It
is desired to create skin strain sensors composed of multifunctional
conductive hydrogels with excellent toughness and adhesion properties
to sustain cyclic loadings during use and facilitate the electrical
signal transmission. Herein, we prepared transparent, compliant, and
adhesive zwitterionic nanocomposite hydrogels with excellent mechanical
properties. The incorporated zwitterionic polymers can form interchain
dipole–dipole associations to offer additional physical cross-linking
of the network. The hydrogels show a high fracture elongation up to
2000%, a fracture strength up to 0.27 MPa, and a fracture toughness
up to 2.45 MJ/m3. Moreover, the reversible physical interaction
imparts the hydrogels with rapid self-healing ability without any
stimuli. The hydrogels are adhesive to many surfaces including polyelectrolyte
hydrogels, skin, glasses, silicone rubbers, and nitrile rubbers. The
presence of abundant zwitterionic groups facilitates ionic conductivity
in the hydrogels. The combination of these properties enables the
hydrogels to act as strain sensors with high sensitivity (gauge factor
= 1.8). The strategy to design the tough, adhesive, self-healable,
and conductive hydrogels as skin strain sensors by the zwitterionic
nanocomposite hydrogels is promising for practical applications.
Nanocomposite hydrogels with unprecedented stretchability, toughness, and self-healing have been developed by in situ polymerization of acrylamide with the presence of exfoliated montmorillonite (MMT) layers as noncovalent cross-linkers. The exfoliated MMT clay nanoplatelets with high aspect ratios, as confirmed by transmission electron microscopy (TEM) and X-ray diffraction (XRD) results, are well dispersed in the polyacrylamide matrix. Strong polymer/MMT interaction was confirmed by Fourier transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC). The effective cross-link densities of these hydrogels are estimated in the range of 2.2-5.7 mol m(-3). Uniaxial tensile tests showed a very high fracture elongation up to 11 800% and a fracture toughness up to 10.1 MJ m(-3). Cyclic loading-unloading tests showed remarkable hysteresis, which indicates energy dissipation upon deformation. Residual strain after cyclic loadings could be recovered under mild conditions, with the recovery extent depending on clay content. A mechanism based on reversible desorption/adsorption of polymer chains on clay platelets surface is discussed. Finally, these nanocomposite hydrogels are demonstrated to fully heal by dry-reswell treatments.
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