**4. Conclusions**

*Current and Future Aspects of Nanomedicine*

collagen [154], or in combination with silk fibroin [70].

changing the ratio of surface area per mass of the scaffolds [159].

**Zein** is the major storage protein of corn, composed of amino acids such as leucine, glutamic acid, alanine and proline, and showing good biocompatibility, flexibility, microbial resistance, and antioxidant activity [61]. Zein has been shown to degradable *in vitro* in PBS and also *in vivo* when implanted subcutaneously in rats in the form of rod-like implants [160]. However, similarly as in many other natural polymers, the application of pure zein nanofibers is limited because of poor mechanical properties of these fibers. Therefore, for skin tissue engineering and wound dressing applications, zein has been mixed with various synthetic and nature-derived polymers, such as polyurethane [161], PLA [162], PCL, hyaluronic acid, chitosan [163], and polydopamine [164], and impregnated with TiO2 nanoparticles [164] or Ag nanoparticles [161] in order to enhance the antimicrobial activity

**Poly(3-hydroxybutyrate-co-3-hydroxyvalerate)** (PHBV), is a polymer produced naturally by bacteria as a storage compound under growth-limiting conditions. It is a thermoplastic linear aliphatic polyester of polyhydroxyalkanoate type. PHBV is approved by the FDA for medical use. PHBV is biodegradable by bacterial enzymes, but it is also susceptible to hydrolytic degradation in water environment, although this degradation is relatively slow. When degradation of porous PHBV scaffolds for tissue engineering was simulated *in vitro* in PBS at 37°C, it lasted several months [165]. In the human body, however, the degradation of PHBV can be accelerated by nonspecific esterase and lysozyme enzymes, both present in cells of the immune system (for a review, see [165]). For biomedical application, PHBV is often used as an alternative to synthetic polymers, but it has several drawbacks, such as relatively high cost, brittleness, relatively difficult processing, and also hydrophobicity, which can hamper the cell adhesion and growth. However, PHBV is piezoelectric, which can stimulate the adhesion, growth, and phenotypic maturation of cells. Pure electrospun PHBV meshes supported the adhesion, growth, and epidermal differentiation of bone marrow mesenchymal stem cells, which was induced by an appropriate composition of cell culture media, containing epidermal

Nanofibrous scaffolds promising for skin tissue engineering and wound healing were also prepared directly from chitin, which was electrospun either alone with further modifications with fibronectin, laminin and particularly with type I

**Gellan gum** is a water-soluble anionic polysaccharide produced by the bacterium *Sphingomonas elodea* (formerly *Pseudomonas elodea*). The repeating unit of the polymer is a tetrasaccharide, which consists of two residues of D-glucose, one of residue of L-rhamnose and one residue of D-glucuronic acid. For skin tissue engineering and wound healing, gellan gum was electrospun with PVA in order to decrease its viscosity and repulsive forces between the polyanions along the polymer chains and to increase the stability, uniformity, and structural consistency of the nanofibers in aqueous environment. The nanofibrous scaffolds were further stabilized by crosslinking with various physical, chemical, and ionic methods, such as by heat, UV irradiation, methanol, glutaraldehyde, and by calcium chloride [155]. These scaffolds supported the adhesion and growth of human dermal 3T3-L1 fibroblasts [155, 156] and human HaCaT keratinocytes [157] and provided a better support for these cells than conventionally proposed gellan-based hydrogels and dry films. In addition, these scaffolds were endowed with antimicrobial activity by incorporation with amoxicillin, and accelerated healing of full-thickness skin excision wound in rats in comparison with non-treated wounds [157]. Similarly as chitosan, gellan gum has been reported to be degradable by lysozyme [158]. Three-dimensional printed gellan gum scaffolds also showed degradation *in vitro* in phosphate-buffered saline (PBS) or in simulated body fluid, and the degradation rate could be modulated by

**82**

of the scaffolds.

Nanofibrous scaffolds made of nature-derived polymers hold a great promise for skin tissue engineering and wound healing. These scaffolds are created from biological matrices, and from this point of view, they resemble the extracellular matrix more closely than synthetic polymers. Some of these polymers, such as collagen, gelatin, elastin, keratin, nonsulfated and sulfated glycosaminoglycans, and also nonmulberry silk fibroin, contain motifs that are recognized and bound by cell adhesion receptors. Therefore, nature-derived polymers can increase the bioactivity of synthetic polymers, when combined with them in nanofibrous scaffolds. Conversely, synthetic polymers can improve the electrospinnability and mechanical properties of the natural polymers. Similarly as synthetic polymers, nature-derived polymers can be more or less degradable in human tissues. Degradable polymers include collagen, gelatin, elastin, keratin, glycosaminoglycans, but also chitosan, gellan gum and PHBV, that is, polymers produced by other than mammalian organisms. Polymers produced by other organisms, such as bacteria, fungi, algae, plants or insects, are usually nondegradable in human tissues, or their degradability is limited due to lack of appropriate enzymes. These polymers include glucans, such as cellulose or dextran, and other polysaccharides and proteins, such as pullulan, alginate, pectin, and silk fibroin. Well-degradable polymers are recommended as direct scaffolds for tissue engineering, while less-degradable polymers are suitable for "intelligent" wound dressing for drug delivery and cell delivery.

#### **Acknowledgements**

This review article was supported by the Grant Agency of the Czech Republic (grants No. 17-02448S and 17-00885S).

*Current and Future Aspects of Nanomedicine*
