April 12, 2012

Sol gel measurements

Sol gel experiments are a useful technique to determine the amount of crosslinking in a polymer. In this technique, samples are weighed carefully, immersed in a specific solvent at a specific temperature, and allowed to swell for a period of time, typically 24 hours. After this immersion period, the samples are removed again and re-weighed. The sample is then dried and re-weighed. From these weight changes, the percentage of the material that is soluble  ('sol') and the crosslinked portion ('gel') can be determined. Additionally, the degree of crosslinking in the gel can be determined.

ASTM D2765 describes this technique for crosslinked polyolefins such as polyethylene. The standard calls for heating jars of xylene in an oil bath. With standard oil baths, it is very difficult to achieve a uniform temperature in both the oil bath and the sample jars within the required +/- 0.5C. Cambridge Polymer Group has modified this technique with individual heater sleeves for each jar, with isolated temperature control for each jar. With this approach, we can achieve much more precise and accurate temperature control, which results in more accurate sol/gel measurements.

CPG Sol/Gel heater sleeves and control system


CPG sol/gel temperature control station
Posted by admin
April 2, 2012

Fingerprint analysis via SEM

Analysis of fracture surfaces of samples is sometimes complicated by inadvertent contamination the the sample surfaces during sample preparation. Dust and other debris from the lab bench, in addition to particles generated from cutting instruments, can contaminate an otherwise clean surface. If gloves are not worn during sample preparation, finger oils and salts can be left behind. As an example, a prepared polymer surface was freshly prepared by cryofracturing the sample, so that no tooling was used. The sample was then briefly touched with a bare fingertip. The sample, when imaged under SEM, shows the crystalline structure outlined in the white rectangle shown above. The elemental composition of this crystalline material is shown below, and contains salt, chlorine, and potassium. The carbon and oxygen may be from the polymer background, and/or from finger oils transmitted with the fingerprint.
Posted by admin
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.



Posted by admin
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.

Posted by CatherineCerasuolo
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.

Posted by CatherineCerasuolo
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.
Posted by admin
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.

Posted by CatherineCerasuolo