Emergency Local 3D Printing: Often Missed Key Features of Facial Masks
A remarkable variety of 3D printed (and sewn) mask designs have flooded the internet in the last couple of weeks. Rather than pick and choose a "best bet" among the existing designs (knowing that these will continue to rapidly evolve as they hit the front-line "field") I wanted to highlight some key features when assessing designs for yourself. While any shield across the face is better than none at all, a little extra thought and effort can greatly improve its efficacy.
One note, most of this information applies to designs intended to act as emergency replacements for N95 masks in clinical settings. If you are working on simple "surgical type" masks for general populace wear, this information is still useful, if not required.
A medical mask needs to ultimately accomplish two things, let oxygen in, keep bad stuff out. To do this, all air flow would ideally be directed through a filter. In single piece masks, the entire mask is produced from a non-woven filter material. Most 3D printed masks are a polymer housing that contain air intake features with filter material in them. Any cracks, pinholes, or gaps between mask, filter and face represent potential sites of pathogen entry.
In designing and manufacturing masks for 3D printing, you need to take into account your design, your material and your manufacturing process to achieve an effective seal. In the design, components that trap and lock a filter in place need to provide a continuous, compressed fit around the material's outline when assembled. A filter should not be loose in a housing. For sealing around the face, the material needs to be flexible. Healthcare professional come in a wide range of sizes and shapes, and their faces are no different. A flexible mask that can conform to a face will always seal better than a rigid one. When designing the walls of the mask, be sure to provide sufficient thickness to allow for a complete bond between 3D printed layers. How thick is "thick-enough" will vary by machine and material, so double check the design guides from the manufacturer. Even if it was solid off the machine, a thin wall can develop cracks and pin-holes under the stresses of continuous wear. Finally, calibrate and test your 3D printers' output to make that the tolerances are tight enough to produce the mechanical interfaces you designed. The best design in the world is useless if printed on a faulty machine.
There have been enough pictures of the bruised and beaten faces of healthcare workers in the news for this to be pretty obvious. Nearly any design can seal if pressed hard enough into the skin. The hard part is producing a design that can seal and remain comfortable over a 14 hour day. Just like with the seal, fit is greatly improved by making the face contacting surfaces from a flexible material. The surfaces can also be made selectively flexible by playing with the wall's thickness and adding ribs or flex features. Many textile masks include pleats to allow for expansion when needed.
In addition to having a soft material or geometric features that allow the mask to conform when under pressure, a close fit can be achieved by including formable wire inserts into areas of the mask. A recent computer software simulation showed just how much impact a tiny metal nose clip has in improving the seal of a mask. Other masks (especially those made completely with non-woven textiles) have a continuous wire running around the entire edge of the mask. This allows for direct customization by the wearer.
For 3D printed masks, you can include channel features at key points for wire inserts. The 3D printed channel walls should be thick enough to prevent the wire from slicing through the mask material when being formed or cleaned, but thin enough to conform to the bent wire shape. Material for these inserts should annealed stainless steel if possible. Spring steel or piano wire will not bend and hold a shape.
Finally, make sure to leverage one of the key strengths of 3D printers - the ability to create a range of sizes as easily as you make one. Improve the fit by scaling the interfacing elements of the masks and creating 3-6 different sizes of the same design.
Even when a masks seal and fit with the facial anatomy is optimized, you may still have issues with pressure. Pressure concentrations, even small ones, applied over extended, continuous use, can lead to bruising, break-down in skin integrity, and edema (swelling) of the surrounding tissues. Once the skin is damaged, pressure will prevent the site from healing. The first way to address this is simply increasing the surface area. The contact area with the face should not be a thin edge, but a smooth, wide surface. Next, pay special attending to areas contacting bony facial surfaces, including the bridge of the nose, the cheek bones, the brow ridge and the forehead. Be aware of how the mask sits on these areas, as well as how you route straps/bands across these surfaces to secure the mask. Wide, adjustable straps and elastic bands will also help, especially since facial tissues may shrink or swell after long mask wear, requiring a fit adjustment.
For any mask you print, be sure to "stress test" your design by wearing it for a full 8-10 hours before handing it off to a health care worker. You can take it off when it gets uncomfortable. They can't.
