Magneto-mechanical mixing and manipulation of picoliter volumes in vesicles

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Birgit Hausmann


T. Franke, L. Schmid, D. A. Weitz and A. Wixforth "Magneto-mechanical mixing and manipulation of picoliter volumes in vesicles" Lab Chip, 9, 2831-2835 2009



Magnetic manipulation, positioning, agitation and mixing of ultrasmall liquid volumes has been realized utilizing superparamagnetic beads in giant unilamellar vesicles. In the presence of a magnetic field the beads align to form extended chains while a rotating magnetic field provokes the chains to break up into smaller fragments caused by the interplay of viscous friction and magnetic attraction.

Results and Discussion

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While a magnetic field gradient generates a force on the magnetic dipole chains a rotational field introduces spinning. An electroformation method was used to fabricate the vesicles. The lipid in chloroform was deposited onto two indium tin oxide (ITO) coated glass slides and the organic solvent was evaporated in vacuum. An aqueous solution containing the superparamagnetic beads was added to the dried lipid. The two ITO plates were mounted in parallel and an electric field was applied. Finally, the voltage was increased to facilitate the separation of vesicles. A theoretical minimum magnetic field of <math> 59 \mu T </math> is needed to align the superparamagnetic beads (of <math> 1 \mu m </math> size) within the vesicles in chains. Since the force to move the vesicle containing beads is proportional to the magnetic field gradient and it also has to be equal to the hydrodynamic drag force, the necessary field can be estimated. The direction of the magnetic field gradient also determines the direction of movement of the vesicles. A rotating magnetic field causes the superparamagnetic chain to rotate inside the vesicle, eventually causing a rotation of the vesicle itself. A fluorescein was added continuous phase fluid to prove that the vesicle is not leaking any content. At high cons the fluorescence. When a vesicle is moved across the microchamber, repeated rotations of the chains were initiated without detection of any fluorescent signal, which shows that the vesicle is leakproof. But, adding the membrane-porating surfactant Triton-X causes water to permeate through the membrane and a strong increase in fluorescent signal can e observed (Fig. 3). When the content of the vesicle is released, the intravesicular fluid volume mixes diffusively with the surrounding bulk solution. The magnetic beads within a vesicle can be used to enhance mixing by active agitation. A rotating external magnetic field is applied to magnetic chains. When a critical frequency of chain rotation is reached the bead chains split into smaller fragments. A model is provided to estimate the number of beads: it can be calculated balancing the tangential drag force with the attractive magnetic force acting between the beads that make up the chain.