Structural, macro- and micro-mechanical properties of supramolecular bi-component l-Lysine-sodium tetraphenyl borate based hydrogels

Abstract

We present a new system of bi-component hydrogels formed of l-lysine hydrochloride mixed with sodium tetraphenyl borate with unusual viscoelastic properties which might be useful in tissue engineering, nano-particles synthesis or bioseparation. Beside cationic-anionic molecular interactions, 1H-NMR experiments have also shown the presence of π-electrons cloud interactions as well as hydrogen bonds stabilizing the supramolecular structure. Bulk rheological and mechanical properties have been characterized using steady and oscillatory shear rheometry as well as uniaxial compression tests. Microstructural information has been deduced from multi particle tracking (MPT) optical microrheology and cryo-preparation scanning electron microscopy (SEM). These gels are shear thickening at low concentrations and shear thinning at higher gelator concentration. Shear and Young’s modulus data obtained from different techniques agree fairly well. Zero shear viscosity as well as modulus data obtained for gels with equimolar lysine and borate concentration exhibit concentration dependence η0∼c5±0.5and G0∼c3.4±0.35, respectively. The scaling exponent for η0agrees well with predictions for neutral polymer in theta solvent or wormlike micelles in the slow breaking regime whereas the variation in G0 is unexpectedly high. We attribute this to the unique microstructure consisting of a cellular structure composed of thin sheets connected by tiny fibers. At low concentrations the mesh size of this structure is about 10 μm and fiber diameter increases from ∼200 nm to ∼2 μm, only at the highest gelator concentration of 7 wt.% the mesh size shrinks to ∼5 μm and the fiber diameter to ∼400 nm. Consistently, tracer particles in low concentrated gels diffuse freely in an aqueous environment. At higher gelator concentration, however, gelation takes place so fast that tracers are trapped in the elastic sheets / fibrous regions and the modulus GMPT´ agrees fairly well with bulk data. Varying the molar ratio of both gel-forming components results in a pronounced viscosity maximum at fixed total gelator concentration. Corresponding microstructural changes could again be visualized using cryo-SEM consistent with MPT data. At high lysine fraction, a coarse coral reef type sheet structure is found while at low lysine to borate ratio a network of thick struts is formed. Finally, diffusing wave spectroscopy experiments performed on samples with different lysine to borate ratio revealed that the plateau modulus G0 is almost independent of temperature whereas the relaxation time TR becomes faster as temperature increases.