The viscoelastic properties of polymer surfaces were investigated by nanoscale dynamic mechanical analysis (nano-DMA) involving contact force modulation in the frequency range of 10–200Hz. Nano-DMA experiments were performed with a Berkovich diamond tip of nominal radius of curvature equal to ∼100nm under a mean contact force of 8–10μN and alternating force equal to 2% of the mean force. Variations in the loss tangent, storage modulus, and loss modulus of low- and high-density polyethylene and ultrahigh molecular weight polyethylene with the force frequency demonstrated significantly different viscoelastic behaviors for shallow depths (<40nm) than for relatively large depths (i.e., 75–100nm). The effects of alternating force frequency and indentation depth on the viscoelastic properties of the different polyethylene materials are interpreted in terms of the microstructure characteristics and the molecular chain mobility at the polymer surfaces. The results show that nano-DMA is an effective technique for nanoscale studies of the viscoelastic behavior of polymer surfaces.
Wear of ultra-high molecular weight polyethylene (UHMWPE) continues to be a major obstacle limiting the longevity of total joint replacements. Efforts to solve the wear problem in UHMWPE have resulted in numerous studies dealing with the microstructure, morphology, and mechanical properties of this polymer. However, the fundamental wear mechanisms at different material length scales in total joint replacements remain elusive. Consequently, a systematic investigation of the initial stage of the wear process was performed in this study in order to obtain insight into the origins of wear in UHMWPE at submicrometer scales. Sliding experiments were performed with both unmodified and crosslinked (by gamma radiation treatment) UHMWPE subjected to reciprocating sliding against Co-Cr alloy in a bath of bovine serum under ranges of mean contact pressure and sliding speed typical of knee joints. Nanoindentation and optical, scanning electron, and transmission electron microscopy were used to examine the effect of crosslinking on the nanomechanical properties, dominant wear mechanisms, and microstructure of UHMWPE. The fundamental wear micromechanisms of unmodified and crosslinked UHMWPE are interpreted in the context of coefficient of friction, wear factor, creep, adhesion force, and microstructure results.
Teaching mechanical and structural biomaterials concepts for successful medical implant design, this self-contained text provides a complete grounding for students and newcomers to the field. Split into three sections: Materials, Mechanics and Case Studies, it begins with a review of sterilization, biocompatibility and foreign body response before presenting the fundamental structures of synthetic biomaterials and natural tissues. Mechanical behavior of materials is then discussed in depth, covering elastic deformation, viscoelasticity and time-dependent behavior, multiaxial loading and complex stress states, yielding and failure theories, and fracture mechanics. The final section on clinical aspects of medical devices provides crucial information on FDA regulatory issues and presents case studies in four key clinical areas: orthopedics, cardiovascular devices, dentistry and soft tissue implants. Each chapter ends with a list of topical questions, making this an ideal course textbook for senior undergraduate and graduate students, and also a self-study tool for engineers, scientists and clinicians.
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