I'm currently developing a treadmill for clinic gait research. If I were to build a flat treadmill with no moving surfaces what materials or coatings would be worth exploring as walking surface. Ideally, the surface would be durable, safe to touch with bare skin, capable of supporting a healthy adult, and low cost. The user would most likely use the treadmill wearing socks, made of either cotton, wool, spandex, or nylon. Delrin has been explored but is not adequate and produces a residual friction load on the user, producing artifacts in gait.
If you don't necessarily have a suggestion for a material, please suggest a way of calculating the equivalent friction coefficient between a surface and a fabric. This method could be experimental or based on theoretical methods using a materials database.