Pull-off Force Research Articles

Effect of Medium on Interaction Forces between Atomic Force Microscopy (AFM) Tip and Gold Nanoparticle

Atomic Force Microscopy (AFM) has become powerful tool not only to study the surface topography but also the interaction forces between the tip and the sample. In this works, the measurement of the forces was determined by using AFM imaging in air and in liquid environment. Then, the force-distance (FD) curve was analysed to determine the interaction forces between the tip and the samples. The forces was computed from the pull-off forces of FD curve. When the FD curve are required in air, a thin layer of water (liquid contaminant layer) adsorbed on the sample surface exerts a meniscus or capillary force. Instead, when working in a liquid, the capillary force is eliminated and other forces becomes relevant. The force present is the van der Waals force and electric double layer force. The net force between the tip and the samples is the vector sum of several forces. In air, the dominant component of the adhesion force between the tip and the sample is believed to be the capillary force while in liquid, the dominant components present is the van der Waals force. The adhesion force ranging from 10 to 100 nN have been reported for imaging in air and the adhesion force of 5 to 104 nN observed in our studies lies well within this range. Meanwhile, in liquid, the adhesion force obtained from this work is in the range of 0.1-40 nN and it is lies within the range reported previously which is as low as 100 pN. Detecting forces is an important step toward developing new analytical and biomedical devices such as biosensors.

Bioinspired Further Enhanced Dry Adhesive by the Combined Effect of the Microstructure and Surface Free-Energy Increase.

Silicone elastomers are known for having low surface free energies generally leading to poor adhesive performances. This surface characteristic can be enhanced by plasma treatments. The microstructured silicone elastomer surfaces can demonstrate superior adhesive performance that is more than 10 times higher in terms of pull-off forces, compared to their unstructured counterpart. Here, we have demonstrated that the combination of these two methods further enhances adhesive performance, especially when the surfaces are biomimetic micro/nanopatterned with, e.g., beetle-inspired mushroom-shaped adhesive microstructure (MSAMS). The plasma treatment time and pressure parameters were varied for the unstructured and MSAMS poly(vinylsiloxane) surfaces to find optimum parameters for maximum adhesion performance. Air plasma treatment induced average adhesive enhancement forces up to 30% on the unstructured surface, but the MSAMS surface demonstrated an enhancement of adhesive forces up to 91% higher than that of an untreated, microstructured control, despite the plasma-treated surface area of the structured surface being only 50% of that of the unstructured surface. High-speed video-recordings of individual microstructures in contact with a glass surface shows that the origin of the adhesion enhancement is due to the special detachment mechanism of individual microstructures that allows sustaining a wider contact area at detachment. We believe that this integration of the plasma treatment with MSAMS suggests a versatile way of functionalization that can further advance the adhesive ability of low-surface-energy polymer surfaces.

Response to Dr Greenwood\u2019s Comments on \u201cExtending the Double-Hertz Model to Allow Modeling of an Adhesive Elliptical Contact\u201d

© 2018, The Author(s). An adhesive elliptical contact is normally found in microscale applications that involve cylindrical solids, crossing at an angle between 0° and 90°. Currently, only one model is available to describe the elliptical contact’s surface interaction: the approximate Johnson–Kendall–Roberts (JKR) model which is limited to soft materials. In this paper, a new adhesive elliptical model is developed for a wide range of adhesive contacts by extending the double-Hertz theory, where adhesion is modeled by the difference between two Hertzian pressure distributions. Both Hertzian pressures are assumed to have an equivalent shape of contact areas, the only difference being in size. Assuming that the annular adhesive region is obtained by the area difference between the two Hertzian contact areas, the pull-off force curves can be calculated. In the limiting case of an adhesive circular contact, the results are very close to results from the existing models. However, for an adhesive elliptical contact in the JKR domain, lower pull-off forces are predicted when compared to the JKR values. Unlike the developed model, the shape of the JKR contact area varies throughout contact. Results show, particularly for conditions close to the JKR domain, that it is important to take into account that the adhesive region is the result of the two Hertzian contact areas having a non-equivalent shape.

Adhesion between rubber and glass in dry and lubricated condition.

