Tuesday, March 21, 2017

Implants by Design: Mimicking Tissue with a New Class of Materials

A new kind of material discussed at last week’s American Physical Society March Meeting in New Orleans, Louisiana could someday make its way into your body. From artificial hips to pacemakers, medical implants give countless people relief, health, confidence, and more time to do the things they love with the people they love. Developing implants that are durable, reliable and well-matched to the body is an active and important area of biomedical research.

Sergei Sheiko and his team at the University of North Carolina at Chapel Hill specialize in designing soft, deformable materials. In an invited talk on Wednesday, he presented work on a class of materials they designed to be super-soft, super-flexible and solvent-free. Implants created with these materials would be hardier than the traditional water-based implants, which can leak, evaporate, freeze, and expand depending on the conditions, potentially leading to infections and other health risks.

Implants can be made from human or animal tissue or from non-biological materials such as metal, plastic, or ceramic. Non-biological soft implants, such as breast implants, are currently made by mixing water with a type of chemical compound called a polymer. This mixture turns into a special type of gel that can mimic human tissue. Although this design works well in many cases, it’s limited because of its dependence on water content.

With this new design, the team created materials soft enough to mimic biological tissue that have absolutely no water. In fact, they don’t contain any liquid at all. The materials are solid, but look and feel similar to a gel. As in traditional soft implants, the backbone of the material is formed by polymers that are cross-linked like a net. In place of water, though, they have side chains also made from polymers. Like a brush for cleaning bottles, these side chains stick out in all directions from each strand in the net causing expansion and softening of the net.

The image on the left illustrates the structure of a polymer gel – typically used for the fabrication of implants, the strands represent polymers and the dots represent water. The image on the right illustrated the bottlebrush, water-free design.
Image Credit: Sergei Sheiko.

The arrangement of these polymers is the key to this design. In fact, the team found that the mechanical properties of the materials, such as rigidity and elasticity, are determined by just three structural parameters. These parameters relate to the spacing and length of the side chains and each one can be controlled independently in the lab. This might not sound like much, but it’s a big deal.

There is a well-accepted “golden rule” in the study of materials, says Sheiko, that more rigid materials are less flexible. In other words, soft or floppy things like pizza dough tend to be stretchier than rigid ones like frisbees. However, by changing one or more of these three parameters, you can control the elasticity and rigidity separately. This means that you can set the mechanical properties of the material to whatever you want them to be within a wide range of options. You can create a material that is both rigid and elastic.

After creating and testing this new class of materials using silicone polymers, the team developed a physical model that describes how the mechanical properties are influenced by each parameter. Using this model, they can now design a material with specific mechanical properties. In other words, you can bring in a tissue sample—maybe of an artery, brain, lung, or skin—and the team can measure the properties of the tissue and then recreate an implant with the exact same mechanical behaviors.

One of the unique things about biological materials is that they can change their structure in response to stretching. For example, skin starts out flexible but becomes more rigid as it is stretched. Sheiko’s lab is now exploring whether lab-created materials might be able to mimic this in some way, perhaps cycling through different network arrangements over time. In addition, the researchers are looking to form collaborations with hospitals and scientists working on tissue engineering and stem cell research in hopes of putting their new class of materials to good use.

Kendra Redmond

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