The average diameter of PCL/IP NFs was ~1.7 μm with a wide distribution from 500 nm to 3 μm as shown in Figure 2a and d. If the concentration of PCL solution was reduced from 10% to 6% (w/v), the diameters of PCL/ibuprofen fibers can be significantly reduced to below 300 nm on average. But there were both fibers and beads, thus making the interpretation of diffusion data very difficult. Figure 2b and e shows that the diameters of pNIPAM/IP nanofibers were in the range of 100–900 nm with an average of 470 nm. Compared to PCL NFs, the standard deviation in the diameters for pNIPAM NFs was much smaller. In the case of pNIPAM/IP/PCL composite nanofibers, the average diameter and diameter distribution were quite similar to those of pure pNIPAM NFs (Figure 2c and f). Noteworthy, no apparent drug particles were observed on the surface of these NFs, thus benefiting a reduced burst effect associated with surface drug molecules.
These NFs were further characterized using Fourier transform infrared (FTIR) spectroscopy (Figure 3). The PCL/IP FTIR spectrum shows there are PCL finger prints peaks at 2948 and 1728 and 1190 cm−1, which can be assigned to the asymmetric C-H stretching, carbonyl stretching and C–O stretching, respectively [19]. In the FTIR spectrum of PNIPAM/IP nanofibers, characteristic peaks of pNIPAM located at 1650 and 1550 cm−1 can be assigned to amide carbonyl stretching and amide N-H bending, respectively [20]. In the case of pNIPAM/IP/PCL composited NFs, finger prints of both pNIPAM and PCL can be clearly seen at 1190, 1550, 1650, 1728 and 2946 cm−1. The IP peaks can’t be clearly observed might be due to its low weight percentage (5 wt%).
The release rates of ibuprofen from three types of NFs were investigated in pH 7.4 deionized water at 22°C and 34°C (Figure 4 and Table 1). It is seen that 1 μmol of ibuprofen was quickly released from pNIPAM/IP NFs in the first one hour at 22°C, and then the rest was released at a much slower rate, 0.05 μmol hr−1. Totally, 24% IP was released in four hours. In contrast, IP was released at a more controllable mode when the temperature was increased to 34°C. The average release rate was ~ 0.2 μmol hr−1 and ~ 0.4 μmol IP was released in the first one hour. Only 17% IP was released in 4 hrs. This phenomenon can be explained by the high water solubility of pNIPAM when the temperature was below its LCST (32°C), leading to the fast IP release from the polymeric matrix. However, pNIPAM becomes much hydrophobic when temperature was above its LCST. Thus pNIPAM functions like a drug depot to prohibit the fast release of hydrophobic IP molecules, resulting in the relatively more controllable release mode. For PCL/IP NFs, ~15% IP was released in the first 1 hr at 22°C and 34°C and there was less than 10% change in devliery rates for both temperatures. On average, 34% IP was released in 4 hrs. In the case of composite pNIPAM/IP/PCL NFs, the diffusion rates at both 22°C and 34°C were quite linear and well controlled without burst effects. The controlled release was due to the presence of PCL barrier that can provide extra buffer zone to reduce burst effect, especially at lower temperature (below the LCST of pNIPAM). Compared to the IP release rate at 34°C, the average IP release rate from this composite NF was 75% faster at 22°C. Similiarly, it is because pNIPAM was quite hydrophilic at room temperature. So water molecules can diffuse through the PCL barrier and carry out IP molecules more quickly. At high temperature, both PCL and pNIPAM were hydrophobic, thus leading to a reduced release rate of hydrophobic IP. On average, 13% and 25% IP were released from pNIPAM and PCL composite NFs at 34 and 22°C, respectively.