A Voxel-based Formulation for Contact Finite Element Analysis

To date, voxel-based finite element models have not been feasible for contact problems, owing to the inherent stair-step boundary discontinuities. New preprocessing techniques are reported herein to mesh these boundaries smoothly, for purposes of contact stress analysis. Further, new techniques are reported to concentrate the mesh resolution automatically near the articular surface, thus reducing the problem size to levels compatible with executing nonlinear problems on contemporary engineering workstations. Close approximations to Hertzian analytical solutions were obtained for spherical and cylindrical geometries meshed in this manner, and an illustrative anatomical contact problem of the human hip joint is presented.     

A critical assessment of standard processing methods for the preparation of palynological samples

Standard processing techniques for the isolation of organic walled dinoflagellate cysts from geological samples are examined, with particular attention to the size and type of sieve mesh used. Variations within the ‘standard’ processing techniques used by different laboratories are identified, and an assessment of the retention capacities of meshes of different sizes and different materials is carried out. Some dinoflagellate cysts and large numbers of Lycopodium spores, used for the calculations of absolute abundance data, were found to pass through 20 μm meshes. This is due to a combination of factors including: the diagonal aperture diameter of a 20 μm mesh measuring over 28 μm; the three-dimensional properties of different mesh weaves (nylon and polyester); and the non-spherical shape of the particles. Experiments demonstrate that the maximum mesh size that should be used in palynological processing is 15 μm. Nylon mesh is more practical to use than polyester as processing time is reduced, but nylon is degraded by contact with acid solutions. Meshes with apertures < 15 μm may be used, though this may be impractical for large samples containing significant quantities of fine siliciclastic or organic material.     

Discretization error when using finite element models: Analysis and evaluation of an underestimated problem

Mesh convergence tests are often insufficiently performed in finite element analyses. There are many parameters which may have an effect on the mesh convergence behavior. The aim of this study was to identify the influence of different parameters on the mesh convergence behavior.For this purpose we used a simplified axis-symmetrical model of a single pedicle screw flank with surrounding bone to simulate a pull-out test. In parameter studies, the flank radii and the contact conditions at the bone–screw interface were varied. These parameter studies were carried out using an implicit and explicit solver. Thereby, the convergence criteria and the number of the substeps for the implicit nonlinear iteration process as well as the velocity and the material density for the explicit approach were considered.The mesh convergence behavior was influenced by varying the flank radii and the contact conditions. The implicit calculations led to a reaction force, which converged rapidly to a certain value with increasing mesh density, whereas the maximum von-Mises stress showed substantial convergence problems. The number of substeps and the convergence criteria of the iteration process strongly influenced the implicit solutions. In contrast, the maximum von-Mises stresses resulting from explicit calculations converged to a certain value after only a few refinement steps. Different pull-out velocities substantially affected the mesh convergence behavior, while the material density showed only a negligible influence.The results indicated the need to perform an appropriate mesh convergence test when using finite element methods. We were able to show that different parameters strongly influence the mesh convergence behavior and we demonstrated that convergence tests do not always lead to a satisfactory or acceptable solution.     

Fabrication of Bioinspired Structured Superhydrophobic and Superoleophilic Copper Mesh for Efficient Oil-water Separation

Oily water treatment has attracted the attention of many researchers.The development of effective and cheap oil/water separation materials is urgent for treating this problem.Herein,inspired by superhydrophobic typical plant leaves such as lotus,red rose and marigold,superhydrophobic and superoleophilic copper mesh was fabricated by etching and then surface modification with 1-dodecanethiol (HS(CH2)11CH3).A rough silver layer is formed on the mesh surface after immersion.The obtained mesh surface exhibits superhydrophobicity and superoleophilicity and the static water contact angle was 153° ± 3°.In addition,the as-prepared copper mesh shows self-cleaning character with water and chemical stability.The as-prepared copper foam can easily remove the organic solvents either on water or underwater.We demonstrate that by using the as-prepared mesh,oils can be absorbed and separated,and that high separation efficiencies of larger than 92％ are retained for various oils.Thus,such superhydrophobic and superoleophilic copper mesh is a very promising material for the application of oil spill cleanup and industrial oily wastewater treatment.     

