Abstract
Mechanical rheometry, specifically rotational rheometry, squeeze flow, and capillary rheometry, and two microrheology methods, namely multiple-particle tracking (MPT) and diffusing wave spectroscopy (DWS) have been used to get new insight into structural and dynamical properties of alginate dissolved in solvents widely used for bioprinting, namely deionized water, phosphate-buffered saline (PBS), and Dulbecco Modified Eagle Medium (DMEM) cell media. Results demonstrate that alginate rheological properties depend on the solvent quality at concentrations higher than 1 wt.%. In this high concentration regime, in aqueous salt-free and PBS solutions, experimental scaling exponents for the concentration dependence of the specific viscosity ηsp and the plateau modulus G0 agree well with theoretical predictions for neutral polymers in good solvent whereas for the terminal relaxation time TR, the exponent is slightly higher than theoretically predicted, presumably due to the formation of aggregates. For alginate dissolved in DMEM, all exponents for ηsp, G0, and TR agree with predictions for polymers in theta solvents, which might be related to the formation of polyelectrolyte complex as a result of interactions between alginate and amino acids. Chain persistence length lp values, as determined directly from high frequency rheometry for the first time, are independent of alginate concentration and temperature. Lower absolute lp values were found for DMEM solutions compared with the other solvents. Moreover, scaling exponents for ηsp, G0, and TR do not change with temperature, within 20 and 60 °C. These findings suggest no change in the conformation of alginate chains with temperature.Graphical abstractAlginate rheological properties depend on the solvent quality
Highlights
Alginate is an anionic polysaccharide derived from marine brown algae [1]
The variation of the dynamic shear moduli G′ and G′′as a function of frequency as determined from both mechanical and optical (DWS) rheometry are given in Fig. 1a for a 3 wt.% alginate aqueous solution at 20 °C
For the terminal relaxation time TR, a slightly stronger concentration dependence is observed compared with theoretical predictions, presumably due to the formation of aggregates in this high-alginate concentration regime
Summary
Alginate is an anionic polysaccharide derived from marine brown algae [1]. The linear copolymer backbone is composed of consecutive blocks of α-L-guluronic (G-blocks) and β-D-mannuronic (M-blocks) linked by a β-(1-4) glycosidic bond. In many studies, the persistence length lp of alginate in dilute solutions has been determined based on intrinsic viscosity [14, 15], size-exclusion chromatography [16, 17] measurements, and electron microscopy images [18, 19]. In all these studies, lp values between 12 and 16 nm were found independent of the technique used and ionic strength. An accurate characterization of the persistence length of alginate chains is of high importance, as the chain flexibility is supposed to affect cell adhesion as suggested by Lee et al [24]
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