Integrating Alkali Lignin into Electrospun PET Nanofibers for Enhanced Viral Protection in Respiratory Masks

Abstract

This study presents the development of an antiviral nonwoven nanofiber fabric composed of polyethylene terephthalate (PET) and alkali lignin, fabricated via solution electrospinning for potential use in face mask filtration media. Alkali lignin, a biobased antimicrobial agent, was incorporated into the PET matrix to enhance both antimicrobial and antiviral efficacy. Various PET concentrations and electrospinning parameters were optimized to achieve uniform, bead-free fibers with nanoscale diameters. Among the tested formulations, 20 wt % alkali lignin loading yielded nanofibers with an average diameter of 290 nm, demonstrating improved hydrophilicity and enhanced fiber morphology. SEM analysis confirmed the uniformity of the fiber structure, while FTIR spectroscopy suggested hydrogen bonding interactions between PET and lignin. Antimicrobial assays showed that the PET–lignin 20 wt % composite completely inhibited the growth of both Staphylococcus aureus and Escherichia coli within 1 h. Moreover, antiviral testing indicated more than a 2-log reduction in human coronavirus titers after 2 h of exposure. The fabricated PET–lignin nanofibers offer a sustainable alternative to conventional polypropylene-based mask materials, featuring enhanced biocompatibility and potential recyclability. These findings highlight the prospective utilization of lignin-integrated PET nanofibers in advanced healthcare and biomedical applications, including antiviral filtration, medical textiles, and tissue engineering scaffolds, while contributing to environmental sustainability through the reutilization of biowaste-derived compounds.

1. Introduction

The global demand for healthcare protection, particularly medical face masks, has surged in recent years due to the ongoing COVID-19 pandemic. Consequently, the use of personal protective equipment (PPE) like face masks, gloves, gowns, shields, and shoe covers has intensified globally. With the rise in face mask usage during this epidemic, the challenges of the increasing volume of disposable face masks released into the environment have increased even further. It is anticipated that up to 173 000 microfibers resulting from used surgical masks are disposed of in landfills daily. According to recent reports, it is estimated that over 3.7 billion masks are used and discarded each day, which translates to about 4.1 million tons of plastic waste annually around the world. These single-use plastic masks could potentially be released into the aquatic ecosystem, causing serious problems to human/animal health and environment. While disposable face masks were initially implemented for effective infection control, the significant environmental challenges associated with their disposal, coupled with a loss of filtration efficiency over prolonged use, necessitate the development of sustainable alternatives. So, a promising alternative is the use of biodegradable or recyclable polymers in face mask manufacturing. Until now, a great number of polymers, including polyester, fiberglass, polycarbonate (PCL), polyethylene (PE), polypropylene (PP), polyacrylonitrile (PAN), polyvinyl acetate (PVA), polylactic acid (PLA), polystyrene (PS), nylon 6 (PA), and polyurethane (PU), have been used to produce fibrous filters for face mask applications. − Among these polymers, biodegradable polymers, used mostly in biomedical and tissue engineering applications, lead to greenhouse gas (GHG) emissions (such as CO₂ and methane) during their degradation process and cause microplastics to be released into water and soil environments. These drawbacks associated with biodegradable polymers make recyclable polymers such as PET a more interesting alternative to replace single-use masks while ensuring the circular economy of the materials and achieving higher durability. The most common recyclable polymers are high-density polyethylene (HDPE), PET, and polyolefins. Among the aforementioned polymers, PET is a cheap and highly recycled polymer that can be easily recycled mechanically or through the solvolysis process.

Face masks are widely used in air and smoke filtration, respiratory filters, and medical applications. They are manufactured from cellulose, glass, quartz, and polymers. Typically, these filters have high porosity of 60–90% and a 3D structure of randomly oriented fibers with diameters ranging from nanometers to micrometers. In general, the fiber’s diameter can vary between 0.5 and 50 μm. Airborne particles larger than 600 nm cannot penetrate through fibrous filters, while smaller ones (300–600 nm) are trapped in the filter’s pores. Tiny particles <300 nm like airborne pathogens, especially SARS-CoV-2 with a size of 80–150 nm, can easily penetrate filter layers and enter the human body. , The SARS-CoV-2 virus can directly be transmitted from an infected person by breathing, sneezing, and coughing or occur indirectly by touching contaminated surfaces. To prevent the release of droplets containing the virus from the wearer into the environment, surgical face masks are used. Surgical face masks are commonly single-use 3-layered fabrics made from PP meltblown filters sandwiched between nonwoven fabrics. The key role of surgical face masks is to control the transmission of particulate matter (PM) and protect against the spread of viral respiratory diseases. Transmission of coronavirus infection occurs when the infected patient is close enough to the uninfected surface. Droplets (>5–10 μm) and bioaerosols (≤5 μm) in the form of PM are responsible for this transmission. , As the virus sizes are nanoscale, conventional face masks cannot completely inhibit their entrance into wearer’s body. Therefore, to protect against human infections, antimicrobial face masks possessing antibacterial and antiviral properties to kill and deactivate pathogens are urgently needed. Antimicrobial face masks not only provide multifunctional properties by deactivating microorganisms upon primary contact but also reduce the possibility of secondary infections. It is assumed that they are a potential substitute for conventional face masks due to their good filtration efficiency and ability to deactivate or kill microorganisms. Therefore, the incorporation of strong antimicrobial active agents (AMA) into face masks and the improvement of their biocompatibility is a prominent topic to be explored. , Materials with antimicrobial and biocidal activities include metal and metal oxides, quaternary ammonium (QA) groups, N-halamine compounds, carbon-based materials such as graphene and graphene oxide (GO), carbon quantum dots (CQDs), antiviral peptides, and nanoparticles. Metals and metal oxides are the most commonly used antimicrobial compounds with strong activity. Some examples of antiviral metals are silver (Ag), copper oxide (CuO), zinc oxide (ZnO), tungsten oxide/carbide, and magnesium peroxides. Ag nanoparticles or Ag-based materials exhibit strong antibacterial, antiviral, and antifungal activities compared to other materials. However, many factors, such as toxicity, agglomeration of metallic nanoparticles in human’s organs, and their nonbiocompatibility, have restricted the usage of metal nanoparticles in medical applications. Therefore, more efforts are being directed toward introducing natural antimicrobial compounds derived from medicinal plants with nontoxicity. Biobased antimicrobial agents include natural biopolymers and plant-derived materials. Some biobased compounds, including chitosan polysaccharide or its derivatives, essential oils, and plant extracts with antimicrobial activity, are widely studied. In contrast to commonly used biopolymers such as chitosan, the use of cellulose and alginate, as well as the application of lignin in this field, is still limited due to the complex and heterogeneous structure of lignin.

