## 1 Introduction
Membranes are obtained by modification in volume, for example, by deposition from multicomponent solutions, which is an effective and technologically simple method. This method was used to obtain membranes based on polyamide-6 (PA-6) by modifying them with polyvinylpyrrolidone (PVP). The polymers were mixed in a formic acid solution, followed by evaporation of the solvent [23]. However, this method is technologically limited in the case of obtaining membranes based on hydrogel materials. At the same time, the method of obtaining film coatings by deposition from multicomponent polymer solutions can be used to modify hydrogel membranes with thin heterolayers. The deposition method of modifying layers from solutions ensures the obtainment of thin films of uniform thickness, with the predicted structure and properties, on hydrogel membranes. This method also allows for changing the nature and formulation of the reinforcing layer within wide limits, while providing composite membranes with additional special properties. During the preparation of composite two-layer membranes by the method of deposition of a modifying layer on a hydrogel membrane-substrate, great attention must be paid to the selection of the formulation of the modifying solution and the structure of the hydrogel substrate to ensure good adhesion between them. The connection of the modifying layer with the hydrogel substrate is carried out, mainly, as a result of the solution flowing into the pores of the substrate and its hardening in the form of \"hooks\". The currently existing assortment of hydrogel materials ensures the production of a huge number of different types of membranes based on them, which, depending on the required set of properties, are used in various fields of science and practice [14]. The set of properties of hydrogel membranes depends on the nature and porous structure of the polymer matrix. Hydrogels based on copolymers of 2-hydroxyethyl methacrylate (HEMA) and PVP are currently widely used in science and practice among hydrogel materials [31; 32]. These hydrogels are considered promising materials for the production of medical bandages [33], contact lenses [34], vascular prostheses [35], materials for the regeneration of damaged tissues [36], enzyme immobilization systems [37], drug delivery systems [38], etc. Due to their unique properties, in particular, the ability to swell, high permeability to aqueous and other solutions, and biotolerance, hydrogel materials based on grafted copolymers of polyHEMA with PVP (pHEMA-gr-PVP) are effectively used for the manufacture of membrane devices, in particular for medical purposes [18; 39]. However, a significant drawback that sharply limits the use of such membranes is their low mechanical strength. Therefore, the search for reinforcing materials, as well as the development of effective methods of modifying hydrogel membranes based on pHEMA-gr-PVP copolymers, remains relevant nowadays. In addition to increasing the physico-mechanical properties, the reinforcing material should provide the selective and penetrating characteristics of the hydrogel membrane. Solutions based on aliphatic polyamides with PVP are promising for modifying hydrogel membranes based on pHEMA-gr-PVP copolymers [23; 40]. Polyamides, due to the presence of polar amide groups in macromolecules, which cause strong intermolecular interactions involving hydrogen bonds, are characterized by a complex of valuable properties, in particular wettability, chemical resistance, elasticity, and high mechanical strength [41; 42]. PVP occupies a special place among a wide range of water-soluble polymers due to its unique properties--surface activity, high hydrophilicity, a wide range of solubility, and a pronounced tendency to complex formation [43]. In addition, PVP is widely used in membrane technologies [40; 44; 45; 46; 47]. Films formed on the basis of polyamide/PVP solutions in formic acid are characterized by hydrophilic properties and increased strength characteristics [23].