The effects of cold arm width and metal deposition on the performance of a U-Beam electrothermal MEMS microgripper for biomedical applications

Microelectromechanical systems (MEMS) have established themselves within various fields dominated by high-precision micromanipulation, with the most distinguished sectors being the microassembly, micromanufacturing and biomedical ones. This paper presents a horizontal electrothermally actuated 'hot and cold arm' microgripper design to be used for the deformability study of human red blood cells (RBCs). In this study, the width and layer composition of the cold arm are varied to investigate the effects of dimensional and material variation of the cold arm on the resulting temperature distribution, and ultimately on the achieved lateral displacement at the microgripper arm tips. The cold arm widths investigated are 14 μm, 30 μm, 55 μm, 70 μm and 100 μm. A gold layer with a thin chromium adhesion promoter layer is deposited on the top surface of each of these cold arms to study its effect on the performance of the microgripper. The resultant ten microgripper design variants are fabricated using a commercially available MEMS fabrication technology known as a silicon-on-insulator multi-user MEMS process (SOIMUMPs)TM. This process results in an overhanging 25 μm thick single crystal silicon microgripper structure having a low aspect ratio (width:thickness) value compared to surface micromachined structures where structural thicknesses are of the order of 2 μm. Finite element analysis was used to numerically model the microgripper structures and coupled electrothermomechanical simulations were implemented in CoventorWare ®. The numerical simulations took into account the temperature dependency of the coefficient of thermal expansion, the thermal conductivity and the electrical conductivity properties in order to achieve more reliable results. The fabricated microgrippers were actuated under atmospheric pressure and the experimental results achieved through optical microscopy studies conformed with those predicted by the numerical models. The gap opening and the temperature rise at the cell gripping zone were also compared for the different microgripper structures in this work, with the aim of identifying an optimal microgripper design for the deformability characterisation of RBCs.peer-reviewe

Simplified thermo-elastoplastic numerical modelling techniques applied to friction stir welding of mild steel

Friction stir welding is a relatively new advanced joining technique that requires minimal power input, ultimately leading to less inherent residual stresses and distortion. The process involves a spinning tool which first plunges into the surface of the, to be welded assembly and then traverses along the joint. Frictional heat is generated, softening the material at temperatures significantly below the melting temperature of the parent material. As the tool traverses along the joint at a predetermined speed, the assembly is joined by means of a plastic straining process. This advanced welding technology has been validated for various aluminium alloys but it is only recently, due to advances in tool technology, that the possibility of joining mild steel using friction stir welding has become a viable option. This study looks into friction stir welding of mild steel and develops simplified numerical methods for the prediction of thermal gradients, residual stresses and deformation. In principle the process modelling requires a multi-disciplinary approach involving coupled thermo-fluid, microstructural-structural modelling process. Much of the latest thermo-mechanical studies of friction stir welding rely on a number of over simplifications particularly related to the heat flux distribution across the tool shoulder, and also on the backing plate boundary conditions. The objective of this paper is to scrutinise the effects of modelling in more detail and establish the most important factors leading to accurate yet computationally efficient prediction of thermal gradients and inherent residual stresses. The results show that both the heat input and heat loss modelling, due to heat dissipation to the surroundings, are crucial for the determination of the final inherent welding residual stresses. The heat generated is modelled through a predefined linear heat flux variation across the tool shoulder. However if a more precise and localized residual stress information is sought, a full thermo-fluid-structural analysis is required. This is time consuming and probably does not give significant information on manufacturing optimization. On the other hand, simplified global solutions offer the possibility to optimise friction stir welding parameters and boundary conditions during the preliminary stages of the development of the fabrication procedures, at relatively minimal time and processing power. This work is financed under the European Commission in Call FP7-SST-2012-RTD-1 High Integrity Low Distortion Assembly (HILDA) project.This study is being funded by the European Commission in Call FP7-SST-2012-RTD-1 under the project titled High Integrity Low Distortion Assembly Hilda. The authors would also like to acknowledge the project partners in particular Mr. Stephen Cater at TWI for providing the FSW parameters and setup configuration together with Dr. Alex Galloway the project coordinator of the HILDA project. This work reflects only the authors views and the European Union is not liable for any use that may be made of the information contained therein.peer-reviewe

Design and analysis of a MEMS-based electrothermal microgripper

Microelectromechanical systems (MEMS) have established themselves in various science and engineering domains. MEMS-based microgrippers provide several advantages in terms of compact size and low cost, and they play vital roles in microassembly and micromanipulation fields within both micromanufacturing and biological sectors. Microactuators based on different actuation principles have been devised to drive MEMS microgrippers. This paper presents a finite element model of a MEMS-based electrothermally actuated microgripper performed using CoventorWare ®. The microgripper design follows standard micromachining processes that make use of reactive ion etching where polysilicon acts as the main structural material while a chromium and gold layer is deposited on the structure for thermal actuation. The simulations are used to assess the performance of the microgripper and to optimise its operating parameters.peer-reviewe