Lignin is a natural compound that originates from plants and wood via various processing methods such as krafting, organosolving, sodaing, and sulfite processes. Based on the source and extraction methods, the derived lignin may possess different compositions and properties.

Lignin is the most abundant biopolymer after cellulose and is one of the constituents of lignocellulosic biomass, comprising a 15–35% portion, which is technically a byproduct of the paper and wood-making industries. Biodegradability, low cost, renewability, abundant availability in industrial waste, and eco-friendliness, as well as its antimicrobial and antifungal properties, have made lignin a desirable biomaterial applicable in many fields. It has demonstrated biocidal activity against a broad spectrum of pathogens, especially Gram-positive and Gram-negative bacteria and plant’s fungi. Thus, the incorporation of lignin-based substances into a polymeric matrix could provide antimicrobial activity. Lignin can initiate chemical bonding reactions due to its rich content of oxygen free radicals. Moreover, it has polyphenolic groups with an aromatic structure rich in hydroxyl and methoxyl functional groups, providing hydrogen for terminating oxidation reactions. The OH groups present in lignin’s structure contribute to its reactivity, functionality, hydrophilicity, and antioxidant activity. Reactive oxygen species (ROS) are highly reactive molecules that contain oxygen. A high concentration of ROS results in damage to membranes, proteins, DNA, and RNA. ROS inactivation seems to be a desirable antiviral mechanism due to the production of nontoxic species. The antiviral and antibacterial mechanism of lignin is attributed to ROS generation by the oxidation of phenolic groups in lignin under light irradiation. For instance, Alzagameem et al. demonstrated that hydroxypropyl methylcellulose (HPMC)/lignin films with 5% lignin exhibited strong antibacterial activity against Staphylococcus aureus (S. aureus), achieving a log reduction of 4.4 at a concentration of 0.1 g/mL. Conversely, higher lignin concentrations (30%) showed reduced efficacy, suggesting that optimal lignin content is critical for antimicrobial performance.

In addition to their antibacterial properties, lignin derivatives have shown antiviral potential. Boarino et al. demonstrated that lignin-based surface coatings effectively inactivated enveloped viruses, such as herpes simplex virus type 2 (HSV-2), influenza A virus, and SARS-CoV-2. Notably, these lignin coatings maintained their antiviral activity over a 6-month period under ambient conditions, highlighting their potential as sustainable antiviral materials. Such findings highlight lignin’s potential as a bioactive additive in the development of antimicrobial and antiviral materials.

Lignin has been studied in various areas of biomedical applications, such as gene and drug delivery, bioimaging, biosensors, 3D printing, and tissue engineering. It has been combined with various polymers, such as PVA, PAN, PEO, and PCL for nanofiber production used in medical applications. For instance, in a study by Yang et al., lignin was incorporated into PVA to develop antimicrobial films. The resulting PVA-lignin 3 wt % films exhibited a remarkable decrease in the number of bacterial colonies to approximately 0.25 × 10⁶ and 1.0 × 10⁶ CFU/mL for S. aureus and E. coli, respectively. Moreover, Santos et al. investigated the impact of incorporating cellulose and/or lignin on the structural and thermal properties of recycled PET mats. Their study demonstrated that the addition of lignin led to a reduction in the average fiber diameter compared to the neat PET mat (PET_ref: 242 ± 59 nm vs PETL/48h: 175 ± 48 nm). Furthermore, the glass transition temperature (T _g) of electrospun PET mats increased with the inclusion of lignin, indicating enhanced thermal stability. They concluded that the characteristics of PET-based mats can be effectively modified by integrating lignin and adjusting the dissolution time prior to electrospinning. These findings demonstrate that lignin-based additives could not only provide antimicrobial activity but also enhance the characteristics and properties of polymeric fibers.

Unlike prior research, which has largely focused on the structural and mechanical properties of PET–lignin composites, this work investigates the antimicrobial and antiviral efficacy of the electrospun PET–lignin fabric. Specifically, the bactericidal and virucidal activities of the prepared fabrics were evaluated against Staphylococcus aureus (Gram-positive), Escherichia coli (Gram-negative), and human coronavirus 229E (ATCC VR-740) over various contact durations. In this study, alkali lignin is incorporated into a PET polymeric solution to maximize the efficiency of the mat in inhibiting viruses and bacteria. Applying alkali lignin as a bioavailable and eco-friendly material can enhance the biocompatibility of face masks. In this context, a PET–lignin composite nonwoven filter was fabricated using the solution electrospinning technique. Electrospinning parameters were meticulously optimized to achieve uniform and bead-free nanofibers. Subsequently, a comprehensive analysis was conducted to evaluate various properties of the resulting fabric, including fiber’s geometry and morphology, average fiber diameter, contact angle, mechanical characteristics, and the influence of alkali lignin on the thermal properties of PET filters. Additionally, the antimicrobial performance of the composite was thoroughly assessed. This study addresses the growing demand for nonwoven fabrics with improved biocompatibility, biosafety, and antimicrobial efficacy capable of effectively inhibiting both viral and bacterial pathogens. Furthermore, the developed antimicrobial nonwoven fabric presents a promising solution to environmental challenges associated with the widespread use and disposal of single-use face masks, as well as the complexities in their recycling, through the utilization of recyclable PET polymer.

