Ultrastrong Poly(2‐Oxazoline)/Poly(Acrylic Acid) Double‐Network Hydrogels with Cartilage‐Like Mechanical Properties

Abstract

The exceptional stiffness and toughness of double-network hydrogels (DNHs) offer the possibility to mimic even complex biomaterials, such as cartilage. The latter has a limited regenerative capacity and thus needs to be substituted with an artificial material. DNHs composed of cross-linked poly(2oxazoline)s (POx) and poly(acrylic acid) (PAA) are synthesized by free radical polymerization in a two-step process. The resulting DNHs are stabilized by hydrogen bridges even at pH 7.4 (physiological PBS buffer) due to the pK ashifting effect of POx on PAA. DNHs based on poly(2-methyl-2-oxazoline), which have a water content (WC) of around 66 wt% and are not cytotoxic, show biomechanical properties that match those of cartilage in terms of WC, stiffness, toughness, coefficient of friction, compression in body relevant stress conditions and viscoelastic behavior. This material also has high strength in PBS pH 7.4 and in egg white as synovial liquid substitute. In particular, a compression strength of up to 60 MPa makes this material superior.

Achieving Swollen yet Strengthened Hydrogels by Reorganizing Multiphase Network Structure

“…[20][21][22] A typical example is double-network (DN) hydrogel, which is usually prepared by introducing a neutral monomer (e.g., acrylamide) with high concentrations into a tightly cross-linked polyelectrolyte (PE) hydrogel network, followed by an additional polymerization to form the DN structure. [20,[23][24][25][26] Compared with the primary PE hydrogel, the formed DN gel swells in size but shows a significant mechanical improvement based on a "sacrificial bond" principle. Inspired by muscle training, Gong's group has further developed self-growing hydrogels that gained both sample size and strength under repetitive mechanical loading by coupling of mechanical stimuli and external substance supply, on the basis of DN gels.…”

Tough hydrogel with high water content and ordered fibrous structures as an artificial human ligament

“…[11][12][13][14][15][16][17] The fabrication of strong and tough multifunctional artificial tissue still faces challenges for existing conventional materials. The strength and toughness of the hydrogels are tremendously improved by introducing mechanisms for energy dissipation during the load process, such as the formation of double network crosslinks, 18,19 ion enhancement, 20,21 and self-assembly. 9,22 Other common reinforcement mechanisms such as the formation of nanocomposites 23,24 and induced hydrophobic polymerization [25][26][27] can also effectively improve the performance of hydrogels.…”

Detailed Understanding of Solvent Effects for the Cationic Ring-Opening Polymerization of 2-Ethyl-2-oxazoline

“…In the two last decades, poly­(2-ethyl-2-oxazoline) (PEtOx) has received considerable attention in the field of biomedical and pharmaceutical applications, − including polymer–prodrug conjugates, − hydrogels, − and matrix excipients. − Furthermore, hydrolysis of PEtOx affords linear poly­(ethylene imine) (PEI), − which is the “gold standard” among cationic polymer transfection agent for delivery of nucleic acids. − As reproducible synthesis and narrow dispersity (for therapeutics) are key parameters for such biomedical applications, an in-depth understanding of the synthesis of PEtOx via living cationic polymerization is important. In this work, we addressed the question what the best solvent is for the polymerization of EtOx with respect to polymer solubility, monomer reaction rate, and polymer dispersity.…”