## 1 Introduction  
Optimized mechanisms within living organisms often provide invaluable inspiration when designing functional materials from a chemical point of view. Among these, polyphenols have received significant scientific attention owing to their diverse physiological roles in nature [1; 2; 3]. A well-known characteristic of natural polyphenols--commonly found in mussel-foot proteins with a high content of 3,4-dihydroxyoxyphenyl-L-alanine (DOPA), a phenolic amino acid, is their underwater adhesive ability [4]. The wet adhesiveness of the mussel-foot protein is attributed to the catechol side chain of DOPA. Catechol can maintain adhesion through various non-covalent and covalent bonds at the interface, in addition to generating a strong cohesive force due to oxidative self-crosslinking [5]. Additionally, polyphenols play an essential role in the hardening of the insect exoskeleton (i.e., cuticle) to protect them from external attacks [6]. Specifically, the curing process begins with the action of oxidases--such as phenoloxidase--on phenolic precursors such as N-acetyldopamine and N-\(\beta\)-alanyldopamine [7]. Another polyphenic material, i.e., melanin, is a major component of the ink used by cephalopods to block the vision of predators. The oxidative polymerization of the L-DOPA results in the formation of 5,6-dihydroxydoxide-2-carboxylic acid (DHICA), which is an intermediate that is involved in the formation of melanin [8]. In short, natural polyphenols with diverse physiological roles can be developed by oxidative crosslinking mechanisms. Inspired by the mechanism, synthetic polyphenols developed from phenolic building blocks have been introduced. Polydopamine [9; 10; 11; 12; 13], poly(L-DOPA) [14], poly(norepinephrine-rine) [15; 16; 17], poly(galidic acid) [18], and poly(tanmic acid) [19; 20; 21] are typical examples; the materials showing similar physicochemical properties to the natural polyphenols have been successfully used as adhesives, optical materials, sensors, and other bioinspired applications [12; 13; 14]. Especially due to their excellent biocompatibility [22; 23], biodegradability [24], and wet-adhesion abilities [25; 26], the self-assembled phenolic building blockshave been successfully utilized as biomaterials, such as nanomedicine, antimicrobial coating, and tissue scaffold (Figure 1). While their usability was confirmed in previous studies, the detailed assembly mechanisms of individual phenolic building blocks have been overlooked in the community. In fact, each phenolic building block has a different number of hydroxyl groups in the benzoic site. The structural dissimilarities influence the chemical interactions that integrate into the building blocks. For example, quinone, which is the critical intermediate that can initiate the oxidative chain reaction [27], can be developed in catechols and gallols but not in phenols. Thus, the oxidative crosslinking kinetics is relatively slow when an aromatic compound with a single hydroxyl group (i.e., phenol) is used; enzymes, oxidants, and other catalysts are often required to induce the associated polyphenols [28]. On the other hand, oxidation can be spontaneously achieved in catechol molecules in the presence of ambient oxygen. If there is an additional hydroxyl group in the catechol group (i.e., gallol), the oxidative crosslinking reaction can be further facilitated [29; 30]. The difference in assembly kinetics significantly impacts their physicochemical properties. Thus, placing an appropriate polyphenol in a suitable application is essential. In this review, we classified representative building blocks that can develop synthetic polyphenols into three categories based on the number of hydroxyl groups (i.e., phenol, catechol, gallol, see Figure 2). Additionally, the relevant self-assembly mechanisms and resulting biomaterials have been described. The information presented in this review will be beneficial when designing a new polyphenol-based functional biomaterial.