By Charles Pekow — Helmets could provide better protection if the foam inside was softer than the expanded polystyrene (EPS) normally used, suggests one study titled Softer Foam in Bicycle Helmets Reduces the Impact Force in a Simulation Model, from the University of Hawaii
Researchers banged the head of a mannequin wearing a variety of standard helmets and again using modified helmets stuffed with softer polyolefin foam. From all three angles tested, “there were statistically lower striking forces sustained with the modified softer foam bicycle helmet compared to the standard bicycle helmet,” according to the results of the study.
The authors suggest further research is needed. See https://www.researchgate.net/publication/372490514_Softer_Foam_in_Bicycle_Helmets_Reduces_the_Impact_Force_in_a_Simulation_Model