Crystal-confined freestanding ionic liquids for reconfigurable and repairable electronics

Abstract

Liquid sensors composed of ionic liquids are rising as alternatives to solid semiconductors for flexible and self-healing electronics. However, the fluidic nature may give rise to leakage problems in cases of accidental damages. Here, we proposed a liquid sensor based on a binary ionic liquid system, in which a flowing ionic liquid [OMIm]PF₆ is confined by another azobenzene-containing ionic liquid crystalline [OMIm]AzoO. Those crystal components provide sufficient pinning capillary force to immobilize fluidic components, leading to a freestanding liquid-like product without the possibility of leakage. In addition to owning ultra-high temperature sensitivity, crystal-confined ionic liquids also combine the performances of both liquid and solid so that it can be stretched, bent, self-healed, and remolded. With respect to the reconfigurable property, this particular class of ionic liquids is exploited as dynamic circuits which can be spatially reorganized or automatically repaired.

Fig. 1

Scheme of preparation and characterization of CCILs (crystal-confined ionic liquids). a Preparation of CCILs as a complex of [OMIm]PF₆ and [OMIm]AzoO through a super-saturated solution cooling method. b Photos of [OMIm]PF₆ (I) and CCILs with addition of 60 wt.% of [OMIm]AzoO (II). c DSC curves of [OMIm]PF₆ and CCILs loading different amounts of [OMIm]AzoO (20, 40, 60, 80, 100 wt%). d Crystal structure of [OMIm]AzoO obtained by single crystal X-ray diffraction. e–j Powder XRD analysis of CCILs loading different amounts of [OMIm]AzoO. e 0 wt%, f 20 wt%, g 40 wt%, h 60 wt%, i 80wt.%, j 100 wt%) (gray line: raw data; red line: enveloping lines; blue line: imitating peaks)

Fig. 2

Prediction and measurement of CCILs leakage from glass tubes with open ends. a Microscopic images of CCILs containing different contents of [OMIm]AzoO. Scale bar: 100 μm. b Schematic illustration of the different status of [OMIm]AzoO in [OMIm]PF₆, including solution state, suspended state, loose accumulation state and close-packed condition (from left to right). h: height of ILs in the glass tube; d: diameter of the glass tube; α: contact angle between the ILs and glass tube; β: contact angle between CCILs and glass tube. c A mathematical model demonstrating the pinning capillary effect by [OMIm]AzoO crystals. d Experimental demonstration of retention of CCILs with different contents of [OMIm]AzoO upon placing the glass tubes vertically (glass tubes have diameter of 3.0 mm and length of 50.0 mm). e The theoretical values of the maximum height of the CCILs with different contents of [OMIm]AzoO that the glass tube can hold without leakage phenomenon

Fig. 3

Electrical sensing performance of CCILs. a Relative conductivity change of CCILs with different contents of [OMIm]AzoO at different temperature (ΔG/G₀ is the relative conductivity change, ΔG = G-G₀). b On–off cycles of thermal response of CCILs operated between 60 °C and room temperature (25 °C) (red, orange, green and blue lines represent CCILs loaded with [OMIm]AzoO of 0, 20, 40, 60 wt%, respectively). c The process of breaking and healing of CCILs with an addition of 60 wt.% [OMIm]AzoO. Temperature is changed from 25–40 °C. The CCILs are sealed in a microchannel made by self-healing polymer. d Resistance change of CCILs filled in an elastic capillary tube at different bending angles. e Strain response of CCILs filled in a stretchable silicone tube

Fig. 4

Microscope observation of the self-healing process at a micron-size scale. a Original CCILs loaded with 60 wt.% [OMIm]AzoO (25 °C). b Damaged sample (25 °C). c–g Raising temperature from 25 to 60 °C. h, i Cooling temperature to 25 °C. Scale bar: 100 μm

Fig. 5

Freestanding and reconfigurable measurements. a Freestanding CCILs (60 wt.% of [OMIm]AzoO) with different shapes, from left to right: powder-like product, plum blossom, and simplified logo of the Renmin University of China. b Scheme of the transport of CCILs containing 60 wt.% of [OMIm]AzoO from the upper circuit to the bottom circuit. c Thermal response of the upper circuit indicated by a green LED at different temperatures (30, 40, 50, 60 °C). d The thermal response of the bottom circuits indicated by a blue LED at different temperatures (30, 40, 50, 60 °C)

Fig. 6

A self-healing robotic arm made of CCILs. a A wooden doll installed with a CCILs (60 wt.% of [OMIm]AzoO) arm connected with an intelligent temperature controlling system. b The process of cutting and healing of the robotic arm at different status: 0 s without injury; 140 s with injury; 140–1870 s self-healing behavior with manual intervention. c The diagram of a controlling circuit for probing and repairing mechanical damages. d Real-time resistance monitoring of the robotic arm at each state according to the self-healing process (b)