Abstract: Provided herein is an uncoated functional paper formed from a slurry comprising about 10 wt % to 90 wt % cellulosic fibers and about 1% to 25% sizing solution per total weight of the fibers. The sizing solution can be applied during papermaking and/or applied in a size press or coater. The paper can be calendered at a temperature greater than 50° C. Methods of making are also provided. Also provided are flexible electronics including a functional paper substrate printed with a conductive ink.