Kinetics of chitosan plasma treatments to elucidate surface modification mechanisms

Naturally derived biopolymers such as chitosan are biocompatible and readily accepted by the body, making them useful in a range of biomedical applications. The hydrophobic nature of chitosan materials, however, presents limitations to the range of available uses. In this study, treating chitosan films with N2 plasmas was seen to be effective in introducing hydrophilic functional groups to the surface. A fivefold reduction in chitosan water contact angle (WCA) was seen when treated with a 125 W, 50 mTorr N2 plasma for 5 min. However, to improve the reproducibility and tunability of these treatments, understanding the gas-phase mechanisms responsible for such surface changes is crucial. To this end, this study utilizes time-resolved optical emission spectroscopy to monitor the densities of plasma species during treatments, to establish kinetic trends. In an N2 plasma without a substrate, gas-phase species were seen to reach a steady state in ∼20 s. Upon introducing chitosan to the reactor, reactive species were observed undergoing slow surface-mediated processes. A decrease in chitosan WCA was observed to be directly correlated with this N2* production and N* consumption, providing mechanistic evidence for the modification process. Such insights have allowed a modification mechanism for the nitrogen functionalization of chitosan to be proposed. These insights are crucial in allowing for large scale reproducibility and surface tunability.