PRECISION MAGNETOPHORESIS SEPARATION OF SUPERPARAMAGNETIC COLLOIDS.
The manipulation of magnetic particles by the use of inhomogeneous magnetic fields
(magnetophoresis) has emerged as a topic of great interest in a wide range of research and
technological areas [1,2]. The idea behind magnetic separation is to take advantage of the
distinctive magnetic response of the particles in solution to remove them from complex
mixtures by the use of applied inhomogeneous magnetic fields . In the different applications,
magnetic particles are typically functionalized with proper chemical groups, designed to bind to
specific non-magnetic components, thus enabling the separation of non-magnetic materials by
combining the use of magnetic particles and magnetic fields.
Current standard magnetophoretic techniques (such as High Gradient Magnetic Separation)
suffer from different difficulties, including lack of reproducibility and scalability and the loose
of control over the magnetic conditions under which the magnetic particles are removed.
Basically, the external magnetic field applied induces highly inhomogeneous gradients (as large
as 104 T/m) in the separator . The magnetic fields generated in this way are not predictable or
reproducible and the magnetic force experienced by the colloids is not uniform across the
system. These inhomogeneous conditions common to the HGMS approach makes difficult to
develop numerical and/or analytical solutions to the problem, which would help in a better
understanding of the magnetophoretic mechanisms and a higher performance; for instance, by
means of a better design of separators or a better choice of the magnetic particles used in
In order to overcome these limitations and facilitate the use of magnetic colloids in
biotechnological applications we have made use of a new concept of magnetic separation (the so
called Precision Magnetophoresis) to effectively remove different types of superparamagnetic
nanoparticles from solution. The process is based on the use of a uniform magnetic gradient,
allowing better quality control and scalability, together with a better control over the
experimental conditions, providing a proper framework for the development of theoretical
models (analytical or numerical).
Within this approach, we have studied the magnetic separation of superparamagnetic colloids
by theory and experiments. Under sufficiently large magnetic fields, a fast
separation process occurs in which the superparamagnetic colloids form chain-like structures
(reversible aggregation) aligned parallel to the applied external magnetic field. It is observed
that, under the effects of a magnetic gradient, these structures move faster than would move an
individual colloid in the same conditions, resulting in a dramatic reduction (orders of
magnitude) of the separation time. Thus, the reversible aggregation of colloids becomes a key
aspect when one thinks to enhance the magnetic separation of magnetic colloids. The kinetics of
both separation processes (cooperative and non-cooperative magnetophoresis) has been
characterized theoretically and experimentally as a function of the different properties of the
colloidal dispersion (magnetic response, size of particles, concentration, solvent viscosity,
etc, ...) and the magnetic separator (magnetic gradient and size of the separator) . Following
the experimental results obtained for different combinations of magnetic colloids and magnetic
fields, the required conditions for the reversible aggregation to occur has been recently
discussed on the base of computer simulations and thermodynamical models .
In the much simpler situation in which the separation process is driven by the individual motion
of the colloids, an analytical solution for the kinetics of the magnetophoresis separation process
is provided . The obtained solution is valid under certain restrictive but realistic conditions which
are explicitly discussed here. We also show the utility of the analytical model by comparing our
predictions with experimental results obtained with superparamagnetic particles of different
sizes and magnetizations. We expect that the availability of a simple, analytical model will
allow for a better understanding of the underlying physics of magnetic separation processes and
also allow a rational design of applications.
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