Numerical study of the hydrodynamic drag force in atomic force microscopy measurements undertaken in fluids

When atomic force microscopy (AFM) is employed for in vivo study of immersed biological samples, the fluid medium presents additional complexities, not least of which is the hydrodynamic drag force due to viscous friction of the cantilever with the liquid. This force should be considered when interpreting experimental results and any calculated material properties. In this paper, a numerical model is presented to study the influence of the drag force on experimental data obtained from AFM measurements using computational fluid dynamics (CFD) simulation. The model provides quantification of the drag force in AFM measurements of soft specimens in fluids. The numerical predictions were compared with experimental data obtained using AFM with a V-shaped cantilever fitted with a pyramidal tip. Tip velocities ranging from 1.05 to 105 µm/s were employed in water, polyethylene glycol and glycerol with the platform approaching from a distance of 6000 nm. The model was also compared with an existing analytical model. Good agreement was observed between numerical results, experiments and analytical predictions. Accurate predictions were obtained without the need for extrapolation of experimental data. In addition, the model can be employed over the range of tip geometries and velocities typically utilized in AFM measurements

Effect of femoral head size, subject weight and activity level on acetabular cement mantle stress following total hip arthroplasty

In cases where cemented components are used in total hip arthroplasty, damage or disruption of the cement mantle can lead to aseptic loosening and joint failure. Currently, the relationship between subject activity level, obesity and prosthetic femoral head size and the risk of aseptic loosening of the acetabular component in cemented THA is not well understood. This study aims to provide an insight into this. Finite element models, validated with experimental data, were developed to investigate stresses in the acetabular cement mantle and pelvic bone resulting from the use of three prosthetic femoral head sizes, during a variety of daily activities and one high impact activity (stumbling) for a range of subject body weights. We found that stresses in the superior quadrants of the cortical bone‐cement interface increased with prosthetic head size, patient weight and activity level. In stumbling, average von Mises stresses (22.4 MPa) exceeded the bone cement yield strength for an obese subject (143 kg) indicating that the cement mantle would fail. Our results support the view that obesity and activity level are potential risk factors for aseptic loosening of the acetabular component and provide insight into the increased risk of joint failure associated with larger prosthetic femoral heads

Experimental validation for chatter stability prediction

This research focused on the experimental validation for chatter stability prediction. An optimum machining was aimed to maximize the material removal rate, whilst maintaining a sufficient stability margin to assure the surface quality. High material removal rate in machining produced self-excited vibration or chatter of the cutting tool and the workpiece. This resulted in a poor surface finish and dimensional accuracy, chipping of the cutter teeth, and also may damage the workpiece as well as machining tool. Frequency response function of a single degree freedom flexural was measured and the cutting stiffness of tools were determined in order to be used in predicting chatter stability using semi discretization method. The aluminium 7075 specimens were used in the milling cutting experiment to validate the chatter stability diagram of mill uniform and variable cutters, where a set of spindle speed and depth of cut had tested. The vibration conditions of machining were identified by analysing the vibration signals and FFT spectrum whether it was stable or in a chatter condition. There are good agreement between predicted stability and cutting experiment for the down-milling operation using uniform 4 flute cutting tool. Stable conditions were shown outside the boundary of chatter region. The optimized cutting tool was predicted to suppress chatter. Machining experiment tests showed there were no chatter vibration conditions during machining process until 1.5 mm depth of cut. According to the results of machining experiment, it was proven that the variable tool had more capability to machining without producing chatter vibration as compared to the regular tool

Maintenance processes modelling and optimisation

A Maintenance Procedure is conducted in order to prevent the failure of a system or to restore the functionality of a failed system. Such a procedure consists of a series of tasks, each of which has a distribution of times to complete and a probability of being performed incorrectly. The inclusion of tests can be used to identify any maintenance errors which have occurred. When an error is identified it can be addressed through a corresponding correction sequence which will have associated costs and add to the maintenance process completion time. A modified FMEA approach has been used to identify the possible tests. By incorporating any selection of tests into the maintenance process it can then analysed using a discrete-event simulation to predict the expected completion time distribution. The choice of tests to perform and when to do them is then made to successfully complete the maintenance objective in the shortest possible time using a genetic algorithm. The methodology is demonstrated by applying it to the repair process for a car braking system. The developed method is suitable for application in abroad range of industries

Zeroth-order finite similitude and scaling of complex geometries in biomechanical experimentation

