Biomimetic three-layer hierarchical scaffolds for efficient water management and cell recruitment

Highlights

• Biomimetic three-layer scaffolds are composed of electrospun film and polyurethane film.

• Radially oriented nanofiber film can affect the behavior of L929 cells.

• Multistage structure nanofiber film has unidirectional water-transport capacity.

• Porous polyurethane foam film has superior water absorption and water retention.

Abstract

Taking inspiration from the structures of roots, stems and leaves of trees in nature, a biomimetic three-layered scaffold was designed for efficient water management and cell recruitment. Using polycaprolactone (PCL) and polyacrylonitrile (PAN) as raw materials, radially oriented nanofiber films and multistage adjustable nanofiber films were prepared through electrospinning technology as the base skin-friendly layer (roots) and middle unidirectional moisture conductive material (stems), the porous polyurethane foam was integrated as the outer moisturizing layer (leaves). Among which, radially oriented nanofiber films could promote the directional migration of fibroblasts and induce cell morphological changes. For the spatially hierarchically nanofiber films, the unidirectional transport of liquid was effectively realized. While the porous polyurethane foam membrane could absorb 9 times its weight in biofluid and retain moisture for up to 10 h. As a result, the biomimetic three-layered scaffolds with different structures can promote wound epithelization and drain biofluid while avoiding wound inflammation caused by excessive biofluid, which is expected to be applied in the field of skin wounds.

Introduction

Wound healing is a highly ordered but complex physiological activity, and in the case of chronic wounds, the stimulatory effects of inflammation increase the permeability of the blood vessels’ walls, resulting in a large amount of biofluid exudation that slows down cell proliferation [1]. At the same time, maceration with biofluid can easily lead to wound aseptic infections which not only tend to prolong wound healing but also make wound care more challenging [2], [3]. According to the moist healing theory, an ideal wound dressing should conduct moisture and have good moisturizing properties so as to enable the export of wound biofluid while maintaining a moist environment [4]. Furthermore, during the repair of damaged tissues, the recruitment of cells to the affected site is critical for tissue reconstruction [5], [6], [7], [8]. Therefore, it is important to develop multifunctional dressings that prevent wound dehydration and the accumulation of biofluid while promoting cell proliferation and migration [9], [10], [11].

Electrospinning is a simple and effective technique for preparing wound scaffolds. Polycaprolactone is a biodegradable polymer while its nanofiber membrane can mimic the natural extracellular matrix (ECM) of tissues. Owing to the good promotion of adhesion, proliferation, migration and differentiation of epithelial cells, it has been widely used in tissue regeneration and wound healing [12], [13]. Therefore, in applying this technique, researchers have prepared nanofiber membranes to promote wound healing, although the process also involved selecting biocompatible materials, adding bioactive substances and drugs as well as changing the morphology and arrangement of fibers [13]. For instance, by using poly(ε-caprolactone) (PCL)-gelatin (Gel) having good biocompatibility as raw materials, Hadizadeh et al. prepared a new bifunctional wound dressing, with curcumin and surfactin extracted from plants added to effectively promote wound healing [14]. Similarly, through the application of electrospinning, Saudi et al. encapsulated the drug diclofenac sodium for up to 14 days [15], while Shi and Zhang et al. Prepared asymmetric composite nanofiber membrane with drainage function to provide a clean environment for wound healing [2], [3], [16]. Nanofibers prepared based on electrospinning technology have randomness and disorder which limit the directional arrangement of nanofibers and migration of cells [12]. By changing the electrospinning receiver, the electric field, the magnetic field as well as other spinning conditions, researchers have successfully obtained nanofiber membranes with different structures such as aligned, orthogonal and radial structures, etc., and these variations have enabled the adhesion behavior of cells on nanofibers to be explored [17], [18], [19]. For example, He et al. prepared neatly arranged hydrophilic inner fibrous membranes, while Du et al. constructed radially structured ones with a gradient effect. They both promote wound healing by altering the arrangement of fibers to promote cell adhesion, proliferation, directional growth and migration [7], [20]. Although electrospinning technology can build nanofibrous membranes having different structures and functions, the currently attainable thickness (less than 1 mm) and porosity (mainly between 55%−80%) cannot absorb too much wound biofluid, thus limiting the scope of application [21], [22].

To overcome the above limitations, researchers have largely studied the development of three-dimensional nanofibrous scaffolds to effectively increase the thickness and porosity of nanofiber membranes, and these can be achieved using gas foaming, direct electrospinning, water bath collecting device, short nanofibers assembling into 3D scaffolds, three-dimensional printing, origami and cell chip engineering as well as centrifugal electrospinning methods [23], [24], [25]. In this context, by preparing a scaffold with a three-dimensional network of connected pore structure, a porosity of up to 99.2% and a water absorption rate of more than 500% can be achieved, thus allowing the absorption of more biofluid than a two-dimensional nanofiber membrane [26], [27], [28], [29]. For example, Chen et al. obtained three-dimensional nanofibrous scaffolds with a porosity of up to 92% by gas foaming [30], while Feng et al. obtained those with an average porosity of up to 99.2% by direct electrospinning and by adjusting the ambient humidity [31]. Interestingly, the porosity and water absorption of the three-dimensional nano-scaffolds obtained by Li et al. were as high as 90% and 1600%, respectively. Due to their unique structures, the highly-porous three-dimensional nano-scaffolds can absorb more wound biofluid, thereby providing an oxygenated environment for cell growth and tissue regeneration [27]. However, the above-mentioned preparations of three-dimensional nano-scaffolds can only increase the pore size and thickness of structures as a means of increasing the absorption of wound biofluid, and as such, they do not have the ability to conduct moisture unidirectionally. At the same time, the fact that large amounts of wound biofluid may not be discharged in time or even flow back into the dressing is often overlooked, and this can easily lead to wound aseptic infections and induce inflammation.

In this study, inspired by the top-down structures of roots, stems and leaves of trees in nature, we designed three-dimensional scaffolds with different structures as well as properties such as drainage function (unidirectional water transportation), moisture retention and induced cell migration to overcome the above-mentioned issues (Fig. 1a). The scaffolds consisted of radially oriented structured nanofiber films, spatially hierarchically structured ones as well as porous polyurethane foam (Fig. 1b). Using polycaprolactone (PCL) as the spinning solution, a radially oriented structured nanofiber film, with the ability to induce cell migration, was constructed under a symmetrical electric field. In the case of the spatially hierarchically structured one, hydrophobic PCL and hydrophilic polyacrylonitrile (PAN) were used as substrates, and by adjusting the spinning parameters, the resulting nanofiber films displayed dual driving effects namely a gradual-transition wettability gradient effect and a capillary effect. Finally, a polyurethane foam film, with a porous structure along with hygroscopic and moisturizing capabilities, was integrated by thermal crosslinking and stickup. From these, the biomimetic three-layer hierarchical scaffolds (RSL scaffolds) can not only promote wound epithelization and biofluid export but also create a moist oxidative microenvironment to promote wound healing, which are expected to have broad application prospects in the field of skin wounds.