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PubMed1988

Variations in the friction coefficients of tablet lubricants and relationship to their physicochemical properties.

A previously described modified annular shear cell (MASC) has been used to measure the friction coefficients of some powdered tablet lubricants on a steel surface. Commonly used lubricants, as well as others belonging to the stearate group, differed in their friction coefficients at equivalent sample sizes and in the degree of their ability to reduce friction with increasing amounts when evaluated by the apparatus. In all comparisons, magnesium stearate had the lowest friction coefficient. Divalent salts of stearic acid appeared to be better than the other lubricants tested, and among themselves differed only in the extent of their ability to decrease friction. The lubricants also differed in their physical and chemical properties. On evaluation by stepwise regression analysis, such physical properties as projected surface area of lubricant particles, Martin's diameter and bulk density accounted for most of the variation in friction coefficients of the lubricants rather than moisture content or melting point. It could therefore be suggested that particle size and/or surface area parameters be incorporated in product specifications to ensure reproducible functionality.

Chemical Phenomena↗
PubMed1994

On the "door-corridor" model of gel electrophoresis. III. The gel constant and resistance, and the net charge, friction, diffusion and electrokinetic force of the migrating molecules.

The door-corridor model of gel electrophoresis enabled an estimation of the net charge of DNA molecules run in various gels. When the runs were carried out in Trisacetate-EDTA buffer having a concentration from 10 to 120 mM, the net charge in 1% agarose gel varied from 1.1 to 0.58 e per base pair. The friction between migrating molecules and gel fibers was dependent on the gel type and concentration, as well as the electric field strength and temperature during electrophoresis. In the 123 to 1,474 bp size range, the friction in 1% agarose changed from 1.91 to 378.03 x 10(-10) N.m-1.s. It was found that the friction per 123 bp DNA segment is not constant, but raises with size. The gel resistance force increases at higher electric field strengths, indicating that elastic forces govern the migration of macromolecules through gels. In the gels studied, the friction, and therefore thermal diffusion, of DNA and protein-SDS complexes scale with from 2.20 to 2.32 power of size. The ratio of thermally induced diffusion and velocity in various gels shows that there is a profound reduction of diffusion compared to velocity with increasing DNA size. This is directly linked to the high exponent relating friction and size. The high resolving power of gel electrophoresis can be correlated to the difference between the frictional coefficients of a diffusing and migrating macromolecule.

DNA↗
PubMed1993

Experimental determination of friction characteristics at the trabecular bone/porous-coated metal interface in cementless implants.

An apparatus was developed to measure load-displacement friction properties at the cancellous bone/porous-coated metal plate interface. Bone cubes were obtained from different proximal regions of four resurfaced cadaveric tibiae. Three different porous-surfaced metal plates (one fiber mesh and two bead) and a smooth-surface metal plate were used. In the presence of a constant normal contact pressure (0.10, 0.15, or 0.25 MPa), a variable tangential load up to the maximum resistance of the interface was applied and both relative normal and tangential displacements were recorded. Repetitive and fatigue loadings were also considered. Measured results show that the interface friction curve is highly nonlinear, exhibiting large relative tangential displacements in the range of 50-400 microns before the maximum load is reached. Relative displacements in the normal direction remain below 10 microns. The maximum resistance in friction is independent of the bone excision site, type of porous-surfaced metal plate, magnitude of normal load, placement of bone cubes on metal plates or vice versa, repetition of applied load, and conservation period of bone cubes in saline solution. The smooth-surfaced metal plate has significantly smaller friction resistance than porous-coated ones. The fatigue loading of up to 4000 cycles at 1 Hz, in the presence of 0.25 MPa contact pressure, slightly decreases the interface friction coefficient. Finally, the initial secant stiffness of the interface at 75% of the maximum resistance load is found to be larger for the bone cubes from the lateral and medial regions and for the metal plate with smooth surface.

Aged↗
PubMed1997

Scopolamine increases prehensile force during object manipulation by reducing palmar sweating and decreasing skin friction.

