Implantable materials may be used in an iatrogenic site. Applications include radiopaque materials for fiducial marking. For instance, a method of treating fiducially marking a patient comprising placing a fiduciary marker in the patient at a tissue in the patient, said marker comprising a collection of pharmaceutically acceptable, covalently-crosslinked hydrolytically biodegradable hydrogel particles into the tissue of the patient, the collection comprising a plurality of the particles having a covalently attached radiopaque agent, with the radiopaque agent being present in the collection at a concentration of at least about 0.1% w/w.