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The environment of the upper gastrointestinal tract, including the stomach, can challenge materials placed into this environment. The pH environment can range from 1.5-2.0 prior to eating, but can spike up to pH~7 during and immediately after meals, requiring an hour or more to fall back to normal levels. Contrary to commonly held beliefs, the stomach does not continuously contain fluid, but only partially fills in anticipation of eating or drinking. The peristaltic action of the stomach grinds food against the stomach walls and itself, and enzymes act to help degrade food, along with the hydrochloric acid present in stomach acid.

Polymers sometimes find their way into the stomach environment, in the form of sutures, drug-release components, satiety treatments (e.g. balloons), and other temporary or permanent implants. Knowledge of how these materials will respond to the stomach environment will help to predict their performance. In some cases, the polymers are designed to respond to the stomach environment itself, swelling or deswelling in response to pH, salinity, temperature, or fluid content. In other cases, the polymer may degrade in response to these conditions.

Researchers at Cambridge Polymer Group have designed custom systems to simulate the stomach's environment. Using test methods with reference to ASTM D523, polymers systems are tested before and after model gastric fluid exposure to demonstrate the change in mechanical, chemical, and morphological properties.

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

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.

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.