Development of Highly Stretchable Liquid Metal Electronics for Health Care


Student thesis: Doctoral Thesis

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Award date11 Sept 2023


Wearable electronics are paramount for human life in such an era of everything being interconnected, smart, and intelligent. In addition, they are usually employed on human epidermal skin to realize various applications such as flexible displays, flexible e-textiles, e-skin, optoelectronic skin, and, most importantly, human healthcare. Electrodes with high stretchability are of great importance in these devices. Liquid metals (LMs) are considered one of the most promising materials for constructing stretchable electrodes because of their high conductivity and fluidity. However, due to its high surface energy, the adhesion between liquid metal and substrates or functional chips is poor. This makes liquid metal electrodes and their electronic equipment easy to fail under large deformation, limiting their practical application. Therefore, developing stretchable and adhesive LM-based electrodes is crucial for their application in health care, such as epidermal sensors for strain/pressure sensing, bio-potential monitoring and even advanced wound care.

Polymers and supramolecular polymers with abundant metal-chelating or hydrogen-bonding motifs that can form interfacial interactions with gallium oxide layers are promising carriers to construct highly stretchable LM electronics. In this research, by adjusting the interfacial interaction between polymers and the oxide layer of LMs, we developed a series of highly stretchable and adhesive LM conductors and electronics. We also explored their biomedical applications, such as human motion detection, healthcare monitoring and advanced wound care. Specifically, there are three parts in this thesis.

In the first part, poly (vinylidene fluoride-co-chlorotrifluoroethylene) (PVDF-co-CTFE), with high stretchability and piezoelectric properties was selected as the polymer matrix. Through a simple dispersion method of solution sonication and casting, we developed LM-polymer composites with high conductivity and ultra-stretchability (up to 10 000%). We systematically studied the dynamic interfacial interactions between the gallium oxide layer and the polyvinylidene fluoride (PVDF) copolymer matrix. This robust interfacial interaction allowed LM channels to remain integral at high strains (5000-10 000%), enabling low conductivity/resistance changes (7.7-20.3) of LM-polymer composites. Combining the features of the high conductivity and the piezoelectricity of the polymer matrix, LM-polymer composites demonstrated great potential as stretchable electrodes and wearable sensors in different strain ranges.

In the second part, we used the same method to construct LM-PU composites, which were further used to construct a multi-mode tactile sensor. The tactile sensor had two sensing layers: a piezoresistive layer for mimicking the slow adapting (SA) mechanoreceptor and a triboelectric sensing layer for simulating the fast adapting (FA) mechanoreceptor. Because of its good interfacial interaction between graphene oxide and the oxide layer of liquid metal, graphene oxide facilitated micro-nano scaled LM droplets uniformly dispersed in the PU matrix, improving piezoresistive properties. Moreover, LM can deform over a wide range of pressures. Therefore, the multi-mode tactile sensor exhibited a wide range of responses. Under the synergistic effects of the triboelectric and piezoresistive sensors, our tactile sensor can detect complex physical stimuli.

Supramolecular polymers with dynamic non-covalent interactions and high interfacial adhesion are highly desired carriers for developing fully integrated bioelectronics. Therefore, in the third part, we prepared a highly adhesive LM electrode by introducing a highly adhesive supramolecular polymer. The as-prepared LM electrodes showed a high adhesion strength of 8.9 MPa, enabling a high stretchability of 1154% when integrated with functional chips. Besides, the supramolecular polymer bonded to the surface of micro-nano scaled LM particles significantly improved its biocompatibility in a physiological environment. Taking advantage of its high surface adhesion and biocompatibility, the as-prepared LM electrodes can conformably contact with skin, enabling continuous recording of epidermal biopotential with high fidelity and signal-to-noise ratio. Besides, it can offer electrical stimulation to accelerate wound healing.

Overall, we prepared various functional LM-based electrodes with tailored properties, which show high stretchability and interfacial adhesion for healthcare monitoring, such as epidermal sensors for strain/pressure sensing, bio-potential monitoring and even advanced wound care.