Novel Magnetic Mixer for Magnetic Bead-Based Microfluidic Bioassays Miguel Berenguel1*, Xavier Granados2, Jordi Faraudo2, Julián Alonso1 and Mar Puyol1

1 Sensors & Biosensors Group, Universitat Autònoma de Barcelona, Bellaterra 08193 (Spain)2 Institut de Ciència dels Materials de Barcelona (ICMAB-CSIC), Campus UAB, Bellaterra 08193 (Spain)* Miguel.Berenguel@uab.cat

Figure 3. Sketches of the three magnet configurations with the corresponding magnetic field (color map), the magnetization of the particles (black arrows) and the acting force (white arrows). A) single magnet, B) two magnets antiferromagnetic-like coupled, in order to enhance the magnetic gradient, and C) the same magnet couple including a static third magnet for creating a bias magnetizing field, which produces an out of plane magnetization.

Figure 2. Schematic representation of the magnetic actuator. A) and B) show schematically the magnetic actuator from different perspectives. Notice that the rotation axis and the center of the magnet’s circumference are not the same. C) Scheme of the MBs’ movement while the actuator ro-tates. D) Pictures of the MBs’ movement.

Figure 7. Calibration curves for the immuno-assay performed without mixing (0 mm/s) and at two different mixing speeds.

A B C D

Figure 1. Fabrication process of the microfluidic device schematically represented. A) CAD design, B) CNC micromilling machine, C) alignment of the layers and bonding, and D) final device.

G BSGrup deSensors iBiosensors

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Rotation Axis

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Figure 6. Signal obtained at different move-ment velocities of the MBs. The dashed line indicates the relative signal at 0 mm/s.

Figure 4. Analytical concept of the immunoassay. The anti-E. Coli O157 MBs (Invitrogen) react successively with the bacteria and the alkaline phos-phatase labelled antibody. Then the immunocomplex is injected in the chip and the substrate 4-MUP is added to generate the fluorescent product.

Off-Chip On-Chip

Figure 5. Detection system used for the fluo-rescence on-chip measurements

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IntroductionMicrofluidic bead-based bioassays are a very powerful technique, with potential applications in fields ranging from medical diag-nosis to environmental analysis and food safety. Nevertheless, these microanalytical systems are limited by the long incubation times required for the molecules to interact by diffusion, being worth noticing that the reaction with the magnetic beads (MB) is heterogeneous.

Herein, we propose a magnetic actuator composed of a mobile set of magnets, which enhances the reaction performance by ac-tively mixing the MBs with the reagents dissolved in the medium.

The magnetic actuator would also enable the control of the MBs within the microfluidic system, thus making it possible to move them from one chamber to another depending on the needs of the assay.

Research Goals• Design and study of a magnetic actuator to control the MBs and enhance the kinetics of a reaction by actively mixing on-chip

• Application of the actuator to the detection of E. Coli O157

Magnetic Actuator

Microfluidic Device Fabrication ResultsE. Coli O157 ImmunoassayAs a proof of concept, a MB-based immunoassay is designed for the detection of E. Coli O157. The formation of the immunocomplex was carried out off-chip and subsequently injected into the microfluidic device to carry out the enzymatic re-action. In this way, we focused on evaluating the enhancement provided by the magnetic actuator for a MB-based reaction on-chip.

due to the overlapping of the stray field with the flux in the poles, as well as an increase of the gradi-ent of the magnetic field in between both magnets, when they are coupled. The static magnet (Fig. 3 C) increases the bias magnetizing field, enhancing the force over the MBs. However, the static magnet would only have an effect when the magnetization of the MBs was not saturated, as it already appears to happen in the simulation of configuration B (Fig. 3 B).

Thus, Configuration B was chosen for the experimental setup, as it fulfills the criteria of simplicity, magnetic field strength (saturation of the MBs) and appropiate size.

Three different configurations of magnets were studied (Fig. 3) taking into account the fol-lowing criteria: simplicity for their integration in the rotat-ing unit; magnets’ dimensions for the accurate movement of the MBs; and expected mag-netic field strength.

As a general trend, Fig. 3 shows that the magnitude of the flux density of the field is larger for the couple of mag-nets (Fig. 3 B and C) than for the single magnet (Fig. 3 A),

MB-based enzymatic reaction on-chipThe reaction product was measured with a miniaturized fluorescence detection system (see Fig. 5). The movement of MBs can be tuned by changing the rotation speed of the magnetic actuator, and it enhances the reaction kinetics due to the mixing. The optimal movement velocity was found to be 1.7 mm/s (see Fig. 6). The calibration curves show a 2.7-fold sensitivity enhancement (see Fig. 7) when using the magnetic actuator. Concentrations of E. Coli as low as 600 cfu/mL have been determined.

Conclusions and Future Work• A magnetic actuator has been designed and studied by means of COMSOL simulations and experi-mentally by an enzymatic reaction using commercial MBs.

• Different concentrations of E. Coli O157 have been determined using a fluorescence immunoassay. The magnetic actuator showed a 2.7-fold sensitivity enhancement over the retention-only systems.

• Further research must address the integration of all the immunoassay’s reactions on-chip.

Watch how it works!Scan this QR code with your smartphone to see a video of the magnetic actuator moving the MBs. You can also have a look at the poster online, or check out our website and see what else we are working on.

AcknowledgementsThe authors acknowledge the financial support of the Govern-ment of Catalonia (FI program), MICINN (project CTQ2012-36165), co-funded by FEDER, and MEC (Consolider Nanoselect, MAT2008-01022).

The microfluidic device is fabricated using the thermoplastic Cy-clic Olefin Co-polymer (COC). Its design is very simple for this first stage of the research. It consists of two main microfluidic ele-ments, namely the reaction chamber, to carry out the reactions of the bioassay, and the optical detection chamber, for the fluores-cence measurements. The fabrication process is shown in Fig. 1.

Simulation of the Magnet Configurations