**1. Introduction**

The skin is the largest organ of the human body with several vitally important functions. The skin acts as barrier against adverse effects of the surrounding

environment on the organism, such as chemical factors, radiation factors, particularly ultraviolet light, and microbial infection. Other important functions of skin include thermoregulation, sensation of temperature, touch, pressure, and pain, keeping appropriate moisture in the underlying tissues, excretion of ions, water, and various biomolecules (e.g., lipids and proteins), and also production and storage of various biomolecules, such as pigments, vitamin D, and keratins for formation of epidermal appendages (for a review, see [1, 2]). Skin severely and chronically damaged by trauma, burns, bedsores, and by various diseases, e.g., diabetes, cannot exert these functions, which can lead to amputation and even death. Therefore, there is essential need to regenerate the damaged skin, particularly by methods of skin tissue engineering and induction of active wound healing. For these purposes, nanofibrous scaffolds seem to be one of the most promising materials. Nanomaterials in general are defined as features not exceeding 100 nm at least in one dimension, i.e., in diameter in case of nanofibers. However, nanofibers usually used in tissue engineering are often thicker (i.e., several hundreds of nm). In fact, they are submicron-scale fibers, but the term "nanofibers" has become widely used for them. Nanofibers can be obtained by various techniques, such as biological synthesis (e.g., nanocellulose produced by bacteria), self-assembly, phase separation, interfacial polymerization, suitable for electrically conductive materials, melt processing or antisolvent precipitation, and particularly by electrospinning, which has emerged as a relatively simple, elegant, scalable, and efficient technique for fabrication of polymeric nanofibers (for a review, see [2–5]).

The advantage of nanofibrous scaffolds is that they mimic the fibrous component of the natural extracellular matrix (ECM), and therefore they can serve as ECM analogues for tissue engineering. In addition, nanofibrous meshes can act as a protective barrier against penetration of microbes into wounds, can keep the moisture in the damaged skin, and, at the same time, allow gas exchange and absorb the exudate from the wounds. These meshes can also be loaded with various bioactive molecules, such as growth and angiogenic factors, cytokines, hormones, vitamins, antioxidants, antimicrobial and antitumor agents, amino acids (l-arginine), wound healing peptides (e.g., melanocyte-inhibiting factor), and with antimicrobial peptides [6–8]; for a review, see [1, 2]. Therefore, nanofibers can serve not only as tissue engineering scaffolds for skin cells but also as carriers for controlled drug delivery into skin.

The nanofibrous scaffolds for skin tissue engineering and wound healing have been prepared from a wide range of synthetic and nature-derived polymers. Both these groups of polymers contain polymers relatively easily degradable in the human organism, and polymers which are non-degradable or slowly degradable. This review is focused on synthetic polymers, which have been used for creation of nanofibrous scaffolds for skin tissue engineering and wound healing applications. Typical and widely used degradable synthetic polymers include polylactides [9, 10] and their copolymers with polyglycolides [11], or polycaprolactone [6, 12] and its copolymers with polylactides [13]. Examples of biostable synthetic polymers are polyurethane [7], polydimethylsiloxane [14], polyethylene terephthalate [15] or polyethersulfone [16]. Polymers degradable in the human body are suitable as direct scaffolds for skin tissue engineering, while biostable polymers can be rather recommended as "intelligent" wound dressings delivering cells (keratinocytes, fibroblasts or stem cells) and bioactive molecules into wounds.

However, polymers in nanofibrous scaffolds are predominantly used in various combinations—synthetic with natural, degradable with non-degradable—and also in combination with various nanoparticles, e.g., mineral nanoparticles [17], carbonbased nanoparticles [1, 18] or metal-based nanoparticles [19, 20]. The reason of these combinations is to improve the stability, spinnability, wettability, mechanical

**35**

**Figure 1.**

*Nanofibrous Scaffolds for Skin Tissue Engineering and Wound Healing Based on Synthetic…*

properties, and bioactivity of nanofibers. The combination of various polymers, nanoparticles, and other components is a strategy commonly used to obtain hybrid materials possessing properties better than those of the individual constituents, regarding their use in scaffolds for tissue engineering or as material for wound dressing [21]. For example, synthetic polymers do not contain adhesion motifs recognizable by cell adhesion receptors, and combination with nature-derived polymers, which are proteins (collagen, gelatin, keratin, fibrin [6, 10, 22, 23]) or polysaccharides (hyaluronic acid, sulfated glycosaminoglycans, such as heparin [24, 25]) can endow them with these motifs, because these polymers are often components of ECM.