Analytical, numerical and experimental study of a horizontal electrothermal MEMS microgripper for the deformability characterisation of human red blood cells

Microgrippers are typical microelectromechanical systems (MEMS) that are widely used for micromanipulation and microassembly in both biological and micromanufacturing fields. This paper presents the design, modelling, fabrication and experimental testing of an electrothermal microgripper based on a ‘hot and cold arm’ actuator design that is suitable for the deformability characterisation of human red blood cells (RBCs). The analysis of the mechanical properties of human RBCs is of great interest in the field of medicine as pathological alterations in the deformability characteristics of RBCs have been linked to a number of diseases. The study of the microgripper’s steady-state performance is initially carried out by the development of a lumped analytical model, followed by a numerical model established in CoventorWare® (Coventor, Inc., Cary, NC, USA) using multiphysics finite element analysis. Both analytical and numerical models are based on an electothermomechanical analysis, and take into account the internal heat generation due to the applied potential, as well as conduction heat losses through both the anchor pads and the air gap to the substrate. The models are used to investigate key factors of the actuator’s performance including temperature distribution, deflection and stresses based on an elastic analysis of structures. Results show that analytical and numerical values for temperature and deflection are in good agreement. The analytical and computational models are then validated experimentally using a polysilicon microgripper fabricated by the standard surface micromachining process, PolyMUMPs™ (Durham, NC, USA). The microgripper’s actuation is characterised at atmospheric pressure by optical microscopy studies. Experimental results for the deflection of the microgripper arm tips are found to be in good agreement with the analytical and numerical results, with process-induced variations and the non-linear temperature dependence of the material properties accounting for the slight discrepancies observed. The microgripper is shown to actuate to a maximum opening displacement of 9 μ m at an applied voltage of 3 V, thus being in line with the design requirement of an approximate opening of 8 μ m for securing and characterising a RBC.peer-reviewe

Preliminary studies on an innovative vertical axis wind turbine concept : SATVAWT

Vertical axis wind turbine concepts are particularly attractive for the urban environment for various reasons including adaptability to varying wind directions. On the other hand, these types of turbines suffer from various disadvantages including prohibitive costs. In this paper, we present an innovative concept of a Self Adjusting Lift Type Vertical Axis Wind Turbine (SATVAWT) being developed by the a consortium of local academia and industry. Although the project is still in its infancy some insight of the design is presented, particularly the aerodynamic and structural aspects of the blades which are being optimised for highest possible efficiency. Some results from numerical analysis tools are also presented. To provide easy start up, the turbine blades are at first retracted. Due to the accelerating motion, momentum is conserved by means of an increase in the polar moment of inertia about the axis of rotation. This causes the turbine blades to flex. In this position, advanced aerodynamic calculations using vortex methods have shown that the turbine should operate at a higher efficiency for the rated condition. Finite Element Analysis (FEA) tools have also been used to assess the structural feasibility of the turbine. The preliminary results presented here are encouraging and will form the basis for further computational and experimental analysis.Bajada New Energy Ltd., CD Power Saving Co. Ltd., Energy Investment Co. Ltd., Solar Engineering Ltd. & Solar Solutions Ltd.peer-reviewe

Creep fatigue analysis of DEMO divertor components following the RCC-MRx design code

In the DEMO fusion reactor, in-vessel components will be subjected to very high thermo mechanical steady and cyclic loads. A design check that is required by the RCC-MRx code used for nuclear installations and fusion reactors is a creep-fatigue check. The fatigue damage is caused by the pulsed operation of the fusion reactor while creep damage occurs during the hold time of loads at elevated temperatures. The temperature of the main divertor components is kept below that which causes creep by using cooling fluid that flows through channels fabricated within the components themselves. Other components such as the shielding liner and reflector plate supports on the divertor cassette cannot be cooled as such and so their temperature can rise high enough so that they sustain creep damage. In the presence of creep, the fatigue life of a component is reduced. In this work, a creep fatigue assessment of a representative simple geometry is carried out. The representative geometry is that of a thick cylinder under the action of steady and fluctuating loads similar to those seen by DEMO in-vessel components while in service. The cylinder example creep fatigue results are used as a benchmark and compared with those obtained using the creep fatigue assessment (CFA) tool developed at KIT (Karlsruhe Institute of Technology). Methodologies used for creep fatigue assessments within RCC-MRx are presented and explained and results discussed. The work should provide a contribution towards any necessary creep fatigue assessments of DEMO divertor components currently being developed.peer-reviewe

