Properties enhancement of antimicrobial chitosan-deposited polylactic acid films via cold plasma treatment

The present study aimed to understand the effect of dielectric barrier discharge cold plasma (DBD-CP) technology on the antimicrobial chitosan deposition and the properties enhancement of polylactic acid (PLA) films. The results indicated that DBD-CP was an effective method for improving the adhesion and surface hydrophilicity of PLA, facilitating the deposition of chitosan coating. This modification was attributed to the increased surface roughness, as well as the presence of polar functional groups observed through atomic force microscopy, surface free energy and Fourier transform infrared spectroscopy analysis. The study further revealed that both water resistance and mechanical properties were significantly improved after DBD-CP treatment, which was positively correlated with the amount of chitosan deposited on the PLA surfaces. Following comprehensive evaluation, the treatment at 75 W was determined as the optimal condition for enhancing the properties. Additionally, the modified film exhibited strong antimicrobial activity against Staphylococcus aureus. Consequently, the DBD-CP technology could be a promising tool for better utilization of PLA-based materials in the antibacterial food packaging industry.

Submit a manuscript: https://www.tmrjournals.com/fh Background Antimicrobial packages have gained considerable attention in the packaging industry as a means of safeguarding food against microbial contamination. However, the majority of these packages are currently manufactured using petrochemical materials, which pose a significant burden on the environment due to their non-biodegradable nature [1]. In light of growing environmental concerns, polylactic acid (PLA), a polyester derived from 100% renewable resources, becomes an attractive candidate for those non-biodegradable polymers [2]. PLA has garnered significant attention in the medical appliance, functional textile, and packaging sectors in recent years, owing to its exceptional properties, including versatility, biodegradability, biocompatibility, transparency, thermoplastic processability and mechanical strength [3][4][5][6]. Constant research has been conducted on the applications of PLA-based composite antibacterial packages, which are categorized based on the types of antibacterial ingredients used. These categories include inorganic antibacterial materials such as AgNPs, TiO 2 and ZnO [7][8][9], organic antibacterial materials such as quaternary ammonium salts, biguanides, phenol ethers and halogenated amines [10,11], and natural antibacterial materials like chitosan, cinnamaldehyde, olive leaf extract and oregano [12][13][14][15]. In recent years, coatings/films containing/loaded with antibacterial agents or antioxidants have been extensively developed and applied, owing to their effective antibacterial properties [16]. It was found that essential oils have been found to exhibit strong antibacterial effects against foodborne bacteria, making them suitable for use as slow-release antibacterial agents in food preservation [17,18]. Yang reported that metal-based supported materials are commonly employed for sterilization purposes due to their excellent antibacterial properties and low biological resistance [19].
Chitosan, the deacetylated product of chitin, is a linear polysaccharide with strong antimicrobial and antifungal activities against a wide range of microorganisms [20,21]. It has been widely utilized as a highly promising antimicrobial and active bio-based packaging material in the food preservation industry. Recently, there have been successful fabrications of PLA-chitosan films in two types: incorporating films or coatings on film surfaces. Sébastien et al. compared the antifungal activity of PLA-chitosan binary blends with that of chitosan-coated PLA films and observed that the chitosan-coated films exhibited better antimicrobial activity than the blend composites. This phenomenon was attributed to the limited diffusivity of chitosan in blends, caused by the hydrophobic nature of the PLA bulk [22]. However, the pristine PLA film exhibits some inadequate surface application properties, which make the deposition effect of coating compounds far less than optimal, influencing the antimicrobial activity of coated films. Specifically, there are two issues: (1) the low surface energy and poor wettability of PLA hinder the attachment of polar or non-polar materials; (2) PLA molecules lack side-chain groups that can participate in chemical modifications [3,23]. Consequently, the PLA surface activation and functionality by a non-chemical method are needed.
Cold plasma is an environmentally friendly technology that generates fewer toxic by-products compared to wet chemistry [24]. Depending on the gas composition, the dielectric barrier discharge cold plasma (DBD-CP), consisting of a range of ions, atoms, electrons and UV radiation, can be produced by ionizing the gas in the gap between two electrodes of the generator. Over the past few decades, DBD-CP has been extensively studied for surface treatment of polymeric materials, including surface cleaning, surface functionalization and surface deposition [25,26]. As numerous studies suggest, DBD processing conducted in the air under atmospheric pressure offers a cost-effective and non-thermal method to enhance the adhesive bonding and printing performance of polymers without altering their bulk properties. Moreover, it is suitable for high-volume in-line industrial applications, making it suitable for implementation in the packaging sector [24,[27][28][29]. To our knowledge, research on DBD-CP-treated biodegrade PLA and its deposited films is much less than that of the common polymeric packaging materials. There is significant interest to investigate the relationship among DBD plasma treatment, chitosan deposition effect and functional properties of films for better utilization of PLA as potential alternatives in the emerging field of industrialized antimicrobial and active packaging.
In this study, the DBD cold operating at atmospheric pressure in the air was performed on PLA film surfaces with different treatment powers, then chitosan was deposited on the modified PLA film surfaces as an antimicrobial coating. We investigated the macroscopic effects of DBD-CP treatment on PLA films and chitosan-deposited films by measuring water contact angle (WCA), surface free energy, mechanical properties and water vapor permeability. The optimum power of treatment was determined. Additionally, we studied the changes in surface morphology using atomic force microscopy (AFM). The chemical changes at the surface were analyzed using attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) and X-ray diffraction (XRD). Furthermore, we evaluated the antibacterial activity of chitosan-deposited PLA films against Staphylococcus aureus.

