A model of transport nonuniversality in thick-film resistors

We propose a model of transport in thick-film resistors which naturally explains the observed nonuniversal values of the conductance exponent t extracted in the vicinity of the percolation transition. Essential ingredients of the model are the segregated microstructure typical of thick-film resistors and tunneling between the conducting grains. Nonuniversality sets in as consequence of wide distribution of interparticle tunneling distances.Comment: 3 pages, 1 figur

Load sensing surgical instruments

Force and pressure sensing technology applied to smart surgical instruments as well as implants allow to give a direct feedback of loads to the surgeon lead to better reliability and success of surgical operations. A common technology used for sensors is low-cost piezoresistive thick-film technology. However, the standard thick-film firing conditions degrade the properties of medical alloys. In order to avoid this problem, the solution is to decrease the firing temperature of thick films. This work presents the development and characterisation of low-firing thick-film systems (dielectrics, resistors and conductors), formulated to achieve chemical and thermal expansion compatibility with an austenitic stainless steel medical alloy. Adherence tests and results on electrical properties of these systems: resistance, temperature coefficient of resistance (TCR) are presented. It was found that the main issue in these systems lies in mastering the materials interactions during firing, especially at the silver-based resistor terminations. The interaction of silver, resistor and dielectric tends to give rise to highly resistive zones at the terminations, affecting reliability. This can be circumvented by post-firing the resistor terminations at a moderate temperatur

Sensors and packages based on LTCC and thick-film technology for severe conditions

Reliable operation in harsh environments such as high temperatures, high pressures, aggressive media and space, poses special requirements for sensors and packages, which usually cannot be met using polymer-based technologies. Ceramic technologies, especially LTCC (Low-Temperature Cofired Ceramic), offer a reliable platform to build hermetic, highly stable and reliable sensors and packages. This is illustrated in the present work through several such devices. The examples are discussed in terms of performance, reliability, manufacturability and cost issue

Anisotropic random resistor networks: a model for piezoresistive response of thick-film resistors

A number of evidences suggests that thick-film resistors are close to a metal-insulator transition and that tunneling processes between metallic grains are the main source of resistance. We consider as a minimal model for description of transport properties in thick-film resistors a percolative resistor network, with conducting elements governed by tunneling. For both oriented and randomly oriented networks, we show that the piezoresistive response to an applied strain is model dependent when the system is far away from the percolation thresold, while in the critical region it acquires universal properties. In particular close to the metal-insulator transition, the piezoresistive anisotropy show a power law behavior. Within this region, there exists a simple and universal relation between the conductance and the piezoresistive anisotropy, which could be experimentally tested by common cantilever bar measurements of thick-film resistors.Comment: 7 pages, 2 eps figure

STUDIES ON PROTEIN UPTAKE BY ISOLATED TUMOR CELLS : II. Quantitative Data on the Adsorption and Uptake of I131-Serum Albumin by Ehrlich Ascites Tumor Cells

Surface adsorption is studied in some detail because it is believed to be a major artifact in measurements of protein uptake by mammalian cells. Adsorption increases linearly with the I131-albumin concentration between 0.001 and 300 mg/ml. After short exposure to 300 mg/ml and two cell washings, the adsorption amounts to 38 mg albumin per gm cell proteins. Further washings remove 80 per cent of this value, leaving a small irreversibly bound residue. At equilibrium, adsorbed albumin can be labeled by a simple albumin exchange. This labeling reaches a steady state within seconds and stays at constant level over 30 minutes. Significant increases above this initial level are measured over periods of 2 hours. In our experimental conditions these increases can be considered due to albumin uptake. This uptake rises linearly with the albumin concentration between 0.5 and 50.0 mg/ml, and reaches 0.2 mg/gm cell protein or 4 x 105 molecules per cell. Compared to the incorporation of free amino acids in similar conditions, this value does not appear to contribute significantly to the N-metabolism of the tumor cells. Adsorption was generally greater than uptake. Both processes are linear functions of the same variable over the whole range of concentration tested. It is suggested that albumin is taken up by pinocytosis

STUDIES ON PROTEIN UPTAKE BY ISOLATED TUMOR CELLS : I. Electron Microscopic Evidence of Ferritin Uptake by Ehrlich Ascites Tumor Cells

Ferritin, added to the incubation medium of ascites tumor cells, was used as an electron microscopic marker to study the uptake of large protein molecules by morphologically intact cells. A definite uptake could be detected after 1 hour of incubation in Tyrode bicarbonate solution containing 0.04 to 13.3 mg ferritin/ml. Ferritin was found in a variety of membrane-surrounded structures, suggesting that pinocytesis and related membrane movements are occurring under physiological conditions and can account for the penetration of intact macromolecules into isolated tumor cells. Supplementation of the medium with serum albumin (33 mg/ml) increased the average amount of ferritin per cell and per pinocytotic structure. Ferritin was strongly adsorbed by fragments of lysed cells, which were readily taken up by intact cells. Besides its role as carrier, this debris appeared to stimulate membrane movements. Only rare examples were found to suggest the release of ferritin from the pinocytotic structures into the cytoplasm. Thus, the disintegration of such structures cannot be considered an obvious step towards a rapid metabolic utilization of protein by the cell. Particles of colloidal gold presented to the cell under the same conditions were not taken up to any significant extent, thus providing good evidence for a selective ingestion of particles of comparable sizes