A method of bonding a first object to a second object includes the steps of: providing the first object including thermoplastic material in a solid state, providing the second object including a proximal surface, applying a mechanical pressing force and a mechanical excitation capable to liquefy the thermoplastic material until a flow portion of the thermoplastic material is flowable and penetrates into structures of the second object, and stopping the mechanical excitation and letting the thermoplastic material resolidify to yield a positive-fit connection between the first and the second object. The second object has a region of low density, wherein the protrusion penetrates the region of low density at least partly before the thermoplastic material is made flowable, and wherein the first object includes a protruding portion after the step of letting the thermoplastic material resolidify, the protruding portion at least partly penetrates the region of low density.