Swelling of Homogeneous Alginate Gels with Multi-Stimuli Sensitivity

Abstract

A new two-step method is suggested for the preparation of homogeneous alginate gels. In the first step, alginate chains are weakly bonded by Ca2+ ions in an aqueous solution with a low pH. In the next step, the gel is immersed into a strong solution of CaCl2 to finalize the cross-linking process. Homogeneous alginate gels preserve their integrity in aqueous solutions with a pH ranging from 2 to 7 and ionic strength in the interval from 0 to 0.2 M, at temperatures ranging from room temperature up to 50 °C, and can be used in biomedical applications. The immersion of these gels into aqueous solutions with low pH induces the partial breakage of ionic bonds between chains (treated as gel degradation). This degradation affects the equilibrium and transient swelling of homogeneous alginate gels and makes them sensitive to the history of loading and environmental conditions (pH, ionic strength and temperature of aqueous solutions). As sensitivity to the environmental stimuli is a characteristic feature of polymer networks connected by catch bonds, homogeneous alginate gels may serve as a simple model, mimicking the behavior of more sophisticated structures in living matter.

The Effect of Temperature on the Mechanical Properties of Alginate Gels in Water/Alcohol Solutions

“…Alginate hydrogels were prepared by a method described previously in [31]. Briefly, 1 wt.% alginate solution (pH was adjusted to 3.5 by adding HCl) was mixed with 50 mM of CaCl 2 solution in a proportion of 28:1 (v/v), and the mixture was poured into a mold and kept overnight.…”

“…Samples in water/alcohol mixtures at temperature T = 22 • C reached equilib-rium after one week of swelling. It was shown in [31] that one week is sufficient for its equilibration. We expect that the organohydrogel will not reach equilibrium after one week of swelling at temperature T = −20 • C. Thus, we stopped the swelling test at temperature T = −20 • C after one week and measured the degree of swelling Q at this instant.…”

“…Due to this shortcoming, we focus on the analysis of ionically cross-linked organohydrogels to be used as gel electrolytes. In the previous paper [31], we prepared uniform alginate hydrogels cross-linked with Ca 2+ ions. The swelling properties of these gels were studied in aqueous solutions with various pH values and ionic strengths.…”

The Effect of pH on the Viscoelastic Response of Alginate–Montmorillonite Nanocomposite Hydrogels

“…However, the range of their applications is limited to weak acidic conditions with pH > 5.5 only [ 12 , 13 ]. When Ca-alginate gels are exposed to strong acidic environments, calcium ions that cross-link alginate chains in the gel matrix are replaced with protons, which leads to disintegration of the gel structure [ 14 ]. This hinders applications of Ca-alginate gels in pH below the pK a of alginate chains (the value of pK a belongs to the interval between 3.6 for mannuronic acid and 3.8 for guluronic acid [ 15 ]).…”

“…Afterwards, the gels were immersed in aqueous solutions with pH = 2. It is known that bonds between alginate chains and Ca 2+ ions are partially broken upon immersion of the gels in acidic environments with pH = 2 [ 14 ].…”