2. Materials and Methods

2.1. Materials

FDA-approved poly­(ethylene terephthalate) (PET, Laser⁺ 7000 (C92A)) was provided by DAK Americas. Alkali lignin (kraft lignin) was purchased from Sigma-Aldrich (#471003, CAS No: 8068051, M _w = 10 000 g/mol, low sulfonate content ∼4%, extracted from pulpwood, pine trees). Trifluoroacetic acid (TFA, 99%) and dichloromethane (DCM) were purchased from Sigma (Canada, ON). The prepared mat properties were compared with a commercial three-layer surgical face mask (Procedure Earloop Face Mask ASTM Level 3 provided by Medicom AssureMask Balance). Required materials for antibacterial and antiviral tests including LB broth (Luria–Bertani), beef extract powder, peptone powder, D/E neutralizer, minimum essential medium eagle (EMEM), fetal bovine serum (FBS), and DMEM were purchased from Sigma-Aldrich. All materials were used as received without further purification.

2.2. Electrospinning Process

Electrospinning is a versatile and efficient technique for producing ultrathin fibers with diameters ranging from the micro- to nanoscale. Due to their large specific surface area, low density, high permeability, and porous structure, electrospun fibers have gained significant attention in medical applications, particularly in the development of tissue engineering scaffolds and surgical face masks. The electrospinning process facilitates the fabrication of nonwoven fabrics characterized by a high surface area and porosity. While melt electrospinning is commonly utilized for the production of PET-based fibers, the use of solution electrospinning has emerged as a feasible, rapid, and practical alternative for manufacturing ultrathin fibers. In this study, an electrospinning equipment named Tong Li (Tech Co., Ltd., Shenzhen, China) model No: TL-OMNI, with a vertical setup and cylindrical drum, was employed to fabricate nanomicroscale fibers. The Tong Li electrospinning system features a broad operating range, specifically characterized by a high voltage differential of +50 kV (module positive) and −20 kV (module negative), coupled with a drum collector rotational speed ranging from 1 to 5000 rpm and a needle sliding range of 530 mm.

Polymeric solutions with different concentrations (7, 10, 15) (% w/v) were prepared 1 day in advance. TFA/DCM mixtures with 1:2 (% v/v) ratio were selected according to the PET solubility limitation and electrospinnability. After stirring overnight at room temperature, the resulting solution was inserted into a 10 mL plastic syringe (BD, Company). A 21-gauge needle connected to the syringe was used. The effect of the electrospinning solution flow rate (0.8 and 1.2 mL/h) on fiber mean diameter and uniformity was examined. All electrospinning experiments were carried out under ambient temperature of about 22 °C and humidity of around 50%. Finally, different electrospinning parameters including flow rate, needle tip-to-collector distance, drum speed, and applied voltage were controlled to achieve continuous and stable nanofibers. After the creation of the nonwoven fabric on the aluminum foil, the mat was removed from the drum surface and kept in a desiccator to enable evaporation of the solvent residue. To ensure the complete evaporation of solvents, samples were placed in an oven at a temperature of around 40 °C overnight.

2.3. Characterization

2.3.1. Scanning Electron Microscopy (SEM)

Scanning Electron Microscopy (SEM) model TM3030 Plus from Hitachi was used to analyze the morphology of the nonwoven fabric. The SEM images were obtained at an accelerating voltage of 15 kV with high resolution. Several properties related to the fiber’s characteristics such as diameter and smoothness of fiber’s surface, pore size distribution, and bead formation within the fabric were acquired from the SEM images. Moreover, fiber’s average diameters were measured by ImageJ software for n = 100 (n is the number of fibers).

2.3.2. Thermogravimetric Analysis (TGA)

The thermal degradation of electrospun mats was investigated via a thermogravimetric analysis instrument (TA Q-500, US). The weight loss percentage and thermal degradation of the samples with a mass in the range of 5–10 mg were assessed from 25 to 800 °C at a heating rate of 10 °C/min, while a nitrogen flow at a rate of 25 mL/min was applied as the carrier gas.

2.3.3. Differential Scanning Calorimetry (DSC)

The degree of crystallinity and thermal properties of the samples, such as melting temperature (T _m) and glass transition temperature (T _g), were determined by a Differential Scanning Calorimetry (DSC) instrument (Q1000, TA Instruments, New Castle, DE, USA). The samples were first placed in standard aluminum pans, and after being placed in the DSC chamber, a heating mechanism driven by a constant flow of dry nitrogen at 10 °C/min as a purge gas was applied over a temperature range of 25 to 300 °C. The crystallinity percentage of the samples can be determined from eq :

$$X_{\mathrm{c}}(\%)=\frac{(H_{\mathrm{f}}-H_{\mathrm{c}})}{H_{\mathrm{f}}^{°}}\times 100$$ 1

where H _f is fusion enthalpy, H _c is the crystallization enthalpy, and H°_f is the fusion heat of the fully crystalline polymer, which is equal to 144 J/g for PET.

2.3.4. Fourier Transform Infrared Spectroscopy (FT-IR)

To identify the functional groups present in fabrics and chemical infrared spectra, a PerkinElmer 65 FTIR-ATR instrument was employed. All the spectra were collected from 32 scans with a resolution of 4 cm^–1 in the 600–4000 cm^–1 wavelength range. After the incorporation of lignin into the PET matrix, possible chemical bonding and interactions between all components in the polymeric blend were identified by FT-IR. The change in the intensity of the wavelength of each functional group observed in the vibrational spectrum indicated the occurrence of a reaction.

2.3.5. Water Contact Angle (WCA)

The hydrophilicity and wettability of the fabricated samples were measured by the sessile drop shape method with the help of an OCA optical contact angle instrument (Data Physics Instruments, Filderstadt, Germany). To begin the measurement, a volume of 2 μL of distilled water was dropped onto the mat surface at ambient temperature and humidity (22.4 °C and 26%, respectively). Contact angle values and images were reported by using the contact angle meter SCA 20 software.

