Fine Adjustment of Interfacial Potential between pH-Responsive Hydrogels and Cell-Sized Particles

We quantitatively determined interfacial potentials between cell-sized particles and stimulus-responsive hydrogels using a microinterferometer. The hydrogel is based on physically interconnected ABA triblock copolymer micelles comprising an inner biocompatible PMPC block and two outer pH-responsive PDPA blocks. The out-of-plane temporal fluctuation in the position of the cell-sized particles was calculated from changes in the interference pattern measured by Reflection Interference Contrast Microscopy (RICM), thus yielding the particle-substrate interaction potential <i>V</i> (Δ<i>h</i>). Measurements in pH buffers ranging from 7.0 to 7.8 resulted in a systematic reduction in height of the potential minima ⟨Δ<i>h</i>⟩ and a concomitant increase in the potential curvature <i>V</i>″ (Δ<i>h</i>). The experimental data were analyzed by applying the modified Ross and Pincus model for polyelectrolytes, while accounting for gravitation, lubrication and van der Waals interactions. Elastic moduli calculated from <i>V</i>″ (Δ<i>h</i>) were in good agreement with those measured by Atomic Force Microscopy. The ability to fine-tune both the gel elasticity and the interfacial potential at around physiological pH makes such triblock copolymer hydrogels a promising biocompatible substrate for dynamic switching of cell–material interactions

Emergence of Strong Nonlinear Viscoelastic Response of Semifluorinated Alkane Monolayers

Viscoelasticity of monolayers of fluorocarbon/hydrocarbon tetrablock amphiphiles di­(<i>FnHm</i>) ((C_<i>n</i>F_2<i>n</i>+1CH₂)­(C_<i>m</i>–2H_{2<i>m</i>–3})­CH–CH­(C_<i>n</i>F_2<i>n</i>+1CH₂)­(C_<i>m</i>–2H_{2<i>m</i>–3})) was characterized by interfacial dilational rheology under periodic oscillation of the moving barriers at the air/water interface. Because the frequency dispersion of the response function indicated that di­(<i>FnHm</i>) form two-dimensional gels at the interface, the viscosity and elasticity of di­(<i>FnHm</i>) were first analyzed with the classical Kelvin–Voigt model. However, the global shape of stress response functions clearly indicated the emergence of a nonlinearity even at very low surface pressures (π ≈ 5 mN/m) and small strain amplitudes (<i>u</i>₀ = 1%). The Fourier-transformed response function of higher harmonics exhibited a clear increase in the intensity only from odd modes, corresponding to the nonlinear elastic component under reflection because of mirror symmetry. The emergence of strong nonlinear viscoelasticity of di­(<i>FnHm</i>) at low surface pressures and strain amplitudes is highly unique compared to the nonlinear viscoelasticity of other surfactant systems reported previously, suggesting a large potential of such fluorocarbon/hydrocarbon molecules to modulate the mechanics of interfaces using the self-assembled domains of small molecules