Comparing design to imaged volumes
X-ray CT was deemed as the most appropriate 3D imaging technique for visualizing whole gyroids due to the high resolutions achievable without requiring sample embedding or destructive sectioning. Figure 1a displays three printed gyroids atop a sample holder in preparation for sequential scanning. Each gyroid design uses identical structurally repeating units, which results in a bed porosity of 50% and provides mechanical strength in addition to providing uniform flow paths both axially and radially . Three gyroid designs with specified feature sizes of 500 µm, 300 µm and 200 µm were designed and 3D printed for comparison. CAD renders available in Figure 2a-c display the visual comparison between designs, where both the material and void phases are equally scaled in size.
Conventional bioseparation structures at a comparative length scale such as packed bed chromatography columns exhibit heterogeneity due to non-uniform particle sizes and wall effects, which have been visualized and characterized in a previous study . By specifying a regular feature shape and size that can then be accurately fabricated through 3D printing, more uniform structural characteristics are theoretically achievable. By X-ray CT imaging 3D printed materials then direct comparisons can be made to the original designs, with a 3D reconstruction of each feature size available in Figure 2d-f.
The imaged gyroids in each case display the feature sizes as specified from CAD files throughout the 3D geometry. The capability to design and fabricate intricate structures with confidence enables physical characteristics to be specifically tailored to the needs of the bioprocess product to be purified. Figure 2d-f does display some artefacts that suggest that fabrication presents some fidelity challenges, particularly as feature size moves closer to the achievable 3D printing resolution. Some over-curing of the DLP resin was observed that results in a reduced porous channel diameter, especially in the internal regions of the 3D printed model.
Imaging complex geometries at the microscale can result in reduced signal to noise ratios closer to the center of each sample. Printing smaller samples may reduce this concern whilst improving pixel size from 5 µm achieved here, however producing gyroids with smaller diameters was found to exhibit structural integrity issues during the drying process required for effective X-ray CT acquisition.
In order to examine 3D printing fidelity in detail, internal tomography was performed on a 500 µm feature at an improved pixel size of 2 µm. By focusing on a single unit within the overall gyroid structure then more characteristics such as surface roughness could be visualized. As can be seen in the material phase of Figure 3a horizontal striations 50 µm apart are visible, with these being the interface between printed layers. Material and voidage phases were binarised in Figure 3b.
The imaged dataset was aligned in the same position as the original design file. Figure 3c displays a 3D render of the solid-fluid interface and Figure 3d overlaying the design and imaged material. Reductions to the designed feature size may become more problematic at the individual unit scale, comparable to an individual chromatography bead within a packed bed. However it may be favorable to increase the surface area available for fluid to interface with printed material whilst maintaining an ordered flow pattern. Individual features may get closer in size to conventional chromatography beads at improved printing resolutions, which will enable direct comparisons between the two approaches at this length scale.