Flow-orthogonal bead oscillation in a microfluidic chip with a magnetic anisotropic flux-guide array
- First Online:
- Cite this article as:
- van Pelt, S., Derks, R., Matteucci, M. et al. Biomed Microdevices (2011) 13: 353. doi:10.1007/s10544-010-9503-5
- 480 Downloads
A new concept for the manipulation of superparamagnetic beads inside a microfluidic chip is presented in this paper. The concept allows for bead actuation orthogonal to the flow direction inside a microchannel. Basic manipulation functionalities were studied by means of finite element simulations and results were oval-shaped steady state oscillations with bead velocities up to 500 μm/s. The width of the trajectory could be controlled by prescribing external field rotation. Successful verification experiments were performed on a prototype chip fabricated with excimer laser ablation in polycarbonate and electroforming of nickel flux-guides. Bead velocities up to 450 μm/s were measured in a 75 μm wide channel. By prescribing the currents in the external quadrupole magnet, the shape of the bead trajectory could be controlled.
KeywordsSuperparamagnetic beadsMicrofluidicsQuadrupole magnetShape-anisotropyMagnetic flux-guides
Since point-of-care devices need to give a result within minutes, a new concept is proposed in which beads are made to scan the fluid. Scanning of the fluid can be obtained by forcing the beads in an oscillating motion orthogonal to the flow, see Fig. 1(b). This way, long diffusion lengths no longer play a role and also the radial position of a target species within the channel will not influence the chance of it reaching a bead.
2 Theory and design
For a functional bio-separator, this concept has to be parallelized to allow for high volume throughputs. This was done by multiplexing the concept with a square array of soft-magnetic structures covering multiple parallel channels. These magnetic structures function as flux-guides to the externally applied field. The flux density is concentrated in the flux-guides due to their high permeability and mutual alignment. Because the flux-guides consist of elongated objects that form the cross-shapes, they show shape-anisotropic behavior; only the two tips aligned with the externally applied field can effectively magnetize. This gives an array of small local quadrupoles that show the same magnetization as the macro-scale quadrupole with controllable coils.
4 Experiments and results
To confirm the magnetic properties of the electroformed nickel, hysteresis curves of different samples were measured with a vibrating sample magnetometer (VSM). For conformity the samples had the same shape and size as the flux-guides in the prototype chip. Measurements were done on both single flux-guides and on an array of flux-guides. For an optimal flux-guide, the saturation magnetization should be high to allow for a high flux density. The hysteresis should be low so that the direction of magnetization in the flux-guides can easily be changed. The measured values corresponded well with these demands, the saturation magnetization was 430–493 kA/m and the coercive flux density was 5.5–7.2 mT.
The actual bead oscillation experiments were carried out by placing the prototype chip inside the quadrupole magnet, see Fig. 7. This quadrupole was the same as used in earlier experiments (Petousis et al. 2007) and consisted of a circular soft magnetic yoke with four tips pointing to its center and four coils between each of these tips. The chip was located in the center air gap measuring 7 × 7 mm.
To find the maximum frequency at which the bead could still follow, the magnetic field rotation rate was gradually increased. The maximum frequency was 3 Hz at rc = 2 which gave an average velocity of 450 μm/s. At higher frequencies, the bead would still oscillate at the prescribed frequency, but it would not reach both walls anymore. When comparing this to the simulations, it was found that in the experiment, the maximum frequency was a factor 3 higher. One reason was that the simulations were performed in 2D where the bead could not be modeled as a sphere. Instead the cross-section of the bead was taken. This resulted in the drag force being slightly higher than in the experiments.
Experiments on multiple 1 μm beads resulted in the formation of chains of approximately 25 μm long. These chains moved in trajectories very similar to the single bead case. The formation of chains will decrease the drag force as the frontal surface area per bead is decreased, this allows for higher bead velocities. These chains however, also rotated along with the prescribed field, similar as reported earlier (Petousis et al. 2007). This rotation might enhance mixing of the fluid which could improve the capture efficiency in the case of a bio-separation.
We have shown a new concept for the oscillation of superparamagnetic beads orthogonal to the axis of a microfluidic channel. This concept can easily be multiplexed for high throughput applications while electric or magnetic interconnects can be avoided. Our polycarbonate based fabrication combines both the fluidic and magnetic functionality within a single layer to maximize magnetic efficiency. A working prototype chip was successfully manufactured and experiments corresponded well to simulations. Furthermore, control of the bead trajectory shape was possible. This allows for application as a bio-separator or fluidic mixer.
Future work will consist of in-flow experiments. Already some preliminary simulations inside a flow where performed which resulted in stable bead oscillation with fluid velocities up to 0.2 mm/s (an example can be found in the Supplementary material). After this has been validated experimentally, the concept can be tested using an immunoassay. A second possible functionality might be to use the oscillating beads as active mixer elements. Additionally, the manipulation concept should be characterized in more detail using different types of beads and geometries for the channels and flux-guides.