A micromagnetoflowcell for microfluidic measurements
Introduction
The advantages of microfluidic systems for analytical applications are [1]:
- 1.
The possibility of using minute quantities of sample and reagents (down to picoliters).
- 2.
Relatively fast reaction times when molecular diffusion lengths are of the order of the microchannel dimensions.
- 3.
A large surface to volume ratio for assays.
Superparamagnetic microparticles have been used extensively in magnetic separation, magnetic resonance imaging, drug delivery and hyperthermic tumour therapy [2]. Such small particles can be used in bio-assays, or even in vivo. The microparticles used in this work (M450 Dynabeads®) are spheres of approximately 5 μm diameter and can be manipulated using electromagnets. Each microparticle consists of individual monodomain superparamagnetic nanoparticles (γ-Fe2O3) encapsulated by a polystyrene surface which is highly hydrophobic. It is magnetized in the presence of a magnetic field and loses its magnetization when the magnetic field is removed [3]. It is, in principle, possible to drive a magnetized bead through a fluid-filled microchannel and, by monitoring its transport properties, to measure the viscosity of the microscopic quantity of fluid in the channel. This is the principle of the micromagnetoflowcell.
Section snippets
Design and fabrication
The principles and design of the micromagnetoflowcell are shown in more detail in Fig. 1. The NdFeB permanent magnet (strength 30 mT) saturates the magnetization of the superparamagnetic particles which are transported along the channel by the action of the magnetic coils in providing the necessary magnetic field gradient. A non-visual method of measuring the particle motion is being researched, but, for the initial proof of principle trials, the motion of the particles is recorded and measured
Experiments
The experimental micromagnetoflowcell chip, as shown in Fig. 5, was mounted on an optical microscope stage and the coils activated with a current of 0.1 A. The motion of the particles along the microchannels was viewed by optical microscope through the PDMS. Video images of the particle trajectories were captured using a high speed CCD camera at 500 fps.
The resin resist SU-8 is highly hydrophobic due to its chemical structure, which consists of an aromatic ring and epoxy functional part resulting
Conclusions
The basic principles of the micromagnetoflowcell, for use as a microviscometer, have been explained and its design and fabrication described. Magnetic field induced local motion has clearly been observed, but there is still work to be done if this is to be extended to longer range motion along the complete length of the microchannel. This will require a “phased” electrical drive current distributed among the coils to drive particles in a quasi-continuous fashion. In addition, more work is
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