DEVELOPING ARTIFICIAL MICROTOOLS FOR BIOLOGICAL APPLICATIONS

Recent technological developments enable the replacement of large, complicated and expensive instrumentation with cheaper and smaller ones. Among these are the widely studied lab-on-a-chip systems applying microfluidic methods. In these systems channels, reactors and reservoirs are built on a microscope slide within a few mm2 area in order to carry, mix, react and detect minute volumes of sample. These microscopic labs require the application of mechanical devices of a few micrometer dimension.

Electron microscopic image of a surface-integrated, light driven micromotor and light-guide

In our lab we are developing micromachinery for microfluidic applications. The structures with a typical size of less than ten micrometer and sub-micrometer resolution are made of polymer that hardens in a pre-defined 3D shape upon illumination with focused laser light. The beam consisting of femtosecond pulses initiates two-photon absorption in the polymer exclusively in the vicinity of the focal spot. This two photon polymerization method enables the production of structures with features down to 200 nanometers.

Polymerization of light-driven microstructures

The first group of polymerized structures is actuated by radiation pressure when the impulse of light is transferred to the objects via reflection, thereby initiating their movement. A typical example for these structures is a surface-attached microwheel of 10μm diameter [Kelemen 2006]. The wheel rotates on an axis when illuminated by a beam emerging from a light guide, also polymerized to the surface by a focused laser beam. Similar wheels are intended to serve as power sources for complex structures performing various microfluidic tasks. We are developing gear shift systems for this kind of machinery, similar to those of the macroscopic world. Besides the successful polymerization of various gears, we have polymerized other mechanical parts, such as spiral springs and tested their performance.

Polymerization of micromanipulators for optical tweezers applications

Our new research area is the polymerization of microtools for optical trapping systems. Optical tweezers are based on the trapping force of a focused laser beam exerted on transparent micrometer-sized objects with refractive indexes higher than that of the surrounding medium. When the task is to interact with biological objects, most trapping systems use chemically functionalized microbeads. The beads enable only translational manipulative motion. With the tools we polymerize, the optical trap will be capable of manipulation with more complicated forms of motion. Due to the variability of the tools’ shape, non-trivial movements such as the twisting of the sample will become possible. We tested the manipulation capabilities of the microtool-optical trap system in an experiment where a complex 3D superstructure was assembled, an operation which was only possible with high-precision translational and rotational movements. [Rodrigo 2009].

Electron microscopic images of the basic units polymerized for the 3D assembly experiment, and optical microscopic image of the process of the assembly

Polymerization with modified laser beams

The most important reason for the investigation of the application of modified laser beams is the acceleration and simplification of the polymerization process. Here the initial single laser beam is altered by a Spatial Light Modulator (SLM) such that several identical laser beams or extended illuminated patterns are projected into the sample and perform polymerization. When we create several distinct but identical focal spots, the result of this parallel-type polymerization is the same number of identical objects (even 25 or 30) [Kelemen 2007]. When a complex and continuous light pattern is focused into the sample, the polymer hardens in the same shape, without the need of time-consuming scanning of the beam.

Surface activation of the polymerized microstructures

The SU8 polymer used for polymerization is an epoxy-based resin which is chemically quite inert, therefore it practically does not interact with biological objects without further treatment. In many cases this is a requirement, but for the micromanipulators designed for optical trapping applications, the interaction is a must. For this the surface of the polymer has to be activated through one of the processes described in the literature. We are adopting the surface treatment protocols to the microstructures, equipping them with small functional groups, macromolecules, proteins, as well as metal colloids. Our initial results show the coating of microrods’ and manipulators’ surface with the protein streptavidin in high density.

Electron and fluorescent optical microscopic image of polymerized helical rods. The optical image shows the fluorescence of the labeled protein streptavidin applied on the rods’ surface

Future plans, applications

The polymerized microstructures can be used in microchannel environment to pump or mix liquids of picoliter volumes. The manipulators made for optical trapping systems, depending on their artificially created surface quality, will enable researchers for various localized measurements on cell surfaces.