As evidenced from the table above, different types of plasma treatments have been adopted by different researchers, and the treatment effects on the resultant composite and substrate properties seem to correlate or vary depending on the type of plasma treatment, type of raw materials, and intended end use applications. As shown in the table, majority of the studies were carried out using dielectric barrier discharge (DBD) cold plasma treatment, where researchers like Rashvand et al. [65], Goiana et al. [72], Wu et al. [77] and Chen et al. [47,79] reported improvements in tensile strength, elasticity and barrier properties like water vapour and oxygen transmission rates. Enhanced stability and shelf life of packaging films and substrates were observed by Glicerina et al. [67] and Akhavan-Mahdavi et al. [68], and modified surface properties of composite materials were reported by Goiana et al. [72] and Chen et al. [79], where they noted changes in surface morphology and hydrophobicity depending on the type of film and plasma treatment frequency, with improved wettability, roughness, and surface free energy after coating or plasma treatment. Additionally, changes in film structure were observed by Wu et al. [77] and Chen et al. [47], while observing rearrangement of protein chains, slight changes in crystal structure, and improved packing density within the film structure. Chen et al. [79] described better-ordered porous structure and enhanced compatibility between zein and coating materials, while Chen et al. [47] reported ordering of zein molecule secondary structure after plasma treatment. Cold plasma treatment, in general, were carried out by a few researchers including Yudhistira et al. [73], Cui et al. [76] and Lin et al. [81]. The major effects on composite properties reported by them include improved coating adhesion, controlled release of active agents and surface modification for enhanced antimicrobial activity. It is interesting to note that unlike the DBD cold plasma treatment, the enhancement of mechanical and barrier properties have not been significantly reported in general cold plasma treatment.

The key findings from two reported studies employing DBD atmospheric air cold plasma (DBD-ACP) include surface modifications, interfacial interactions, and stability and delamination resistance. Quite different from other reports, the use of DBD-ACP exhibited increased hydrophilicity, suggesting improved water absorption and wetting properties. Both Heidemann et al. [80] and Li et al. [70] observed enhanced adhesion and interaction, whether it be between film layers, or between the film and food materials, thereby leading to an improvement in barrier properties and film functionalities. Nevertheless, helium cold plasma treatment carried out by da Fonseca de Albuquerque et al. [71] did not significantly influence the water vapour permeation, but created an improved physical barrier to water. Moreover, even though this treatment technique increased the surface roughness, it was not significant enough in increasing the water contact angle of the modified surface. Surface dielectric barrier discharge (SDBD) plasma from Plasma Assisted Sanitation System (PASS) is another less common technique reported by Capelli et al. [74], predominantly used for food packaging decontamination from SARS-CoV-2 RNA. In this study, they observed that the plasma treatment decontaminated virus, without significantly affecting the properties of packaging and food substrate. Yet another plasma treatment method termed carbon tetrafluoride (CF₄) reactive-ion etching (RIE) was reported by Kim et al. [78], where nanopillars are introduced to the film surface, with potential applications in developing antibacterial overcoating. Such films were observed to exhibit improved optical and antibacterial properties.

Overall, these reports indicate that various coating technologies can effectively enhance the functional characteristics of food packaging films, resulting in augmented product protection, prolonged shelf lives, and possibly improved sustainability. However, it’s important to note that explicit effects of each coating technology can depend on the type of film material, coating composition and treatment parameters.

The prospects of using plasma treated sodium alginate (SA) films for food packaging applications was studied by Sharmin and colleagues [75], where, unlike the traditional methods of plasma treatments, this study dealt with the use of plasma activated water (PAW). The main focus of the researchers remained on understanding the effect of citric acid (CA) and PAW on the mechanical, rheological and barrier properties of SA films (Also given in Table 1). The tensile strength and elongation at break values of the resultant films were seen to increase by 43 and 66% when PAW was used to prepare SA solutions, while a reduction in water vapour transmission rate (WVTR) up to 44% was obtained for 1% CA incorporated SA samples, all indicating the development of better films when compared to neat SA films.