2.3.6. Water Vapor Transmission Rate (WVTR)

To analyze the water vapor permeability through the PET-based fabrics, a PERMATRAN-W 101 K instrument with a 6-cell sample capacity was used. This equipment is designed to test the WVTR of porous and breathable barriers like polymer membranes and filters, which comply with ASTM D6701. Medical face masks, such as surgical masks and N95 respirators, are primarily designed to balance filtration efficiency with breathability. In this regard, air filtration performance and filter comfort for the wearer can be evaluated by determining the water vapor transmission rate. It is important to note that higher WVTR values can help reduce moisture buildup inside the mask, which improves the breathability and durability of face masks. However, in a highly humid environment, the moisture droplets carrying viruses might help viruses move through tiny pathways in the filter. So, in theory, staying too long in high-humidity conditions could slightly reduce the barrier and filtration performance.

2.3.7. Mechanical Properties

The mechanical strength of fiber mats illustrates their resistance against any form of stress without tearing or deforming. To determine the mat’s ability to withstand external stress, various strength parameters, including tensile strength, Young’s modulus, and elongation at break, were measured by an Instron tensile testing machine (Model: E3000, UK, England) with a load capacity of up to 3 kN.

2.3.8. Porosity Measurement

Porosity is defined as the fraction of pore or void volume to material’s total volume. Porosity is determined by measuring the samples’ apparent density using a Gas Pycnometer (Micromeritics AccuPyc II 1340, USA). The sample’s apparent density (skeletal density) was determined using helium gas displacement. The measurement was conducted within a 10 mL sealed chamber at 25 °C, which allowed the helium to fully penetrate the small pores of the electrospun mats with a mass of approximately 0.5 g. After measuring the sample’s bulk volume and density, the porosity was calculated using the following eq :

$$\phi (\%)=(1-\frac{{\rho }_{\mathrm{bulk}}}{{\rho }_{\mathrm{apparent}}})\times 100$$ 2

Where ρ_bulk is sample’s bulk density, calculated from measuring the sample’s total volume and mass. ρ_apparent is obtained from Gas Pycnometer and φ shows the sample’s porosity percentage.

2.3.9. Antibacterial Test

To evaluate the antibacterial performance of the prepared samples, ISO 20743:2021 guidelines (Textiles: Determination of the antibacterial activity of textile products) were followed. In this regard, two common bacterial strains S. aureus (Gram-positive) and E. coli (Gram-negative) were selected. The colony count method was applied for determining the antibacterial efficacy of the samples. First, about 5 μL of bacteria was thawed from the −80 °C fridge and then inoculated into 5 mL of LB broth. After incubating overnight at 200 rpm and 37 °C, the bacterial concentration was adjusted to 3–5 × 10⁸ CFU/mL by checking that the OD₆₀₀ was equal to 1. Second, the bacterial suspension was diluted 3 times with nutrient broth (NB) to achieve a bacterial concentration of 10⁵. Then, the treated and untreated samples were sterilized under UV for 15 min. After placing the samples in separate tubes, about 5 mL of the bacterial solution was poured on each mat, then incubated at 37 °C with 90% relative humidity. After incubating for a specific contact time, 1 mL of the bacterial suspension containing samples was mixed with 19 mL of D/E neutralizing solution. Following 5 s of vortexing, 10-fold serial dilutions were conducted by adding 100 μL of the vortexed mixture into 900 μL of phosphate-buffered saline (PBS). Finally, the bacterial viability was determined by dispersing 100 μL of the diluted suspension on agar plates. All the agar plates were incubated overnight at 37 °C, and then the formed bacterial colonies were counted with the aid of a microscope. Scheme illustrates the biocidal process steps employed to evaluate the antimicrobial efficacy of lignin-incorporated PET against selected bacterial species.

1. Schematic of Antibacterial Methodology SSteps for Textiles.

2.3.10. Antiviral Tests

To evaluate the antiviral activity of the various samples, the ISO 18184:2019 protocol entitled “Textiles Determination of antiviral activity of textile products” was followed, and for the determination of virus infectivity of prepared fabrics, the Median Tissue Culture Infectious Dose (TCID₅₀) method was applied. In this study, human coronavirus 229E (ATCC _VR-740) and human cell line MRC-5 (ATCC CCL-171) were used. Initially, the specimens were cut into 5 × 5 cm² pieces before being sterilized by UV lamps for 15 min. Afterward, 100 μL of virus suspension with a 3 × 10⁵ TCID₅₀/mL concentration was inoculated on the surface of each sample, followed by incubation at 25 °C for contact times of 1 and 2 h. In the next step, 5 mL of the neutralizing solution was added to each tube containing the samples. Finally, a 10-fold dilution of the viral suspension was conducted, and about 100 μL of each dilution step was inserted into a 96-well plate containing MRC-5 cells with a confluency of more than 70%. After 1 h of incubation at 35 °C under 5% CO₂, the virus was removed from the 96-well plate by an adsorbent pad and washed with PBS. Consequently, 100 μL of EMEM was added to the plate containing infected cells and was kept in an incubator for 5–7 days until cytopathic effects progressed to 80%.

At the end of the incubation period, the 96-well plate was observed under a microscope, and the TCID₅₀ values were determined by following the Spearman–Karber method. Scheme illustrates the experimental steps for antiviral efficacy determination of textiles based on the ISO-18184 methodology.

2. Schematic of Virus Titer Assay Steps According to the ISO-18184 Method for Textiles.

3. Results and Discussion

3.1. Electrospinning Process Optimization

In this study, the effect of various process parameters was examined to fabricate nanoscale and bead-free nonwoven fabrics. Electrospun PET mats were fabricated under varying polymer concentrations and flow rates, while other parameters including an applied voltage of 20 kV, a drum rotational speed of 50 rpm, and a needle-to-collector distance of 18 cm were maintained constant. PET solution concentration plays a key role in electrospinnability and overcoming the surface tension forces to produce uniform and continuous fibers, while flow rate affects fiber’s diameter and geometry. Thus, the fiber’s diameter and morphology were improved by optimizing the PET concentration in the TFA/DCM solution.

