Atrial activity during thePR segment of the MCG

Summary Sloping signals were observed in thePR segment of normal magnetocardiograms recorded from five dogs. Larger ramps were observed after first-degree AV nodal blockade was induced pharmacologically, and complete atrial repolarization was observed following isolated atrial contractions in two animals with second-degree block. These ramps, ranging up to 6.7 pT in size and arising from the atria, may complicate magnetic detection of His-Purkinje activity. This work was supported by the National Institute of Health under Grant No. HL 23184.     

First magnetic measurements of action currents in isolated cardiac Purkinje fibers

Summary We present the first measurements of the magnetic field associated with the action potential in a bundle of isolated cardiac Purkinje fibers. Our findings demonstrate the feasibility of using magnetic techniques to measure the axial current in a bundle of spontaneously active cardiac cells. Paper presented at the ≪IV International Workshop on Biomagnetism≫, held in Rome, September 14–16, 1982.     

Spatiotemporal dynamics of damped propagation in excitable cardiac tissue

Compared to steadily propagating waves (SPW), damped waves (DW), another solution to the nonlinear wave equation, are seldom studied. In cardiac tissue after electrical stimulation in an SPW wake, we observe DW with diminished amplitude and velocity that either gradually decrease as the DW dies, or exhibit a sharp amplitude increase after a delay to become an SPW. The cardiac DW-SPW transition is a key link in understanding defibrillation and stimulation close to the refractory period, and is ideal for a general study of DW dynamics.     

Microfluidic platform for real-time signaling analysis of multiple single T cells in parallel

Deciphering the signaling pathways that govern stimulation of na?ve CD4+ T helper cells by antigen-presenting cells via formation of the immunological synapse is key to a fundamental understanding of the progression of successful adaptive immune response. The study of T cell-APC interactions in vitro is challenging, however, due to the difficulty of tracking individual, non-adherent cell pairs over time. Studying single cell dynamics over time reveals rare, but critical, signaling events that might be averaged out in bulk experiments, but these less common events are undoubtedly important for an integrated understanding of a cellular response to its microenvironment. We describe a novel application of microfluidic technology that overcomes many limitations of conventional cell culture and enables the study of hundreds of passively sequestered hematopoietic cells for extended periods of time. This microfluidic cell trap device consists of 440 18 micromx18 micromx10 microm PDMS, bucket-like structures opposing the direction of flow which serve as corrals for cells as they pass through the cell trap region. Cell viability analysis revealed that more than 70% of na?ve CD4+ T cells (TN), held in place using only hydrodynamic forces, subsequently remain viable for 24 hours. Cytosolic calcium transients were successfully induced in TN cells following introduction of chemical, antibody, or cellular forms of stimulation. Statistical analysis of TN cells from a single stimulation experiment reveals the power of this platform to distinguish different calcium response patterns, an ability that might be utilized to characterize T cell signaling states in a given population. Finally, we investigate in real time contact- and non-contact-based interactions between primary T cells and dendritic cells, two main participants in the formation of the immunological synapse. Utilizing the microfluidic traps in a daisy-chain configuration allowed us to observe calcium transients in TN cells exposed only to media conditioned by secretions of lipopolysaccharide-matured dendritic cells, an event which is easily missed in conventional cell culture where large media-to-cell ratios dilute cellular products. Further investigation into this intercellular signaling event indicated that LPS-matured dendritic cells, in the absence of antigenic stimulation, secrete chemical signals that induce calcium transients in T(N) cells. While the stimulating factor(s) produced by the mature dendritic cells remains to be identified, this report illustrates the utility of these microfluidic cell traps for analyzing arrays of individual suspension cells over time and probing both contact-based and intercellular signaling events between one or more cell populations.     

Remote sensing of aluminum alloy corrosion by SQUID magnetometry

Macroscopic magnetic fields were detected on corroding aluminum alloy (AA 2024) samples by SQUID magnetometry. The fields originated from corrosion reactions due to asymmetric sample geometry, electrolyte flow and differences in surface activity. Magnetic images were obtained by a SQUID magnetometer operating in liquid helium with a spatial resolution of approximately 1 mm. The measurements demonstrated SQUID capability for corrosion sensing across integrated media consisting of gaseous and solid dielectrics (air, plastics), an electronic conductor (aluminum alloy) and an ionic conductor (solution). The results show the potential of SQUID magnetometry for practical corrosion detection in restricted locations (hidden corrosion) and in subjects where solution flow is applied.Contribution to the 3rd Baltic Conference on Electrochemistry, GDASK-SOBIESZEWO, 23–26 APRIL 2003.Dedicated to the memory of Harry B. Mark, Jr. (February 28, 1934 – March 3rd, 2003)     

Model-controlled hydrodynamic focusing to generate multiple overlapping gradients of surface-immobilized proteins in microfluidic devices

Historically, it has been difficult to generate accurate and reproducible protein gradients for studies of interactions between cells and extracellular matrix. Here we demonstrate a method for rapid patterning of protein gradients using computer-driven hydrodynamic focusing in a simple microfluidic device. In contrast to published work, we are moving the complexity of gradient creation from the microfluidic hardware to dynamic computer control. Using our method, switching from one gradient profile to another requires only a few hours to devise a new control file, not days or weeks to design and build a new microfluidic device. Fitting existing protein deposition models to our data, we can extract key parameters needed for controlling protein deposition. Several protein deposition models were evaluated under microfluidic flow conditions. A mathematical model for our deposition method allows us to determine the parameters for a protein adsorption model and then predict the final shape of the surface density gradient. Simple and non-monotonic single and multi-protein gradient profiles were designed and deposited using the same device.