**6. Future Perspectives for Liquid and Solid Bio-Based Coatings**

The present review evidenced the needs of formulating new bio-based coatings, which can be highly compatible with cellulosic and bioplastic substrates, in which thermoplastic starch films are one of the main examples [213]. The use of proper food or agricultural waste for their formulation agrees with the circular economy principles, can keep the cost of new materials down and can result in evident environmental advantages.

It is easy to predict, on the basis of the present literature survey, that chitin/chitosan coatings could be interesting both in liquid and solid forms. Cellulosic substrates [26], but also bioplastic [214] and textile substrates [113], can be easily treated with liquid coatings. The penetration of the liquid in the cellulosic or textile tissue is an important aspect to be controlled. Whereas chitosan, dissolved in acidic water, can penetrate inside the tissues, the chitin nanofibrils, generally suspended in water, remain on the substrate surface. In both cases the antimicrobial action can be modulated by controlling the concentration of these biopolymers in the liquid product. Solid coatings in powder or in film can be highly innovative. CNs or chitosan could be properly dispersed in thermoplastic matrices, having a low melting temperature for an easy and not expensive coating in terms of energy application.

Proteins can actually be used more on plastic and cellulosic substrates for developing high oxygen barrier coating for plastic and cellulose packaging [81,93], but they could also be potentially employed in solid coating formulations, despite their difficult processability and temperature sensitivity [215].

A cutin lacquer was developed for metallic substrates [103], but it is potentially applicable by liquid coatings on cellulosic and bioplastic substrates to obtain coloured (i.e., not transparent) coatings. The high hydrorepellency of cutin could probably allow these

properties to be modulated on many substrates. The application of cutin in solid coating would be very new and interesting for the same reason. These last considerations are summarized in Table 4.

**Table 4.** Predictable perspectives for chitin/chitosan, protein and cutin on different substrates.


In general, the preparation of liquid coatings based on chitin/chitosan, protein or cutin is at a higher technological readiness level, with respect to solid coatings.

The latter are extremely promising but more challenging than liquid coatings, as the modulation of morphological features based on coating concentration is a complex issue, as well as for the possible thermal degradation that could occur during processing and further application.

The considered biopolymers are thus extremely promising for developing innovative and environmentally friendly coatings for several substrates with some pros and cons, shown in Figure 2. These coatings can be extremely useful for improving the properties of renewable products, thus boosting their use in several applications.

**Figure 2.** Advantages and disadvantages of liquid and solid coatings.
