Perhaps you should try some spelling variations:
acoustic metamaterial » acoustic metamaterials, acoustic materials, elastic metamaterials
critical materials » optical materials, chemical materials, artificial materials
acoustic material » acoustic materials, acoustic metamaterials, elastic material
plastic material » plastic materials, elastic materials, genetic material

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: %((critical materials) OR (((((acoustic material) OR (plastic material))) OR (acoustic metamaterial))))%22.
• The words AND, OR and NOT may confuse the search; try adding quotes: "%((critical materials) or (((((acoustic material) OR (plastic material))) or (acoustic metamaterial))))%22".
• Adding a wildcard symbol may retrieve word variants: %((critical materials) or (((((acoustic material) OR (plastic material))) or (acoustic metamaterial))))%22*.