3.2. Scanning Electron Microscopy (SEM)

Figure presents SEM images of electrospun PET mats produced under various electrospinning conditions. Figure a exhibits the resulting fiber morphology for a PET concentration of 15% and a flow rate of 1.2 mL/h. The resulting fabric revealed relatively large pore sizes in the microscale range and an average fiber diameter of approximately 1.4 ± 0.8 μm, substantially above the desired threshold for efficient nanoparticle filtration in face mask applications.

1.

Morphology of the electrospun fibers as a function of the polymer concentration and flow rate: a)­15 wt % PET, 1.2 mL/h; b) 15 wt % PET, 0.8 mL/h; c) 10 wt % PET, 1.2 mL/h; d) 10 wt % PET, 0.8 mL/h; e) 7%, 0.8 mL/h; f) 7%, 1.2 mL/h. All samples were prepared under the same condition: TFA:DCM (1:2), voltage: 20 kV, needle-to-collector distance: 18 cm, and drum speed: 50 rpm.

To reduce the fiber diameter, the flow rate was decreased to 0.8 mL/h, which led to a significant improvement, yielding an average diameter of approximately 957 ± 165 nm, as shown in Figure b. Further optimization was pursued by adjusting the polymer concentration. At a PET concentration of 10 wt %, fibers with average diameters of 340 ± 95 nm and 675 ± 115 nm were obtained at flow rates of 0.8 mL/h and 1.2 mL/h, respectively.

The SEM images of fibers with 10% PET are shown in Figure c-d. However, Figure e-f indicate that the reduction of PET concentration to 7% has increased the incidence of bead formation, likely due to insufficient chain entanglement and lower solution viscosity. Based on the balance between fiber diameter, uniformity, and morphological integrity, the electrospinning conditions of 10 wt % PET concentration with a flow rate of 0.8 mL/h were selected as the optimal processing parameters.

To enhance the antimicrobial properties of the PET mat, varying amounts of alkali lignin (AL) were incorporated into the PET solution at concentrations of 10, 15, and 20 wt %. Based on the antiviral and antibacterial performance evaluations, the PET–AL composite containing 20 wt % AL exhibited the most effective activity. Consequently, this formulation was selected as the active nonwoven mat and was used for subsequent analyses. Figure a displays the morphology of the AL powder, while Figure b-c show the electrospun PET–lignin 20 wt % mat. All the electrospinning parameters were set as follows: 10 wt % PET in TFA/DCM (1:2), 20 wt % of AL with respect to PET content, and a flow rate of 0.8 mL/h. Incorporation of AL into the PET solution resulted in a notable reduction in fiber diameter, with an average diameter of about 290 ± 35 nm, aligning with the primary objective of this study. In conclusion, the electrospinning of PET-based solutions, particularly with the addition of lignin, successfully produced ultrafine, uniform, and bead-free nanofiber mats suitable for advanced filtration applications.

2.

SEM images of a) AL powder, b) PET–AL 20 wt % at 50 μm magnification, and c) PET–AL 20 wt % at 5 μm magnification.

3.3. Thermogravimetric Analysis (TGA)

TGA was performed to investigate the thermal degradation behaviors of lignin, PET, and their composite. The results related to the sample weight loss versus temperature are reported in Figure . The multiple peaks in the DTG graph of alkali lignin powder demonstrate multistage degradation, likely due to its heterogeneous and complex structure. The degradation of AL could be divided into 3 main stages, namely, moisture evaporation, fast degradation, and slow degradation. The initial weight loss (∼5%) from 0 to 150 °C corresponds to moisture removal or loss of volatile components, a common behavior of biobased compounds. Second, between 150 and 500 °C, the main weight loss of about 30% of lignin occurs and involves the breakdown of ether linkages and decomposition of phenolic groups. At the third stage, over 500 °C, the decomposition of more stable structures, such as condensed aromatic rings and residual carbonaceous char, might happen with combustion during the heating procedure at 600 °C.

3.

Thermal degradation results of a) AL powder and b) comparative TGA graphs of electrospun samples based on PET.

The TGA graph of control PET fabric reveals thermal stability maintained up to approximately 380 °C. Beyond this temperature, a sharp and rapid weight loss is observed between 380 and 470 °C, indicating a single-step thermal decomposition process of PET, which is due to the breakdown of ester linkages, resulting in the formation of volatile degradation products such as carbon monoxide and terephthalic acid derivatives.

The PET–lignin 20 wt % shows an intermediate thermal degradation profile between pure PET and lignin powder. It is observed that the degradation occurs slightly earlier than that of pure PET, indicating that lignin reduces the composite’s initial thermal stability while also suggesting a two-stage degradation process. The first stage from 0 to 350 °C with 15% weight loss is attributed to lignin decomposition. The second degradation stage, occurring between 350 °C and 470 °C, is due to the breakdown of the PET matrix. The weight loss observed above 600 °C is due to the lignin residue in the sample. The differences in degradation profiles imply potential chemical interactions between lignin and PET, such as hydrogen bonding, which may influence the composite’s decomposition mechanism and thermal stability. Incorporation of lignin into the PET matrix alters the degradation dynamics, which highlights the potential of AL as a functional bioadditive in polymer composites, contributing to char formation, thermal insulation, and potentially enhancing fire retardancy.

3.4. Differential Scanning Calorimetry (DSC)

The following Figure indicates the DSC curves of PET–control fiber, PET pellets, and PET–AL 20 wt %, which were used to determine the effect of the electrospinning process and AL integration on PET crystallinity degree and thermal properties. The determined values of T _m, T _g, and X _c of electrospun samples are presented in Table .

4.

DSC curves of PET fiber, PET pellets, and PET–lignin 20 wt % under N₂ atmosphere at 50 mL/min with a heating rate of 10 °C/min.

1. Glass Transition Temperature (T _g), Melting Temperature (T _m), and Crystallinity (X _c) of Samples Determined from Melting Enthalpy Values.

