Injectable polyoxazoline grafted hyaluronic acid thermoresponsive hydrogels for biomedical applications.

Injectable thermosensitive hydrogels based on hyaluronic acid (HA) grafted with lower critical solution temperature (LCST) polyoxazoline (copolymers of poly(isopropyl-co-butyl oxazoline)) or P(iPrOx-co-BuOx) have been elaborated with tunable solution/gel temperature transitions and gel state elastic modulus. A suitable HA-g-P(iPrOx-co-BuOx-67/33)-0.10 sample with an iPrOx/BuOx ratio of 67/33, a polymerization degree (DP) of 25, a substitution degree (DS) of 10%, and displaying thermally induced gelling character with elastic (G') and viscous (G'') moduli crossover points at 25 °C and a G' at 37 °C around 80 Pa has been chosen for medical application. Hydrogels obtained with HA-g-P(iPrOx-co-BuOx-67/33)-0.10 exhibited high stability at 37 °C and excellent injectability properties with full and quick reversibility. The incorporation of a secondary network (HA), until 35 wt%, into the thermosensitive hydrogel also demonstrated very good stability and injectability.

Hyaluronic Acid Functionalization with Jeffamine® M2005: A Comparison of the Thermo-Responsiveness Properties of the Hydrogel Obtained through Two Different Synthesis Routes.

Hyaluronic acid (HA) of different molar masses (respectively 38,000, 140,000 and 1,200,000 g.mol-1) have been functionalized with a commercial poly(etheramine), Jeffamine® M2005, in order to devise physical thermo-responsive hydrogels. Two routes have been studied, involving the use of either water for the first one or of N,N'-Dimethylformamide (DMF), a polar aprotic solvent, for the second one. In the case of the water route, the reaction was performed using a mixture of N-(3-Dimethylaminopropyl)-N'-ethylcarbodiimide (EDC) and N-hydroxysuccinimide (NHS) as coupling reagents. The reaction was optimized while making sure no free M2005 remained in the final material, leading to M2005 grafting degrees of about 4%, which enabled the formation of hydrogels by increasing the temperature. In the case of the organic solvent route, propylphosphonic anhydride T3P® was used as a coupling reagent in DMF, resulting in a M2005 grafting degree of around 8% with better thermo-responsive properties of HA-g-M2005 compared to those obtained when the reaction was performed in water. However, the reaction systematically led to covalent cross-linking in the case of the HA, with the highest starting molar masses resulting in a very different rheological behaviour and with higher gel strength retaining thermo-responsive behaviour but being only poorly soluble in water.

Two competing reactions of tetrabutylammonium alginate in organic solvents: Amidation versus γ-lactone synthesis.

Biocompatibility and thickening properties predetermine alginates as ingredients in food, cosmetic and pharmaceutical products. Further chemical modifications are often desired for a product optimization. The introduction of hydrophobic groups can be realized by employing organic tetrabutylammonium alginate (TBA-Alg) solutions. The synthesis of alginic acid alkyl amides from TBA-Alg with 2-chloro-1-methylpyridinium iodide (CMPI) as a coupling agent, however, has so far not resulted in a high degree of amidation. The analysis of the coupling reaction revealed the formation of mannuronic acid γ-lactone structures, which required a conformation change from (1)C4 to (4)C1. The opening of the γ-lactone required a high excess of butylamine. In the case of CMPI, triethylamine had to be added prior to the coupling agent in order to suppress the assumed alginic acid formation. The degrees of amidation achieved were up to 0.8, and for propylphosphonic anhydride as the coupling agent up to 1. The molecular weights of the alginic acid butyl amide were ≥35kDa.

Synthesis enhancements for generating highly soluble tetrabutylammonium alginates in organic solvents.

The adaptation of alginates to food and pharmaceutical specifications is limited to aqueous chemistry due to the insolubility of sodium alginate (Na-Alg) and the insufficient solubility of tetrabutylammonium alginate (TBA-Alg) in organic solvents. In the present investigation, these restrictions were resolved by optimizing the solubility of TBA-Alg by improving its synthesis from Na-Alg via heterogeneous acidification with hydrochloric and formic acid, followed by neutralization with tetrabutylammonium hydroxide. The best acidification results were achieved with formic acid, because the reaction controlling solubility of the by-product in the acidic solvent was improved in comparison to hydrochloric acid. The solubility of TBA-Alg in polar aprotic organic solvents improved by increasing the degree of TBA substitution (DSTBA), decreasing the molecular weight of TBA-Alg and increasing the relative permittivity of the solvent. The best TBA-Algs, with DSTBA=0.95 and relative high molecular weights, gave optically clear solutions with a turbidity of about 1 NTU.