Experimental

Cisplatin and cis-DIDP were purchased from Kunming Guiyan Pharmaceutical Co. Ltd. (China) and stored away from light at −4 °C. PCL (molecular weight 5 × 10⁴), sodium chlorine (A.R.), glycine (B.R.), and glucose (A.R.) were purchased from Sinopharm Chemical Reagent Co. Ltd. (China). RPMI1640 (the culture medium) and newborn calf serum were purchased from Shanghai Shichen Reagent Co. Ltd. (China). Human hepatocellular carcinoma cell line SMMC-7721 was newly purchased from Shanghai Cell Center (Chinese Academy of Sciences).

Cisplatin and cis-DIDP were dissolved respectively away from light in deionized water (different solutions such as normal saline, 5% glucose, and 0.1 mol L⁻¹ glycine were respectively used as alternative solvent to examine the solvent effect) to form 1 mmol L⁻¹ solution. The solution absorbance was determined from time to time by the Agilent 8453 UV–Vis spectrophotometer (Agilent, USA).

PCL was dissolved in dimethylformamide (DMF) to form a polymer solution (PCL wt% 5–15%), and heated by water bath. Then, a predetermined amount of cis-DIDP (1–15% to PCL) was dispersed in DMF. The cis-DIDP dispersion was added into the PCL polymer solution with continuous stirring to form homogeneous PCL polymer solution containing cis-DIDP. In the electrospinning procedure, the polymer solution was firstly transferred to a syringe. Then, the syringe pump was used to deliver the solution through a hollow needle (8#, outside diameter of the needle is 8 mm), the flow rates were 0.5–3.0 mL h⁻¹. A high voltage DC generator was used to produce 10–25 kV voltage to inject polymer solution through the hollow needle. An aluminum foil was used as a collector to gather the random fibers. The distances from the spinneret to the collector were 10–25 cm. All the experiments were performed at room temperature. The fibers were finally taken out and dried under vacuum for 48 h. The blank PCL fibers were fabricated by the same method but without dispersing the cis-DIDP in the DMF dichloromethane solution.

The fibers with average diameters from 50 to 500 nm could be fine-tuned by adjusting electrospinning parameters, such as concentration of cis-DIDP, solvent, electrospinning voltage, polymer solution flow rates, and the distances between needle and collector. Different operation parameters are listed in Additional file 1: Tables S1–S4. Additional file 1: Figures S5–S8 shows the SEM images of the products fabricated under different conditions. After trial and error, the following electrospinning conditions were used: 10/100 (cis-DIDP/PCL), 20 kV (voltage), 1.0 mL h⁻¹ (flow rate), 15 cm (distance) to prepare products for further studies.

The products generally were characterized by SEM, XRD, and FT-IR [26]. An S-4800 high-resolution field-emission scanning electron microscopy (FE-SEM, Hitachi, Japan) was used to observe the morphology of collected fibers. The samples for SEM observation were sputtered and coated with a thin layer of gold for better imaging. The average fiber diameters and its distribution were calculated from the random fibers of a typical SEM image.

The structure of cis-DIDP powders, PCL, and nanofibers were examined by Advance D8 X-ray diffraction (XRD, Bruker, Germany). The XRD patterns were determined with an X-ray diffractometer with Cu Ka radiation (λ = 1.54056 Å, 40 kV, 40 mA) over the 2θ range of 10°–70° with the scanning rate of 0.2°s⁻¹.

FT-IR (Thermo Fisher, USA) was used to analyze the molecular structure of cis-DIDP, blank PCL nanofibers, and cis-DIDP@PCL nanofibers. Drug cis-DIDP was commonly mixed with potassium bromide (KBr) and compressed to pellet; nanofibers were cut into pieces and mixed with KBr and compressed to pellets, then were scanned at the wave number of 4000–400 cm⁻¹.

The mass of cis-DIDP in solution were determined by UV–Vis spectrophotometer. The release profile of cis-DIDP was obtained from cis-DIDP@PCL immersion in deionized water, normal saline, or phosphate buffer solution (PBS), respectively. The cis-DIDP@PCL (~100 mg each) was statically incubated in 100 mL deionized water, normal saline, or PBS (pH 7.4), as sustained-release solution, respectively. At preset interval, 1 mL incubated solution was taken out and measured by UV–Vis spectrophotometer, and meanwhile, 1 mL solution was added into the sustained-release solution. The experiments were performed for three times, using the immersion solution of blank fibers as control. The accumulative release of cis-DIDP from the fiber was calculated as a function of the incubation time. In this paper, cis-DIDP was uniformly dispersed in the electrospinning solution and evenly scattered in the PCL fibers [27, 28]. Predetermined amount of cis-DIDP@PCL (~100 mg) was dissolved in 100 mL sustained-release solution. The concentration of cis-DIDP was measured by UV–Vis spectroscopy for three times. Because of uniform dispersion of cis-DIDP in solution and scattered in the fibers, the encapsulation efficiency (%EE) of the product could be calculated by Eq. (1).

Here, C ₀ is the concentration of cis-DIDP in cis-DIDP@PCL (μg mL⁻¹), V ₀ is the volume of cis-DIDP@PCL solution (mL), M ₀ is the mass of added cis-DIDP@PCL (mg), and wt% is the mass fraction of cis-DIDP in fiber.

In vitro, the anticancer activity of the cis-DIDP and cis-DIDP@PCL fibers were examined by MTT assay; cisplatin was selected as control. Human hepatocellular carcinoma cells (SMMC-7721 line cell) were chosen as the target tumor cells. The tumor cells were cultured in RPMI 1640 containing 10% newborn calf serum, 25 μg mL⁻¹ penicillin and 25 μg mL⁻¹ streptomycin, then adjusted to 5 × 10⁴ cells mL⁻¹; 200 μL aliquots of the cell suspension were added into each well of a 108-well plate and incubated in the humidified atmosphere containing 5% CO₂ at 30°C for 24 h. Cisplatin, cis-DIDP, and cis-DIDP@PCL were added to the tumor-cell-cultured well and incubated for another 12, 24, 36, 48, and 72 h, respectively. Cisplatin, cis-DIDP, and cis-DIDP in cis-DIDP@PCL contents were 50 μg mL⁻¹. The 20 μL MTT solution (5 mg mL⁻¹) was added to each well and maintained incubation for 4 h. Finally, the supernatant in the wells was discarded carefully, and 150 μL DMSO was added to each of the wells to dissolve the residue. The optical densities of DMSO solutions were determined by a microplate reader at 490 nm, and the cell inhibition was calculated.