OPTICAL MICROMANIPULATION

Light carries momentum, i.e. it exerts force upon objects. In everyday life this force is negligible. In the microworld, however, it is different: if a micrometer-sized particle is illuminated by light of moderate intensity (10 mW), the light pressure may be significant. If a particle with an index of refraction higher than that of its surroundings is placed in a focussed light beam, it will be trapped in the focus. Micrometer-sized particles can be trapped this way. Typical forces fall in the pN range: this is just the range to manipulate biological objects (cells, cell organelles) in water and also of forces exterted by biological machines. Consequently, this is a procedure with immense potential in biology.

In the basic case the position of spherical objects is controlled by optical tweezers. It would offer an additional degree of control if the orientation could also be controlled, thereby extending the manipulation possibilities. In our laboratory we investigate the interaction of optical tweezers with objects of special shape.

We can produce micrometer-sized particles of arbitrary shape by two-photon excited photopolymerization of light-hardening photoresits, and we use such objects to study new phenomena of trapping. In addition, based on the trapping of such objects we develop novel tools of micromanipulation. We introduce two typical methods.

Particles of helical, propeller shape will rotate in optical tweezers. Objects to which such rotors are attached can be rotated, and micromechanical machines can be driven with them. They can be used for the generation of complex machines for use in biology.

If the optical tweezers is generated by linearly polarized light, flat objects will be trapped such that they will orient along the plane of polarization. With this method objects can be oriented. If we attach a molecule to such a particle, we can exert or measure torque on this molecule. In this way we can twist molecules, and their torsional properties can be determined. This is very important in biology, since there are numerous processes involving rotation. For example, to access the information stored in DNA it has to be untwisted, consequently information about the torsional properties of DNA is fundamental for understanding the encoded information. With our method we can determine the torsional elasticity of DNA, an important parameter for the function. Numerous DNA/protein interactions and protein motions involving rotation can be studied by this method.

Optical control in microfluidics

In modern biochemical and medical diagnostical research there is a need for devices that operate with small amounts but are able to process large amounts of samples in a short time. These requirements can be met by decreasing size, and microfluidics (lab-on-a-chip) is the development seeking solution in this direction. In the size range of these devices the physics of processes is quite different from that of the macroscopic world, consequently the solution is not merely a scaling/down of known cases; development is progressing in different directions.

We believe that optical control can be very useful in microfluidics and we perform research in this area studying the processes on which future devices can be based.

Using the photopolymerisation technique, we build microfluidics systems where channels and optical waveguides are integrated into a single system. Here light is used to investigate material (cells, molecules) in the channel; in addition, light is also used to manipulate objects. For example, the fluorescence of cells can be measured using the integrated optical waveguides and selected ones are separated by the pressure of light also carried in integrated waveguides. Disposable all-optical microfluidic cell sorters were built this way.

To extend optical control, we developed the concept of light-controlled electroosmosis. In a liquid-filled channel the surface charge of the wall is neutralized by opposite charges collecting at the wall that can be moved by an electric field parallel to the wall. In microchannels the total volume can be agitated this way, and this is electroosmosis. We cover the channel wall with photosensitive material, and in this way electroosmosis can be modulated by light.

We have developed different flow control schemes: in a single channel we can turn the flow on and off. We have built a light-controlled flow switch: in a biforcating channel, the direction of flow is selected by light. In microfluidics flow is always laminar, therefore mixing, a key process in biochemical reactions is always a problem. Light-controlled electroosmosis again offers a solution: by illuminating the light-sensitive channel wall with light of appropriate pattern, the flow pattern can be controlled within a single microchannel.

The elaborated methods are useful to study flow properties of microchannels, at the same time they have great potential offering methods for controlling microfluidics systems by light. Our aim is the development of completely light-controlled microfluidics systems.