The aim of this study was to determine whether relatively long-term changes in skin friction induced by a pharmacological blockade of sweat excretion would alter the grip forces applied to objects of a variety of different surface textures and frictions. Five men and three women were asked to lift the vertically mounted armature of a linear motor between the thumb and index finger and to hold it against an opposing force for 2 s. A 1.0-kHz tone indicated to the subject that the manipulandum had been correctly positioned between the upper and lower position limits. The linear motor generated a 2.5-N force tangential to the skin surface simulating an object weighing approximately 250 g. Three different polyamide plastic surfaces (either smooth or etched with 1.0 mm high Braille beads evenly spaced at 2- or 3-mm intervals) contacted the fingers in these experiments. Subjects lifted and held in a precision grip one of the three surfaces for blocks of ten consecutive trials, but the order of presentation of the three different textures was varied to offset the effect of expectancy. On a second block of ten trials the subjects were requested to release the object slowly to measure the ratio of the grip force normal to the grasped surface to the tangential load force at the moment of slip. This ratio or its inverse provided the coefficient of friction or the slip ratio for a particular subject and surface condition. Twelve hours prior to a second recording session all subjects placed transdermal patches of 1.5 mg scopolamine behind each ear to reduce palmar sweating by blocking the muscarinic receptors of exocrine sweat glands. The subjects were re-tested following procedures that were identical to the first session. Scopolamine significantly reduced the friction of the skin on the smooth and 2-mm beaded surfaces, but the friction of the 3-mm beaded texture was unaffected. Scopolamine also caused subjects to increase both the peak and static grip forces for all the textures including the 3-mm beaded surface, suggesting that for two of the three surfaces they were responding to the increased slipperiness of the skin due to reduced sweat production.

Adult↗
PubMed2002

Role of friction and tangential force variation in the subjective scaling of tactile roughness.

The present study examined the contribution of normal (Fz) and tangential (Fx) forces, and their ratio, kinetic friction (Fx/Fz), to the subjective magnitude estimations of roughness. The results suggested that the rate of variation in tangential stroking force is a significant determinant of roughness perception. In the first experiment, six volunteer subjects scaled the roughness of eight surfaces explored with a single, active scan of the middle finger. The surfaces were 7.5x2.4-cm polymer strips embossed with truncated cones 1.8 mm high with a spatial period of 2.0 mm in the transverse direction and 1.5-8.5 mm in the longitudinal, scanning direction. The surfaces were mounted on a six-axis force and torque sensor that measured the perpendicular, contact force (normal to the skin surface) and the tangential force along the axis of stroking. The results confirmed the findings of an earlier study that magnitude estimates of perceived roughness increase approximately linearly up to a longitudinal spatial period of 8.5 mm. Across subjects, no consistent correlations were found between perceived roughness and either the mean normal or tangential force alone. Although significant positive correlations were found between roughness and mean kinetic friction for all subjects, they were not as consistently robust as one might have expected. Furthermore, instantaneous kinetic friction varied widely over the course of a single stroke because of within trial oscillations in the tangential force. The amplitude of these oscillations increased with the longitudinal spatial period and their frequency was determined by a combination of the spatial period and the stroking velocity. These oscillations were even more conspicuous in the first derivative or rate of change of the tangential force (dFx/d t), which was quantified as the root mean square (RMS) of the tangential force rate. The mean normalized RMS proved to be strongly correlated with subjective roughness, averaging 0.88 for all subjects. In order to dissociate the fluctuations in tangential force from both the surface structure and the mean kinetic friction, a second experiment was performed on six additional subjects who estimated the roughness of identical lubricated and unlubricated (dry) surfaces. Lubrication with liquid soap reduced the mean kinetic friction by approximately 40%, the RMS of the tangential force rate by slightly more than 21% and the subjective estimates of roughness by 16.4%. Taken together, the results suggest that in tactile exploration, the RMS of the tangential force rate may be an important determinant of subjective roughness.

Adult↗
PubMed2006

The effect of shoe sole tread groove depth on the friction coefficient with different tread groove widths, floors and contaminants.