Formation of adhesions at surgical meshes in a rat experimental model

Abdominal wall hernias are surgical problem that are easily solved with laparoscopic surgery. The determining factor for the success of the operation is the right choice and use of surgical mesh as the support material. The most common complication of surgical mesh placement is the formation of adhesions. Aim of this paper is to determine whether there is a statistic difference in formation of adhesions between different surgical meshes in lab environment. Wistar rats were used as the experimental model. After the anaesthesia a 1x1 cm defect of the abdominal wall was made, but the skin was left intact. The mesh was placed directly on the internal organs. The experiment considered four different mesh types. After set time periods of one, two or four weeks the animals were sacrificed and the amount of formed adhesions were evaluated based on the modified Diamond scale. Immediately after the first week we found a statistically significant difference in the adhesion occurrence rate between compared materials. The smallest amount of adhesions was caused by polypropylen + polydoksanon mesh, and the most by polypropilen mesh. Polypropylen + polyglactin mesh showed significant reduction of adhesion formation between the tested weeks. We can conclude that polypropylen + polydoxanon meshes are superior for ventral hernia operation, because those defects are in close contact with the internal organs and it is very important to have the smallest amount of adhesions.     

On foundations of discrete element analysis of contact in diarthrodial joints

Information about the stress distribution on contact surfaces of adjacent bones is indispensable for analysis of arthritis, bone fracture and remodeling. Numerical solution of the contact problem based on the classical approaches of solid mechanics is sophisticated and time-consuming. However, the solution can be essentially simplified on the following physical grounds. The bone contact surfaces are covered with a layer of articular cartilage, which is a soft tissue as compared to the hard bone. The latter allows ignoring the bone compliance in analysis of the contact problem, i.e. rigid bones are considered to interact through a compliant cartilage. Moreover, cartilage shear stresses and strains can be ignored because of the negligible friction between contacting cartilage layers. Thus, the cartilage can be approximated by a set of unilateral compressive springs normal to the bone surface. The forces in the springs can be computed from the equilibrium equations iteratively accounting for the changing contact area. This is the essence of the discrete element analysis (DEA). Despite the success in applications of DEA to various bone contact problems, its classical formulation required experimental validation because the springs approximating the cartilage were assumed linear while the real articular cartilage exhibited non-linear mechanical response in reported tests. Recent experimental results of Ateshian and his co-workers allow for revisiting the classical DEA formulation and establishing the limits of its applicability. In the present work, it is shown that the linear spring model is remarkably valid within a wide range of large deformations of the cartilage. It is also shown how to extend the classical DEA to the case of strong nonlinearity if necessary.     

Sensitivity-based adaptive mesh refinement collocation method for dynamic optimization of chemical and biochemical processes

Collocation on finite element (CFE) is an effective simultaneous method of dynamic optimization to increase the profitability or productivity of industrial process. The approach needs to select an optimal mesh of time interval to balance the computational cost with desired solution. A new CFE approach with non-uniform refinement procedure based on the sensitivity analysis for dynamic optimization problems is, therefore, proposed, where a subinterval is further refined if the obtained control parameters have significant effect on the performance index. To improve the efficiency, the sensitivities of state parameters with respect to control parameters are derived from the solution of the discretized dynamic system. The proposed method is illustrated by testing two classic dynamic optimization problems from chemical and biochemical engineering. The detailed comparisons among the proposed method, the CFE with uniform mesh, and other reported methods are also carried out. The research results reveal the effectiveness of the proposed approach.

