Polyurethane (PU) microcapsules containing hexamethylene diisocyanate (HDI) as core materials are facilely manufactured via interfacial polymerization reaction of commercial methylene diphenyl diisocyanate (MDI) prepolymer and 1,4-butanediol in an oil-in-water emulsion. The resultant capsules have diameters of 5–350 μm and shell thickness of 1–15 μm, both linearly related to the agitation rate in the double logarithm coordinates. The typical core fraction of microcapsules and the yield of synthesis are around 60 wt% and 70 wt%, respectively, while varying with reaction conditions. The effects of parameters including reaction duration and temperature, surfactant concentration, agitation rate, and environmental factors on the formation of microcapsules were systematically investigated and optimized. Quality assessments of each batch of microcapsules were performed using thermogravimetric analysis and scanning electron microscopy. Anticorrosion coatings mixed with synthesized microcapsules were prepared on a steel substrate. Preliminary results indicated significant corrosion retardancy happened in the self-healing coatings under an accelerated corrosion process, showing the great potential of this facile microencapsulation technique in development of catalyst-free, one-part self-healing coatings for corrosion control.

Flexible multidirectional strain sensors are crucial to accurately determining the complex strain states involved in emerging sensing applications. Although considerable efforts have been made to construct anisotropic structures for improved selective sensing capabilities, existing anisotropic sensors suffer from a trade-off between high sensitivity and high stretchability with acceptable linearity. Here, an ultrasensitive, highly selective multidirectional sensor is developed by rational design of functionally different anisotropic layers. The bilayer sensor consists of an aligned carbon nanotube (CNT) array assembled on top of a periodically wrinkled and cracked CNT–graphene oxide film. The transversely aligned CNT layer bridge the underlying longitudinal microcracks to effectively discourage their propagation even when highly stretched, leading to superior sensitivity with a gauge factor of 287.6 across a broad linear working range of up to 100% strain. The wrinkles generated through a pre-straining/releasing routine in the direction transverse to CNT alignment is responsible for exceptional selectivity of 6.3, to the benefit of accurate detection of loading directions by the multidirectional sensor. This work proposes a unique approach to leveraging the inherent merits of two cross-influential anisotropic structures to resolve the trade-off among sensitivity, selectivity, and stretchability, demonstrating promising applications in full-range, multi-axis human motion detection for wearable electronics and smart robotics.