# Encapsulation of \(\alpha\)-Lipoic Acid in Halloysite Nanotubes
## 1 Introduction
Therefore, encapsulation in nano-sized particles was proposed as an effective method for increasing the dispersion in water and bioavailability of various non-polar bioactive substances [9]. Nanoscale encapsulation in cyclodextrin and amylose was proposed as one of the effective processes in the preparation of aqueous dispersions of hydrophobic substances [10,11]. The level of protection of \(\alpha\)-lipoic acid against thermal degradation and oxidation during encapsulation in starch with a high amylose content, both in the native and octenylsucctimulated states, was investigated by modeling the digestive tract [12]. Octenylsucctimulated starch was found to be more effective in stabilizing \(\alpha\)-lipoic acid than native starch, probably due to the amphiphilic and hydrophobic nature of the substituents in the modified starch. Moreover, the stability during gastric and intestinal digestion in vitro and the subsequent slow release of \(\alpha\)-lipoic acid during digestion further suggest that high-amylose octenylsucccinylated starch is a possible digestive carrier of controlled-release \(\alpha\)-lipoic acid. The photochemical stability of \(\alpha\)-lipoic acid under the influence of natural daylight at room temperature was investigated as part of an aqueous dispersion of a nanostructured lipid carrier (NLC), which is a known effective delivery system for drugs and other biologically active substances. Encapsulation in NLCs was found to be an effective process that retained 88.5% of \(\alpha\)-lipoic acid after 120 days of exposure to daylight, while unencapsulated acid degraded by 99% during this time. It was shown that the photodegradation of \(\alpha\)-lipoicacid occurs due to the breaking of the S-S bond of the 1,2-dithiolane ring in the molecule, which is accompanied by an unpleasant odor [13]. Halloysite nanotubes are one of the best-known transport systems for the delivery of organic substances. It is a natural material with a special tubular nanostructure (HNTs) with the chemical formula Al\({}_{2}\)Si\({}_{2}\)O\({}_{5}\) (OH)\({}_{4}\), which makes it an inexpensive and valuable alternative to the most common carbon nanotubes. The length of nanotubes is mainly from 200 nm to 2 \(\upmu\)m, their inner diameter is 15-50 nm, and the outer diameter is 50-200 nm [14; 15]. One of the main limitations of this nanomaterial is its wide polydispersion in size, and therefore the separation and purification procedures are challenging [16; 17]. However, its unique structure can be used to modify the surface with organic molecules (for example, polymers, biopolymers, or surfactants) using both electrostatic and van der Waals interactions [18]. In addition, the outer and inner surfaces have opposite charges due to different chemical compositions (Al-OH and Si-O-Si groups, respectively). Thus, the inner (Al) surface is positively charged and the outer (Si) surface is negatively charged, and targeted electrostatic interactions are envisaged [19; 20]. Based on their structure, halloysite nanotubes can be loaded with negatively charged substances, such as biologically active molecules, antioxidants, etc. Biologically active substances loaded inside the inner lumen of nanotubes can receive additional protection from external factors [21; 22; 23; 24]. This is a very important characteristic that ensures the creation of new smart nanocontainers for loading, storage, and long-term release of chemical agents of various effects. Equally important is the fact that halloysite, being completely non-toxic, creates additional opportunities for the wide application of nanocomposites based on it with the use of active ingredients [25] and for the delivery and controlled release of pharmaceuticals [25; 26; 27; 28]. The biocompatibility of halloysite nanotubes has been tested on different living organisms, providing low toxicity even for oral administration [29; 30; 31]. Besides medical applications, including tissue engineering, cosmetic formulations for hair treatments have been recently reported [32; 33; 34].