Mikkel Fougt Hansen

Characterization of brainstem preproglucagon projections to the paraventricular and dorsomedial hypothalamic nuclei.

In the brain preproglucagon expression is limited to a cluster of neurons in the caudal part of the nucleus of the solitary tract (NTS) as well as a smaller number of neurons that extend laterally from the NTS through the dorsal reticular area into the A1 area. These neurons process preproglucagon to glucagon-like peptide-1 (GLP-1), GLP-2, oxyntomodulin and glicentin. The neurons project mainly to the hypothalamus, where especially two nuclei involved in appetite regulation--the paraventricular (PVN) and dorsomedial (DMH) hypothalamic nuclei--are heavily endowed with GLP-immunoreactive nerve fibres. To gain further insight into this neurocircuitry, we injected the retrograde tracers cholera toxin, subunit B (ChB) and Fluorogold (FG) into the PVN and the DMH, respectively. Of thirty-five injected rats, six had successful injections that predominantly restricted within the boundaries of the PVN and DMH. Hindbrain sections from these rats were triple labelled for ChB, FG and GLP-2. A total of 24+/-1% of the PVN-projecting NTS-neurons contained GLP-2-ir whereas 67+/-4% of the DMH-projecting neurons were also stained for GLP-2, suggesting that the NTS-projections to the DMH arise mainly from preproglucagon neurons. Approximately 20% of backfilled cells in the NTS contained both retrograde tracers, therefore presumably representing neurons projecting to both the PVN and the DMH. The results of the present study demonstrate that the majority of the preproglucagon-expressing neurons in the NTS project in a target-specific manner to the hypothalamus. It is therefore possible that individual subgroups of GLP-containing neurons can mediate different physiological responses.

Critical dynamics of an interacting magnetic nanoparticle system

Effects of dipole-dipole interactions on the magnetic relaxation have been investigated for three Fe-C nanoparticle samples with volume concentrations of 0.06, 5 and 17 vol%. While both the 5 and 17 vol% samples exhibit collective behaviour due to dipolar interactions, only the 17 vol% sample displays critical behaviour close to its transition temperature. The behaviour of the 5 vol% sample can be attributed to a mixture of collective and single-particle dynamics.

Bead Capture on Magnetic Sensors in a Microfluidic System

The accumulation of magnetic beads by gravitational sedimentation and magnetic capture on a planar Hall-effect sensor integrated in a microfluidic channel is studied systematically as a function of the bead concentration, the fluid flow rate, and the sensor bias current. It is demonstrated that the sedimentation flux is proportional to the bead concentration and has a power law relation to the fluid flow rate. The mechanisms for the bead accumulation are investigated and it is found that gravitational sedimentation dominates the bead accumulation, whereas the stability of the sedimented beads against the fluid flow is defined by the localized magnetic fields from the sensor.

Bead capture and release on a magnetic sensor in a microfluidic system

Planar Hall effect magnetic sensors for detection of biological agents using surface treated magnetic beads are integrated with a fluid injection system. The response of the sensors is used to evaluate bead capture rates for different bead concentrations c and fluid flow rates Q, and to monitor subsequent removal of the beads. It is found that the capture rate scales directly with c and that it depends linearly on Q. At low Q the capture rate is only partially due to gravitational sedimentation of beads. At higher Q an additional term proportional to Q becomes important, which is attributed to capture of beads by the magnetic fields near the sensor. It is shown that beads can be washed off the sensor surface.

A High-Throughput SU-8Microfluidic Magnetic Bead Separator

We present a novel microfluidic magnetic bead separator based on SU-8 fabrication technique for high through-put applications. The experimental results show that magnetic beads can be captured at an efficiency of 91 % and 54 % at flow rates of 1 mL/min and 4 mL/min, respectively. Integration of soft magnetic elements in the chip leads to a slightly higher capturing efficiency and a more uniform distribution of captured beads over the separation chamber than the system without soft magnetic elements.