In electrospun nanofibrous meshes, the polymers can be combined by various approaches. In blending electrospinning, the polymers are mixed, filled in the same syringe, and electrospun together, which results in creation of fibers with two or more components randomly distributed within a fiber. In multi-jet electrospinning, each polymer is filled in a separate syringe and electrospun individually, which results in creation of meshes with two or more types of nanofibers. These types of nanofibers can be electrospun either concurrently and distributed randomly (i.e., mixing electrospinning) or alternatively and arranged into separate layers (i.e., multilayering electrospinning). In co-axial electrospinning, hybrid nanofibers with a core-shell architecture are created by spinning of two different solutions filled into outer and inner compartments of a co-axial syringe. Finally, an electrospun polymer can be secondarily coated with other polymers or bioactive substances [22,

Nanofibrous meshes for skin tissue engineering can also be combined with other material types, such as porous 3D scaffolds or hydrogels in order to reconstruct two main skin layers, i.e., epidermis containing keratinocytes and dermis containing fibroblasts [23, 29–31]. Another advanced approach promising for construction of dermo-epidermal replacements is centrifugal jet spinning, capable of large-scale

*DOI: http://dx.doi.org/10.5772/intechopen.88744*

26–28] (**Figure 1**).

production of nanofibrous 3D scaffolds [32, 33].

*Modes of combination of various polymers and compounds in nanofibrous scaffolds.*

### *Nanofibrous Scaffolds for Skin Tissue Engineering and Wound Healing Based on Synthetic… DOI: http://dx.doi.org/10.5772/intechopen.88744*

properties, and bioactivity of nanofibers. The combination of various polymers, nanoparticles, and other components is a strategy commonly used to obtain hybrid materials possessing properties better than those of the individual constituents, regarding their use in scaffolds for tissue engineering or as material for wound dressing [21]. For example, synthetic polymers do not contain adhesion motifs recognizable by cell adhesion receptors, and combination with nature-derived polymers, which are proteins (collagen, gelatin, keratin, fibrin [6, 10, 22, 23]) or polysaccharides (hyaluronic acid, sulfated glycosaminoglycans, such as heparin [24, 25]) can endow them with these motifs, because these polymers are often components of ECM.

In electrospun nanofibrous meshes, the polymers can be combined by various approaches. In blending electrospinning, the polymers are mixed, filled in the same syringe, and electrospun together, which results in creation of fibers with two or more components randomly distributed within a fiber. In multi-jet electrospinning, each polymer is filled in a separate syringe and electrospun individually, which results in creation of meshes with two or more types of nanofibers. These types of nanofibers can be electrospun either concurrently and distributed randomly (i.e., mixing electrospinning) or alternatively and arranged into separate layers (i.e., multilayering electrospinning). In co-axial electrospinning, hybrid nanofibers with a core-shell architecture are created by spinning of two different solutions filled into outer and inner compartments of a co-axial syringe. Finally, an electrospun polymer can be secondarily coated with other polymers or bioactive substances [22, 26–28] (**Figure 1**).

Nanofibrous meshes for skin tissue engineering can also be combined with other material types, such as porous 3D scaffolds or hydrogels in order to reconstruct two main skin layers, i.e., epidermis containing keratinocytes and dermis containing fibroblasts [23, 29–31]. Another advanced approach promising for construction of dermo-epidermal replacements is centrifugal jet spinning, capable of large-scale production of nanofibrous 3D scaffolds [32, 33].

**Figure 1.** *Modes of combination of various polymers and compounds in nanofibrous scaffolds.*

*Applications of Nanobiotechnology*

environment on the organism, such as chemical factors, radiation factors, particularly ultraviolet light, and microbial infection. Other important functions of skin include thermoregulation, sensation of temperature, touch, pressure, and pain, keeping appropriate moisture in the underlying tissues, excretion of ions, water, and various biomolecules (e.g., lipids and proteins), and also production and storage of various biomolecules, such as pigments, vitamin D, and keratins for formation of epidermal appendages (for a review, see [1, 2]). Skin severely and chronically damaged by trauma, burns, bedsores, and by various diseases, e.g., diabetes, cannot exert these functions, which can lead to amputation and even death. Therefore, there is essential need to regenerate the damaged skin, particularly by methods of skin tissue engineering and induction of active wound healing. For these purposes, nanofibrous scaffolds seem to be one of the most promising materials. Nanomaterials in general are defined as features not exceeding 100 nm at least in one dimension, i.e., in diameter in case of nanofibers. However, nanofibers usually used in tissue engineering are often thicker (i.e., several hundreds of nm). In fact, they are submicron-scale fibers, but the term "nanofibers" has become widely used for them. Nanofibers can be obtained by various techniques, such as biological synthesis (e.g., nanocellulose produced by bacteria), self-assembly, phase separation, interfacial polymerization, suitable for electrically conductive materials, melt processing or antisolvent precipitation, and particularly by electrospinning, which has emerged as a relatively simple, elegant, scalable, and efficient technique for

