Soft matter dynamics: A versatile microgravity platform to study dynamics in soft matter.

We describe an experiment container with light scattering and imaging diagnostics for experiments on soft matter aboard the International Space Station (ISS). The suite of measurement capabilities can be used to study different materials in exchangeable sample cell units. The currently available sample cell units and future possibilities for foams, granular media, and emulsions are presented in addition to an overview of the design and the diagnostics of the experiment container. First results from measurements performed on ground and during the commissioning aboard the ISS highlight the capabilities of the experiment container to study the different materials.

Gold nanoparticle assisted synthesis of MoS2 monolayers by chemical vapor deposition.

The use of metal nanoparticles is an established paradigm for the synthesis of semiconducting one-dimensional nanostructures. In this work we study their effect on the synthesis of two-dimensional semiconducting materials, by using gold nanoparticles for chemical vapor deposition growth of two-dimensional molybdenum disulfide (MoS2). In comparison with the standard method, the employment of gold nanoparticles allows us to obtain large monolayer MoS2 flakes, up to 20 µm in lateral size, even if they are affected by the localized overgrowth of MoS2 bilayer and trilayer islands. Important modifications of the optical and electronic properties of MoS2 triangular domains are reported, where the photoluminescence intensity of the A exciton is strongly quenched and a shift to a positive threshold voltage in back-gated field effect transistors is observed. These results indicate that the use of gold nanoparticles influences the flake growth and properties, indicating a method for possible localized synthesis of two-dimensional materials, improving the lateral size of monolayers and modifying their properties.

Wave-Vector Dependence of the Dynamics in Supercooled Metallic Liquids.

We present a detailed investigation of the wave-vector dependence of collective atomic motion in Au_{49}Cu_{26.9}Si_{16.3}Ag_{5.5}Pd_{2.3} and Pd_{42.5}Cu_{27}Ni_{9.5}P_{21} supercooled liquids close to the glass transition temperature. Using x-ray photon correlation spectroscopy in a previously uncovered spatial range of only a few interatomic distances, we show that the microscopic structural relaxation process mimics the structure and presents a marked slowing down at the main average interparticle distance. This behavior is accompanied by dramatic changes in the shape of the intermediate scattering functions, which suggest the presence of large dynamical heterogeneities at length scales corresponding to a few particle diameters. A ballisticlike mechanism of particle motion seems to govern the structural relaxation of the two systems in the highly viscous phase, likely associated with hopping of caged particles in agreement with theoretical studies.

Hierarchical electrospun tendon-ligament bioinspired scaffolds induce changes in fibroblasts morphology under static and dynamic conditions.

The regeneration of injured tendons and ligaments is challenging because the scaffolds needs proper mechanical properties and a biomimetic morphology. In particular, the morphological arrangement of scaffolds is a key point to drive the cells growth to properly regenerate the collagen extracellular matrix. Electrospinning is a promising technique to produce hierarchically structured nanofibrous scaffolds able to guide cells in the regeneration of the injured tissue. Moreover, the dynamic stretching in bioreactors of electrospun scaffolds had demonstrated to speed up cell shape modifications in vitro. The aim of the present study was to combine different imaging techniques such as high-resolution X-ray tomography (XCT), scanning electron microscopy (SEM), fluorescence microscopy and histology to investigate if hierarchically structured poly (L-lactic acid) and collagen electrospun scaffolds can induce morphological modifications in human fibroblasts, while cultured in static and dynamic conditions. After 7 days of parallel cultures, the results assessed that fibroblasts had proliferated on the external nanofibrous sheath of the static scaffolds, elongating themselves circumferentially. The dynamic cultures revealed a preferential axial orientation of fibroblasts growth on the external sheath. The aligned nanofibre bundles inside the hierarchical scaffolds instead, allowed a physiological distribution of the fibroblasts along the nanofibre direction. Inside the dynamic scaffolds, cells appeared thinner compared with the static counterpart. This study had demonstrated that hierarchically structured electrospun scaffolds can induce different fibroblasts morphological modifications during static and dynamic conditions, modifying their shape in the direction of the applied loads. LAY DESCRIPTION: To enhance the regeneration of injured tendons and ligaments cells need to growth on dedicated structures (scaffolds) with mechanical properties and a fibrous morphology similar to the natural tissue. In particular, the morphological organisation of scaffolds is fundamental in leading cells to colonise them, regenerating the collagen extracellular matrix. Electrospinning is a promising technique to produce fibres with a similar to the human collagen fibres, suitable to design complex scaffolds able to guide cells in the reconstruction of the natural tissue. Moreover, it is well established that the cyclic stretching of these scaffolds inside dedicated systems called bioreactors, can speed up cells growth and their shape modification. The aim of the present study was to investigate how hierarchically structured electrospun scaffolds, made of resorbable material such as poly(L-lactic acid) and collagen, could induce morphological changes in human fibroblasts, while cultured during static and dynamic conditions. These scaffolds were composed by an external electrospun membrane that grouped inside it a ring-shaped bundle, made of axially aligned nanofibres, resembling the morphological arrangement of tendon and ligament tissue. After 7 days of parallel cultures, the scaffolds were investigated using the following imaging techniques: (i) high-resolution X-ray tomography (XCT); (ii) scanning electron microscopy (SEM); (iii) fluorescence microscopy and (iv) histology. The results showed that fibroblasts were able to grow on the external nanofibrous sheath of the static scaffolds, by elongating themselves along their circumference. The dynamic cultures revealed instead a preferential axial orientation of fibroblasts grown on the external sheath. The aligned nanofibre bundles inside the hierarchical scaffolds allowed an axial distribution of the fibroblasts along the nanofibres direction. This study has demonstrated that the electrospun hierarchically structured scaffolds investigated can modify the fibroblasts morphology both in static and dynamic conditions, in relation with the direction of the applied loads.

