## 2 Results and Discussion
### Preparation and Characterization of Immobilized polyHIPE Scaffold
Attribute to the polystyrene skeletal frame and the stiff cross-linker DVB, the polyHIPE scaffolds exhibit excellent mechanical properties with a high Young's modulus (_E_). In detail,as is shown in Figure 2b, by comparing Young's modulus of PHI-0590 (36.6 MPa), PHI-1090 (32.6 MPa), and PHI-2090 (7.3 MPa), a decrease in Young's modulus was displayed with the increase in the amount of Span80 used in the preparation of the emulsion. Moreover, the polyHIPE with increasing internal phase fraction had an enhanced Young's modulus (\(E_{\text{PHI-1075}}\) = 8.3 MPa, \(E_{\text{PHI-1080}}\) = 26.7 MPa, and \(E_{\text{PHI-1090}}\) = 32.6 MPa). It is worth mentioning that the Young's modulus of given polyHIPEs in this study exceeds that of most scaffolds widely used in cell culture today, such as PVA-based materials (\(E\) = 1.88 MPa) and F127-BUM gel (\(E\) = 0.67 MPa) [42; 43], and its robust mechanical property could ensure the structural integrity during the process of subsequent immobilization and several cycles of fermentation. The smooth flow of substrate inside the material is very important, i.e., the interconnectivity of the porous material should be sufficient to ensure a sufficient mass transfer rate. As is shown in Table 1, the interconnectivity of polyHIPEs increased along with the volume ratio of the internal phase increasing (PHI-1075, PHI-1080, and PHI-1090). Moreover, because the emulsifier could reduce the HIPE droplet size and increase the interconnectivity of porous materials [27; 44], the interconnectivity was improved as the Span80 content increased (PHI-0590, PHI-1090, and PHI-2090). \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Sample** & **Span80 (wt.\%)\({}^{\text{[a]}}\)** & \begin{tabular}{c} \(f_{\text{internal phase}}\) \\ **(vol.\%)\({}^{\text{[b]}}\)** \\ \end{tabular} & \
\begin{tabular}{c} **D (\(\mu\)m)\({}^{\text{[c]}}\)** \\ \end{tabular} &
\begin{tabular}{c} **d (\(\mu\)m)\({}^{\text{[a]}}\)** \\ \end{tabular} & **Interconnectivity (\%)\({}^{\text{[d]}}\)** \\ \hline PHI-0590 & 5 & 90 & \(18.9\pm 5.9\) & \(3.4\pm 1.8\) & 6.1 \\ \hline PHI-1090 & 10 & 90 & \(12.5\pm 3.1\) & \(3.2\pm 1.9\) & 12.3 \\ \hline PHI-2090 & 20 & 90 & \(11.6\pm 4.9\) & \(3.6\pm 1.4\) & 25.3 \\ \hline PHI-1075 & 10 & 75 & \(16.1\pm 4.6\) & \(3.3\pm 1.4\) & 10.5 \\ \hline PHI-1080 & 10 & 80 & \(14.1\pm 3.8\) & \(3.1\pm 1.2\) & 11.5 \\ \hline \hline \end{tabular} \({}^{\text{[a]}}\) Mass fraction of Span80 with respect to monomers. \({}^{\text{[b]}}\) Internal (aqueous) phase volume fraction compared to the emulsion’s volume. \({}^{\text{[c]}}\) Average diameter of void. \({}^{\text{[d]}}\) Average diameter of interconnected pores. \({}^{\text{[e]}}\) Interconnectivity of polyHIPEs.  
\end{table} 
Table 1: Composition of emulsion templates and properties of the resulting polyHIPEs. Figure 1: SEM images of open-cell polyHIPE scaffolds. Given that the medium is a kind of aqueous solution, the wettability of the material is a crucial prerequisite for effective following cell entrance and mass transfer. The water wettability of the polyHIPE was then investigated by putting a drop of water onto the sample. As shown in Figure 3, taking PHI-1090 as an example, when roughly 20 uL of water is dropped upon a sample, the water permeates the monolith within 1.5 s. Great water wettability was also proved by other polyHIPEs (PHI-0590, PHI-2090, PHI-1075, and PHI-1080) with infiltration time within 2 s for a drop of water (Figure S2), and the wetting rate increased with an increase in the interconnectivity of polyHIPE. The favorable water permeability ensures a smooth mass transfer during subsequent cell immobilization and fermentation.