We study the adhesion between differently processed glass and filled bromobutyl rubber in dry conditions, in water, and in silicone oil. The boundary line between contact and non-contact in adhesion experiments can be considered as a mode I crack, and we show that viscoelastic energy dissipation, close to the opening (or closing) crack tip and surface roughness, strongly affects the work of adhesion. We observe strong adhesion hysteresis and, in contrast to the Johnson-Kendall-Roberts theory prediction for elastic solids, this results in a pull-off force (and work of adhesion) which depends on the loading force and contact time. In particular, for the system immersed in water and silicone oil, we register very weak adhesive bonding. For glass ball with baked-on silicone oil, the pull-off force is nearly independent of the contact time, but this is not observed for the unprocessed glass surface.

Do Well-functioning THAs Retrieved at Autopsy Exhibit Evidence of Fretting and Corrosion?

Our understanding of fretting and corrosion at head-neck junctions in modular THAs in vivo is based largely on the analysis of retrieved implants removed for various diagnoses. Little is known about the condition of head-neck tapers in well-functioning THAs. Regarding a cohort of well-functioning autopsy-retrieved modular THAs, we asked: (1) Does trunnion geometry or femoral head material affect the pull-off force of the femoral head? (2) Is there a relationship between trunnion damage and length of implantation time, head diameter, and neck length? (3) Does visual damage scoring accurately determine the presence or absence of corrosion on cobalt-chrome trunnions? Sixty-six femoral stems and engaged femoral heads were retrieved at autopsy from 53 patients at Anderson Orthopaedic Research Institute from 1998 to 2014. Ten stems were excluded for low stem design group size or insufficient head-stem clearance for pull-off testing, leaving a cohort of 56 THAs with a median implantation time of 10 years (range, 1-24 years). The femoral stems included three cobalt-chrome (CoCr) designs from a single manufacturer with either a 12/14 or 14/16 trunnion design (N = 36 and 20, respectively) mated with alumina or CoCr heads (N = 13 and 43, respectively). The force required to pull off the femoral heads was measured using a uniaxial load frame according to ASTM F2009-00. Mating surfaces were visually examined to assess the presence and severity of fretting and corrosion using a modified Goldberg scoring system. Three 12/14 trunnions of similar implantation lengths and varied damage scores were selected for imaging with a scanning electron microscope (SEM) and energy dispersive x-ray analysis (EDAX) to confirm the absence or presence of corrosion damage. No difference was seen in pull-off force between groups based on trunnion geometry and head material (median [range], alumina-12/14: 3127 [2320-6992] N, alumina-14/16: 2670 [1095-7919] N, CoCr-12/14: 2255 [1332-5939] N, CoCr-14/16: 2812 [1655-4246] N; p = 0.132). A positive correlation was found between damage score and length of implantation (ρ = 0.543, p < 0.001). However, no correlation between damage score and either head diameter or neck length was found (ρ = -0.012, p = 0.930 and ρ < 0.001, p = 0.995, respectively). In all, 39 of 56 specimens demonstrated no fretting or corrosion, and 16 specimens had mild damage scores. One specimen demonstrated severe corrosion without visual evidence of fretting. The presence of intergranular corrosion on this trunnion was determined by SEM imaging and EDAX. The absence of corrosion products on two trunnions with no observed damage was confirmed. This study found little evidence of fretting and corrosion in a cohort of well-functioning CoCr-CoCr and alumina-CoCr head-neck couples. Further studies are necessary to characterize fretting and corrosion at head-neck junctions of well-functioning implants of other designs and manufacturers. The results from this study suggest that patients with well-functioning THAs using polyethylene bearing surfaces with alumina or CoCr heads appear to be at low risk for trunnion corrosion for the specific CoCr alloy stems and trunnion geometries analyzed here.

Surface Forces Apparatus Measurements of Interactions between Rough and Reactive Calcite Surfaces.

nm-Range forces acting between calcite surfaces in water affect macroscopic properties of carbonate rocks and calcite-based granular materials and are significantly influenced by calcite surface recrystallization. We suggest that the repulsive mechanical effects related to nm-scale surface recrystallization of calcite in water could be partially responsible for the observed decrease of cohesion in calcitic rocks saturated with water. Using the surface forces apparatus, we simultaneously followed the calcite reactivity and measured the forces in water in two surface configurations: between two rough calcite surfaces (CC) and between rough calcite and a smooth mica surface (CM). We used nm-scale rough, polycrystalline calcite films prepared by atomic layer deposition. We measured only repulsive forces in CC in CaCO3-saturated water, which was related to roughness and possibly to repulsive hydration effects. Adhesive or repulsive forces were measured in CM in CaCO3-saturated water depending on calcite roughness, and the adhesion was likely enhanced by electrostatic effects. The pull-off adhesive force in CM became stronger with time, and this increase was correlated with a decrease of roughness at contacts, the parameter which could be estimated from the measured force-distance curves. That suggested a progressive increase of real contact areas between the surfaces, caused by gradual pressure-driven deformation of calcite surface asperities during repeated loading-unloading cycles. Reactivity of calcite was affected by mass transport across nm- to μm-thick gaps between the surfaces. Major roughening was observed only for the smoothest calcite films, where gaps between two opposing surfaces were nm-thick over μm-sized areas and led to force of crystallization that could overcome confining pressures of the order of MPa. Any substantial roughening of calcite caused a significant increase of the repulsive mechanical force contribution.