Uncertainties of synchrotron microCT-based digital volume correlation bone strain measurements under simulated deformation.

Digital Volume Correlation (DVC) is used to measure internal displacements and strains in bone. Recent studies have shown that Synchrotron radiation micro-computed tomography (SR-microCT) can improve the accuracy and precision of DVC. However, only zero-strain or virtually-moved test have been used to quantify the DVC uncertainties, leading to potential underestimation of the measurement errors. In this study, for the first time, the uncertainties of a global DVC approach have been evaluated on repeated SR-microCT scans of bovine cortical bone (voxel size: 1.6 µm), which were virtually deformed for different magnitudes and along different directions. The results showed that systematic and random errors of the normal strain components along the deformation direction were higher than the errors along unstrained directions. The systematic percentage errors were smaller for larger virtual deformations. The random percentage error was in the order of 10% of the virtual deformation. However, higher errors were localized at the boundary of the volumes of interest, perpendicular to the deformation direction. When only the central region of the samples was considered (100 µm layers removed from the borders where the deformation was applied), the errors in the direction of virtual deformation were comparable to the errors in the unstrained directions. In conclusion, the method presented to estimate the uncertainties of DVC is suitable for testing anisotropic specimens as cortical bone. The good agreement between the uncertainties in measurements of strain components obtained with this approach and with the simpler zero-strain-test suggests that the latter is adequate in the tested deformation scenarios.

High-resolution x-ray tomographic morphological characterisation of electrospun nanofibrous bundles for tendon and ligament regeneration and replacement.

