A Nature-Inspired Anti-Slipping Winter Shoe Sole ^†

Slips and Falls (SFs) were not considered a major source of lethal injuries until recently. Icy surfaces (wet/dry ice) during winters cause severe SFs while walking. Nearly 70,000 serious SFs happen in Finland and Sweden every year, and about 17,000 people lose their lives in the USA (1 million SFs occur in the USA each year as per the GRITIT survey). Approximately 83% of people in Russia and thousands of people in the UK fall during winters and die as per HSE-UK. In 2018, the WHO published that there were 37.3 million SFs cases demanding medical attention including millions requiring intensive care which resulted in permanent injuries, trauma, loss of limbs, and deaths due to falls making it the second most prominent reason for fatal injuries. Two-thirds of SFs accidents happen because of icy/snowy surfaces. Research shows that huge monetary deficits of EUR 2.4 billion in Finland, EUR 280 million in Sweden, and GBP 42 million in the UK occurred due to SFs. In the USA alone, according to CDC data, USD 50 billion each year is being spent on healthcare due to SFs.

The present solutions for shoe soles are based either on design modifications which include changing the tread patterns, or the use of crampons/studs/spikes/cleats, etc., or material-based solutions which include the effort to improve friction between the sole–ice interface. Material-based solutions lose their effectiveness after a while once the ice becomes wet (generation of micro-layers of water due to body weight or external temperature) and it becomes a hundred times slipperier. Tread patterns also become vulnerable once the ice enters and locks the patterns. Dry ice is comparatively good for friction. Additionally, spikes/studs/crampons need to be applied and taken off every time before going out of the house. We cannot use them inside because they could damage the floor.

Here we introduce an innovative winter shoe sole based on wet/dry adhesion inspired by frog toes and gecko feet. The hexagonal microstructures of the frog toe are combined with the gecko’s nano-fibrillar spatulae arranged in a hierarchical structure to increase the surface area of contact and friction. The sole consists of rubber composites with oxide nano-powders such as zirconia and silica. The composite is a blend of nano powder in different proportions (nano-powders in 1, 2, 3, 4, and 5% by weight) to obtain better adhesiveness, wear resistance, and strength. After mixing, vulcanization of the rubber composite followed by surface-texturing by laser/moulding to produce micro/nano structures. The composite sole material behaves as superhydrophobic surface offering contact angle greater than 150⁰. When the sole touches the icy surface, it drives out water providing a better sole–ice contact. In this condition, the sole will provide a higher surface contact area due to nanostructures and driving out water from the interface. The composite with zirconia was produced with both kinds of textures and will be subjected to friction tests for their effectiveness on wet/dry icy surfaces.