Micro and Nanosystems - Volume 4, Issue 2, 2012

Over the past few decades mechanical characterisation of material systems has become a well-established and in many cases routine experimental procedure and when used in combination with microscopy (e.g. Electron or Atomic) allows not only the evaluation of the mechanical properties (e.g. tensile strength) of the studied system, but also the in-situ examination of the microstructural and fracture development of the material in question and in the case of AFM the adhesive properties of the system. Another way of examining the mechanical response (e.g. acoustic velocity, related to Young’s modulus) of a system of interest in-situ is by directly measuring its properties on the device that it is designed to be part of (e.g. acoustic filter). However, as the scale of the experiments and studied systems decreases and their complexity increases, it becomes absolutely necessary to design and engineer not only different types of novel materials and specimen geometries, but also novel devices and fixtures that would accommodate the intended measurements. The principal aim of this special issue is to give an overview of the latest research and developments in specific areas of micro- and nano-scale characterisation. The topics covered in the contributing papers address the most recent trends in in-situ mechanical characterisation of micro- and nano-systems; examples include classic metals (e.g. Ni) advanced thermal barrier coatings (TBC), thin film bulk acoustic resonators (FBAR) & bacterial cells (e.g. E.coli). The paper by Maitland et al., presents the results from a study aimed at the development of a methodology for examining stable crack growth in thermal barrier coatings at high resolutions. The contribution by Craig et al., which is also based on the development of a Microscopical technique (AFM), presents the results from a pilot study where novel thermal probes were used in an attempt to characterise their interactions with biological systems. In contrast, the paper by Nirschl et al., deals with the other aspect of in-situ materials characterisation described above. In their paper the authors clearly demonstrate how FBAR can be a used as a tool to characterise the mechanical properties of thin films in-situ on a micro- and nanoscale within a certain range of parameters. The contribution by Korsunsky et al., reviews some recently developed diffraction techniques, based on the use of (sub)microscopic focused X-ray beams and aimed at the characterisation of material structure and mechanical behaviour at unprecedented spatial resolution. Overall, it is strongly believed that the articles in this special issue present original experimental research and that these will be of interest to the readers of Micro & Nanosystems and scientists in the more general fields of Engineering and Materials Science.

Atomic Force Microscopy was successfully employed to measure for the first time the interactive forces between different types of heated probes and E. coli bacterial cells. The results indicate that within a narrow temperature range there is a step decrease in the pull-off force and thus the adhesion between the tip and the sample. This adhesion measurement is a method of probing the interactions between the tip and the cell and the step change can be associated with structural changes taking place on the surface of the bacterium. This methodology gave reproducible results although some aspects of the force distance curves are not yet understood. This study indicates that the use of heated probes can be used to measure thermal transitions in biological systems and this approach could be extended to study how temperature affects specific cell surface interactions using functionalised tips.

Environmental Scanning Electron Microscopy (ESEM) allows samples to be imaged at high resolutions in their natural hydrated state and when used in combination with appropriate apparatus to measure the mechanical properties of the system under investigation. The approach has been successfully utilised in the study of a number of exciting materials systems and has been able to address a variety of scientific questions in areas ranging from Physics through to Life Sciences. The principal aim of this brief overview is to highlight the latest and most recent developments in micromechanical characterisation by means of ESEM and to outline the advantages of this approach. The basic principles of ESEM operation and in-situ tensile testing have also been considered.

The advent of (sub)microscopic focused X-ray beams at third generation synchrotron sources has opened up possibilities for the characterisation of material structure and mechanical behaviour with unprecedented spatial resolution. Crucially, the non-destructive nature and fast rate of X-ray data collection allow in situ deformation to be studied. In this review, we concentrate on the inelastic deformation response of ductile metallic (poly)crystals. We describe a range of diffraction-based techniques we have developed including monochromatic beam reciprocal space mapping, scanning “pink” beam micro-diffraction compound topography, and white beam Laue micro-diffraction. We compare the results obtained using each of the above techniques, and assess and review the insights that they afford into micro-scale material deformation.

We have developed a special test fixture in an attempt to study the fracture behaviour of ceramic materials in an Environmental Scanning Electron Microscope (ESEM). The fixture, which can be easily fitted onto any conventional tensile/compression stage, was designed to load a double torsion specimen, thus providing conditions for stable crack growth. This not only allows evaluation of the fracture toughness and R-curve behaviour of a material, but also observation of the propagating crack at high resolutions. In this proof of concept study we have used two methodologies to study the mechanical behaviour of thermal barrier coatings – Constant Displacement Rate Method & Relaxation Method. It was found that the Relaxation Method worked adequately and it was possible to follow the development of a growing crack, whereas the Constant Displacement Rate Method was found to be unsuitable, due to the fact that in most cases the force increased continuously until abrupt failure of the specimen occurred.

