## 4 Conclusions  
Bacterial nanocellulose has a wide range of applications in medicine, as wound dressings, scaffolds, and prosthesis, in the food industry, as a packaging component, a low calorie additive and emulsifier, and in electronic devices and many other applications. The bottleneck for the implementation of BNC products on the market is the high cost of BNC production. One approach to solve this problem is the development of a new production strategy which allows enhancement of the yield and productivity of BNC. On the other hand, it is important to maintain the quality of membranes. In our study we proposed a new surface air-flow bioreactor for BNC which enables the increase in the productivity by 65% through the control of culture ventilation. The quality of the BNC can be assessed by mechanical parameters, such as tensile strength and the Young modulus. BNC produced in the SAF bioreactor exhibits improved mechanical strength which can be related to a higher quality of that biomaterial. Additionally, we studied a wide range of different process parameters which can alter the yield or mechanical parameters of bacterial nanocellulose. The greatest influence on both studied responses was proven for such factors as the air-flow ratio, glucose concentration, and culture time. The highest achieved yield was 4.11 g/L and the highest productivity was 0.91 g/L/day. In terms of the mechanical strength, the stress of BNC membranes varied from 0.8 to 6.39 MPa depending on culture condition, while the Young modulus was in the range 1.92-23.52 MPa. This data defines the rational area of input factor variability in static culture of _K. xylinus_ E25 and could serve as a useful basis for further simultaneous optimization of the yield and tensile strength.  
Conceptualization, I.C. and S.B.; formal analysis, I.C.; methodology, I.C.; supervision, S.B.; visualization, I.C. and M.R.; writing--original draft, I.C.; writing--review and editing, I.C., M.R. and S.B. All authors have read and agreed to the published version of the manuscript. This research received no external funding. The authors would like to thank [PERSON] and [PERSON] for technical support. The authors thank [PERSON] for careful reading of this manuscript and English corrections. The authors declare no conflict of interest. The authors declare no conflict of interest.  
Appendix A Evaluation of the Impact of Process Parameters on BNC Yield Calculated as a Dry Weight per Surface Unit  
BNC yield calculated as a dry weight per surface unit (g/m\({}^{2}\)) allows presentation of the effect of examined factors without obvious relationships between the dry weight and surface area of the bioreactor. The effects of variables were calculated based on the data presented in the Table 11 and visualized in the Figures 11 and 12, as a Pareto chart and the main effect plots, respectively.  
From Figures 11 and 12 it can be seen that most of the parameters had a significant effect on dry weight per surface unit, namely bioreactor area, cultivation time, surface to volume ratio, air-flow ratio, inoculum volume, and humidity of air. The highest positive influence on the studied response was the cultivation time (47.36%), followed by the air-flow ratio (14.98%) and inoculum volume (14.11%) (Table 12).  
The impact of the bioreactor surface on dry weight per surface unit was highly reduced in comparison to the influence on the dry weight. That factor had a significant positive effect on yield (g/m\({}^{2}\)), but its contribution was only 7.1%.