	Sample name	T g (◦C)	T m (◦C)	X c (%)
1	PET pellets	78.47	245.2	31
2	PET fiber	74.5	247.35	27.4
4	PET–lignin fiber 20 wt %	51.5	244.8	11.2
5	PET–lignin film20 wt %	49.8	243.3	10

The DSC curve of PET pellets illustrated a low-intensity endothermic peak at approximately 78 °C, which is due to the PET glass transition temperature. A relatively intense peak around 168 °C is due to the cold crystallization of PET, followed by the endothermic peak at around 245 °C, which is related to the melting of the PET crystalline fraction. As a semicrystalline PET is utilized as reference pellets, crystallinity (X _c) around 31% was expected. By comparing the DSC results of PET pellets and electrospun PET fibers, it is concluded that the dissolution of PET in the solvent mixture and the electrospinning process has decreased the T _g and crystallinity value of PET, while the melting temperature was increased slightly.

The DSC graph of the sample containing lignin showed an exothermic peak at 255 °C, which is due to the thermal decomposition of lignin. The results exhibited that the addition of lignin into the electrospun mats had a slight effect on the T _m value of PET, while the crystallinity of PET was significantly decreased. This phenomenon could be due to the increase in PET solution viscosity by adding lignin, which makes it difficult for PET chains to align during electrospinning and contributes to a reduction in X _c values.

3.5. FT-IR (Fourier Transform Infrared Spectroscopy) Analysis

According to Figure , the FTIR spectra of pure AL powder reveal a slightly fluctuating baseline with fewer peaks, which is related to its distinct chemical composition, with broad O–H stretching approximately around 3000–3600 cm^–1, CC aromatic ring vibrations at 1500–1600 cm^–1, and stretching of the C–O bonds from phenolic or ether groups between 1100 and 1250 cm^– 1.

5.

FTIR spectra of electrospun PET control fabric, AL powder, and PET–AL 20%.

Analysis of the PET Reference spectrum exhibits characteristic peaks of PET, especially an intense peak at 1720 cm^–1 corresponding to the carbonyl (CO) stretching bond from ester groups. Similarly, the peaks observed at 720, 1100–1250, and 2950 cm^–1 indicate the presence of conjugated C–H, C–O, and aliphatic C–H stretching vibrations, respectively.

The incorporation of lignin in PET fiber has altered the intensity of the peak around 1720 cm^–1, contributing to the CO stretching vibrations caused by the esterification reaction between PET and lignin. TFA used as an electrospinning solvent and known as an esterifying reagent can react with hydroxyl groups present in the lignin structure. Additionally, the extension of spectrum in the 3000–3500 cm^–1 range is due to O–H interactions, such as hydrogen bonding.

FT-IR analysis indicates that alkali lignin introduces abundant hydroxyl groups due to its heterogeneous phenolic structure. These functional OH groups are capable of forming hydrogen bonds with the ester carbonyl groups present in the PET polymer. In parallel, the aromatic rings present in both alkali lignin and PET promote π–π stacking interactions, leading to further interfacial adhesion enhancement. During the DCM/TFA solution-blending process, PET is dissolved in the mixed solvent system, while alkali lignin is dispersed or partially solubilized due to TFA’s strong protonating and hydrogen-bonding capabilities. Moreover, the presence of TFA in the solution mixture improves lignin dispersion through acid-mediated protonation and limited covalent interactions, effectively suppressing lignin agglomeration and promoting stable PET–lignin integration. Continuous stirring overnight facilitates these interactions, enabling the formation of a homogeneous single-phase solution suitable for electrospinning. Overall, the integration mechanism between PET and AL in composite fibers is primarily governed by noncovalent interfacial interactions, including hydrogen bonding and π–π stacking interactions, providing sufficient compatibility between both compounds. During solution electrospinning, as solvent evaporation proceeds, PET chains begin to reorganize, and lignin becomes immobilized within the emerging polymer network, typically residing in the amorphous regions of PET, resulting in the creation of uniform and elongated fiber production.

3.6. Water Contact Angle (WCA)

Figure presents the water contact angle (WCA) measurements of electrospun samples alongside a commercial surgical face mask. To reduce errors, the test was repeated 5 times for each sample. As illustrated, the commercial surgical mask (Figure a) showed a WCA of 137.6°, mostly the same as the control PET fabric (Figure b) with a WCA of approximately 146.2°, indicating the hydrophobic nature of the PET polymer. However, the incorporation of AL into the PET matrix significantly modified the surface properties, rendering the samples more hydrophilic. This transformation is evidenced by the behavior of water droplets on the PET–lignin (20%) sample, as captured at distinct time intervals in Figure c-e. The sequential images taken at 0 s (Figure c), 5 s (Figure d), and 10 s (Figure e) clearly demonstrate progressive absorption of the water droplet, confirming the enhanced water-absorbing capability imparted by lignin incorporation, while the WCA remained the same for commercial and electrospun PET mats after 30 s.

6.

Water contact angle images of a) commercial surgical mask, b) electrospun PET, and c) PET–lignin 20 wt % at 0 s, d) 5 s, and e) 10 s.

3.7. Water Vapor Transmission Rate (WVTR)

Six fabric specimens with a diameter of 10 cm were placed in a WVTR instrument under applied conditions of 38 °C and RH of 95% for 24 h. The values of WVTR are reported in Table as the amount of water vapor that passes through a material per unit area per unit time (typically g/m²·day). The transmission rate output is presented as a graph of WVTR vs time to verify the equilibrium state and permeability data report. High transmitters were analyzed by this equipment with a typical throughput of six samples analyzed every 1 h. All tests were repeated 3 times to minimize errors. According to the WVTR values reported in Table , the incorporation of AL in the PET mat led to an increase in water vapor permeability, which is due to the hydrophilicity concept of lignin-based derivatives. A comparative analysis of the permeability properties of electrospun PET and PET–AL revealed notable enhancements upon lignin incorporation. In terms of moisture management, the PET–lignin fabric exhibited superior performance. The WVTR improved by 11.5%, from approximately 39.25 to 43.78 g/m²· day. This enhancement suggests that lignin contributes to increased fiber porosity or changes in the surface morphology that favor vapor diffusion. Moreover, liquid permeation experienced a substantial increase by 35.5%, g·mL/m²·day caused by potential hydrophilic interactions introduced by lignin, which can facilitate faster liquid passage through the matrix.

