An obstacle to people using durable cups is cleaning the cup after each use, particularly if the cup is kept in a purse or back pack.  A modification to the steam unit used to froth espresso drinks could address this.  Steam is an effective, non-chemical method of cleaning and sanitizing a plastic cup.  Using steam would also have the advantage of pre-warming the cup before the hot drink is added..  If the mechanism has an efficient design, it would accommodate cups of all sizes, work quickly and not add significantly to the barista's burden or slow down the lines.