fabrication of polymeric nanofibers (for a review, see [2–5]).

or stem cells) and bioactive molecules into wounds.

The advantage of nanofibrous scaffolds is that they mimic the fibrous component of the natural extracellular matrix (ECM), and therefore they can serve as ECM analogues for tissue engineering. In addition, nanofibrous meshes can act as a protective barrier against penetration of microbes into wounds, can keep the moisture in the damaged skin, and, at the same time, allow gas exchange and absorb the exudate from the wounds. These meshes can also be loaded with various bioactive molecules, such as growth and angiogenic factors, cytokines, hormones, vitamins, antioxidants, antimicrobial and antitumor agents, amino acids (l-arginine), wound healing peptides (e.g., melanocyte-inhibiting factor), and with antimicrobial peptides [6–8]; for a review, see [1, 2]. Therefore, nanofibers can serve not only as tissue engineering scaffolds for skin cells but also as carriers for controlled drug

The nanofibrous scaffolds for skin tissue engineering and wound healing have been prepared from a wide range of synthetic and nature-derived polymers. Both these groups of polymers contain polymers relatively easily degradable in the human organism, and polymers which are non-degradable or slowly degradable. This review is focused on synthetic polymers, which have been used for creation of nanofibrous scaffolds for skin tissue engineering and wound healing applications. Typical and widely used degradable synthetic polymers include polylactides [9, 10] and their copolymers with polyglycolides [11], or polycaprolactone [6, 12] and its copolymers with polylactides [13]. Examples of biostable synthetic polymers are polyurethane [7], polydimethylsiloxane [14], polyethylene terephthalate [15] or polyethersulfone [16]. Polymers degradable in the human body are suitable as direct scaffolds for skin tissue engineering, while biostable polymers can be rather recommended as "intelligent" wound dressings delivering cells (keratinocytes, fibroblasts

However, polymers in nanofibrous scaffolds are predominantly used in various combinations—synthetic with natural, degradable with non-degradable—and also in combination with various nanoparticles, e.g., mineral nanoparticles [17], carbonbased nanoparticles [1, 18] or metal-based nanoparticles [19, 20]. The reason of these combinations is to improve the stability, spinnability, wettability, mechanical

**34**

delivery into skin.

In the waste majority of studies dealing with skin tissue engineering based on nanofibrous scaffolds, keratinocytes have been cultivated in a conventional static cell culture system, submerged into the culture media, although under physiological condition *in vivo*, keratinocytes are exposed to air. Therefore, in advanced skin tissue engineering, it is necessary to cultivate keratinocytes under appropriate mechanical loading, i.e., strain stress [34] or pressure stress [35], and simultaneously to cultivate them on the scaffolds exposed to the air-liquid interface [34, 36].

This chapter summarizes earlier and recent knowledge on skin tissue engineering and wound dressing applications, based on nanofibrous scaffolds made of synthetic non-degradable and degradable polymers, including our results.

### **2. Nanofibers from synthetic non-degradable polymers**

Synthetic non-degradable polymers, explored for creation of electrospun nanofibrous meshes for skin regenerative therapies, included polyurethane, polydimethylsiloxane, polyethylene terephthalate, polyethersulfone, and even polystyrene. This group of polymers also includes non-degradable hydrogels, such as poly(acrylic acid), poly(methyl methacrylate), and poly(di(ethylene glycol) methyl ether methacrylate).

