Resorbable polymers, or plastics that break down in the body's aqueous environment, have been in use for some time. Their value has been demonstrated in a number of applications, including tissue-engineered scaffolds, suture materials, and other wound closure devices. These materials generally break down as water hydrolyzes the bonds that form the plastic until they are eventually completely absorbed.
A new resorbable polymer may have the potential to replace existing materials used in medical device manufacturing. Cheaply made, the material can be tailored to degrade over wide range of timeframes, which may make it suitable for many applications. Recently, a relatively commonplace material has been adapted to fit into the hydrolyzable family of polymers, according to the University of Illinois at Urbana-Champaign. Polyurea are a type of material commonly found in paints, caulking, and certain glues. Interestingly, their current uses are often a result of their stability in the aqueous environment and their resistance to being broken down. New research, however, by University of Illinois engineers has changed how these polymers may be used. The newly developed form of polyurea material is different because it contains hindered urea bonds, according to a recent study published by the team in the Journal of the American Chemical Society. The bonds are composed of 1-tert-butyl-1-ethylurea, and they are highly dynamic, which means that water can access and attack the bonds. The material can be configured into linear polymers or cross-linked gels, both of which are resorbable over time. The simplicity of the reaction process makes the manufacture of these materials very low in cost, because synthesis only consists of mixing bulky multifunctional amines and isocyanates, according to the press release. The precursor materials are very cheap and easily obtained, and the reactions can be performed at ambient conditions. There are no harmful byproducts created during the processing, and the copolymer composition, which affects the degradation rate, can be easily changed. Additionally, the polymer, once it is synthesized, can be broken down in as little as a few days.
While resorbable materials are not a recent addition to the medical device landscape, their uses continue to increase.
Vascular grafts, particularly ones composed of artificial materials like Teflon or poly-tetrafluoroethylene (ePTFE), have long been fraught with limitations. Researchers at Northwestern University's college of engineering have announced a new polymer that may improve the success of synthetic vascular grafts by reducing one of the key contributors to failure: oxidative stress.
Blood vessels involved in vascular grafting and other vascular interventions are often subject to atherosclerosis and restenosis, or a closing off of the grafted vessel over time. Eventually this leads to a failure of the graft or vessel, and the patient is back where they started. Both of these conditions have been associated with oxidative stress that develops during the healing cycle. When a synthetic graft is implanted, this is further complicated by the body's natural inflammatory response to implanted materials, which in itself can contribute to oxidative stress. The team came up with a novel approach: make antioxidants part of the material you are implanting. If antioxidants are released by your implant, then perhaps it can reduce oxidative stresses in the local areas where it is implanted. This, in turn, may increase biocompatibility and reduce the types of complications previously seen with grafting procedures.
“In the past, people have added antioxidant vitamins to a polymer and blended it in. That can affect the mechanical properties of the material and limit how much antioxidant you can add, so it doesn't work well. What we are doing is different. We are building a material that is already inherently, intrinsically antioxidant," said Professor Guillermo Ameer, a researcher in the department of biomedical engineering at Northwestern's McCormick School of Engineering and Applied Science and professor of surgery at the Feinberg School of Medicine. Specifically, the team are using a newly synthesized material that contains antioxidants in its very structure. This material is called poly (1,8-octanediol-co-citrate-co-ascorbate) (POCA), a flexible elastomeric plastic that is partially made from citric acid and ascorbate. Ascorbate is also better known as vitamin C, a potent anti-oxidant. Like most degradable polymers, POCA releases degradation products. Unlike most degradable polymers, however, POCA releases degradation products that reduce oxidative stresses and appear to be beneficial.
In vitro testing from Ameer's study showed that antioxidants were being released into the culture medium. Antioxidant activity was even present after the polymer had completely degraded. When cultured with endothelial cells, the cells that line the interior of blood vessels, there were no apparent negative effects, and the cells were able to grow on the POCA material. When these same cells were exposed to added oxidative stress, the polymer and its degradation products reduced the oxidative cell death that would normally be seen.
In vivo POCA also led to some interesting outcomes when coated on an expanded ePTFE graft. This preliminary testing led to decreased intimal hyperplasia when compared to an uncoated ePTFE graft. Intimal hyperplasia in many cases is associated with restenosis of vessels that have been repaired.
The new biomaterial may find applications outside of vascular grafting, such as in creating scaffolds for tissue engineering, coating and manufacturing safer medical devices, promoting healing in regenerative medicine, and protecting cells, genes, and viruses during drug delivery.