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Plastics Today, September 2015

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Materials 46 Global Plastics rePort 2015 Plasticstoday.coM where they are needed—such as coiled around a nerve, and then the polymers could degrade away, leaving the electron- ics in place. "It could give you a really nice con- trolled integration of biological tissue onto devices. If we can control how the body responds to devices, that allows us to control a lot better the electrical func- tionality of these devices," Voit says. Another area of Voit's research involves silyl ether thiol-ene networks, which Voit has found can degrade with- in 30 minutes. Voit sees potential in the use of these materials as drug-delivery vehicles. Toughening up biomaterials One limitation of many resorbable bioma- terials is their strength profile. While offer- ing considerable strength upfront, their durability degrades quickly over time. One company, Novus Scientific, sought to solve this problem through a material innovation that involved knit- ting two degradable polymers together to create a surgical mesh that provides tissue support for six to nine months. The first of the two polymers, a copo- lymer of glycolide, lactide and trimeth- ylene carbonate (TMC), was engineered for fast resorption, offering mechanical strength for two months. The second, a copolymer of lactide and TMC, main- tains mechanical strength for six to nine months. The mesh material completely degrades within three years. On the market internationally, the TIGR Matrix is being employed for abdominal wall repair and hernia preven- tion. "That is why, in this case, it holds the abdominal wall together for that six to nine months. … A healthy person needs six months to recover after a sur- gery involving the abdominal wall," says Stefan Sowa, Vice President of Opera- tions and Site Manager at Novus Scien- tific's Uppsala, Sweden, headquarters. The quick-dissolving fiber offers strength initially after implantation. "In the first period, it keeps the tissue fixated very tightly. It is stiff," Sowa says. "Once the first type of fiber dissolves, you are left with the more elastic mesh." This variable strength and elasticity profile assists the body in healing itself, according to the company. "One of the challenges with anything resorbable is what is left behind," says Tac-Whei Ong, President of Novus Scientific Inc.'s U.S. subsidiary, based in San Diego, CA. "What is going to support this tissue from reherniating again?" he asks. Several years ago, biologic mesh was hailed as a promising new treatment for applications including hernia repair, Ong says. "But those products failed in terms of remodeling." In addition, biological mesh tends to cost considerably more than synthetic mesh. It is also problematic to use metal- based mesh for hernia repair. "If you are looking at strength alone, titanium is the strongest material you could use," Ong says. "Titanium-based mesh failed miser- ably when used for this application." The abdominal wall is constantly moving, and inserting a stiff metallic mesh in that part of the body can cause the tissue sur- rounding it to rip away. So how does the TIGR mesh over- come these problems? It is designed to offer dynamic load transfer to the sur- rounding tissue, thus helping to spur healthy connective tissue growth rather than scar tissue. The knitted material offers support while also stretching dynamically as the body's natural tissue does. "This allows the surrounding tissue to be trained to be remodeled," Ong says. The body, after all, responds to load. Muscles and bone that are unused atro- phy. Consider how astronauts in space lose bone and muscle mass, for instance. TIGR was developed to respond to mechanical transduction. "Within the fibers of the collagen, there [are] actin fibers that respond to load. As it responds to load as you move, it aligns collagen in the direction of the force," Ong says. "This is active mechanical remodeling." This leads the body to develop type-1 collagen, which is func- tional connective tissue, rather than type- 3 collagen, which is scar tissue. The company is broadening the scope of applications for the material beyond hernia reconstruction, and the material is now used in breast surgery, as well. The company is also open to selling the material to medical device companies on a license basis. "Anyone who wants a new indication would have to do a study so that it can be registered for that appli- cation with the appropriate regulatory body," says Sowa. "But we are open to sublicensing—for instance, doing it for knees or something like that." Material marriages of convenience Researchers at the Fraunhofer Institute for Manufacturing Technology and Advanced Materials in Bremen, Ger- many, are working to develop a strong biodegradable biomaterial of a different sort: Metal-ceramic composites strong enough to be used as suture anchors to treat tendon rupture. While traditional suture anchors suc- ceed in holding torn tendons in place, the implants, which are made of tita- nium or an inert polymer, either remain permanently in the body after the ten- don has healed or are retrieved surgically. Similar to the aforementioned example with the TIGR mesh, biode- gradable tissue anchors must offer suffi- cient strength over a relatively long time period. Fraunhofer researchers employed a similar strategy, too, using two materi- als with different degradation speeds. Iron alloys were used to corrode slowly, ensuring high mechanical strength, while tricalcium phosphate (TCP) ceramic decomposes quickly, and can stimulate bone growth while aiding the ingrowth of the implant. Fraunhofer researchers used powder injection molding to create the com- ponents. This process offers the ability to produce complex structures cost- effectively and in large numbers. It also allows for properties such as density and porosity to be controlled selectively, which is crucial when developing materi- als with high mechanical strength. In addition to improving how some shoulder surgeries are performed, the underlying materials and manufactur- ing technology used to make the tissue anchors could serve as a building block for other biodegradable implants, as well. The German researchers plan to con- tinue exploring how these materials react within the body. This article originally appeared in the January/February 2015 issue of Medical Product Manufacturing News.

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