Electrospun PCL Filtration Membranes Enhanced with an Electrosprayed Lignin Coating to Control Wettability and Anti-Bacterial Properties

Abstract

This study reports on the two-step manufacturing process of a filtration media obtained by first electrospinning a layer of polycaprolactone (PCL) non-woven fibers onto a paper filter backing and subsequently coating it by electrospraying with a second layer made of pure acidolysis lignin. The manufacturing of pure lignin coatings by solution electrospraying represents a novel development that requires fine control of the underlying electrodynamic processing. The effect of increasing deposition time on the lignin coating was investigated for electrospray time from 2.5 min to 120 min. Microstructural and physical characterization included SEM, surface roughness analysis, porosity tests, permeability tests by a Gurley densometer, ATR-FTIR analysis, and contact angle measurements vs. both water and oil. The results indicate that, from a functional viewpoint, such a natural coating endowed the membrane with an amphiphilic behavior that enabled modulating the nature of the bare PCL non-woven substrate. Accordingly, the intrinsic hydrophobic behavior of bare PCL electrospun fibers could be reduced, with a marked decrease already for a thin coating of less than 50 nm. Instead, the wettability of PCL vs. apolar liquids was altered in a less predictable manner, i.e., producing an initial increase of the oil contact angles (OCA) for thin lignin coating, followed by a steady decrease in OCA for higher densities of deposited lignin. To highlight the effect of the lignin type on the results, two grades of oak (AL-OA) of the Quercus cerris L. species and eucalyptus (AL-EU) of the Eucalyptus camaldulensis Dehnh species were compared throughout the investigation. All grades of lignin yielded coatings with measurable antibacterial properties, which were investigated against Staphylococcus aureus and Escherichia coli, yielding superior results for AL-EU. Remarkably, the lignin coatings did not change overall porosity but smoothed the surface roughness and allowed modulating air permeability, which is relevant for filtration applications. The findings are relevant for applications of this abundant biopolymer not only for filtration but also in biotechnology, health, packaging, and circular economy applications in general, where the reuse of such natural byproducts also brings a fundamental demanufacturing advantage.

Keywords: FTIR; PCL; SEM; antibacterial test; electrospinning; electrospraying; eucalyptus; lignin; morphology; oak; wettability.

Figure 1

Representative pictures of PCL membranes coated with eucalypt lignin at different times showing color changes as the deposition time increases: (A) bare PCL; (B) AL-2.5; (C) AL-5; (D) AL-10; and (E) AL-60.

Figure 2

Scanning Electron Microscope (SEM) results: (A) AL-EU-2.5; (B) AL-EU-5; (C) AL-EU-10; (D) AL-EU-60; (E) AL-EU-120; and (F) PCL.

Figure 3

Illustrative optical microscope images: (A) PCL coated for 10 min with AL-OA; (B) PCL coated for 120 min with AL-OA.

Figure 4

SEM micrographs of AL-OA-60 (A) and AL-EU-60 (B), the latter already shown in Figure 2D, indicate that lignin nanoparticles tend to coalescence more markedly in the case of eucalyptus lignin than for oak lignin.

Figure 5

Trends in roughness Ra (Arithmetic Average Roughness) as a function of coating time with different lignins.

Figure 6

Trends in porosity of the layered membrane for each lignin grade and for different deposition times.

Figure 7

Comparison of permeability results (cm/s).

Figure 8

Contact angle histograms of AL-EU (A) and AL-OA (B) measured using water and 1PWD (1 Primary Wound Dressing©).

Figure 9

ATR-FTIR spectra comparison of the membrane PCL, AL-2.5, AL-5, and AL-10. PCL-specific peaks assignment: stretching vibration of the –C=O group (band 1), deformation vibrations of the –CH₂ groups (band 2), stretching vibrations of C–O and C–C groups in the crystalline phase (band 3), asymmetric stretching vibrations of C–O–C (band 4), symmetric stretching vibrations of C–O–C (band 5), and stretching vibrations of C–O and C–C groups in the amorphous phase (band 6).

Figure 10

ATR-FTIR spectra comparison of the membrane PCL, AL-60, and AL-120. Lignin-specific peaks assignment: aromatic skeletal vibrations C=O stretching (band L1), C=C aromatic ring vibration (S > G) (band L2), C=C aromatic ring vibration (G > S) (band L3), aliphatic C-H stretching vibrations in CH₃ (band L4), guaiacyl ring breathing (band L5), C−C, C−O, and C=O stretching in condensed G units (band L6), CH elongation in the G ring (band L7), and the typical S ring CO stretching (band L8).

Figure 11

(A) Biological experiment 1—Petri dishes obtained from S. aureus culture without any mat (control) and cultures with blank PCL and PCL-Lignin mats at two different serial dilutions (10⁻⁵ and 10⁻⁴) and last sampled time (3 h); (B) Bacterial population (CFU/mL) measured for control, PCL, and coated samples at different time points: 0 (T0), 1.5 (T1), and 3 (T2) hours (* p < 0.05).

Figure 12

(A) Petri dishes obtained from S. aureus cultures with blank PCL and PCL-Lignin mats at two different serial dilutions (10⁻⁵ and 10⁻⁶) and last sampled time (4.5 h); (B) S. aureus bacterial population (CFU/mL) measured for PCL and coated samples at different time points: 3 (T2) and 4.5 (T3) hours (* p < 0.05 or ** p < 0.01).

Figure 13

(A) Petri dishes obtained from E. coli cultures with blank PCL and PCL-Lignin mats at two different serial dilutions (10⁻⁵ and 10⁻⁶) and the second last sampled time (3 h); (B) E. coli bacterial population (CFU/mL) measured for PCL and coated samples at different time points: 3 (T2) and 4.5 (T3) hours (* p < 0.05).