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Advances in medical polymers and devices |
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The global medical device market is valued at around US$200 bln and the US consumes about 45% of this. Markets are growing in Eastern Europe, Brazil, Russia, India and China, according to the consultant Len Czuba. The Middle East and Africa have potential for growth, but there are limitations of infrastructure and political instability. Current healthcare trends include minimally invasive surgery: device developments include a silicone access port for multiple instruments from Covidien; microendoscopes from Biovision Technologies; and remote handgrips that simulate real hand feeling from Novare Surgical Systems. Biodegradable/bioabsorbable materials are seeing increased usage, but can be difficult to process. Materials and device development were discussed extensively at the AMI conference on Medical Grade Polymers 2009. Demand for medical plastics is expected to reach US$6.55 bln in the US in 2012.
The FDA has regulated medical devices in the US since 1976 and its standards are recognised worldwide. The process is risk and data based � benefit must outweigh risk. Approvals include good manufacturing practice and control of materials supplies to ensure consistent products. Tests vary with category � implants are highest risk. If a new device is a modification of an exisiting one then approval is simpler involving a 510(k) submission, but completely new devices must undergo a full premarket approval (PMA) process. There are currently around 30-50 of these submissions per year, compared to 3-4,000 510(k) applications, according to Laura Byrd, an engineer in the PMA office. Sometimes applications arrive on pallets in trucks as the paperwork is comprehensive. Polymer manufacturers can submit a Master File to the FDA and give written approval for manufacturers to access these reports.
Medical Murray is involved in developing and manufacturing new high-tech disposables and implants, and works with companies and physicians. Tanner Hargens is a biomaterials expert with the company. Material specification includes mechanical, chemical, biocompatibility, electrical and thermal properties, as well as processability. It is expensive to take a new device to market due to the cost of design and tests to obtain performance data, as well as FDA approval. The company estimates a cost of around US$150,000 to US$1 mln and a 1 to 4 year time frame to take a device through the 510(k) level, and 2.5-10 years and US$1-80 mln for a PMA. Current projects include a new polyurethane synthetic ear cartilage. Polymers used in implants include polyethylene, PEEK, silicone, polysulfone, PTFE, polypropylene and polycarbonate, along with the bioabsorbables polylactide (PLA), polyglycolide (PGA) and copolymers of PLA/PGA. Processing can build in material stresses or knit lines with lower strength, and methods like solvent casting can give issues with solvent removal. Sterilisation affects materials in different ways depending on the technique, from ethylene oxide and gamma irradiation to autoclave. The rewards are seeing a device operational and improving the quality of life for patients.
Polymaterials aims to develop biocompatible and biodegradable polymer scaffolds using rapid tooling techniques to model each implant to the patient. The objective is to form a temporary structure that acts as a mould for a new body part using host cells. In one study, the company has used biodegradable polyurethane to model an ear cartilage, which was then seeded with cartilage cells. Cell retention can be an issue and is usually improved by modifying the scaffold surface, however in this case a gel composite containing fibrinogen, growth factors, thrombin and calcium chloride was used as a carrier for the cells, giving a high efficiency of seeding. The scaffold is expected to last for 1-2 years. The company is also looking at scaffolds for bone and fat (for example in breast tissue replacement).
Dr Vipul Dav� at Cordis Corporation is involved in developing cardiac stents that also release drugs. He is using supercritical carbon dioxide to purify poly(lactide-co-glycolide). After loading with the drug, solvent removal cannot use high temperatures or the drug will degrade, so alternative techniques have to be employed. The rate of drug release can be controlled by factors such as the crystallinity of the polymer.
Smart heart patches are being developed by Dr Wakatsuki of the Medical College of Wisconsin. Cardiomyocytes are mixed with a polymer suspension and moulded into a band shape, then grown with a current applied to simulate the heart beating. This generates a pulsating band of material. Long-term there is hope of creating healthy cardiac muscle patches to apply to failing hearts, as currently there are 20-40,000 US patients waiting for heart transplants and only 2,500 hearts are available each year.
Dr Guangyu Lu is the manager for plastics at Teleflex Medical in the Critical Care R&D Department. He has reviewed the factors in selecting a material from physicochemical properties to cost. Blood contact polymers should not: adsorb protein, release additives into the bloodstream, carry infection, cause clots or cancer, or provoke an immune response or irritation.
Coatings are used on medical devices for protection and to improve biocompatibility. Hydrophilic/lubricious polymers include polyvinyl pyrrolidone (PVP), polyethylene oxide (PEO), polyvinyl alcohol (PVA), etc. Coatings are applied by dip coating, spray coating, brush, roll or blade. Key performance measures include substrate adhesion, durability and mechanical properties, thickness and swelling in body fluids, particle and leachables release, biocompatibility, and degradation of implants. Bayer Material Science has been studying polyurethane hydrophilic coatings. An aqueous dispersion can be used or a low boiling solvent, permitting low temperature or ambient curing. The resulting film has extensive entanglement during curing and gives a uniform, transparent, insoluble film which does not require external crosslinking. The coating has been tested for biocompatibility.
Solvay Advanced Polymers has a range of materials for medical applications including polycarbonate, polyarylamide, polysulfone, polyphenylsulfone and PEEK. These have been tested for compatibility with sterilization methods and disinfectants, including hundreds of repeat cycles of use and cleaning. The company has named a range of materials as Solviva Biomaterials potentially available for implant use, which includes PEEK, self-reinforcing polyphenylene, polyphenylsulfone and polysulfone.
Biological safety testing is carried out by NAMSA, based primarily on the ISO 10993 Part 1 standard. Currently there is blurring between medical and pharmaceutical devices, as more are combined. First an assessment plan is drawn up, and then materials are selected for fitness for purpose, physicochemical and toxicological properties. Next other factors are considered such as the manufacturing processes, leachables and degradation products. Eight principles are applied in all and these will be expanded in the new version of the standard due for release around March 2010. Currently tests are being carried out on bioabsorbable polymers to examine decomposition products.
Ciba Expert Services has recently evaluated a surgical mask containing antimicrobials in the polymer (either silver/zinc or triclosan). Tests were developed to simulate breathing through the mask and exposure to saliva, and analytical techniques were honed to detect the key components. Inhalation exposure of 10 hours or leaching exposure to 10 masks gave acceptable safety margins for exposure to triclosan, silver and zinc.
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