(405g) The Effect of Crosslinker Concentration on Drug Release Kinetics of Thermo-Responsive, Lignin-Based Soft Composites

The incorporation of the biopolymer lignin in soft composites has gained recent attention due to its various favorable qualities, such as biocompatibility, antimicrobial and antibacterial properties, and sustainability of sourcing. Further, lignin can be used in conjunction with stimuli-responsive materials, such as poly(N-isopropylacrylamide) (PNIPAm), which is an ideal material for drug elution, as these materials undergo a volume change close to human physiological conditions. However, pure PNIPAm hydrogels exhibit slow water uptake and poor mechanical strength, hindering their usefulness and prolonged usage in transport-facilitated delivery applications. To combat these issues, the fabrication of interpenetrating networks (IPNs) containing PNIPAm and other crosslinked polymers have been explored. However, traditional methods investigate the role of exclusively petroleum-based materials in the fabrication of these soft composites. Recently, the role of lignin in stimuli-responsive gels has been explored, though the extent to which the purity and molecular weight of lignin have been explored is still lacking. In this study, we fabricated soft composites containing: (1) a thermo-responsive polymer, PNIPAm; (2) a hydrophilic polymer, poly(vinyl alcohol) (PVA); and (3) a sustainable biopolymer, lignin. Specifically, soft composites were fabricated at lignin concentrations of 40 mass % and 50 mass %. To form the IPN of the hydrogel, the concentrations of crosslinkers glutaraldehyde and N,N’-methylenebisacrylamide were varied between 1 and 10 mass % of their respective polymers. Additionally, three lignin molecular weights were selected, ranging from values of approximately 5,700 Da to approximately 160,000 Da. After fabrication, both the mechanical properties and the drug-release kinetics of each soft composites was characterized. Specifically, the Young’s modulus of each hydrated membrane was measured using mechanical indentation at both room temperature (~21 °C) and 40 °C. Furthermore, the permeability of methylene blue (MB), a model organic pollutant, was measured through the membrane mounted in a permeation cell and analyzed with ultraviolet-visible spectroscopy. In addition, the equilibrium water uptake was measured at room temperature and 40 °C. Finally, the diffusion of caffeine through the network was analyzed via ultraviolet-visible spectroscopy at room temperature and 40 °C. It was observed that the addition of lignin to the soft composites altered all four properties measured – Young’s modulus, MB permeability, water uptake, and drug diffusion. In particular, the diffusion of caffeine into water was suppressed with the addition of lignin, where complete release of the caffeine was seen to be significantly slower in membranes containing lignin. Furthermore, the molecular weight of the lignin was also seen to impact the caffeine release kinetics of these soft composites.