2. WVTR Results of Electrospun PET-Based Samples.

Sample composition	Fabric thickness (μm)	WVTR (g/m2·day)	Permeation (g· mil/m2·day)
PET	77.2 ± 1.4	39.25 ± 2.91	119.3 ± 6.78
PET–lignin 20%	93.2 ± 3.1	43.78 ± 1.65	161.7 ± 2.41
Commercial mask	404.2 ± 1.0	17.32 ± 2.25	275.61 ± 3.58

Overall, the integration of AL into the PET matrix not only alters the fabric structure but also significantly enhances its permeability characteristics, indicating potential for face mask manufacturing, where breathability and moisture transport are critical. Moreover, by comparing the results with the commercial surgical mask, the PET-based fabric has enhanced permeability and porosity due to having a higher WVTR value.

3.8. Mechanical Properties

Table presents the tensile test results of electrospun samples based on PET and PET incorporated with 20 wt % of lignin. The test was repeated 6 times for each sample to minimize errors.

3. Tensile Test Parameters of Electrospun PET-Based Samples.

Sample name	Young’s modulus (MPa)	Tensile strength (MPa)	Elongation at break (%)
PET fiber	69.4 ± 4.12	3.56 ± 0.5	197.6 ± 20.2
PET–lignin 20%	42.35 ± 5.6	2.84 ± 0.2	103.77 ± 12.6
Commercial mask	9.44 ± 0.8	1.72 ± 0.6	115.56 ± 8.6

According to the tensile test results, the mechanical strength, especially the tensile strength of PET–AL 20% was slightly decreased, which might be due to the lignin amorphous and brittle structure with poor chain flexibility. Another effective factor could be the nonuniform and weak interfacial adhesion between PET and lignin. Moreover, the Young’s modulus or stiffness of samples containing lignin was decreased because of the disruption of the crystallinity and alignment of PET chains after the addition of lignin into the polymeric solution. Based on the last column of Table , the elongation at break of PET fabric is reduced from approximately 197.6 ± 20.2% to 103.77 ± 12.6%, when 20 wt % of AL was added to PET. The reduction in mechanical strength and elongation indicates that the dispersion of AL particles within PET fibers has disrupted the crystallinity and alignment of the PET chain, leading to the fiber’s brittle structure, but it still remains ductile. By comparing the tensile test results with the commercial surgical face mask, it can be concluded that nonwoven PET-based samples have enhanced mechanical properties.

3.9. Porosity Measurement Results

The porosity percentages of the electrospun mats were determined by using eq . The porosity values were 86.9% for PET and 97.1% for PET–AL 20%. The increase in PET–AL 20% porosity demonstrates the improvement of the PET porous structure through the incorporation of AL into the polymeric matrix. To further characterize the pore’s structure, the average pore size was verified by analyzing SEM images using ImageJ software to measure the area between adjacent fibers. The quantitative analysis yielded average pore sizes of 975 ± 75 nm for PET and 750 ± 58 nm for PET–AL 20%. Therefore, the average pore size decreased by approximately 23%, yielding a tighter pores distribution of PET–AL 20% sample. The higher porosity and smaller pore sizes can directly address the critical requirements for high-efficiency filtration media. The improved porosity of PET integrated with 20 wt % of AL potentially contributes to sufficient air permeability and less pressure drop by providing a higher surface area, which is a key factor for assessing the material’s feasibility in practical respiratory masks.

3.10. Antibacterial Efficacy Results

The antibacterial efficacy of the PET–lignin composite against S. aureus and E. coli was evaluated by the ISO 20743:2021 method. Initially, the minimum effective concentration of alkali lignin required to inhibit bacterial growth was determined. For this purpose, varying amounts of AL (5, 10, 15, and 20 mg) were tested in a 5 mL bacterial suspension over a 24 h contact period. Since no bacterial colonies were observed on the agar plates after 24 h, indicating complete inhibition, shorter contact times of 1 and 3 h were subsequently investigated.

Figure a presents the antibacterial performance of AL against E. coli at these reduced contact intervals. After 1 h of exposure, 5 and 10 mg of AL did not result in a significant reduction of E. coli colonies compared to the blank. However, an approximately 2-log reduction was observed with 15 mg, while complete inhibition (no detectable colonies) was achieved with 20 mg of AL. At the 3 h contact time, 10 mg exhibited an ∼2.5-log reduction in E. coli colonies, whereas 15 and 20 mg led to total bacterial inactivation.

7.

A comparative analysis of the antimicrobial efficacy of a) AL powder effectiveness in 1, 3 h, b) electrospun PET–lignin samples after 24 h of contact time, c) PET–lignin against E. coli, and d) PET–lignin against S. aureus after 1 h and 3 h of contact time.

These findings suggest that incorporating a minimum of 20 mg of AL into the polymeric matrix imparts rapid and potent antibacterial activity against E. coli, achieving complete inhibition within just 1 h of contact.

Figure b illustrates the antibacterial performance of electrospun PET mats incorporated with varying concentrations of AL after 24 h of contact with E. coli and S. aureus. The PET–lignin mat containing 10 wt % AL exhibited a bacterial reduction of approximately 2.5 to 3 log units. In contrast, mats containing 15 and 20 wt % AL demonstrated complete inactivation of both bacterial strains, indicating a concentration-dependent enhancement in antimicrobial efficacy.

The effect of contact time and lignin concentration on bacterial viability is further demonstrated in Figure c. After 1 h of exposure, PET mats with 10 and 15 wt % AL achieved reductions of ∼0.5-log and ∼2-log units against E. coli, respectively. Extending the contact time to 3 h led to a modest improvement in antibacterial activity for the 10 wt % mat, achieving about a 1-log reduction. Notably, at both 1- and 3-h exposure intervals, PET mats containing 15 and 20 wt % AL, completely eradicated E. coli colonies, confirming their superior efficacy at higher lignin concentrations.

