Self-Healing Poly(acrylic acid) Hydrogels with Shape Memory Behavior of High Mechanical Strength

Abstract

A promising strategy to design synthetic hydrogels with the ability to self-heal is to substitute the covalently cross-linked polymer chains by supramolecular ones. Although supramolecular hydrogels generally exhibit rapid self-healing without the need for any stimulus, they suffer from low mechanical strength which prevents them from any stress-bearing applications. Here, we describe a novel way for the production of self-healing hydrogels with shape memory behavior of high tensile strength (0.7–1.7 MPa) and stretch at break (800–900%). Hydrophobically modified poly(acrylic acid) (PAAc) chains with cetyltrimethylammonium (CTA) counterions form the physical network of such hydrogels. They were prepared via micellar copolymerization of acrylic acid with 2 mol % stearyl methacrylate (C18) as the hydrophobic comonomer in an aqueous NaBr solution of cetyltrimethylammonium bromide (CTAB). Extraction of free CTAB micelles from the physical gels results in a drastic increase in their Young’s moduli (from 8–30 to 180–600 kPa) and tensile strengths (from 0.1–0.2 to 0.7–1.7 MPa) due to the complex formation between PAAc and CTAB. Loading and unloading cycles conducted on hydrogels both at the state of preparation and at equilibrium in water show a significant hysteresis and good superposition of the successive loading curves, demonstrating damage done during loading is recoverable in nature. The hydrogel samples self-healed via heating and surfactant treatment of the damaged areas withstand up to 1.5 MPa stresses and rupture at a stretch of 600%. Because of the drastic change in the elastic modulus of PAAc hydrogels with a change in temperature, they also exhibit shape memory properties with a recovery ratio of 100%.

Biomimicry, Biofabrication, and Biohybrid Systems: The Emergence and Evolution of Biological Design

“…For example, supramolecular interactions have been used to expand the biomimetic capabilities of such materials by giving them the ability to rapidly self-heal and exhibit high mechanical strength in addition to demonstrating shape memory. [17,18] The triggered changes in conformation and functional behavior demonstrated in engineered assembly approaches are also reversible, similar to the self-assembly approaches described above, providing an attractive alternative for manufacturing more complex 3D structures.…”

Functional self-healing materials and their potential applications in biomedical engineering

“…Thus, it may open a door for self-healing materials with shape memory properties in the design of novel biomaterials for potential prosthetics and orthotics applications. Recently, a self-healing poly(acrylic acid) (PAAc) hydrogel with enhanced mechanical strength in terms of shape memory behavior was reported [74]. The supramolecular polymer network is formed using hydrophobically modified PAAc with oppositely charged cetyltrimethylammonium (CTA) via hydrophobic and electrostatic interactions.…”

Self‐Healing Poly(acrylic acid) Hydrogels: Effect of Surfactant

“…Hydrogels deswell in water with 60% reduction in the gel mass (m rel ¼0.37 AE0.2) after gels were immersed in a large excess of water because PAAc is a polyelectrolyte whose repeating units bear an electrolyte group and forms a complex with cationic surfactants. [16][17][18] Here, the surfactant alkyl chains are trapped electrostatically in a supramolecular polymer network formed via hydrophobic interactions. 70% CTAB bound to PAAc network chains was calculated from the nitrogen contents of the dried gels, which were subjected elemental analysis measurements using a Thermo Flash EA-1112 Series elemental analyzer.…”

“…[11,12] Recently, our research group presented a simple strategy for the production of self-healing hydrogels via hydrophobic interaction. [13][14][15][16] Incorporation of hydrophobic sequences within the hydrophilic polymer chains via micellar polymerization generates dynamic hydrophobic associations between the hydrophobic domains of polymer chains and surfactant micelles acting as physical cross-links of the resulting hydrogels. These reversible breakable cross-links are responsible for rapid self-healing.…”

“…In this study, pH responsive physical gels were prepared by micellar copolymerization of hydrophilic monomer acrylic acid (AAc) with large hydrophobic monomer stearyl methacrylate (C18) in solution of worm-like anionic surfactant [15] (sodium dodecyl sulfate, SDS) and cationic surfactant [16] (cetyltrimethyl ammonium bromide, CTAB) micelles. Hydrophobe content of the monomer mixtures was 2 mol.% and total monomer concentration in gels was 20 w/v%.…”