## 2 Materials and Methods
### LC-MS Analysis  
_Aloe vera_ contains more than 75 different compounds, including vitamins (vitamin A, C, E, and B12), enzymes (e.g., amylase a class of enzymes that catalyze the hydrolysis of starch; catalase enzyme that catalyzes the decomposition of hydrogen peroxide to water and oxygen, and peroxidase, enzymes that catalyze the oxidation of a substrate by hydrogen peroxide or an organic peroxide), minerals (e.g., zinc an essential mineral for our body, copper essential to the proper functioning of organs and metabolic processes, selenium an element involved in the protection of cells in the against excess H\({}_{2}\)O\({}_{2}\), in heavy metal detoxification and calcium), sugars (monosaccharides such as mannose-6-phosphate and polysaccharides such as glucomannans), anthraquinones (aloin and emodin), fatty acids (e.g., lupeol a pentacyclic triterpenoid, in which the hydrogen at the 3 beta position is substituted by a hydroxy group and campesterol, a steroid derivative characterized by the hydroxyl group in position C-3 of the steroid skeleton), hormones (auxins and gibberellins), and others (e.g., salicylic acid, a lipophilic monohydroybenzoic acid, lignin, organic polymers that form key structural materials in the support tissues of vascular plants and saponins a class of bitter-tasting compounds that produce soap-like foam when added to water. Most saponins occur naturally in plants.) [18,19]. Among the various water-soluble pharmacological findings compounds for _Aloe vera_, there is aloesin 2-acetilonil-8-394 142-144, \(\beta\)-D-glucopraniosyl-7-hydroxy-5-methylchromone, with a molecular weight of 394, that may be used as tracer for a whole mixture [20].  
Aloesin release from lenses into the physiological solution was determined by high pressure liquid chromatography conducted using a Vanquish (Thermo Fisher Scientific Wilmington, DE, USA) Ultra High Performance Liquid Chromatography (UHPLC) system composed of a binary pump, an autosampler, and a column oven [21] For aloesin determination, 15 \(\upmu\)L of each sample were injected onto an Accucore aQ column (100 \(\times\) 2.1 mm; 2.6 \(\upmu\)m particle size) (Thermo Scientific, San Jose, CA, USA) maintained at 25 \({}^{\circ}\)C. The eluents were 0.1% formic acid (eluent A) and acetonitrile (eluent B). The flow rate was 200 \(\upmu\)L/min. The mobile phase was a binary linear gradient in the following sequence: isocratic 30% B for one min, a linear gradient over the course of 19 min to 100% B, maintained at 100% B for five min, and, finally, a linear gradient to 30% B in two min. The re-equilibration time in 30% B was six min. After HPLC separation, the eluent was directly sent to a Q Exactive Plus Orbitrap mass spectrometer (Thermo Scientific, San Jose, CA, USA) equipped with a heated electrospray ionization source (HESI-II). Before the start of each acquisition series, the mass spectrometer was externally calibrated with a negative ion calibration solution (Thermo Fisher Scientific, San Jose, CA, USA). Negative full-scan mass spectra were recorded in the mass range of 100-700 \(m\)/z. The following operation parameters were applied: the sheath and auxiliary gas flow rate were 45 and 10, respectively; the spray voltage was 3.5 kV, the capillary temperature was 400 \({}^{\circ}\)C; the S-lens (radio frequency) RF level was 100. The software used for operating the ultra-high-pressure liquid chromatography with high resolution/high accuracy Orbitrap(r) mass spectrometry (UHPLC/HR-MS) (Thermo Fisher Scientific, San Jose, CA, USA) was Xcalibur (version 4.1). The full scan data were processed, and the amount of aloesin was measured via the extracted ion current (EIC) peak area (\(m\)/z 393.11 [M-H]-).