# Recent Advances in Bio-Based Wood Protective Systems: A Comprehensive Review
## 5 Bio-Based Pigments and Dyes
### Microbial Pigments  
Since ancient eras, organic dyes have served as colour enhancers in wood applications. Their environmentally friendly and health-conscious attributes, stemming from qualities like safety, non-toxicity, absence of carcinogenicity, and biodegradability, have aligned well with the growing emphasis on ecological considerations and human well-being. Consequently, microbial pigments present a promising avenue for industrial implementation, given their abundant availability, diverse range, rapid reproduction rates, and the flexibility of their cultivation without temporal or spatial constraints. With this perspective in consideration, a recent study conducted by [PERSON] et al. [39] assessed the potential advantages and obstacles associated with the utilization of microbial dyeing techniques for pigment incorporation in wood processing. Scholars have extensively presented and examined two viable approaches for integrating microbial pigments into the wood dyeing sector. One method involves inoculation, a straightforward and efficient technique where microorganisms deposit pigments onto or within the wood. However, this approach necessitates the industrial foundation of microbial screening and induction, along with the required equipment for ensuring industrial scalability. The alternative approach entails employing extracted pigments secreted by microorganisms for wood dyeing. This method offers relatively easier industrialization prospects. However, it entails technical challenges such as achieving mass production, establishing clean pigment-extraction methods using resoluble solvents, and addressing the deep penetration and stability of pigments within the wood. However, these processes appear to be very promising, as demonstrated by recent literature studies. For instance, [PERSON] et al. [40] conducted an experiment where pigments were derived from _Scrystallization cuboideum_ (producing red pigmentation) and _Chlorociboria aeruginosa_ (yielding blue-green pigmentation). These pigments were then dissolved in raw linesed oil, and the resulting solution was applied to samples of Douglas-fr (_Pseudotsuga menziesii_) and western white pine (_Pinus monticola_). The outcomes of the study revealed the promising possibilities of fungal pigments in the wood coating sector, as they contributed to an extended lifespan of the applied coating. This finding is particularly significant considering the growing shift towards sustainable materials, indicating that pigments sourced from wood-decaying fungi hold potential as beneficial additives for wood coatings. Similarly, in pursuit of a more environmentally conscious and ecologically sustainable wood-dyeing technique, [PERSON] et al. carried out a 35-day microbial dyeing experiment on bleached poplar [142]. They accomplished this by utilizing the blue-strain fungus _Lasidiplodia theobromae_ in a reverse manner within the poplar. Over the course of the dyeing period, the brightness value and reflectivity of the bleached poplar gradually diminished. Following 35 days of biological staining, the hyphae of _L. theobromae_ managed to penetrate the poplar veneer through its transverse structures (wood ray and vessel), creating an approximate 0.5 mm thickness, yet the chemical composition of the poplar remained largely unaffected. Moreover, the post-dyeing veneer exhibited minor alterations in terms of wettability and tensile modulus. The impact of _L. theobromae_ was also examined in an additional study, focusing on dyeing techniques aligned with environmental preservation requirements and adaptable to industrial advancements [143]. Bleached poplar wood was subjected to dyeing using melanin secreted by _L. theochromae_. The investigation revealed that extending the dyeing duration of coarse melanin led to enhanced absorption by veneers. Consequently, as the dyeing time increased, the brightness value exhibited an uptick while the reflectivity diminished.