Customised Corksole

Every year in Germany, between 30 and 109 grams of microplastics per capita are released into the environment through the abrasion of shoe soles (Bertling et al., 2018; UBA, 2020). Conventional soles are usually made of synthetic polymers, which are difficult to break down and contribute to the microplastic problem. A promising alternative is cork – a natural material with excellent cushioning properties that decomposes without microplastic pollution. As cork only grows slowly and recycling is only possible above a certain grain size, the remaining small particles are often thermally recycled. In this project, an innovative process is being developed to combine finely ground cork with biodegradable biopolymers and transfer it to 3D printing as a sustainable shoe sole. The sole is produced using a special paste 3D printing process in which a viscous mixture of alginate and calcium chloride serves as a binder for the cork particles. Alginate, a water-soluble powder, is dissolved in a liquid and its viscosity is adjusted with glycerine. The calcium chloride is mixed with guar gum, which triggers a polymer reaction. This reaction forms a stable biopolymer that produces the desired strength and elasticity. The process allows the material properties to be precisely controlled, ensuring optimum adaptation to the application. The 3D printing process is based on an innovative printing principle in which the cork biopolymer mass is pressed out of a cartridge into a printing bed with alginate solution using air pressure. One mass floats weightlessly in the other, so to speak, so that complex geometries can be produced without support material. This opens up completely new design possibilities for shoe soles that conventional production methods do not offer.

However, this printing process also requires precise coordination of parameters such as air pressure, temperature of the pastes and size of the cannula used in order to ensure even material distribution and stable structures. After printing, the mass is initially soft and flexible, with a consistency reminiscent of a jellyfish, and must therefore undergo a drying phase. During this process, it shrinks by a few centimetres and takes on a leather-like firmness that is ideal for use as a shoe sole. Another innovation of this process is the possibility of combining the printed sole with an upper material made of natural fibers. By inoculating the sole with mycelium, for example Ganoderma lucidum, the mycelium can bond with the upper material and form a seamless unit. This eliminates the need for adhesives and seams, which further improves the biodegradability of the entire shoe. Shaping the sole requires precise digital control. While conventional 3D printing processes are usually supported by slicer software, the print paths for this process have to be specially designed in Rhino and converted into G-code using Grasshopper. Printing is most successful when the entire sole is produced in a continuous line to avoid irregularities in the material distribution. This sustainable shoe sole not only reduces microplastics, but also optimizes the recycling process of cork. The cork used is a waste product from conventional cork processing and is given a second life through this process. The cushioning properties are retained, while the sole of the shoe can be individually adapted to the wearer’s needs using 3D printing. After use, the entire shoe can be biodegraded, creating a closed ecological cycle. This combination of cork, biopolymers and mycelium represents a pioneering solution for more sustainable shoe production.