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Teaser, summary, work performed and final results

Periodic Reporting for period 2 - PrintPack (Arranging the Particles: Step Changing Chemical Measurement Technology)


The PRINTPACK project aims at developing a fabrication method to produce perfectly ordered 3D-materials composed of 3D-networks of spherical (functionalized) microparticles. One key application for this novel type of meta-materials is liquid chromatography (LC). In the past...


The PRINTPACK project aims at developing a fabrication method to produce perfectly ordered 3D-materials composed of 3D-networks of spherical (functionalized) microparticles. One key application for this novel type of meta-materials is liquid chromatography (LC). In the past decades, the progress in LC has basically been following Moore’s law over the last decade, but will now soon come to a halt. LC is the current state-of-the-art chemical separation method to measure the composition of complex mixtures. Driven by the ever growing complexity of the samples in e.g., environmental and biomedical research, LC is constantly pushed to higher efficiencies. Using highly optimized and monodisperse spherical particles, randomly packed in high pressure columns, the progress in LC has up till now been realized by reducing the particle size and concomitantly increasing the pressure. With pressure already up at 1500 bar, groundbreaking progress is still badly needed, e.g., to fully unravel the complex reaction networks in human cells.

For this purpose, it is proposed to leave the randomly packed bed paradigm and move to structures wherein the 1 to 5 micrometer particles currently used in LC are arranged in perfectly ordered and open-structured geometries. This is now possible, as the latest advances in nano-manufacturing and positioning allow proposing and developing an inventive high-throughput particle assembly and deposition strategy. The PI\'s ability to develop new parts of chromatography will be used to rationally optimize the many possible geometries accessible through this disruptive new technology, and identify those structures coping best with any remaining degree of disorder. Using the PI\'s experimental know-how on microfluidic chromatography systems, these structures will be used to pursue the disruptive gain margin (order of factor 100 in separation speed) that is expected based on general chromatography theory.

Testing this groundbreaking new generation of LC columns together with world-leading bio-analytical scientists will illustrate their potential in making new discoveries in biology and life sciences. The new nano-assembly strategies might also be pushed to other applications, such as photonic crystals.

1) Determine the ideal 3D spherical particle arrangement for LC under the constraints of the currently conceivable fabrication possibilities and build a dispersion model for chromatographic beds that is complete enough (i.e., wherein all parameters can be calculated a priori) to rationally design this optimal arrangement.
2) Develop new tools and fabrication strategies to produce large-area, perfectly ordered arrays of monodisperse spheres in the 1 to 5 m-size range in 2D, 2.5D and 3D-arrangements.
3) Produce ideally structured LC columns with various cross-sectional area (ranging from single cell analysis format to standard industrial size) producing groundbreaking separation speeds and efficiencies.
4) Demonstrate the potential of the developed concepts and technologies in top-level chemical measurement applications in science and industry

Work performed

In the first half of the project, the main emphasis was on carrying out WP1-3, with some preliminary work to be conducted in WP5. The latter has been conducted by Gitte Coopmans, who has been selecting the optimal separation conditions (pH, T, gradient parameters) of biological samples involving peptides and proteins.

In WP1.1, correlations for the dependency of the dispersion coefficient and the local Sherwood number as a function of the Peclet number have been established. These calculations will define a new yardstick for the chromatographic performance of perfectly ordered structures. Publication of this work will proceed after completion of a sufficiently broad range of structures.
Another publication is planned on the effect of the presence of connection bridges between the stacked spheres as foreseen in one of the most preferred Printpack embodiments (cf. figure in project abstract). It has been shown that their contribution to the overall dispersion will not be greater than 5%.

In WP 1.2, detailed calculations of the contribution of the side-wall effect to the overall dispersion have been made and this led to the introduction of a so-called magical sidewall distance. This distance can be determined such that the flow resistance of the trough-pore adjacent to the side wall is identical to the flow resistance of the trough-pores in the bulk of the beds. This approach can be generalized over all possible geometries and hence provides a generic solution to the side-wall problem. A paper on this approach is planned for the upcoming year.

The general framework for the kinetic plot analysis planned in WP 1.3 is ready. Once all sphere packing geometries have been calculated (the gyroid and the stacked truncated dodecaeder structure still needs to be done), the dispersion and flow resistance data will be filled into the general framework and the best structure will provide the most advantageous curve in the kinetic plot. Obviously, this will lead to a new publication.
In WP1.5, detailed computational fluid dynamics simulations of gas and particle velocity fields have been carried out to optimize the design of the combination of particle tray and vacuum-suction tray needed in the first step of the envisioned printing process.

In WP2.1, several designs to present the micro-particles to the micro-machined collection tray represented (cf. upper half of Fig. 3a of part B2 of the project proposal text) have been explored. A first breakthrough was realized with the use of a closed chamber with a perforated bottom to provide a uniform and steady air inlet. Although the results are promising, the frequency of misplaced particles is relatively high. Computational fluid dynamics simulations revealed this is due to the fact that the inertia forces of the particles are too high to remain on the streamlines all the way into the holes of the collection tray. Another problem was the excessively high flux of particles towards the surface, often creating a situation where multiple particles are competing for the same hole, thus leading to an undesirable local particle excess. To slow down the particles, the vacuum-suction trays were interfaced with a sieve membrane perforated with holes that are only 50% wider than the particles to be trapped. This innovative strategy considerably reduces the number of local defects and excess layers as well as preventing any particle agglomerates to reach the surface.

In the frame of WP2.2, a nanometric displacement set-up as has been designed and installed. Its core part consists of a 6-axis hexapod nano-precision positioning system and was fabricated by Physik Instrumente GmbH (Germany). It is now fully operational as planned in Milestone 2.3.

In WP2.3, a major breakthrough has been realized by developing a method to uniformly fill an array of circular pockets with single particles. This is a key step towards the realization of the structures as shown in Fig. 2b and Fig. 4 (left) of part B2 of the

Final results

In WP1, we have discovered a new mode hydrodynamic dispersion regime in ordered flow-through media. This regime is characterized by the fact that the exponent for the velocity-dependence of the axial dispersion coefficient does not increase monotonically with the Peclet-number (Pe), but instead goes through a transition where the exponent decreases with Pe over a range of intermediate Pe-values. In addition, we could also build a model explaining this unexpected phenomenon by combining the existing finite parallel zone model with a physically sound closure model to represent how the species transport in a series of finite parallel zones is coupled.

In WP2, we could fill a 2D-array of 15x15 particles with 99% accuracy using the vacuum-driven assembly approach envisioned in the project proposal. Current efforts are being undertaken to increase the surface area and further enhance the success rate.

Next to the gas-phase approach leading to the above result, we also developed a liquid-phase method in parallel. The advantage of this approach is that it operates under lower Stokes number conditions and hence the particles have a much better streamline compliance. Other advantages of the liquid-phase method is that more options exist to prevent particle agglomeration, i.e.., through the use of surfactants.

With the achievement of step 1 one of the envisioned printing process, we can now work on the controlled deposition of the arranged layers (step 2) and their subsequent fixation using photocurable glues.