Slipping and falling are common phenomena in both workplaces and our daily activities. The risks associated with slipping and falling are related to the materials of footwear/floor, contamination condition, and geometric design of the sole. Shoe soles of various tread design are very common. Tread pattern of the shoe affects friction especially under liquid-contaminated conditions. Verification of the effects of tread groove depth is significant in assisting designers in designing proper footwear for workers exposed to slippery floor conditions. In this study, we measured the friction coefficients using the Neolite footwear pads on the terrazzo, steel, and vinyl floors under three liquid-contaminated conditions. A Brungraber Mark II slipmeter was used. The footwear pads had tread grooves with a width of either 3 or 9mm. The depth of the tread grooves ranged from 1 to 5mm. The results showed that tread groove depth affected the friction coefficients significantly. Higher friction values were recorded for footwear pads with deeper tread grooves on wet and water-detergent-contaminated floors. The averaged coefficient of friction (COF) gain per tread groove depth increase in millimeter under these two surface conditions ranged from 0.018 to 0.108, depending on the tread groove width, floor, and contaminant.

Accident Prevention↗
PubMed2006

The effect of hyaluronic acid and phospholipid based lubricants on friction within a human cartilage damage model.

The lubricating abilities of different formulations of high molecular weight hyaluronic acid (HA), dipalmitoyl phosphatidylcholine (DPCC) and mixtures of both HA and DPCC were assessed in an in vitro model. Levels of start-up friction were determined using an osteoarthritis (OA) damaged human cartilage model set within a specially designed friction rig. To examine the long term benefits of HA, the extent of penetration of HA into cartilage tissue was investigated using fluorescently labelled HA and confocal microscopy. It was found that in this model, all formulations of HA and the majority of DPCC lubricants reduced friction (HA 5 and 10 mg ml(-1), DPPC 200 mg ml(-1) reductions of 51.9%, 46.7% and 46.5% respectively), compared to a Ringers solution control. Lubrication was found not to be concentration dependent for HA formulations, but concentration was key for DPCC lubrication (100 mg ml(-1) reduced friction by only 15.9%). By combining HA and DPCC (HA/DPPC; 5 mg ml(-1)/100 mg ml(-1) and 10 mg ml(-1)/200 mg ml(-1)), a further improvement was noted (69.5% and 61.9%, respectively) as the mean levels of friction were reduced by up to a further 17% than the most effective individual formulation (HA 5 mg ml(-1)). Penetration of HA into bovine cartilage by up to 300 microm from the surface was observed over a 48 h period. It was observed that HA specifically targeted the chondrocytes as it was primarily found within the lacunae surrounding the cells.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed2004

Frictional properties of regenerated cartilage in vitro.

Although tribological function is the most important mechanical property of articular cartilage, few studies have examined this function in tissue-engineered cartilage. We investigated changes in the frictional properties of cartilage regenerated from the inoculation of rabbit chondrocytes into fibroin sponge. A reciprocating friction-testing apparatus was used to measure the friction coefficient of the regenerated cartilage under a small load. The specimen was slid against a stainless steel plate in a water vessel filled with physiological saline. The applied load was 0.03 N, the stroke length was 20 mm, and the mean sliding velocity was 0.8 mm/s. The friction coefficient of the regenerated cartilage decreased with increasing cultivation time, because a hydrophilic layer of synthesized extracellular matrix was formed on the fibroin sponge surface. The friction coefficient of the regenerated cartilage was as low as that of natural cartilage in the early stages of the sliding tests, but it increased with increasing duration of sliding owing to exudation of interstitial water from the surface layer.

Animals↗
PubMed2005

Determination of frictional conditions between electrode array and endosteum lining for use in cochlear implant models.

Frictional conditions between the electrode array (in cochlear implants) and the endosteum lining covering the walls of the interior scala tympani structure strongly influence the sliding behaviour of the electrode array. Friction coefficients, determined by a simple but effective method based on the impending slippage model of electrode arrays sliding over the endosteum lining are reported in this paper. In this study, friction coefficients of the Nucleus standard straight and the Contour arrays have been determined with and without lubricants applied on the endosteum lining. In the absence of applied lubricants, friction coefficients were found to be 0.19 for the Nucleus standard straight array and 0.12 for the Contour array. Application of lubricants (glycerin and sorbelene) has the potential to lower the friction coefficient for Nucleus standard straight array (0.12 and 0.15) and for the Contour array (0.04 and 0.08). These results are used in finite element models to predict accurately the trajectories of electrode arrays and sliding contact pressures on cochlear structures to evaluate the likelihood of damage sustained during insertion.

