High-performance athletic shoes often contain a complex mix of materials. Different parts of the sole require different properties, such as cushioning, flexibility and support. While this improves performance, it also makes production more complicated and recycling more difficult.

Researchers at Fraunhofer have developed an alternative through the ZiProMat (Circular Programmable Materials) project. Their concept uses a programmable monomaterial sole that delivers multiple functions while being made from a single material. The long-term goal is to produce both the sole and upper from polyester-based materials, making footwear easier to recycle at the end of its life.

Performance Through Design Rather Than Material Mixing

Instead of combining several materials, the Fraunhofer team creates different performance zones through structural design. Ribs and ridges are incorporated into the material, allowing specific areas of the sole to respond differently under load. The heel can provide greater cushioning, while the forefoot can remain firmer for stability and propulsion.

The technology has already been demonstrated in collaboration with sportswear manufacturer Puma. According to the researchers, the project shows that programmable materials can be used in high-quality consumer products without compromising performance.

The sole is produced from sheets of thermoplastic copolyester elastomer. These sheets are heated and thermoformed using vacuum moulding techniques. Each layer receives a predefined structure before multiple sheets are stacked and joined together. The resulting three-dimensional architecture determines the mechanical properties of the finished sole.

Closing The Loop With Polyester-Based Footwear

The latest phase of the project focuses on circularity. Researchers from the Fraunhofer Cluster of Excellence Programmable Materials (CPM) and the Fraunhofer Cluster of Excellence Circular Plastics Economy (CCPE) are working to ensure that both the sole and upper can be manufactured from polyester-based materials.

Since many sports shoe uppers already contain polyester textiles, this approach could enable a significantly larger proportion of the shoe to enter a closed-loop recycling system. The project therefore addresses one of the key challenges in footwear design: the difficulty of separating and recovering mixed materials.

Advanced Recycling Methods

The research team is also developing new recycling technologies. One process uses selective solvent-based dissolution to remove contaminants and recover high-purity polyester. Another uses pyrolysis to break polymers down into smaller molecules, which can then be converted into valuable chemical feedstocks such as terephthalic acid.

According to the researchers, these methods have already enabled the recovery of pure polyester from worn athletic shoes made from a variety of materials. This demonstrates that high-quality recycling and material recovery can be achieved simultaneously.

Beyond Sportswear

Although the current focus is on athletic footwear, the technology could be applied to other products, including work shoes. Ultimately, ZiProMat aims to establish a platform for programmable materials and closed-loop recycling systems that can be used across multiple industries.

By combining material efficiency, structural engineering and advanced recycling, the project offers a promising route towards more circular product design and resource use.

Source: Fraunhofer CCPE
Photo: Fraunhofer CPM