Based on the fact that the plastic scelet of IUDs frequently gives side effects van Os developed an IUD consisting of cupper only without a plastic frame.
He therefore developed a cupper wire in a small ring form witht he intention to prevent expulsion. Pilot studies showed it was effective but significantly more frequently expelled than classical cupper IUDs. Further investigation was therefore abandoned.
This experimental IUD also proved that the effective substance in an IUD is in fact the cupper ions.