Polyurethane (PU) has been most frequently used from the mentioned polymers, which is due to its elasticity, and also possibility to prepare it in a degradable form, e.g., as poly(ester-urethane) urea (PEUU), which facilitates its applicability in skin tissue engineering [37], while the non-degradable forms of PU (and other non-degradable polymers in general) are rather used in wound dressing applications. Non-degradable PU nanofibrous meshes has been tested as advanced wound dressings loaded with various healing, angiogenic, anti-inflammatory, antioxidant, and antimicrobial substances. For example, blending PU with propolis improved the mechanical strength and hydrophilicity of the nanofibrous membrane, its cytocompatibility with fibroblasts and its antibacterial activity [38]. Blending PU with virgin olive oil endowed the nanofibrous meshes with photoprotective and antioxidant properties [19]. Dextran in composite PU/dextran fibers had angiogenic activity, and also served as a carrier for incorporation of β-estradiol, which accelerated healing of acute cutaneous wounds by its potent anti-inflammatory activity [7]. Another promising nanofibrous membrane applicable for wound dressing was prepared from electrospun PU, treated by plasma and subsequent spraying with chitosan solution containing an inclusion complex of β-cyclodextrin encapsulating berberine, i.e., an isoquinoline alkaloid with antimicrobial and anti-inflammatory activity [39]. Other antimicrobial substances incorporated into PU-based nanofibers included silver nanoparticles [20, 40], copper oxide nanocrystals [19] and antibiotics, such as silver-sulfadiazine [41] and amoxicillin [37]. In general, all these scaffolds showed none or low toxicity towards human HaCaT keratinocytes [20] or fibroblasts [19, 40], and no adverse reactions when implanted into laboratory animals *in vivo* [37, 41]. In addition, copper oxide is known by its angiogenic activity [19]. Nanofibrous meshes created by electrospinning from blends of PU with various concentrations of hydroxypropyl cellulose were also tested for transdermal drug delivery using donepezil hydrochloride, i.e., a drug used for treatment of Alzheimer disease [42]. PU was a component of a novel bilayer wound dressing, consisting of a commercial PU membrane as an outer layer, and an electrospun gelatin/keratin nanofibrous mat as an inner layer. The outer layer acted as a barrier against bacteria and other contaminants, while the inner layers promoted the adhesion, spreading, migration and growth of fibroblasts *in vitro*, and vascularization and wound healing in rats *in vivo* [23].

**37**

*Nanofibrous Scaffolds for Skin Tissue Engineering and Wound Healing Based on Synthetic…*

Polydimethylsiloxane (PDMS) in electrospun nanofibrous scaffolds was used in blends with thermoplastic PU (TPU) in 90:10, 80:20, and 70:30 blend ratios of TPU and PDMS. The activity of mitochondrial enzymes and proliferation of human dermal fibroblasts significantly increased with the percentage of PDMS in the scaf-

Polyethylene terephthalate (PET) in combination with honey improved the morphology of chitosan-containing fibers, decreased the diameter of electrospun fibers, increased the fiber deposition area in the collector, and increased the water uptake capacities of the material, which is important for exudating wounds [15]. In another study, low-molecular weight cationic compounds were synthesized from re-purposed PET and used for self-assembling into high aspect ratio supramolecular nanofibers for encapsulation and delivery of piperacillin/tazobactam (PT), an anionic antibiotic. In a *Pseudomonas aeruginosa*-infected mouse skin wound model, the treatment with the PT-loaded nanofibers was more effective in comparison with free PT, as evidenced by significantly lower counts of *P. aeruginosa* at the wound

Polyethersulfone (PES) nanofibrous membranes, made by fine tuning of electrospinning parameters, supported the proliferation of fibroblasts similarly as standard tissue culture polystyrene, and when applied as experimental wound dressings in mice, they showed a higher exudate absorption capacity, higher epithelial regeneration, greater fibroblast maturation, improved collagen deposition, and faster edema resolution than control commercial wound dressings, namely Vaseline gauze dressing and a conventional gas permeable

Polystyrene (PS) in its amorphous state is a transparent and colorless material. It is a hard, stiff, and very brittle polymer with remarkable water vapor permeability, very high electrical resistance, and low dielectric loss. For wound dressing applications, PS was electrospun with poly(ɛ-caprolactone) and chamomile extract, containing phenolics and flavonoids, particularly apigenin with remarkable wound healing effect [44]. Electrospun polystyrene nanofibrous scaffolds were also applied for cultivation of skin cells in dynamic bioreactors and at the air/

Poly(acrylic acid) (PAA) was recently used for preparation of nanofibers incorporated with reduced graphene oxide, intended for delivery of antibiotics, which was controlled by photothermal activation of the nanofibers [18]. In another study, electrospun nanofibers consisting of PAA and poly(1,8-octanediol-*co*-citric acid), i.e., a synthetic biodegradable elastomer, showed intrinsic antibacterial activity and were used for topical delivery of physiologically relevant concentrations of growth

