Monomaterial running shoes could be fully recycled – into new shoes

With running more popular than ever, production of running shoes has reached a new level. Global footwear manufacturing has increased significantly over the last 15 years, and the industry itself has become a form of advanced engineering. Big companies are competing to build lighter, faster, and more energy-efficient shoes using complex combinations of foams, fabrics, and plastics. While performance continues to improve, sustainability still remains a secondary concern.

The typical running shoe lifespan is at most 800 km (497 miles), which means avid runners go through a pair every four to six months. There are several ways you can recycle worn-out shoes, and some big brands have developed special programs where they collect old shoes and shred them into rubber and foam particles. These materials can be used for building playgrounds or running tracks, but this is downcycling rather than true recycling, as old pairs are not used as raw material for new ones. A full cycle is pretty hard to achieve right now, and here is why.

One of the biggest environmental challenges with modern running shoes is that they are made from many different materials. The upper is typically made from woven fabric, the sole combines soft, bouncy foam and hard rubber, and some elements are often made from plastics. Such a design helps produce high-performance shoes, but it also makes them very difficult to recycle.

Researchers at Germany's Fraunhofer group – who have brought us shoe tech innovations before – are now exploring an innovative new approach: what if a running shoe could be made almost entirely from a single material? This is the goal of the ZiProMat project, which focuses on developing “programmable monomaterial” for athletic footwear.

Modern running shoes are all about cushioning and energy efficiency. Ideally, the heel should be soft and shock-absorbing, the midsole should provide a smooth transition during the stride, and the forefoot needs to be firm for efficient push-off. This complex combination can be achieved using just one material – thermoplastic copolyester elastomer.

Instead of combining different materials, the researchers basically modify the structure of the same material. Making the upper of the shoe is relatively straightforward, and the main challenge is the sole. Sheets of copolyester are heated and thermoformed into molds. By using ribs and ridges, each layer can be shaped to be softer in some areas and firmer in others. After each layer is formed individually, they are then stacked together to form a complete shoe sole.

This process seems fairly complex, but it has major environmental advantages. Since both the upper and sole of the shoe are polyester-based, the entire shoe could be fully recycled at the end of its life. Instead of ending up in landfills, shoes can be reused as raw material for new pairs.

The project has already moved beyond the theoretical stage. Fraunhofer collaborated with Puma to test the feasibility of monomaterial athletic shoes. The testing has shown that the concept can definitely work in real-world prototypes and shoe production.

There is still a long way to go before this technology reaches mass production. If successful, the researchers are also hoping to apply it to work footwear, where durability is the main priority.

The ultimate goal of the ZiProMat project is to establish a technology platform for programmable materials. In a few years this approach could shift the footwear industry to focus not just on speed and durability, but also on sustainability.

Source: Fraunhofer