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Cambridge, MA, United States

Ansari F.,University of California at Berkeley | Gludovatz B.,Lawrence Berkeley National Laboratory | Kozak A.,Cambridge Polymer Group | Ritchie R.O.,University of California at Berkeley | And 2 more authors.
Journal of the Mechanical Behavior of Biomedical Materials | Year: 2016

Ultrahigh molecular weight polyethylene (UHMWPE) has remained the primary polymer used in hip, knee and shoulder replacements for over 50 years. Recent case studies have demonstrated that catastrophic fatigue fracture of the polymer can severely limit device lifetime and are often associated with stress concentration (notches) integrated into the design. This study evaluates the influence of notch geometry on the fatigue of three formulations of UHMWPE that are in use today. A linear-elastic fracture mechanics approach is adopted to evaluate crack propagation as a function of notch root radius, heat treatment and Vitamin E additions. Specifically, a modified stress-intensity factor that accounts for notch geometry was utilized to model the crack driving force. The degree of notch plasticity for each material/notch combination was further evaluated using finite element methods. Experimental evaluation of crack speed as a function of stress intensity was conducted under cyclic tensile loading, taking crack length and notch plasticity into consideration. Results demonstrated that crack propagation in UHMWPE emanating from a notch was primarily affected by microstructural influences (cross-linking) rather than differences in notch geometry. © 2016 Elsevier Ltd. Source

Ling D.,Massachusetts General Hospital | Bodugoz-Senturk H.,Massachusetts General Hospital | Bodugoz-Senturk H.,Harvard University | Nanda S.,Cornell University | And 3 more authors.
Proceedings of the Institution of Mechanical Engineers, Part H: Journal of Engineering in Medicine | Year: 2015

Polyvinyl alcohol hydrogels are biocompatible and can be used as synthetic articular cartilage. Their mechanical characteristics can be tailored by various techniques such as annealing or blending with other hydrophilic polymers. In this study, we quantified the coefficient of friction of various candidate polyvinyl alcohol hydrogels against cobalt-chrome alloy or swine cartilage using a new rheometer-based method. We investigated the coefficient of friction of polyvinyl alcohol-only hydrogels and blends with polyethylene glycol, polyacrylic acid, and polyacrylamide against swine cartilage and polished cobalt-chrome surfaces. The addition of the functional groups to polyvinyl alcohol, such as acrylamide (semi-interpenetrating network) and acrylic acid (blend), significantly reduced the coefficient of friction. The coefficient of friction of the polyvinyl alcohol-only hydrogel was measured as 0.4 ± 0.03 against cobalt-chrome alloy, and 0.09 ± 0.004 against cartilage, while those measurements for the polyvinyl alcohol-polyacrylic acid blends and polyvinyl alcohol-polyacrylamide semi-interpenetrating network were 0.07 ± 0.01 and 0.1 ± 0.003 against cobalt-chrome alloy, and 0.03 ± 0.001 and 0.02 ± 0.001 against cartilage, respectively. There was no significant or minimal difference in the coefficient of friction between samples from different regions of the knee, or animals, or when the cartilage samples were frozen for 1 day or 2 days before testing. However, changing lubricant from deionized water to ionic media, for example, saline or simulated body fluid, increased the coefficient of friction significantly. © 2015 IMechE. Source

Braithwaite G.J.C.,Cambridge Polymer Group | Daley M.J.,OrthogenRx | Toledo-Velasquez D.,OrthogenRx
Journal of Biomaterials Science, Polymer Edition | Year: 2016

Hyaluronic acid of various molecular weights has been in use for the treatment of osteoarthritis knee pain for decades. Worldwide, these products are regulated as either as drugs or devices and in some countries as both. In the US, this class of products is regulated as Class III medical devices, which places specific regulatory requirements on developers of these materials under a Pre-Market Approval process, typically requiring data from prospective randomized controlled clinical studies. In 1984 pharmaceutical manufacturers became able to file an Abbreviated New Drug Application for approval of a generic drug, thus establishing standards for demonstrating equivalence to an existing chemical entity. Recently, the first biosimilar, or generic biologic, was approved. Biosimilars are biological products that are approved by the FDA because they are highly similar to a reference product, and have been shown to have no clinically meaningful differences from the reference product. For devices, Class II medical devices have a pathway for declaring equivalence to an existing product by filing a 510 k application for FDA clearance. However, until recently no equivalent regulatory pathway was available to Class III devices. In this paper, we consider the critical mechanical performance parameters for intra-articular hyaluronic products to demonstrate indistinguishable characteristics. Analogous to the aforementioned pathways that allow for a demonstration of equivalence, we examine these parameters for an existing, marketed device and compare molecular weight and rheological properties of multiple batches of a similar product. We propose that this establishes a scientific rationale for establishing Class III medical device equivalence. © 2015 The Author(s). Published by Taylor & Francis. Source

Cambridge Polymer Group | Date: 2003-04-15


Agency: National Science Foundation | Branch: | Program: SBIR | Phase: Phase I | Award Amount: 99.62K | Year: 2001

This Small Business Innovation Research (SBIR) Phase I project describes a unique instrument capable of quantifying the extensional rheological behavior of solutions, pastes and melts. In this instrument a small quantity of fluid is rapidly stretched between two plates to form a liquid bridge, and the diameter decrease is subsequently monitored as the fluid drains under gravity and surface tension. Comparison of rheological models with the data allows one to extract viscosity, surface tension, elasticity and other parameters relating to extensional flows. Currently, researchers rely purely on simple shear characterization or capillary rheometry, neither of which can provide unambiguous quantitative information about extensional flow behavior. The integration of hardware and analysis software will make the instrument both versatile and unique. The instrument will be invaluable to industry where all processing flows (such as extrusion, filling, pumping, blow molding, spraying etc.) involve extensional flow fields. It will find utility in industry as both a quality control tool and a research grade device. Additionally it will be of use to academia, where no simple quantitative analytical device exists for examining the draining (and filament forming) behavior of fluids. In addition the instrument described has a number of intrinsic advantages that make it ideal for a shop floor installation. It is compact (our envisioned design will have a footprint smaller than 0.1 m 2 ) and robust (with few moving parts it will be tolerant of dust and vibration). It should also be easy to use, especially in an indexing mode for intra-lab comparisons (or floor level quality control). The removable plates will allow easy cleaning and the ability to change plate surface chemistry. Finally the sample volumes will be small.

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