Tunable Hydrogels of Cellulose Nanocrystals Cross-Linked with Water Soluble Polysaccharides

Hydrogels based on nanosized cellulose have potential in a wide range of different applications. Often, cellulose nanofibers (CNFs) are used for hydrogel preparation, as the highly flexible CNF fibrils have an inherent tendency to entangle, forming gels already at very low concentrations. Cellulose nanocrystals (CNCs), on the other hand, are highly crystalline structures and as such do not exhibit similar gelling properties and are therefore utilized in hydrogels to lesser extent. However, the gelling property can be a limiting factor for easy CNF manipulation, while the more fluid-like CNC can be utilized at broader concentration ranges, allowing hydrogel tunability at much wider parameter space.<br/><br/>In this work we have investigated different water-soluble polysaccharides in their ability to function as cross-linkers and facilitate CNC hydrogel formation. For this, we have tested 15 different water-soluble polysaccharides, such as xanthan, hydroxypropyl cellulose, carboxymethyl cellulose (CMC), hydroxyethyl cellulose (HEC) xylan, guar, and three types of mixed linkage glucans (MLGs), among others. CNCs from different sources were used, including bacterial cellulose and cellulose from bleached softwood pulp both treated with HCl gas hydrolysis and 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) radical-mediated oxidation. The CNCs and polysaccharides were mixed at different ratios and the performance of the formed hydrogels were tested in their rheological properties and their porosity was investigated using thermoporosimetry.<br/><br/>One possible application for hydrogels includes immobilization of photosynthetic cells. Photosynthetic cells can be utilized as a renewable, sustainable, and economical method for biofuel, food, and medicinal compounds production. Thus far, these cell factories have been mostly maintained in suspension cultures. For improved efficacy of light utilization, energy loss and water consumption, cells can be immobilized in different structures, such as alginate. However, capturing cells in these structures can inhibit the transfer of substrates and the produced chemicals, while still facilitating undesired cell division. Hydrogels made from nanocellulose offer a great alternative for cell immobilization, as cellulose is abundantly available, biocompatible, and biodegradable. Also, nanocellulose hydrogels provide controllable transparency, good gas barrier properties and can be tuned in their permeability and wet strength. Furthermore, they offer good mechanical stability and high porosity, both which are beneficial for a long-term cell survival and efficient production of desired chemicals. Therefore, the prepared nanocellulose hydrogels were evaluated in their suitability for cellular immobilization, biocompatibility, and long-term cell viability by entrapping a wild-type cyanobacterial strain Synechocystis sp. PCC6803 within the gels and monitoring the long-term photosynthetic activity. The most promising cross-linkers in terms of good hydrogel formation, mechanical properties, biocompatibility, and long-term cyanobacteria performance were found out to be HEC, CMC, and all three MLGs.<br/><br/>To further finetune the properties and to optimize the hydrogel performance, the fundamental interactions of the hydrogel components were studied using a CNC model films in surface plasmon resonance and quartz crystal microbalance. These interaction studies revealed that e.g., the binding of MLGs on CNCs was concentration dependent, but the extent of binding was not affected by the molecular weight of MLG, implying that any MLG type is suitable for robust construction of hydrogels from CNCs. The polysaccharide-CNC thin films were further characterized using atomic force microscopy and ellipsometry.