Repair of ligaments and tendons requires scaffolds mimicking the spatial organisation of collagen in the natural tissue. Electrospinning is a promising technique to produce nanofibres of both resorbable and biostable polymers with desired structural and morphological features. The aim of this study was to perform high-resolution x-ray tomography (XCT) scans of bundles of Nylon6.6, pure PLLA and PLLA-Collagen blends, where the nanofibres were meant to have a predominant direction. Characterisation was carried out via a dedicated methodology to firmly hold the specimen during the scan and a workflow to quantify the directionality of the nanofibres in the bundle. XCT scans with 0.4 and 1.0 µm voxel size were successfully collected for all bundle compositions. Better image quality was achieved for those bundles formed by thicker nanofibres (i.e. 0.59 µm for pure PLLA), whereas partial volume effect was more pronounced for thinner nanofibres (i.e. 0.26 µm for Nylon6.6). As expected, the nanofibres had a predominant orientation along the axis of the bundles (more than 20% of the nanofibres within 3° and more than 60% within 18° from the bundle axis), with a Gaussian-like dispersion in the other directions. The directionality assessment was validated by comparison against a similar analysis performed on SEM images: the XCT analysis overestimated the amount of nanofibres very close to the bundle axis, especially for the materials with thinnest nanofibres, but adequately identified the amount of nanofibres within 12°. LAY DESCRIPTION: Repair of ligaments and tendons requires dedicated materials (scaffolds) mimicking the spatial organisation of the collagen (the main material composing such natural tissue). Electrospinning is a promising technique that allows production of fibres with nanometric dimension using high voltage to stretch very tiny drops of polymeric solutions. Electrospinning allows processing both polymers that can be resorbed by the host tissue, and nonresorbable ones, to obtain the desired structural and morphological features by arranging the nanofibres in bundles. The aim of this study was to perform high-resolution x-ray computed tomography (XCT) scans of bundles, where the nanofibres were meant to have a predominant direction. The investigation included bundles of different compositions: a biostable polymer (Nylon) and bioresorbable ones (pure Poly-L-lactic acid (PLLA) and PLLA-Collagen blends). The electrospun bundles were produced using a validated method (Sensini et al 2017: https://doi.org/10.1088/1758-5090/aa6204). To this end, we developed a dedicated methodology to scan such small specimens, and a workflow to quantify the directionality of the nanofibres in the bundle. For all the compositions, XCT scans with extremely high resolution (i.e. down to 0.4 µm) were successfully collected. As expected, better images were obtained for those bundles where the nanofibres were thicker than the scanning resolution (i.e. 0.59 µm for pure PLLA). The images of the thinnest nanofibres (i.e. 0.26 µm for Nylon) were poorer because the fibre diameter was smaller than the resolution (partial volume effect). The nanofibres had a predominant orientation along the axis of the bundles (more than 60% of the nanofibres were within 18° from the bundle axis). The nanofibres had a Gaussian-like dispersion in the other directions. As this is the first time that XCT is used to quantify the directionality of this kind of bundles, the directionality assessment was further validated by comparison against a similar analysis performed on SEM images. Overall, this study has demonstrated the usefulness and reliability of using high-resolution x-ray computed tomography (XCT) scans to investigate the morphology of polymeric scaffolds made of electrospun nanofibres.

Charge-separation enhancement in inverted polymer solar cells by molecular-level triple heterojunction: NiO-np:P3HT:PCBM.

Hole collection and transport are crucial physical processes in bulk-heterojunction (BHJ) solar cells, which represent major bottlenecks due to their limitations in power conversion efficiency (PCE). Hence, a more efficient alternative is needed to accept and transport holes to the collection electrode in BHJ solar cells. Here, we bring both electron and hole collection centres close to the point of exciton generation by infiltrating P3HT poly(3-hexylthiophene):PCBM ([6,6]-phenyl-C61-butyric acid methyl ester) blend into a highly porous interconnected p-type NiO-nanoparticle (NiO-np) network, through solvent-assisted grafting. In this study, a hybrid polymer solar cell is demonstrated with a P3HT:PCBM:NiO-np triple-heterojunction active layer which showed greatly improved rectification behaviour, long electron lifetime and generated higher PCE of 4% under AM 1.5 solar illumination with a 75% increase in PCE with respect to the P3HT:PCBM device. The optimum NiO-np amount and active-layer thickness were found to be 2% and 250 nm, respectively.

Simultaneous double rod and en-bloc direct vertebral rotation technique for correction of main thoracic adolescent idiopathic scoliosis: retrospective analysis of 14 cases.

Adolescent idiopathic scoliosis (AIS) is a triplanar deformity associated with rib hump, especially when a principle thoracic curve is present. The aim of this study is to evaluate the results of AIS correction retrospectively, using simultaneous double rod derotation manoeuvre technique followed by en-bloc direct vertebral rotation (DVR). Fourteen patients were included in this study. Coronal and sagittal thoracic Cobb angle, global coronal balance, sagittal balance, rib hump prominence, Scoliosis Research Society outcome instrument score (SRS-22) and Walter Reed visual assessment scale (WR-VAS) values were recorded pre- and postoperatively and evaluated. Results were evaluated at a mean follow-up of 2 years. Good to excellent radiographic and clinical results were obtained in all patients. No major perioperative complications occurred. This technique has proved to be effective for surgical correction of the deformity in Lenke type 1 AIS with good clinical and radiological results and low rate of complications.

Porphyrin conjugated SiC/SiOx nanowires for X-ray-excited photodynamic therapy.

