Dextran macromers containing oligolactate-(2-hydroxyetheyl methacrylate) units (Dex-lactateHEMA,
Fig. 1) with DP = 8 (the lactide chain length of the lactate unit) and DS = 15 (the average number of cross-linkable chains per dextran) were synthesized using the procedure described previously
32,33 with the feeding ratios of precursors modified accordingly. The DP and DS of the macromers were estimated from the
1H NMR spectra. Poly(NIPAAm-co-Dex-lactateHEMA) hydrogels (
Table 1) loaded with FITC-insulin or insulin lispro were synthesized using UV-initiated free radical polymerization method in a Teflon mold with well-defined wells (2 mm in diameter and 1.6 mm in height). To synthesize FITC-insulin–loaded hydrogels, a first type of pre-polymer solutions was prepared from NIPAAm/Dex-lactateHEMA with weight ratios of 8/1 (for hydrogels G-8-1), 6/3 (for hydrogels G-6-3), or 4/5 (for hydrogels G-4-5), and 5 wt% FITC-insulin in 0.1 wt% Irgacure 2959 in DMF; 2 μL of solvent was used for each milligram of the total weight of NIPPAm and Dex-lactateHEMA macromer (
Table 1). To synthesize insulin lispro-loaded hydrogel discs, a second type of pre-polymer solutions was prepared by replacing the FITC-insulin with 6.4 IU/gel insulin lispro in the first type of pre-polymer solutions. In addition to synthesizing blank hydrogel discs, a third type of pre-polymer solutions was prepared by not including FITC-insulin in the first type of pre-polymer solutions. The pre-polymer solutions were added into the Teflon mold and then exposed to 500 mW/cm
2 UV light EXFO Lite UV source (EXFO, Inc., Richardson, TX, USA) for 3 minutes. The resulting hydrogels were carefully peeled out from the well and washed with deionized water (DI) twice. The fluorescent intensity of the wash solutions was measured and the amount of insulin in the wash solutions was determined from a standard curve. The insulin encapsulation efficiency was calculated by subtracting the amount of insulin in the wash solutions from the originally loaded insulin amount and then dividing by the originally loaded insulin amount. The result showed that the encapsulation efficiency of all the hydrogels was approximately 98%: 97.64% ± 0.79%, 98.64% ± 0.31%, and 98.41% ± 0.04% for hydrogels G-8-1, G-6-3, and G-4-5, respectively. The loading content of the hydrogels was calculated to be approximately 4.7%: 4.65% ± 0.09%, 4.67% ± 0.27%, and 4.72% ± 0.39% for hydrogels G-8-1, G-6-3, and G-4-5, respectively. The obtained hydrogel discs were frozen with liquid nitrogen and dried in a freeze-dryer overnight. The hydrogel discs without insulin are referred to as “blank hydrogels,” those with FITC-insulin are referred to as “FITC-insulin–loaded hydrogels,” and those with insulin lispro are referred to as “insulin lispro-loaded hydrogels” in the subsequent sections.