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PubMed2006

Viscoelastic study of the mechanical unfolding of a protein by AFM.

We have applied a dynamic force modulation technique to the mechanical unfolding of a homopolymer of immunoglobulin (Ig) domains from titin, (C47S C63S I27)5, [(I27)5] to determine the viscoelastic response of single protein molecules as a function of extension. Both the stiffness and the friction of the homopolymer system show a sudden decrease when a protein domain unfolds. The decrease in measured friction suggests that the system is dominated by the internal friction of the (I27)5 molecule and not solvent friction. In the stiffness-extension spectrum we detected an abrupt feature before each unfolding event, the amplitude of which decreased with each consecutive unfolding event. We propose that these features are a clear indication of the formation of the known unfolding intermediate of I27, which has been observed previously in constant velocity unfolding experiments. This simple force modulation AFM technique promises to be a very useful addition to constant velocity experiments providing detailed viscoelastic characterization of single molecules under extension.

Animals↗

PubMed1999

Mechanisms for age-related changes of fingertip forces during precision gripping and lifting in adults.

We investigated changes across the adult life span of the fingertip forces used to grip and lift objects and their possible causes. Grip force, relative safety margin (grip force exceeding the minimum to avoid slip, as a fraction of slip force), and skin slipperiness increased beginning at age 50 years. Skin slipperiness explained relative safety margin increases until age 60 years. Hence, after age 60 years, additional factors must elevate grip force. We argue that one factor is impaired cutaneous afferent encoding of skin-object frictional properties on the basis of three findings. First, only subjects 60 years and older increased their relative safety margins when the friction of the gripped surfaces was varied randomly versus experiments that varied only object weight. Skin slipperiness did not account for this behavior. Second, these older subjects scaled the initial portion of their force trajectories for the slippery surface during experiments when friction was varied. Third, their grip force adjustments to new surfaces were delayed approximately 100 msec as compared with young subjects. Previous research has demonstrated that friction is signaled locally by fast-adapting afferents (FA I afferents), which decrease in number during old age. By contrast, adjustments triggered by object set-down, an event encoded by FA II afferents throughout the hand and wrist, were not delayed in our old subjects. Other findings included that anticipatory control of fingertip forces using memory of object weight was unimpaired in old age. Finally, old and young adults modulated their fingertip forces with equal smoothness and with similar relative intertrial variability.

Adult↗

PubMed1999

Static calculations for mesh fixation by intraabdominal pressure in laparoscopic extraperitoneal herniorrhaphy.

In laparoscopic extraperitoneal herniorrhaphy, there have been no static calculations for nonfixed prostheses with regard to the ideal size of the prosthesis, mesh overlap on the hernial orifice, friction coefficient, and shear forces. The aim of the present study was to perform these static analyses. The friction coefficient of a polypropylene mesh was measured in an experimental setting. Based on the distribution of adhesive force and load on the mesh, we calculated the required mesh overlap over the hernial opening. The friction coefficient was determined as mu0 = 0.3. The friction surface required to immobilize the prosthesis is 4.16 times the stress surface. Assuming a safety factor of 2.5, the minimum distance between the margin of the prosthesis and that of the hernial opening should be equal to the diameter of the hernial opening. For smaller lesions we recommend a minimum mesh overlap of 2 cm over the hernial orifice. From a hernia size 2 cm and larger, the distance between the margin of the prosthesis and the hernial opening should be equal to the diameter of the orifice. From 4 cm and larger, the prosthesis should be secured with a stapler (or a similar mode of fixation) to prevent recurrence.

Abdominal Muscles↗

PubMed1991

[Optimization of arch guided tooth movement by the use of powerhooks].

