Polyzwitterions as a Versatile Building Block of Tough Hydrogels: From Polyelectrolyte Complex Gels to Double-Network Gels

Abstract

The high water content of hydrogels makes them important as synthetic biomaterials, and tuning the mechanical properties of hydrogels to match those of natural tissues without changing chemistry is usually difficult. In this study, we have developed a series of hydrogels with varied stiffness, strength and toughness based on a combination of poly(2-acrylamido-2-methylpropane sulfonic acid) (PAMPS), a strong acidic polyelectrolyte, and poly-N-polyzwitterion with weak acidic moiety. We demonstrate that modifying the true molar ratio, R, of PCDME to PAMPS results in four unique categories of hydrogels with different swelling ratio and Young's modulus. When R < 1, a negatively charged polyelectrolyte gel (PE) is formed; when 1 < R < 3, a tough and viscoelastic polyelectrolyte complex gel (PEC) is formed; when 3 < R < 6.5, a conventional, elastic interpenetrating network gel (IPN) is formed; and when R > 6.5 a tough and stiff double-network gel (DN) is formed. Both the PEC and DN gels exhibit high toughness and fracture stress, up to 1.8 MPa and 1.5 MPa, respectively. Importantly, the PEC gels exhibit strong recovery properties along with high toughness, distinguishing them from DN gels. Without requiring a change in chemistry, we can tune the mechanical response of hydrogels over a wide spectrum, making this a useful system of soft and hydrated biomaterials.

Polyelectrolyte Gels: Fundamentals, Fabrication and Applications

“…However, most of these tough gels lack self-recovery and are susceptible to cyclic loading (fatigue) because of the damage of irreversible sacrificial bonds during deformation. To overcome this problem, Yin et al employed polyzwitterions (poly- N -(carboxymethyl)- N , N -dimethyl-2-(methacryloyloxy) ethanaminium (PCDME)) as a building block to construct self-recoverable gels using anionic PAMPS [ 111 ]. The self-recovery property on cyclic loading is achieved by simply varying the molar ratio of PCDME and PAMPS, where the reversible sacrificial ionic interactions between PCDME networks are more dominant compared to the irreversible covalent bonds between the PAMPS networks.…”

Highly Stretchable Hydrogels as Wearable and Implantable Sensors for Recording Physiological and Brain Neural Signals

“…In general, the double-network hydrogels exhibit an elastic modulus of 0.1-1 MPa, which is also larger than that of neural tissues. [40][41][42] Inspired by the double-network hydrogels, in this work, we utilize polymer chain entanglements, hydrophobic association, and covalent bonds as crosslinkers to modulate the tradeoff between modulus and fatigue resistance/stretchability. The prepared hydrogels exhibit relatively low modulus, good stretchability, and excellent fatigue resistance, which contain two kinds of crosslinking sites, i.e., microgels (≈600 nm) and lauryl groups.…”

“…In general, the double‐network hydrogels exhibit an elastic modulus of 0.1–1 MPa, which is also larger than that of neural tissues. [ 40–42 ]…”

Untangling the mechanics of entanglements in slide-ring gels towards both super-deformability and toughness

“…8,9 However, their poor mechanical properties often restrict their practical applications which require high mechanical strength and large deformation recoverability. 10 In order to toughen these gels, multi-networks, 11,12 phase-separation, 13 ion bonding 14 and nanoparticles 15 are incorporated into them to increase energy dissipation during deformation, thus enhancing their toughness. However, these network structures are easily damaged, which results in the rupture of the gels.…”