Braided NiTi alloys microfilaments with near-linear responses: toward flexible high-pressure sensors

Shape memory alloys (SMAs) are smart materials with superelasticity originating from a reversible stress-induced martensitic transformation (MT) accompanied by a significant electrical resistance change. However, the stress-strain and resistance-stress relationships of typical NiTi wires are non-linear due to the stress plateau during the stress-induced MT. This limits the usage of these materials as pressure sensors. Herein, we propose a high-strength flexible sensor based on superelastic NiTi wires that achieves near-linear mechanical and electrical responses through a low-cost double-braided strategy. This micro-architectured strategy reduces or even eliminates stress plateau and it is demonstrated that the phase transformation of microfilaments can be controlled: regions with localized stress undergo the MT first, which is successively followed by the rest of the microfilament. This structure-dependent MT characteristic exhibits slim-hysteresis superelasticity and tunable low stiffness, and the braided wire shows improved flexibility. The double-braided NiTi microfilaments exhibit stable electrical properties and repeatability under approximately 600 MPa (8% strain) and can maintain stability over a wide temperature range (303–403 K). Moreover, a cross-grid flexible woven sensor array textile based on microfilaments is further developed to detect pressure distribution. This work provides insight into the design and application of SMAs in the field of flexible and functional fiber.