Arch guided tooth movement is today still the most often used technique to move teeth bodily in mesiodistal direction. One of the most important disadvantage of this method, however, is friction between bracket and arch wire, which impairs tooth movement and may result in dangerous overload of the anchorage units. Powerhooks are used to accomplish a more effective orthodontic tooth movement by reduction of the tipping and rotating moments. This investigation answers the question which mechanical dimensions powerhooks should have in order to minimize friction. If the point of force application lies coronally to the center of resistance (CR) and rotatory moments are eliminated, friction can be reduced by as much as 90%. Concerning length, powerhooks must not be extended beyond. Otherwise, if the point of force application lies apically to CR, friction increases dramatically. In the horizontal direction, a unilateral powerhook should not be extended to more than the half of its length.

Dental Stress Analysis↗

PubMed1990

[Tribologic study of hip prostheses].

The multitude of total hip replacement systems cannot mask the fact that it is after all a matter of friction between two surfaces. The wear and distorsion resulting from this friction are still to be precisely evaluated for each marketed hip system. These two parameters have indeed to be evaluated when new devices are used. The "in vivo" behavior of a hip system is best approached when studied on a model reproducing the hip movements and the cyclic loads endured by the hip throughout walking. Data obtained using this type of model allow us to approach the behavior of metal-polyethylene interfaces: there is always a grinding period whose duration varies from 300,000 to 2,000,000 cycles. The friction factors during this initial grinding period may be two to three times greater than the final ones. Data obtained upon wear over 5 millions cycles correspond to clinical findings. Concerning the high density polyethylene-metal interface, variations of molecular size or supply of supplementary molecules may increase the friction factors up to three times the initial values. But wear may vary much more.

Aluminum Oxide↗

PubMed1980

Biomechanical analysis of the sliding characteristics of compression hip screws.

UNLABELLED: We examined the effect of screw-plate angle on the sliding characteristics and jamming potential of four popularly used stainless-steel and cobalt-chromium-molybdenum compression hip screws. The actual coefficient of sliding friction for these alloys was measured in each device. The force on the screw required to overcome the static frictional force also was determined, by varying the lengths of screw engaged in the barrel under conditions of static equilibrium representing 130-degree and 150-degree screw-plate angles. For the 130-degree loading configuration, this force was significantly (p < 0.001) higher than that required for the 150-degree loading configuration for all four screw types. The actual coefficient of friction was relatively constant for each material, although slight variations due to differences in design between screw types were found. A positive correlation (p < 0.01) was seen between the apparent coefficient of friction (the ratio of sliding force to normal force) and the length of the screw extending from the barrel. All stainless-steel screws jammed in the 130-degree tests when not completely engaged in the barrel. None of the 150-degree tests produced jamming and none of the cobalt-chromium-molybdenum screws jammed in either the 130-degree or the 150-degree test. Examination of jammed devices by scanning electron microscopy showed galling on the superior surface of both the screw and the barrel. CLINICAL RELEVANCE: Understanding the conditions that facilitate sliding of hip screws aids in ensuring their proper use. The higher the nail-plate angle, the easier it is to impact the hip-fixation device and thus allow bone impaction and stability at the fracture site. The potential for jamming a sliding hip screw is decreased by maximum engagement of the screw in the barrel. Differences in the material and design of sliding hip-fixation devices have relatively little effect on the sliding characteristics compared with the nail-plate angle and the engagement of the screw in the barrel.

Alloys↗

PubMed1999

Potential thermal artifacts in hip joint wear simulators.