Conducting and stretchable nanometer-thin gold/thiol-functionalized polydimethylsiloxane films

Future low-voltage dielectric elastomer transducers (DETs), based on nanometer-thin elastomer membranes, will rely on soft and compliant electrodes with reasonable electrical conductivity and sound adhesion to the elastomer. State-of-the-art adhesion promoters, including nanometer-thin Cr/Ti films, result in defects for applied areal strains larger than 3% and lead to increases in the stiffness of DETs. To generate forces in the Newton range, these low-voltage DETs have to be stacked in thousands of layers. Herein, we present a compliant electrode, which consists of gold bonded covalently to thiol-functionalized polydimethylsiloxane (SH-PDMS) films. The membranes were fabricated using molecular beam deposition and in situ and/or subsequent ultraviolet light (UV) radiation. Peel-off tests demonstrate the expected strong binding of Au to the SH-PDMS network, with this highly stretchable Au/SH-PDMS layer capable of withstanding strains of at least 60%, without losing conductivity. Optical micrographs show signs of cracks for strained pure Au and Au/Cr electrodes but not for the Au/SH-PDMS layer. The mechanical properties and adhesion forces of Au/SH-PDMS were extracted by means of atomic force microscopy (AFM), using a spherical Au tip coated with methyl groups (CH3). The elastic modulus of ( 12 ± 9 ) MPa increased slightly against the 20-nm-thin Au/PDMS example, but it can be tailored by the cross-linking density of Au/SH-PDMS via the UV irradiation dose. Unloading nanoindentation curves revealed pull-off forces between the CH3-functionalized AFM tip and the Au/SH-PDMS layer at the time of separation. For Au/SH-PDMS, the spectral distribution of pull-off forces exhibits repulsive forces with the CH3 groups of the PDMS network as well as adhesive forces resulting from interactions with the nanometer-sized Au clusters. This approach provides the means to bind gold clusters homogenously within the SH-PDMS film. Such compliant electrodes are the prerequisite for fabricating low-voltage DETs that can be stretched by more than 50%.

Towards Alternative Approaches for Coupling of a Soft Robotic Sleeve to the Heart.

Efficient coupling of soft robotic cardiac assist devices to the external surface of the heart is crucial to augment cardiac function and represents a hurdle to translation of this technology. In this work, we compare various fixation strategies for local and global coupling of a direct cardiac compression sleeve to the heart. For basal fixation, we find that a sutured Velcro band adheres the strongest to the epicardium. Next,we demonstrate that a mesh-based sleeve coupled to the myocardium improves function in an acute porcine heart failure model. Then, we analyze the biological integration of global interface material candidates (medical mesh and silicone) in a healthy and infarcted murine model and show that a mesh interface yields superior mechanical coupling via pull-off force, histology, and microcomputed tomography. These results can inform the design of a therapeutic approach where a mesh-based soft robotic DCC is implanted, allowed to biologically integrate with the epicardium, and actuated for active assistance at a later timepoint. This strategy may result in more efficient coupling of extracardiac sleeves to heart tissue, and lead to increased augmentation of heart function in end-stage heart failure patients.

A microelectromechanical systems (MEMS) force-displacement transducer for sub-5 nm nanoindentation and adhesion measurements.

We present a highly sensitive force-displacement transducer capable of performing ultra-shallow nanoindentation and adhesion measurements. The transducer utilizes electrostatic actuation and capacitive sensing combined with microelectromechanical fabrication technologies. Air indentation experiments report a root-mean-square (RMS) force resolution of 1.8 nN and an RMS displacement resolution of 0.019 nm. Nanoindentation experiments on a standard fused quartz sample report a practical RMS force resolution of 5 nN and an RMS displacement resolution of 0.05 nm at sub-10 nm indentation depths, indicating that the system has a very low system noise for indentation experiments. The high sensitivity and low noise enables the transducer to obtain high-resolution nanoindentation data at sub-5 nm contact depths. The sensitive force transducer is used to successfully perform nanoindentation measurements on a 14 nm thin film. Adhesion measurements were also performed, clearly capturing the pull-on and pull-off forces during approach and separation of two contacting surfaces.

