The medical device task group of ASTM met in Jacksonville, FL from November 12-15th. The meeting starting with a workshop on modularity and tapers in total joint replacement devices, and discussed potential issues with femoral head fretting.
The cleanliness task group worked on two draft standards. The first, a guideline for validation of medical device cleanlines, is now ready for balloting. The second, preparation of test soils for cleanliness testing, is making good progress, and will be ready for balloting soon. The task group also discussed starting a new standard for measuring cleanliness levels of plastic implants.
The PEEK task group heard a presentation about material properties similarities and differences between different grades of PEEK, and further discussed how to replace the test method for heavy metals as lead with a more current assay.
The UHMWPE task group discussed the up-coming round robin study of small punch on UHMWPE (samples are being sent). Net ash on consolidated UHMWPE will be removed from ASTM F648, and new draft standards for electron spin resonance characterization and fatigue crack propagation are in consideration.
In the bone cement group, the preliminary results of a round robin study on the titration method for benzoyl peroxide quantification was presented and discussed.
For the May 2014 meeting, a workshop on Additives in Biomedical Plastics will be held. A soliciation will go out shortly, but all those interested in presenting can also contact Stephen Spiegelberg from Cambridge Polymer Group, who is one of the chairs.
The molecular weight distribution of polymers strongly influence their properties, such as tensile strength, crack resistance, and solubility. Gel permeation chromatography is a commonly used technique to measure molecular weight distribution, but relies on the ability of the polymers to be dissolved in a solvent that is readily usable in a GPC column. Additionally, the molecular weight distribution is inferred from polymer standards, which should have a similar, if not identical, repeat unit to the unknown sample for the most reliable results.
Melt rheometry can be be used to determine molecular weight distributions, using the fact that the viscosity of a polymer is strongly dependent on the molecular weight and distribution. For instance, at low shear rates, the zero shear viscosity of a polymer melt scales with the weight average molecular weight to the 3.4 power. Similarly, the dynamic viscoelastic properties of polymer melts can be used to determine the overall molecular weight distribution. To do so, a frequency sweep is conducted on the sample, providing the G’ and G” as a function of frequency. Using theoretical models by Mead or Thimm, the molecular weight distribution can then be determined, assuming that the model parameters are known for the polymer in question.
Contact CPG for more information on this technique.
CPG recently purchased a new impact tester, a CEAST 9050 (Instron). With both V-notching and blade notching capabilities, we can perform Izod impact testing on materials in compliance with ASTM D256, as well as impact testing on UHMWPE per ASTM F648. The pneumatic release option on the impact tester allows very reproducible results.
CPG researcher Dr. Stephen Spiegelberg was recently elected to the post of Recording Secretary for ASTM Committee F04 “Medical and Surgical Materials and Devices”. Click here for more information on the committee. The committee’s 160 members meet biannually, attending over two days of technical meetings capped by a symposium or workshop on relevant topics in the medical/surgical materials and device industry.
Stop by booth 10 and take a look at what we’ve been working on!
Cambridge Polymer Group is your premier contract research resource solving problems with our multi-disciplinary research team and full service laboratory. We provide routine analytical testing on materials, custom test design, consultation, and out-sourced assistance for translational research. We assist clients in developing new materials, design of prototypes for proof-of-concept studies, experimental design and data collection for patents and fund-raising, assistance with 510K approvals, and development of new materials for targeted applications. For problem-solving with your materials, we are a full-service, ISO 9001 certified CRO.
Medical Grade Polymers 2013 brings together medical device manufacturers and polymer suppliers, and aims to cover all aspects from innovative devices to biocompatibility testing. Experts in medical device design, manufacture and testing along with polymer producers and manufacturing machinery suppliers attend these events to discuss the latest developments in markets, technical and regulatory issues, and production technology.
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.
August 2013 – Cambridge Polymer Group Announces The Opening Of A New West Coast Office
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.
Woven structures are increasingly making their way into medical devices. Ligament and tendon replacements, surgical mesh for hernias, vascular grafts, and composite structures all make use of weaving technology using polymeric fibers. Proper characterization of the woven system can help ensure it will be the load requirements of the final application. Common testing, beyond biocompatibility, include characteristics of the mesh architecture itself, such as mesh thickness, pore size, mesh density, and characteristics of the weave. Mechanical testing includes tensile, tear, stiffness, burst strength, and suture pull out resistance. For newer polymer systems, elution characteristics of the polymer may necessary, along with an assessment of the response of the material to the environment in which it is placed (e.g. gastric, blood, fat, etc.). While similar to basic polymer testing, the macrostructure of these devices requires some modification to standard mechanical tests. Contact Cambridge Polymer Group for assistance in your woven material testing.
Plasticizers are typically added to polymers (especially PVC) to increase material flexibility. Such materials are found in a broad range of applications, such as construction materials, cosmetics, medical devices, children’s toys. The type and percent content of plasticizer directly affects the material’s mechanical properties.
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.
Because of the possibility that such plasticizers may leech out of a given material, the use of some plasticizers has been restricted or banned in cosmetics, medical devices, and children’s toys within the EU and the state of California.
CPG has developed chemical assays based on gas chromatography with mass spectroscopy (GC-MS) to identify and quantify plasticizers in plastics. Companies who purchased plasticized plastics from third party vendors are increasingly using these types of assays to verify that their materials comply with state and federal regulations.
A more detailed white paper on this assay can be found on CPG’s web site.
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.