Materials
The PLA used in this study had a D-lactide content of 2% and an L-lactide content of 98%, and it was obtained from Natureworks@ under the product code 4032D. Chitosan with a deacetylation degree of 91% and an average molecular weight of 335,400 Da, as determined by gel permeation chromatography, was obtained from Golden-Shell Pharmaceutical Co. Ltd. (Yuhuan, China). Anhydrous ethanol, dichloromethane, and glacial acetic acid were supplied by Wind-Ship Chemical Reagent Co. Ltd. (Tianjin, China). The Staphylococcus aureus strain used in the antibacterial activity evaluation was obtained from the strain preservation room of the College of Food Science and Engineering at Tianjin University of Science and Technology, with the strain number 800926.

Preparation of PLA films
The PLA film was prepared using a casting procedure. Initially, PLA (3% w/v) was dispersed in a 1% (v/v) dichloromethane solution and stirred continuously for 3 h until complete dissolution. The resulting PLA film-forming solution was then poured onto a smooth melamine resin plate and dried under windless conditions at room temperature (25°C) for 24 h. After drying, the films were carefully peeled off from the plates and equilibrated in a desiccator maintained at 50 ± 5% relative humidity and 23 ± 2°C for one week before plasma modification.

DBD-CP treatment on the PLA films
The DBD atmospheric-pressure air cold plasma (DBD-50, fabricated by Suman Co., Ltd., Nanjing, China) was utilized for the surface treatment of PLA films. The experimental set-up ( Figure 1) is Figure 1 The schematic diagram of the DBD-CP pretreatment apparatus. DBD-CP, dielectric barrier discharge cold plasma; PLA, polylactic acid. composed of two plane-parallel aluminum electrodes which were both covered with a quartz layer as a dielectric barrier (3 mm thickness, 9 cm length). In the DBD cold process, the distance of the gap between two layers was set at 5 mm and the input current was fixed at 1.5 A. The air atmospheric condition was adjusted to 45% relative humidity and 25°C. After that, the PLA film was placed onto the ground quartz layer and subsequently exposed to the plasma with different treatment powers supplied at 25, 50, 75, 100 and 125 W for about 30 s, respectively, and the modified PLA films P 25 , P 50 , P 75 , P 100 and P 125 were obtained correspondingly. The untreated PLA film was termed P 0 (the control).

Chitosan deposition onto the PLA films
The chitosan (1% w/v) was dissolved into an acetic acid aqueous solution (1% w/v) with gentle stirring for 12 h. Subsequently, the control and the plasma-treated PLA films were immersed in prepared chitosan solution with a 60°C water bath for 10 min. After the immobilization reaction, the chitosan-deposited PLA films were thoroughly rinsed with a large amount of distilled water and dried under ambient conditions (23 ± 2°C). The chitosan-deposited PLA films corresponding to different treatment powers were labeled as P 0 -C, P 25 -C, P 50 -C, P 75 -C, P 100 -C, and P 125 -C. The depositing ratio of the films was calculated using the following equation: Where W 1 is the dry weight of PLA films. W 2 is the dry weight of PLA-based film after depositing chitosan. Before the measurement, two number of the washing cycle was performed after chitosan depositing in order to remove the unbound chitosan chains from PLA films, and all samples were placed at 105°C to obtain the dry weight.

