Natural silk cocoons have fibrillary structure atseveral levels of length scales, which offered mechanical properties superior to many artificial silk materials. Despite the fact that many solvents had been endeavored to extract natural nanofibrils for the construction of functional materials, silk microfibrils have not been produced in scale thus far. Most reported solvents tended to exfoliate silk fibers directly into nanofibrils, due to their abilities of silk-dissolution and/or hydrolysis. In this study we showed that urea could preferentially peel and dissociate natural silk fibers into silk microfibrils with the diameter of 102 nm and large aspect ratios, without giving conspicuous hydrolysis and loss of β- sheets. Having colloidal stability analogous to individual silk nanofibrils, the resultant microfibrils could further be engineered into films and composites through conventional filtration and liquid casting. By reserving the orienting and nonslipping fibril bundle structures, these mesoscale building blocks promised mechanical properties for films and composites superior to those of silk nanofibrils. By hybridizing with Ag nanowires, electrothermic fibrous composites were produced. Having mechanical flexibility, folding endurance, and rapid electrothermic response (e.g., 115 °C within 50 s) under low voltages, these biocompatible electrothermic pads show applications in wearable and thermotherapy fields for medical thermotherapy domains.