## 2 Results and Discussion  
### Preparation and Characterization of Immobilized polyHIPE Scaffold  
Herein, a P(St-\(co\)-GMA) polyHIPE monolith was prepared via an emulsion-templating method for microbial cell immobilization. Prior to the templating copolymerization, a w/o HIPE was prepared by homogenizing the oil and water with a high internal phase fraction. The polyHIPE was synthesized by curing the external phase of the HIPE template, in which the St functioned as a mechanical monomer, whereas GMA was employed as a functional monomer to immobilize cells covalently. Moreover, the incorporation of the cross-linker DVB could improve polyHIPE's structural stability [37]. The polyHIPE was characterized by FT-IR. As shown in Figure S1, the strong absorption peak at 2924 cm\({}^{-1}\) is attributed to the C-H stretching vibration of the -CH\({}_{2}\)- structure in copolymers, whereas the peak at 1734 cm\({}^{-1}\) is associated with the C=O group in GMA. The ring vibrations of the benzene ring were in the absorption bands of 1601, 1493, and 1452 cm\({}^{-1}\), and the bending vibration of hydrogen atoms on the mono-substituted benzene ring absorbs at 1029 and 759 cm\({}^{-1}\), respectively. The characteristic peak of the epoxy group of GMA is present at 907 cm\({}^{-1}\). The absorption peaks at 797, 839 cm\({}^{-1}\) are indicative of dibasic substituted compounds at the adjacent and interposition of the DVB benzene rings, which is approximately similar to the spectra in the literature, demonstrating the successful modification of functional group GMA on the monolith [38]. The obtained polyHIPE was supposed to be applied to immobilize cells, and the structure must be figured out as the first factor that the monolith should be set up to facilitate the movement of cells within the polyHIPE scaffold. As seen in Figure 1, all generated polyHIPEs have an open-cell void structure, and it can be observed that the void structure of polyHIPE varies with different emulsifier Span80 concentrations or varied internal phase ratios. As is summarized in Table 1, it could be seen that the average void size reduced from 18.9 um to 11.6 um as emulsifier concentration increased from 5 wt.% to 20 wt.%. In addition, along with the internal phase fraction increasing from 75 vol.% to 90 vol.%, the void size decreased from 16.1 um to 12.5 um. This finding reveals that the void size of polyHIPE can be tuned by appropriately changing the parameters for the preparation of HIPE. In IMCF, the size of the pore throats should be larger than the size of the bacteria so that the cells can enter the voids without resistance before being completely immobilized, and the size of the voids should be large enough to provide the cells for metabolism and growth activities. It can be seen from Table 1 that the size of the pore throats tends to be a similar level (3-4 um), which is sufficient for the entering of _P. acidilactici_ to fulfill their admission requirements. Moreover, despite the variations, the void sizes of the entire given polyHIPEs are highly satisfying and provide a spacious site for the cells. When the polyHIPEs are used for cell immobilization and subsequent fermentation, the cells and broth infiltrate the polyHIPEs, and the capillary effect acts on the scaffold [39; 40]. Moreover, since the polyHIPE is constantly shaken in the broth throughout the fermentation, it would be eroded by the broth or collide with the container walls, which requires the scaffold to have significant mechanical strengths to preserve its intrinsic porous structure against the complex fermentation process. The mechanical properties of the polyHIPEs herein were characterized by measuring the compressive strength (Figure 2). Figure 2a depicted a typical polyHIPE stress-strain curve, with a linear region at low strains, a brief stress plateau, and a densification stage with a sharp increase in stress [41].