## 1 Introduction  
Current adhesive systems allow for the improvement of clinical procedures due to their component chemicals and mechanism of action, reducing the operating time of application, handling of use, and side effects on the patient [1]. Universal or multi-mode adhesives are essentially one-step adhesives, combining an acidic primer and adhesive in a single solution and allowing a self-etching, total, or selective etching technique application. These systems can be applied to different dental surfaces, namely, wet and dry demineralized dentine [2, 3], enamel, and even ceramic materials, composites, and metals [4]. Universal adhesives can be considered dependent materials [5] as their components modify their mechanical properties [6, 7]. The adhesive composition is mainly based on a mixture of resin monomers HEMA (facilitates resin diffusion within the collagen network) and Bis-GMA (decreases shrinkage by polymerization and is fast curing to improve the mechanical properties of the polymer), inhibitors or stabilizers (antioxidants capable of eliminating free radicals, preventing spontaneous initiation and propagation of polymerization), solvents (which decrease viscosity and promote resin filtration), photointiators(which contribute to the formation of a stable adhesive interface), and filler particles [8,9]. Universal adhesive systems have a composition similar to self-etching adhesives [3]. They contain phosphorus monomers, mainly 10-MDP (10-methacryloyloxydecil dihydrophagate), that generate ionic bonds with hydroxyapatite by means of nano-layers (nano-layering) [10]. This monomer presents a chemical bond with the hydroxyapatite present in dentine and enamel. To facilitate the use of universal adhesives in clinical situations, whether etching is required or not, the manufacturers mix resin monomers with lower acidity in appropriate concentrations [11], increasing the potential for hydrogenation. The depth of demineralization in the dentine is linked to this potential of the acidic monomer [12]. Depending on the degree of acidity, we classify them as: strong pH = 1, intermediate pH \(\sim\) 1.5, slight pH \(\sim\) 2, and ultra-low pH = 2.5. Those with a low pH or pH close to 1.5 contain functional monomers that mainly demineralize hard dental tissues; those with a pH greater than 1.5 partially demineralize the substrate, interacting chemically with the rest of the hydroxyapatite. On the other hand, ultra-low acidic monomers expose dentin collagen in a very superficially way [13, 14, 15]. This adhesive action in permanent dentition is not the same as in deciduous dentition for reasons such as dentin and enamel composition and structure, interfering with clinical bonding [16]. Dental substrates in deciduous dentition have a lower mineral content and less thickness than those in permanent dentition, along with a more accentuated prismatic layer in the enamel and a greater tubular density in the dentine, therefore reducing the intratubular area available for adhesion [10,16,17]. There is controversy in the results of different studies as to whether permanent substrates present more or less adhesion strength than deciduous ones. In this line, [PERSON] et al. conducted a systematic review where they concluded that deciduous dentin had different fracture resistance and independent studies were needed [17]. A study published by [PERSON] et al. reported significantly greater resistance to fracture in dentin in permanent teeth, confirming the need not to analyze both dentitions in the same group [18]. Based on the hypothesis that the different composition and pH of the universal adhesives used may interfere with the mechanical properties of the different universal adhesive systems, with the rise of adhesive systems in dental medicine, these materials need to be analyzed to identify the best strategy for clinical use in order to choose the most suitable adhesive system in daily clinical practice.