Grouping the terms for material properties expressed in Equation 4, the MI for minimizing the persistence of a beam with a solid square cross-section is 1 𝑘 𝑑 𝐸 1/4 . Notably, this MI implies that minimizing the beam's environmental lifetime requires considering a material's kd and E. Using reported values for kd and E of several plastics, functionally equivalent beams made from polycaprolactone (PCL) and polyhydroxyalkanoates (PHA) could be the least persistent, followed by cellulose diacetate (CDA), polyamide (PA), and polyurethane (PUR) (Figure ).