(20a) Development of Electrospun PCL Aerosol Filter Media for Respiratory PPE Applications

The global COVID-19 pandemic demonstrated the effectiveness of respiratory personal protective equipment (PPE), namely facemasks and respirators, as a tool for mitigating the spread of viral disease through the capture of suspended aerosol particulates from air. Still, the experience highlighted deficiencies with conventional meltblown polypropylene respirators, namely the reliance on electrostatic surface charges to achieve high filtration efficiency for a low air resistance, which results in limited shelf/use life, poor materials versatility, and capital-intensive manufacturing. These limitations contributed to supply bottlenecks during the COVID-19 crisis, and support the need to develop alternative filter media. Additionally, being restricted to the use of polypropylene, which is not biodegradable, exacerbates the environmental impact of widespread facemask use.

Electrospinning is a readily scalable process that requires relatively low amounts of capital investment and is capable of manufacturing nonwoven media that comprise sub-micron fibers with controlled morphology from a wide selection of materials. Electrospun media have found applications in drug delivery, catalysis, consumer textiles, and industrial filtration. Previous experimental and modeling efforts suggest that nanofiber media have a much higher aerosol collection efficiency compared to microfibers for a comparable air resistance, eliminating the need to impose electrostatic surface charges. In order to leverage these strengths during PPE demand surges, additional data on the design, performance, and manufacturing of electrospun filter media are needed to enable rapid production deployment.

Here, we report our work on the design of electrospun poly(ε-caprolactone) (PCL) nanofiber filters. PCL is a commercially available, hydrolyzable polymer with favorable mechanical properties that has been previously electrospun for other applications. We present structure-property relationships that relate important characteristics of electrospun PCL media – namely, thickness, fiber diameter distribution, and porosity - to aerosol filtration performance, which is evaluated on the basis of filtration efficiency of particles ranging from 0.01-1 μm in diameter and pressure drop at superficial velocities relevant to facemask use. Testing protocols approximate N95 standards set forth by the National Institute for Occupational Safety and Health (NIOSH). We demonstrate that these electrospun media achieve good filtration performance through primarily mechanical means. Moreover, we identify specific combinations of thickness, fiber diameter, and solidity that meet NIOSH targets and are suitable for respiratory PPE. Along with these results, we provide electrospinning protocols intended for rapid transfer to industrial production lines. Lastly, we compare our experimental results to existing transport models for predicting air permeability and filtration efficiency of nonwoven filters, which have most commonly been applied to microfiber media.