Add a period to Device.association and review the association.status valueset to reflect the information conveyed in the period (e.g. is active/inactive needed when association.period can be used to define the times that the device was active?)

As an example of this type of element, see Encounter.location. Consider whether other elements within the resource should also pick up this history tracking capability. Consider whether there is a PHI disclosure concern having mulitple patient's association with a device in the resource (consider assigning a wheelchair)

Also, change the humanSubject element name to patient.