Soft electronic materials for emerging fields

Stretchable and conformal electronic materials are highly desirable for growing interest in soft, wearable electronics and soft robotics. However, stretchable conductors, a key component in these systems, typically show limitations in terms of the ultimate stretchability, stable conductivity while stretching, and the ability to reconfigure or recycle to a pristine state for reusable and adaptable circuitry.  Here, we developed a soft composite material with liquid metal for soft, robust circuitry that is self-healing, reconfigurable, and ultimately recyclable.  This work was performed in Professor Michael Bartlett’s Soft Materials and Structures Lab at Virginia Tech in collaboration with postdoctoral researcher A B M Tahidul Haque.

By removing the hard case of traditional electronics, soft devices are introduced into a rugged world and new means to protect their functionality must be introduced. This necessitates the design of soft materials that can self-heal autonomously when damage occurs to maintain continued performance. In cases where severe damage occurs, or when you want to rapidly change the device functionality, innovative processing approaches are required to reconfigure the device and ultimately recycle the materials to create new devices. This highlights the need to design and develop highly tunable material systems that can thrive and function in diverse applications and unforgiving environments. Our work shows that a liquid metal (eutectic Ga-In alloy - EGaIn) - elastomer composite can provide solutions to these challenges. By combining a soft elastomer with droplets of liquid phase metal,  we achieve metal like electrical conductivity while maintaining a soft, skin-like mechanical response. By further controlling how the liquid metal droplets connect, we can tailor the electronic and mechanical properties of the composite to create robust, soft devices that self-heal, can be reformed while in use, and then ultimately be recycled to create multi-use soft electronic materials.

 High tunability of elastomer-plasticizer-liquid metal system

To incorporate the above highlighted features and realize robust soft electronics, we implemented a three-component system with elastomer, plasticizer, and liquid metal. The elastomer matrix is a styrene-isoprene-styrene (SIS) physically crosslinked block co-polymer that is thermoplastic in nature, offering the opportunity to reprocess the elastomer. To provide further mechanical tunability to the matrix, we chose polybutadiene (PBD) as a plasticizer additive. This combination enables important characteristics such as being soft, highly deformable, and tough while also providing local or global re-processability. We utilize a solution processing technique to create a suspension of discrete micron sized (~30 µm) liquid metal droplets homogenously dispersed in the SIS-PBD matrix. This mixture can then be cast and create a soft, highly deformable elastomer with dispersed liquid metal droplets. 

Figure 1: Key aspects of our soft, liquid metal circuit. (a) Creation of soft and stretchable conductive traces. (b) Self-healing when damaged. (c) Reconfiguration capabilities shown by erasing and creating a new trace. (d) Recyclable and reprocessable for multiple generations of electronic circuits.

Embossing as a rapid on-demand approach

For selective fabrication of tunable soft conductive traces, we introduce a scalable and tunable embossing approach. After casting and curing the pristine composite contains dispersed droplets of liquid metal. During embossing, the application of a compressive load through a patterned stamp results in a connected network of liquid metal droplets that are now electrically conductive (Fig 1a). This technique can be utilized as an on-demand approach to rapidly create electrically conductive traces.  The trace resistance can further be tuned during embossing and intricate circuity can be created through designed stamp architectures. During embossing the compressive loads applied and the resultant resistance in the region are simultaneously measured. This synchronous setup assists in tuning the resistance and applied load, where either of these parameters can be used to trigger and control the test algorithm. This technique is used to create highly stretchable resistors in the range of 10 Ω to 1 kΩ with conductivity of 190 to 0.95 S.cm^-1 respectively in an unstrained state. To show this resistance difference further qualitatively, we used these traces to power two LEDs and found that the LED paired with the 10 Ω resistor shines brighter. To highlight their operation in a circuit under strain, we interfaced an LED with embossed traces and stretched the circuit in tension where the LED maintains connection up to 240% strain.

We evaluated the electrical performance of the conductive traces by stretching them in tension. The resistance of a typical metallic conductor increases with stretch, but in the case of our liquid metal embossed lines, it is constant or decreases. The normalized resistance (R/R₀) for a trace is initially equal to 1 and decreases as a strain is applied to reach a value of 0.56 before it finally breaks at 1200% strain. The initial electrical conductivity calculated from the trace dimensions is 150 S.cm^-1 which reaches as high as 45 400 S.cm^-1 at 1200% strain. These interesting properties highlight the unique potential of liquid metal for soft yet high performing stretchable conductors.  

Soft circuit life cycle control

Our soft composite approach utilizes the unique ability of liquid phase inclusions to reconfigure their microstructure to make, maintain, and break robust liquid metal networks. Through our material system and the embossing approach, we demonstrated our liquid metal composites as promising candidates for regenerative electronics. We demonstrated how these perform in repetitive usage by testing under cyclic strain up to 1000 cycles. The stable electromechanical performance under tension even when significant sections of the embossed conductive traces are removed shows the instant self-healing ability (Fig 1b). In addition, reprocessing the physically cross-linked elastomer matrix enables the ability to process the liquid metal microstructure for network formation and subsequent reconfiguration (Fig 1c). In situations where the damage is beyond repair or the circuit needs to be completely modified, the material can be regenerated to the pristine microstructure by  bulk reprocessing (Fig 1d). As highlighted through their robust operation, self-healing ability, circuit reconfiguration, and recyclability we achieve complete life cycle control of the materials. These properties improve resilience in soft devices which can be utilized for diverse functionalities in soft electronics and robotics. Further, this combination of features in a single material system enables robust soft electronics that can reduce electronic waste through improved lifetime and recyclability.

Read the full paper here:
Tutika, R., Haque, A.B.M., Bartlett, M.D. "Self-healing liquid metal composite for reconfigurable and recyclable soft electronics", Communications Materials, 2021, 2, 64.