Poly(methyl methacrylate) (PMMA) nanofibers were used for construction of antiscarring wound dressings. PMMA containing polyethylene glycol and kynurenic acid, an antifibrotic agent, suppressed proliferation of fibroblasts *in vitro*, and when administered as wound dressing in rats *in vivo*, they inhibited expression of collagen and fibronectin, and enhanced the production of matrix metalloprotease 1 (MMP-1), an ECM-degrading enzyme [48]. In addition, core-shell nanofibers containing PVA and PMMA were used for delivery of ciprofloxacin hydrochloride,

Poly(di(ethylene glycol) methyl ether methacrylate) (PDEGMA), a thermoresponsive polymer, was blended with poly(l-lactic acid-*co*-ε-caprolactone), P(LLA-CL), and used for construction of nanofibrous carriers for controlled drug delivery, namely for delivery of ciprofloxacin. These fibers also supported the growth of fibroblasts, and by decreasing the temperature, they enabled the cell

*DOI: http://dx.doi.org/10.5772/intechopen.88744*

sites, and by a histological analysis [43].

folds [14].

bandage [16].

factors [47].

an antibiotic [49].

detachment and delivery into wounds [50].

liquid interface [45, 46].

#### *Nanofibrous Scaffolds for Skin Tissue Engineering and Wound Healing Based on Synthetic… DOI: http://dx.doi.org/10.5772/intechopen.88744*

Polydimethylsiloxane (PDMS) in electrospun nanofibrous scaffolds was used in blends with thermoplastic PU (TPU) in 90:10, 80:20, and 70:30 blend ratios of TPU and PDMS. The activity of mitochondrial enzymes and proliferation of human dermal fibroblasts significantly increased with the percentage of PDMS in the scaffolds [14].

Polyethylene terephthalate (PET) in combination with honey improved the morphology of chitosan-containing fibers, decreased the diameter of electrospun fibers, increased the fiber deposition area in the collector, and increased the water uptake capacities of the material, which is important for exudating wounds [15]. In another study, low-molecular weight cationic compounds were synthesized from re-purposed PET and used for self-assembling into high aspect ratio supramolecular nanofibers for encapsulation and delivery of piperacillin/tazobactam (PT), an anionic antibiotic. In a *Pseudomonas aeruginosa*-infected mouse skin wound model, the treatment with the PT-loaded nanofibers was more effective in comparison with free PT, as evidenced by significantly lower counts of *P. aeruginosa* at the wound sites, and by a histological analysis [43].

Polyethersulfone (PES) nanofibrous membranes, made by fine tuning of electrospinning parameters, supported the proliferation of fibroblasts similarly as standard tissue culture polystyrene, and when applied as experimental wound dressings in mice, they showed a higher exudate absorption capacity, higher epithelial regeneration, greater fibroblast maturation, improved collagen deposition, and faster edema resolution than control commercial wound dressings, namely Vaseline gauze dressing and a conventional gas permeable bandage [16].

Polystyrene (PS) in its amorphous state is a transparent and colorless material. It is a hard, stiff, and very brittle polymer with remarkable water vapor permeability, very high electrical resistance, and low dielectric loss. For wound dressing applications, PS was electrospun with poly(ɛ-caprolactone) and chamomile extract, containing phenolics and flavonoids, particularly apigenin with remarkable wound healing effect [44]. Electrospun polystyrene nanofibrous scaffolds were also applied for cultivation of skin cells in dynamic bioreactors and at the air/ liquid interface [45, 46].

Poly(acrylic acid) (PAA) was recently used for preparation of nanofibers incorporated with reduced graphene oxide, intended for delivery of antibiotics, which was controlled by photothermal activation of the nanofibers [18]. In another study, electrospun nanofibers consisting of PAA and poly(1,8-octanediol-*co*-citric acid), i.e., a synthetic biodegradable elastomer, showed intrinsic antibacterial activity and were used for topical delivery of physiologically relevant concentrations of growth factors [47].

Poly(methyl methacrylate) (PMMA) nanofibers were used for construction of antiscarring wound dressings. PMMA containing polyethylene glycol and kynurenic acid, an antifibrotic agent, suppressed proliferation of fibroblasts *in vitro*, and when administered as wound dressing in rats *in vivo*, they inhibited expression of collagen and fibronectin, and enhanced the production of matrix metalloprotease 1 (MMP-1), an ECM-degrading enzyme [48]. In addition, core-shell nanofibers containing PVA and PMMA were used for delivery of ciprofloxacin hydrochloride, an antibiotic [49].

