Mechanical Validation of the iMASC System
Characterization of mask material after sterilization
An advantage of the iMASC system over the half-mask respirators is the methods of sterilization. We have performed tensile tests of the mask material after 10 autoclave cycles and 5 minutes in a 1:10 bleach solution and 70% isopropyl alcohol. We found that 10 autoclave cycles make the mask slightly stiffer, while the bleach soak resulted in no change and the isopropanol alcohol soak makes the material less stiff. Despite these small changes in tensile strength, there were no gross differences in the mask compared to the non-sterilized mask.
Figure 1: Mechanical testing on samples cut directly from masks exposed to a variety of sterilization methods including 10 cycles of autoclaving, 10-minute soak in 10% bleach solution, and 10-minute soak in isopropanol.
Finite element analysis for mask deformation upon different face shapes and sizes
We used non‐linear finite element (FE) analyses to evaluate the deformation response of the flexible mask frames while wearing and determine the forces required to keep the mask in place across a range of subject faces. We reported the numerical snapshots of the face mask when subjected to the strap’s tensile loads, denoted by T, and monitored the deformation of the mask at different levels of the reaction force exerted from the mask to the face, F = 0 (undeformed), 4.5 (initial contact), and 10 (full contact) N. The color maps represent the distribution of displacement’s magnitude, U, showing relatively large deformation of the mask required to fit in to the subject face. We also calculated the normal contact forces, FN , and contact pressures, P, as a function of F to evaluate the interaction between the mask and face. As expected, no FN was recorded at F = 0. By pulling the straps, the mask starts to be engaged with the face, and at F = 4.5 N the maximum FN occurs around the cheek. Further pulling the straps (F = 10 N) induces a relatively higher FN along the edge of the mask in the check and chin (lower lips) rather than the nose and cheekbones. This is a signature of the need to the Aluminum strip to bond across the bridge of the nose to enhance the contact pressure.
Next, we estimated the reaction force required to achieve an average contact pressure of P =10 KPa (relatively-uniformed distributed along the edge of the mask) as a higher limit of the contact pressure that results in a suitable fit between the mask and skin faces. This reaction force is equivalent to the force applied through the straps. We reported the reaction forces for twenty different subjects, ranging from 9.5 to 15 N. These variations are duo to the difference in shape and size of the subject’s faces especially in the jaw and cheekbone parts. Through application of these forces via the straps combined with the aluminum strip across the nose bridge, one can guarantee the mask will be tightly stayed in place.
Figure 2: (A) Numerical images showing the deformation of the elastomeric mask at different levels of reaction forces, F= 0, 4.5, and 10 N in two different views (top and bottom rows). The colors represent the magnitude of displacement field, U. (B) The corresponding distribution of the normal contact forces, FN, between the mask and face. (C) Reaction forces for the subject numbers n=1,2,3,.., 20 computed from simulations.