Nanoporous characterization
Bioseparation media typically have nominal pore ratings in the range of
tens to hundreds of nanometers rather than the 500 µm to 200 µm designed
channels as examined here . Each gyroid material phase has an internal
porosity that provides the required surface area for product or impurity
binding in the diffusive domain, analogous to internal chromatography
bead structure. Imaging a small piece of 3D printed gyroid at a pixel
size of 16 nm using nanoscale X-ray CT enabled fine features to be
resolved in Figure 6a, however the volume generated was deemed to be
insufficient due to a greatly restricted field of view.
Therefore a pixel size of 63 nm was selected for imaging as a suitable
balance of high resolution whilst generating a larger volume for
analysis and flow simulation. Figure 6b displays an equivalent slice at
the lower resolution that still demonstrates good signal to noise ratio
and clear differentiation between the black voidage phase and the gray
material. Figure 6c shows a much larger field of view at the same
resolution, revealing the intricate and detailed substructure within the
material phase that is further demonstrated in the 3D render of Figure
6d.
Tortuosity factor was simulated across all planes within a binarised
cube, resulting in an average measurement of 2.52 ± 0.33. Tortuosity is
an important factor in governing mass transfer properties, therefore the
internal nanostructure in the material phase provides another length
scale that can be optimized for the product and process of interest . By
obtaining digital representations of real porous material then more
accurate measurements can be made in comparison to conventional
techniques. Equation based derivation using porosity as an input has
historically been applied, however does not consider the internal
geometry of materials being estimated for tortuosity . Diffusive bulk
flow and streamlines are provided in Figure 7, suggesting a complex yet
reasonably consistent geometry without any noticeable areas of fluid
bypass. A bulk solution diffusivity of 7.00 x10-11m2s-1 resulted in a simulated
material diffusivity of 2.17 x10-11m2s-1 ± 0.16
x10-11 m2s-1.
The binarised volume enabled pore analysis of the voidage phase. Figure
8a displays a pore network map. Pores can be observed to have a range of
sizes and also connecting with other pores as represented by sticks
between the spheres. The pore size distribution in Figure 8b measured an
average pore diameter of 793 nm ± 315 nm. This pore size distribution
suggests larger diameters compared to a previous study that measured an
average of 289 nm, however that approach relied on 2D scanning electron
microscopy measurements on the sample surface with a different material
composition .
By applying a design cycle approach that uses imaging data to inform
decisions then pore size characteristics can be specifically tailored to
the needs of the product and bioprocess across both permeable and
diffusive domains by analyzing real materials. Ensuring that porogens
are entirely removed from the material phase through washing steps is
necessary to minimize nanostructure variability.