Complex sentence: The open cell foam is not only microporous, but its polymeric substance is also extremely hard, meaning that it works like sandpaper but on a smaller scale, getting into tiny grooves and pits in the subject being cleaned.
Split into simpler sentences: The open cell foam is not only microporous, but its polymeric substance is also extremely hard. This means it is actually working like sandpaper but on a very small scale, getting into tiny grooves and pits in the subject being cleaned.