Can We Put a Value on the Adhesion and Tackiness of Greases?

This work focuses on presenting a new methodology, based on approach–retraction curves, to objectively and accurately measure adhesion and tackiness of industrial greases. The definitions of pull-off force and tackiness are redefined, explained and measured, whereas the effect of testing parameters such as retraction speed, grease film thickness, applied load and temperature is discussed. This work aims to point out that the adhesion and tackiness of greases are not just a single value, but that they strongly depend on the experimental conditions. Based on this proposed method, a mapping of the tackiness of greases with respect to the experimental or working conditions can be performed.

Relative humidity influence on adhesion effect in MEMS flexible structures

An atomic force microscope operating in contact mode is used in this paper for determining the pull-off forces as a function of relative humidity. The paper reports the measurements and the modeling of adhesion forces versus humidity in controlled ranges between 20 and 90% RH. In the low RH range ( 20%), the adhesion force increases very sharply once ‘liquid-like’ adsorbed water layer forms, because it increases the capillary force. After the relative humidity reaches about 70–80% the drop in the experimental values of the adhesive force is a result of desorption of water molecules and the corresponding decrease of water menisci contribution. To investigate the effect of relative humidity on adhesion for flexible structures such as aluminum microbridges with different lengths against silicon substrate we used an analytical method which encompasses the effect of capillarity as well as the solid-to-solid interaction. The capillary force is expressed as the sum between the Laplace force and the surface tension, while the solid-to-solid interaction is estimated using the Derjaguin–Muller–Toporov (DMT) model. The analytical results are in good agreement with the experimental ones.

Onset of detachment in adhesive contact of an elastic half-space and flat-ended punches with non-circular shape: analytic estimates and comparison with numeric analysis

A recent paper by Popov, Pohrt and Li (PPL) in Friction investigated adhesive contacts of flat indenters in unusual shapes using numerical, analytical and experimental methods. Based on that paper, we analyze some special cases for which analytical solutions are known. As in the PPL paper, we consider adhesive contact in the Johnson–Kendall–Roberts approximation. Depending on the energy balance, different upper and lower estimates are obtained in terms of certain integral characteristics of the contact area. The special cases of an elliptical punch as well as a system of two circular punches are considered. Theoretical estimations for the first critical force (force at which the detachment process begins) are confirmed by numerical simulations using the adhesive boundary element method. It is shown that simpler approximations for the pull-off force, based both on the Holm radius of contact and the contact area, substantially overestimate the maximum adhesive force.

A Soft Bionic Gripper with Variable Effective Length

This article presented a four-fingered soft bionic robotic gripper with variable effective actuator lengths. By combining approaches of finite element analysis, quasi-static analytical modeling, and experimental measurements, the deformation of the single soft actuator as a function of air pressure input in free space was analyzed. To investigate the effect of the effective actuator length on the gripping performance of the gripper, we conducted systematical experiments to evaluate the pull-off force, the actuation speed, the precision and error tolerance of the soft gripper while grasping objects of various sizes and shapes. A combination of depressurization and pressurization in actuation as well as applying variable effective actuator length enhanced the gripper’s performance significantly, with no sensors. For example, with tunable effective actuator length, the gripper was able to grasp objects ranging from 2 mm – 170 mm robustly. Under the optimal length, the gripper could generate the maximum pull-off force for the corresponding object size; the precision and the error tolerance of the gripper were also significantly improved compared to those of the gripper with full-length. Our soft robotic prototype exhibits a simple control and low-cost approach of gripping a wide range of objects and may have wide leverage for future industrial operations.

An ellipsoidal cap model for adhesion of a soft particle on a rigid substrate

Surface energy outside the contact zone, which is not accounted for in the classical Johnson–Kendall–Roberts (JKR) model, can play an essential role in adhesion mechanics of soft particles. An open problem in the adhesion mechanics of soft elastic particles is how to achieve an explicit expression for the surface energy outside the contact zone in terms of the two JKR-type variables (a, δ), where a is the radius of the contact zone and δ is the relative approach of two bodies. The present work aims to develop an ellipsoidal cap model for the surface energy outside the contact zone of a soft elastic particle on a rigid substrate in terms of the two JKR-type variables (a, δ). An explicit expression for the surface energy outside the contact zone is derived, and simple asymptotic equations are obtained to determine the two unknowns (a, δ). The validity and accuracy of the derived expression and asymptotic equations are verified by good agreement with the Young–Dupre equation in the absence of an external applied force, and are also justified by good agreement of the predicted pull-off force with known results available in recent literature.