Frictional heat was monitored during wear tests of ultrahigh molecular weight polyethylene acetabular cups bearing against femoral balls of metal or ceramic in a hip simulator, using bovine serum as a lubricant. About 1 to 2 h of continuous cycling were required for the temperature in the zone of contact between the cup and ball to rise to its maximum steady value, and this equilibrium temperature was markedly higher with increased load and/or cycling rate. Frictional heating caused substantial precipitation of the proteins from the serum and, in some of the tests running at 1.5 or 2 Hz, an adherent proteinaceous layer was observed attached to the surface of the balls. The maximum temperature was also substantially higher in tests run with the cup mounted above the ball rather than below. Surprisingly, the tests running at higher frictional torque and temperature (i.e., those with the most protein precipitation and/or adherent layers) produced the least wear of the polyethylene. This might have been due to the solid proteins that formed a protective layer between the ball and cup. Because patients with hip prostheses typically do not walk for hours without rest, the maximum temperatures in vivo are likely to be much lower than those reached in the hip simulator. Therefore, the affects of protein precipitation on the resultant wear properties of the materials should be considered potential artifacts of the hip simulator tests. Increasing the volume of the lubricant bath reduced the maximum temperatures for tests running at 1.5 Hz but had little affect at 2 Hz. Reducing the cycling rate is an effective way to avoid overheating of the specimens, but this necessarily extends the time required to complete a test.

Animals↗

PubMed1993

Handling characteristics of braided suture materials for tight tying.

To establish criteria for characterizing synthetic sutures, the handling characteristics of silk suture were analyzed. The characteristics that distinguish silk suture from other braided suture materials are its good "knot security" and relatively low "tiedown resistance." Analytic consideration of knot security suggests that not only superficial friction but also resistance force produced by cross-sectional deformity of braided threads plays an important role in silk's superior performance. Results of a "pullout friction test," developed to quantitatively evaluate resistance produced by surface friction and cross-sectional deformity suggest that the superiority of silk thread can be explained in terms of high static withdrawal resistance under low loads and relatively low dynamic withdrawal resistance under high loads.

Elasticity↗

PubMed2006

Self-assembling peptides as injectable lubricants for osteoarthritis.

The self-assembly of peptides is explored as an alternative route towards the development of new injectable joint lubricants for osteoarthritis (OA). The versatility of the peptide chemistry allows the incorporation of behavior reminiscent of hyaluronic acid (HA), while the triggered in situ self-assembly provides easy delivery of the samples by injection due to the low viscosity of the peptide solutions (that are initially monomeric). Using design criteria based on the chemical properties of HA, a range of de novo peptides were prepared with systematic alterations of charge and hydrophilicity that self-assembled into nematic fluids and gels in physiological solution conditions. The frictional characteristics of the peptides were evaluated using cartilage on cartilage sliding contacts along with their rheological characteristics. Peptide P(11)-9, whose molecular, mesoscopic, and rheological properties most closely resembled HA was found to be the most effective lubricant amongst the peptides. In healthy static and dynamic friction testing (corresponding to healthy joints) P(11)-9 at 20-40 mg/mL performed similar to HA at 10 mg/mL. In friction tests with damaged cartilage (corresponding to early stage OA) P(11)-9 was a less efficient lubricant than HA, but still the best among all the peptides tested. The results indicate that de novo self-assembling peptides could be developed as an alternate therapeutic lubricant for early stage OA.

Amino Acid Sequence↗

PubMed2000

Mixture of hyaluronic acid and phospholipid prevents adhesion formation on the injured flexor tendon in rabbits.

Dipalmitoyl phosphatidylcholine, a highly surface-active polar lipid, has been implicated as a potential boundary lubricant for synovial joints. We examined the effects of dipalmitoyl phosphatidylcholine on the flexor tendon and its protective effect against postoperative adhesion in two experimental steps. First, the flexor digitorum fibularis and the distal pulley of rabbits were set for a friction test. The test was performed with saline solution, sodium hyaluronate, or a mixture of dipalmitoyl phosphatidylcholine and sodium hyaluronate as the lubricant. The friction coefficient was significantly lower with the mixture of dipalmitoyl phosphatidylcholine and sodium hyaluronate than with saline solution or sodium hyaluronate. We concluded that the decreased friction coefficient indicates that dipalmitoyl phosphatidylcholine could complement the boundary-lubricating ability of the tendon. In the second experiment, we used an experimental adhesion model of the flexor digitorum fibularis in the rabbit. During the operation, either saline solution, sodium hyaluronate, or a mixture of dipalmitoyl phosphatidylcholine and sodium hyaluronate was injected into the tendon sheath. The specimen was sent to another tester, and the work required to tear off the adhesion was measured. The work required was significantly greater for the tendons that had been injected with saline solution than for those given injections of dipalmitoyl phosphatidylcholine and sodium hyaluronate. Our findings suggest that dipalmitoyl phosphatidylcholine plays an important role in the boundary lubrication of the tendon and that after tendon injury, the administration of a mixture of dipalmitoyl phosphatidylcholine and sodium hyaluronate may improve tendon lubrication and prevent adhesion formation.

