In the present work, transparent holographic poly(diallyldimethylammonium chloride) (PDADMAC)/heparin and PDADMAC/poly(styrenesulfonate) (PSS) films were synthesized via polyelectrolyte coacervates. PDADMAC/heparin films were obtained without temperature treatment. Thin holographic free-standing films with a 1 μm grating period and uniform surface of a polyelectrolyte complex were readily and quickly made by pressing polyelectrolyte coacervate, the hydrated viscoelastic fluid-like form of polyelectrolyte complex precursor, between a flat surface and holographic mask. Heparin replaces PSS in film composition to prepare the sheer film. Thus, the PDADMAC/heparin holographic film demonstrates transparency and reversible response for humidity under diffraction detection. In addition to diffraction humidity signal measurements, the cobalt(II) chloride was impregnated in polyelectrolyte coacervate to make an additional colorimetric signal response. In this case, the free-standing film serves both as the substrate for the hygroscopic salt and as a diffraction humidity sensor. The PDADMAC/heparin/Co(II) chloride film demonstrates a linear humidity range from 50 to 90%. Additionally, due to hydrated inorganic salt ion size, cobalt chloride prevents film porosity, which initiates under film swelling. Based on the results and calculations obtained, the study proposes the mechanism of water incorporation, including the reptation model and polyelectrolyte complex behavior. Results of density functional theory calculations prove that binding of cobalt aqua complexes [Co(H2O)6]2+ with the dimeric associates heparin/PDADMAC via noncovalent interactions (hydrogen bonds) additionally is much more energetically favorable compared with the alternative association of heparin/PDADMAC with water molecules.
A large number of different skin diseases such as hits, acute, and chronic wounds dictate the search for alternative and effective treatment options. The wound healing process requires a complex approach, the key step of which is the choice of a dressing with controlled properties. Hydrogel-based scaffolds can serve as a unique class of wound dressings. Presented on the commercial market, hydrogel wound dressings are not found among proposals for specific cases and have a number of disadvantages—toxicity, allergenicity, and mechanical instability. Bilayer dressings are attracting great attention, which can be combined with multifunctional properties, high criteria for an ideal wound dressing (antimicrobial properties, adhesion and hemostasis, anti-inflammatory and antioxidant effects), drug delivery, self-healing, stimulus manifestation, and conductivity, depending on the preparation and purpose. In addition, advances in stem cell biology and biomaterials have enabled the design of hydrogel materials for skin tissue engineering. To improve the heterogeneity of the cell environment, it is possible to use two-layer functional gradient hydrogels. This review summarizes the methods and application advantages of bilayer dressings in wound treatment and skin tissue regeneration. Bilayered hydrogels based on natural as well as synthetic polymers are presented. The results of the in vitro and in vivo experiments and drug release are also discussed.
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