The present invention provides a prosthetic heart valve device with improved fit, apposition and/or paravalvular leakage mitigation. Thus, the prosthetic device comprises an anchoring structure for supporting prosthetic valve leaflets, wherein the anchoring structure comprises and/or is formed at least partially from a hydrophilic material designed to swell when contacted by the water solute in the patient's blood. Locating the hydrophilic material to provide swelling to improve the fit and/or apposition of the device to the patient's tissues results in, inter alia, improved paravalvular leakage mitigation. Some of the methods and mechanisms described herein may also be used to provide a seal against the septal hole created during transseptal delivery of the device.
