## 2 Results and Discussion
### Fermentation Process of Immobilized P. acidilactici  
The fermentation of lactic acid by immobilized _P. acidilactici_ was conducted under an anaerobic environment. The distribution of cells in the voids at the end of fermentation can be observed by SEM-EDS mapping (Figure 4b,e), from which it can be seen that the number of cells in the voids multiplied substantially, proving the extraordinary biocompatibility of the polyHIPE scaffold. Then the optimum temperature of the immobilized _P. acidilactici_ fermentation was characterized by the lactic acid production fermented under a series of temperature gradients. As shown in Figure S4, the \({}^{1}\)H-NMR spectrogram showed double peaks at \(\delta\) = 1.10-1.11 ppm, which are characteristic peaks of \({}_{\mathrm{L}}\)-lactic acid. Take PHI-1090 as an example, as shown in Figure 5 and Table S1, in the temperature range of 38-48 \({}^{\circ}\)C, the \({}_{\mathrm{L}}\)-lactic acid produced by the immobilized cells was higher than that of the suspended cells owing to the low thermal conductivity of the polyHIPE. In addition, 28.6% of the remaining activity of the immobilized cells was retained at 40 h, which is higher than alginate immobilized cells (55% at 4 h) [47], demonstrating the excellent temperature stability of the PHI immobilized cells. The optical purity of \({}_{\mathrm{L}}\)-lactic acid produced by immobilized _P. acidilactici_ was 99.6 \(\pm\) 0.1%, which was equivalent to that of \({}_{\mathrm{L}}\)-lactic acid  
Figure 4: SEM images of _P. acidilactici_ immobilized PHI-1090 (**a**), _P. acidilactici_ immobilized PHI-1090 after one cycle fermentation (**b**), and _P. acidilactici_ immobilized PHI-1090 after 10 cycle batches (**c**). (**d–f**) were sulfur element EDS images, corresponding to (**a–c**), sequentially. generated by suspended _P. acidilactici_, indicating that immobilization had no influence on the \(\mathrm{L}\)-lactic acid generation. The structural dependency of \(\mathrm{L}\)-lactic acid production at 42 \({}^{\circ}\)C by immobilized cells was evaluated by immobilizing _P. acidilactici_ on various structural carriers. As shown in Figure 6 and Table S2, the lactic acid yields of the immobilized cells were all higher than those of the free cells, with a maximum yield increase of 17.6%. Moreover, by comparing the \(\mathrm{L}\)-lactic acid yield from the fermentation of _P. acidilactici_ immobilized PHI-1075, PHI-1080, and PHI-1090, the lactic acid yield improved as the polyHIPE interconnectivity increased, which is because higher interconnectivity leads to a more efficient mass transfer. This result can also be seen from PHI-0580, and PHI-1090, where the highest yield of lactic acid was obtained after PHI-1090 fermentation, and the morphology of the PHI-1090 after undergoing a fermentation process could be seen in Figure 7a,b. However, _P. acidilactici_ immobilized on PHI-2090 had the highest interconnectivity but lower fermentation efficiency, which may be caused by the lower mechanical properties of PHI-2090, and the shaking during fermentation may damage the porous structure, resulting in low lactic acid yield (Figure 7c,d). Figure 5: The \(\mathrm{L}\)-lactic acid concentration for _P. acidilactici_ immobilized PHI-1090 fermented at different temperatures. Figure 6: The \(\mathrm{L}\)-lactic acid concentration for suspended and immobilized _P. acidilactici_ on different PolyHIPE scaffolds at 42 \({}^{\circ}\)C.  
The fermentation process occurred when the monolith of immobilized _P. acidilactici_ PHI-1090 was introduced directly to the fermentation medium at 42 \({}^{\circ}\)C to convert glucose to L-lactic acid within 40 h. The cell capacity in the support was measured by 0.11 g/g after 40 h, which was much higher than the primary content (0.04 g/g), indicating cell growth during fermentation. The glucose and L-lactic acid concentrations during fermentation were monitored. As shown in Figure 8, the glucose was consumed from 50 g/L to 3.5 g/L and 3.4 g/L for suspended and immobilized _P. acidilactici_, respectively.