Dynamic Sliding Enhancement on the Friction and Adhesion of Graphene, Graphene Oxide, and Fluorinated Graphene.

Graphene and functionalized graphene are promising candidates as ultrathin solid lubricants for dealing with the adhesion and friction in micro- and nanoelectromechanical systems (MEMS and NEMS). Here, the dynamic friction and adhesion characteristics of pristine graphene (PG), graphene oxide (GO), and fluorinated graphene (FG) were comparatively studied using atomic force microscopy (AFM). The friction as a function of load shows nonlinear characteristic on GO with strong adhesion and linear characteristic on PG and FG with relatively weak adhesions. An adhesion enhancement phenomenon that the slide-off force after dynamic friction sliding is larger than the pull-off force is observed. The degree of adhesion enhancement increases with the increasing surface energy, accompanied by a corresponding increase in transient friction strengthening effect. The dynamic adhesion and friction enhancements are attributed to the coupling of dynamic tip sliding and surface hydrophilic properties. The atomic-scale stick-slip behaviors confirm that the interfacial interaction is enhanced during dynamic sliding, and the enhancing degree depends on the surface hydrophilic properties. These findings demonstrate the adhesive strength between the contact surfaces can be enhanced in the dynamic friction process, which needs careful attention in the interface design of MEMS and NEMS.

The relative composition of actin isoforms regulates cell surface biophysical features and cellular behaviors

BackgroundCell surface mechanics is able to physically and biomechanically affect cell shape and motility, vesicle trafficking and actin dynamics. The biophysical properties of cell surface are strongly influenced by cytoskeletal elements. In mammals, tissue-specific expression of six actin isoforms is thought to confer differential biomechanical properties. However, the relative contribution of actin isoforms to cell surface properties is not well understood. Here, we sought to investigate whether and how the composition of endogenous actin isoforms directly affects the biomechanical features of cell surface and cellular behavior. MethodsWe used fibroblasts isolated from wild type (WT), heterozygous (HET) and from knockout (KO) mouse embryos where both β-actin alleles are not functional. We applied a combination of genome-wide analysis and biophysical methods such as RNA-seq and atomic force microscopy. ResultsWe found that endogenous β-actin levels are essential in controlling cell surface stiffness and pull-off force, which was not compensated by the up-regulation of other actin isoforms. The variations of surface biophysical features and actin contents were associated with distinct cell behaviors in 2D and 3D WT, HET and KO cell cultures. Since β-actin in WT cells and smooth muscle α-actin up-regulated in KO cells showed different organization patterns, our data support the differential localization and organization as a mechanism to regulate the biophysical properties of cell surface by actin isoforms. ConclusionsWe propose that variations in actin isoforms composition impact on the biophysical features of cell surface and cause the changes in cell behavior.

Effects of Seating Load Magnitude on Incremental Cyclic Fretting Corrosion in 5°40’ Mixed Alloy Modular Taper Junctions

BackgroundMechanically assisted crevice corrosion of modular tapers continues to be a concern in total joint arthroplasties. A surgical factor that may affect taper fretting corrosion during cyclic loading is seating load magnitude. In this study, modular head-neck taper junctions were seated, capturing load-displacement, over a range of axially oriented loads, and electrochemical and micromotion data were captured during short-term incremental cyclic fretting corrosion (ICFC) tests. The hypothesis is low seating loads result in greater motion and fretting corrosion in ICFC tests. The effect of assembly load on pull-off force post-ICFC testing was also evaluated. MethodsThe study employed custom-built test fixtures which measured head-neck micromotion and an electrochemical chamber to monitor electrochemical reactions. Head-neck motion measurements were captured using 2 noncontact differential variable reluctance transducers mounted to the head. Seating experiments ranged from 1000 to 8000 N. ResultsSignificant differences due to seating loads were reported in seating displacement, ICFC subsidence, and fretting current at 4000 N cyclic load. Seating load decreased but did not eliminate currents. Fretting onset load remained fixed (approximately 1200 N) for tapers seated above 2000 N. Fretting subsidence was negligible for seating loads of 4000 N or higher, and increased subsidence was observed below 4000 N. ConclusionThis short-term test method evaluated the acute performance of modular implants which were assembled under various loads and demonstrated the link between seating loads, fretting motions, and electrochemical reactions. While increased seating loads reduced fretting corrosion and taper subsidence, it did not prevent fretting corrosion even at 8 kN seating.

