Artificial cilia for microfluidics - report

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Artificial cilia for microfluidics
exploring the use of a horizontally micro-structured ferromagnetic PDMS composite


Cilia are thin, flexible rods found in nature which induce mixing and pumping on a microscale. This way of inducing motion in a fluid can be applied to the domain of microfluidics. As a step towards microfluidic control by artificial cilia, magnetic actuation of artificial cilia was investigated in theory and tested in practise by feasibility prototypes; both confirmed the possibility of inducing motion in ferromagnetic artificial cilia using a magnetic field.

The bending of a cilium as a result of a magnetic field was estimated theoretically for a paramagnetic as well as a ferromagnetic material; the former in the high-gradient field of a current wire, and the latter in the homogeneous field of a magnet. When combined with an estimation of the parameters involved, the resulting cilia movement favoured the ferromagnetic cilium, which could achieve a greater deflection than the paramagnetic cilium and whose behaviour was independent of scaling. Both showed a p3-dependence on aspect-ratio.

Feasibility prototypes were created for experimental verification. The soft polymer polydimethylsiloxane (PDMS) was used as a material for artificial cilia. It was doped with 70nm Fe@C particles to make it magnetic. Characterisation by a vibrating sample magnetometer (VSM) showed that the magnetic behaviour was independent of the concentration of magnetic particles in PDMS, with which the saturation magnetisation Ms, remanent field Mr and susceptibility χ scaled linearly. Extrapolating the measurements to a 100vol% Fe@C-PDMS composite resulted in Ms=0.6MA/m, Mr=0.16 Ms and χ≈5. The coercive field was measured to be Hc=17.5mT. Field-curing of the composite did not substantially enhance its magnetic properties.

To obtain micro-fabricated high-aspect ratio structures, PDMS was structured horizontally using a sacrificial layer lift-off technique. For this procedure, two photosensitive PDMS formulations were investigated, of which s-PDMS was selected. This resulted in micro-structured artificial cilia of about 10μm in thickness and 250μm in length. Actuation with a 50mT field resulted in a deflection of about 180μm, which was in the same order of magnitude as calculated from theory and VSM measurements. Also artificial cilia with a size of one order of magnitude larger were found to behave accordingly. Although the actuating field exceeded the coercive field of the composite, a permanent moment perpendicular to the field was shown to remain and cause the cilium to deflect.

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