Cochlear Implants↗
PubMed2006

Frictional properties of Hartley guinea pig knees with and without proteolytic disruption of the articular surfaces.

OBJECTIVE: To apply a pendulum technique to detect changes in the coefficient of friction of the articular cartilage of the intact guinea pig tibiofemoral joint after proteolytic disruption. DESIGN: Twenty-two hind limbs were obtained from 11 3-month old Hartley guinea pigs. Twenty knees were block-randomized to one of two treatment groups receiving injections of: (1) alpha-chymotrypsin (to disrupt the superficial layer of the articular surface) or (2) saline (sham; to control for the effects of the intra-articular injection). The legs were mounted in a pendulum where the knee served as the fulcrum. The decay in pendulum amplitude as a function of oscillation number was first recorded and the coefficient of friction of the joint was determined from these data before injection. Ten microliters of either isotonic saline or 1 Unit/microL alpha-chymotrypsin was then injected into the intra-articular joint space and incubated for 2h. The pendulum test was repeated. Changes in the coefficient of friction between the sham and alpha-chymotrypsin joints were compared. One additional pair of knees was used for histological study of the effects of the injections. RESULTS: Treatment with alpha-chymotrypsin significantly increased the coefficient of friction of the guinea pig knee by 74% while sham treatment decreased it by 8%. Histological sections using Gomori trichrome stain verified that the lamina splendens was damaged following treatment with alpha-chymotrypsin and not following saline treatment. CONCLUSIONS: Treatment with alpha-chymotrypsin induces mild cartilage surface damage and increases the coefficient of friction in the Hartley guinea pig knee.

Animals↗
PubMed2004

Frictional forces between cohesive powder particles studied by AFM.

A range of commercially important powders (hydrated alumina, limestone, titania and zeolite) and glass ballotini were attached to atomic force microscope cantilevers, and inter-particle friction forces studied in air using lateral force microscopy (LFM). The in situ calibration procedure for friction forces is described. LF images, line profiles, LF histograms, surface roughness, pull-off forces, and the load dependence of friction in the range 0-25 nN were studied for both particle-particle and particle-wall (steel) contacts. The single-particle friction results are discussed in terms of contact mechanics theory. Particle-particle contacts showed load-dependent friction, involving single asperity contacts (non-linear behaviour) or multi-asperity contacts (linear behaviour). Particle-wall contacts usually showed little load dependence and were more adhesive. The results are also related to shear stress-normal stress data (yield loci) for the same materials from bulk shear testers.

Friction↗
PubMed2002

The frictional properties at the thoracic skin-fascia interface: implications in spine manipulation.

OBJECTIVE: To assess the friction at the thoracic skin-fascia interface to determine the potential reaction force vectors during thoracic manual therapy. DESIGN: A basic in vivo study of human subjects, documenting the frictional properties at the interface between the thoracic skin and underlying fascia. BACKGROUND: Chiropractors, and other spine manipulative therapists, during thoracic manipulation have been attempting to apply force vectors to spine tissues in specific directions in addition to those applied normal to the skin. For obliquely applied forces to be directly transmitted to the underlying vertebrae, either friction is required at the skin-fascia interface or the applied force must "hook" on a bony process. METHODS: Subjects were placed in the prone position with the thoracic skin exposed. The posterior thoracic region was loaded with normal forces, incrementally from 125.3 to 392.9 N. The interface between the load and the skin was either a plexiglass plate or modelled hands. A force was then applied to either apparatus in the cephalad direction. The applied forces and corresponding displacements were measured using a load cell and an optoelectronic camera system, respectively. Chiropractors then performed actual thoracic manipulation to determine if they could maintain their location of contacts (spinous process/transverse process) on the underlying vertebra. RESULTS: Each of the subjects exhibited negligible friction between the thoracic skin and underlying fascia for both the plexiglass and modelled hand contacts. Furthermore, in each case, the apparatus travelled a distance greater than that between two transverse or spinous processes without showing an abrupt change in the slope of the force-displacement curves. The hands of chiropractors performing thoracic manipulation travelled a similar distance during the dynamic thrust. CONCLUSIONS: The skin-fascia interface over the thoracic spine exhibits negligible friction. Therefore, the reaction force from a thoracic vertebra will be normal to the overlying skin. Furthermore, the data show that the ability to "hook" either a thoracic transverse or spinous process in the superior-inferior direction during a manipulative thrust may be greatly over-rated. RELEVANCE: During thoracic spinal manipulation, one cannot direct a force vector to a thoracic vertebra at a given angle by simply directing their thrust in that direction.

