Development of auxetic shoulder straps for sport backpacks with improved thermal comfort

Auxetic foams and lattices, characterized by a negative Poisson's ratio, offer unique mechanical and thermal properties that make them promising candidates for next-generation sports and outdoor equipment. This study investigates the application of auxetic materials in the shoulder straps of trekking backpacks to improve both mechanical performance and thermal comfort. First, EVA foam samples were cut using water jet technology to create different auxetic geometries. After conducting tensile tests and digital image correlation analyses, the re-entrant geometry was identified as the most effective. Based on these findings, three types of shoulder straps were developed: a standard model and two auxetic models that differed in cutting pattern (one with undulating and one with linear edges) to evaluate their effects on deformation behavior. After further testing, the auxetic design with wavy edges proved to be superior and was integrated into a fully functional backpack prototype. Tests were then conducted on human subjects to determine whether the auxetic structure improved thermophysiological comfort and load distribution compared to a standard model made from the same materials. The study ultimately led to the development of a patented design for backpacks with auxetic shoulder straps and highlighted their potential to improve the user experience in outdoor applications.