We derive a model for voltage-induced wetting, so-called electrowetting, from the principle of virtual displacement. Our model includes the possibility that charge is trapped in or on the wetted surface. Experimentally, we show reversible electrowetting for an aqueous droplet on an insulating layer of 10 µm thickness. The insulator is coated with a highly fluorinated layer impregnated with oil, providing a contact-angle hysteresis lower than 2 • . Analyzing the data with our model, we find that until a threshold voltage of 240 V, the induced charge remains in the liquid and is not trapped. For potentials beyond the threshold, the wetting force and the contact angle saturate, in line with the occurrence of trapping of charge in or on the insulating layer. The data are independent of the polarity of the applied electric field, and of the ion type and molarity. We suggest possible microscopic origins for charge trapping.

We demonstrate control of fluid motion in three-dimensional structures with thousands of microchannels. Fluids are manipulated via an electrocapillary pressure, originating from electrostatic control of the solid/fluid interfacial tension in the microchannels. Reversible fluid displacement has been achieved for all channel orientations with respect to gravity. The velocities of several centimeters per second are nearly two orders of magnitude higher than the velocities demonstrated by other electrofluidic actuation principles.

The realization of biomolecular detection assays for diagnostic purposes is technologically very challenging because such tests demand full integration for ease of use and need to deliver a high analytical performance with cost-effective use of materials. In this article an optomagnetic immunoassay technology is described based on nanoparticles that are magnetically actuated and optically detected in a stationary sample fluid. The dynamic control of nanoparticles by magnetic fields impacts the key immunoassay process steps, giving unprecedented speed, assay control and seamless integration of the total test. The optical detection yields sensitive and multiplexed assays in a low-cost disposable cartridge. We demonstrate that the optomagnetic technology enables high-sensitivity one-step assays in blood serum/plasma and whole saliva. Drugs of abuse are detected at sub-nanogram per millilitre levels in a total assay time of 1 min, and the cardiac marker troponin I is detected at sub-picomole per litre concentrations in a few minutes. The optomagnetic technology is fundamentally suited for high-performance integrated testing and is expected to open a new paradigm in biosensing.

We present a plasmonic biosensor based on hundreds of individual gold nanorods with single-molecule sensitivity that are simultaneously monitored in real-time within a dark-field microscopy setup. The approach allows for the statistical analysis of single-molecule interactions without requiring any labeling of the analyte. We study an antibody-antigen interaction and find that the waiting-time distribution is concentration-dependent and obeys Poisson statistics. The ability to probe hundreds of nanoparticles simultaneously will provide a sensor with a dynamic range of 7 decades in concentration and will enable the study of heterogeneity in molecular interactions.

The demand for easy to use and cost effective medical technologies inspires scientists to develop innovative lab-on-chip technologies for point-of-care in vitro diagnostic testing. To fulfill medical needs, the tests should be rapid, sensitive, quantitative, and miniaturizable, and need to integrate all steps from sample-in to result-out. Here, we review the use of magnetic particles actuated by magnetic fields to perform the different process steps that are required for integrated lab-on-chip diagnostic assays. We discuss the use of magnetic particles to mix fluids, to capture specific analytes, to concentrate analytes, to transfer analytes from one solution to another, to label analytes, to perform stringency and washing steps, and to probe biophysical properties of the analytes, distinguishing methodologies with fluid flow and without fluid flow (stationary microfluidics). Our review focuses on efforts to combine and integrate different magnetically actuated assay steps, with the vision that it will become possible in the future to realize integrated lab-on-chip biosensing assays in which all assay process steps are controlled and optimized by magnetic forces.

We demonstrate advanced fluid manipulations using magnetic polymeric artificial cilia on the walls of a microfluidic channel. In nature, cilia are little hairs covering the surface of micro-organisms which enable them to manipulate a fluid on the micro-scale. The asymmetric movement of natural cilia is crucial to obtain a net fluid flow. We have developed a ferromagnetic polymer made from iron nanoparticles and polydimethylsiloxane, and describe a process that can structure the material into high aspect ratio lying artificial cilia with a length of 300 microm. These artificial cilia were actuated with a homogeneous rotating magnetic field (micro(0)H < 50 mT) generated with a compact external electromagnet. An asymmetric movement involving torsion could be created when the cilia were provided with a remanent magnetisation perpendicular to the plane of rotation of the magnetic field vector. The artificial cilia could be actuated in fluid up to a frequency of approximately 50 Hz. In an aqueous solution in a microfluidic chamber we were able to generate rotational as well as translational fluid movements with fluid velocities up to approximately 0.5 mm s(-1).

The antigen-capturing activity of antibodycoated nanoparticles is very important for affinity-based bioanalytical tools. In this paper, a comprehensive study is reported of the antigen-capturing activity of antibodies that are nondirectionally immobilized on a nanoparticle surface. Superparamagnetic nanoparticles (500 nm) were covalently functionalized with different quantities of monoclonal antibodies against cardiac troponin I (cTnI). At a low antibody surface coverage, up to 4% of the immobilized antibodies could capture antigen molecules from solution. At high antibody coverage (≥50 × 10 2 antibodies per nanoparticle, i.e., ≥ 64 × 10 2 antibodies per μm 2 ), the fraction of antigen-capturing antibodies drops well below 4% and the number of active antibodies saturates at about 120 per nanoparticle. The fraction of active antibodies is small, yet surprisingly their dissociation constants (K d ) are low, between 10 and 200 pM. In addition, the surface-binding activity of the antibody-coated nanoparticles was analyzed in an optomagnetic sandwich immunoassay biosensor, measuring cTnI in undiluted blood plasma. The data show that the immunoassay response scales with the number of active antibodies, increasing initially and saturating at higher antibody densities. The observations are summarized in a molecular sketch of the attachment, ordering, and functionality of antibodies on the nanoparticle surface.

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