J.B. Medley

Imaging protein deposits on contact lens materials.

Purpose. The majority of studies investigating protein deposition on contact lens materials require that the deposit of interest be removed, potentially resulting in erroneous results if some proteins are not removed adequately. The purpose of this study was to investigate the use of in situ imaging methods to examine protein deposition on conventional poly(2-hydroxyethyl methacrylate) (polyHEMA)-based and silicone hydrogel contact lens materials. Methods. Six silicone hydrogel and five polyHEMA-based hydrogel contact lens materials were examined by Atomic Force Microscopy (AFM) and/or Scanning Electron Microscopy (SEM) techniques, after being deposited with proteins in an in vitro model. AFM studies examined lenses deposited solely with lysozyme at approximate physiological concentrations and SEM studies were conducted on lenses exposed to a dilute mixture of lysozyme and albumin-conjugated gold spheres. Results. AFM studies demonstrated that the lens materials had markedly differing surface topographies. SEM results showed that galyfilcon A and balafilcon A lenses deposited both lysozyme and albumin in relatively large aggregates, as compared with lotrafilcon A and B, in which the proteins were deposited in a more evenly spread, monolayer formation. Polymacon lenses deposited more protein than any of the silicone hydrogel materials and much of the protein was aggregated together. AFM data indicated that balafilcon A, lotrafilcon A and polymacon deposited lysozyme in a similar manner, with very little lysozyme being deposited in discrete areas. Galyfilcon A behaved very differently, with the lysozyme exhibiting both aggregates as well as string-like formations over the lens surface. Conclusions. Imaging techniques that allow proteins to be examined in situ show much promise for determining the extent and physical characterization of protein on contact lens materials. These techniques indicate that the pattern of deposition of proteins onto silicone hydrogel contact lens materials differs between materials, depending upon their bulk and surface composition.

Fluid film lubrication in artificial hip joints

The importance of fluid-film lubrication mechanisms experienced in hip implants is reviewed. For the different types of hip implants currently used in clinical practice, including conventional/cross-linked polyethylene-on-metal/ceramic, metal-on-metal and ceramic-on-ceramic bearing couples, a detailed review is presented of theoretical fluid-film lubrication analyses along with friction measurements and wear studies, both having a strong emphasis on the fluid-film mechanisms involved. Future studies and improvements of hip implants from a fluid-film lubrication point of view are discussed.

Spring-Supported Thrust Bearings Used in Hydroelectric Generators: Comparison of Experimental Data with Numerical Predictions

Laboratory experiments that simulate the conditions acting on large spring-supported thrust bearings in hydroelectric generators were performed and film thicknesses, temperatures and pressures were measured. A software package provided theoretical predictions of the experimental results. This software used a control volume, numerical formulation to model the three dimensional flow of mass, momentum and energy in the thrust bearing and included thermoelastic deflection of the pad and flow of oil in the groove between the pads. A detailed comparison of the experimental data with the numerical predictions showed quite good overall agreement, particularly with the deformed shape and temperatures of the pad. The agreement provided evidence of the fidelity of the software package and supported its continued use in analysis and design. Presented as a Society of Tribologists and Lubrication Engineers paper at the ASME/STLE Tribology Conference in Toronto, Ontario, Canada, October 26–28, 1998

Friction of Contact Lenses: Silicone Hydrogel versus Conventional Hydrogel

Silicone hydrogel contact lenses can be worn for up to 30 nights without removal, but for some users, wear time is limited due to poor comfort. This lack of comfort may be related to friction between the anterior lens surface and the under-surface of the eyelid. The purpose of the present study was to initiate an investigation of these friction forces. In-vitro experiments were performed to measure friction forces for silicone hydrogel (SH) lenses and for conventional hydrogel (CH) lenses. The SH and CH lenses were represented by Focus Night and Day (CIBA Vision) and by Optima FW (Bausch and Lomb), respectively. Half of the lenses were subject to in-vitro deposition (doping) of both human serum albumin and hen-egg lysozyme proteins while the other half were undoped and only soaked in phosphate buffer saline solution to remove the presence of the packaging fluid. Scanning electron microscopy was used to obtain images of the undoped and doped lenses to provide insight into the differing friction forces that were measured. For the friction experiments, the contact lenses were placed on a silicone eye-form, which approximated the shape and compliance of the human eye. A custom friction testing apparatus was developed in which a stationary lens was loaded against a sinusoidally reciprocating, flat glass plate in the presence of a saline solution. Two normal forces were applied during testing and peak friction forces were measured at cyclic steady state. Scanning electron microscopy imaging involved preparation of protein conjugated gold nano-spheres in order to provide a label for each of the proteins. The measured friction forces increased for increasing normal force and in the absence of protein doping. Although the friction forces measured for the SH lenses were consistently higher than those for the CH lenses, the differences were not statistically significant. The images of doped lenses showed aggregated mounds of both proteins on both lens types but they were more apparent on CH lenses. A monolayer coverage of both proteins was most evident on the SH lenses. After correlating the images with the friction results, it was suggested that early stages of protein deposition could help lubricate the lens-eyelid interface and reduce friction. However, protein deposition patterns (profiles), as observed in the present study, had no obvious influence on the friction forces.

Spring-Supported Thrust Bearings Used in Hydroelectric Generators: Laboratory Test Facility©

A test facility was developed for the experimental investigation of the large spring-supported thrust bearings used in hydroelectric generators. The test bearing had an outer diameter of 1.168 m, thus making it large enough to simulate most of the features of the thrust bearings in the field, yet small enough to be placed conveniently in an engineering laboratory. Instrumentation included a distribution of thermocouples and pressure taps along with displacement probes of eddy current type that were located in the rotor to measure oil film thickness. A detailed description of the test facility was provided along with some representative results and a description of some experimental difficulties. A survey of the relevant literature published in the past 40 years suggested that this apparatus was unique in its ability to study relatively large spring-supported thrust bearings in the laboratory. Presented as a Society of Tribologists and Lubrication Engineers paper at the World Tribology Congress in London, ...

