Exploring the anomalous cytotoxicity of commercially-available poly(N-isopropyl acrylamide) substrates

Poly(N-isopropyl acrylamide) (pNIPAM) is a stimulus-responsive polymer that has been of great interest to the bioengineering community. When the temperature is lowered below its lower critical solution temperature (∼32 °C), pNIPAM rapidly hydrates, and adherent cells detach as intact cell sheets. This cell-releasing behavior in a physiologically relevant temperature range has led to NIPAM's use for engineered tissues and other devices. In a previous study, however, the authors found that although most techniques used to polymerize NIPAM yield biocompatible films, some formulations from commercially-available NIPAM (cpNIPAM) can be cytotoxic. In this work, the authors investigate the reasons underlying this anomaly. The authors evaluated the response of a variety of cell types (e.g., bovine aortic endothelial cells, BAECs; monkey kidney epithelial cells, Vero cells; and mouse embryonic fibroblasts, 3T3s) after culture on substrates spin-coated with sol-gel (spNIPAM) and commercially-prepared (cpNIPAM). The relative biocompatibility of each cell type was evaluated using observations of its cell morphology and function (e.g., XTT and Live/Dead assays) after 48 and 96 h in culture. In addition, the substrates themselves were analyzed using NMR, goniometry, and XPS. The authors find that all the cell types were compromised by 96 h in culture with cpNIPAM, although the manner in which the cells are compromised differs; in particular, while Vero and 3T3 cells appear to be undergoing cytotoxic death, BAECs undergo apoptic death. The authors believe that this result is due to a combination of factors, including the presence of short chain oligomers of NIPAM in the commercially-available preparation. This work will provide valuable insights into the cytotoxicity of commercially-prepared polymer substrates for this type of bioengineering work and therefore into the applicability of cells grown on such surfaces for human subjects.