A prosthetic heart valve (100, 100") includes a valve assembly mounted to an expandable stent (102). The prosthetic heart valve includes a cuff (300) coupled to the stent. The cuff includes a pocket (306) formed between an outer side (302) and an inner side (304) of the cuff. The pocket includes a plurality of biocompatible and irreversibly expandable microspheres (310). After implantation of the prosthetic heart valve into a patient, any gaps existing between the cuff and the native heart valve are exposed to bodily fluid, such as blood. The blood enters the pocket of the cuff through pores (320) in the outer side of the cuff. The blood interacts with the microspheres, causing irreversible expansion of the microspheres. The microspheres expand to fill any gaps between the prosthetic heart valve and the native heart tissue, sealing the gaps and preventing leakage through the gaps.
