What do aircraft turbines and hip implants have in common? They can both be manufactured with a 3D printer. At Hamburg University of Technology, research on the process is ongoing. The findings are also part of an entirely new generation of shoes.

Back and forth the nozzle moves, back and forth, over and over again. Each movement applies a new layer of red, blue, or orange plastic. In the end, a shoe emerges—but not one that looks like any ordinary shoe. At least not like one you know. Rather, it resembles something straight out of a futuristic comic, beamed into the real world and now ready to be slipped onto an actual foot. On the foot, it doesn’t feel like a conventional shoe either, but soft and squishy, with a spring in each step. Every shoe is made of a single block of the same soft, fully recyclable plastic. And it’s custom-made, too. A completely new generation of shoe.

What makes the shoes of Hamburg-based company Zellerfeld special: they come out of a 3D printer. Printed in Hamburg and soon also in Austin, Texas. Zellerfeld is scaling up due to high demand—from around 200 printers today to 2,000 in the near future. The company receives professional support from the Fraunhofer Institute for Additive Production Technologies (Fraunhofer IAPT) in Bergedorf. Together with Hamburg University of Technology (TUHH), Fraunhofer IAPT continuously researches the further development of 3D printing and drives its industrialization. The results of this research are found, for example, in ever lighter aircraft components, in medical implants—and in the shoes of the future.

Material Only Where It’s Needed

Additive production, or additive manufacturing, is simply another term for 3D printing. A product is created by applying—or adding—material, in contrast to processes where material is removed to shape the product, such as milling. This means there is virtually no waste. And: “With additive manufacturing, you only have material where it’s needed,” explains Dirk Herzog, senior engineer at TUHH’s Institute for the Industrialization of Smart Materials, closely linked with Fraunhofer IAPT. At TUHH, he is responsible for the “research roadmap,” as he describes his job. And he can vividly explain what exactly is being researched. By “material only where it’s needed,” he means: wherever material like metal or plastic isn’t required—for example, to ensure stability—it can simply be left out in 3D printing, leaving a cavity instead. This creates a filigree structure instead of a solid part.

That means less material is needed (more cost-efficient!), and the part becomes lighter. Just like the aircraft engine components printed using a process further developed at TUHH. Herzog: “With every kilo less weight, an aircraft consumes less fuel.” A real contribution to sustainability.

Delicate Lattice Structures

When a printer manufactures a hip implant in a fine lattice structure, human tissue can grow more easily into the implant after insertion, making the artificial hip more durable. “These delicate structures cannot be produced with any other process,” emphasizes Herzog. And this same principle can be used to create ultra-modern shoes: the lattice structure in the plastic sole provides cushioning with every step.

“3D printing has, of course, been around for a while,” Herzog explains during a tour through Fraunhofer IAPT’s large hall, where all kinds of 3D printers stand side by side—from small to enormous, for both metal and plastic printing. “But for a long time, it wasn’t industrially feasible.” The interest was there, but so was “a lack of understanding of the possibilities and limitations.” At TUHH, the concept of transfer—making research economically usable—was integrated early into teaching: “Our graduates already have a basic understanding of how and where additive manufacturing can be applied.” At TUHH, 3D printing is continuously developed “always with the goal of broad industrial applicability.” Fittingly, the institute is now called the Institute for the Industrialization of Smart Materials. Originally, printing was done at the Institute of Laser and Plant Systems Engineering (iLAS). Until April of this year, both institutes coexisted at TUHH. In 2009, the Laser Center North (LZN) spun off from iLAS as a limited company, offering consulting and industry-related research in additive production. In 2018, LZN was integrated into Fraunhofer IAPT.

Justin Bieber, Post Malone, and Cro

Fraunhofer IAPT has been advising the shoe start-up Zellerfeld for about three years. Zellerfeld takes advantage of more 3D-printing benefits in its production. Because the printers can easily be reconfigured for each shoe, every shoe can be custom-made, based on a scan that the customer creates at home with an app. On the website, you can choose whether you want to scan your foot or order an “old standard size”—with “boring” written in parentheses next to this option, perfectly illustrating what makes Zellerfeld’s pioneers so appealing: the entire process is hip, new, and fun, and aims to revolutionize shoe shopping.

Anyone who feels like a designer can print their own creations and sell them through the website. Since each pair of shoes is made on demand, there is no overproduction; at the end of a season, nothing needs to be dumped or destroyed, and no giant warehouses are necessary: the shoes are shipped directly to customers after production. From scanning your foot, it currently takes about three to four weeks until the desired shoes arrive in your mailbox—that’s how long you have to wait. Pop star Justin Bieber, US rapper Post Malone, and German rapper Cro are willing to wait—they all wear Zellerfeld shoes. Cro, as well as American artist and designer Heron Preston and the Hugo Boss team, have already created their own designs, all available via Zellerfeld’s website. Prices range from 130 to 325 euros per pair.

24 Hours Until A Shoe Is Finished

It takes about 24 hours to print a single shoe. Zellerfeld is working on shortening the process further—with support from Fraunhofer IAPT. The material used is so-called TPU, thermoplastic polyurethane, a plastic that becomes moldable when heated and retains its shape after cooling. The TPU is wound onto a roll like a thick plastic thread. The printer feeds this thread, called filament, through a nozzle where it is melted. Then, layer by layer, it is deposited according to the computer-generated design. Until the shoe is completely finished.

Can any 3D enthusiast simply print shoes at home with their own printer? “If you have the right device, you can certainly try,” says Dirk Herzog. “You just won’t achieve this level of quality.”