An aerodynamic study on flexed blades for VAWT applications

There is renewed interest in aerodynamics research of VAWT rotors. Lift type, Darrieus designs sometimes use flexed blades to have an ’egg-beater shape’ with an optimum Troposkien geometry to minimize the structural stress on the blades. While straight bladed VAWTs have been investigated in depth through both measurements and numerical modelling, the aerodynamics of flexed blades has not been researched with the same level of detail. Two major effects may have a substantial impact on blade performance. First, flexing at the equator causes relatively strong trailing vorticity to be released. Secondly, the blade performance at each station along the blade is influenced by self-induced velocities due to bound vorticity. The latter is not present in a straight bladed configuration. The aim of this research is to investigate these effects in relation to an innovative 4kW wind turbine concept being developed in collaboration with industry known as a self-adjusting VAWT (or SATVAWT). The approach used in this study is based on experimental and numerical work. A lifting line free-wake vortex model was developed. Wind tunnel power and hot-wire velocity measurements were performed on a scaled down, 60cm high, three bladed model in a closed wind tunnel. Results show a substantial axial wake induction at the equator resulting in a lower power generation at this position. This induction increases with increasing degree of flexure. The self-induced velocities caused by blade bound vorticity at a particular station was found to be relatively small.peer-reviewe

Dynamic analysis of a floating hybrid spar TLP concept for wind monitoring applications in deep sea

This paper presents an investigation to assess the motions experienced by a floating hybrid spar-tension leg platform (TLP) structure when supporting a wind monitoring lattice tower in deep water conditions of the Central Mediterranean through a numerical study using the software package ANSYS® AQWA™. The structure supporting the tower involves a four-legged system with a single cylindrical spar used to provide the required buoyancy and retain the four tethers under tension. The four horizontal spokes linking the spar to the tethers are located above water level to facilitate the installation of a LiDAR System and guy wires for supporting the wind monitoring tower. A parametric analysis was carried out for different geometrical and met ocean conditions, and the simulations were restricted to regular (single frequency) wave and constant wind speed conditions only. The Morison formulation was used to resolve the hydrodynamic loads in a time domain. The study shows how the natural periodic times of the floating wind-monitoring mast structure decrease with increasing buoyancy-to-weight ratios. From the time-series simulations on the floating structure it was evident that slender spars experience smaller displacements. This is a favourable result as it means that it would have a lower impact on the wind measurements taken by the cup-type anemometers installed on the tower. Finally, a sensitivity analysis was carried out to examine the variations of surge motion predictions resulting from deviations in the hydrodynamic coefficients. It was observed that the platform surge motion is more sensitive to deviations in the added mass coefficient than the drag coefficient.peer-reviewe

Thermo-mechanical fluid–structure interaction numerical modelling and experimental validation of MEMS electrothermal actuators for aqueous biomedical applications

Recent developments in MEMS technologies have made such devices attractive for use in applications that involve precision engineering and scalability. In the biomedical industry, MEMS devices have gained popularity in recent years for use as single-cell manipulation and characterisation tools. A niche application is the mechanical characterisation of single human red blood cells, which may exhibit certain pathological conditions that impart biomarkers of quantifiable magnitude that are potentially detectable via MEMS devices. Such applications come with stringent thermal and structural specifications wherein the potential device candidates must be able to function with no exceptions. This work presents a state-of-the-art numerical modelling methodology that is capable of accurately predicting MEMS device performance in various media, including aqueous ones. The method is strongly coupled in nature, whereby thermal as well as structural degrees of freedom are transferred to and from finite element and finite volume solvers at every iteration. This method therefore provides MEMS design engineers with a reliable tool that can be used in design and development stages and helps to avoid total reliability on experimental testing. The proposed numerical model is validated via a series of physical experiments. Four MEMS electrothermal actuators with cascaded V-shaped drivers are presented. With the use of the newly proposed numerical model as well as the experimental testing, the MEMS devices’ suitability for biomedical applications is confirmed.peer-reviewe

Coupled finite element-finite volume multi-physics analysis of MEMS electrothermal actuators

Microelectromechanical systems (MEMS) are the instruments of choice for high-precision manipulation and sensing processes at the microscale. They are, therefore, a subject of interest in many leading industrial and academic research sectors owing to their superior potential in applications requiring extreme precision, as well as in their use as a scalable device. Certain applications tend to require a MEMS device to function with low operational temperatures, as well as within fully immersed conditions in various media and with different flow parameters. This study made use of a V-shaped electrothermal actuator to demonstrate a novel, state-of-the-art numerical methodology with a two-way coupled analysis. This methodology included the effects of fluid–structure interaction between the MEMS device and its surrounding fluid and may be used by MEMS design engineers and analysts at the design stages of their devices for a more robust product. Throughout this study, a thermal–electric finite element model was strongly coupled to a finite volume model to incorporate the spatially varying cooling effects of the surrounding fluid (still air) onto the V-shaped electrothermal device during steady-state operation. The methodology was compared to already established and accepted analysis methods for MEMS electrothermal actuators in still air. The maximum device temperatures for input voltages ranging from 0 V to 10 V were assessed. During the postprocessing routine of the two-way electrothermal actuator coupled analysis, a spatially-varying heat transfer coefficient was evident, the magnitude of which was orders of magnitude larger than what is typically applied to macro-objects operating in similar environmental conditions. The latter phenomenon was correlated with similar findings in the literature.peer-reviewe