Visit CPG at the Medical Grade Polymer 2013 Technical Conference
17-18 September 2013
Crowne Plaza, Boston/Woburn, Massachusetts
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Polarized light microscopy is an effective tool to examine the crystalline structure of materials. In this technique, a sample is placed between two polarizers which are oriented 90 degrees to each other, or are "crossed". Light is transmitted through the polarizers and samples into an objective. Light travelling through the first polarizer becomes polarized in the plane of the polarizer. When it hits the second polarizer, no light will be transmitted as the second polarizer (also called the analyzer) is oriented 90 degrees to the first polarizer, unless the sample is optically anisotropic. Optically anisotropic materials are ones where the optical properties are different when probed in different directions. They have a different refractive index, or speed of light, in different orientations normally due to molecular alignment. Crystalline materials will show birefringence, as will oriented materials (e.g. stretched in one direction) if there is a notable chemical dissimilarity along one axis of the molecule relative to the cross direction (such as polystyrene). In this case, the light will be slowed down along one axis of its path, causing it to rotate as it passes through the sample. As such, a portion of the light will be transmitted through the analyzer, showing up as a bright spot. The amount of rotation depends on the thickness of the sample, the amount of orientation, the wavelength of the light, and the chemical nature of the material. Crystalline samples will often show the classic "Maltese Cross" pattern, such as those seen in the image of polyethylene oxide above. The dark sections in the crystalline structure are the portions where the orientation is either normal or parallel to the polarization axis of the transmitted light, so that the light is not rotated as it passes through the sample. The multi-colored ares in the image above are thicker sections of PEO, which causes the light to undergo multiple rotations, and will separate the white light into various colors, like a prism.
For more information on the theory of birefringence, go to this application note.
Cambridge Polymer Group has expanded its operation with the opening of a West Coast office to support growing demand for materials consultation. The new office will be run by Ayyana Chakravartula, PhD. (617) 629-4400 Ext. 23, and is located in Oakland, CA. This office has been established to assist our west coast and mountain plains customers.
However, substantial concerns have been raised over the safety of some plasticizers. Several ortho-phthalates, for example, have been classified as potential endocrine disruptors that may cause developmental toxicity. Other concerns have been raised about possible carcinogenicity and the effects of plasticizers on the environment.
In 2000, the orthopedic community received a wake up call when one manufacturer, Sulzer, began to receive notices from surgeons that one of their acetabular shells, the InterOp, was failing to show osseointegration in a number of patients after a few months. The InterOp was designed with a titanium porous back to allow fixation by bony ingrowth. A thorough investigation ensued to determine why osseointegration was occurring for some patients. A number of consultants and laboratories, including Cambridge Polymer Group, were enlisted in this investigation.
Following a few months of analysis, it was determined that two key manufacturing step changes resulted in the poor outcomes. Firstly, Sulzer introduced an additional lathe-turning step following the porous titanium coating sintering process. The lathe-turning step introduced a lubricating oil into the porous backing that was insufficiently removed during the cleaning cycle. Any lubricants introduced prior to the titanium sintering process would be burned cleanly away. The second manufacturing step change was the removal of a nitric acid passivation process. Passivation is normally included in metallic devices to clean away any iron-based fragments introduced by machining tools.
Cambridge Polymer Group quantified oil content on hundreds of devices, comparing the manufacturing lots where clinical failures occurred. Interestingly, the bulk of the clinical failures occurred only in lots lacking the passivation step, despite the fact that other lots with passivation had higher levels of oil as well.
In the end, it was postulated that an endotoxin residing in the oil was responsible for the lack of osseointegration. Such an endotoxin would be readily removable with nitric acid passivation.
In the end, patients with failed InterOps received replacement devices, and a new ASTM sub-committee was formed to develop standards for determining cleanliness of medical devices. Device cleanliness has become a standardized test for medical device manufacturers.
More information on medical device cleanliness can be found here.
LabView, made by National Instruments, is a versatile programming language that has good application for laboratory equipment automation, motion control, image collection, and data analysis. Engineers and scientists at Cambridge Polymer Group routinely use LabView in their design of custom analytical instruments to characterize materials. The LabView code allows the users to set up the experimental conditions, control the equipment, collect and save the data, and analyze the data. The final data set is easily viewed in Excel, Word, or other formats.
Recently, two CPG scientists completed their coursework allowing them to become Certified LabView developers. CPG has designed custom software for clients for existing instruments and for automated data analysis algorithms.
A link to the video from a CPG-generated LabView program that collects crack propagation lengths automatically in a Fatigue Crack Propagation test can be found here.