# In Situ Transformation of Electrospun Nanofibers into Nanofiber-Reinforced Hydrogels
); [EMAIL_ADDRESS] (J.C.); [EMAIL_ADDRESS] (A.-L.S. ); [EMAIL_ADDRESS] (M.J.A.v.d.P. ); [EMAIL_ADDRESS] (M.M.J. ); [EMAIL_ADDRESS] (T.R.) School of Medicine, Nazarbayev University, Nur-Sultan 010000, Kazakhstan
to hydrogel formation [10]. Research has also focused on double- and multi-network hydrogels to overcome challenges related to the low mechanical stability of hydrogels. Multi network hydrogels can, for instance, be produced by 3D printing and offer advantageous properties such as self-healing, self-assembly and shear thinning [11]. Double-network gel wound dressings are characterized by a heterogeneous structure in which a firm, strongly cross-linked layer is combined with a slightly cross-linked layer that is stretchable and ensures the integrity of the dressing even under mechanical stress [12]. A different approach to improve the mechanical properties is based on the preparation of nanofiber-reinforced hydrogels [13; 14]. Nanofiber scaffolds are often fabricated by electrospinning, which relies on applying a high voltage to a polymer solution to produce dry and flat fiber scaffolds on an oppositely charged collector [15]. Such scaffolds exhibit high porosities but small pore sizes in the low micrometer range, generally between 5 \(\upmu\)m and 50 \(\upmu\)m, depending on the polymer type, concentration and preparation conditions [16; 17]. Nanofibers have been shown to provide mechanical support for cells, facilitating cell migration across the wound bed [3; 4; 18; 19]. Therefore, a combination of these two systems is promising with respect to achieving the desired properties within a single biomaterial [13; 14]. The most commonly used strategies to obtain nanofiber-hydrogel composites include multi-step processes involving the separate fabrication of nanofiber scaffolds and hydrogels followed by their combination to form a composite material [13; 14]. In this context, nanofiber scaffolds can be used either as untreated or freeze-dried mats and can also be broken into shorter pieces by homogenization or sonication and added to a pre-cursor hydrogel solution, which is subsequently cured and transformed into a hydrogel [7; 20; 21; 22; 23; 24; 25; 26]. Another method involves electrospinningning nanofibers directly into the precursor hydrogel solution to achieve a similar effect [27; 28; 29]. However, the direct transformation of electrospun nanofibers into nanofiber-hydrogel composites, i.e., nanofiber-reinforced hydrogels, has not been described yet. Such an approach would result in a material which converts into a nanofiber-reinforced hydrogel in situ, hence not requiring any incorporation of the fibers into a hydrogel prior to its application. Another benefit of this approach would be that a potential wound dressing precisely fills the shape of the wound upon in situ transformation into a hydrogel, which may be beneficial for the treatment of deep wounds [30; 31]. Therefore, the aim of this study was to produce core-shell nanofibrons scaffolds that transform into a nanofiber-reinforced hydrogel upon contact with water. To achieve this, coaxial electrospinning was employed, where a core and a shell solution are spun simultaneously to form core-shell nanofibers [32]. Aqueous ethanol and diluted acetic acid (AA), i.e., solvents that are generally recognized as safe [33], were used for electrospinningning. The plant protein zein was selected to form the core of the fibers together with small amounts of polyethylene oxide (PEO) to facilitate electrospinning [34], whereas the animal-derived protein gelatin was used as a shell polymer. Both zein and gelatin are biocompatible and biodegradable [35; 36]. Hydrophobic zein is insoluble in water and is promising for electrospinningning drug delivery systems for tissue engineering [35]. Gelatin is a degradation product of collagen and carries arginine-glycine-aspartate (RGD) motifs that are beneficial for cell attachment and proliferation [37].