Microgels or microcapsules? Role of morphology on the release kinetics of thermoresponsive PNIPAm-co-PEGMa hydrogels

Abstract

The effect of morphology of PNIPAm, PEGMa and PNIPAm-co-PEGMa hydrogels on the uptake and delivery (release kinetics) of a model drug (FITC–dextran) was investigated. Two types of hydrogel architectures: microgels and microcapsules, without and with core–shell morphology, were synthesized. The microcapsules had 30–50% greater uptake compared to the corresponding microgel architecture. The estimated pore size for the PNIPAm, PNIPAm-co-PEGMa and PEGMa hydrogels were 78, 92 and 130 Å, respectively. The drug release was performed at 25, 37 (physiological temperature), and 45 °C (targeted stimulating temperature). Diffusion coefficients at temperatures below VPTT of the microgels showed close correlation with the pore size, but this was not the case for the microcapsules. The release kinetics is dominated by temperature responsiveness at T greater than the VPTT and by hydrogel morphology at T < VPPT. There is a striking advantage of using temperature responsive PNIPAm on the release kinetics. In fact, in both PNIPAm and its copolymer with PEG, a quick burst release is observed at T > VPTT. More than 80% of the drug was released in the first 10 min using the temperature responsive microcapsule, compared to 1 h for the corresponding microgel. Unlike prior reports in the literature, the release of FITC–dextran is characteristic of a Super Case II Fickian diffusion and Anomalous release mechanism for the copolymer microgels when T > VPTT and for the PNIPAm and PEGMa microgels when T < VPTT. These results demonstrate the feasibility of modulating the release profile of encapsulated proteins (for tissue repair), chemotherapeutics (for drug delivery) and nucleic acids (for gene delivery) by tailoring the polymer morphology.