Highly stretchable and mechanically tunable antennas based on three-dimensional liquid metal network

doi:10.1016/j.matlet.2020.127727

Materials Letters

Volume 270, 1 July 2020, 127727

https://doi.org/10.1016/j.matlet.2020.127727 Get rights and content

Highlights

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3-D LM composite with high electrical conductivity of 10⁵–10⁶ S/m is fabricated.
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Soft antennas with ultimate strain of 300% are achieved based on 3-D LM composite.
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The antenna presents a strain-independent reflection coefficient of around −30 dB.
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The antenna exhibits stretch-tuned resonant frequency over wide frequencies.
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Both softness of LM and protection from stiff layer contribute to the performance.

Abstract

We develop a class of stretchable dipole antennas based on embedding three-dimensional liquid metal network into an elastically soft elastomer as conductive branches, which can be highly stretched up to a strain of 300% while presenting high-quality reflection coefficient around −30 dB and a wide range of tunable resonant frequency from 1.55 to 0.45 GHz simultaneously. Neither mechanical damage nor performance degradation was observed during 100 stretch-release cycles under 100% strain for the antennas. The high electrical conductivity and deformation reversibility of the liquid metal, super stretchability of the soft elastomer and protection from the rigid layer are responsible for such a unique performance, enabling potential application for wireless strain sensors.

Graphical abstract

Introduction

Stretchable antennas, having the capability of wireless communication or sensing, are key components in the advanced flexible electronic systems, such as wearable wireless sensors, body injectable radio-frequency identification chips and human–machine communication systems [1], [2], [3], [4]. They can not only conformally conform to the non-planar surfaces of movable parts of human bodies and other objects, but also respond to the mechanical deformation with its spectral response in a wide range of frequency [3], [5]. Commonly, they are composed of three parts, including stretchable dielectric medium, rigid electrical feed connecter, and stretchable conductor [6], [7]. When improving the stretchability and reflection coefficient stability of the stretchable antennas, we have to face two limitations. First, the electrical conductivity of the current conductors deteriorates consistently with stretch that generally leads to a stretch-decreased reflection coefficient in the antenna [7], [8], [9]. Second, the area near the rigid feed connector of the antenna is susceptible to mechanical deformation and consequently results in a low stretchability.

Soft liquid metal (LM)-based composites have been recently developed as stretchable conductors and antennas [6], [10], [11], [12], [13], [14], [15], [16]. The softness of the LM contributes to the stretchability and gives rise to a stretch-increased electrical conductivity. For instance, a dipole antenna that can be stretched to a maximum strain of 120% was fabricated by injecting LM into elastomeric microfluidic channels [14]. Although much progress has been achieved, the maximum stretchability (120% strain) of the LM-based antennas is still much lower than that of its stretchable conductors (400% strain), as both microfluidic channels and rigid feed connector area in the antenna are susceptible to stretch. In this study, by embedding three-dimensional (3D) LM network into an elastically soft elastomer as conductive branches and coating stiff elastomer on the rigid feed connector region as protection layer, a class of highly stretchable dipole antennas with an invariant reflection coefficient that can be maintained up to a strain of 300% were reported.

Section snippets

Materials and methods

The 3-D LM composite was prepared by using sugar cube as templet. Firstly, the eutectic gallium-indium LM alloy with a melting point of ~15 °C is prepared by mixing 75.5% gallium and 24.5% indium by weight at 200 °C. The sugar templates were produced via hot pressing the sugar particles under 10 MPa at 100 °C for 1 h. Then, the 3-D LM network was obtained by vacuum infiltrating LM into the porous channels inside the sugar template which was followed to be dissolved in iced water (0 °C), as

Results and discussions

Fig. 1(a) shows that the original 3-D LM composite with a diameter of 90 mm can be stretched to be a rectangular with 250 mm in length and 200 mm in width. Such super stretchability and softness can be ascribed to the deformability of the LM when the stress is applied. Notably, Fig. 1(b) shows that the 3-D LM composite presents a unique stretch-increased electrical conductivity, for example, from 8.1 × 10⁵ S m-1 at unstretched condition to 1.3 × 10⁶ S m-1 under 200% strain. This high electrical

Conclusion

A novel antenna with a stretchability of 300% strain, wide stretch-tuned resonant frequency, excellent reflection coefficient around −30 dB and strain-independent performance was presented. Such a super performance is attributable to the deformability of LM and the combination of soft matrix as branch where strain mainly occurs and rigid layer to protect the weak region from overstrained. Our approach brings about a unique design to develop highly stretchable and mechanically tunable antennas

CRediT authorship contribution statement

Bin Yao: Investigation, Formal analysis, Data curation, Writing - original draft. Xinwei Xu: Investigation, Formal analysis, Data curation, Writing - original draft. Qingfeng Zhang: Resources, Writing - original draft. Hao Yu: Resources, Writing - original draft. He Li: Formal analysis, Data curation. Lulu Ren: Formal analysis, Data curation. Steven Perini: Resources. Michael Lanagan: Resources. Qing Wang: Conceptualization, Supervision, Writing - review & editing. Hong Wang: Conceptualization,

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

B.Y. was supported by a fellowship from the China Scholarship Council (CSC). H.W. acknowledged the support of the National Science Foundation of China (No. 61631166004), Shenzhen Science and Technology Program (Nos. KQTD20180411143514543 and JCYJ20180504165831308) and Shenzhen DRC project [2018]1433.

References (17)

Y. Sohn et al.
Mater. Lett.
(2020)
M. Park
Nat. Nanotechnol.
(2012)
T. Someya
Nature
(2016)
Z. Xie
Adv. Mater.
(2019)
M. Rashed Khan
Appl. Phys. Lett.
(2011)
Y. Li
Adv. Funct. Mater.
(2019)
S. Cheng
Appl. Phys. Lett.
(2009)
Z. Li
Adv. Funct. Mater.
(2015)

There are more references available in the full text version of this article.

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