The development of innovative nanosystems opens new perspectives for multidisciplinary applications at the frontier between materials science and nanomedicine. Here we present a novel hybrid nanosystem based on cytocompatible inorganic SiC/SiOx core/shell nanowires conjugated via click-chemistry procedures with an organic photosensitizer, a tetracarboxyphenyl porphyrin derivative. We show that this nanosystem is an efficient source of singlet oxygen for cell oxidative stress when irradiated with 6 MV X-Rays at low doses (0.4-2 Gy). The in-vitro clonogenic survival assay on lung adenocarcinoma cells shows that 12 days after irradiation at a dose of 2 Gy, the cell population is reduced by about 75% with respect to control cells. These results demonstrate that our approach is very efficient to enhance radiation therapy effects for cancer treatments.

Revealing the fast atomic motion of network glasses.

Still very little is known on the relaxation dynamics of glasses at the microscopic level due to the lack of experiments and theories. It is commonly believed that glasses are in a dynamical arrested state, with relaxation times too large to be observed on human time scales. Here we provide the experimental evidence that glasses display fast atomic rearrangements within a few minutes, even in the deep glassy state. Following the evolution of the structural relaxation in a sodium silicate glass, we find that this fast dynamics is accompanied by the absence of any detectable aging, suggesting a decoupling of the relaxation time and the viscosity in the glass. The relaxation time is strongly affected by the network structure with a marked increase at the mesoscopic scale associated with the ion-conducting pathways. Our results modify the conception of the glassy state and asks for a new microscopic theory.

Controlling the dynamics of a bidimensional gel above and below its percolation transition.

The morphology and the microscopic internal dynamics of a bidimensional gel formed by spontaneous aggregation of gold nanoparticles confined at the water surface are investigated by a suite of techniques, including grazing-incidence x-ray photon correlation spectroscopy (GI-XPCS). The range of concentrations studied spans across the percolation transition for the formation of the gel. The dynamical features observed by GI-XPCS are interpreted in view of the results of microscopic imaging; an intrinsic link between the mechanical modulus and internal dynamics is demonstrated for all the concentrations. Our work presents an example of a transition from a stretched to a compressed correlation function actively controlled by quasistatically varying the relevant thermodynamic variable. Moreover, by applying a model proposed some time ago by Duri and Cipelletti [Europhys. Lett. 76, 972 (2006)] we are able to build a master curve for the shape parameter, whose scaling factor allows us to quantify a "long-time displacement length." This characteristic length is shown to converge, as the concentration is increased, to the "short-time localization length" determined by pseudo-Debye-Waller analysis of the initial contrast. Finally, the intrinsic dynamics of the system is then compared with that induced by means of a delicate mechanical perturbation applied to the interface.

Heterogeneous and anisotropic dynamics of a 2D gel.

We report x-ray photon correlation spectroscopy (XPCS) results on bidimensional (2D) gels formed by a Langmuir monolayer of gold nanoparticles. The system allows an experimental determination of the fourth order time correlation function, which is compared to the usual second order correlation function and to the mechanical response measured on macroscopic scale. The observed dynamics is anisotropic, heterogeneous and superdiffusive on the nanoscale. Different time scales, associated with fast heterogeneous dynamics inside 2D cages and slower motion of larger parts of the film, can be identified from the correlation functions. The XPCS results are discussed in view of other experimental results and models of three-dimensional gel dynamics.

Accuracy of finite element predictions in sideways load configurations for the proximal human femur.

Subject-specific finite element models have been used to predict stress-state and fracture risk in individual patients. While many studies analysed quasi-axial loading configurations, only few works simulated sideways load configurations, such as those arising in a fall. The majority among these latter directly predicted bone strength, without assessing elastic strain prediction accuracy. The aim of the present work was to evaluate if a subject-specific finite element modelling technique from CT data that accurately predicted strains in quasi-axial loading configurations is suitable to accurately predict strains also when applying low magnitude loads in sideways configurations. To this aim, a combined numerical-experimental study was performed to compare finite element predicted strains with strain-gauge measurements from three cadaver proximal femurs instrumented with sixteen strain rosettes and tested non-destructively under twelve loading configurations, spanning a wide cone (0-30° for both adduction and internal rotation angles) of sideways fall scenarios. The results of the present study evidenced a satisfactory agreement between experimentally measured and predicted strains (R(2) greater than 0.9, RMSE% lower than 10%) and displacements. The achieved strain prediction accuracy is comparable to those obtained in state of the art studies in quasi-axial loading configurations. Still, the presence of the highest strain prediction errors (around 30%) in the lateral neck aspect would deserve attention in future studies targeting bone failure.

