There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Nanotechnology and biosensors

Pull-in instability as an inherently nonlinear and crucial effect continues to become increasingly important for the design of electrostatic MEMS and NEMS devices and ever more interesting scientifically. This review reports not only the overview of the pull-in phenomenon in electrostatically actuated MEMS and NEMS devices, but also the physical principles that have enabled fundamental insights into the pull-in instability as well as pull-in induced failures. Pull-in governing equations and conditions to characterize and predict the static, dynamic and resonant pull-in behaviors are summarized. Specifically, we have described and discussed on various state-of-the-art approaches for extending the travel range, controlling the pull-in instability and further enhancing the performance of MEMS and NEMS devices with electrostatic actuation and sensing. A number of recent activities and achievements methods for control of torsional electrostatic micromirrors are introduced. The on-going development in pull-in applications that are being used to develop a fundamental understanding of pull-in instability from negative to positive influences is included and highlighted. Future research trends and challenges are further outlined.

Electrostatic pull-in instability in MEMS/NEMS: A review

Textbook of biochemistry with clinical correlations , 4th edn TM Devlin, ed pp xvii + 1186, illustrated Wiley-Liss, New York, 1997 £2995, hardback

This beautifully illustrated and well-written book, with an impressive array of authors, is aimed at both undergraduate and postgraduate level As the editor states in the preface, it is not intended to be a compendium of biochemistry but rather emphasises the biochemistry of mammalian cells The first 22 chapters cover …

Textbook of biochemistry with clinical correlations

https://jcp.bmj.com/content/jclinpath/12/2/193.1.full.pdf

Practical Clinical Biochemistry

High field conduction in n+nn+ amorphous hydrogenated silicon (α‐Si:H) films has been investigated at different temperatures. The results can be explained by assuming space‐charge limited conduction (SCLC) with a uniform density of traps. The value of density of states at the Fermi level, g(EF), obtained from the SCLC measurements ranges between 7−9×1016 cm−3 eV−1. Similar values are obtained by measurements of field effect and frequency and temperature dependence of Schottky barrier capacitance on material grown under identical conditions.

Space‐charge limited conduction in n+nn+ amorphous hydrogenated silicon films

Porous silicon (PS) has many interesting and unique properties that make it a viable material in the field of MEMS. In this paper, we investigate the application of PS in improving the sensitivity of bulk micromachined piezoresistive pressure sensors. The property of a low Young's modulus of PS and its dependence on porosity have been exploited to obtain a higher sensitivity compared to pressure sensors with single crystalline silicon membranes. Simulation was carried out to represent the Si/PS composite membrane by two layers with Young's modulus corresponding to silicon and PS. The behavior of this membrane was studied for various values of porosity and thickness of the PS layer. Composite Si/PS membranes were fabricated by converting a part of the silicon membrane thickness into PS by electrochemical etching in an HF-based electrolyte. Polysilicon piezoresistors were formed on the membrane in the form of a Wheatstone bridge for the measurement of sensitivity. When compared to membranes of silicon, the sensitivity of the composite Si/PS membrane is found to be higher showing improvement with an increase in the porosity and thickness of the PS layer.

http://iopscience.iop.org/article/10.1088/0960-1317/17/8/025/pdf

Enhancement of the sensitivity of pressure sensors with a composite Si/porous silicon membrane

Knudsen forces are gas molecular forces which originate from the differential temperatures in rarefied gases. We report measurements of these forces at normal ambience on test structures made by surface micromachining of polysilicon. Using these results, a surface micromachined Knudsen vacuum sensor has been simulated, fabricated and characterized. The vacuum sensor has an area of 1 mm2. The fabricated device has a sensitivity of 40 fF Pa−1 in the pressure range of 0.1–10 Pa. The measured data is analysed and the magnitude of the Knudsen's force is extracted. The paper also suggests ways to enhance the range and improve the sensitivity of such sensors.

https://iopscience.iop.org/article/10.1088/0960-1317/24/4/045003/pdf

Knudsen force based MEMS structures

A modified electrolyte–insulator–semiconductor–capacitor (EISCAP) developed in-house was used for the determination of triglycerides and urea in blood, in addition to urea in milk. The sensitivity of the sensor was 55 ± 0.5 mV per unit change of pH. The EISCAP was optimized to operate at 25 °C in 0.25 mM phosphate buffer (pH 6), 1 M KCl and 1 mg lipase for triglycerides and in 1 mM Tris–HCl buffer (pH 7.4), 0.5 M KCl and 0.5 U urease for urea. Comparisons of the results obtained from the developed biosensor and clinical records gave correlation coefficients of 0.979 and 0.974 for blood triglycerides and blood urea respectively. Similarly, estimation of milk urea determined using the sensor in comparison with the spectrophotometric method gave a correlation coefficient of 0.992. The sensor gave reproducible and reliable results up to six months.

Potentiometric estimation of blood analytes—triglycerides and urea: Comparison with clinical data and estimation of urea in milk using an electrolyte–insulator–semiconductor–capacitor (EISCAP)

Ultrathin silicon nanoporous membranes with multiple pores were fabricated using batch processes involving chemical vapor deposition and rapid thermal annealing. Transmission electron microscope images showed the existence of nanopores with an average pore size of 13 nm. Measurement of ionic conduction of electrolytes with varying conductivity across the membranes confirmed the existence of pores and the repeatability of the process. The functional diameter of the pores was determined by analyzing the permeability of several industrially and medically important globular biomolecules of varying sizes such as α-amylase, bovine serum albumin, catalase and xanthine oxidase. Biomolecules with hydrodynamic diameters up to 8 nm passed through the nanopores, whereas the passage of the larger molecules was hindered. The surface charges on the molecules determine the functional diameter of the pores, and hence the permeability, as substantiated by varying the pH of the buffer solution. The filtered proteins were found to be uncleaved from sodium dodecyl sulfate polyacrylamide gel electrophoresis, and the enzyme assay of the filtered amylase showed that the activity remained unchanged.

Enakshi Bhattacharya

Papers

Space‐charge limited conduction in n+nn+ amorphous hydrogenated silicon films

Enhancement of the sensitivity of pressure sensors with a composite Si/porous silicon membrane

Knudsen force based MEMS structures

Potentiometric estimation of blood analytes—triglycerides and urea: Comparison with clinical data and estimation of urea in milk using an electrolyte–insulator–semiconductor–capacitor (EISCAP)

Fabrication of Ultrathin Silicon Nanoporous Membranes and Their Application in Filtering Industrially Important Biomolecules