Rheological analysis of thermo-responsive alginate/PNIPAAm graft copolymers synthesized by gamma radiation

Abstract

Focused on biomedical applications of thermo-responsive polymers, low-doses of gamma radiation from a 60 Co source were applied in a simple one-pot method to synthesize graft copolymers of alginate and poly(N-isopropylacrylamide) (PNIPAAm) with different compositions. The molar percentage of grafted NIPAAm (% molar NIPAAm) was determined by thermogravimetric analysis (TGA) and elemental analysis (EA), being the copolymer structure-property relationship studied in terms of thermo-associative and rheological behavior in aqueous solutions. The addition of more NIPAAm monomer in the initial mixture of reaction, as well as, increasing absorbed dose lead to a greater grafting. However, increasing radiation dose produces copolymers with diminished viscoelastic properties caused by the alginate backbone scission. From rheological curves, two transition temperatures, Ta and Tgel, were determined as a consequence of the thermo-responsiveness of PNIPAAm side chain. Storage (G') and loss (G'') modulus curves undergo a slope inversion at Ta temperature, where both moduli begin to increase caused by an associative behavior of PNIPAAm domains. While, Tgel temperature is related to the onset of the gelation process at the G'/G'' crossover. In order to design samples with liquid-gel transitions close to the human body, able to form gels in situ once inoculated, it was possible to tailor both transition temperatures selecting copolymers with an appropriate PNIPAAm content and optimizing the copolymer concentration in the aqueous solution. A good agreement between transition temperatures and viscoelastic properties was achieved for 5 wt% aqueous solutions of copolymers with low NIPAAm content synthesized at the lowest absorbed dose (0.5 kGy). Highlights  Low doses of γ radiation allow to synthesize alginate-g-PNIPAAm copolymers  Structure-properties relationship was well defined by rheology in aqueous solution  Copolymers sol-gel transitions could be tailored close to body temperature  We obtained injectable solutions to form gels in situ once inoculated

Rheological Investigation of Thermoresponsive Alginate-Methylcellulose Gels for Epidermal Growth Factor Formulation

“…It is known that composition affects the initial thermal transitions and gelling temperatures of compound gel systems [20]. Figure 7a shows the impact of polymer blending and methylcellulose concentration on thermoresponsiveness.…”

Synthesis and Evaluation of a Thermoresponsive Degradable Chitosan-Grafted PNIPAAm Hydrogel as a “Smart” Gene Delivery System

“…At ambient temperature, all gels were in a liquid form as G′′ > G′ with tan δ > 1 as the G" was more prominent compared to G' and dominated the flow properties [9,77,78,79]. As the temperature elevated, the increased thermal movement of the polymer chains resulted in a decreased viscosity Swelling in water is an important characteristic of hydrogels, as the biomaterials with high water absorption could mimic living tissues [65].…”

Chemically Modified Biopolymers for the Formation of Biomedical Hydrogels

“…359 Upon exposure to elevated temperature (e.g., >34 °C), interactions between propyl hydrophobic groups strengthen, leading to the formation of a physically crosslinked network. 359 PNIPAAm has been grafted to many biopolymers including alginate, 360 HA, 361 CS, 362 chitosan, 363 silk fibroin, 364 and gelatin 365 for the formation of thermoresponsive hydrogels (Figure 16a). For example, Zhu et al grafted PNIPAAm onto keratin by first using thiol−ene radical addition to conjugate NIPAAm monomers onto free thiol groups on keratin, and subsequently polymerizing PNIPAAm off the keratin backbone using free radical kinetic chain polymerization.…”

“…PNIPAAm has been grafted to many biopolymers including alginate, HA, CS, chitosan, silk fibroin, and gelatin for the formation of thermoresponsive hydrogels (Figure a). For example, Zhu et al grafted PNIPAAm onto keratin by first using thiol–ene radical addition to conjugate NIPAAm monomers onto free thiol groups on keratin, and subsequently polymerizing PNIPAAm off the keratin backbone using free radical kinetic chain polymerization .…”