# Recent Advances in Bio-Based Wood Protective Systems: A Comprehensive Review
## 4 Bio-Based Fillers
### Bio-Based Additives for Antimicrobial Wood Systems
[97] infused Scots pine wood specimens with varying concentrations of valonia, chestnut, tare, and sulphited oak tannins at levels of 3%, 5%, 10%, and 15%. These treated samples were subsequently exposed to attacks by the brown-rot fungi _C. puteana_ and _Postia placenta_, as well as the white-rot fungi _T. versicolor_ and _Pleurotus ostreatus_. The aim was to identify the most suitable tannin type and optimal concentration required to ensure effective resistance against decay. Tannins demonstrated their efficacy in suppressing brown-rot fungal assaults when no leaching was performed prior to the decay assessment. In a similar vein, [PERSON] et al. [96] conducted a comparison between the preservative impact of tannin extract and a conventional preservative blend containing CCB (chromated copper borate) against the biodeterioration of _Acacia mearnsii_ wood. The tannin concentrations exhibited comparable outcomes to those achieved with the CCB mixture across all assessments. Notably, the tannins bolstered the material's resistance to biological degradation, leading to its classification as highly resistant to fungal activity. Taken together, the outcomes indicate the potential of tannins to serve as a viable natural preservative solution. Similarly, other wood extracts have demonstrated interesting biocidal behaviour. For instance, [PERSON] et al. [93] investigated and analysed pyrolysis distillates derived from the barks of spruce and birch trees, as well as hemp. They examined various fractions obtained from these distillates to evaluate their potential as antifungal agents for deterring wood decay. The notable capacity of the pyrolysis distillates to effectively inhibit decay, even at concentrations of 1% and below, highlighted their potential for serving as a resource for developing sustainable formulations of wood preservatives. In contrast, [PERSON] et al. [123] conducted an evaluation indicating that extractives from knotwood in Scots pine effectively hindered the proliferation of wood-decay fungi. On the other hand, heartwood extracts from black locust demonstrated considerably higher effectiveness as scavengers of free radicals compared with extracts from Scots pine. These extracts were deposited within the cavities and on the cell walls of impregnated sapwood. The treatment of sapwood blocks with extracts from both Scots pine and black locust led to a reduction in the fungal decay of the wood. Lastly, the ability of extracts obtained from wound-related beech wood (_Fagus sylvatica_ L.) to act as fungicides against specific brown (_G. trabeum_) and white (_T. versicolor_)-rot fungi was evaluated [124]. The analysis of the fungicidal attributes of extracts from wound-related wood revealed that the pronounced inhibitory impact on wood-decaying fungi could be attributed to both methanolic extracts of wound-related wood and the healthy sapwood. Apart from displaying notable resilience against fungal attacks, contemporary wood coatings are frequently expected to possess the capability to hinder the proliferation of bacteria and viruses. Certainly, with the global outbreak of the novel coronavirus pneumonia in 2020, there was a substantial rise in the demand for antibacterial products. As a result, recent endeavours have been focused on developing environmentally friendly antibacterial fillers and additives to address this requirement. For example, citric acid stands out for its cost-effectiveness and its natural origin, alongside its antimicrobial properties. [PERSON] et al. [125] developed an oligomer derived from citric acid, which was subsequently blended with epoxy acrylate and tri(propylene glycol) diacrylate (TPGDA) to create a UV-curable coating. The investigation demonstrated that the area where the growth of _Staphylococcus aureus_ was inhibited expanded as the proportion of UV-curable unsaturated oligomer in the coating formulation increased. Nonetheless, [PERSON] et al.