Printed Chromatography Media (Keynote)

Keynote PresentationConventional chromatography media suffer from a number of limitations, including packing defects caused by random or improper packing and constraints on the individual particle geometries that are able to be packed effectively. Chromatography media in the form of beads usually have a distribution in size or, if not, are extremely expensive. Other media particles such as crushed glass or resins, pack with poor flow properties. Monolithic media, likewise, suffer a number of limitations, one being difficulty in casting large-scale columns because of non-uniformities that arise during in situ polymerisation caused by the rate of removal of heat of reaction. Monoliths also suffer from random packing effects. Many of the above limitations cause axial dispersion or poor flow properties such as high pressure drops and flow channelling. Three-dimensional (3D) printing is a new approach to solid synthesis, which is capable of creating media with exquisite control of packing geometry. For example, perfect alignment of identically dimensioned spheres into close- or dense-packing or a variety of regular packing arrangements; controlled sphere dimensions with each sphere placed into a desired position within a lattice; cubes that are aligned to touch at their corners; fibres aligned axially. Indeed, by 3D printing, within the constraints of printed resolution, one could produce an exact physical replicate of any computer model, including both perfect and deliberately imperfect geometries. This would allow one to test experimentally a packing geometry modelled by computational fluid dynamics e.g. a close-packed column of identical spheres with a single "imperfection" such as a flow channel. One could also print the separation media within the containing vessel, with all flow connectors in place and create a wide variety of geometries not currently available by conventional media synthesis or packing methods. In this paper, we show, for the first time, the versatility of this approach to column design through examples of flow through packed media designed in silica, then printed and tested experimentally

3D Printed Porous Media Columns with Fine Control of Column Packing Geometry

In this paper we demonstrate, for the first time, the use of 3D printing (also known as additive manufacturing or rapid prototyping) to create porous media with precisely defined packing morphologies, directly from computer aided design (CAD) models. We used CAD to design perfectly ordered beds with octahedral beads (115 µm apothem) packed in a simple cubic configuration and monoliths with hexagonal channels (150 µm apothem) in parallel and herringbone arrangements. The models were then printed by UV curing of acrylonitrile-butadiene-styrene powder layers. Each porous bed was printed at 1.0, 1.5 and 2.0 mL volumes, within a complete column, including internal flow distributors and threaded 10–32 flow connectors. Close replication of CAD models was achieved. The resultant individual octahedral beads were highly uniform in size, with apothems of 113.6 ± 1.9 µm, while the monolith hexagonal cross-section channels had apothems of 148.2 ± 2.0 µm. Residence time distribution measurements show that the beds largely behaved as expected from their design void volumes. Radial and fractal flow distributor designs were also tested. The former displayed poor flow distribution in parallel and herringbone pore columns, while the fractal distributors provided uniform flow distribution over the entire cross section. The results show that 3D printing is a feasible method for producing precisely controlled porous media. We expect our approach to revolutionize not only fundamental studies of flow in porous media but methods of chromatography column production

3D printing of porous media at the microstructural scale

What is superficially referred to as ‘packing quality’, a myriad of geometrical parameters governing the interrelations between pores, has only been measured post-hoc in the form of separation efficiency. While several computational studies of chromatography bed microstructures have explored the effects of various packing parameters on dispersion, experimental replication and model validation has remained elusive. Additive manufacturing, or 3D printing, offers the opportunity to manufacture porous media composed of micro-structural elements of different shapes and sizes, and to precisely locate and orient them within the bed. For example, spherical beads with a narrow size distribution can be constructed individually at desired locations within the bed, allowing the creation of specific packing arrangements, i.e. perfectly ordered lattices or random packing mimicking conventionally packed chromatography columns. Further opportunities are to create geometric elements with different shapes or sizes and place them at individual locations within the same bed. Alternatively, the structural focus can shift from the solid-phase to the mobile phase, with the design of complex flow channels within a monolithic bed. These observations led us to propose the use of 3D printing as both a chromatography column production method and as a tool to enable fundamental studies of packed bed microstructures. The main challenges to this approach include achieving sufficient printing resolution to compete with current media in terms of theoretical plate height and developing materials that have appropriate internal porosity and surface functionalities to enable high binding capacity and specificity. Other challenges are as for conventional media, for example good swelling properties, low non-specific adsorption, and the absence of toxicity and leaching. Here, we show examples of progress made to date in creating 3D printed chromatography columns. These include i) micro-structural analyses of columns containing porous beds with a variety of lattice arrangements and channel structures, printed at a maximum current printing resolution of 16 µm and ii) comparison of residence time distributions and flow characteristics for a range of columns, including several printed with different integrated flow distributors and column cross-sections. We demonstrate reasonable fidelity between printed and designed columns and identify current limitations with regard to resolution. Finally, we compare packed beds incorporating deliberately introduced imperfections within packing lattices, including a ‘line defect’ that runs the length of the column and a ‘cluster defect’ consisting of localized voids at various locations within the packing. Experimentally determined reduced plate heights are compared with computational fluid dynamics flow studies

Fabrication of polymer monoliths within the confines of non-transparent 3D-printed polymer housings

In the last decade, 3D-printing has emerged as a promising enabling technology in the field of analytical chemistry. Fused-deposition modelling (FDM) is a popular, low-cost and widely accessible technique. In this study, RPLC separations are achieved by in-situ fabrication of porous polymer monoliths, directly within the 3D-printed channels. Thermal polymerization was employed for the fabrication of monolithic columns in optically non-transparent column housings, 3D-printed using two different polypropylene materials. Both acrylate-based and polystyrene-based monoliths were created. Two approaches were used for monolith fabrication, viz. (i) in standard polypropylene (PP) a two-step process was developed, with a radical initiated wall-modification step 2,2'-azobis(2-methylpropionitrile) (AIBN) as the initiator, followed by a polymerization step to generate the monolith; (ii) for glass-reinforced PP (GPP) a silanization step or wall modification preceded the polymerization reaction. The success of wall attachment and the morphology of the monoliths were studied using scanning electron microscopy (SEM), and the permeability of the columns was studied in flow experiments. In both types of housings polystyrene-divinylbenzene (PS-DVB) monoliths were successfully fabricated with good wall attachment. Within the glass-reinforced polypropylene (GPP) printed housing, SEM pictures showed a radially homogenous monolithic structure. The feasibility of performing liquid-chromatographic separations in 3D-printed channels was demonstrated