Mechanically robust ultrathin nanofibrous films by using microfluidic-based continuous printing

Ultrathin nanofibrous films with unique properties, such as controlled thickness, structures, and excellent mechanical robustness, play a vital role in flexible wearable devices, electronic skin, and rechargeable batteries. However, nanofibrous films are always facing limitations in their mechanical properties, even though they are strong when used as textiles, mainly owing to their structural shortcomings by using conventional fabrication methods. Herein, we present the fabrication of free-standing ultrathin nanofibrous films with good mechanical properties by using a microfluidic-based continuous printing strategy. Owing to the precisely controllable microfluidic flow in the micrometre-scale, the resulting aramid nanofibre (ANF) films can reach thicknesses as low as 140 +/- 25 nm. Specifically, the tensile strength of such ultrathin ANF films is recorded at an impressive value of 667 +/- 40 MPa, representing a 120% improvement compared to the films prepared by using casting method. Such excellent mechanical robustness comes from the double-sided protonation, which shows a symmetrically dense structure compared to the asymmetric structure of cast films. Furthermore, we demonstrate the continuous fabrication of thin regenerated cellulose nanofiber (RCNF) and cellulose diacetate (CDA) films using the microfluidic-based printing strategy. Both microfluidic-based films show significant enhancements in strength, with a 42% increase for RCNF and a 94% increase for CDA compared to their cast films. We envision that this microfluidic-based continuous printing strategy provides a promising pathway for the development of advanced ultrathin nanofibrous films towards practical applications. Free-standing mechanically robust ultrathin nanofibrous films have been fabricated by using microfluidic-based continuous printing owing to the symmetric solvent exchange, showing great potential for practical applications.