1,2-Dipalmitoylphosphatidylcholine↗

PubMed1993

Degenerative change and rotator cuff tears. An anatomical study in 160 shoulders of 80 cadavers.

In order to determine the effects of friction and rubbing in the development of rotator cuff tear, we studied 160 shoulders of 80 cadavers (age at death 43-93 years, mean 69.3 years). Seventy-two cadavers were fixed with formalin and eight were fresh cadavers. The surface of the cuff and the undersurface of the acromion were observed macroscopically. Eight shoulders of fresh cadavers were examined by scanning electron microscopy. Ninety-eight specimens (61%) showed degenerative changes of the supraspinatus tendon. The number of tendons with degeneration and tear increased from the fifth to sixth decade of life, and the size of the tear increased with age. However, there was no sustained increase in the incidence with aging from the age of 60 to 90 years, and the percentage with degenerative changes of the cuff remained at approximately 60% in each decade. Ninety-six specimens (60%) showed degeneration of the subacromial surface. The percentage with degeneration of the undersurface of the acromion remained at approximately 60% from the sixth to ninth decade. There was a significant correlation between the severity of the changes in the rotator cuff and the subacromial surface. Scanning electron microscopy showed changes suggesting effects of friction and rubbing on the rotator cuff, such as regularly arranged wool-like spherical structures on the surface of the tendon and rounded ruptured ends of the tendon fibers. These results indicate that degenerative change of the rotator cuff is aggravated by a friction and rubbing mechanism with the undersurface of the acromion and leads to development of a complete tear.

Acromion↗

PubMed2002

Deployment of fingertip forces in tactile exploration.

The purpose of this study was to examine how contact forces normal to the skin surface and shear forces tangential to the skin surface are deployed during tactile exploration of a smooth surface in search of a tactile target. Six naive subjects participated in two experiments. In the first experiment, the subjects were asked to explore a series of unseen smooth plastic surfaces by using the index finger to search for either a raised or recessed target. The raised targets were squares with a height of 280 micro m above the background surface and that varied in side lengths from 0.2 mm to 8.0 mm. A second series of smooth plastic surfaces consisted of small recessed squares (side lengths: 2.0, 3.0, 4.0 and 8.0 mm) that were etched to a depth of 620 micro m. Although made of an identical material, the plastic substrate had a lower coefficient of friction against the skin because only the recessed square had been subjected to the electrolytic etching process. The surfaces were mounted on a six-axes force and torque sensor connected to a laboratory computer. From the three axes of linear force, the computer was able to calculate the instantaneous position of the index finger and the instantaneous tangential force throughout the exploratory period. When exploring for the raised squares, the subjects maintained a relatively constant, average normal force of about 0.49 N with an average exploration speed of 8.6 cm/s. In contrast, all subjects used a significantly higher average normal force (0.64 N) and slightly slower mean exploration speed (7.67 cm/s) when searching for the small recessed squares. This appeared to be an attempt to maximize the amount of skin penetrating the recessed squares to improve the probability of target detection. In a second experiment, subjects were requested to search for an identical set of raised squares but with the fingertip having been coated with sucrose to impede the scanning movement by increasing the friction. Overall, the subjects maintained the same constant normal force that they used on the uncoated surface. However, they increased the tangential force significantly. The similarity of the search strategy employed by all subjects supports the hypothesis that shear forces on the skin provide a significant stimulus to mechanoreceptors in the skin during tactile exploration. Taken together, these data suggest that, in active tactile exploration with the fingertip, the tangential finger speed, the normal contact force, and the tangential shear force are adjusted optimally depending on the surface friction and whether the target is a raised asperity or a recessed indentation.