Contact angle and surface free energy measurements
The contact angle measurements were performed using a JY-82A contact angle analyzer, following the methodology described in the literature [30]. Deionized water and diiodomethane were used as standard media to measure the contact angles of polar and nonpolar solutions, respectively. Each sample was tested three times to ensure accuracy and reproducibility. The surface free energy was calculated using the following equation: Where θ is the polar or nonpolar liquid contact angle; σ s p and σ s d are the dispersion and polar components of the film surface, respectively; σ i is the liquid surface tension; σ l d and σ l p are the dispersion and polar components of the media, respectively.

Mechanical properties
The tensile strength (TS) and elongation at break (EAB) of films were evaluated by an electronic universal testing machine (Shenzhen Reger Instrument Co. Ltd., Shenzhen, China). The films were cut into specimens measuring 2 cm × 7 cm using scissors. The testing probe used was A/TG, with a clamping distance of 60 mm, a tensile load of 50 N, and a displacement rate of 10 mm/min. The stretching speed during the test was maintained at 10 mm/min. Each sample was tested three times to ensure reliable results and account for any variability.

Water vapor permeability
The water vapor permeability (WVP) of films was measured with a simulated cup method according to a published report by the following equation [31]: Where L is the film thickness (m); w is the weight gain of the cup after time interval t (g); A is the exposed area of the film (m 2 ); P is the saturation vapor pressure in Pascals (Pa) at the test temperature. In this experiment, t is stated as 24 h.

AFM observation
The surface roughness was analyzed by atomic force microscopy (JSPM-5200, JEOL, Tokyo, Japan). The microscope was operated using the tapping mode to measure variations in surface topography by alternations of the contact amplitude of the cantilever tip on the surface of the films. The films were cut to 5 × 5 mm and pasted on the sample stage. All AFM images with a scan size of 20 ×20 μm were acquired from the air side of the films. The surface roughness was calculated by the digital nanoscope software: root-mean-square roughness (R q ), average roughness (R a ).

XRD analysis
XRD spectrum was measured using an X-ray diffractometer (Rigaku D/max 2500 v/pc, Japan Science and Technology Corporation, Saitama-ken, Japan), operating at 45 kV and 40 mA. The incidence angle (2θ) was from 5°to 35°at a scanning rate of 4°min -1 .

ATR-FTIR analysis
ATR-FTIR spectroscopy was recorded using a VECTOR22 FTIR spectrometer (Brook Instruments Inc., Beijing, China) in the wave number range from 4000 to 400 cm -1 . After the background scan, the film was placed in the sample holder and tested with an average of 32 scans at 4 cm -1 resolution.

Antimicrobial activity assay
Antimicrobial activity was tested with Staphylococcus aureus (ATCC29213) using the disc diffusion method. Briefly, the films were cut into 6 mm circular pieces under aseptic conditions and disposed of onto an agar surface where 0.1 mL suspension (4 × 10 6 CFU/mL) was contained. Every sample was carried out three times. After a 24 h incubation at 35 ± 2°C, the size of the growth inhibition zone was recorded. The width of the inhibition zone (H) was used to evaluate the antibacterial effect by the equation.
Where H is the width of the inhibition zone, D is the average value of the outer diameter of the inhibition zone and d is the sample diameter (mm).

Statistical analyses
Statistical analysis was conducted using the SPSS 17.0 software package. One-way analysis of variance was employed to determine significant differences among the different groups. Additionally, Duncan's multiple range test was used to perform post-hoc analysis and identify specific pairwise differences between the groups. The significance level was set at a p-value of less than 0.05 to determine statistically significant results.

Effect of DBD-CP on the surface coating of chitosan on PLA films
The effect of DBD-CP treatment on the surface coating of chitosan on PLA films was investigated by comparing the deposition ratios of chitosan on plasma-treated or untreated PLA films ( Table 1). The PLA films were immersed in 1% chitosan/acetic acid aqueous solution for chitosan coating. Before measurement, two washing cycles were performed after chitosan depositing to remove unbound chitosan chains from the PLA films. Table 1 shows that the chitosan deposition process did not have a significant effect on film thickness (P > 0.05).
Only a small amount of chitosan was deposited on the untreated PLA film (P 0 ) (0.21%). However, with DBD-CP treatment, the chitosan deposition ratio significantly increased with the plasma treatment power and reached a maximum value at P 75 -C (2.65%). This result confirms the enhanced adhesion effect of CP technology on coating chitosan layers onto PLA films. However, prolonged plasma treatment at 100 W or 125 W resulted in a decrease in the adhesion between chitosan and PLA films, leading to a decrease in the deposition ratio of chitosan.

