Figure 1.

Design and molecular engineering of SHARK. (a and b) Comparison of the structures and equivalent electric circuits of typical sandwich-shaped hydrogel-based artificial skin (a) and SHARK (b). SHARK comprises a bulk junction of hydrogel capacitors made of dielectric peptide-coated conductive graphene layers dispersed in the hydrogel matrix. The strong yet dynamic interactions between the peptide and the graphene layers endow SHARK with high mechanical stability and great self-healing properties. (c) AFM-based single-molecule force spectroscopy experiments are used to characterize the interfacial bonding between graphene and the polymeric hydrogel network through self-assembled peptide layers. The experimental schemes for measuring the adsorption forces between Py or Py-GAGAGY and the graphite surface are shown in the left and middle panels. Py or Py-GAGAGY is allowed to adsorb on HOPG and is then picked up by a PEG-functionalized cantilever tip through maleimide-thiol chemistry. Prior to the single-molecule experiment, Py-GAGAGY self-assembled into a fibrous network on the HOPG surface, as confirmed by AFM imaging (right). (d) Typical force-extension curves at a pulling speed of 400 nm s⁻¹. Black lines correspond to worm-like chain fitting to the retraction traces. The heights of the peak correspond to the detachment forces. (e) Rupture force histograms for Py and Py-GAGAGY from graphite surfaces at a pulling speed of 400 nm s⁻¹. The Gaussian fittings show average dissociation forces of 56.5 ± 22.5 pN (n = 118) and 129.8 ± 29.7 pN (n = 207). (f) Dynamic force spectroscopy experiments for the dissociation of adsorbed Py and Py-GAGAGY from graphite at various force loading rates. The dissociation of Py from graphite is almost loading rate independent, indicating that the binding and unbinding are in fast equilibrium. However, the dissociation of Py-GAGAGY is a nonequilibrium process and is strongly dependent on the loading rate. Fitting the experimental results to the Bell-Evans model (solid lines) yields a spontaneous dissociation rate of 2.0 s⁻¹ and a potential width of 0.08 nm. Self-assembly of Py-GAGAGY greatly enhances the binding strength of Py with graphite and shifts the binding/unbinding equilibrium to a time scale slower than the force loading rates in the experiments.