Joseph West

Transport and magnetic properties of dilute rare-earth–PbSe alloys

Polycrystalline alloys of PbSe with rare-earth elements (Ce, Pr, Nd, Eu, Gd, and Yb) have been prepared and their magnetic susceptibility (from 4 to 120 K), galvanomagnetic and thermomagnetic transport (from 80 to 380 K) properties have been measured. Most samples are paramagnetic, and the concentration of rare-earth atoms in the PbSe lattice is deduced from fitting a Curie-Weiss law. The electrical conductivity, Hall, Seebeck, and transverse Nernst-Ettingshausen effects are interpreted in terms of the carrier density and mobility, the density of states effective mass, and the scattering exponent. In summary, Pb1−xEuxSe is a semiconductor with a wider gap than PbSe, but the carrier density is unaffected by the presence of Eu. The other rare earths, which are essentially trivalent atoms, act as donors, with a doping efficiency close to unity in the case of Ce and Nd, but much less for Gd and Yb. The mechanisms that govern the observed decrease in mobility are also discussed.

Electrophoretic drawing of continuous fibers of single-walled carbon nanotubes

Control over the current during electrophoretic drawing of continuous fibers of single-walled carbon nanotubes (SWCNTs) is shown to be critical in producing long fibers with specific diameters. In the process, as-produced SWCNTs are first dispersed in N,N-dimethylformamide (DMF) by sonication. A tungsten probe tip is then immersed in the SWCNT∕DMF solution, and a dc bias is applied between the tip and another electrode at the bottom of the beaker containing the solution. After a dark cloud several millimeters in diameter develops around the tip, the electrode is withdrawn to form continuous macroscopic fibers of SWCNTs. The resulting fiber length and diameter are found to be principally determined by the magnitude of the current. Under constant voltage conditions where the current is allowed to vary, the fibers are short (several millimeters long) and their diameters vary drastically along their lengths. Of significance is the fact that when the current is maintained at constant values, fibers several centimeters in length with uniform diameters ranging from 26to42μm are obtained at a withdrawal rate of 0.85μm∕s. For this withdrawal rate, long fibers (∼3cm and greater) are obtained at an optimum value of the current (400nA) using hybrid conditions where the voltage is maintained constant earlier in the process while the current is maintained constant later in the process. Control of the total current at low values during this process has the potential to produce long fibers with uniform submicron diameters.Control over the current during electrophoretic drawing of continuous fibers of single-walled carbon nanotubes (SWCNTs) is shown to be critical in producing long fibers with specific diameters. In the process, as-produced SWCNTs are first dispersed in N,N-dimethylformamide (DMF) by sonication. A tungsten probe tip is then immersed in the SWCNT∕DMF solution, and a dc bias is applied between the tip and another electrode at the bottom of the beaker containing the solution. After a dark cloud several millimeters in diameter develops around the tip, the electrode is withdrawn to form continuous macroscopic fibers of SWCNTs. The resulting fiber length and diameter are found to be principally determined by the magnitude of the current. Under constant voltage conditions where the current is allowed to vary, the fibers are short (several millimeters long) and their diameters vary drastically along their lengths. Of significance is the fact that when the current is maintained at constant values, fibers several cen...

Non-contact method for directing electrotaxis

We present a method to induce electric fields and drive electrotaxis (galvanotaxis) without the need for electrodes to be in contact with the media containing the cell cultures. We report experimental results using a modification of the transmembrane assay, demonstrating the hindrance of migration of breast cancer cells (SCP2) when an induced a.c. electric field is present in the appropriate direction (i.e. in the direction of migration). Of significance is that migration of these cells is hindered at electric field strengths many orders of magnitude (5 to 6) below those previously reported for d.c. electrotaxis, and even in the presence of a chemokine (SDF-1α) or a growth factor (EGF). Induced a.c. electric fields applied in the direction of migration are also shown to hinder motility of non-transformed human mammary epithelial cells (MCF10A) in the presence of the growth factor EGF. In addition, we also show how our method can be applied to other cell migration assays (scratch assay), and by changing the coil design and holder, that it is also compatible with commercially available multi-well culture plates.

Design and characterization of an electromagnetic probe for distinguishing morphological differences in soft tissues

We present a method for designing and optimizing an in-house designed electromagnetic probe for distinguishing morphological differences in biological tissues. The probe comprises concentric multi-wound coils, the inner being the primary coil and the outer being the detector coil. A time-varying voltage is imposed on the primary coil, resulting in an induced current in the detector coil. For highly conductive samples, eddy currents are induced in the sample and inductively couple with the electromagnetic probe. However, in weakly conducting samples, the primary coupling mechanism is found to be capacitive though there can be a non-negligible inductive component. Both the mutual inductive coupling and the capacitive coupling between the sample and the probe are detected as a change in the induced voltage of the detector coil using lock-in detection. The induced voltage in the detector coil is influenced more by the morphological structure of the specimen rather than by changes in electrical conductivity within different regions of the sample. The instrument response of the lock-in amplifier is also examined with simulated input voltage signals to relate its output to specific changes in inductive and capacitive coupling, in order to relate sample characteristics to a single voltage output. A circuit element model is used to interpret the experimental measurements. It is found that the sensitivity of the measurement for a given set of probe characteristics (resistances, inductances, and capacitances) can be optimized by adding a small amount of capacitance in the external circuit in parallel with the detector coil. Illustrative measurements are presented on animal (porcine and bovine) tissue and on human liver tissue containing a metastatic tumor to demonstrate the capabilities of the probe and measurement method in distinguishing different tissue types despite having similar electrical conductivities. Since biological tissues are multi-scale, heterogeneous materials comprising regions of differing conductivity, permittivity, and morphological structure, the electromagnetic method presented here has the potential to examine structural variations in tissue undergoing physical changes due to healing or disease.

