This research project explores the environmental issue of microplastic pollution, specifically focusing on its release from shoe soles into natural ecosystems. Despite increasing global awareness of microplastics, the particular impact of footwear, especially those used for outdoor activities in natural areas like trail running, has been largely overlooked. Microplastics released in these settings can directly impact ecosystems, affecting wildlife and soil health. This study investigates the potential of bio-based, biodegradable plastics, which can decompose harmlessly in soil, as an alternative material for trail running shoe soles. The aim is to mitigate harmful microplastic pollution and explore how the design of trail running shoes can be adapted to incorporate bio-based, biodegradable plastics. The project begins by examining the problem of microplastic pollution, its origins, consequences, and the role of footwear. It distinguishes the difference between bio-based and petroleum-based plastics and the role of bio-based biodegradable and compostable plastics in embracing a circular economy. Through exploratory research, design iterations, prototyping, and critical analysis, this study evaluates the feasibility of using bio-based, biodegradable plastics in manufacturing trail running shoe soles. In fact, shoe soles have been identified as the primary source of microplastic pollution due to the constant abrasion with the ground. A research was conducted in order to identify a bio-based soil-biodegradable plastic which could replace the currently used synthetic rubber. In terms of performance and environmental sustainability, PHA stands as the closest in meeting these criteria, yet it is not fully suitable for shoe soles application due to its limited flexibility. Nevertheless, the project conceptualises “BioStep”, a trail running shoe that features a replaceable biodegradable outsole. The usage of such an outsole does not release any harmful microplastic when used during outdoor activities or in the decomposition process at the end of its lifespan. This project sets the basis for the development of bio-based soil-biodegradable shoe soles for trail running shoes. Further research will be needed to identify a ready-to-use material and to address its performance in running scenarios. Finally, this study emphasizes the need for collaborative efforts among designers, manufacturers, and material scientists to mitigate microplastic pollution and to set the path towards more sustainable footwear.