Maximizing the fixation strength of modular components by impaction without tissue damage.

ObjectivesTaper junctions between modular hip arthroplasty femoral heads and stems fail by wear or corrosion which can be caused by relative motion at their interface. Increasing the assembly force can reduce relative motion and corrosion but may also damage surrounding tissues. The purpose of this study was to determine the effects of increasing the impaction energy and the stiffness of the impactor tool on the stability of the taper junction and on the forces transmitted through the patient’s surrounding tissues.MethodsA commercially available impaction tool was modified to assemble components in the laboratory using impactor tips with varying stiffness at different applied energy levels. Springs were mounted below the modular components to represent the patient. The pull-off force of the head from the stem was measured to assess stability, and the displacement of the springs was measured to assess the force transmitted to the patient’s tissues.ResultsThe pull-off force of the head increased as the stiffness of the impactor tip increased but without increasing the force transmitted through the springs (patient). Increasing the impaction energy increased the pull-off force but also increased the force transmitted through the springs.ConclusionsTo limit wear and corrosion, manufacturers should maximize the stiffness of the impactor tool but without damaging the surface of the head. This strategy will maximize the stability of the head on the stem for a given applied energy, without influencing the force transmitted through the patient’s tissues. Current impactor designs already appear to approach this limit. Increasing the applied energy (which is dependent on the mass of the hammer and square of the contact speed) increases the stability of the modular connection but proportionally increases the force transmitted through the patient’s tissues, as well as to the surface of the head, and should be restricted to safe levels.Cite this article: A. Krull, M. M. Morlock, N. E. Bishop. Maximizing the fixation strength of modular components by impaction without tissue damage. Bone Joint Res 2018;7:196–204. DOI: 10.1302/2046-3758.72.BJR-2017-0078.R2.

Impaction Force Influences Taper-Trunnion Stability in Total Hip Arthroplasty

BackgroundThis study investigated the influence of femoral head impaction force, number of head strikes, the energy sequence of head strikes, and head offset on the strength of the taper-trunnion junction. MethodsThirty titanium-alloy trunnions were mated with 36-mm zero-offset cobalt-chromium femoral heads of corresponding taper angle. A drop tower impacted the head with 2.5J or 8.25J, resulting in 6 kN or 14 kN impaction force, respectively, in a single strike or combinations of 6 kN + 14 kN or 14 kN + 14 kN. In addition, ten 36-mm heads with −5 and +5 offset were impacted with sequential 14 kN + 14 kN strikes. Heads were subsequently disassembled using a screw-driven mechanical testing frame, and peak distraction force was recorded. ResultsFemoral head pull-off force was 45% the strike force, and heads struck with a single 14 kN impact showed a pull-off force twice that of the 6 kN group. Two head strikes with the same force did not improve pull-off force for either 6 kN (P = .90) or 14 kN (P = .90). If the forces of the 2 impactions varied, but either impact measured 14 kN, a 51% higher pull-off force was found compared to impactions of either 6 kN or 6 kN + 6 kN. Femoral head offset did not significantly change the pull-off force among −5, 0, and +5 heads (P = .37). ConclusionFemoral head impaction force influenced femoral head trunnion-taper stability, whereas offset did not affect pull-off force. Multiple head strikes did not add additional stability, as long as a single strike achieved 14 kN force at the mallet-head impactor interface. Insufficient impaction force may lead to inadequate engagement of the trunnion-taper junction.

A representative volume element model for the adhesion between a micro-pillared surface and a compliant substrate

A representative volume element (RVE) model was developed to understand how the adhesion response between an effectively rigid substrate with micro-pillar arrays and a compliant substrate is affected by the spacing and aspect ratio of pillars. A number of verification and validation steps were taken to ensure the RVE model was robust. We found that the pull-off force decreases when pillar spacing is small enough such that the local deformation within the compliant substrate around one pillar is affected by neighboring pillars. Additionally, pull-off force can increase dramatically if the compliant substrate makes contact with the back surface between pillars, which occurs when the pillar spacing is large and pillar aspect ratio is small. Our RVE model can provide useful insights and quantitative data for understanding the adhesive contact mechanics between micro-patterned surfaces and soft substrates.