We successfully designed and tested an antiviral and HME filter with 3D printed casing in 1 week. We produced two versions of each filter using different filter materials. Except for the combination of 1.5 μm filter paper and 5 mm sponge, which had an unacceptably high peak inspiratory pressure, both produced circuit leak and compliance tests within the normal operating limits of the anesthesia machine they were tested on [15].
Healthcare systems around the globe are currently experiencing unforeseen demand of medical items. To mitigate the impact of this shortage on LMICs we used simple materials accessible outside the normal hospital supply chain to successfully develop and test 3D printed HMEs and viral filters for use on anesthesia machines and ventilators. These casings have been trialed to resemble the properties of those available commercially.
COVID-19 spreads via aerosolization and contact with contaminated surfaces [16]. Aerosols occur when solid or liquid molecules are dispersed through air. Their size can range from 0.3–100 μm in diameter. Particles of 1–5 μm remain in the air and therefore, present the biggest challenge [5]. Considering this method of transmission, appropriate filtration of ventilator circuits is of vital importance [17].
The antiviral properties of 1.5 μm filter paper have been previously investigated [18, 19]. Whilst more effective at trapping larger particles (1.5 μm) they are effective at trapping particles as small as 0.3 μm, particularly at highly humid environments as found in the HME [5, 20]. The 1.5 μm filter paper is usually used by water processing plants, and in food and drink manufacturing. As a relatively specialist material, it may not be readily available.
The antiviral properties of the 5 μm sediment cartridge have been less well investigated. The pore size of this material seems too large. Nevertheless, this is the pore size when the material is saturated with water. When used in air, the pore size is one-tenth the size in water [21]. This would provide a pore size of 0.5 μm, which should be sufficient. It is also more widely available and easier to source, in case the 1.5 μm filter paper is unavailable. The product we used is also available as a 1-μm sediment cartridge which is suitable as an antiviral filter [22].
The humidifying properties of a foam-based HME depend on the foam’s density. Humidification capability is proportional to density, but it increases resistance to air flow. The density of the foam used in the commercial HME is 26–32 kg m− 3 [23]. We calculated the density of the foam we used gravimetrically as 21 kg m− 3. The porosity of the foam was calculated as 98% which is in line with previous studies of polyurethane foams [24, 25]. It is not unreasonable, therefore, to expect that both foams will produce similar humidification at standard operating room conditions.
Another potential advantage to our filter is that it can be disassembled and the 3D printed parts re-sterilized using either commercially available wipes or by soaking in bleach. Once dry, the filter and HME material can be replaced and it can be reused.
We have completed the feasibility and suitability tests demonstrating that the different filters comply with the anesthesia machine specifications. However, we did not evaluate their actual antiviral, antibacterial, and heat and moisture preservation capabilities and further investigations in this area should be considered prior to use. Nevertheless, all designs passed the methylene-blue dye test demonstrating that the dye passed through the filter rather than around it. This is similar to most 3D printed N95 masks reported in the literature which have at best undergone leak testing but no further clinical evaluation.
It should be pointed out, however, that these filters are only intended for use as a last resort to overcome an abrupt interruption of the supply chain, rather than to replace commercial alternatives. As a class II medical device, we would strongly encourage anyone planning on using our design to apply for an Emergency Use Approval from their relevant regulatory body.
Whilst production of designs in the healthcare setting suitable for 3D printing requires a certain amount of technical skills, reproducing them in the field does not [26, 27]. With mobile data being nearly ubiquitous, an internet connection is no longer required in order to share models with colleagues working in low resource or rural settings. STL files are relatively small (2–70 megabytes [Mb]), and can be printed in geographically remote locations with no internet access using a mobile phone, laptop, SD card, and a 3D printer. The whole process requires about 600 Mb of data storage. We have also printed these filters using polylactic acid (PLA) and polyvinyl alcohol (PVA) filler on a desktop Makerbot Method printer (Stratasys, Minnesota, USA). One cartridge of PLA will make approximately 9–13 filters. The STL file we produced is available as Supplemental Material to this report.
In these tumultuous times, it is a responsibility of the international medical community to provide assistance to our colleagues who are experiencing shortage of equipment, either through a disrupted supply chain or by being priced out of the market. We hope that these HMEs and viral filters may be of use to clinicians who may face critical supply chain issues during the COVID-19 pandemic.
