Recent bicycle helmet designs and directions for future research: A comprehensive review from material and structural mechanics aspects

Cycling becomes more and more popular for recreation, exercising and commuting in many countries. Despite the popularity of cycling, it often comes with high level of risk. Conventional bicycle helmets, with expanded polystyrene foam (EPS) liners, can effectively mitigate impact by linear acceleration. However, there has been an urgent call for further improvement in bicycle helmets, as rotational acceleration has been found to be more dominant in severe head injuries. Recent advancements in manufacturing technologies have enabled various novel conceptual ideas and designs of bicycle helmets, which are previously deemed impracticable due to complexities in structures and materials, to become feasible. There are various bicycle helmet designs in terms of structures and materials used in both the helmet shell and liner, targeting at reducing both linear and rotational acceleration. Moreover, inspired by biological structures in nature, bio-inspired structures have been developed rapidly with excellent energy absorption capacity. As a piece of protective equipment, Bicycle helmet is a representative example where researchers are attempting to apply bio-inspired structures. The objective of the paper is to review the development of bicycle helmets and recent exploration for improvement. This includes the history of bicycle helmets, current test standards, designs of conventional bicycle helmets and the latest research (e.g. material replacement and novel structures), as well as application of bio-inspired structures to helmets. This review also identifies the limitations of current designs and standards, and challenges for future investigations.