Adult↗
PubMed1999

Nanoscale design of snake skin for reptation locomotions via friction anisotropy.

Multi-mode scanning probe microscopy is employed to investigate the nanostructure of dermal samples from three types of snakes. Sophisticated friction modifying nanostructures are described. These include an ordered microfibrillar array that can function to achieve mission adaptable friction characteristics. Significant reduction of adhesive forces in the contact areas caused by the 'double-ridge' nanoscale microfibrillar geometry provides ideal conditions for sliding in forward direction with minimum adhesive forces and friction. Low surface adhesion in these local contact points may reduce local wear and skin contamination by environmental debris. The highly asymmetric, 'pawl-like' profile of the microfibrillar ends with radius of curvature 20-40 nm induces friction anisotropy in forward backward motions and serves as an effective stopper for backward motion preserving low friction for forward motion. The system of continuous micropores penetrating through the snake skin may serve as a delivery system for lubrication/anti-adhesive lipid mixture that provides for boundary lubrication of snake skins.

Actin Cytoskeleton↗
PubMed1999

The influence of friction and interference on the seating of a hemispherical press-fit cup: a finite element investigation.

The formation of gaps in the polar region of acetabular cups is seen as a drawback of press-fit fixation of non-cemented acetabular cups. Recent findings indicate a link between long-term polar gaps and the gaps present directly after implantation. In this study the process of press-fitting is simulated with a linear-elastic two-dimensional axisymmetric finite-element model. The aim of this paper is to investigate the possible importance of friction and interference on the formation of these gaps. A range of cup-bone friction coefficients (mu = 0.1-0.5) is assigned to the cup-bone interface in order to represent the unknown amount of friction occurring during press-fitting. The cup is modeled with a radius of 27 mm, whereas the radius of the cavity is varied between 26.50 and 26.75 mm, thus, creating 0.50 and 0.25 mm radial interference fits. The difference in cavity radius represents the discrepancy between the radius of the last-reamer-used and radius of the cavity it creates. The subchondral plate is considered as being completely removed during reaming. The effects of impact blows via the surgeon's mallet during surgery are modeled as a series of four load pulses, in which peak force is gradually increased from 0.5 to 4.0 kN. The effects of load removal as well as those of load application are investigated. On load application, the cup penetrates into the cavity, and on load removal, the cup rebounds. Depending on the friction, interference and load applied, the position of the cup after the load pulse is somewhere between its position at peak force and its position at the beginning of the pulse. Although the simplifications and conditions involved in the creation of the model necessitate caution when interpreting the results for all clinical cases, it is found that the seating of hemispherical cups in trabecular bone could be more satisfactory for intermediate values of friction (mu = 0.2-0.3) and smaller interference fits (0.25 mm).

Acetabulum↗
PubMed1997

Friction in hip prostheses.

Although the reduction of frictional torques was the driving force behind the design of the Charnley prosthesis, later concerns about wear and subsequent loosening of this and other hip replacements have dominated debate within the bioengineering community. To stimulate discussion on the role of friction in loosening, a review of the frictional characteristics of different prostheses was undertaken. The use of simple laboratory screening-type machines in the frictional assessment of different material combinations is discussed together with experiments performed on single axis simulators using both conventional and experimental prostheses. In particular, recent developments in the use of soft layer components are highlighted. Further, the possible link between excessively high frictional torques and loosening is discussed in the light of current results obtained from explanted prostheses.

Alloys↗
PubMed2003

Evaluation of friction of conventional and metal-insert ceramic brackets in various bracket-archwire combinations.