     

Management and sperm production of boars under differing environmental conditions

The management of boars to ensure good sperm production under differing environmental conditions is a major concern for pig keepers in both tropical countries and countries where there are extreme environmental changes. Such changes create stress in animals and influence the production of spermatozoa. High temperatures during hot summer months may result in lower feed consumption and create stresses that result in the inhibition of spermatogenesis. Although tropical countries do not have a problem with major variations in day length, this can cause problems such as decreased litter size and infertility in other regions of the world. Evaporative cooling systems built into boar accommodation are often used to reduce fluctuations in both temperature and humidity during the hot and humid months seen in tropical countries. The system has become popular in AI boar studs, where it is reported to reduce stress and improve feed consumption. Other management factors, such as housing comfort, social contact, mating conditions and the frequency of mating, are also very important boar management aids that assist good quality semen production; these will be covered briefly in this review. This review will consider primarily those management factors, for example, the management of temperature and humidity using evaporative cooling systems and other techniques that enable AI boar studs to maximize sperm fertility through adjustments to the environment.     

Post-normalization quality assessment visualization of microarray data

Post-normalization checking of microarrays rarely occurs, despite the problems that using unreliable data for inference can cause. This paper considers a number of different ways to check microarrays after normalization for a variety of potential problems. Four types of problem with microarray data that these checks can identify are: clerical mistakes, array-wide hybridization problems, problems with normalization and mishandling problems. Any of these can seriously affect the results of any analysis. The three main techniques used to identify these problems are dimension reduction techniques, false array plots and correlograms. None of the techniques are computationally very intensive and all can be carried out in the R statistical package. Once discovered, problems can either be rectified or excluded from the data.     

生物系统学面临的难题

随着对生物多样性和可持续发展的全球性关注,对生物系统学的需求愈来愈大。但许多决策者认为生物系统学是一门古老的学科,谁都可以成为分类学家。这严重地影响了生物系统学的发展。论述生物系统学面临的重大科学问题。包括概念、目标和技术等问题,有利于澄清对生物系统学的一些误解,推进学科的发展,为发挥生物系统学的基础与支撑作用营造一个健康的环境。本文提出了生物系统学的六个科学难题：物种定义是概念难题,物种数量和物种间的相互关系是跨世纪的目标难题,信息管理和鉴定手段是两上技术问题,生物系统学家逐渐减少是一个社会性难题。     

A contact model with ingrowth control for bone remodelling around cementless stems.

This work presents a computational model for bone remodelling around cementless stems. The problem is formulated as a material optimisation problem considering the bone and stem surfaces to be in contact. To emphasise the behaviour of the bone/stem interface, the computer model detects the existence of bone ingrowth during the remodelling; consequently, the contact conditions are changed for a better interface simulation. The trabecular bone is modelled as a strictly orthotropic material with equivalent properties computed by homogenisation. The distribution of bone relative density is obtained by the minimisation of a function that considers both the bone structural stiffness and the biological cost associated with metabolic maintenance of bone tissue. The situation of multiple load conditions is considered. The remodelling law, obtained from the necessary conditions for an optimum, is derived analytically from the optimisation problem and solved numerically using a suitable finite element mesh. The formulation is applied to an implanted femur. Results of bone density and ingrowth distribution are obtained for different coating conditions. Bone ingrowth does not occur over the entire coated surfaces. Indeed, we observed regions where separation or high relative displacement occurs that preclude bone ingrowth attachment. This prediction of the model is consistent with clinical observations of bone ingrowth. Thus, this model, which detect bone ingrowth and allow modification of the interface conditions, are useful for analysis of existing stems as well as design optimisation of coating extent and location on such stems.     

Prediction of femoral head collapse in osteonecrosis

The femoral head deteriorates in osteonecrosis. As a consequence of that, the cortical shell of the femoral head can buckle into the cancellous bone supporting it. In order to examine the buckling scenario we performed numerical analysis of a realistic femoral head model. The analysis included a solution of the hip contact problem, which provided the contact pressure distribution, and subsequent buckling simulation based on the given contact pressure. The contact problem was solved iteratively by approximating the cartilage by a discrete set of unilateral linear springs. The buckling calculations were based on a finite element mesh with brick elements for the cancellous bone and shell elements for the cortical shell. Results of 144 simulations for a variety of geometrical, material, and loading parameters strengthen the buckling scenario. They, particularly, show that the normal cancellous bone serves as a strong supporting foundation for the cortical shell and prevents it from buckling. However, under the development of osteonecrosis the deteriorating cancellous bone is unable to prevent the cortical shell from buckling and the critical pressure decreases with the decreasing Young modulus of the cancellous bone. The local buckling of the cortical shell seems to be the driving force of the progressive fracturing of the femoral head leading to its entire collapse. The buckling analysis provides an additional criterion of the femoral head collapse, the critical contact pressure. The buckling scenario also suggests a new argument in speculating on the femoral head reinforcement. If the entire collapse of the femoral head starts with the buckling of the cortical shell then it is reasonable to place the reinforcement as close to the cortical shell as possible.     

