In addition to achieving material-efficient designs, foaming with biodegradable bioplastics achieves designs with minimal environmental lifetimes. Ashby showed that the most materialefficient design for stiffness and strength optimizes a material index (MI) that relates density (𝜌) to Young's modulus (𝐸) and yield strength (𝜎 𝑦 ), respectively. Material indices are material properties or groups of them that optimize performance for a given objective (e.g., minimizing mass, cost, or environmental impact). The solid films and foam sheets investigated in our studies were evaluated using MIs for those geometries (Supplementary Table ). By this measure, and as expected, the foams optimized these MIs compared to the solid material, and overall, the PS foams optimized these MIs (Figure ).