Effect of DBD-CP treatment on surface hydrophilicity and free energy
The effect of DBD-CP treatment and chitosan deposition on the WCA of PLA films is illustrated in Figure 2. The surface free energy parameters are presented in Table 2. Generally, PLA films exhibit a hydrophobic nature, which impacts their applicability in packaging. As shown in Figure 2, P 0 had a WCA value of 78.41 ± 0.71°. After DBD-CP treatment, a significant decrease (P < 0.05) in the WCA of the PLA films was observed. This indicates that DBD-CP treatment plays a crucial role in improving the hydrophilicity of PLA surfaces, suggesting that plasma-treated PLA films may achieve higher adhesion compared to untreated ones [32]. It is noteworthy that the DBD-CP treatment at 75 W exhibited the most significant improvement in hydrophilicity, as evidenced by the decrease in WCA from 78.41 ± 0.71°to 38.78 ± 0.89°. This change suggests that a saturation state for surface polar groups was reached on PLA films after treatment at 75 W, likely due to molecular steric hindrance and oxidation levels [33].
Regarding the chitosan-deposited films, the deposition of chitosan, a hydrophilic polysaccharide rich in hydroxyl and amide groups, further enhanced the surface hydrophilicity of PLA. Surface free energy is a measure of the intermolecular forces on the surface of an object and is closely related to the wettability of a solid surface. A higher surface free energy corresponds to better hydrophilicity. As verified, surface roughness and surface energy are the two key factors in determining the surface hydrophobicity and adhesion of materials [34]. Thus, the increased hydrophilicity could be explained by the appearance of polar functional groups on the PLA surface, as proved by the increase in the polar component of surface free energy of P 75 (Table 2). Moreover, this increase in polar components contributes to a substantial enhancement in the strength of adhesive joints. From an energy perspective, it can be inferred that DBD-CP treatment promotes the deposition of a larger amount of chitosan on the PLA surface, creating a more compact interface. Similar conclusions have been observed in recent studies [35].

Effect of DBD-CP treatment on mechanical properties
Mechanical properties play a crucial role in determining the quality and suitability of films for specific applications. Figure 3 demonstrates the impact of DBD-CP treatment on the EAB and TS of chitosan-deposited PLA films. Under all cases of experimental condition plasma treatments, chitosan-deposited films showed a higher EAB than P 0 -C. Similarly, the TS values of P 25 -C, P 50 -C and P 75 -C were higher than that of P 0 -C, which can be attributed to the deposition of chitosan layers. This tendency aligns well with the results described in section "effect of DBD-CP on the surface coating of chitosan on PLA films". These findings indicate that depositing chitosan through DBD-CP modification with discharge powers in the range of 25-75 W could be a promising tool for better mechanical properties of PLA-based materials in the food packaging industry.
In the case of prolonged treatment conditions (100 W or 125 W), TS value decreased from 54.67 ± 1.37 MPa for P 0 to 52.5 ± 2.12 MPa for P 125 -C, as shown in Figure 3. This decrease in TS could be attributed to the degradation of PLA chains or the deformation of the    PLA bulk structure by exposing PLA films to an excessive plasma treatment. Furthermore, the crystallization of semi-crystalline polymers can also impact mechanical properties of the polymer [36]. The similar tendency of TS value was reported by the research on ultrahigh molecular weight polyethylene modification using cold plasma technology. This study showed that the TS of ultrahigh molecular weight polyethylene fibers slightly decreased (5.6%) when it was exposed to plasma with prolonged treatment time (120 s) [24]. Another recent study on PPA reported that prolonged plasma treatment led to weight loss in the polymer film [37].

