Ultra-stretchable hydrogels with hierarchical hydrogen bonds

Abstract

Hydrogels are attractive for applications in intelligent soft materials and flexible electronics. Herein, we report a new hydrogel with a hierarchical hydrogen bond system consisting of (1) weak hydrogen bonds between N,N-dimethylacrylamides (DMAA) and acrylic acids (AAc) and (2) strong multiple hydrogen bonds between 2-ureido-4[1H]-pyrimidinone units. By optimizing the ratios of DMAA and AAc and the balance of weak and strong hydrogen bonds, the hydrogels have unique properties. The transparent hydrogels have tunable Young's modulus (70-1,250 kPa) and are highly stretchable (up to 4,340% strain), tough (fracture energies of 10.8 kJ/m 2 , matching natural rubber) and insensitive to notches when it is highly stretched (λ = 19.6). Stretchable hydrogels are promising materials for diverse applications, such as biomaterials 1 , force sensors 2,3 , supercapacitors 4 , actuators 5 , optical fibers 6 , stretchable conductors 7 and soft electronics 8. Since the hydrogels undergo dynamic actions in these applications, such as stretching, compression and torsion, they should have appropriate mechanical strength, flexibility and stretchability under deformation. Intensive efforts have been devoted to developing tough and elastic hydrogels. Gong et al. 9 reported a double network hydrogel (DN hydrogel) by introducing a soft hydrogel network with slippery chains into a rigid hydrogel network. The internal fracture of covalent bonds in the brittle network, which dissipates significant energy 10. The soft network absorbs the energy and prevents the formation of macroscopic crack. DN hydrogels show robust mechanical strength and toughness. However, permanent chemical fracture in DN hydrogels led to poor fatigue resistance. The toughness and elasticity of DN hydrogel originate from the mechanism of disruption of sacrificial bonds, which dissipated energy 11. Attempts to introduce other chemical structures or physical crosslinks as sacrificial bonds have been made, such as ionic bonds 12,13 , triblock copolymers 14 and polyampholytes 15. Hybrid hydrogel systems by introducing crystallites 16 and microspheres 17 , micelles 18 , nanocomposite 19,20 into the hydrogels have also been reported. These reversible crosslinked sacrificial bonds or microstructures synergically dissipated energy, which endows hydrogels with high stretchability and toughness. Hydrogen bonds are promising dynamic bonds to impart fascinating properties, such as self-healing 21-23 , toughness 24,25 , and shape memory 26. Hydrogen bonds are stable in the presence of ions and they are different from other aggregates like micelles, inorganic fillers, which have sizes at the nano or micron level and impede the optical properties. Hydrogel base on hydrogen bonds can be transparent, which is favorable to sensors and electronics. However, hydrogels with only one energy-dissipating mechanism by H-bonds cannot achieve satisfactory overall mechanical properties. They are either soft and tough or strong and brittle 27-29. Here, we report a new multiple energy-dissipating h...

Highly Sensitive On‐Skin Temperature Sensors Based on Biocompatible Hydrogels with Thermoresponsive Transparency and Resistivity

“…[ 28,29 ] The compressive modulus is decreased with an increase in the amount of PAAc in TRHs, which is attributed to the strength of H‐bonding. [ 30–32 ] To investigate the stretchability, the TRH 1‐1 was stretched by various strain levels as shown in Figure S3 (Supporting Information). The representative photographs of the TRH 1‐1 under corresponding tensile strain levels shown in Figure S3 (Supporting Information) clearly indicate that the TRH 1‐1 can stretch up to 150% and maintain the initial shape without noticeable fracture.…”

Transparent and photochromic poly(hydroxyethyl acrylate–acrylamide)/WO3 hydrogel with antibacterial properties against bacterial keratitis in contact lens

“…The increased mechanical strength of p(HEA‐AAm)/WO 3 hydrogel could be due to the presence of extended hydrogen bonding groups in the DES which is responsible for strong hydrogen bond crosslinking, and further enhances the crosslinking density of the hydrogel matrix 48 . This also improves the softness of the hydrogel, thereby improving its stretchability and causing it to withstand more compressive stress 49 . The presence of WO 3 in the hydrogel further improves the formation of ionic linkage which could enhance the mechanical strength of the hydrogel.…”

Hydrogels Classification According to the Physical or Chemical Interactions and as Stimuli-Sensitive Materials

“…An example is a project developed by You et al in which they report a hydrogel with a hydrogen-bonding system consisting of weak hydrogen bonds between N,N-dimethylacrylamide (DMAA), and acrylic acid (AAc) and strong hydrogen bonds between 2-ureido-4 [H]-pyrimidinone units. The hydrogels have unique properties through optimization between the radii of the monomers and a balance of the interactions [ 69 ].…”