepared by reacting an enzyme with glutaric aldehyde on the surface of a protein membrane found in nature. Exemplary is the reaction of glucoseoxidase with glutaric aldehyde on the surface of hog intestine membrane.

A roller ski comprising a foot plate (6) supported by a forward and a rear pneumatic rubber wheel (7, 8) and provided with rigid supporting members (9) arranged to support the user's leg laterally at a level between the foot and the knee and at the same time to allow the leg to move freely in the longitudinal direction of the roller ski.