Inter-penetrating hydrogels (IPHs) as a new class of injectable polysaccharide hydrogels with thixotropic nature and interesting mechanical and biological properties

Abstract

Hydrogels are important and very promising materials for biomedical applications, such as cell scaffolds and drug delivery systems. A new class of injectable materials was obtained by mixing polysaccharide chemically crosslinked hydrogels. The mixing was performed by exploiting the thixotropic behaviour of the polysaccharide hydrogels: in other words they were passed through a syringe to subject them to shear stress and render them fluid, then they were mixed together in the fluid state. Once the material regained its consistency, a new hydrogel was obtained, called an interpenetrating hydrogel (IPH), containing both the native hydrogels. This new class of materials showed different physical-chemical, mechanical and biological properties from the native hydrogels utilized for the mixing. The hydrogels taken into consideration were: negatively charged carboxy methyl cellulose (CMC) and hyaluronic acid (Hyal), uncharged guar gum (GG), and chitosan (CHT), which bore positive charges at physiological pH. The IPHs obtainedi.e. CMC-CHT, CMC-GG and CMC-Hyal -were characterized by means of Fourier transform infrared spectroscopy (FT-IR) and swelling measurements. The mechanical properties of the IPHs were investigated by rheometric analyses, while their morphology was studied by scanning electronic microscopy (SEM). The biological performance of the IPHs was evaluated using fibroblasts (NIH 3T3 cell line) and compared with that of the native hydrogels.

Polysaccharide-Based Hydrogels: The Key Role of Water in Affecting Mechanical Properties

“…By repeating the same sequence of steps several times (that is, increasing and decreasing the oscillation stress on the same sample, step 3 and step 4 Figure 2), G' and G" values follow the same trend and maintain the same values as those determined in step 2. This result indicates a sort of "recovery" effect of the G' and G" values once the material is subjected to further cycles of stresses [7,26]. In the case of squeezed hydrogels (Figure 3(a-c)), both the storage modulus G' and the loss modulus G'' values are reduced due to the passage from the syringe with respect to those of the native hydrogels (Table 1).…”

“…Hydrogel synthesis follows the same chemical route that has been well-documented in carbodiimide chemistry [7,25]. Basically, the synthesis of CMC-and Hyal-based hydrogels exploits the formation of an amide bond between the carboxylate groups of the polymer chains and the primary amine groups of 1,3 diaminopropane (DAP), which is added to the polymer aqueous solution as the cross-linking agent of the polymer chain.…”

“…In order to verify how the hydrogels respond to the variation of an applied stress, a four-step dynamic stress sweep test was performed. The measurement is carried out by increasing (step 1) and decreasing (step 2) the oscillation twice, through the addition of a further increasing step (step 3) and decreasing step (step 4) [7,25]. The sample is subjected cyclically to the same applied stress (increasing and decreasing steps).…”

“…The stoichiometry of the reaction can be tuned in order to form hydrogels with different degrees of cross-linking, taking into consideration the number of carboxylic groups in each polysaccharide, i.e., one carboxylic group per disaccharide unit in Hyal and two in CMC polymer. On the other hand, CHT synthesis (see Figure 1c) involves the formation of an amide bond between the amine groups of the polymer and the carboxylic groups of the cross-linking agent, i.e., adipic acid (AA) [7]. The determination of the degree of cross-linking (defined as the number of polymer groups bound to the cross-linker) is obtained by pontentiometric titration of the free protonable groups remaining after hydrogel formation (carboxylate groups for CMC, Hyal or amine groups for CHT polymer), or by means of 13 C nuclear magnetic resonance [27].…”

“…The swelling degree (SD) of each hydrogel was calculated according to the equation: % S. D. = (h s -h 0 )/h 0 ) × 100 (1) where h s is the height of the swollen hydrogel and h 0 is the initial height of the hydrogel in dried state (freeze-dried) [7].…”