An electrical feedthrough assembly for an implantable medical device includes an outer ferrule of metallic material having an outer surface hermetically sealed to an implantable device housing. There is an inner feedthrough assembly which is hermetically sealed within the ferrule and which has a structure of sintered layers that include: i. an electrical insulator of ceramic insulator material, ii. one or more electrically conductive vias of metallized conductive material embedded within and extending through the electrical insulator, and iii. a transition interface region around each of the conductive vias comprising a gradient mixture of the ceramic insulator material and the metallized conductive material forming a gradual transition and a mechanical bond between the electrical insulator and the conductive via.
