## 1 Introduction  
Universal dental adhesives have been developed to minimize the number of bottles and steps of application in order to make them more user-friendly and less time-consuming during their use in dental treatments [1, 2, 3]. Such multimode systems have showed a high bonding performance to enamel and dentin surfaces [4; 5] both when used in etch-and-rinse, in selective-etch, or self-etch mode [6].  
These adhesives could be classified as strong (pH \(<\) 1), mild (pH = 2), or ultra-mild (pH \(>\) 2.5), based on their acidity [3]. An etching step using orthophosphoric acid was recommended to apply such adhesives onto enamel surfaces [5]. However, bonding on dental substrate is more difficult due to its organic composition, dentin permeability, and the tubular structure [7]. Several studies demonstrated that modern universal adhesives applied in self-etch or etch-and-rinse mode can achieve substantial bonding to dentin [8; 9].  
One of the main advantages of the universal adhesives is that the clinician can select the application mode according to the clinical situation [10]. These adhesives may be used, not only on tooth substrates including resin composite, but also on different substrates, such as silica-based glass ceramics, metal alloys, and zirconia [6; 11]. Several modern adhesives contain a functional acid monomer known as 10-methacryloyloxydelyl dihydrogen phosphate (10-MDP), which can interact and form a chemical bond with calcium ions in hydroxyapatite crystals [4; 12]. However, different application procedures are required for those universal systems with different chemical compositions, in particular when containing an acidic functional monomer different from 10-MDP [13]. One of the main requirements of a dental adhesive is to create a resin-dentin bond in the oral cavity with appropriate durability over time due to minimal collagen degradation and monomers hydrolysis [2]. Long-term water storage at 37 \({}^{\circ}\)C has been used in several studies to evaluate the durability of bonding performance through shear bond strength (SBS) or micro-tensile bond strength tests [2; 4; 5]. Some studies believe that the thickness and length of hybrid layer and resin tags, respectively, influence the bonding performance of adhesives at short and long time [14]; however, this aspect has not been yet clarified with modern universal bonding systems, so that it is necessary to study and publish the outcome of such an investigation.  
Hence, the aim of the present study was to compare the bond strength of five universal adhesives applied onto the dentin surface at baseline (24 h) and after prolonged water storage (six months in H\({}_{2}\)O). Moreover, it was investigated via scanning electron microscopy (SEM) whether the adhesive layer thickness and/or the length of the resin tags could influence the SBS after an aging period of six months. The first null hypothesis was that there would be no differences in bond strength between the five different universal adhesives after long-term water aging. The second null hypothesis was that the adhesive thickness and its resin infiltration into dental tubules would play no important role on the bond-strength durability.