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Currently Playing: How Bicycle Helmets Are Engineered to Protect Your Brain

Bill explains how bicycle helmets are engineered to protect riders from brain injuries. He traces the history of helmet design from pith helmets and leather motorcycle helmets through the introduction of EPS foam, and explains why the cheap, single-use nature of that foam is the key to helmets working as a public safety intervention. He explains how EPS foam is manufactured from polystyrene beads impregnated with pentane, expanded by steam, and fused in a mold. He then explains two methods for reducing the rotational forces that cause concussions: MIPS, which uses a slippery inner layer that allows the helmet to slide on impact, and WaveCel, a lattice of distorted triangular cells that collapses to absorb oblique impacts. Lastly, he explains why WaveCel's unusual tiling is auxetic — expanding in both directions when stressed — which allows it to conform to the curved surface of a head. Video Sections 00:02 Bicycle helmets: a brief history The first bicycle helmets were pith helmets and leather motorcycle helmets — useful for scrapes but poor protection from impact. Designs for foam-lined helmets to prevent concussions appear by the early 1950s. 00:53 The three components of a modern helmet A modern helmet has a thin polycarbonate outer shell, a layer of EPS foam, and an inner layer to reduce injury from rotational forces. Bill removes a section from two helmets to show these components. 01:38 How foam protects the brain Foam protects by spreading an impact's impulse over a longer time, dramatically reducing the peak force. Studies show helmets produce a nearly fifty percent reduction in head injuries. 02:50 EPS: the material inside your helmet The foam is expanded polystyrene — the same material as packing peanuts, but denser. A packing peanut is two percent EPS; a helmet's foam is about ten percent. 3:13 Manufacturing EPS foam EPS begins as polystyrene spheres impregnated with liquid pentane. Steam causes the pentane to vaporize and expand the beads, which are then aged in silos before being poured into a mold and fused into a foam liner. 04:25 The problem of rotational injury A helmet's foam protects against direct impacts, but oblique strikes cause the skull to rotate. The brain lags behind by inertia, and that differential motion shears brain tissue — causing concussions. 04:55 MIPS: a sliding inner layer MIPS — Multi-directional Impact Protection System — uses a slippery inner layer held by rubber straps. On oblique impact, the outer helmet slides ten to fifteen millimeters, reducing the rotational force transmitted to the skull. 05:19 WaveCel: a collapsible lattice WaveCel replaces some of the foam with a lattice of distorted triangular cells. The lattice is collapsible in multiple directions, absorbing the energy of an oblique strike before it can rotate the rider's head. 05:37 Auxetic materials: growing in both directions WaveCel's distorted triangular tiling is auxetic — when stretched in one direction, it expands in the other. This allows the flat lattice to conform to the curved surface of a head, unlike a regular hexagonal mesh. 06:56 The future of helmet design Engineers are exploring gradient-density foams, gels, liquids, and even magnetically activated fluids to further reduce rotational injury. Helmet design continues to evolve. 07:19 Closing credits