Measurement of LNT and calcium ion release from CSL-gel
The standard curve of LNT was determined by UV-vis spectrophotometry (UV-vis, PGEN-ERAL) according to the method as described previously[25]. The sustained-release ability and rate of CSL-gel and SL-gel were tested according to the previous methods[25]: Briefly, 50 pieces of CSL-gel or SL-gel were added into 10 mL of deionized water (pH 5) at 20 °C and an UV−vis spectrophotometer was used to measure the absorbance of LNT in solution at 24−336 h, respectively. The same method was used to measure the cumulative release of calcium ions and investigate the effects of different temperatures (Temperature: 10, 20, 30, 40, 50, and 60 °C, pH=5), different pH values (pH: 3.0, 5.0, 7.0, and 9.0, adjusted by Tris-HCl, at 20 °C), and different concentrations of cations (The concentrations of NaCl solution were 0.1, 0.2, 0.4, and 0.8 mol/L, respectively, pH=5, at 20 °C) on the sustained-release ability and rate of CSL-gel as well. The cumulative release rate (CRR) of LNT was calculated according to formula 3. The release kinetics of LNT from CSL-hydrogel and SL-hydrogel were analyzed using Higuchi (Formula 4) and Korsmeyer−Peppas models (Formula 5) [28].
CRR (%) =\(\frac{\sum_{0}^{t}{C_{t}V_{\text{total}}}}{Q_{1}}\ \times 100\%\)(3)
Ct represents the release LNT concentration at time t, Vtotal represents solution volume, and Q1 represents the total quality of loaded LNT.
Higuchi model: Qt=\(\text{Kt}^{1/2}\) (4)
Korsmeyer-Peppas model: Qt=\(\text{Kt}^{n}\) (5)
Qt represents the cumulative release rate of LNT until time t, K represents the kinetic constant, and n is an index that reflects the following release mechanisms: Fickian diffusion (n < 0.43), non-Fickian or anomalous diffusion (0.43 < n < 0.85), and case II transport (n > 0.85).