Adult↗

PubMed1993

Evaluation of wear in an all-polymer total knee replacement. Part 1: laboratory testing of polyethylene on polyacetal bearing surfaces.

The friction and wear characteristics of a prototype, all-polymer hip prosthesis were evaluated using a multi-station hip-joint simulator. The components consisted of 41 mm diameter polyacetal copolymer balls in combination with acetabular cups of UHMW polyethylene, and were compared to the conventional combination of cobalt-chromium alloy balls against UHMW polyethylene cups. The all-polymer combination exhibited about 20% lower friction and 39% lower wear of the polyethylene cups. The total volume of polymer wear from the all-polymer combination (i.e. including the wear of the polyacetal ball) was 23% less than the volume of polyethylene wear from the metal-polymer components. The appearance of the worn surfaces of the polyethylene cups in the scanning electron microscope were similar for the two combinations of materials, and the surfaces of the polyacetal balls showed only mild wear. Formaldehyde, a product of the degradation of polyacetal, was present in traces in some samples of lubricant from the test chambers with polyacetal balls. In this laboratory model, the friction and wear properties of a polyacetal copolymer ball in combination with an UHMW polyethylene demonstrated good potential for clinical application.

Acetals↗

PubMed2005

Protein-mediated boundary lubrication in arthroplasty.

Wear of articulated surfaces can be a major lifetime-limiting factor in arthroplasty. In the natural joint, lubrication is effected by the body's natural synovial fluid. Following arthroplasty, and the subsequent reformation of the synovial membrane, a fluid of similar composition surrounds the artificial joint. Synovial fluid contains, among many other constituents, a substantial concentration of the readily adsorbing protein albumin. The ability of human serum albumin to act as a boundary lubricant in joint prostheses has been investigated using a pin-on-disc tribometer. Circular dichroism spectroscopy was employed to follow the temperature- and time-dependent conformational changes of human serum albumin in the model lubricant solution. Effects of protein conformation and polymer surface hydrophilicity on protein adsorption and the resulting friction in the boundary lubrication regime have been investigated. Unfolded proteins preferentially adsorb onto hydrophobic polymer surfaces, where they form a compact, passivating layer and increase sliding friction-an effect that can be largely suppressed by rendering the substrate more hydrophilic. A molecular model for protein-mediated boundary friction is proposed to consolidate the observations. The relevance of the results for in vivo performance and ex vivo hip-joint testing are discussed.

Adsorption↗

PubMed2004

Resistance to sliding of orthodontic brackets with bumps in the slot floors and walls: effects of second-order angulation.

OBJECTIVE: Two stainless steel (SS) bracket designs with two bumps across the slot floors and rounded slot walls and one polycrystalline alumina (PCA) bracket design with one bump along the slot floor were compared to conventional SS and PCA brackets. METHODS: For each design, the resistances to sliding of four archwire-bracket couples, two in the dry state and two in the wet (saliva) state, were measured at 32 second-order angles between -12 and +12 degrees. RESULTS: When clearances existed, the kinetic coefficients of friction of the SS brackets ranged from 0.13 to 0.15 in the dry state and from 0.17 to 0.18 in the wet state. For the PCA brackets, the kinetic coefficients of friction were 0.18 for both designs in the dry state and were 0.18 for the brackets with bumps and 0.24 for the brackets without bumps in the wet state. SIGNIFICANCE: Overall, the bumps and rounded slot walls did not reduce classical friction. When the brackets with rounded slots were modeled, the critical contact angles for binding were larger than the values based on the apparent dimensions, but were less than those based on the true dimensions. When clearances did not exist, the rates of binding were greater for the SS brackets with bumps and rounded walls than for the conventional brackets. The rates of binding increased because the rounded walls forced the archwires into complex shapes. Both PCA bracket designs had similar rates of binding; but unlike the conventional design, the brackets with the bumps did not permanently deform the archwires at large angles.

