Xenotransplantation . They have been used but not very often and not with very much success. Xenotransplantation has been one of the holy grails of transplant surgery since its inception. Through genetic modification of porcine donors, however, researchers are getting closer to mitigating the violent antibody response that is one of the major roadblocks.
Good question . That has been a topic of continued research for decades. Unfortunately animal or xenotransplants, cause very severe rejection and would not last long. The future holds the promise of genetically engineering such animal organs so they will not trigger severe rejection. Today pig hear valves can be altered so they can be maintained in a human.