Stay-wet fabric filter media

MedTextra fabric system is a new filter media designed for the safety, technical, medical and military markets.
MedTextra fabric system is a new filter media designed for the safety, technical, medical and military markets.

MedTextra fabric system is a new filter media designed for the safety, technical, medical and military markets. It is a stay-wet fabric and filter media whose woven and non-woven form factors overcome the biggest obstacle to antimicrobial performance: keeping the active ingredient from drying out. This new media captures moisture from breath and ambient air to keep antimicrobials working in respiratory facemasks.

A small film application tool company in St. Paul, Minnesota, US, LIDCO Products, which makes hand tools to apply pressure-sensitive film has developed a stay-wet filter media aimed at overcoming the biggest obstacle to antimicrobial performance: keeping the active ingredient from drying out. Perhaps even more surprisingly, the leader of this team is a self-described former ‘junk dealer’ and art major. The other two members of the team, chemical engineers who had long careers at 3M and H B Fuller, contributed their knowledge of polymer materials and manufacturing methods. Small interdisciplinary teams of senior engineers and a designer-thinker, like this one, can be very fast and creative. It did not hurt that the ultimate budgetary decision-maker led the team.

Disposable respiratory masks that cost-effectively and materially reduce the inhalation of viral particles have the potential to significantly reduce sickness, suffering and shortened lives for millions of people around the globe; but such masks are not available today.

Socially acceptable

David Gray, the stay-wet fabric development team leader and owner of the tool company, LIDCO Products, set out to make respiratory masks that were more socially acceptable and less stigmatizing. His wife needed to wear masks while her immune system was weakened by chemotherapy so he had a powerful motivation to launch this innovation journey. He thought that scarves and turtleneck sweaters could have a dual-use as respiratory masks. But when looking into disposable N95 and N99 masks, he found that their filtration efficiency for viruses was not very good. The tiny N1H1 and SARS viruses passed right through the masks available to protect his wife. His fashion design project had quickly become very technical.

So Gray did some networking and was referred to Bob Hume a retired 3M and H.B. Fuller engineer who was consulting when not fly-fishing. Together they pursued a ‘capture and kill’ strategy for viral particles in the filter matrix. A first step was creating a mask that doesn’t concentrate microbes on the outside of the mask which then contaminates one’s fingers when taking it off. Mark Litman, a former chemical engineer and patent attorney for 3M, the third member of the invention team, joined soon after. LIDCO Products and 3M are both based in St. Paul, Minnesota.

The team found published research showing that the drying out of filter media is the main cause of ineffectiveness of many antimicrobial compounds. Once they dry out they are not effective when rewetted. So the team turned to Superabsorbent Polymers (SAP) fibers, like the ones used to absorb many times their weight in liquid in baby diapers, to sustain the moisture level in the filter media. They also thought that fibers with wet surfaces would make a more effective filter media. SAP would improve the performance of both the antimicrobial and the filter.

Keep the media moist

But adding SAP was still was not enough. Gray’s team wanted a way to keep the media moist long after being charged with water during manufacture. They pulled the solution right out of the air. By putting a humectant on the SAP fibers the user’s breath and moist ambient air would reload water into the media and keep the antimicrobial, such a Quaternary Ammonium compound, moist and effective. Lysol and Clorox both make Quat disinfectants. The team decided to call their new nonwoven media MedTextra and applied for a trademark and a patent.

The next step in the media development process was sourcing SAP fibers with good fuzziness to make the path through the filter more tortuous for the viral particles. Unfortunately they had to buy a 1,000 lb. bale of SAP. “I would welcome a call if you have a use for 900 lbs. of SAP,” said Gray. Fortunately he still had a warehouse to store the bale in from his days as a junk dealer, buying and selling surplus inventory and scrap nationally under the Ax-Man brand. Many inventors who sought cheap parts there for their prototypes still fondly remember Ax-Man surplus stores. “I know what products not to invent, because I bought them for pennies at liquidation,” noted Gray.

Gray commissioned a weaver to spin SAP fiber into thread, weave fabric and to make a prototype scarf/air filter. The team also found a pilot plant for air-laid nonwoven filter fabrics and arranged test runs of several basis weights and ratios of structural to SAP fibers. So Gray’s several large rolls of filter media were added to Gray’s warehouse. Filter makers evaluated the fabric by touch and Hume did absorption and simple pressure drop tests. They found that the first round of prototypes had too much SAP and were too thick for mask use. So back to the pilot plant they went. Gray wrote more checks.

The MedTextra prototypes demonstrated that it could be made on contemporary air-laid machines out of commercially available materials. But did it work? The first step was to show that the antimicrobial Quat was as effective on SAP as it was on cotton fiber at short and longer exposure times. Cotton’s performance is well understood in antimicrobial testing. SAP did as well as cotton except during the first 30 seconds after draining from immersion in Quat. At five minutes its performance was the same as cotton. The percentages of viral material deactivated compared well with other antimicrobial media for representative viruses, a 99.8% reduction in viral loading. The team thinks that during the first 30 seconds after immersion the antimicrobial is still being absorbed by the SAP and is therefore less available on its surface.

Three more years

It took three more years until LMS Technology, an independent lab, was engaged to perform filtration efficiency and pressure-drop measurements on MedTextra samples of various specifications. A solid 26% improvement in filter efficiency was found after the MedTextra fabric system was conditioned to 50% relative humidity (RH) for 24 hours over the same media at 20% RH. The 20% RH results are can be used as a control because at 20% RH the tested SAP does not absorb moisture and so it acts as a normal fiber. This particular SAP maxed out its loading at about 50% RH and the fibers swelled. While efficiency jumped with higher RH, the pressure-drop remained about the same, indicating that mask user comfort and HVAC performance objectives were achievable.

To make further development for a wide range of specific filter applications less risky and costly for licensees, a predictive filter-performance model was developed based on a public domain model. Compared to the LMS test data, the model under-predicts observed performance. It’s a single fiber model and its two-layer media module was adapted to approximate the performance of two-fiber blend variables. Now fiber type and grade; basis weight; SAP grams per meter; and other design options can be modeled so that the next round of prototypes perform closer to the desired performance specification. Using the model should be more cost-effective than repeated trial and error development.

There are many ways that the antimicrobial ingredient and the humectant can be built into the MedTextra fabric system. They may be sprayed on or the media web could be submerged in these solutions. Moisture barrier packaging may be needed for some applications or in dry environments, but such packaging would instantly enable the highest level of efficiency after one puts a respirator mask on. Otherwise a just few minutes of breathing through it is expected to raise the filtration efficiency to its maximum level.

Diverse background

This project underscored the importance of fully understanding the problem so that the resulting solution is relevant. The diversity in the backgrounds of the MedTextra team members and their many years of experience helped them see connections, find suppliers and evaluate options. New team members were welcomed and patiently well briefed by the earlier members. The personal chemistry between the team members helped speed decision-making and create a high tolerance for uncertainty. This tolerance helped keep the search for solutions broad and unconventional. And the financial decision-maker was deeply involved so investment decisions were made quickly. Like a start-up, they had the luxury of few meetings, fewer memos and an organizational bias toward action.

The MedTextra fabric system is poised for application-specific development by brands and manufacturers who know their customer’s needs well. The future includes prototypes specified by modeling and collaboration with testing labs and ASHRAE to develop methods and standards for dynamic testing of viral filtration effectiveness of respiratory mask and HVAC media that incorporate capture and kill antimicrobials such as the MedTextra technology.