Aluminum Oxide↗

PubMed2006

Design factors influencing performance of constrained acetabular liners: finite element characterization.

Constrained acetabular liners are utilized to deal with the infrequent but devastating problem of recurrent dislocation. While an encouraging treatment of last resort, the clinical performance of contemporary constrained liners has been somewhat mixed. There are multiple factors contributing to this variability, one of which is the limited understanding of the intrinsic mechanical characteristics of these specialty devices. To address this issue, a three-dimensional, materially nonlinear, multi-surface contact finite element model of a representative constrained liner was created. The model was physically validated, and then used for parametric testing to explore the effects of individual design features. The model was exercised for both intra-operative assembly and lever-out dislocation. It was found that the coefficient of friction between the femoral head and the liner substantially affected both the force required to seat the femoral head into the liner during assembly, and the peak moment resisting dislocation (226% increase in assembly force for friction coefficients of 0.2 versus 0.0; 49% reduction in dislocation moment for friction coefficients of 0.013 versus 0.135). As expected, the cup opening radius also had a dominant effect on both maneuvers: decreasing the opening radius from 13.9 to 13.6 mm increased assembly force by 506 N and increased the dislocation moment by over 3.5 N-m, whereas the influence of other design parameters was much more modest.

Acetabulum↗

PubMed2006

Incisal orientation and biting efficiency.

Broad-edged 'spatulate' upper and lower incisors are distinctive of catarrhines and platyrrhines who use them in various ways to peel fruits, remove bark, and strip leaves from branches. The incisors of modern humans not only control the bite size of foods during ingestion, but often grip items in a number of non-food related tasks. Such uses have long been implicated for Neandertals as well. Despite the evolutionary importance of incision and the fact that the incisors feature prominently in clinical dentistry (via orthodontic practices designed both to correct incisal misalignments and adjust their orientation), little is known about what affects their functional efficiency. Few mechanical analyses of incisal action have been published and none that seem to take note of the mechanisms of both fracture and friction at the tooth-food interface. Here, we modeled the incisal tip as a wedge, finding that the efficiency of biting foods that fracture elastically is strongly dependent on both the apex angle of the incisor and the coefficient of friction. Based on apex angle measurements from a small sample of human central incisors, the overall efficiency of upper central incisors is predicted to be greatest when the angle between the apex bisector and the direction of applied force is zero. However, this is complicated greatly by friction, particularly for the lower incisors. The analysis probably applies not only to the use of incisors by humans, but also to some extent to frugivorous primates. This model should clarify the mechanics behind incision and can provide a basic foundation upon which more advanced models can be built on in the future.

Animals↗

PubMed2005

Nanotribological characterization of human hair and skin using atomic force microscopy.

Healthy hair and skin is highly desired. Characterization of their morphological, frictional, and adhesive properties (tribological properties) is essential to enhance understanding of hair and skin and to advance the science. Literature on the tribological characterization of hair and skin is scarce to date. The paper presents nanotribological data and analysis on hair (Caucasian, Asian, and African hair at virgin, chemo-mechanically damaged, and treated conditions) and synthetic hair and skin, as well as roughness data of human skin replica. Roughness statistics are presented to characterize the vertical and spatial surface parameters. Average coefficient of friction values were determined for each ethnicity and hair type, and are discussed. The directionality dependence of friction is also discussed. Magnitude and spatial distribution of adhesive force are used to estimate thickness and distribution of the conditioner film.

Asian People↗