By Clare Scott

3D bioprinting is one of the most fascinating, and perhaps the most misunderstood, areas of 3D printing. Scientists are still years away from being able to print fully functional, implantable human organs. However, more types of functional human tissue can be 3D printed than many realize. Some types of tissue are extremely difficult to print, but researchers have found a possible solution: bioprint those tissues not on Earth, but in space.

Recently, aerospace manufacturer and space infrastructure technology company Redwire Space successfully bioprinted a human meniscus aboard the International Space Station (ISS). A meniscus is a protective piece of cartilage in the knee joint that helps the knee move freely. A torn meniscus is one of the most common orthopedic injuries. It’s also one of the most difficult to remedy, as the meniscus cannot be replaced—it can only be surgically repaired or removed. This means that people who tear the meniscus are at increased risk of arthritis or knee replacements.

Bioprinting, however, could present new options. Soft tissue, like a meniscus, is difficult to print on Earth and requires scaffolding to prevent it from collapsing under gravity. In an outer space environment, though, without gravity, scaffolding is not required. Redwire Space used human cells to print the meniscus with its BioFabrication Facility, which was launched to the ISS in 2022.

Once the printing was complete, the tissue was transferred to the Advanced Space Experiment Processor, another Redwire facility, for culturing before being shipped back to Earth for analysis.

The meniscus is a good type of tissue to test the capabilities of the BioFabrication Facility and space-based bioprinting as it is avascular, meaning it has no blood vessels. Blood vessels are one of the most difficult aspects of bioprinting, and have held the technology back from long-term goals such as printing, for example, a human heart.

Redwire Space, among many other bioprinting organizations, still aims to print working human organs in the future, although that future may still be a long way off. The printed meniscus allows the company to improve and scale its bioprinting system, in addition to offering a possible future solution for torn meniscus injuries. Next, Redwire wants to try something more complex, namely, bioprinting a piece of cardiac tissue.

Redwire Space won’t be the first to bioprint cardiac tissue. More than one research team just this year have found different ways of printing human heart ventricles that beat on their own. While this type of research could indeed eventually lead to bioprinted hearts, it serves another purpose in the meantime: allowing researchers to more accurately test drug safety and efficacy.

One of these research teams, from Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) and the Wyss Institute for Biologically Inspired Engineering at Harvard University, bioprinted the ventricle with a specially developed ink. The hydrogel ink, which was infused with gelatin fibers, maintains its shape when printed, meaning that it can be used to bioprint ventricles and other complex structures without scaffolding.

Bioprinting using scaffolding is a viable method as well, but it has several limitations and potential complications. Both scaffold-based and scaffold-free bioprinting have pros and cons, and ultimately they both will be needed to advance bioprinting to its fullest potential.  A wide range of companies and research teams are working on different methods of bioprinting, whether they’re attention-grabbing (bioprinting in space!) or more under the radar. Ultimately, the cumulative work of all of these teams will be what launches bioprinting into the mainstream and develops truly lifesaving solutions.