Cambridge Polymer Group

March 14, 2012

Accelerated Aging

Accelerated aging protocols are key to medical device development. For initial product design, manufacturers of medical devices would like to know how their devices will respond to an in vivo environment, for a time period that could extend to decades in the case of permanent implants. For product labeling and regulatory, the manufacturer needs to be able confidently report an acceptable shelf-life for their devices before implantation or use. Shelf-life timeframes are often 2 years, and sometimes as long as 5 or more years. Clearly, this extent of time is too long for a real time aging study at room temperature.

As a result, manufacturers turn to accelerated aging protocols. For shelf-storage, there are a few ASTM methods that are used by device manufacturers. The most popular technique is described in ASTM F1980, which uses increased temperature to accelerate the kinetics of degradation of the materials. In this method, samples are usually placed in a convection oven for a period of time in their final packaging. A relatively simple expression is used to compute the accelerated aging factor, AAF

AAF=Q10^[(Ta - Trt)/10]

where Ta is the accelerated aging temperature, Trt is room temperature, and Q10 is an aging pre-factor that depends on the material. Ideally, the Q10 parameter is determined by comparing real time testing to accelerated aging testing, picking a parameter (such as tensile strength) to monitor. By measuring this parameter at multiple times during the aging study, one can construct a relationship between this property and aging time. Determining the equivalent amount of time to reach the same change in property at the different aging temperatures allows you to calculate your AAF. Know AAF and Ta, you can now calculate Q10 for your material. The Q10 will sometimes depend on temperature, as shown in the bottom graph. This plot was generated using the variable Q10 method, whereby short term real-time aging data was extrapolated to long term values as indicated.

February 21, 2012

Implants in the After Life

Ever wonder what happens to metal hip and knee replacements after their recipient expires? How about metal pins and screws to hold bones together? In the past, these components, often composed of expensive metals such as cobalt chrome, tantalum, and titanium, were buried with the patient. Now, a Dutch Company, OrthoMetals, has teamed up with crematoriums to recover the metal components from the ash. The recovered components are not for re-use as medical devices, but rather are melted down and used in non-medical applications, such as cars, wind turbines, and general construction materials. The company, which was started in 1997, now recycles up to 250 tons of metal a year.

February 14, 2012

Crush Strength of Catalyst Material

Catalyst powder for chemical reactions is often formed into a packed bed and placed into a reactor vessel. In packing the catalyst, the formation of smaller particles, or fines, can occur if the packing pressure exceeds a critical value. This fine formation is undesirable, as it increases the potential for bed compaction and subsequent increase in pressure in the reactor.

In ASTM D7084, "Determination of bulk crush strength of catalysts and catalyst carriers," the crush pressure required to generate 1 wt.% of fines is determined, where 'fines' are defined as particles passing through a mesh size that is half the diameter of the catalyst pellet. Multiple compression loads are used, and the results are interpolated to determine the pressure that yields 1 wt.%.

Cambridge Polymer Group performs ASTM D7084. Please contact us for more information.

February 9, 2012

Tissue Block for Suture Practice

Traditionally, medical students have practiced suturing on tissue mimics made from silicone or polyurethane elastomers. These materials lack the lubriscious nature of natural tissue. Using our proprietary hydrogel technology, CPG has developed single and multi-layer tissue blocks that contain a similar amount of water as natural tissue, and hence provides a similar feel as natural tissue. These blocks can be formed into a variety of sizes, and are re-usable. Contact Cambridge Polymer Group for more information.

February 9, 2012

FDA clears ECiMA

The FDA has cleared ECiMA(tm), a highly crosslinked polyethylene containing Vitamin E, for use in hip arthroplasties. ECiMA is sold by Corin, and was developed by researchers at Cambridge Polymer Group and the Massachusetts General Hospital. ECiMA was developed as a second generation highly crosslinked UHMWPE to replicate the good wear properties of the first generation highly crosslinked UHMWPEs, while having improved mechanical properties and oxidation resistance.
View the 510(k) application.

This technology is available for license.

January 25, 2012

Hip Implant Recall

Johnson & Johnson has continued to investigate their metal-on-metal implants, which were recalled in 2010 due to some patients reactions to metal debris generated during articulation. In a Reuter's report today, J&J had fourth quarter charges of $800 million associated with medical costs related to the recall.

January 23, 2012

Radiopacity in Medical Devices

Temporary or permanent implants often contain a radiopacifier, which is a material with a higher electron density contrast compared to the surrounding material so that it absorbs X-ray energy. In an X-ray, a radiopacifier appears as a bright section, as shown in the catheter above (the internal wire is a radiopacifier). Radiopacifiers are often made of metals such as gold, tungston, or powders such as zirconium oxide, barium sulphate and bismuth. When considering the design of a new medical device, manufacturers will need to assess the radiocontrast of the device so that the medical practitioner can see the device during implantation, in the case of catheters, guidewires, and other temporary devices with the use of fluoroscopes, or after permanent implantation, in the case of hip and knee replacements, stents, heart valves, and other permanent devices.

ASTM F640 "Standard Test Methods for Determining the Radiopacity for Medical Use" describes test methods for quantitative assessment of the contrast a radiopacifier has in a medical device, for either permanent implantation or temporary. In this method, the device is placed into an X-ray imaging system and imaged using standard times, voltages, and currents used for the X-ray diagnosis of humans. For two of the test methods, body mimics can be used, which may be animal, cadaver, or synthetic components that replicate the portion of the body where the device is to be placed. From the X-ray image of the device, a densitometry system is used to measure the optical density difference between the sample radiopacifier and the background.

CPG performs ASTM F640 using our custom densitometry system. Please contact us for your testing needs.

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