Ultrasensitive Wearable Strain Sensors of 3D Printing Tough and Conductive Hydrogels.

In this study, tough and conductive hydrogels were printed by 3D printing method. The combination of thermo-responsive agar and ionic-responsive alginate can highly improve the shape fidelity. With addition of agar, ink viscosity was enhanced, further improving its rheological characteristics for a precise printing. After printing, the printed construct was cured via free radical polymerization, and alginate was crosslinked by calcium ions. Most importantly, with calcium crosslinking of alginate, mechanical properties of 3D printed hydrogels are greatly improved. Furthermore, these 3D printed hydrogels can serve as ionic conductors, because hydrogels contain large amounts of water that dissolve excess calcium ions. A wearable resistive strain sensor that can quickly and precisely detect human motions like finger bending was fabricated by a 3D printed hydrogel film. These results demonstrate that the conductive, transparent, and stretchable hydrogels are promising candidates as soft wearable electronics for healthcare, robotics and entertainment.

Interfacial Viscoelasticity of Self-Assembled Hydrophobic/Hydrophilic Particles at an Air/Water Interface.

Hydrophobic/hydrophilic mixtures of latex particles at an air/water interface self-assemble, creating space filling, interconnected aggregates as the relative surface fractions of the dissimilar particles approach 0.5, which is reflected both in qualitative observation and fractal dimension of the microstructure. It is hypothesized that this change in microstructure occurs due to an asymmetry in the electrostatic interaction between similar and dissimilar particles caused by polarization of hydrophilic particles by hydrophobic particles. The changes in both microstructure and interparticle interactions significantly impact the interfacial viscoelasticity. As greater shape complexity is observed, interfacial complex moduli can increase by as much as 3 orders of magnitude and interfaces become more elastic.

Modifying interfacial interparticle forces to alter microstructure and viscoelasticity of densely packed particle laden interfaces.

HYPOTHESIS: It is possible to control the absolute and relative magnitude of repulsive and attractive interactions and hence microstructure of interfacial particles at and air/water interface by adjusting subphase composition. It should be possible to modify interfacial viscoelasticity from elastic to viscous behavior through these changes to interfacial microstructure. EXPERIMENTS: Particle laden interfaces are made from micron sized polystyrene at an air/water interface. The inter-particle interactions are controlled by the subphase salt concentration and addition of both non-ionic and ionic surfactants. These interfaces are then characterized using an interfacial rheometer with a custom visualization system. FINDINGS: Three distinct microstructures are observed. Low repulsion and high attraction systems exhibit a soft glassy rheology with a disordered but dense microstructure. Creating high repulsion results in a dense hexagonal crystal. Finally, in systems with reduced repulsion and attraction, a hexatic phase can be observed. Each of these microstructures exhibit unique interfacial viscoelastic behavior. These results indicate that control over the properties of these interfaces, and hence Pickering emulsions, is possible through manipulation of interparticle forces.