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 .