Effect of different chitosan concentrations and plasma treatment durations on fresh pistachios was studied by Akhavan-Mahdavi and co-workers [68], and the radar chart depicted below provides an in-depth understanding on the sensory analysis results after 180 days of storage of pistachios after the treatments. In their study, the influence of varying amounts of chitosan (0.5 and 1.5% w/v) and cold plasma treatment duration (60 and 120 s) were observed and reported for 180 days of pistachio storage. The resultant effects on the pistachio shelf life were estimated by measuring various factors such as hardness, colour components, total mould and yeast, moisture and aflatoxin content, peroxide values, and sensory evaluations. From the reported results, it can be seen that the treatment with 1.5% chitosan and 120 s of cold plasma treatment produced highest conservation of pistachio hardness and colour indices. It has also been reported that among all the examined parameters, the plasma treated sample P₁₂₀ C_1.5 and the control sample exhibited the highest and lowest scores respectively.

While studying the decontamination of food packages from SARS-CoV-2 RNA with a cold plasma-assisted system, Capelli and team observed that plasma treatment decontaminated the virus, without significantly affecting the properties of either the packaging or the food substrate [74]. As reported for K5 and K10 samples, the viral RNA reduction driven merely by air exposure for both the packaging materials tested was very minimal. Both packing materials underwent a considerable but modest reduction in detectable RNA after 5 min of CAP treatment of around 10 and 16% for PP and PET, respectively. The RNA molecules, whose quantity was below the detection thresholds for each target sequence discovered by this PCR approach, were fully destroyed by CAP treatment for 10 min. Both PET and PP were used to study how O₂, and CO₂ changed in the headspaces of the packages as they were being stored. As predicted, the respiration of apple tissues resulted in CO₂ building up and O₂ being used up. Due to the greater thickness and reduced permeability of PET, the O₂ loss was accelerated for apples in packaging. Also, O₂ had been completely absorbed in just three days. However, for both packing materials, there was no discernible variation in CO₂ and O₂ concentration between treated (P5, P10) and control samples (C) [74]. The apple samples were also measured for titrable acidity (TA) and firmness values (N) during storage. TA showed an increase during storage in all apple samples packaged in PE, which was attributed to the generation of carbonic acid from CO₂, which is present in higher concentration in PET packages than in PP ones. Although the exposure to CAP did not significantly affect the samples in either case, this difference was correlated with the type of components present.

The morphological changes induced by plasma treatment on PET surface has been reported by Kim et al. [78]. It was found that the CF₄-RIE PET had a regular array structure made up of spherical capped nanopillars with 30 nm diameters, 237 nm heights, and 75 nm pitches (the space between two pillars), as opposed to the flat surface of neat PET. The Progomphus obscurus (sand dragon) wing, which exhibits a nanopillar array structure composed of high-aspect-ratio spherically capped nanopillars with an average diameter of 50 nm and an average height of 241 nm, served as the source of inspiration for the CF₄-RIE’s nano surface structure. The optical improvement observed by the researchers were also attributed to these nanopillar array structure, where the rough surfaces tend to reduce transmittances by increasing the absorption capacity for incident light due to light scattering.

While studying the effect of cold plasma treatment on thyme essential oil (TO)/silk fibroin (SF) nanofibers against Salmonella Typhimurium in poultry meat, Lin and co-workers [81] conducted sensory evaluation tests were on chicken and duck meat. According to them, when compared to the control group, chicken and duck meat samples wrapped in plasma-TO/SF nanofibers membranes demonstrated an improvement in flavor and general appeal. The researchers also deduced that the sensory evaluation scores of poultry meat with plasma-TO/SF nanofibers were higher than the control group since the test group received a higher score after the plasma treatment, indicating that plasma-TO/SF nanofibers membrane could improve food quality without losing the good flavor.

As part of the U.S. National Science Foundation supported program on Future Technologies Enabled by Plasma Process (FTPP), at the University of Alabama at Birmingham (UAB), we focused on the developmental studies, a step further, to have smart sensor-integrated packaging against food pathogens or toxins via inkjet printing onto plasma treated films [2,82]. Moreover, our group recently published an invited mini review on mitigation strategies in engineered healthcare materials towards antimicrobial applications and another on non-thermal plasma processing for nanostructured biomaterial [83,84]. Plasma research programs for materials and biomatter for application in agriculture, medical materials [85,86,87,88,89,90,91,92,93,94], food packing, and other plasma-technologies for automobile and aerospace applications will establish Alabama State as a Southeastern regional hub for plasma science expertise and create thousands of high-paying technical careers in the state and region.