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.
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.
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.
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.
Polymers can be classified as linear or branched. Branched polymers contain chains hanging off the backbone of the polymer, which could include a single side chain or multiple side chains. Linear polymers do not have branches. Branching can strongly influence the processing behavior and ultimate properties of a polymer. In the melt state, branching will increase the melt viscosity of a polymer compared to its linear analog. Branching will also reduce crystallinity in semi-crystalline polymers, as the branches partially inhibits the packing of the polymer chains. Branching can occur during polymerization due to a variety of reasons, including the use of divalent monomers, mixed monomers with different side groups, backbiting during polymerization, radiation grafting, or other reasons. Branching can also occur during processing, particularly if the material undergoes scissioning or further reaction. As a consequence, measurement of branching is important to assess process conditions and ultimate properties in the polymer in question.
Branching can be assessed rheologically, with nuclear magnetic resonance spectroscopy (NMR), and inferred with techniques that monitor end properties such as crystallinity. Detailed information can be determined by triple detection gel permeation chromatography (GPC). In this technique, a polymer is run through a standard GPC column that is equipped with a refractive index detector, a light scattering detector, and a viscometer. The refractive index detector provides information about the concentration of the polymer chains at a given elution time, light scattering provides absolute measurement of the weight-average molecular weight at each elution time, and the viscometer provides the effective density of the polymer chains at each elution time. With these measurements, a Mark-Houwink plot can be generated, as shown below. By comparing the test sample in question to a sample that is known to be a linear polymer, it is possible to assess if the material is branched. For a given molecular weight, a branched polymer will have a smaller volume and hence a reduced viscosity compared to a linear polymer. The amount of deviation from the linear analog is an indication of how much branching there is in the test sample. In this matter, the degree of branching can be calculated for each molecular weight.
The Spring ASTM meeting of F04 (Biomedical Materials and Devices) met in Indianapolis, IN this week. Some key highlights of the meeting are as follows:
Cleanliness of Biomedical Devices
The standard for shipping of potentially infectious tissues and devices was approved, and is now available as ASTM F2995-13.
ASTM F561 (Device retrieval) will be amended to contain a new method for isolating polyethylene particles from tissue.
Two draft standards were discussed. The first, Validating Cleanlines for Biomedical Devices and Instruments, is making good progress, and should be ready for another sub-committee vote prior to the next meeting. The second, Test Soils, is awaiting more information on formulas from some committee members. Two additional draft standards (Designing devices for cleaning, and Establishing threshold limits for residue levels) are expected to be ready for discussion by the November meeting.
Polymers
The committee on UHMWPE discussed several topics, including the need for 3 new standards:
Cambridge Polymer Group is working on these three draft standards.
Additional discussion included a small punch round robin study on UHMWPE, which is expected to take place in the next few months. The PEEK standard was also discussed, with debate on the need for different classifications of PEEK, along with additional test methods for extractables and metals levels in PEEK.
A workshop on polymer additives is being considered for a meeting 1 year from now. Interested parties should contact Cambridge Polymer Group for more details.
Bone Cement
CPG researchers suggested a modification to the existing benzoyl peroxide titration method in F451. A small round robin will take place in the next 6 months trying this new protocol.