Scaled experimentation provides an alternative approach to full-scale biomechanical (and biological) testing but is known to suffer from scale effects, where the underlying system behaviour changes with scale. This phenomenon is arguably the overriding principal obstacle to the many advantages that scaled experimentation provides. These include reduced costs, materials and time, along with the eschewal of ethical compliance concerns with the application of substitute artificial materials as opposed to the use of hazardous biological agents. This paper examines the role scale effects play in biomechanical experimentation involving strain measurement and introduces a formulation that overtly captures scale dependencies arising from geometrical change. The basic idea underpinning the new scaling approach is the concept of space scaling, where a biomechanical experiment is scaled by the metaphysical mechanism of space contraction. The scaling approach is verified and validated with finite-element (FE) models and actual physical-trial experimentation using digital image correlation software applied to synthetic composite bone. The experimental design aspect of the approach allows for the selection of three-dimensional printing materials for trial-space analysis in a complex pelvis geometry. This aspect takes advantage of recent advancements in additive manufacturing technologies with the objective of countering behavioural distorting scale effects. Analysis is carried out using a laser confocal microscope to compare the trial and physical space materials and subsequently measured using surface roughness parameters. FE models were constructed for the left hemipelvis and results show similar strain patterns (average percentage error less than 10%) for two of the three trial-space material combinations. A Bland–Altman statistical analysis shows a good agreement between the FE models and physical experimentation and a good agreement between the physical-trial experimentation, providing good supporting evidence of the applicability of the new scaling approach in a wider range of experiments

Unraveling the crosstalk between cell sheets of human adipose stem cells and keratinocytes

We have previously shown that 3D constructs of human adipose stem cells (hASCs) cell sheets led to the formation of de novo hair follicles and rete-ridges like structures. An up-regulation of keratinocyte growth factor (KGF) was also observed in the experimental condition in relation to the control groups. We hypothesized that the natural adhesive character of the cell sheets promoted the direct interaction between the host and the transplanted cells1. In this sense, the present work aims at elucidating this communication between hASCs and human keratinocytes (hKC) and determining the extent of its mediation by KGF. In an in vitro scratch assays we showed that the secretome of hASCs in contact with hKC promotes cell migration and closure of the scrape. Moreover, when KGF-antibody was added diminished hKC migration was observed, suggesting that KGF might be one of the key cytokines involved in the interaction with hASCs. Furthermore, in order to assess the communication via gap junctions (GJ), a calcein-AM transfer assay was carried out in the presence/absence of a GJ inhibitor. The transference of the dye from hASCs to adjacent hKC, confirmed both by fluorescence microscopy and flow cytometry, showed that these cells also communicate via GJ. Moreover while hKCs expressed connexin (cx)43 and cx26, highly expressed at the wound margins, hASCs were only positive for cx43  as shown by immunocytochemistry and flow cytometry. Finally, the direct communication between transplanted hASCs cell sheets and hKC at the wound margins is being addressed in a human ex-vivo skin model with an artificial wound, to better mimic the previous in vivo conditions and confirm our hypothesis. So far we were able to demonstrate that hASCs and hKCs communicate directly through cx43 and indirectly via KGF secreted by hASCs, which promote KCs migration. (1)       Cerqueira, M. T.; Pirraco, R. P.; Santos, T. C.; Rodrigues, D. B.; Frias, A. M.; Martins, A. R.; Reis, R. L.; Marques, A. P.; Cerqueira   Pirraco, RP,  Santos, TC,  Frias, AM,  Martins, AR,  Reis, RL, Marques, AP, M. T. Biomacromolecules 2013, 14, 3997â 4008.    Portuguese Foundation for Science and Technology (FCT) for MTC (SFRH/BPD/96611/2013),RPP(SFRH/BPD/101886/2014)info:eu-repo/semantics/publishedVersio

Design for Product Service Supportability (DfPSS) Approach: A State of the Art to Foster Product Service System (PSS) Design

Product-Service System lifecycle is characterized by several phases from the initial concept to the final disposal. However, as for conventional products, the profit generation and the market success of PSSs critically depend on the decisions taken during the initial lifecycle stages, when PSSs are conceptualized, designed, developed and engineered. These are hence the phases deserving more attention in order to manage the intrinsic complexity of such systems, taking it in account during the entire PSS life cycle design phase. According to this, one of the main gaps detected in the PSS design process is the lack of methods able to support the early integration of service features during the product design. In this specific context DfX approaches, where X= x-bility stands for enhancing products design considering at the same time service features to be embedded on it (x) according to certain performance measures (-bility), are supposed to significantly contribute. The Serviceability point of view appears to be a critical aspect of the design of product-oriented PSS that has not been improving yet: significant enhancement in this products' characteristic will only occur if some changes will arise in the way they are designed. Indeed companies still need guidelines able to enhance the PSS design process in a more systematic way. On this basis, due to the main gap of integrating service features in the product design process, the paper presents and defines DfX approaches enlightening, among the several target properties they have been called to improve so far, the most suitable DfX streams detected to solve the reported PSS design issue and to define Design for Product Service Supportability (DfPSS)