Biomechanical robustness of a new proximal epiphyseal hip replacement to patient variability and surgical uncertainties: a FE study.

The biomechanical behaviour of current hip epiphyseal replacements is notably sensitive to the typical variability of conditions following a standard surgery. The aim of the present study was to assess the biomechanical robustness to the variability of post-operative conditions of an innovative proximal epiphyseal replacement (PER) hip device featuring a short, curved and cemented stem. The risk of femoral neck fractures, prosthesis fractures and aseptic loosening were assessed through a validated finite element procedure following a systematic approach. Risk changes due to anatomical variations were assessed mimicking extreme conditions in terms of femoral size and level of osteoporosis. Failure risks associated with surgical uncertainties were assessed mimicking extreme conditions in terms of uncertainties on the prosthesis position/alignment, cement-bone interdigitation depth, and friction between the prosthesis and the hosting cavity. The femoral neck strength increased after implantation from 9% to 49% and was most sensitive to changes of the anatomo-physiological variables. The risk of stem fractures was low in all studied configurations. The risk of stem loosening was low and most sensitive to surgical uncertainties. In conclusion, the new device can be considered an effective alternative to current epiphyseal replacements. Care is recommended in a proper seating of the prosthesis in the femur.

A new hip epiphyseal prosthesis: design revision driven by a validated numerical procedure.

An innovative epiphyseal device has been recently proposed claiming an effective bone-prosthesis load transfer and a nearly physiological bone stresses distribution. However preliminary experimental tests showed a 23% weakening of the femoral neck after implantation. Aim of this study was to revise the prosthesis geometry with the goal of enhancing the femoral neck strength after implantation, while maintaining unchanged the initial conceptual design. To this aim, the risk of femoral neck fractures, prosthesis fractures, aseptic loosening and excessive bone resorption were addressed through a validated finite element procedure following a systematic approach. The initial prosthesis geometry was revised to reduce each investigated failure risk below the threshold of acceptance (100%). The new geometry was re-assessed to verify the effectiveness of the revision. The first design was predicted to locally induce high bone strains and cement stresses, which translated in a risk of bone and cement failure exceeding the threshold of acceptance (>100%). The revised design preserved a good stability of the device, contemporary reducing the risk for bone (45%) and cement (60%) failure. If results will be confirmed by statistical and clinical experimentations, current clinical indications for hip epiphyseal devices might be extended.

Extensive risk analysis of mechanical failure for an epiphyseal hip prothesis: a combined numerical-experimental approach.

There has been recent renewed interest in proximal femur epiphyseal replacement as an alternative to conventional total hip replacement. In many branches of engineering, risk analysis has proved to be an efficient tool for avoiding premature failures of innovative devices. An extensive risk analysis procedure has been developed for epiphyseal hip prostheses and the predictions of this method have been compared to the known clinical outcomes of a well-established contemporary design, namely hip resurfacing devices. Clinical scenarios leading to revision (i.e. loosening, neck fracture and failure of the prosthetic component) were associated with potential failure modes (i.e. overload, fatigue, wear, fibrotic tissue differentiation and bone remodelling). Driving parameters of the corresponding failure mode were identified together with their safe thresholds. For each failure mode, a failure criterion was identified and studied under the most relevant physiological loading conditions. All failure modes were investigated with the most suitable investigation tool, either numerical or experimental. Results showed a low risk for each failure scenario either in the immediate postoperative period or in the long term. These findings are in agreement with those reported by the majority of clinical studies for correctly implanted devices. Although further work is needed to confirm the predictions of this method, it was concluded that the proposed risk analysis procedure has the potential to increase the efficacy of preclinical validation protocols for new epiphyseal replacement devices.

Effect of long-term physiological activity on the long-term stem stability of cemented hip arthroplasty: in vitro comparison of three commercial bone cements.

