Magnetic polymer actuators for microfluidics

The manipulation of fluids on the sub-millimetre scale -microfluidics- finds application in the miniaturisation and integration of biological analysis, chemical synthesis, optics and information technology. In a microfluidic device, fluids need to be transported, mixed, separated and directed in and through a micro-scale system. The effcient mixing of fluids -particularly needed for analysis or synthesis -presents a large challenge in microfluidics. Mixing cannot occur by turbulence because of the low Reynolds number that prevails in micro-channels, and molecular diffusion is rather slow in achieving mixing on the scale of a microfluidic channel. A solution for obtaining mixing on efficient time scales has been to passively or actively manipulate the fluids to induce chaotic advection and increase the interfacial area of two fluids progressively, thereby decreasing the length scale over which diffusion has to take place to mix the fluids. In this thesis we investigate magnetic polymer micro-actuators that can be incorporated on the walls of microfluidic channels and can be actuated with magnetic fields. A magnetic stimulus that addresses micro-actuators is very robust, because of the low interaction between magnetic fields and (bio)chemical fluids. The use of polymeric materials for producing micro-actuators potentially allows for cost-effective micro-devices with integrated fluidic actuation. The aim of the thesis is to provide generic and advanced fluid control inside microfluidic devices, e.g: for the purpose of integrated pumping or for the purpose of mixing. Superparamagnetic and ferromagnetic particles have been dispersed in polymers with a low elastic modulus and the composites have been characterised mechanically and magnetically. A low elastic modulus polymer enables large deflections of micro-actuators with practical magnetic fields. In this thesis, various types of the elastomer polydimethylsiloxane (PDMS) have been used for constructing the polymeric micro-actuators with a low elastic modulus. The efficiency of magnetic actuation on small scales is discussed for two actuator concepts. It is shown that actuation by magnetic torque scales neutrally with miniaturisation, allowing for actuation with externally generated magnetic fields. In contrast, actuation by magnetic gradient force scales inversely proportional to the size of the actuator. Therefore magnetic gradient actuation cannot be induced by an external electromagnet and requires a localised generation of magnetic fields. Because viscous effects dominate over inertial effects at small scales (Reynolds number <1), symmetric and in-phase movements of micro-actuators would induce no net fluid flow. Therefore the generation of asymmetric or out-of-phase movements of microactuators has been investigated for each actuator concept. The interaction of magnetic particles embedded in PDMS has been studied and compared to the interactions in a ferrofluid. The enhancement of magnetic susceptibility due to the particle interactions was found to be limited because of the clustering of magnetic particles in the polymer that induces local demagnetisation. The alignment of clusters of particles in a magnetic field was investigated and the resulting magnetic anisotropy was quantified. Modelling has established that such an intrinsic magnetic anisotropy for an actuator can provide an increase in actuation amplitude up to one order of magnitude, for the same stimulus. The magnetic PDMS composites developed in this thesis have been used to fabricate high aspect ratio micro-actuators that are standing or lying on a substrate. Standing superparamagnetic PDMS micro-actuators were produced by mould replication. The standing micro-actuators have been actuated locally with the high magnetic field gradient generated by an integrated current wire (resulting in actuation by magnetic gradient force). The local stimulus allows for individual addressing of the micro-actuators and potentially enables out-of-phase movements of adjacent actuators. Possible geometries for the actuator device have been explored with models that describe the deffection of the actuators and the heat dissipation in the current wire. The fabricated micro-actuators were found to respond to the magnetic stimulus of the current wire but also to the thermal stimulus associated to the heat dissipation in the current wire, because of temperature dependent swelling of the micro-actuators in a solvent. The different time scales of magnetic and thermal actuation allowed the creation of an asymmetric movement. The standing micro-actuators have also been actuated by a homogeneous magnetic field generated by an external electromagnet (resulting in actuation by magnetic torque). A non-constant phase lag was demonstrated between actuators having different amplitudes of defection, which can potentially provide efficient mixing on small scales. The high frequency actuation of the standing micro-actuators was found to be limited to 5 Hz, which we attribute to the viscous behaviour of the PDMS. Lying ferromagnetic PDMS micro-actuators were produced with lithographic and sacrificial layer techniques. The lying micro-actuators have been actuated by a homogeneous magnetic field generated by an external electromagnet (resulting in actuation by magnetic torque). The permanent magnetisation of the actuators allowed for much larger deflections than for the standing superparamagnetic actuators. For a specific initial magnetisation of the actuators and using a rotating magnetic field, the actuators were shown to exhibit selectively either a symmetric or an asymmetric movement. The actuation at high frequencies of the microactuators was limited by the viscous drag in fluid and, in our experiments, by the high frequency limitations of the electromagnet. The micro-actuators could operate up to a frequency of 50 Hz, which is one order of magnitude higher than for the standing super paramagnetic actuators. The higher actuation frequency indicated that the type of PDMS used to fabricate the lying ferromagnetic micro-actuators exhibits less viscous behaviour. In a microfluidic cavity, the lying ferromagnetic micro-actuators induced local vortices or translational net fluid flows, depending on their initial magnetisation. Two micro-actuators pointing in opposite directions were actuated fully independently with the same external stimulus, depending on the rotation direction of the magnetic field. The different re-magnetisation in each case could explain the possibility for individual actuation. Provided with this independent actuation, two sets of vortices can be controlled individually in a microfluidic device, which is particularly promising to mix fluids with e.g: a blinking vortex protocol. The observed translational net fluid flows can in principle provide integrated pumping in microfluidic devices