Liquid-metal/NdFeB/Ni composites for high wettability, patternable, stretchable electronic wire

In the past few years, the development of flexible and stretchable electronics devices has grown exponentially.^1,2 To meet the needs of human motion tracing, physiological signal measurements, and robotic sensing, wearable devices must have good flexibility and stretchability.^3–5 Electronic wires are crucial for the design of stretchable electronic devices. It not only achieves electrode connection of functional components but also ensures efficient transmission of circuit signals.⁶ 

Due to its high conductivity and stretchability, liquid-metal (LM) is considered as ideal materials for manufacturing stretchable wires.^7–11 Research on liquid metal ink and printing technology can enable people to manufacture liquid metal circuits on various flexible substrates through coating, screen printing, or direct writing printing methods.^12–15 However, the poor wettability and adhesion caused by high surface tension remain the primary problems of liquid metal wires. Compared to traditional electronic ink, liquid metal ink has a greater risk of leakage and damage, which can easily cause circuit failure. Moreover, the increase in resistance of liquid metal wires caused by stretching and squeezing is also an issue that cannot be ignored. Moreover, the increase in the resistance of liquid metal wires caused by stretching and squeezing is also an issue that cannot be ignored. This phenomenon, which is commonly applied to piezoresistive sensors,^16–18 can lead to an increase in wire loss and signal attenuation.

Adding microparticles is an effective method to improving the wettability of liquid metals. Utilizing the oxidation effect of liquid metals to enhance wettability is a fundamental method for manufacturing liquid-metal composites and thus derived control methods for adding inorganic oxide particles.¹⁹ However, inorganic oxide microparticles reduce the conductivity and thermal conductivity of liquid metals. By adding metal particles, such as Cu and Ag, the wettability, conductivity, and thermal conductivity of liquid metals can be greatly improved.^20,21 In addition, by adding nickel particles, magnetic field assisted patterning of liquid metals can be achieved.²² However, the stability of LM composites is poor, and the metal particles may settle or precipitate in the liquid-metal matrix, thereby altering the performance of LM wires.

The study of liquid metal wires based on NdFeB particles is a landmark study. Some researchers have reported that magnetized NdFeB particles have a good locking effect on liquid metals,^23,24 and LM/NdFeB composites has a more stable structure compared to LM/soft magnetic composites. He reported the first application case of LM/NdFeB composites as a coatable electronic wire.²³ Then, Liu’s team proposed a liquid metal composites doped with nano-silver modified NdFeB particles.²⁵ Due to the porous structure constructed from NdFeB particles, it can effectively prevent the leakage of liquid-metal electronic wires. The silver modification process increases the manufacturing difficulty and cost of LM composites. Moreover, the addition of non-magnetic particles will reduce the locking ability of NdFeB particles on LM.

To sum up, developing a LM composites with properties of wettability, patternable, easily manufactured is still a challenging task. Therefore, this paper presented a liquid metal/NdFeB/Ni composites for stretchable electronic wires. By adding nickel particles, the properties of LM/NdFeB composites were improved. The mechanism and experimental results are given in the following sections.

Figure 1 illustrates the working principle of the proposed LM/NdFeB/Ni composites. In Fig. 1(a), the porous structure composed of NdFeB and Ni particles can effectively store liquid-metal and reduce its surface tension. As a soft magnetic medium with high permeability, the submicron or nanoscale nickel microspheres can enhance the magnetic force between magnetized NdFeB particles. The presented LM composites can be patterned through surface coating (directed writing) and microchannel filling, as shown in Fig. 1(b). Figure 1(c) gives another patterning method, i.e., magnetic field assisted direct writing. Figure 1(d) exhibits the reason for anti-squeezing and anti-stretching of LM composites based electronic wire—the plastic magnetic mud is not easily deformed during extrusion.

In this work, Galinstan alloy (Ga_68.5In_21.5Sn₁₀) with a meting point of −19 °C was used as the matrix of the proposed LM composites. The low meting point makes it to have the ability of working in low-temperature environments. The size of neodymium–iron–boron (Nd₂Fe₁₄B) microparticles was 5 μm. The chosen size of the Ni microparticle was 50 and 500 nm. The physical parameters of the raw materials are given in Table I.

99.9% pure Ni microparticles, magnetized Nd₂Fe₁₄B microparticles, and Galinstan were put in the agate mortar and grounded in air for 5 min. Then, the mixture was put in a mechanical mixer and stirred for 10 min. We set the mass ratio of NdFeB and Ni to 1:2 and adjusted the content of Galinstan to prepare LM/NdFeB/Ni composites with different components. LM/NdFeB composites was also prepared for comparison. The components of the prepared composites are given in Table II.

As shown in Fig. 2, the microstructure of LM/NdFeB/Ni composites was observed by scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS). Figure 2(a) shows the micrographs of 5 μm NdFeB particles and 50 nm Ni particles. SEM photographs of LM/NdFeB/Ni composites are shown in Fig. 2(b), and NdFeB microparticles coating with LM can be seen in the photograph. The EDS analyses are given in Fig. 2(c), and Ga, In, Sn elements from Galinstan, Nd, and Fe elements from NdFeB, and Ni element were found in the photographs. Moreover, the formation of porous structures and the distribution of Galinstan in LM composites can be observed through the energy spectra of Ga, Nd, Fe, and Ni elements.

