## 2 Materials and Methods  
### Sample Preparation  
A total of fifty single-rooted premolars with intact enamel surfaces were obtained immediately after extraction due to orthodontic and periodontal reasons. Following the extraction procedure, the teeth underwent a cleaning process utilizing pumice powder. Subsequently, they were preserved in a solution of physiologic saline supplemented with a small quantity of thymol crystals, at a temperature of 4 \({}^{\circ}\)C, for less than 2 months. The exclusion criteria encompassed teeth that had visible or detectable caries, restorations,hyperplastic lesions, stains, cracks, and bleaching. The premolars underwent decoration utilizing a diamond disk (Horico, Berlin, Germany). The included specimens were sectioned into two halves with intact buccal and palatal coronal surfaces along the mesiodistal plane, resulting in a total sample size of 100 samples. The flat surface of the buccal and lingual surfaces was attained by using a sequence of silicon carbide abrasive sheets, ranging from 600 to 3000 grit. This was followed by fine polishing using a water-based diamond paste with a particle size of 1-0.25.  
All of the samples were then exposed to an aerated beverage by immersing them in Coca-Cola(tm) (Hindustan Coca-Cola(tm) Beverages Private Limited, Bengaluru, India), pH 2.3, for 2 min each day over 1 month to induce erosion on the enamel surface. The beverage was replaced daily. Following the induced erosion, the specimens were embedded in acrylic cold-care resin (DPI-RR Cold Cure, DPI, Mumbai, Bengaluru, India) and filled in a custom-made metal mold in such a way that the enamel surface of all of the samples faced upward. Subsequently, all of the study samples were arbitrarily divided into 5 groups based on the reparative and regenerative agents used (n = 20), as follows:  
Group 1: In this group, the samples were not treated with any remineralizing agent.  
Group 2: Curodont (Curodont Repair, Credentis AG, Windisch, Switzerland) (P11-4 oligopeptide solution) was diluted with 0.05 mL of deionized water. A single drop of this solution was then applied to the demineralized enamel surfaces and left undisturbed for 5 min. This was then followed by washing the surface with distilled water [21].  
Group 3: The study samples involved in this group were treated with the regenerative Enamel Science(tm) Advanced Toothpaste (a CSSP-based product) for 5 min. This was followed by the application of Advanced Enamel Serum for 3 min. This was then followed by washing the surface with distilled water [18].  
Group 4: The specimens in this group were applied with topical fluoride (TF) solution, specifically Nupro Neutral(tm) (Johnson and Johnson, Inc., New Brunswick, NJ, USA), containing 2.0% NaF (9047 ppm). The fluoride gel was then washed from the surface with distilled water after 4 min using tissue paper [22].  
Group 5: In this group, TF was applied in the same manner as that used for Group 4. This was followed by irradiation of the TF-treated surface with a CO\({}_{2}\) laser (PC 015-D CO\({}_{2}\) Laser System, Shanghai JueHua Laser Tech. Development Co., Ltd., Shanghai, China) with an adjusted power of 0.5 W and an energy density of 0.44 J/cm\({}^{2}\) and pulse duration of 100 s, with a 0.001-sec interval between pulses. The surfaces were then washed with distilled water [22; 23].