# Electrospinning of Chitosan for Antibacterial Applications--Current Trends
## 7 Application Fields of Electrospun Chitosan Nanofibers and Chitosan Derivatives
### Air Filtration  
To protect humans from germs, bacteria, and viruses, ventilation filters are diversely used in buildings, vehicles, and airplanes. Despite the use of a ventilation system, several studies revealed that there were still significant amounts of bacteria in closed environments. For instance, airplane walls are padded with heat-insulating blankets, which might offer a dark environment with condensed moisture for bacteria to thrive and be discharged into the aircraft, threatening passengers' health [179]. The integration of electrospun fibers into air filtration systems has been demonstrated to overcome problems with conventional systems. In a study by [PERSON] et al. [179], the survival rates of typical airborne bacteria _E. coli_ and _Bacillus subtilis_ (_B. subtilis_) on blankets with hydrophilic or hydrophobic properties under simulated flight cabin climate conditions were assessed for a classic flight duration of 445 min, repeated in five cycles. The numbers of bacteria decreased tremendously during the first cycle because of the 30% relative humidity, which is unfavorable for bacteria survival, leading to cell dehydration and nutrient deficiency. Nevertheless, at the end of the fifth cycle, 0.4% of the initial _B. subtilis_ load survived versus a much lower _E. coli_ rate. The interpretation is that _B. subtilis_ produces endospores capable of freezing in harsh conditions such as dehydration and radiation and of resuming their reproduction after re-stabilization of the environment. This reveals an alarming risk for human health in closed areas, where home textiles such as blankets, curtains, and filters represent potential hosts for pathogenic bacteria and allow long-term proliferation if they are not completely killed and even a small fraction survives. In such situations, antibacterial filters integrated within indoor ventilation systems come in hand [179]. Airborne bacterial survival can be reduced using effective filtration systems. Thus, in this study, different filter media were prepared and the _E. coli_ survival rates were assessed for 4 groups of positively charged and chitosan-modified filters. A substrate layer of melt-blown fibers covered with a PTFE membrane comprised the positively charged composite filter medium. The three filters were made of pure nylon-6, chitosan-dipped nylon-6, and nylon-chitosan electrospun nanofibrous membranes. In comparison to the _E. coli_ decreasing rate on the uncharged control sample, the _E. coli_ bacterial survival rate on the positively charged filter dropped to 17.8% after six hours, whereas the bacteria dropping rates were sharper on the nanofibrous filters, with 8.4% after two hours (pure nylon filter), 7.1% after two hours (chitosan/nylon composite filter), 2.8% after two hours, and 0% after four hours (chitosan-dipped nylon filter). These findings reveal that the chitosan on the filter's surface may significantly suppress microorganisms. Additionally, the filtration preparation method had an influence on its efficiency; filters with chitosan coatings were superior to filters containing blended nanofibers. This might be due to the possible formation of hydrogen bonds between the carbonyl groups in nylon-6 and the amino groups in chitosan, thereby reducing the generated amount of NH\({}^{3+}\) groups in reaction with water molecules. Due to this cohesive mixture, fewer chitosan molecules were present at the fiber surface, resulting in less chitosan-bacteria interactions and influencing the antibacterial ability of the blended electrospun membrane in comparison to the dipped filter medium [180]. Decreasing the nanofiber diameter has several positive benefits, including an increased specific surface-area-to-volume ratio and decreased average pore size. Ultrafine chitosan/PVA hybrid nanofibers incorporating TiO\({}_{2}\) or Ag nanoparticles were produced by utilizing needle-free electrospinninging at a large scale. The daily production may exceed 1.2 kg, allowing these ultrafine nanofibers to be used in air filtration and environmental cleaning.