Silica is designed to withstand higher temperatures. You're not going to find a common natural fiber that you can use to wick and that will hold up against such high ranges of temperatures (up to 1,100-1,200 F).
Cotton works extremely well, but you can't under any circumstances dry burn it - this applies to other fibers as well (butchers twine, hemp etc). Cotton will do the job, will produce vapor and will give you all that silica does (based on my exp. - cotton is all I use), but you have to keep it wet, otherwise you're going to get an equally nasty dry burning hit.
Unlike silica, cotton doesn't really require a break in period and you can pull the silica wick from the Protank head and twist cotton through as a replacement (that's how I started) without having to rebuild a coil, but you need to do is carefully and realize that it doesn't take too much cotton to work as a replacement.
To replace the silica:
1). Gently pull the silica from the coils. Gently. If the silica sticks, run hot water over the coil head to remove the gunk. You can also drop it into some boiling water and that should loosen things up enough to pull it free.
2). Grab some cotton from a cotton ball and twist it into a piece that's about the same size as the silica you pulled out. No larger. The cotton doesn't need to be in there tight. You should be able to pull the cotton through the coil with little or no resistance. If you use too much you risk burning the cotton on the first fire.
3). Trim the cotton down to the same size as the silica - no need for "flavor" wicks with cotton, so nothing goes on top of the coil.
4). Prime the cotton by juicing it up before you put the head back in and put the tank back together.
5). Once the tank is together, let it set for 1-2 minutes (my personal preference) and then vape.