In Fig. 3, the x-ray diffraction (XRD) results show the evident for phases of Ni@50 nm, NdFeB, and Galinstan in the LM composites.

Figure 4 shows the magnetic properties of the LM/NdFeB/Ni composites. The hysteresis curves of three kinds of LM composites (LM/NdFeB, LM/NdFeB/Ni@50 nm, and LM/NdFeB/Ni@500 nm) with a liquid metal content of 73.8% are shown in Fig. 4(a). LM/NdFeB composites has the highest coercivity and magnetization remanence. The magnetic properties of LM composites doped with 50 nm Ni particles are slightly better than those doped with 500 nm Ni. With the increase in the liquid metal content, its saturation magnetization will also decrease [Fig. 4(b)].

The measurement results of hysteresis loops indicate that Ni regulated LM composite materials exhibit both soft and hard magnetic properties. When there is no external magnetic field, the presence of Ni particles can enhance the magnetic attraction before NdFeB particles (as described in Sec. II A). Moreover, this regulatory effect is related to the relative size of NdFeB and Ni particles.

Figure 5 shows the wettability and adhesion properties of LM and LM composites on different surfaces. The results show that LM composites perform better wettability on the silicone and glass surfaces [as shown in Figs. 5(a) and 5(b)] than LM. Figure 5(c) shows the wettability of them in open microchannels, and due to the low surface tension and plasticity of LM composites, it can effectively fill the microchannels. Figure 5(d) gives the adhesion comparison between LM and LM/NdFeB/Ni composites, which proves that our proposed LM/NdFeB/Ni composites has better adhesion than pure LM.

The writing results using two different patterning methods are shown in Fig. 6. The first method is direct writing—coating LM composites on the surface of polyvinyl chloride plastic (PVC). Figure 6(a) shows the letters written by using this method. The second method is magnetic assisted patterning. By using this method, a planar was drawn [in Fig. 6(b)]. These two methods can be used under different scenarios.

As described in Sec. II B, LM composites with different component ratios were prepared. We found that the maximum weight ratio of the liquid metal and magnetic particles is 0.59:0.41. Once the ratio is exceeded, LM composites will transition from plastic mud state to sandy state. In Fig. 7, the compressive properties of two LM composites with a Galinstan content of 73.8% are compared.

In the experiment, the two composites were compressed 50% vertically. As given in Figs. 7(a) and 7(b), the required stress is 3.52 and 5.29 N for LM/NdFeB and LM/NdFeB/Ni composites, respectively. This indicates that under the same weight ratio conditions, LM/NdFeB/Ni composites have better compressive strength than LM/NdFeB composites. This is due to the improvement of NdFeB porous structure by Ni microspheres. On the other hand, NdFeB and Ni magnetic particles of the same weight can lock more liquid metals, than NdFeB particles.

In Fig. 8(a), the resistance of two different LM composites with volume ratios of 0.68:0.064:0.256 and 0.68:0.32 is compared. Using the Kelvin four-terminal sensing method, the LM composites’ conductivity with/without Ni particles (500 nm) is 2.67 × 10⁶ and 1.66 × 10⁶ S/m, separately. We also fabricated two flexible electronic wires to test their strain-insensitive ability. Figure 8(b) is the resistance variation contrast figure of the flexible wires with or without Ni particles. These demonstrate that adding Ni microparticles into liquid metal/NdFeB composites can reduce the initial resistance, and its change rate is also lower when being stretched.

The proposed LM composites have the properties of anti-stretching, anti-leakage, and anti-tamper. Figure 9 gives two application cases of LM composites’ electronic wires. As shown in Fig. 9(a), LM/NdFeB/Ni composites and LM are filled in the silicone-based microchannels, as a part of the LED power supply circuit. Under horizontal tension, the LM wire breaks while the LM composites’ wire remains conductive. In Fig. 9(b), LM composites’ electronic wire is coated on a paper, and due to its high wettability and adhesion, it can effectively resist damage from sharp objects on the wire.

In this paper, we proposed LM/NdFeB/Ni composites for stretchable electronic wires. Ni microparticles were used to improve the structure, magnetic properties, and mechanical properties of LM composites. The results show that the proposed composites exhibit good wettability, conductivity, and stress/strain insensitive properties. Its patterning methods are also diverse. We have demonstrated through some cases that electronic wires made of LM composite materials have excellent characteristics of anti-leakage and anti-damage. LM/NdFeB/Ni composites are very suitable for constructing various flexible and stretchable electronic circuits, and as electrodes for sensors.

In future studies, we intend to investigate how to use LM/NdFeB/Ni composites to achieve 3D direct printing of electronic wires.

This work was supported by Special Fund for Basic Scientific Research Business of Zhongyuan University of Technology (Grant No. K2022YY008) and Science and Technology Research Project of Jiangxi Provincial Department of Education (Grant No. GJJ180936).

The authors have no conflicts to disclose.

Zhifang Zhu: Conceptualization (equal); Methodology (equal); Writing – original draft (lead). Ran Zhao: Data curation (equal); Funding acquisition (equal); Writing – original draft (supporting). Bingliang Ye: Project administration (equal); Supervision (equal); Writing – review & editing (equal). Huan Wang: Funding acquisition (equal); Writing – review & editing (equal).

The data that support the findings of this study are available within the article.