Perhaps you should try some spelling variations:
acoustical materials » acoustic materials, acoustic material, optical materials
optical material » optical materials, optic materials, optics materials
optimal maternal » optical materials, fetal maternal, optimal patterns

You may be able to get more results by adjusting your search query.

• If you are trying to use Boolean operators, they must be ALL CAPS: %((((optical material) OR (optimal maternal))) OR (acoustical materials))%22.
• The words AND, OR and NOT may confuse the search; try adding quotes: "%((((optical material) OR (optimal maternal))) or (acoustical materials))%22".
• Adding a wildcard symbol may retrieve word variants: %((((optical material) OR (optimal maternal))) or (acoustical materials))%22*.