Is full physical contact necessary for buffering effects of pair housing on social stress in rats?

Our previous study showed that pair housing with a familiar male prevented an increase in anxiety caused by social defeat in male rats. The present study attempted to identify the aspects of social interactions with a familiar male that are needed for the emergence of such a pair-housing effect. In Experiment 1, the subject was repeatedly exposed to the cage and bedding used by a familiar pairmate, after two instances of social defeat. Mere exposure to the soiled cage and bedding did not prevent an increase in anxiety in the elevated plus-maze test performed two weeks after social defeat. In Experiment 2, the subject was separated from a familiar pairmate with a wire mesh partition, which allowed visual, auditory, and limited physical contact, in addition to olfactory contact with the pairmate. The separation with a wire mesh partition abolished the buffering effect of pair housing on anxiety. These results indicate that visual, auditory, and olfactory contact with a familiar male was not sufficient in reducing the anxiogenic effect of social defeat in male rats. It was suggested that full physical contact is necessary for the emergence of the buffering effect of pair housing on social stress.     

Operant conditioning of small birds for acoustic discrimination

Standard operant conditioning techniques have been successfully applied to the problem of call discrimination in small birds. Budgerigars (Melopsittacus undulatus) and canaries (Serinus canarius) were trained with a GO/NOGO procedure using positive reinforcement to discriminate between both conspecific contact calls and contact calls of the other species. Budgerigars showed equal facility at each task. By contrast, canaries learned the task involving conspecific contact calls but failed, as a group, to discriminate between budgerigar contact calls. These results show that conditioning techniques can be used to study perceptual learning of species typical calls in small birds.     

Direct comparison of muscle force predictions using linear and nonlinear programming

Estimating forces in muscles and joints during locomotion requires formulations consistent with available methods of solving the indeterminate problem. Direct comparisons of results between differing optimization methods proposed in the literature have been difficult owing to widely varying model formulations, algorithms, input data, and other factors. We present an application of a new optimization program which includes linear and nonlinear techniques allowing a variety of cost functions and greater flexibility in problem formulation. Unified solution methods such as the one demonstrated here, offer direct evaluations of such factors as optimization criteria and constraints. This unified method demonstrates that nonlinear formulations (of the sort reported) allow more synergistic activity and in contrast to linear formulations, allow antagonistic activity. Concurrence of EMG activity and predicted forces is better with nonlinear predictions than linear predictions. The prediction of synergistic and antagonistic activity expectedly leads to higher joint force predictions. Relaxation of the requirement that muscles resolve the entire intersegmental moment maintains muscle synergism in the nonlinear formulation while relieving muscle antagonism and reducing the predicted joint contact force. Such unified methods allow more possibilities for exploring new optimization formulations, and in comparing the solutions to previously reported formulations.     

Determination of elastic moduli of thin layers of soft material using the atomic force microscope

We address three problems that limit the use of the atomic force microscope when measuring elastic moduli of soft materials at microscopic scales. The first concerns the use of sharp cantilever tips, which typically induce local strains that far exceed the linear material regime. We show that this problem can be alleviated by using microspheres as probes, and we establish the criteria for their use. The second relates to the common use of the Hertz contact mechanics model, which leads to significant errors when applied to thin samples. We develop novel, simple to use corrections to apply for such cases. Samples that are either bonded or not bonded to a rigid substrate are considered. The third problem concerns the difficulty in establishing when contact occurs on a soft material. We obtain error estimates for the elastic modulus resulting from such uncertainty and discuss the sensitivity of the estimation methods to error in contact point. The theoretical and experimental results are compared to macroscopic measurements on poly(vinyl-alcohol) gels.     

