Jammed Microgels in Deep Eutectic Solvents as a Green and Low-Cost Ink for 3D Printing of Reliable Auxetic Strain Sensors.

Additive manufacturing is a promising technique for offering novel functionality to various materials by creating three-dimensional (3D) structures. However, the development of sustainable synthesis processes for 3D printing inks or 3D-printed materials remains a major challenge. In this work, a simple two-step mixing approach is developed to prepare a 3D printing ink from green, low-cost, and low-toxicity materials [commercial Carbopol and deep eutectic solvents (DESs)]. A small weight fraction of Carbopol can impart desired rheological properties to the DES used in the 3D printing ink and also can significantly enhance the stretchability of eutectogels up to 2500% strain. The 3D-printed auxetic structure shows a negative Poisson's ratio (within 100% strain), high stretchability (300%), high sensitivity (gauge factor of 3.1), good moisture resistance, and sufficient transparency. It can detect human motion with high skin comfort and breathability. The results of this work highlight a green, low-cost, and energy-saving strategy to fabricate conductive microgel-based inks for 3D printing of wearable devices.