Measuring the volume of a standard shape (e.g. cube, cylinder, sphere) is straightforward, as one only needs to measure the relevant dimensions (length, height, diameter, etc.) and calculate the volume using known geometric equations. Measuring a non-standard shape is also straightforward if you have an analytical balance. Using Archimedes’ principle of buoyancy, the weight of the object, when immersed in a liquid, will decrease by the volume of liquid the object displaces, which is its volume. Using the density attachment for an analytical balance, the mass of the object is first measured. A beaker of a suitable liquid (e.g. water) is then placed on the density attachment, and the object is re-weighed, this time while immersed in the water. By subtracting the difference in the two masses, one calculates the mass of the liquid that was displaced by the object. Knowing the density of the liquid at the test temperature, one can calculate the volume of the test sample by dividing the buoyancy mass by the fluid density.
This method is useful for measuring the change in dimensions of samples due to polymerization, crosslinking, crystallization, or other chemical processes.
DJO Global announced that it will be launching its Motivation(tm) total knee system at the American Academy of Orthopedic Surgeons meeting in 2013. The Motivation System incorporates e+(tm), an UHMWPE blended with Vitamin E.
Dr. Michael Kasser, from Center for Devices and Radiological Health at the FDA, has published an article in the Journal of Biomedical Material Research detailing the history of regulation on ultra high molecular weight polyethylene (UHMWPE) and its use in medical devices. This article is available as a early view download for registered users. Dr. Kasser offers a flow chart for the 510K decision making process to decide if a new productis substantially equivalent to a product already on the market, and therefore, if the 510K route can be used, as opposed to the more costly and time-consuming PMA route.
The paper outlines some of the testing used for establishing equivalency, based on ASTM test methods and tests developed within the orthopedic industry. In particular, custom tests designed to test anti-oxidants used in UHMWPE are required for 510K submissions.
Cambridge Polymer Group has assisted several clients in getting regulatory clearance on their UHMWPE materials, and is very familiar witht the testing requirements.
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.
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.
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.
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.
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, 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.
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.