Similarly, the antibacterial performance of PET–lignin mats against S. aureus is presented in Figure d. Following 1 h of contact time, mats with 10 and 15 wt % AL induced reductions of approximately 0.5-log and 1.5-log units, respectively. Upon increasing the exposure time to 3 h, the antibacterial activity of the 10 wt % mat improved to a 2-log reduction. Mats containing 15 and 20 wt % AL, however, demonstrated complete bactericidal activity against S. aureus after 3 h of contact.

Overall, these results indicate that higher concentrations of AL are required to achieve complete inactivation of both E. coli and S. aureus. Based on these findings, 20 wt % AL was identified as the optimal loading concentration for incorporation into the PET matrix to impart effective and rapid antimicrobial properties to the electrospun mats.

The observed antibacterial activity of AL is attributed primarily to the generation of reactive oxygen species (ROS), including superoxide and hydroxyl radicals, which are inherently present in its structure. These ROS are believed to disrupt bacterial membranes by inducing oxidative stress, leading to cytoplasmic leakage and membrane disintegration, ultimately resulting in cell death.

The images of E. coli colony formation on agar plates observed after 1-h exposure time of various fabrics are illustrated in Figure . Figure a depicts the control sample, showing E. coli colonies at an initial concentration of 10⁵ CFU/mL, serving as the blank for evaluating the antibacterial performance of the prepared specimens. Figure b illustrates the antibacterial activity of a conventional surgical face mask, and Figure c demonstrates the response of the electrospun PET fabric without additives. In Figure d, the antibacterial performance of the PET–AL 20 wt % composite is presented. Based on the visual comparison of bacterial colony formation across the samples, it is evident that both the surgical face mask and pristine PET electrospun fabric do not exhibit antibacterial activity. In contrast, the incorporation of AL into the PET matrix significantly suppresses bacterial growth, indicating effective antibacterial functionality against E. coli.

8.

Photos of E. coli colony formation for a) blank E. coli, b) conventional surgical face mask, c) electrospun PET, and d) PET–lignin 20 wt % after 1 h of contact time.

3.11. Antiviral Efficacy Results

The antiviral activity of the prepared PET-based nanofibrous mats was evaluated using human coronavirus 229E (HCoV-229E), an enveloped virus characterized by a surface of glycoproteins. The results, illustrated in Figure , present viral titers following two different exposure durations aimed at assessing the virucidal efficacy of lignin-incorporated composites. The incorporation of alkali lignin into the PET matrix significantly enhanced the antiviral performance of the samples. Notably, there is limited literature on the antiviral properties of lignin-based composites, particularly those utilizing AL, thereby underscoring the novelty and relevance of this study. The antiviral mechanism of lignin-based compounds is attributed to the generation of reactive oxygen species (ROS), facilitated by hydroxyl functional groups present AL.

9.

Antiviral efficacy results of electrospun PET–lignin fabrics.

The polyphenolic structure of lignin allows it to undergo oxidation–reduction reactions. Under specific conditions such as exposure to UV light, the oxidation of lignin phenolic units can generate ROS, such as superoxide anions (O₂ ^–) and hydrogen peroxide (H₂O₂). These ROS induce oxidative stress within the cell, damaging intracellular components (DNA, proteins) and the cell membrane. According to literature, lignin shows efficacy against both enveloped viruses (like coronaviruses) and nonenveloped viruses by a direct virion inactivation mechanism. In this study, an enveloped virus named the Human Coronavirus (HCoV-229E) was used. Alkali lignin directly targets the lipid envelope and surface glycoproteins of enveloped viruses by disrupting the structural integrity of the viral particles. This destruction of the envelope and the associated surface proteins (which are crucial for host cell binding) prevents the virus from successfully attaching to or fusing with the host cell, thus reducing their infectivity. Moreover, AL will inactivate virus in direct contact by adsorptive inhibition mechanism. Lignin effectively acts as a physical or chemical barrier, preventing the virus from interacting with the host cell membrane and making it impossible for the virus to begin replication. Therefore, the presence of AL in the PET matrix enhances ROS generation, contributing to its virucidal effectiveness.

The higher amount of AL in the polymeric matrix will result in more ROS production, which is in favor of the antiviral performance of nonwoven filters. The experimental data demonstrated an ∼2-log reduction in viral concentration after 2 h of exposure at 25 °C for a sample containing 20 wt % of AL and a 1-log reduction for 10 wt % concentration. Furthermore, increasing the content of AL lignin in the mat resulted in a slight enhancement of antiviral performance, even under a constant contact time. These findings confirm the rapid and effective antiviral action of PET–lignin nanofibrous filters, with over a 2-log reduction in viral titers observed within 2 h of contact. Thus, the developed materials exhibit promising potential as antiviral filter media for use in face mask applications.

4. Conclusion

In this study, a novel bioactive PET-based nonwoven filter was successfully fabricated via solution electrospinning, with a focus on optimizing process parameters such as flow rate and polymer concentration to achieve uniform nanofiber morphology. Under optimal conditions (0.8 mL/h flow rate and 10 wt % PET concentration), uniform PET fibers with an average diameter of 340 nm were obtained. The addition of 20 wt % alkali lignin to the PET matrix not only improved the hydrophilicity of the resulting composite but also led to the formation of finer fibers with an average diameter of approximately 290 nm. The PET–lignin composite exhibited remarkable antibacterial activity, achieving significant inhibition of E. coli and S. aureus within 1 h of contact. Additionally, the material demonstrated promising antiviral performance, achieving more than a 2-log reduction in human coronavirus 229E (HCoV-229E) titers after 2 h, meeting the criteria for effective antiviral agents. These results confirm the potential of PET–lignin electrospun fabrics as multifunctional, recyclable filter materials suitable for use in protective face masks and other biomedical applications.

A.N.: Conceptualization, Methodology, Validation, Formal data analysis, Investigation, WritingOriginal draft, Visualization. A.A.: Conceptualization, Methodology, Validation, Resources, WritingReview and editing, Supervision, Funding acquisition.

The authors acknowledge NSERC (Natural Sciences and Engineering Research Council of Canada) for financial support and funding.

The authors declare no competing financial interest.