Gallium-Carbon: A Universal Composite for Sustainable 3D Printing of Integrated Sensor-Heater-Battery Systems in Wearable and Recyclable Electronics

Abstract

This study presents a novel three-dimensional (3D) printable gallium-carbon black-styrene isoprene styrene block copolymer (Ga-CB-SIS), offering a versatile solution for the rapid fabrication of stretchable and integrated sensor-heater-battery systems in wearable and recyclable electronics. The composite exhibits sinter-free characteristics, allowing for printing on various substrates, including heat-sensitive materials. Unlike traditional conductive inks, the Ga-CB-SIS composite, composed of gallium, carbon black, and styrene isoprene block copolymers, combines electrical conductivity, stretchability, and digital printability. By introducing carbon black as a filler material, the composite achieves promising electromechanical behavior, making it suitable for low-resistance heaters, batteries, and electrical interconnects. The fabrication process involves a simultaneous mixing and ball-milling technique, resulting in a homogeneous composition with a CB/Ga ratio of 4.3%. The Ga-CB-SIS composite showcases remarkable adaptability for digital printing on various substrates. Its self-healing property and efficient recycling technique using a deep eutectic solvent contribute to an environmentally conscious approach to electronic waste, with a high gallium recovery efficiency of ∼98%. The study's innovation extends to applications, presenting a fully digitally printed stretchable Ga-CB-SIS battery integrated with strain sensors and heaters, representing a significant leap in LM-based composites. This multifunctional and sustainable Ga-CB-SIS composite emerges as a key player in the future of wearable electronics, offering integrated circuits with sensing, heating, and energy storage elements.

Keywords: 3D printable ink; Ga–CB–SIS ink; energy storage device; liquid metal composite; recyclable; sinter-free; strain sensor; wearable thermal devices.

Figure 1

Ga–CB–SIS composite: (A) constitutional materials, characterization, 3D printing; (B) applications; and (C) fabrication of an integrated sensor–heater–battery system.

Figure 2

(A) Schematic of the preparation process of the Ga–CB–SIS composite. (B) Resistance of Ga–CB–SIS composite vs different CB/Ga ratio. (C) Cross-sectional scanning electron microscopy (SEM) images of the Ga–CB–SIS composite. (D) Printed trace with light-emitting diodes (LEDs) subjected to different percentages of strain (Max strain ∼200%). (E) Electromechanical characterization of Ga–CB–SIS composite: (i) resistance vs time, (ii) resistance vs strain (30, 50, and 100%) cycling for 10 times, (iii) resistance vs time, and (iv) resistance vs strain 100% cycling for 1000 cycles. In (C), (D), and (E), the CB/Ga weight ratio was 0.043.

Figure 3

2D and 3D digital printability of the Ga–CB–SIS composite ink over different substrates. (A) Digital printer, 2D and 3D printing state over different substrates, (B) TPU, (C) polycarbonate sheet, (D) acetate sheet, and (E) foam.

Figure 4

(A) (i) Digitally printing the interconnections for the strain sensor, (ii) full printed interconnections, (iii) wearable patch and acquisition board applied on the volunteer’s hand, and (iv–vi) performance of strain sensor with bending the fingers. (B) e-textile heater (i) and (ii) original state, (iii) and (iv) under 30% strain, (v) and (vi) under 50% strain. (C) Thermal image of different printed patterns.

Figure 5

(A) Printing steps of the Ag₂O–Ga battery, Ga–CB–SIS as the anode, and Ag₂O–CB–SIS as the cathode. (B) (i) Original state of the digital printable behavior of the assembled Ag₂O–Ga battery, the red LEDs are in the on-state, (ii) stretching and (iii) folding states, as is also shown in Video S5. (C) Potential profile of the Ag₂O–Ga stretchable battery at 2 mA·cm^–2. (D) Capacity of the battery under 0, 30, and 50% strain at 2 mA·cm^–2. (E) Galvanostatic discharge–charge cycle performance of the battery at 0.8 mA·cm^–2: (i) all cycles and (ii) the cycles 70–80 s.

Figure 6

(A) Schematic presentation of DES preparation with oxalic acid and choline chloride, molar ratio 2:1, and (B) recycling of the Ga–CB–SIS composite ink e-waste via the DES method.