What harm can bioaerosols cause?

Global spread of COVID-19 alerts people of the spread of diseases by bioaerosol. Bioaerosols that contain living organisms, such as bacteria, viruses, pollen, spores, and fungi, remain suspended in the air for long periods of time. They are found in both indoor and outdoor air with small sizes (1 nm−1000 μm) and varying concentrations (1×10³−1×10⁶ cells m^-3), and they originate from both natural and anthropogenic activities. Exposure to bioaerosols can cause severe respiratory diseases, such as asthma, allergies, cough, pneumonia and etc. Bioaerosol has the potential to affect millions of humans worldwide and has been associated with an increased risk of lung cancer. Especially in a relatively enclosed room, like indoor, bioaerosol generated by humans can spread and infect people rapidly. The researchers determined in a sample of children’s classrooms that the emission rates attributable to occupants ranged from 0.8 million to 35 million bacteria cells per person-hour and from 3 million to 57 million fungal cells per person-hour. That means humans are exposed to indoor air bioaerosol all the time, yielding a high health risk.

How to control bioaerosols?

As traditional methods to control bioaerosols base on space elimination and filtration, they suffered from space occupying, energy consuming and secondary contamination. UV, plasma, and disinfection spraying are commonly used for space air disinfection. It usually takes several hours to clean the room. However, bioaerosols are continuously generated when human stays in the room, as human is also one of the sources of bioaerosols. The risk of diseases transmission is high during human occupying. Filtration is able to remove bioaerosols during human occupying. Even though a high removal efficiency can be achieved by a filtration system with a strong negative pressure, these filtration methods induce large pressure drops, are easily blocked, have low adhesive force and high energy consumption, and they are also unable to inactivate bioaerosols in situ.

Why we choose artificial spider silk?

In nature, spider silk can actively capture tiny dust particles and microdroplets from air; the microdroplet coalesce to form larger droplets, which concentrate small dust particles and moisture on the spider silk. By mimicking special fiber structure of spider silk, some researchers have developed artificial spider silk (ASS) for water collection from air in arid regions. However, all previous studies were only focused on the collection of water from air but did not consider airborne microorganisms in bioaerosols, which mainly exist in water microdroplets, particle matter and aggregations dispersed in air. Therefore, inspired by the spider silk, we thought that if it can be used as a special bioaerosol catcher and in situ killer with low energy consuming.

Mechanisms underlying the artificial spider silk photocatalyst.

In this study, an artificial spider silk photocatalyst (ASS) has been firstly proposed to actively capture and concentrated bacteria in bioaerosols and to produce biocidal reactive oxygen species (ROS) to kill bacteria in situ in the droplet or air under light irradiation. The influences of the physical/chemical structures of the ASS, relative humidity and the bioaerosols characteristics on the capture/inactivation performances of the ASS have been revealed. The mechanisms of the bioaerosols capture of the ASS have been clarified by systematically investigating the hydrophilic property, the Laplace pressure and surface energy gradients of the ASS. We also used bacteria probe to detect the adhesive forces between the bacteria and the ASS, revealing that the adhesive forces are much higher at high relative humidity than that at low relative humidity. High adhesive forces at high relative humidity tell the truth of hydrophilic property of the ASS is a key factor of bioaerosols capture. The big diameter and rough surface of the spindle knots of the ASS enhance capture and concentration properties of the ASS, indicating that the greater Laplace pressure drop and surface energy gradients are benefit for bioaerosols capture and concentration. Moreover, the photocatalytic inactivation mechanism of the ASS under UV light irradiation is generating ROS in the droplet and in air to attack captured bacteria. After understanding the underlying mechanisms of the ASS for bioaerosols capture and inactivation, this novel strategy with low energy consuming and high efficiency may provide a new insight into solving the problem of indoor bioaerosols control. 

In summary, we designed a bio-mimicking artificial spider silk photocatalyst and investigated its bioaerosol capture and inactivation mechanisms. We found that the ASS can capture more bioaerosol than common fiber due to special structure of the ASS, as well as 99.99% inactivation efficiency obtained under UV light irradiation for 4 h. Under the background of global pandemic of respiratory diseases, this unique ASS photocatalyst shows great potential to continuously bioaerosol control.