Did You Hear About Lab-made Ear?

For over 30 years, researchers have sought to produce an ear in a laboratory from a patient's living cellular material. In 2016, ETH Professor Marcy Zenobi-Wong and her team made waves with an ear created in a 3D printer. Now, however, researchers from ETH Zurich, the Friedrich Miescher Institute in Basel and the Cantonal Hospital of Lucerne have taken another important step forward. Using human ear cartilage cells, the team has produced elastic cartilage in the laboratory, achieving mechanical properties similar to that of natural tissue. The engineered cartilage has similar stability to a real ear and, in an animal model, retained its shape and elasticity after six weeks.

This research is relevant, not least because fires and accidents frequently result in people losing their ears, either in whole or in part. In addition, some children suffer from congenital malformations of the outer ear. This condition, known as microtia, affects around four in every 10,000 children. To this day, reconstruction using the patient's rib cartilage remains the standard approach. Yet, this procedure is painful and can cause scarring and deformation in the thoracic region - and the reconstructed ear is often stiffer than a natural ear. This presents researchers with a challenge.

"We aren't implanting soft tissue in the hope that it remains stable in the body. Instead, we want to achieve that stability in the laboratory," says Philipp Fisch, lead author of the external page study published in Advanced Function Materials. Fisch is a senior researcher in the Tissue Engineering and Biofabrication Group led by ETH Professor Marcy Zenobi-Wong.

Nevertheless, elastin remains a central challenge. This protein is what gives the ear its malleability. Not only do the researchers need to produce it, but they also need to network it correctly and ensure it is stabilised for the long term. Researchers are yet to determine a precise biological "blueprint" to achieve this.

From a tissue sample to the printed ear

The researchers extracted cells from small cartilage remnants removed in operations to correct the shape of patients' ears. This served as the starting material. One hundred thousand cells can initially be isolated from a small piece of tissue approximately three millimetres in diameter. However, a printed ear requires several hundred million cells. The researchers therefore allowed the cells to grow further in the laboratory, placing them in a special nutrient solution. They also developed a special culture environment to supply the inside of the printed ear with nutrients and oxygen and ensure that the tissue matured in a uniform manner.

The research team tested different growth factors to promote cell division. At the same time, they wanted to prevent the ear cartilage cells from behaving like fibroblasts. These connective tissue cells primarily produce type I collagen and can form scar tissue. The result would be fibrocartilage, a softer tissue with type I collagen, instead of the stiffer type II collagen and elastin typically found in ear cartilage.

The researchers then embedded the multiplied cells in a bioink, a gel-like material that serves as the carrier. They used a 3D printer to form ear structures from this ink. Immediately after printing, the tissue was still very soft. "While the input material is crucial, so too is the tissue's ability to develop," explains Fisch. The printed ears were therefore placed in an incubator to mature for several weeks and received a continuous supply of nutrients. The aim was to promote formation of type II collagen, elastin and glycosaminoglycans - sugar-like molecules that bind water and enhance cartilage strength.

Dimensionally stable in animal models

Fisch believes that a combination of four factors was decisive to the team's success. "We optimised cell proliferation, adjusted the material properties, increased the cell density and controlled the maturation environment more effectively," he explains. After around nine weeks of lab-based pre-maturation, the researchers implanted the ear constructs under the skin of rats. They then monitored the tissue over several weeks. The researchers found that the artificial ears remained stable after six weeks, with mechanical properties similar to that of natural cartilage. "Despite this major success, elastin remains a challenge for us, as we were not able to mature it fully," says Fisch. "We observed changes in the tissue. That clearly shows that we need to stabilise it further."

Only a handful of research groups around the world are working to produce elastic ear cartilage. What's more, the research process is time-intensive, with a single experiment lasting roughly three to four months. The researchers conduct complex experiments that combine different conditions in order to decode the biological blueprint, which remains elusive. The controlled formation of a stable elastin network is decisive for the artificial ear to maintain its shape over the long term.

The patient search for an elastin network blueprint

"We've been working on this problem in our group for over ten years," says Fisch. To outsiders, this might sound like a long time. "When it comes to biofabrication of tissue, or tissue engineering as it's also known, swift progress is rare to see."

Engineered ear cartilage is the subject of keen interest. "The study had barely been published before I received a message from the parents of a child with microtia," recalls Fisch. The parents wanted to know how far the research had advanced and when clinical trials might take place.

For his part, Fisch remains optimistic. "If all goes well, we hope to find the blueprint for the elastin network within the next five years," he says. The next steps would be clinical studies, structured testing procedures and formal approval processes. Artificial ear cartilage must overcome these regulatory hurdles before it can make its way from the laboratory to clinical practice.

"Our current study provides a good guide to the current state of research," summarises Fisch. "It shows how close we already are to recreating the human ear - and what's still missing."