O2 Pass Thru
In the zeal to keep the virus out, don't forget that air still needs to come in. Health care workers can go through Olympic level sprints of physical activity over the course of their day. While most of the air flow is determined by the material and number of layers of filter material, this is further restricted by the design of the enclosure around it. The filter needs to be sealed around the edges, and reinforced as necessary, but the filter surface area should remain as exposed as possible.
As part of your mask "stress-test", include a few sprints around whatever space you can find. Once you get your heart-rate up, do you get air blow-back out the sides of the mask? Do your glasses steam up?
Whether or not a 3D printed mask can be sterilized with a particular sterilization method is highly dependent on the material and method used to print. Be aware that the most common methods for on-site sterilization of medical products in clinics are autoclave, Ethylene Oxide (EtO) and vaporized hydrogen peroxide (all gas or vapor based methods). If your product can not be safely cleaned with these methods, you need to include a warning label on the device, and recommendations for how they should be cleaned. Prusa Printers did an amazing job of testing their printed face shield parts in certified labs and posted the test results in an online article. Please note, these results are for their design, using their machines and their specific material. The results can NOT be extrapolated to other 3D printed materials.
Design and geometry also contribute to safe and effective sterilization. If parts of the mask need to be removed/replaced before sterilization, make sure this can be done quickly, repeatedly, in gloves and without tools. Add features to improve hanging or sorting on a rack, or (if recommending chemical cleaning) allow it to sink rapidly in a bucket of disinfectant. Minimize small, ornamental features, blind pockets and crevices where pathogens can hide. If you do add features, extrude them out to provide access for scrubbing brushes, or cut through a part so they can be flushed out.
"Safe" Failure Modes
A key part of every medical device design process is identifying and addressing potential risks. These include risks not only due to material or mechanical features, but also use environment risks. Hospital and clinics are filled with scared, stressed and very sick people whose mental and physical control is often heavily compromised. A patient may grab and pull off a mask, or strike a health worker in the face. In these situations, certain mask features can become dangerous. Many lower cost masks have the elastic stapled in place, with the ends free. This allows the elastic to pull free of the mask if it is pulled hard enough, rather than choke or cut the wearer. Buckles and latches on more complex masks should be designed to release before the strap tension gets too high. Be especially careful of any hard or sharp edged features along the top edge of the mask, over the nose and along the under eye areas. These can be driven towards the sensitive eye area if a mask crumples under a blow to the face. As a life-long wearer of glasses, I can testify that tiny wire nose pads can cause a lot of damage when exposed to an overactive toddler's head.
Be sure to be particularly abusive to a few of your 3D printed prototypes. Punch it, drop it, kick it. When it does crack or crumple, does it create sharp edges? Does it buckle or send small particles flying?
For those who have not worked in the medical device field, it can be difficult to explain the shear amount of calibration, testing and controls that differentiate a medical manufacturer from those of less risky products. "Risky" products include masks. While deceptively simple, they affect the wearer's breathing, apply pressure to some of the most fragile skin on the body, and are in intimate contact with not one, but two of the human body's main orifices and mucosal membrane sites. Their ability to cause harm if fabricated poorly goes beyond failing their primary function of shielding from pathogen exposure.
Be aware of each step of your manufacturing operation, from raw materials to the final delivery into the hands of a front-line worker. Who and what touches the materials? What oils, contaminants or residues can transfer at those points of contact? When the material is cut or sanded, are all of the generated particulates removed? Do you have a set procedure that you follow for each unit to ensure reputability? Do you have tests and inspection points to check each of your product's functions before it is shipped? When it's in transport, how do you make sure the items arrive intact and functional?
Beyond the Mask - 3D Printing of Tools and Accessories
As I frequently remind my students, don't self-limit where a technology can improve your design by fixating on the product itself. I spoke about potential 3D printing applications throughout the medical product development cycle in a recent Webinar for the Temple Health & BioScience District.
For existing masks, both locally made and mass manufactured, there are opportunities for items that either improve the fit and comfort. A great example are the simple "Ear Guards" produced by a local Boy Scout that pull a mask's constricting elastic ear loops away from the tender creases behind a wearer's ears. Instead of printing a entire mask, print an adapter to re-purpose an existing mask. Use 3D printing to create multiple "fit models" of faces to test prototypes. Even with 100% sewn masks, specialized tools and templates can greatly improve the quality and speed of home-made products, such as this Bias Tape Maker tool used in creating face mask straps.
Many thanks to all the designers, hackers, makers and more jumping into the design effort. Please keep applying those brilliant minds to all parts of the problem.