Stop-dwell-start motion: a novel simulator protocol for the wear of metal-on-metal hip implants

Theory, supported by some experimental evidence, suggests that metal-on-metal (m-m) hip implants can have protective elastohydrodynamic lubrication films during continuous walking that may reduce wear. A simple transient model indicated that such films were remarkably constant during continuous motion and could prevent surface contact of smooth, low clearance implants. However, these films could both form and breakdown rapidly during intermittent motion in vivo. Thus, a more representative simulator motion protocol was specified involving repeated cycles of Stopping, dwelling with a constant load for 1 minute and starting 10 minute sessions that applied continuous walking conditions. An automatic control system was designed and to implement this protocol in a hip simulator. Six implants were tested for about 3 million cycles (Mc) under the developed stop-dwell-start motion protocol. Compared with continuous motion testing of similar implants, the wear at 3 Mc approximately doubled (from about 0.6 to 1.2 mm3) and wear rates showed considerable increase (from 0.04 to 0.19 mm3/Mc). The simulator wear of m-m hip implants should be investigated during intermittent as well as continuous motions. Ultimately, designs should address the issue of wear during stop-dwell-start motion protocols.

Influence of Lubricant Additives on Friction in a Disc Machine

The friction behaviour of base stock and formulated lubricating oils under conditions of continuous elastohydrodynamic lubrication (ehl) and micro-ehl were investigated in a side-slip disc machine by varying the surface roughness of the discs. Under micro-ehl conditions, the friction was 2 - 3 times larger than under ehl conditions and the additives produced significant differences in the friction. For ehl, small but distinct and repeatable differences in friction were produced by the additives. These differences permitted the derivation of a limiting shear stress expression as a function of pressure, using an empirical approach suggested in the literature, for each of the lubricants. The friction measured under micro-ehl conditions along with an estimation of the real area of contact allowed an alternative evaluation of the limiting shear stress which agreed quite well with the predictions based on the friclion measured under ehl conditions. Thus, it was suggested that friction measurements under ehl conditions could be used to evaluate and explore the influence of additives on the limiting shear stress. Furthermore, a simple model for friction in micro-ehl was developed which was consistent with the results of the present investigation.

On the role of the lambda parameter in simulator wear of metal-on-metal hip implants

The purpose of the present study was to quantify the change in the lambda parameter (λ) during continuous orbital bearing simulator testing of metal-on-metal hip implants and relate λ to wear. The topographies of eight implants were measured periodically throughout four million cycles (Mc) of testing. A typical initial run-in wear period was followed by lower wear rates. The final values of volumetric wear were low (

Metal-metal cervical disc implants: A material investigation using pin-on-plate tests

The tribological behaviour of two material pairs was explored for the application of metal-metal cervical disc replacement Efforts are being made to find a material pair that minimizes wear while at the same time permitting acceptable postoperative imaging. The current study investigated wear and friction of stainless steel (used in an earlier design cervical disc implant) and a titanium-based metal matrix composite. Pin-on-plate wear tests were performed with both linear-tracking and crossing-path reciprocating motions. The metal matrix composite had a superior wear performance compared to stainless steel for both types of motion. When tested with crossing-path motion, the total wear of metal matrix composite showed a decrease compared with linear-tracking motion while total wear was not affected for the stainless steel. Friction tests were performed on stainless steel, metal matrix composite and, for comparison purposes, a high carbon cobalt-based alloy. Both the metal matrix c omposite and stainless st eel had c oefficients of friction that w ere higher than t he cobalt-based alloy.

Lubrication and wear of alumina-alumina hip bearings

The majority of alumina-alumina hip simulator studies in the literature have been unable to reproduce the wear rates and surface features found in most implant retrievals. The present study investigated simulator wear with two novel tests regimes. The first test investigated the simulator wear of six alumina-alumina implants in the presence of different serum concentrations (30% and 90%) under continuous motion conditions for 3 million cycles (Mc). The serum concentration did not influence the wear significantly. The implants were grouped together to give an initial run-in wear rate of 0.98±0.12 mm 3 /Mc (for 0–0.25 Mc) that decreased to a steady-state wear rate of 0.032±0.006 mm 3 /Mc (for 0.25–3 Mc). The second test subjected the same implants to stop-start motion in the presence of a 30% serum solution for an additional 1 Mc, in order to elucidate the influence of more intimate surface contact. As a result, the implants went through a second, less dramatic run-in phase over the first 0.5 Mc, then the wear rate decreased to a steady-state value of almost zero. The implants were examined after the first test with Atomic Force Microscopy (AFM) and demonstrated a relief polishing on the femoral head that extended longitudinally about 5.5 mm from the apex, where a gradual transition to the original polished surface occurred. The root-mean square roughness in the worn region was found to be about twice that of the polished region. A similar relief pattern was observed on replicas of the acetabular cups, in an area that extended longitudinally in a zone 2.9–5.6 mm longitudinally from the apex of the cup. Using the initial geometry and roughness of the implants, lambda values were predicted that suggested mixed or full fluid film lubrication. Using the measured gravimetric wear, it was possible to predict a wear zone size on the head that was consistent with the AFM observation and to estimate the total linear wear. The low wear rates demonstrate the excellent wear resistance of the alumina-alumina. However, these simulator tests continue to under-predict the wear in-vivo , indicating that even stop-start simulator testing with a 30% concentration of serum did not provide severe enough conditions to produce the higher in-vivo wear rates.