Measurement of Electrical Impedance and Eddy Currents in Tissue Phantoms

Measurement of the electromagnetic (EM) properties of tissue such as electrical conductivity, permittivity, and eddy current characteristics can be used in clinical medicine for characterizing and distinguishing soft tissue morphology. Such measurements can yield complementary information to what can be obtained using analysis with an optical microscope. An example is the assessment of margins during the surgical resection of occult tumors. In current practice, the surgeon relies on pre-operative imaging modalities, sight and palpation to locate and attempt to fully resect the tumor(s). Frozen section pathological assessment offers the only other resource available to the surgeon for margin analysis, but it is incomplete because only a small fraction of the resected tissue is examined and it is often not feasible to wait for the results of the frozen section analysis before completing the surgery. This paper describes a characterization and imaging method based on variations in electromagnetic tissue properties to assess the surgical margins of resected tissues. This is noteworthy because accurate margin assessment has been shown to significantly improve long term patient outcomes[1].Copyright

Eddy Current Measurement for Characterizing Soft Tissues

Real-time and non-invasive imaging of tissues and detection of diseases on millimeter to centimeter scales can be useful in some clinical applications such as determination of margins during cancer surgery and image-guided pathology. In this paper, we describe an eddy current measurement method for characterizing soft tissues. The device consists of a pair of concentrically wound coils, a primary coil excited by a low frequency (<100 kHz) sinusoidal voltage, inducing a voltage and current in the secondary detecting coil. When a conducting sample is present, eddy currents develop in the sample and alter the induced voltage and phase on the detecting coil. The output voltage and phase of the detecting coil are then monitored using lock-in amplification.Experimental measurements on porcine muscle tissue examine the effects of varying tissue macrostructure and conductivity on the eddy current detector. Three sets of experiments are presented. First, muscle samples cut into different sized grids simulating the restriction of eddy current domains show that morphological structure has a strong influence on the detector signal. Second, eddy current measurements made on porcine muscle samples at varying degrees of dehydration show that as conductivity decreases, eddy current signals also decrease. Finally, measurements on porcine muscle samples soaked overnight in deionized water complement the dehydration experiments and confirm detector voltage and phase changes decrease with decreasing conductivity.Copyright

Eddy Current Detection of Cancer in Surgically Excised Tissue

In current clinical practice, a patient usually undergoes a diagnostic computer tomography (CT) scan for evaluation of specific presenting symptoms. The presence of cancer is then confirmed by a diagnostic biopsy or at surgical exploration by histopathologic analysis. Suspicious finding on the diagnostic CT scan may be followed by an 18F FDG (Fluorodeoxyglucose radiolabeled with 18F) positron emission tomography (PET) scan. In a majority of cases, these pre-operative CT and PET scans are used to identify the approximate location of the tumor(s) before surgical intervention. Surgery remains the most effective means of treating solid malignancies despite advances in chemotherapy and radiation therapy [1].Copyright

Response of an Isolated Particulate Subjected to Lateral Electric Fields in a DC Glow Discharge

Isolated spheres of borosilicate glass are suspended in a dc glow discharge in neon. The isolated sphere is laterally displaced by applying a transient voltage across a pair of electrodes mounted on the wall of the discharge tube. By monitoring the response of the particulate when subjected to a step change in the voltage and to sinusoidally varying voltages of different frequencies, the charge on the suspended sphere is inferred. Unlike most previous studies, this paper considers heavier (from 16 to 42 times heavier) particulates and heavily damped conditions. Measurements are reported for 20.3- and 4.9-mum -diameter borosilicate glass spheres. It is found that lateral excitation of an isolated particulate in a dc glow discharge cannot drive the motion to classical resonance as has been observed in previous work involving axial displacements in RF plasmas. The classical forced damped oscillator model used successfully in previous work to explain experimental observations is found to be inadequate for the conditions of this paper. Rather, the base excitation (BE) model is found to exhibit good agreement with the present experimental results at frequencies exceeding the frequency at which the displacement is a maximum. Sheath polarization, neglected in previous work, is included in this paper. It is found that drag due to collisions with neutrals is insufficient to account for the total drag on the particulate under heavily damped conditions. A new means of estimating the charge from the frequency response of the particulate motion using the BE model is described. Based on this method, charge numbers of Z = 9.2 times105 and Z = 1.5 times104 are inferred for the 20.3- and 4.9-mum spheres, respectively.