## 3 Impact of Cold Plasma on Packaging Properties  
### Surface Roughness  
The application of cold plasma increases the surface roughness of biopolymer films and can be attributed to the etching effect phenomena. This is caused by the bombardment of high-energy plasma species like electrons, photons, positively or negatively charged ions, free radicals, and gas molecules or atoms in their excited or fundamental states [74, 75]. The etching effect in a film's surface may arise due to chemical processes such as cleavage of chemical bonds, scission of polymer chains, or chemical degradation of film components by the influence of the free radicals. It can also be caused by a physical process like the removal or re-aggregation of low molecular weight components/fragments on the surface of the film. The other possible reason for the change in the surface roughness by the application of cold plasma may be due to its micro-discharge filaments [43, 52, 75]. The extent of roughness may vary depending on the power supplied (voltage), exposure time, and uniformity of the exposed energy of the plasma species onto the film surface [52, 58]. The etching effect phenomenon is noticed commonly with the application of DBD plasma, radiofrequency (RF) generated plasma, microwave (MW)-driven plasma with different gases like air, argon, O\({}_{2}\), N\({}_{2}\), and CO\({}_{2}\). The overall variation in the surface roughness of the film exposed to cold plasma is summarized in Table 2.  
As shown in Figure 3I, the Atomic Force Microscopic (AFM) topography of the zein film treated with DBD atmospheric cold plasma demonstrated that the roughness of the polymer surface increased with the increase in voltage and treatment time. A 5 min treatment at a voltage of 60, 70, and 80 kV increased the roughness (RRMS: Root Mean Square Roughness) from 16.41 nm to 52.04, 126.16, and 132.87 nm, respectively. This enhancement in the roughness was attributed to the etching effect of plasma treatment [75]. Such incremental increases in surface roughness were also observed for polylactic acid/nisin film treated with cold plasma, where the surface roughness value of the untreated film was 4.26 nm and increased to 6.24 nm with the increase in treatment time from 0 to 30 s. A further increase in treatment time led to a decrease in surface roughness and this could be due to the longer treatment time (the 60 s) having etched off the PLA microscale protuberances and the appearance of nano-needle-like bumps, resulting in a more uniform PLA surface [48]. Also, [PERSON], [PERSON] [76] treated biodegradable polyethylene-oxide terephthalate (PEOT)/polybutylene terephthalate (PBT) film with DBD plasma involving air, He, Ar, and N\({}_{2}\). AFM topography showed N\({}_{2}\) and air plasma treatment of PEOT/PBT film increased the surface roughness whereas He, and Ar plasma treatment decreased the surface roughness as compared to untreated sample as shown in Figure 3 II. It was reported that He and Ar plasma treatment etched away the sharp peaks present on the surface of the PEOT/PBR film resulting in a more uniform surface. No significant change in the surface topography or surface roughness was observed for N\({}_{2}\) treated PEOT/PBR film and the application of air plasma completely changed the surface topography by forming hemispherical protrusions on the film surface. Such variation in etching behavior can be attributed to the difference in active species present in the plasma discharge. Notably, air plasma containing significantly higher oxygen molecules created an atomic oxygen species that is known for excellent etching phenomena [76]. Therefore, it is clear that the variation in surface roughness is a function of four factors: voltage; treatment time; types of plasma gas; and the internal structure of the polymer.