## 1 Introduction  
As the largest organ, the skin can protect people from pathogens and excessive water loss. When it gets injured, an ideal wound dressing should serve as a barrier against dust and microorganisms, maintain a favourable moist environment for the wound healing and allow gas permeability [1]. Also, it should benefit effective promotion of cell adhesion and proliferation, control of bleeding while reducing or even stopping oozing after the hemostasis process. Many types of natural substances are promising agents to prepare a wound dressing which is capable of accelerating wound healing and promoting hemostasis, such as collagen. Collagen, as a widely used material for biomedical applications, has gained broad clinical and consumer acceptance as a safe material [2]. As a major fibrous protein in extracellular matrix (ECM), collagen plays a predominant role in maintaining the biological and structural integrity of ECM [3]. The fibrillar structure of collagen is important for cell attachment, proliferation, and differentiation [4]. To architecturally mimic that structure, a nonwoven matrix from collagen, which is interesting for use in wound healing processes, can be easily produced via electrospinning [5; 6; 7; 8; 9]. In addition to the promotion of cell adhesion and proliferation, this collagen electrospun membrane is intended to induce platelet adhesion and aggregation to promote the coagulation process which is favorable for using as a wound dressing. However, after electrospinning, water-insoluble collagen becomes soluble and the collagen nanofibers become swollen and lose fibrillary structure when exposed to water [10]. This is the result of the lack of the natural crosslinks which are generated in the native collagen fibrous structure by covalent and hydrogen bonds and are indispensable for the natural collagen network to gain its level of mechanical strength [11]. In order to maintain the nanofibrous structure and enhance mechanical properties of collagen, various crosslinking techniques have been used. Up to now, several physical and chemical methods have been reported for crosslinking collagen and collagen-based materials. Dehydroothermal treatment and UV irradiation are common methods of physical crosslinking [12; 13]. However, compared with chemical crosslinking, they are less efficient. The physically crosslinked collagen would be nearly fully degraded after incubation for 24 h [14]. Compared with physical crosslinking, chemical crosslinking could efficiently enhance the mechanical properties and water stability of electrospun collagen. Many chemicals such as formaldehyde, glutaraldehyde, oligosaccharides, carbodiimide and genipin, have been used as chemically crosslinking agents [15; 16; 17; 18; 19; 20]. Among them, 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC)/\(N\)-hydroxysuccinimide (NHS) and genipin were considered less cytotoxic, but the crosslinked fibers showed inadequate water stability in aqueous or high humidity environments [5; 21]. Furthermore, genipin and EDC/NHS has been reported to be cytotoxic at some concentrations depending on cell type (typically 0.5-5 mM) [21; 22]. With dialdehyde groups, glutaraldehyde (GA) is the most widely used crosslinking agent, due to its high efficiency, low price and crosslinking of a large number of amino groups over a varying range of distances which makes it ideal for biological applications [11]. The aldehyde group of glutaraldehyde can induce Schiff-based intermediates with amino groups of lysyl and hydroxylysyl of collagen. Schiff-based intermediates are linked further to form larger crosslinked entities. So, covalent chemical crosslinking of neighboring collagen fibrils is achieved by using GA crosslinking [23]. It has been reported that the potential cytotoxicity of GA maybe due to the residues of unreacted GA and leaching out as the materials degrade. However, several previous studies have demonstrated evidence of no cytotoxic effect of GA crosslinked collagen followed by glycine quenching and washing steps [15; 24].