Effect of DBD-CP treatment on water resistance
Water resistance properties of materials, particularly when in contact with high-moisture products, are critical for controlling the shelf life of food products. To assess the effect of chitosan deposition on the water resistance ability of PLA films, WVP was measured under a humidity environment of 90% ± 2% and is detailed in Table 1. P 0 displayed a relatively low WVP value of (9.95 ± 0.02) × 10 -6 gm/m 2 Pa. Notably, the presence of the chitosan layer significantly reduced WVP of P 0 films (P < 0.05), indicating an enhanced water resistance property and showing a positive correlation with the deposited ratio of chitosan.
It could be found that the chitosan deposited ratio, the hydrophilicity of PLA film, fundamental properties (water resistance, mechanical properties) related to packaging processing of chitosan deposited films displayed the similar tendency within different DBD-CP treatment powers. After a comprehensive consideration of those changes, the plasma discharge power of 75 W was selected as the optimum power condition for exposing PLA to DBD-CP.

Surface morphology of chitosan-deposited PLA films
The surface morphology and roughness of P 0 , P 75 , P 0 -C and P 75 -C were evaluated using AFM. Three-dimensional (3D) plots were obtained to visualize the height of the films relative to a reference plane, and the corresponding R q and R a values were analyzed (Figure 4a-4d). A pronounced difference in the surface structural feature was observed for the PLA film with or without DBD-CP treatment. P0 exhibited a smooth and uniform surface without any pores or cracks. However, the DBD-CP treatment induced a substantial alteration in the surface topography of the PLA film, resulting in a rougher surface. The R q value increased from 18.20 nm for P 0 to 85.80 nm for P 75 (Figure 4b), and the R a value showed a similar increasing trend. The higher R q and R a values for P 75 indicated that the DBD-CP treatment affected the outermost atomic layers of the PLA film. It is worth noting that plasma treatment is known for inducing a uniform morphology change due to its low aggressiveness. Therefore, the observed spots in the AFM images can be attributed to the removal of low molecular weight moieties of PLA through the plasma etching effect [38]. Furthermore, from Figure 4c and Figure 4d, it can be observed that the chitosan deposition by the immersion method would form a homogeneous chitosan layer on the plasma-treated PLA film surface.

Crystal structure of chitosan-deposited PLA films
The effect of chitosan deposition on the crystal structure changes of PLA films is illustrated in Figure 5. The pristine PLA film (P 0 ) exhibited a flat diffraction peak at diffraction angle 2θ angle of 16.43° (  Figure 5a), indicating the amorphous nature of the PLA film [39]. However, after the 75 W DBD-CP treatment, an evident crystalline peak appeared at 16.36°, indicating a significant increase in the crystallinity of the PLA film. This observation suggests that DBD-CP treatment can promote the development of a more regular and compact crystalline form in PLA. This increase in crystallinity can be attributed to the rearrangement of PLA polymer chains in the amorphous regions induced by the DBD-CP treatment [40,41]. Furthermore, in Figure 5d and Figure 5e, a characteristic new peak corresponding to chitosan appeared at a diffraction angle of 22.21°in both the P 0 -C and P 75 -C patterns. However, the peak intensity for P 75 -C was higher than that for P 0 -C. Customarily, the peak intensity enhanced upon the amount of chitosan indicated that more chitosan was introduced onto PLA surfaces after DBD-CP treatment, and the deposited composite film had a more stable crystal structure, which also provided evidence to the improvement of rigidity as discussed in section "effect of DBD-CP treatment on mechanical properties".

ATR-FTIR spectrums of chitosan-deposited PLA films
To investigate and confirm the intermolecular interactions between chitosan and PLA, the ATR-FTIR spectrum of P 0 , chitosan, P 75 , P 0 -C and P 75 -C were analyzed, as shown in Figure 6. The spectrum of P 0 exhibited peaks at 2995.32 cm -1 and 2944.49 cm -1 , which can be   attributed to CH 2 stretching, a peak at 1746.87 cm -1 corresponding to the amide I band (1750-1600 cm -1 ), and a peak at 1455.37 cm -1 attributed to CH 2 bending. As shown in Figure 6c, the peaks at around 2995.32 cm -1 , 2944.49 cm -1 and 1455.37 cm -1 in P 0 were not shifted after the DBD treatment, indicating that the experimental discharge plasma treatment (75 W, 30 s) did not break the carbon chain structure of the PLA molecule. However, two new peaks were observed in P 75 at 3496.41 cm -1 and 1640.53 cm -1 , corresponding to O-H stretching vibration and C-O stretching vibration, respectively. The appearance of the new peaks provides qualitative evidence of the introduction of oxygen-containing functional groups, which may be the result of surface oxidation induced by the plasma treatment. As shown in Figure 6e, a broad absorption peak at 3367.29 cm -1 of P 75 -C appeared after chitosan deposition, which was attributed to O-H stretching vibration or N-H stretching of chitosan. By comparing with the pure chitosan film (Figure 6b), this peak position shifted from 3362.22 cm -1 (P 0 -C) to 3367.29 cm -1 (P 75 -C). Additionally, an absorption peak at 1599.65 cm -1 (P 75 -C) appeared, corresponding to the amide II band of chitosan. These subtle changes in the shape, position, and intensity of the ATR-FTIR peaks indicate intermolecular alterations between the plasma-treated PLA and chitosan molecules. The mechanisms underlying these changes are further discussed in next section, where the possible interactions and bonding between the PLA and chitosan are explored.

