Fusion hair extensions have keratin bonding on the ends and ultrasonic waves soften the keratin to bond during application. The best fusions hair extensions are made of Remy hair that has been carefully sorted and assembled to preserve its extraordinary quality. Fusion hair extensions are attached one by one, stuck with the keratin melted a few millimeters from the root of real hair. Fusion hair extensions last for about six months. Remy fusion hair extensions are rather costly and the installation process is lengthy.