Iconacy Orthopedic Implants Gets FDA Clearance on Cima

Iconacy Orthopedic Implants, a privately held medical device company in Warsaw, IN, received FDA clearance to market their highly crosslinked UHMWPE prepared using the CIMA process developed at Cambridge Polymer Group and the Massachusetts General Hospital. CIMA is a patented highly crosslinked, low wear and oxidation-resistant ultra high molecular weight polyethylene that is non-exclusively licensed to Iconacy for use in their I-Hip total hip replacement technology.

Source: Stacy Page Online

Mooney Rivlin Testing

The Mooney-Rivlin model is a hyperelastic model that can be used to predict the deformation behavior of elastomers to uniaxial, planar, and biaxial extension. This model requires knowledge of two or more Mooney-Rivlin parameters that are specific to the material in question, and usually to the deformation mode in question as well. Some experimental work has shown that the Mooney-Rivlin parameters obtained in uniaxial extension do not lead to good predictions of the same material’s behavior in biaxial extension.

The scientists at Cambridge Polymer Group have developed a system for easily measuring the biaxial deformation behavior of elastomers using a bubble inflation system. Custom software allows generation of biaxial stress-strain data, which can then be fit to obtain the Mooney-Rivlin parameters.

Contact Cambridge Polymer Group for more information on contract biaxial tension testing, and see our application note on Mooney Rivlin Experiments.

Nano-Patterning on Hip Replacements

The BBC reports that a team from Glasgow University has developed a technique to created a pitted nanopatterned surface using poly ether ether ketone (PEEK). Tissue growth studies conducted by this group has shown that the nanopattern helps to elicit bone growth through the manipulation of stem cells. These researchers feel that it they can put this pattern onto the hip stems used in total hip replacement surgeries, loosening of the hip stem due to soft tissue growth around the stem could be mitigated. The Glasgow team feels that if successful, it could greatly increase the in vivo duration of the hip stem.

Presentation on Biomedical Applications of Hydrogels

The current standard of care in orthopedic joint replacement relies on the use of hard bearing surfaces comprised of polyethylene, ceramics, and metals. The natural tissues these synthetic materials replace are usually softer, viscoelastic materials that are best described as hydrogels, or hydrophilic network structures of cross linked macromolecules. In this audio conference presentation, our speaker discusses the increased use of hydrogels in biomedical applications, outlining what they are, their properties, and why they may have value in several biomedical applications, including orthopedics and spine. The presentation discusses potential applications, and looks at tissue models based on hydrogels for testing and training. Finally, attendees learn what issues have to be addressed in designing and using these materials, including concerns about how to test these soft, viscoelastic materials reliably in regimes relevant for their application.

This audio conference covers:

What makes hydrogels different from other materials

• Challenges with using and designing for hydrogels
• Potential applications in biomedical devices and tissue models
• Testing issues that must be addressed

This web conference is on August 7th at 11:30 am EST.

Click here for registration information

Tensile Strength of Fluids

Authors Lubansky et al from Swansea University in the UK analyzed the tensile strength of polyethylene glycol fluids using a CaBER(r) capillary breakup extensional rheometer. In this technique, a small aliquot of fluid is stretched rapidly between two endplates, and the resultant filament breakup kinetics are monitored with a high resolution laser micrometer. The breakup kinetics are a function of the fluids extensional rheological properties, coupled with surface tension. These properties can influence the behavior of fluids in jetting flows, fiber spinning, liquid deposition, and a variety of other processing methodologies.

The article was published in the Journal of Non-Newtonian Fluid Mechanics, and can be accessed here.

More information on the CaBER(r) can be found here.

Zimmer Vitamin E UHMWPE Cleared by FDA

Zimmer, Inc. has received FDA clearance on their highly crosslinked, Vitamin E containing UHMWPE. The 510(k) application is for a hip liner, and is being marketed under the trademarked name “Vivacit-E” (pronounced ‘vicacity’). Zimmer is advertising the new material in their Continuum acetabular shell system, shown above.

Link to 510(k)
Link to Zimmer Continuum Website

Synthetic Fat

Medical device and instrumentation design and development usually requires testing the prototypes in a simulated environment. Interest in treatments involving patients with high fat content has led to requests for tissue models containing a large amount of simulated fat. Using their skills in custom polymer formulations, researchers at Cambridge Polymer Group have developed a simulated fat model for instrument testing and training. The fat model has a realistic feel, will not degrade, and can be prepared in a variety of shapes and thicknesses. Contact Cambridge Polymer Group for more details.

UHMWPE for Total Joint Arthroplasty

In the 10th Anniversary Issue of BoneZone, a trade journal focusing on arthroplasty, CPG staff were asked to write an article on the history and future of ultra high molecular weight polyethylene for use in total joint arthroplasties such as hip and knee replacements. This article breaks down the history of UHMWPE as follows:

First generation of highly crosslinked UHMWPE
First introduced in the late 1990’s, these materials were irradiation crosslinked with either gamma or electron beam radiation, with doses from 50 to 100 kGy. The post-processing on these materials either involved annealing (heating below the melting temperature) or melting (heating above the melting temperature) in an attempt to reduce the number of residual free radicals that could react with oxygen, leading to embrittlement.

Second generation of highly crosslinked UHMWPE
In response to implant design requiring improved mechanical properties, second generation crosslinked UHMWPE were introduced between 2005 to now (2012). These second generation materials did not use melting to reduce the effects of free radicals, but rather addressed free radicals through mechanical deformation, repeated annealing, or antioxidants.

Future generations of UHMWPE
It is likely that future generations of highly crosslinked UHMWPE will incorporate gradients in crosslink density, providing high crosslink on the bearing surfaces, and low crosslinking in the regions requiring high mechanical strength. Alternative antioxidants will likely be considered as well.

To see the full article, follow this link.

Gastric Fluid Interactions With Plastics

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 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.