Long-term endurance of the cement mantle is fundamental for the survival of cemented hip prostheses. Current protocols to characterize bone cements are unsuitable to predict the actual clinical outcome. The aim of this study was to assess if it is possible to rank cement types having diverse clinical outcome by using a simplified in vitro physiological test. Composite femurs were implanted with identical stems (Lubinus-SPII), using different commercial cement types: CMW1 to represent cement with poor clinical outcome; Simplex-P and Cemex-RX to represent cements with a positive clinical outcome. Implanted femurs were subjected to a validated protocol that simulated a demanding but physiological loading spectrum. Inducible micromotions and permanent migrations were recorded throughout the test. After test completion, the cement mantles were sectioned and inspected with dye penetrants to quantify the fatigue-induced cracks. Micromotions did not differ significantly between cement types (possibly because a successful prosthesis was chosen that is very stable in the host bone). Significant differences were observed in terms of cement cracks: CMW1 induced significantly more numerous and larger cracks than Simplex-P and Cemex-RX; no difference was observed between Simplex-P and Cemex-RX. This indicates that this protocol: (a) can discriminate between 'good' and 'bad' cements and (b) yields consistent results when comparable cements are tested. The proposed protocol overcomes the limitations of existing standardized material tests for bone cements. New cements can be assessed in comparison with other cements with known (positive/negative) clinical outcome, tested with the same protocol.

Doping-induced conductivity transitions in molecular layers of polyaniline: optical studies of electronic state changes.

The doping-induced conductivity transitions in molecular layers of polyaniline have been studied by monitoring the correlated optical and spectroscopic changes using spectroscopic and single wavelength extinction ellipsometry, also in total internal reflection mode (TIRE), together with reflection spectrometry. The measurements were performed on deposited multilayers as well as on a Langmuir monolayer at the air-water interface, as a function of acidic doping. We found that the characteristic spectroscopic features of conducting and insulating polyaniline persisted down to the single layer, both in the solid state and at the air-water interface. We also investigated in real time the modulation of conductivity induced by the intercalation of Li ions in the polyaniline film, by a combination of time-resolved ellipsometry and reflectivity spectra measurements. In this case, the enhanced sensitivity provided by the TIRE geometry, combined with the relatively fast time scale accessible by the single wavelength ellipsometry, allowed us to follow in detail in real time the doping/dedoping process.

Doping-induced conductivity transitions in molecular layers of polyaniline: detailed structural study.

We report detailed structural investigations, by synchrotron X-ray reflectivity (XRR), grazing incidence diffraction (GID), and space-resolved grazing incidence X-ray-induced fluorescence (GIXF), on the structure of molecular layers of polyaniline (PANI) that can be converted from insulating to conducting state simply by doping. We first address the simpler, but more intriguing, system, i.e., a floating monolayer of PANI on different subphases, for which we found a typical thickness of 28(1) A, not much affected by the doping process. For the doped film we also found an internal lateral structure, with in-plane spacing of 3.5 A-albeit with a small coherence of 3-4 repeat units only-compatible with face-to-face interchain stacking of phenyl rings, in agreement with the literature. By GIXF we could confirm the crucial role of Cl(-) intercalation in the doping process of the PANI film: under doping conditions (0.1 M HCl subphase) the Cl(-) intake is 8 times larger than in nondoping conditions (0.1 M KCl subphase). Multilayers transferred onto solid substrate were studied also as a function of the applied voltage, as this system constitutes the core of an electrochemically controlled device whose strongly nonlinear characteristic make it useful for applications to adaptive networks for complex information processing. By the application of an electrostatic field of 140 V/m, Cl ionic migration was observed confined to the polymeric film surface.

Effect of stem preheating on the fatigue behaviour of bone cement around hip prostheses.

Tensile fatigue behaviour of bone cement specimens obtained from cement mantles moulded in vitro, simulating the surgical scenario, was investigated. The effect of stem preheating, before its insertion into the cement dough, on specimen fatigue life was studied. A commercial bone cement was selected for this study. Bone cement mixing was conducted in air, following the manufacturer's instructions, and injected simulating the clinical practice. Two conditions were considered: stem maintained at the surgical room temperature (23 degrees C), and stem preheated to 45 degrees C. Four repetitions of the whole procedure were performed for each condition obtaining a total of 32 specimens. All specimens underwent fatigue testing (stress ratio, 0; maximum tensile stress, 15 MPa) until failure. Both two-parameter and three-parameter Weibull distributions were initially used to analyse the fatigue life data set. However, the two-parameter distribution was chosen for both groups on the basis of the coefficient of determination used to test the goodness of fit. Stem preheating seems to have a negligible effect on fatigue behaviour of the studied bone cement in the low range of fatigue lives (up to 10(4)). However, above this number of cycles, stem preheating seems to reduce the probability of failure. These findings are discussed in the text.