Salt-Dependent Structure in Methylcellulose Fibrillar Gels

Investigating the gelation behavior and mechanisms of Ficus awkeotsang Makino pectin under the influence of different cations

This study explored the gelation behavior, gel properties, and mechanisms of Ficus awkeotsang Makino Pectin (JFSP) under monovalent (Li+, Na+, K+, Rb+, Cs+) and divalent metal ions (Mg2+, Ca2+, Fe2+, Zn2+, Sr2+, Ba2+). At a polymer concentration of 0.3 % (w/v), stable gels formed with divalent ions at 1.8 × 10-3 mol/L and monovalent ions at 4.5 × 10-4 mol/L. The addition of metal ions increased the particle size of pectin dispersions, with divalent ions causing a larger increase (180.3 ± 0.3 and 202.2 ± 2.3 nm) compared to monovalent ions (170.2 ± 0.6 to 177.3 ± 0.2 nm). Also the metal ions neutralized negative charges on pectin molecules and transformed layered pectin gels into an ordered and porous network, as confirmed by scanning electron microscopy. Atomic force microscopy and small-angle X-ray scattering further confirmed a cross-linked structure with smaller inter-chain distance in Ca2+-treated gels (2.67 nm) compared to K+-treated (2.95 nm) and control samples (3.65 nm). Microrheology indicated enhanced pectin interactions and network heterogeneity upon ion addition. Infrared spectroscopy showed intensified ionic carboxyl group vibrations, suggesting interactions between free carboxyl groups and cations, reducing electrostatic repulsion and promoting chain entanglement and cross-linking. Therefore, divalent ions, especially Ca2+, promoted gelation and improved gel hardness (20.15 ± 0.62 g), water holding capacity (98.86 ± 1.06 %), and stability (water loss rate of <10 %). These findings highlight the critical role of ion type and concentration in optimizing JFSP gel performance, with important implications for the development of JFSP-based functional foods.

Gelation and post-gelation mechanism of methylcellulose in an aqueous medium: 1H NMR and dynamic compressive rheological studies

Methylcellulose (MC) has become crucial in 3D bioprinting in the last decade. Researchers investigated MC aqueous solutions blended with biopolymers at room temperature, focusing on rheological studies. Even at low concentrations, the gel state of MC, which provides structural strength through hydrophilic and hydrophobic associations, was explored for injection-based 3D printability. Post-gelation phenomena were examined at 80 °C using a dynamic mechanical analyzer (DMA), revealing increased storage and loss moduli with frequency, indicating a robust gel network structure. Optical microscopy reveals that upon heating from 40 to 80 °C, the structural strength is enhanced via the formation of hydrophobic confirmations, starting from the micro-helical structure to the associated microarray. These microarrays are further synchronized to withstand the high frequency of the DMA probe. Compressive rheology outcomes allow us to elaborate on the possibility of injection-based 3D printability of aqueous MC gel at 80 °C. 1H and 13C NMR studies probed hydrophobic interactions among MC chains, showing evidence of H-bonding through temperature-dependent shifts. UV/Vis experiments traced gel formation, depicting a time-dependent network formation process. Overall experiments indicated that adjusting temperature could control gelation time, allowing precise tuning of the printing process and achieving fine layers (10 μm) in the printed membrane with maximum hydrophobic clusters.

Sol-Gel Transition of a Thermo-Responsive Polymer at the Closest Solid Interface

Thermo-responsive polymers are applied as surface modifications for the temperature switching of hydrophilic and hydrophobic properties through adsorption and grafting on solid substrates. The current understanding of the influence of polymer chains bound to the solid surface on the transition behavior of thermo-responsive polymers is rather restricted. In this study, we aim to elucidate the effect of the bound polymer chains at the interface on the thermo-responsive sol-gel transition behavior of aqueous methylcellulose (MC) solutions by employing a quartz crystal microbalance (QCM) to evaluate the shear modulus near the solid interface. When the sample thickness was thinner on the order of the millimeter scale, the sol-gel transition temperature evaluated by the cloud point decreased because the condensation of MC near the solid interface promoted the sol-gel transition. On the other hand, focusing on the closest solid interface on the nanometer scale by QCM, the sol-gel transition temperature increased when approaching the solid interface. Adsorption and interfacial interactions reduced the chain mobility and restrained the sol-gel transition by preventing MC chain aggregation. We demonstrated the physical properties evaluation at the closest interface between the thermo-responsive polymer and solid substrate by combining a simple analytical model of QCM and controlling the analytical depth of the QCM sensors. In conclusion, the mobility change of the bound polymer chains at the solid interface caused by adsorption and interfacial interactions must be considered when a thermo-responsive polymer is applied as in adsorbed or thin films on solid substrates for the functionalization of biomaterials.

Leveraging the gel-to-sol transition of physically crosslinked thermoresponsive polymer hydrogels to enable reactions induced by lowering temperature

Much work has been done on the use of heating to trigger reactions via the temperature-dependent removal of a barrier or constraint separating reagents. Far less work, however, has been done on the use of cooling to achieve a similar goal. Numerous applications, such as those involving components or materials susceptible to persistent low temperatures and cases in which energy for heating is not available, would benefit from this inverse approach. Hence, in this study we explore whether physically crosslinked hydrogels can be reliably used as thermoresponsive constraints that allow reagents to react only upon cooling. We achieve this by loading reagents into adjacent blocks of thermoresponsive hydrogel and showing that these reagents can only react with each other after the temperature of the hydrogel falls below its lower critical solution temperature (LCST). Above the LCST, the reagents remain sequestered in separate gels and no reaction occurs; this "OFF" state is stable for extended periods. When the system is allowed to cool, the hydrogels liquify and flow into each other, allowing mixing of the embedded reagents ("ON" state). We tune the hydrogels' LCSTs using NaCl, quantify the NaCl's tuning effect using rheometry, and determine that reactions are triggered reproducibly at temperatures similar to the tuned LCSTs. We also demonstrate generalizability of the concept by exploring situations involving radically different reaction types. This concept therefore constitutes a new approach to autonomous material behavior based on cooling.