## 1 Introduction  
Pediatric dentistry employs a wide range of materials for direct conservative restorations, including glass ionomer cements, composite resins, bulk-fill resins, and self-etching and/or self-adhesive resins. Glass ionomer cements are materials formed from a reaction between a powder and liquid, which prove highly useful in pediatric dentistry due to their ease of manipulation and fluoride release capabilities [1]. They offer a protective effect on the tooth surface, are bioactive, and are suited to various applications, including dental restorations. In addition to their fluoride-releasing ability, these materials also adhere well to tooth structures, but some studies suggest their main disadvantages include wear and long-term strength loss [2]. Currently, composite resins are the most widely used material for treating carious lesions on permanent teeth, gradually replacing the use of dental amalgam due to their biocompatibility, good aesthetic quality, easier handling, and non-toxicity [3]. Composite resins consist of three primary components: a polymeric resin matrix, a filler, and a coupling agent that binds the filler to the matrix. Their major drawbacks include polymerization shrinkage, low wear resistance (especially in first-generation composite resins), and a lack of antibacterial activity [4]. The introduction of new materials has led to numerous advantages. For instance, bulk-fill resins can be placed in increments of 4 mm, larger than the 2 mm required for traditional resins, thus avoiding the layering technique while still achieving adequate polymerization [5]. More recently, self-adhesive and self-etching resins have been developed, potentially offering a time-saving advantage in cases of reduced patient cooperation. These composites are used in small Class I cavities, either as a base layer or a sealant without the need for a bonding agent [6]. Currently, three self-adhesive flowable composites are available, including Kerr Vertise Flow, which contains glycerol phosphate dimethacrylate (GPDM) monomers, ensuring a strong etching effect [7]. When selecting an appropriate material, various factors must be considered, including the material's mechanical and physical characteristics. Important considerations include polymerization depth, surface hardness, and roughness, as well as the material's response to acidic attacks and wear [8]. Polymerization depth can be defined as the thickness of resin that can be converted from monomer to polymer and depends on multiple factors, such as the type of material, the amount of filler present, exposure, irradiation, and the distance of the light source in photopolymerizable materials [9]. Inadequate polymerization can lead to restoration failure and subsequent pulpal damage. Hardness refers to the material's resistance to mechanical indentation by a harder object [10]. It is also important to consider roughness, which, in addition to being intrinsic to the material, can increase due to brushing, and increased roughness correlates with increased plaque deposition [11]. In pediatric patients, frequent consumption of acidic foods can cause, in addition to damage to dental tissue, a loss of material strength in restorations. Furthermore, the wear resistance that develops from continual brushing on the dental material is crucial to avoid fractures or restoration loss [12; 13]. The purpose of this study is to analyze and compare the polymerization depth, hardness, roughness, and surface characteristics following acidic attack and brushing-induced wear of four materials used in direct restorations in pediatric dentistry. Specifically, the characteristics of two glass ionomer materials, one bulk-fill resin, and one self-etching and self-adhesive resin will be examined.