Features

The shape of a human ear is very individual, usually symmetrical, and, like a fingerprint, so characteristic that one can identify us by them. The outer portion of our ears has a complex structure that surgeons have a hard time replacing when disease or accident robs us of an ear. Researchers at ETH Zurich, the Swiss Federal Institute of Technology, say that they are able to produce an ear replacement with just the right look and feel. Their research was published in June in the Journal of the Mechanical Behavior of Biomedical Materials.

Fig. 1– Natural ears and an ear replacement that looks and feels real. (Credit: L. Nimeskern and Angelika Jacobs, ETH Zurich)
The outer ear, called the auricle, consists mainly of cartilage, which contains neither nerves nor blood vessels, and cannot regenerate after injury. Reconstructing an ear remains a challenge for plastic surgeons, especially as patients want the replacement to look and feel like their original.

How It Works

An international team involving Kathryn Stok, researcher at the Institute for Biomechanics at ETH, characterized the mechanical properties of ear cartilage and applied this knowledge to develop auricle implants. They performed mechanical tests, measured the cartilage flexibility, and mapped the mechanical properties of different parts of the human ear.

In parallel, they tested a novel biomaterial’s performance in the same mechanical tests to learn how the biomaterial must be tuned to match the characteristics of a real ear. The biomaterial consists of a web of nanocellulose fibers spun by a certain kind of bacteria, Gluconacetobacter xylinus.

“The bacteria build this web naturally to create an environment in which they can protect themselves,” explained Héctor Martínez Ávila, a visiting PhD student from Chalmers University, Göteborg, Sweden. He works with the team on testing and characterizing the material that is produced in Göteborg. “One of the advantages of the material is that we can tune it to render softer and stiffer parts, like in a real ear,” said Stok. And, it is well tolerated by the human body.

By first scanning a patient’s healthy ear using magnetic resonance imaging, the scientists created a silicon mold using 3D printing whereby they allow the bacteria to weave a web of nanocellulose. They varied the stiffness of the material by either compressing the material to yield more rigid parts or by inserting bubbles of gel as placeholders to produce softer, parts.

After removing both the bacteria and the gel with extensive washing, the scientists use the resulting structure as a “scaffold” for cartilage cells. In a culture dish, these cells settle onto the material and into its pores. Supported by this framework the cartilage grows into the desired shape. The material thus serves two functions: as a scaffold for regrowing the tissue and as an implant material that supports the newly formed auricle. To see a video of stress-relaxation testing of cartilage, visit http://www.techbriefs.com/tv/cartilage.

This approach allows a more exact reconstruction of an auricle than the currently used methods where surgeons remove cartilage from a patient’s rib and carve it into shape. When this piece is implanted underneath the skin, the body grows connective tissue that joins the cartilage with the skin. As the new ear lies completely underneath the skin at this point, the surgeon has to “free” it in an additional surgery so it stands freely off the head like a healthy ear. Not only can this procedure ruin one of the patient’s ribs but also the shape of the new ear depends on the surgeon’s skills.

What’s Next

As the bacterial nanocellulose can be shaped to mirror a patient’s healthy ear and its stiffness can be adjusted, the researchers expect the material to advance to clinical application in the near future. The research team said that once they have established a procedure for the ear, it could be used for any cartilage tissue. The highest demand would probably be in joint cartilage, for instance in knees, thinks Stok, but also nose, throat, and spinal disc cartilage could be created.