A hierarchical tuning strategy for continuously adjustable phase-transition ionic conductors toward multimodal sensing

Abstract

Conventional phase-transition ionic conductors (PTICs) based on ionic liquids (ILs) suffer from a fixed resistance-switching temperature (T_RS), intrinsically limited by the immutable melting point of ILs, which restricts their applications in scenarios requiring specific thermal triggers. Herein, we propose a general hierarchical strategy to achieve continuous and precise regulation of T_RS. This is accomplished by leveraging the well-defined relationship between the melting temperature (T_m) of polyethylene glycol (PEG) and its molecular weight (M_n) for coarse adjustment, followed by fine-tuning via blending with lower-M_n PEG or incorporating a plasticizer, succinonitrile (SN). The resulting PEG/PDES-Li-based PTICs enable wide-range tuning of T_m and T_RS from 37 to 59 °C with a precision of ∼1 °C. The optimized conductor (PTIC-4) demonstrates an ultrahigh negative temperature coefficient of resistance (TCR) of −7.64% °C⁻¹ within 30–40 °C, allowing for the detection of subtle temperature variations. Moreover, the material undergoes a reversible transparent-to-opaque transition at T_RS, facilitating intuitive visual thermometry. Beyond temperature sensing, the conductor also functions as a high-performance strain sensor for monitoring human joint motions and even Morse code communication. This work establishes a versatile platform and a general design principle for the development of intelligent wearable devices, medical monitoring systems, and human–computer interaction interfaces.