Self-Sanitization in a Silk Nanofibrous Network for Biodegradable PM0.3 Filters with In Situ Joule Heating

Abstract

In the contemporary way of life, face masks are crucial in managing disease transmission and battling air pollution. However, two key challenges, self-sanitization and biodegradation of face masks, need immediate attention, prompting the development of innovative solutions for the future. In this study, we present a novel approach that combines controlled acid hydrolysis and mechanical chopping to synthesize a silk nanofibrous network (SNN) seamlessly integrated with a wearable stainless steel mesh, resulting in the fabrication of self-sanitizable face masks. The distinct architecture of face masks showcases remarkable filtration efficiencies of 91.4, 95.4, and 98.3% for PM_0.3, PM_0.5, and PM_1.0, respectively, while maintaining a comfortable level of breathability (ΔP = 92 Pa). Additionally, the face mask shows that a remarkable thermal resistance of 472 °C cm² W^-1 generates heat spontaneously at low voltage, deactivating Escherichia coli bacteria on the SNN, enabling self-sanitization. The SNN exhibited complete disintegration within the environment in just 10 days, highlighting the remarkable biodegradability of the face mask. The unique advantage of self-sanitization and biodegradation in a face mask filter is simultaneously achieved for the first time, which will open avenues to accomplish environmentally benign next-generation face masks.

Figure 1

Schematic illustration of the synthesis of SNN for face masks. (a) Silkworm cocoons, (b) degumming process, (c) acid hydrolysis, and (d) mechanical chopping of silk fibers. (e) Self-sanitizable face mask fabricated by sandwiching the SS mesh in between the SNN layers; inset shows the magnified view of the (f) SNN and (g) SS mesh embedded with SNN.

Figure 2

Structural and morphological characterization of the SS mesh: (a) X-ray diffraction (XRD) patterns of the pristine and etched SS mesh and (b) average diameter and pitch of the SS mesh as a function of etching time in acidic solution. FESEM micrographs of the (c) pristine and (d) etched SS mesh, respectively (scale bar, 100 μm each).

Figure 3

(a) XRD patterns of the SS mesh embedded in the SNN (face mask); inset shows the XRD peak corresponding to the SNN. Optical microscope images of (b) boundary of the SS mesh/SNN; inset SNN (scale bar, 200 and 100 μm, respectively). FESEM images of (c) SNN; inset corresponds to the magnified view (scale bar, 2 μm and 300 nm, respectively). Mechanical properties of the pristine SS mesh (PSS), etched SS mesh (ESS), SNN, and sandwich of the etched SS mesh and SNN (ESS/SNN), (d) representative stress–strain curves, and (e) Young’s modulus from the linear region.

Figure 4

Filtration performance of the SNN-based face masks. (a) Schematic illustration of FE and pressure drop measurement setup. (b) FE of the SNN molded in one layer, two layers, three layers, and four layers for the PM of size 300, 500, and 1000 nm. Differential pressure as a function of (c) SNN layers at a constant face velocity of 4.71 cm/s and (d) face velocity for one layer of the SNN.

Figure 5

Joule heating performance of a self-sanitizable face mask. (a) Temperature of the face mask as a function of time for the applied bias of 1.1, 2.0, 2.4, and 2.9 V. (b) Response time of the face mask for different saturation temperatures. (c) IR image of the face mask for the applied bias of 2.9 V, scale bar: 1 cm. (d) Saturation temperature vs input power per unit area for the estimation of thermal resistance. (e) Temperature switching cycles of face masks.

Figure 6

FESEM images of cultured E. coli bacteria on the face mask (a,b) before the heat treatment (scale bar: 1 μm and 500 nm, respectively) and (c,d) after the in situ Joule heating at 70 °C for 15 min (scale bar: 1 μm and 500 nm, respectively). Biodegradability test: environmental decomposition of the SNN after exposure to natural soil for (e) 1, (f) 3, (g) 7, and (h) 10 days, respectively.