## 2 Plasma  
Plasma is referred to as the fourth state of matter and is usually visualized as an arc or discharge of bright fluorescent light. Plasma in its physical form is found to be a partially or completely ionized gas mixture of reactive species like electrons, photons, positively or negatively charged ions, free radicals, and gas molecules or atoms in their excited or fundamental states with a net neutral charge. Thus plasma is considered as \"quasi-neutral\" [50; 51; 52]. Plasma is induced under varying processing temperatures and pressures by energizing a neutral gas, thus being classified into thermal (equilibrium) plasma and non-thermal (non-equilibrium) plasma [50; 52; 53]. The thermal plasma requires a high pressure (>105 Pa) and power supply (power > 50 MV), in which the main feature is the existence of thermodynamic equilibrium among all generated species at a temperature range of 2 \(\times\) 10\({}^{3}\) K to 3 \(\times\) 10\({}^{4}\) K. In contrast, the non-thermal plasma does not require high pressure or power and can exist without a localized thermodynamic equilibrium, thus referred to as non-equilibrium plasma. It is further subdivided into two categories namely (i) quasi-equilibrium plasma (100-150 \({}^{\circ}\)C), where the generated species are in a local thermodynamic equilibrium state, and (ii) non-equilibrium plasma (<60 \({}^{\circ}\)C) where electrons possess a higher temperature and heavier species have a moderate temperature without any local thermodynamic equilibrium, thereby rendering a lower temperature to the whole system. This low temperature, non-equilibrium plasma is also known as cold plasma [50; 52; 53; 54; 55]. Cold plasma, also referred to as atmospheric pressure plasma, is generated upon the application of an electrical field to a neural gas at atmospheric pressure. Either direct current (DC) or alternate current (AC) at a frequency (ranging between Hz to GHz) is applied to the gas or gas mixture between two electrode plates. The plasma generation from the neutral gas is associated with the three major phenomena that are excitation, ionization, and dissociation. Neutral gas or a mixture of gases contains few carriers in the form of electrons or ions. Upon application of an electric field, free carrier charges get accelerated and may collide with ions, or atoms in the gas or electrode surface. The excitation process increases the translational and transitional energy of the atoms. Therefore, sufficient energy in the excitation process results in the ionization of atoms by removing their loosely bound electrons. Both excitation and ionization phenomena may be due to the collision within and between different gaseous components (electrons, ions, and atoms, neutral molecules) and radiations. On the other hand, the dissociation is a result of an inelastic collision of a molecule with an electron, ion, or photon [53; 55; 56]. Over the years, plasma technology has been used in the area of food and biomedical applications. Cold plasma is an emerging, green technology that presents various potential applications in sustainable biopolymer-based food packaging. Notably, the application of cold plasma for surface functionalization of polymers by the introduction of selective functional groups allows the modification of surface properties giving improved wettability, sealability, and printability, promoting adhesion without compromising the desired bulk properties of the polymer [52; 57]. It has been found that gas plasma reactions improved the mechanical properties of biopolymer-based films by enhancing the polymer resistance towards failure [46; 58]. It also improved the barrier property of the film against gases by plasma deposition of a barrier layer [59; 60]. Depending on the excitation frequency, atmospheric pressure plasma or cold plasma may be classified as DC and low-frequency discharges (corona discharges and dielectric barrier discharges); radiofrequency discharges (atmospheric pressure plasma jets-APP); and microwave driven plasma.