Thin-film bulk acoustic resonators (FBAR) can be used as mass sensors when the adsorbed mass is linear to the frequency shift caused by the adsorption. This is, however, only the case if the adsorbed layer is thin compared to the thickness of the resonator. In this paper, we investigate the adsorption of films with thicknesses of some nanometres up to few hundreds of nanometres. With this range, we cover films thicknesses being small compared to the resonator and thicknesses in the range of the resonator thickness. The adsorption of materials was simulated for materials with different mass densities and acoustic velocities. Thin films of platinum, aluminium oxide, tungsten and carbon nanotubes were deposited on the FBAR and the results were fitted to the model used in the simulations. The acoustic velocity of the carbon nanotube films was much lower than the other materials investigated in this study. With this interesting property, carbon nanotube thin-films are a promising material for acoustic devices where materials with particularly low acoustic impedance are desired. The paper shows that the FBAR can be a useful tool to characterise mechanical properties of thin films in situ in the micro- and nanoscale within a certain range of parameters.

Hydrogen titanate nanotubes can be turned into thermally stable TiO2 nanotubes by a sol-gel impregnation method. Gold is deposited on such stable TiO2 nanotubes via a deposition-precipitation process. The prepared materials are characterized with powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and nitrogen adsorption. Their catalytic performances for low-temperature CO oxidation are studied. Compared with the hydrogen titanate nanotubes supported gold catalyst, such stable TiO2 nanotubes supported gold catalysts show better catalytic performance.

The paper focuses on optimizing layout of obstacles for enhanced mixing in microchannels. The obstacles can hinder fluid flow and alter the flow direction. In this case, the mixing performance can be achieved further effectively. Three parameters including geometric configuration, layout, and number of obstacles have been studied for analyzing mixing in a ‘T’ channel. In order to facilitate the design, numerical simulations and orthogonal experiment were used to study the effect of three parameters on sensitivity of mixing. The degree of sensitivity using the Taguchi method can be ranked as: asymmetric layout > number of obstacles > geometric configuration. Placing obstacles in the microchannels is an effective method for enhancing mixing and the micromixer with obstacles is relatively easy to produce using standard microfabrication techniques. It can be demonstrated that the results should facilitate the design of microfluidic device.

Nanotechnology is playing major role in modern therapeutics for effective delivery of drugs to the troublesome target sites. Human eye presents various physiological and anatomical constraints limiting the absorption of therapeutic agent delivered on its surface by conventional ophthalmic dosage forms. Dosage form variables- like particle size, charge on drug particles, elasticity, bioadhesion and corneal surface retention can affect the absorption of the therapeutic element for ocular purposes. Various nanotechnology based systems have been developed for improving ocular therapeutics and increasing market demand has directed efforts for exploring commercial viability of nanotechnology based ocular systems. The nano-ocular therapeutic systems have been classified on the basis of their structures, properties and functionalities. The present review compiles elaborated information on the nano-ocular systems, their utilization, development considerations, intellectual and market aspects and the future prospects for ocular therapy.

Telmisartan (TLM), a poorly water-soluble antihypertensive agent exhibits suboptimal oral bioavailability in all conventional dosage forms. The present investigation was aimed to develop an oral solid dosage form incorporating nanoparticles of TLM. TLM loaded nanosuspension was formulated by media milling technique. Preliminary trials were carried out for selecting size of milling agent, type of stabilizer, speed of stirring and time of stirring. A 32 full factorial design was implemented in order to estimatethe effect of critical factors like concentration of surfactant (X1) and amount of milling agent (X2) on two major parameters [particle size (Y1) and saturation solubility (Y2)] of drug loaded nanosuspension. The optimized batch of drug loaded nanosuspension was lucratively subjected to lyophilization using trehalose as cryoprotectant without any significant change in particle size. Saturation solubility and dissolution properties of optimized formulation were drastically improved as compared to pure drug. In-vivopharmacokinetic study also exhibited intense increase in oral bioavailability of TLM. These results propose potential use of nanosuspension to improve dissolution and oral bioavailability of poorly water-soluble drugs, like TLM.

Structural applications of aluminium have grown considerably in the last decades. This paper reviews research on patents of aluminium composites synthesized methods, such as mechanical alloying, hot pressing, sintering, selfpropagating high-temperature synthesis, and spark plasma sintering. It seems apparent from the foregoing overview that aluminum alloys possess a number of very attractive characteristics which, together with their very light weight, make then extremely attractive for many applications. Further, their versatility with respect to options of how to shape them and strengthen them provide an amazing variety of choices when you are looking for an ideal material for a special application. At the same time, a further study for optimizing the processing technologies is required to improve the property of Aluminium composites. Future directions in this field have been suggested.

This paper reports an electroplated copper mask technology which consists of sputtered chromium and copper seed layer with electroplated copper and gold layer, in combination with hard baked thick AZ4620 photoresist based on MEMS technology. The etching depth attains to 680μm after more than 3 hours’ immerging in the concentrated HF 48% etching solution with smooth generated surface without pinholes. In addition, undercut ratio of 0.87 could be achieved. Compared with other masks in the literature, this electroplating mask technology is characterized by lithography compatibility, simple, low cost, short time consumption, large undercut ratio as well as smooth generated surface. Thus, it can be used in MEMS fields such as: precisely assignment of permanent magnet for the electromagnetic microrelay.