A D V A N C E D M A T E R I A L S & P R O C E S S E S | S E P T E M B E R 2 0 2 1 2 9 shown in Fig. 1, the foams have tunable microstructures and morphology. MECHANICAL PROPERTIES The strengthened Cu foams are mechanically sturdy yet still maintain ultralow density. As shown in Fig. 2, a lightweight 15% 2ED-Cu foam disc, with a mass of 120 mg, can be easily supported on top of a feather. Interest- ingly, the same foam is able to sustain a heavy load of 1.8 kg (15,000 times its own mass) without collapsing. The mechanical properties of the foams can be further tuned by adjusting the microstructures. PRESSURE DIFFERENTIALS The foams are strong, and can withstand air speeds of over 20 m/s and a pressure drop of 1 atm (or 10 mH 2 O) without degradation. For face velocities of about 1-10 cm/s, where most com- mon filters operate, the pressure drop across the foams are in the 0.6-6mmH 2 O range (Fig. 3), i.e., within the breathable range  . The pressure drop is linearly dependent on the face velocity in this range, and the pressure drop coeffi- cient is inversely proportional to breathability, where a lower value cor- responds to a smaller pressure drop and better breathability. For exam- ple, the 5% foam exhibits a small val- ue of 0.6 mmH 2 O • s/cm, indicating a breathability approximately twice as good as that of 1.0 mmH 2 O • s/cm for the N95 respirators  . FILTRATION EFFICIENCY The filtration efficiency test was carried out using aerosol-based poly- disperse NaCl particles, which is wide- ly used for testing filters for face masks [11,12] , and benchmarked against several common filters  . These foams show excellent efficiency for much of the 0.1 - 1.6 μm size range, and this is particularly relevant for COVID-19, with- out the use of electrostatics. For exam- ple, a 1.0 mm thick, 15% 2ED-Cu foam can filter particles in the 0.8 - 1.6 μm, 0.5 - 0.8 μm, and 0.1 - 0.4 μm size ranges, with 99.4%, 96.1%, and 78.3% efficien- cy, respectively (Table 1). For the criti- cal 0.1 - 0.4 μm sized particles, an improved efficiency of 97.0% is achieved by increasing the foam thick- ness up to 2.5 mm. The thinnest sample of a 0.8 mm thick, 5% density 2ED-Cu foam can filter 85.5% of 0.1 - 0.4 μm sized particles. To compare these Cu foam filters with other common filters, it is useful to introduce a filtration quality factor, where α is the penetration ratio of par- ticles, v is the air velocity and ΔP is the pressure drop across the filter. This metric, similar to those in literature [11,12] , is independent of filter thickness or number of filters  . It is also relatively independent of air flow conditions. The only Q dependence on air velocity is due to the capture efficiency. The com- parison of the quality factor for 0.3 μm particles Q 0.3 of the tested filtration me- dia is shown in Fig. 4. For the 5% 2ED- Cu foam (0.8 mm thick), Q 0.3 =(3.24±0.34) mmH 2 O -1 • cm • s -1 . This value is com- parable to that of an electrostatical- ly charged N95 respirator, while the breathability is almost twice as good. Fig. 2 — A 120 mg 15%Cu foam disc, 1.4 mm thick, 9 mm in diameter, can be supported on top of a feather, and can support a 1.8 kg load itself without deformation. Fig. 3 — Pressure drop dependence on the face velocity of 2ED-Cu foams with various densities and thicknesses, adapted with permission fromMalloy et al.  .