Introduction {#Sec1} ============ The use of natural fibers to replace synthetic ones in various textile applications is rapidly increasing worldwide. In comparison to synthetic fibers, they are eco-friendly, biodegradable and biocompatible. As such they have found a variety of applications, both in the fashion industry and for the hygiene industry. For example, cellulose-based products are used for diapers and sanitary napkins due to their porous structure that allows liquid to pass through quickly. Cellulose also has excellent absorbency due to the large number of hydrophilic --OH groups present in its molecular structure. Besides this, due to its biodegradability and biocompatibility, cellulose is also an attractive option for wound dressings. The most popular natural cellulose-based product used as a wound dressing is cellulose-based collagen (Col) film \[[@CR1]\]. However, the low mechanical stability of pure cellulose makes its use as a membrane for medical applications limited. To increase the mechanical strength of the membrane it can be reinforced with either chemical \[[@CR2], [@CR3]\] or physical methods \[[@CR4]\]. Another alternative to enhance the strength of cellulose is mixing it with synthetic polymers. A good example of this is a polymer blend consisting of a polyvinyl alcohol (PVA) polymer with a cellulose polymer (e.g., a hydroxypropyl methylcellulose (HPMC) or a carboxymethyl cellulose (CMC)) \[[@CR5]\]. These have also been used for the preparation of wound dressings \[[@CR6], [@CR7]\]. Nevertheless, some of the disadvantages, such as the increase in cost of the final product, the low moisture permeability and poor mechanical stability of the prepared membrane, still limit the acceptance of these membranes. An interesting solution that solves these drawbacks is to prepare nanocomposites, which are composites consisting of a matrix of a synthetic polymer combined with a nanocellulose \[[@CR8], [@CR9]\]. For example, Wang et al. prepared electrospun nanocomposites consisting of poly(vinyl alcohol) and cellulose nanocrystals (CNC) and their application in wound healing demonstrated high water vapor transmission rate \[[@CR10]\]. The advantages of nanocellulose-based electrospun nanocomposites are due to the improvement of the mechanical properties and the barrier properties of the polyvinyl alcohol (PVA) matrix. The barrier properties of PVA can be enhanced significantly when it is combined with CNC due to its network structure that allows for the easy migration of the polymer chains between the nanocellulose network \[[@CR11]\]. Moreover, due to its low dielectric constant and low glass transition temperature, the PVA polymer can act as a plasticizer for CNC, which allows it to disperse easily \[[@CR12]\]. Another recent study investigated the electrospun nanocomposite of hydroxypropyl methylcellulose and carbon nanotubes for wound healing \[[@CR13]\]. Results demonstrated that the prepared membrane has good water absorption capacity and good cell viability. Nevertheless, the main challenge is to tailor the fiber size and network structure during the electrospinning process to obtain optimal wound dressing properties. As mentioned before, electrospinning is a promising technique to prepare nanocomposites for wound healing applications. To overcome this challenge, a novel approach was developed in our laboratory. This involves the use of water-soluble cellulose derivatives (such as HPMC and carboxymethyl cellulose) to form a water-based gel solution. Water-soluble cellulose derivatives have been used as matrix and gelling agent of wound dressings \[[@CR14], [@CR15]\]. The solution is then electrospun to form fibers of the two polymers that form a co-electrospun fibrous matrix. Moreover, by this process, it is possible to generate nanofibers that are suitable for cell adhesion. The wound dressing can be produced from only one nanofibrous layer (wound dressing only containing an electrospun nanofiber layer) or from two nanofibrous layers with different fiber size and distributions (to generate additional effects). Due to the different properties of the prepared nanofibrous mats and also because of its wide availability in the market, hydroxypropyl methylcellulose was used as the matrix polymer in the present study. This includes its unique rheological properties and network structure \[[@CR16]\]. The presence of a hydrogel (gel layer) of water-soluble cellulose derivative was expected to enhance the water absorbency of the nanofiber membrane (due to the fact that it is a highly hydrophilic polymer) \[[@CR17]\]. The aim of the present study was to investigate the possibility of enhancing the water absorption capacity of an electrospun nanofibrous mat containing hydroxypropyl methylcellulose by adding a hydrogel layer (containing HPMC) on top. Methods {#Sec2} ======= Materials {#Sec3} --------- Carboxymethyl cellulose sodium salt (CMC) (average degree of substitution 0.63, molar mass 40 kg mol^−1^) was obtained from Sigma-Aldrich (St. Louis, Missouri, USA). Hydroxypropyl methylcellulose (HPMC) was purchased from Fluka (Steinheim, Switzerland). Acetone was used as a solvent and purchased from VWR (Darmstadt, Germany) to prepare the prepared solution. Polycaprolactone (PCL) was obtained from Sigma Aldrich and acetone was used to prepare the PCL solution. Dichloromethane and methanol were used as solvents and purchased from R&M Chemicals (Essex, UK). A 2,3-bis (2-methoxy-4-nitro-5-sulfophenyl)-5-\[phenylamino\]-2*H*-tetrazolium hydroxide (XTT) kit, phosphate buffered saline (PBS) and trypsin from porcine pancreas, fetal bovine serum (FBS), 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) and TritonX-100 were purchased from Sigma-Aldrich (St. Louis, Missouri, USA). Preparation of electrospun nanofibers {#Sec4} ------------------------------------- The solvent cast electrospinning method was used to prepare the CMC nanofibers. A high voltage of 25 kV was applied to the solution at a flow rate of 10 μl/min. Fibers were collected on a thin aluminum foil placed at a distance of 5 cm from the collector, wrapped in Parafilm and kept in a beaker filled with deionized water at 4 °C. The beaker was covered with aluminum foil. After 1 week the formed nanofibers were dried in a vacuum oven at 60 °C for 24 h. To prepare the PCL/CMC nanofibers, different concentrations (5, 10, and 15 wt%) of CMC were mixed with the PCL solution (20 wt% PCL in CH~3~OH). For this, 0.3 g of CMC was dissolved in 3 ml of CH~3~OH. A high voltage of 25 kV was applied to the solution at a flow rate of 10 μl/min to prepare the nanofibers. The PCL/CMC solution was electrospun at different concentrations of PCL and CMC and at a fixed CMC concentration of 5 wt%. The flow rate of the solution was fixed to 10 μl/min. The PCL/CMC solution was electrospun on top of a CMC electrospun mat to investigate the formation of the desired nanofibrous membrane. PCL/CMC nanofibers with different weight percentages (5, 10, and 15 wt%) of CMC were also prepared by this process and by varying the PCL solution flow rate. For this, 0.3 g of PCL was dissolved in 3 ml of CH~3~OH to prepare the PCL solution. PCL/CMC nanofibers were also prepared by electrospinning the PCL/CMC solution directly on a thin aluminum foil without pre-electrospinning the CMC nanofibers. Preparation of electrospun nanofibrous mats with a hydrogel layer {#Sec5} ----------------------------------------------------------------- The hydrogel layer on the nanofibers was prepared by dissolving HPMC in PBS (pH 7.4) to form a solution with a final concentration of 3 wt% (according to the results obtained by the rheological and mechanical properties measurements). The electrospun mat was first immersed into the PBS solution. After 3 min, the mat was carefully removed from the solution and placed on a glass slide. The resulting hydrogel layer was dried in a vacuum oven at 60 °C for 24 h. The same procedure was applied for the formation of electrospun nanofibrous mats with a different PCL/CMC ratio. Here, a polymer solution with a final PCL concentration of 30 wt% was used. In this case, CMC was used as the aqueous cellulose derivative. Rheological measurement {#Sec6} ----------------------- For the rheological measurements, a Discovery Hybrid Rheometer from TA Instruments (New Castle, DE, USA) was used. The rheological properties of the electrospun nanofibrous mats were measured at room temperature (23 °C) using a 25 mm/2° cone-plate geometry. Two samples of the electrospun nanofibrous mat were cut into samples of 20 × 10 mm (width × length) using a sharp razor blade to prepare the rheological measurements. To determine the storage modulus (G′) and loss modulus (G″), the frequency sweep