Elastic conductive materials for wearable devices

Abstract

Wearable devices are of paramount importance for human life in such an era of everything interconnected, smart, and intelligent. The flexible system springs up with anticipation of various applications such as flexible display, flexible E-textiles, and human healthcare. More researchers are dedicated to designing distinctive devices for on-body signal monitoring, including electrocardiography, electromyography, blood pressure, etc. To realize these functions, the conductive materials play an indispensable role in the epidermal signal collection and body movement detection with elastic conductors and sensors, respectively. While designing various conductors and sensors, two dominant issues should be comprehensively addressed: first, the fabrication of high-conductive conformal elastic conductors for e-skin applications; second, the design of flexible sensors for body movement recognition and monitoring.

To fabricate a high-performance elastic conductor, the Gallium-based liquid metal (LM) was applied due to its excellent electrical property upon elongation and dynamic deformations. The LM addresses the stretchability-conductivity dilemma that matters in the development of high-performance elastic conductors. However, the high surface tension of LM limits its processability for screen printing, direct printing, membrane fabrication, etc. Researchers employed a series of methodologies, such as polymer-LM mixture, hydrogen bond enhancements, metal alloying, and micro-nano size refinement, to reduce the surface tension of LM and increase the adhesion of LM to elastic substrates. Nevertheless, the interaction and structure design of the interface between LM and elastomer are understudied. And it is still a major challenge to simultaneously achieve electrical stability upon stretching and cyclic durability for LM-based stretchable electrodes.

In the first project, we report a nano-liquid metal (LM)-based highly robust stretchable electrode (NHSE) with a self-adaptable interface that mimics water-to-net interaction. Based on the in situ assembly of electrospun elastic nanofiber scaffolds and electrosprayed LM nanoparticles, the NHSE exhibits an extremely low sheet resistance of 52 mΩ sq−1. It is not only insensitive to a large degree of mechanical stretching (that is, 350% electrical resistance change upon 570% elongation) but also immune to cyclic deformation (that is, 5% electrical resistance increases after 330,000 stretching cycles with 100% elongation). However, this electrode suffers from impermeability of air and moisture as applied to human skin, which would cause skin inflammation and irritation. An LM-based stretchable conductor that provides permeability, leakage prevention and electrical robustness, simultaneously is still missing.

The second project address the above issue by introducing an LM-based leakage-free permeable stretchable conductor (LPSC) by developing a novel LM fibre scaffold and in-situ encapsulation technique. An unprecedented permeable encapsulation of LM fibres (~30 μm in diameter) is achieved through ‘anchor-like’ TPU nanofibers via electrospinning. Even under a mechanical pressure like 100 kPa, the LM fibres will be confined inside the encapsulation layer. In the meanwhile, the LPSC exhibits outstanding permeability in air and moisture and retains an ultralow sheet resistance (16 mΩ/square). However, without structure design, the multi-substrate wettability of LM is also significant for its application. And the optimal conductivity-adhesion composition of LM should be further studied for the screen printing of LM.

In the third project, we developed an LM oxidization methodology that enables LM paste with different viscosities. This Oxidized LM (O-LM) shows excellent adhesion and processibility as applied on the elastic substrates, like PDMS, VHB tape, and SBS. And the O-LM presents exceptional electrical property upon uniaxial stretching and cyclic deformation. The resistance of 4h O-LM barely changed as the sample was stretched to 1500% elongation. especially under the stain of 1500%. The above three conductors exhibit excellent conductivity under deformations. However, for the collection of all healthcare signals, flexible sensors are highly required to realize on-body movement, gesture, and pulse signal detection.

In the fourth project, a multimodal flexible wearable sensor (MFWS) was proposed for addressing the multi-stimuli differentiation (e.g., pressure, bending direction, stretching direction) in complex application scenarios. The MFWS was fabricated by a highly pressure-sensitive carbon nanotubes-polydimethylsiloxane (CNTs/PDMS) porous sponge that was sandwiched between two sets of interdigital electrodes. Furthermore, the conductive porous sponge guarantees the excellent performance of the sensor (high sensitivity, large measurement range, as well as high stability) owing to its high specific surface conductive area, outstanding mechanical flexibilities and compressibility. And with the combination of laser cutting treatment, the interdigitated nano-network electrode possesses highly conductive and stretchable properties based on the cost-effective mask-free operation and short the processing cycle. As a result, this flexible sensor exhibits promising utility for a new generation of accurately detecting the complexity of human body motions.

In summary, this thesis has introduced the fabrication of conductive materials for realizing highly robust electrodes, air-permeable and leakage-free conductors, multi-substrate wettable LM, and multi-mode flexible sensors. A flexible system that integrates stretchable electrodes and sensors for human healthcare would be the focus of our next investigation.

Date of Award	Jul 2022
Original language	English
Awarding Institution	University of Nottingham
Supervisor	Guang Zhu (Supervisor), Run Wei Li (Supervisor) & George Chen (Supervisor)