A device for insertion into soft tissue including a micro electrode, a micro light source; a stiffening element having a material dissolvable or degradable in aqueous body fluid or a material swellable in such fluid to form a transparent gel; a coat of a flexible non-conducting polymer material on the stiffening element; a base disposed at the rear end of the device. The flexible coat has a distal opening allowing light emitted from the light source to leave the device upon said collapse or dissolution or swelling. Also disclosed is a therapeutic or diagnostic device formed in the tissue from the insertable device, uses thereof, and a method of disposing the insertable device in soft tissue.