Schematic illustration
Based on our findings and previous studies on DBD-CP technology, a schematic illustration of the formation pathway of chitosan deposition on PLA films through DBD-CP treatment is presented in Figure 7 (left). Additionally, 2D AFM views of the surface morphologies at various stages are shown in Figure 7 (right). The PLA film, being a hydrophobic substance without side-chain groups, exhibits a smooth and inert surface. These characteristics make PLA surface a limitation to adhere to chitosan as antimicrobial packaging materials. This problem can be addressed by utilizing DBD-CP treatment at an optimal intensity (75 W, 30 s). This treatment leads to improvements in surface roughness, hydrophilicity, and surface free energy of PLA films (Figure 2, Figure 4, Table 2). Moreover, the DBD-CP treatment introduces polar functional groups, such as hydroxyl, carbonyl and carboxyl groups, might be introduced onto the PLA surface, which enhances the deposition effect of chitosan ( Figure 6). It was known that chitosan has two kinds of reactive groups (hydroxyl and amide groups). As shown in the schematic illustration (Figure 7), one of the possible intermolecular interaction was between hydroxyl, amide groups of chitosan and the active hydroxyl groups of PLA by secondary interaction (hydrogen bonds), and the other one was through the covalent interaction (amide bonds) between the amino groups of chitosan and active carboxyl groups of PLA. However, the excessive intensity plasma treatments (exceeded 100 W, 30 s) had a risk of deformation of microcosmic appearance (as proved by SEM images in Figure 8) or and the degradation of bulk structure of PLA, therefore decreased the chitosan depositing ratio, mechanical strength and the further antimicrobial activity of the chitosan-deposited PLA film as antibacterial packages (Table 1, Figure 3, Figure 9).

Antimicrobial assay
Packaging plays a crucial role in food preservation, and its antimicrobial activity is a one of the most vital factor in improving the shelf life of packaged food. To assess the antibacterial activity, the chitosan-deposited films were tested against Staphylococcus aureus, a causative microorganism responsible for foodborne intoxication. The width of the growth inhibition zone was measured and calculated based on the clear zone of inhibition, as illustrated in Figure 9. The results showed that the widths of growth inhibition of P 50 and P 75 were significantly higher than that of P 0 -C (P < 0.05). P 75 -C exhibited the optimum width of growth inhibition, measuring 5.02 ± 0.29 mm. Chitosan showed a broad range of activities against microorganisms and has been believed as an antimicrobial agent from composites or coatings. The interaction of anionic groups of chitosan on Staphylococcus aureus cell surface and cationic chitosan molecules might be contributed to effects [42]. However, it is worth noting that the width of growth inhibition decreased significantly when the treatment power exceeded 100 W. The variation tendency of antibacterial activities was in accordance with the amount of chitosan deposited on the PLA surface and the degree of interaction between PLA and chitosan molecules. Therefore, it would be highly desirable that the chitosan-deposited PLA film with moderate plasma treatment could be a potential antimicrobial food package.

Conclusion
The findings of this study highlight the effectiveness of DBD-CP treatment in activating PLA film surfaces and achieving improved chitosan deposition. With the discharge power increasing, there is a correlation between the chitosan deposition effect and the intensity of the plasma treatment. Specifically, the DBD treatment at a discharge power of 75 W demonstrated optimal effects in enhancing hydrophilicity, mechanical strength and antimicrobial activity of the chitosan-deposited films. However, it should be noted that exceeding a discharge power of 100 W resulted in detrimental effects on the PLA bulk structure, leading to a decrease in mechanical strength and antimicrobial activity. This can be attributed to the degradation of PLA chains and the destruction of existing polar functional groups. Therefore, it is important to avoid such high discharge powers in the industrial process to maintain the desired properties of the PLA-based materials.