Enhancing interacting residue prediction with integrated contact matrix prediction in protein-protein interaction

Identifying the residues in a protein that are involved in protein-protein interaction and identifying the contact matrix for a pair of interacting proteins are two computational tasks at different levels of an in-depth analysis of protein-protein interaction. Various methods for solving these two problems have been reported in the literature. However, the interacting residue prediction and contact matrix prediction were handled by and large independently in those existing methods, though intuitively good prediction of interacting residues will help with predicting the contact matrix. In this work, we developed a novel protein interacting residue prediction system, contact matrix-interaction profile hidden Markov model (CM-ipHMM), with the integration of contact matrix prediction and the ipHMM interaction residue prediction. We propose to leverage what is learned from the contact matrix prediction and utilize the predicted contact matrix as “feedback” to enhance the interaction residue prediction. The CM-ipHMM model showed significant improvement over the previous method that uses the ipHMM for predicting interaction residues only. It indicates that the downstream contact matrix prediction could help the interaction site prediction.     

High-performance liquid chromatography of unmodified rosin and its applications in contact dermatology

Rosin is a well recognised skin sensitiser and is also amongst the most common causes of occupational asthma. Due to its complex chemical composition, it is difficult to isolate its many components and this has hindered progress in the identification of the specific respiratory and contact allergens it contains. This paper reports the application of high-performance liquid chromatography and other analytical techniques to the isolation and identification of contact allergens in complex mixtures such as rosin. HPLC methods were developed in order to isolate as many rosin components as possible and these were then patch tested on rosin sensitive individuals. The structure of the most dermatologically active component was then determined using mass spectrometry, nuclear magnetic resonance and infrared techniques. An HPLC method has also been developed which will enable the identification of rosin in commercial products, providing a valuable tool for determining the cause of rosin contact allergy. Furthermore, mass spectral data for the common abieitic-type resin acids are compiled which were used to confirm the identification of the HPLC resin acid peaks and have not been reported previously.     

Effect of meniscus replacement fixation technique on restoration of knee contact mechanics and stability

The menisci are important biomechanical components of the knee. We developed and validated a finite element model of meniscal replacement to assess the effect of surgical fixation technique on contact behavior and knee stability. The geometry of femoral and tibial articular cartilage and menisci was segmented from magnetic resonance images of a normal cadaver knee using MIMICS (Materialise, Leuven, Belgium). A finite element mesh was generated using HyperWorks (Altair Inc, Santa Ana, CA). A finite element solver (Abaqus v6.9, Simulia, Providence, RI) was used to compute contact area and stresses under axial loading and to assess stability (reaction force generated during anteroposterior translation of the femur). The natural and surgical attachments of the meniscal horns and peripheral rim were simulated using springs. After total meniscectomy, femoral contact area decreased by 26% with a concomitant increase in average contact stresses (36%) and peak contact stresses (33%). Replacing the meniscus without suturing the horns did little to restore femoral contact area. Suturing the horns increased contact area and reduced peak contact stresses. Increasing suture stiffness correlated with increased meniscal contact stresses as a greater proportion of tibiofemoral load was transferred to the meniscus. A small incremental benefit was seen of simulated bone plug fixation over the suture construct with the highest stiffness (50 N/mm). Suturing the rim did little to change contact conditions. The nominal anteroposterior stiffness reduced by 3.1 N/mm after meniscectomy. In contrast to contact area and stress, stiffness of the horn fixation sutures had a smaller effect on anteroposterior stability. On the other hand suturing the rim of the meniscus affected anteroposterior stability to a much larger degree. This model emphasizes the importance of the meniscus in knee biomechanics. Appropriate meniscal replacement fixation techniques are likely to be critical to the clinical success of meniscal replacement. While contact conditions are mainly sensitive to meniscus horn fixation, the stability of the knee under anteroposterior shear loads appeared to be more sensitive to meniscal rim fixation. This model may also be useful in predicting the effect of biomaterial mechanical properties and meniscal replacement shape on knee contact conditions.