Large-Scale Nanofiber Manufacturing| Lotus leaf-inspired poly(lactic acid)nanofibrous membranes with enhancedhumidity resistance for superefficient PMfiltration and high-sensitivity passivemonitoring

Professor He Xinjian & Associate Professor Xu Huan from China University of Mining and Technology: Lotus Leaf-Inspired Highly Humidity-Resistant and Electroactive Polylactic Acid Nanofiber Membranes

Particulate matter (PM) pollution in air poses significant challenges for modern society, severely damaging ecological environments, hindering precision manufacturing development, and even threatening human health. Due to their extremely small size and complex composition, PMs easily carry and spread bacteria and viruses, posing major risks to respiratory safety and cardiovascular systems. Biodegradable polylactic acid (PLA) nanofiber membranes (NFMs) show great potential in addressing increasing airborne PMs and plastic/microplastic pollution, but still suffer from poor electret effects, dielectric properties, and surface activity - especially rapid charge dissipation under high humidity, presenting challenges for long-term respiratory protection.

Recently, Professor He Xinjian/Associate Professor Xu Huan's team at China University of Mining and Technology collaborated with Professor Gao Jiefeng from Yangzhou University and Researcher Li Heguo from Academy of Military Sciences to publish their latest research in Journal of Hazardous Materials: "Lotus leaf-inspired poly(lactic acid) nanofibrous membranes with enhanced humidity resistance for superefficient PM filtration and high-sensitivity passive monitoring". Inspired by the "micro-synaptic structures on lotus leaf surfaces", the researchers employed an "electrospinning-electrospraying" strategy to anchor MOF nanocrystals onto PLA nanofibers, creating dense synapses resembling natural lotus leaf structures. This unique MOF synaptic structure derived exceptional combinations of electroactivity, in-situ electret properties, charge regeneration mechanisms, and outstanding humidity resistance.Introducing MOF nanocrystals as electroactive mediators created active shells on PLA nanofibers, significantly improving filtration performance by enhancing surface activity and electroactivity. The lotus-inspired strategy of creating unique MOF synaptic structures on PLA nanofibers shows broad prospects for real-time respiratory monitoring and air filtration.

Figure 1: Biomimetic polylactic acid nanofiber membrane for highly humidity-resistant respiratory protection and high-sensitivity monitoring.

Fiber surface morphology profoundly impacts membrane filter performance. Accordingly, the team regulated membrane morphology by anchoring MOF-5 nanocrystals on PLA fibers, preparing nanofiber membranes with different fiber diameters and MOF synaptic structures.Electrospun Normal PLA fibers showed smooth surfaces with inter-fiber adhesion, while electrospun-electrosprayed BM-PLA NFMs consisted of randomly stacked continuous uniform nanofibers with surface nanobumps forming MOF synaptic structures that provided air molecule transport channels. 

Figure 2: Morphological evolution observation of BM-PLA NFMs.

Fiber diameter gradually decreased with MOF-5 nanocrystal anchoring. XRD showed intensified MOF-5 characteristic peaks at 2θ = 6.9°, 9.1° and 15.8° in BM-PLA membranes. FTIR revealed microstructural evolution through absorption band shifts: C=O stretching vibration (1753→1755 cm⁻¹), benzene skeleton vibration (1455→1452 cm⁻¹), methyl deformation (1381→1383 cm⁻¹), and new Zn-O peak at 526 cm⁻¹, confirming MOF-5 anchoring.

Figure 3: Structural analysis of BM-PLA NFMs.

BM-PLA NFMs exhibited high electroactivity and superior mechano-electric conversion. With 8 wt% MOF nanocrystals (BM-PLA8), initial surface potential reached 3.0 kV (vs Normal PLA's 1.1 kV), slightly decreasing to 2.6 kV after 7-day aging (vs 0.6 kV). BM-PLA8 also showed significantly enhanced output voltage/current (64.5 V/151.6 nA) with high humidity resistance (90% RH) and long-term stability.

Figure 4: Evaluation of electroactivity and triboelectric output performance of BM-PLA NFMs.

iltration tests at various airflow rates (10-85 L/min) demonstrated BM-PLA8's superior PM0.3 filtration efficiency (95% vs Normal PLA's 75.8%) with lower air resistance (250 Pa vs 350 Pa). Outdoor applications confirmed BM-PLA8's reusability and humidity resistance, maintaining stable filtration efficiency during 4-hour continuous operation (32 L/min) and under high humidity.

Figure 5: Air filtration performance of BM-PLA NFMs.

SEM observations revealed filtration mechanisms: Normal PLA relied solely on physical interception and weak electrostatic adsorption due to large smooth fibers and low electroactivity, while BM-PLA NFMs exhibited: (1) Thinner fibers and MOF synapses increasing surface activity; (2) MOF structures enhancing charge traps and air slip effects; (3) Polar groups in MOF-5 improving dielectric constant for stronger electret effects; (4) Large MOF-5 surface area enabling active adsorption.

Figure 6: Filtration mechanism of BM-PLA NFMs.

As a key physiological function, respiration reflects health status. Real-time respiratory monitoring helps detect and prevent respiratory diseases, holding significance for personalized healthcare. While some sensors are widely used in health monitoring due to low cost and unique signals, they still face flexibility and sensitivity limitations. Compared to traditional devices, triboelectric nanogenerator (TENG) membrane filters based on nanofiber membranes show superior applicability for respiratory sensing. As shown in Figure 7, the assembled TENG could harvest subtle vibration energy from human breathing and convert it into electrical signals for respiratory monitoring, clearly distinguishing normal, weak, and rapid breathing patterns through frequency and intensity analysis.

Figure 7: TENG based on NFMs for real-time respiratory status monitoring.

In this study, the biomimetic lotus synapse design not only significantly improved fiber electret effects, interfacial polarization, and electroactivity but also optimized PLA nanofiber morphology and diameter. Consequently, BM-PLA NFMs demonstrated excellent air purification with low resistance, maintained high filtration efficiency under high humidity and during 4-hour outdoor tests, and enabled highly sensitive passive respiratory monitoring via TENG, indicating medical application potential. In summary, BM-PLA NFMs show broad prospects in respiratory protection and personal physiological monitoring.