The purpose of the study was to measure and compare the level of frictional resistance generated between conventional ceramic brackets (Transcend Series 6000, 3M Unitek, Monrovia, Calif), ceramic brackets with stainless steel slot (Clarity, 3M Unitek), conventional stainless steel brackets (Victory Series, 3M Unitek), and 3 different orthodontic wire alloys: stainless steel (stainless steel, SDS Ormco, Glendora, Calif), nickel-titanium (Ni-Ti, SDS Ormco), and beta-titanium (TMA, SDS Ormco). All brackets had a 0.022-in slot, and orthodontic wire alloys were tested in 3 different sections: 0.016 in, 0.017 x 0.025 in, and 0.019 x 0.025 in. Each of the 27 bracket-archwire combinations was tested 10 times, and each test was performed with a new bracket-wire sample. Static and kinetic friction were measured on a specially designed apparatus. All data were statistically analyzed (analysis of variance and Scheffé for the bracket effect, Kruskal-Wallis and Mann Whitney for the alloy and section effects). Metal-insert ceramic brackets generated significantly lower frictional forces than did conventional ceramic brackets, but higher values than stainless steel brackets, in agreement with the findings of the few previous reports. Beta-titanium archwires had higher frictional resistances than did stainless steel and nickel-titanium archwires. No significant differences were found between stainless steel and nickel-titanium archwires. All the brackets showed higher static and kinetic frictional forces as the wire size increased. Metal-insert ceramic brackets are not only visually pleasing, but also a valuable alternative to conventional stainless steel brackets in patients with esthetic demands.

Analysis of Variance↗
PubMed2003

Evaluation of friction of stainless steel and esthetic self-ligating brackets in various bracket-archwire combinations.

This study measured and compared the level of frictional resistance generated between stainless steel self-ligating brackets (Damon SL II, SDS Ormco, Glendora, Calif), polycarbonate self-ligating brackets (Oyster, Gestenco International, Göthenburg, Sweden), and conventional stainless steel brackets (Victory Series, 3M Unitek, Monrovia, Calif), and 3 different orthodontic wire alloys: stainless steel (Stainless Steel, SDS Ormco), nickel-titanium (Ni-Ti, SDS Ormco), and beta-titanium (TMA, SDS Ormco). All brackets had a.022-in slot, whereas the orthodontic wire alloys were tested in 3 different sections:.016,.017 x.025, and.019 x 0.025 in. Each of the 27 bracket and archwire combinations was tested 10 times, and each test was performed with a new bracket-wire sample. Both static and kinetic friction were measured on a custom-designed apparatus. All data were statistically analyzed (Kruskal-Wallis and Mann Whitney U tests). Stainless steel self-ligating brackets generated significantly lower static and kinetic frictional forces than both conventional stainless steel and polycarbonate self-ligating brackets, which showed no significant differences between them. Beta-titanium archwires had higher frictional resistances than stainless steel and nickel-titanium archwires. No significant differences were found between stainless steel and nickel-titanium archwires. All brackets showed higher static and kinetic frictional forces as the wire size increased.

Dental Alloys↗
PubMed1994

Zirconia brackets: an evaluation of morphology and coefficients of friction.

The frictional characteristics of two types of zirconia (Harmony, Hudson Ltd., Sheffield, U.K., and Toray, Yamaura Corp., Tokyo, Japan) brackets were compared with those of polycrystalline alumina (Transcend 2000, Unitek Corp., Monrovia, Calif.) brackets in both dry and wet states. To compare the couples, four arch wire alloys were studied: stainless steel, cobalt-chromium, nickel titanium, and beta-titanium. Under dry conditions, the highest frictional coefficients were seen with the Harmony/beta-titanium couple (uk = 0.64); the lowest values were seen with both Transcend 2000/stainless steel (uk = 0.13) and Toray/cobalt-chromium couples (uk = 0.13). Beta-titanium arch wires produced the highest coefficients of friction against each type of ceramic bracket, except against Toray arch wires in the wet state. The presence of human saliva produced only slight changes in the frictional behavior of zirconia brackets. We conclude that currently available zirconia brackets offer no significant improvement over alumina brackets with regard to their frictional characteristics.

Aluminum Oxide↗