Non-invasive evaluation of alginate/poly-l-lysine/alginate microcapsules by magnetic resonance microscopy

In this report, we present data to demonstrate the utility of 1H MR microscopy to non-invasively examine alginate/poly-l-lysine/alginate (APA) microcapsules. Specifically, high-resolution images were used to visualize and quantify the poly-l-lysine (PLL) layer, and monitor temporal changes in the alginate gel microstructure during a month long in vitro culture. The thickness of the alginate/PLL layer was quantified to be 40.6±6.2 μm regardless of the alginate composition used to generate the beads or the time of alginate/PLL interaction (2, 6, or 20 min). However, there was a notable difference in the contrast of the PLL layer that depended upon the guluronic content of the alginate and the alginate/PLL interaction time. The T2 relaxation time and the apparent diffusion coefficient (ADC) of the alginate matrix were measured periodically throughout the month long culture period. Alginate beads generated with a high guluronic content alginate demonstrated a temporal decrease in T2 over the duration of the experiment, while ADC was unaffected. This decrease in T2 is attributed to a reorganization of the alginate microstructure due to periodic media exchanges that mimicked a regular feeding regiment for cultured cells. In beads coated with a PLL layer, this temporal decrease in T2 was less pronounced suggesting that the PLL layer helped maintain the integrity of the initial alginate microstructure. Conversely, alginate beads generated with a high mannuronic content alginate (with or without a PLL layer) did not display temporal changes in either T2 or ADC. This observation suggests that the microstructure of high mannuronic content alginate beads is less susceptible to culture conditions.