Poly(di(ethylene glycol) methyl ether methacrylate) (PDEGMA), a thermoresponsive polymer, was blended with poly(l-lactic acid-*co*-ε-caprolactone), P(LLA-CL), and used for construction of nanofibrous carriers for controlled drug delivery, namely for delivery of ciprofloxacin. These fibers also supported the growth of fibroblasts, and by decreasing the temperature, they enabled the cell detachment and delivery into wounds [50].

*Applications of Nanobiotechnology*

methyl ether methacrylate).

In the waste majority of studies dealing with skin tissue engineering based on nanofibrous scaffolds, keratinocytes have been cultivated in a conventional static cell culture system, submerged into the culture media, although under physiological condition *in vivo*, keratinocytes are exposed to air. Therefore, in advanced skin tissue engineering, it is necessary to cultivate keratinocytes under appropriate mechanical loading, i.e., strain stress [34] or pressure stress [35], and simultaneously to cultivate them on the scaffolds exposed to the air-liquid interface [34, 36]. This chapter summarizes earlier and recent knowledge on skin tissue engineering and wound dressing applications, based on nanofibrous scaffolds made of synthetic non-degradable and degradable polymers, including our results.

**2. Nanofibers from synthetic non-degradable polymers**

Synthetic non-degradable polymers, explored for creation of electrospun nanofibrous meshes for skin regenerative therapies, included polyurethane, polydimethylsiloxane, polyethylene terephthalate, polyethersulfone, and even polystyrene. This group of polymers also includes non-degradable hydrogels, such as poly(acrylic acid), poly(methyl methacrylate), and poly(di(ethylene glycol)

Polyurethane (PU) has been most frequently used from the mentioned polymers, which is due to its elasticity, and also possibility to prepare it in a degradable form, e.g., as poly(ester-urethane) urea (PEUU), which facilitates its applicability in skin tissue engineering [37], while the non-degradable forms of PU (and other non-degradable polymers in general) are rather used in wound dressing applications. Non-degradable PU nanofibrous meshes has been tested as advanced wound dressings loaded with various healing, angiogenic, anti-inflammatory, antioxidant, and antimicrobial substances. For example, blending PU with propolis improved the mechanical strength and hydrophilicity of the nanofibrous membrane, its cytocompatibility with fibroblasts and its antibacterial activity [38]. Blending PU with virgin olive oil endowed the nanofibrous meshes with photoprotective and antioxidant properties [19]. Dextran in composite PU/dextran fibers had angiogenic activity, and also served as a carrier for incorporation of β-estradiol, which accelerated healing of acute cutaneous wounds by its potent anti-inflammatory activity [7]. Another promising nanofibrous membrane applicable for wound dressing was prepared from electrospun PU, treated by plasma and subsequent spraying with chitosan solution containing an inclusion complex of β-cyclodextrin encapsulating berberine, i.e., an isoquinoline alkaloid with antimicrobial and anti-inflammatory activity [39]. Other antimicrobial substances incorporated into PU-based nanofibers included silver nanoparticles [20, 40], copper oxide nanocrystals [19] and antibiotics, such as silver-sulfadiazine [41] and amoxicillin [37]. In general, all these scaffolds showed none or low toxicity towards human HaCaT keratinocytes [20] or fibroblasts [19, 40], and no adverse reactions when implanted into laboratory animals *in vivo* [37, 41]. In addition, copper oxide is known by its angiogenic activity [19]. Nanofibrous meshes created by electrospinning from blends of PU with various concentrations of hydroxypropyl cellulose were also tested for transdermal drug delivery using donepezil hydrochloride, i.e., a drug used for treatment of Alzheimer disease [42]. PU was a component of a novel bilayer wound dressing, consisting of a commercial PU membrane as an outer layer, and an electrospun gelatin/keratin nanofibrous mat as an inner layer. The outer layer acted as a barrier against bacteria and other contaminants, while the inner layers promoted the adhesion, spreading, migration and growth of fibroblasts *in vitro*, and vascularization

**36**

and wound healing in rats *in vivo* [23].

Another thermoresponsive polymer, poly(N-isopropylacrylamide) (PNIPAM) was used for fabrication of nanofibers for transdermal delivery of drugs, namely levothyroxine (T4), which helps to reduce deposits of adipose tissue [51].
