(813d) Wirelessly Controlled Microfluidic Actuators Using Radiofrequency Electromagnetic Induction

Wirelessly Controlled Microfluidic Actuators using Radiofrequency Electromagnetic Induction

Wasi Syed*, Wen-I Wu*, Scott Campbell**, Todd Hoare** and P. Ravi Selvaganapathy*

* Department of Mechanical Engineering, McMaster University, CANADA

** Department of Chemical Engineering, McMaster University, CANADA

E-mail: campbesb@mcmaster.ca  |  hoaretr@mcmaster.ca

INTRODUCTION: The biomedical applications of microfluidic systems have enormous potential, particularly for their use in handling biomolecules.¹ However, the development of commercializable integrated microfluidic systems have been limited by the lack of reliable microfluidic components, such as micropumps and microvalves. A promising type of microvalve for these systems are thermopneumatic actuators, which operate based on the temperature-induced volume expansion of a phase-change material to block off a segment of a microfluidic channel to control flow. These actuators can provide large displacement forces and have facile fabrication as compared to electrostatic, magnetic, piezoelectric, and bimorph actuators, leading to their widespread use in microfluidics.²

Conventional thermopneumatic actuators are controlled by application of an electrical current to microheaters that produce heat due to resistance losses. However, in some instances, such as in therapeutic implants and microsurgical tools, it is not practical to have wired connection and preferable to have wireless actuation.³ Recently, hyperthermia treatments induced using superparamagnetic iron oxide nanoparticles (SPIONs) and external oscillating magnetic fields (OMF) has also been recognized as a promising therapy for cancers by safely and remotely delivering heat to a local area inside the body.⁴ In this work we plan to dissipate the heat generated by similar nanoparticles upon the application of an external OMF to the surrounding phase-change material in order to control microfluidic flow rates around the actuator. The phase-change material used herein is polyethylene glycol (PEG, MW ~1,000), which has a melting temperature of 39˚C and a corresponding volume change of 25%. Subsequently, when PEG is combined with SPIONs this volume expansion can used to control micro pumps and valves wirelessly using an OMF.

EXPERIMENTAL: The SPIONs are synthesized using a coprecipitation method⁵ with Fe(III)Cl₃ and Fe(II)Cl₂ followed by PEG (MW ~8,000) surface functionalization. The resulting iron oxide nanoparticles are 10-20 nm and aggregate into larger clusters, as shown in Figure 1.

The nanoparticles are mixed with molten PEG at concentrations of 0.5, 1.0 and 1.5 mg/mL and cast as thin sheets (2.5 mm thick) at room temperature. Small disks (∅6 mm) are punched and then placed onto spin-coated PDMS membranes (100 µm thick) as shown in Figure 2.

Uncured PDMS pre-polymer is casted on top to seal the disks. The actuator blocks are diced from this composite after a 24 hour curing process at room temperature. Subsequently, these actuator blocks are integrated with microchambers and microchannels by plasma treatment to form microfluidic pumps and valves respectively, both shown in Figure 3.

The OMF used to induce heating of the SPIONs has three coils 4 cm in diameter, and operates at  a frequency of 220 kHz to produce a maximum manetic flux of 4.5 mT for all experiments.

RESULTS: Initial experiments were performed on OMF induced micropumps and microvalves. In the case of micropumps, the microchamber of the pump is filled with dye for visualization and a camera is used to track the volume expansion as a change in the height of the liquid in the outlet column, as shown in Figure 4a.

It was observed that volumetric expansions of ~25% are possible (similar to that of the literature⁶) and the rate of volumetric expansion is proportional to the concentration of the SPION nanoparticles, as shown in Figure 5. Notably, the micropumps were shown to be capable of providing high output volumes (>18 mm³), which could further increase its potential applications.

In the case of the valve, a flow rate of 1 mL/min (DI water) is infused by a syringe pump through a microfluidic channel with rounded cross-sectional shape (20 mm length, 1 mm width, 0.5mm height) and the upstream pressure is monitored by a pressure sensor (as depicted in Figure 4b). Figure 6 shows the valve is effectively closed against a pressure of 10 psi as long as the magnetic field is on. When the magnetic field is turned off, the PEG within the valve solidifies and the valves are opened within three minutes.

The slow response to fully open the valve after OMF application is due to the time needed to cool the molten PEG, which can be further reduced by increasing its surface-to-volume ratio (ie. smaller PEG volume). The effects of the dimensions of the actuator, the SPION concentration in the PEG material, and the controlled actuation of multiple will also be presented.

CONCLUSIONS: Novel magnetic microfluidic actuators that can be controlled by an external OMF are capable of both providing high output volumes as micropumps and can withstand high pressures as microvalves, which could potentially permit their use in integrated microfluidic systems  or as implanted devices for medical therapy applications.

REFERENCES: (1)             Oh, K. W.; Ahn, C. H. Journal of Micromechanics and Microengineering 2006, 16, R13–R39.  (2)  Gravesen, P.; Branebjerg, J.; Jensen, O. S. Journal of Micromechanics and Microengineering 1993, 3, 168–182. (3)  Smith, T. J.; Coyne, P. J.; Smith, W. R.; Roberts, J. D.; Smith, V. American journal of hematology 2005, 78, 153–154. (4)  Mornet, S.; Vasseur, S.; Grasset, F.; Duguet, E. Journal of Materials Chemistry 2004, 14, 2161–2175. (5)  Hoare, T.; Santamaria, J.; Goya, G. F.; Irusta, S.; Lin, D.; Lau, S.; Padera, R.; Langer, R.; Kohane, D. S. Nano letters 2009, 9, 3651–3657. (6)  Song, W. H.; Kwan, J.; Kaigala, G. V; Hoang, V. N.; Backhouse, C. J. Journal of Micromechanics and Microengineering 2008, 18, 045009.

ACKNOWLEDGEMENTS: This research is funded by the Natural Sciences and Engineering Research Council of Canada (NSERC), the Ontario Graduate Fellowship, and the J.P. Bickell Foundation (Medical Research Grant).