# Encapsulation of \(\alpha\)-Lipoic Acid in Halloysite Nanotubes
## 1 Introduction  
One of the well-known natural antioxidants is \(\alpha\)-lipoic acid (ALA), which is also known as thiotic acid, 6.8-dithiooctic acid, or 1,2-dithiolane-3-pentanoic acid. \(\alpha\)-lipoic acid exists in the form of two enantiomers (Figure 1): R-(+)-ALA and S-(\(-\))-ALA. But in nature, only the R-form of \(\alpha\)-lipoic acid is found in organisms, both in free form and in conjugated forms with lysine residues [direct and indirect antioxidant properties of]. Commercial sources contain only the racemic form of this compound (rac-ALA). Having a special molecular structure, \(\alpha\)-lipoic acid easily transfers electrons or acyl groups to acceptor compounds, which counteracts redox reactions. Together with its reduced form (dihydrolipoic acid, DHLA), \(\alpha\)-lipoic acid is one of the most powerful antioxidant pairs found in nature and is essential for cellular energy metabolism. The ALA/DHLA redox couple is capable of regenerating other endogenous antioxidants, such as vitamins C and E and glutathione [1, 2]. Figure 1: Structural formula of \(\alpha\)-lipoic acid in two forms: (a)—R form, (b)—S form. The influence of \(\alpha\)-lipoic acid on the cellular energy metabolism of most living organisms, from bacteria to humans, is possible due to the fact that the molecule of \(\alpha\)-lipoic acid is very small and easily absorbed through the cell membrane. Due to its amphiphilic nature, \(\alpha\)-lipoic acid is located in the lipid compartment of cells; therefore, it is widely distributed in living organisms [3,4]. Due to its unique properties, mainly antioxidant, \(\alpha\)-lipoic acid is widely used in the practice of treating a number of diseases in people, such as slowed metabolism, states of intoxication, disorders of lipid and carbohydrate metabolism, and reduced liver function. Contributes to a decrease in blood sugar and an increase in the amount of glycogen in the liver. Its action is similar to the pharmacological properties of B vitamins. Therefore, \(\alpha\)-lipoic acid is one of the most widely used substances in pharmaceuticals [5]. However, the disadvantages of this chemical are its instability to thermal exposure, oxidation, and degradation under the influence of daylight, which significantly reduces its antioxidant efficiency and ability to perform other important functions in human treatment. This is also challenging from an analytical point of view [6]. One of the ways to protect \(\alpha\)-lipoic acid in the composition of medicinal products is through the use of delivery systems or encapsulation in various carriers, which has become the subject of recent research by a wide range of scientists. Recent studies indicate that \(\alpha\)-lipoic acid has been studied in a wide range of delivery systems, in particular using chitosan microspheres, polysaccharide gel beads, and a number of other polymer-based carriers [7, 8, 9]. The study of the antioxidant capacity of \(\alpha\)-lipoic acid using the 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical showed that microencapsulation in various polymer matrices and carriers allows it to almost completely preserve its ability to absorb free radicals in an in vitro model. These data are confirmed by numerous literature studies on the use of colloid carriers in other systems as well [7]. Loading into structural cavities created by polymeric carriers is a widely practiced approach to stabilizing unstable ingredients. In addition to the stabilizing effect, the release or delivery of captured ingredients can be controlled, improving their bioavailability for humans [8]. An important issue in the development of effective ways to protect unstable ingredients for oral use is their solubility in water, which determines their biological half-life and bioavailability. An increase in bioavailability is also achieved due to the dispersion and reduction of the particle size of the insoluble substance, which improves the speed of its removal and stability during storage.