Christopher E. Tabor

Advances in flexible hybrid electronics have ushered in a range of new applications from ultra-stretchable circuitry for soft robotics to textile-integrated wearable electronics. The onset of the Internet of Things demands even more from these flex-hybrid electronic materials and devices. Specifically, operation in the gigahertz range for transmission, receiving, and transporting data for high-speed operation and near field communication while still maintaining mechanical compliance will be transformative in many commercial areas such as industrial manufacturing, healthcare, and athletics. In this work, we present a stretchable RF transmission line based on a polymerized liquid metal network (Poly-LMN) patterned onto a stretchable dielectric substrate. The Poly-LMN material has metallic conductivity and notably exhibits a high degree of strain insensitivity that is uniquely well-suited to the requirements of stretchable RF devices. The fabrication and characterization of coplanar waveguide transmission lines are presented. The Poly-LMN material maintains consistent transmission and reflection RF performance whereas other stretchable conductors exhibit increased loss while subjecting devices to strains up to 100%. Additional characterization demonstrates the robustness of the chosen material set when subjected to 1,000 cycles of strain and to extreme strains of 300%. Finally, several applications of the stretchable RF conductor are presented, including highly stretchable and self-healing coaxial cables and a peel-and-stick double stub impedance matching system.

Room‐temperature liquid metal particles based on gallium have become significantly important for developing next‐generation soft electronics. Eutectic gallium‐indium (EGaIn) alloys can be fabricated into core‐shell particles discretely encapsulated by a passivating oxide. Application of mechanical stimuli results in rupturing the molten core through the oxide shell, merging EGaIn particles to form conductive pathways. Generally, the mechanical properties of EGaIn are largely defined by the native oxide which imparts viscoelastic properties to the fluid. In this work, the practical implications of EGaIn deformation and fracture behavior with the native oxide and a non‐native inorganic silica shell are demonstrated. To augment the mechanical properties of EGaIn, silica nanoshells are introduced as a chemically inert coating to enable brittle fracture of particles. In situ single‐particle nanoindentation characterization reveals the environmental and geometrical considerations for particle deformation and fracture. Silica‐coated EGaIn particles reach stiffness values at least an order of magnitude higher than that of native EGaIn particles. The thickness of the silica shell can be tailored to further modify the mechanical behavior of EGaIn, enabling potential pressure‐sensitive conductivity. These results provide additional pathways to understand the design and implementation of functionalized EGaIn particles for future applications in mechanoresponsive electronics, plasmonics, and therapeutics.

Next generation textile‐based wearable sensing systems will require flexibility and strength to maintain capabilities over a wide range of deformations. However, current material sets used for textile‐based skin contacting electrodes lack these key properties, which hinder applications such as electrophysiological sensing. In this work, a facile spray coating approach to integrate liquid metal nanoparticle systems into textile form factors for conformal, flexible, and robust electrodes is presented. The liquid metal system employs functionalized liquid metal nanoparticles that provide a simple “peel‐off to activate” means of imparting conductivity. The spray coating approach combined with the functionalized liquid metal system enables the creation of long‐term reusable textile‐integrated liquid metal electrodes (TILEs). Although the TILEs are dry electrodes by nature, they show equal skin‐electrode impedances and sensing capabilities with improved wearability compared to commercial wet electrodes. Biocompatibility of TILEs in an in vivo skin environment is demonstrated, while providing improved sensing performance compared to previously reported textile‐based dry electrodes. The “spray on dry—behave like wet” characteristics of TILEs opens opportunities for textile‐based wearable health monitoring, haptics, and augmented/virtual reality applications that require the use of flexible and conformable dry electrodes.

The characterization of an RF transmitter composed of insulating and conducting regions of liquid‐metal embedded elastomer (LMEE) is presented along with in situ measurements of LMEE microstrip lines as they are subjected to local material strains up to 40%. The LMEE is comprised of microscale droplets of a gallium–indium alloy that is liquid at room temperature and suspended within a cured elastomer matrix of polydimethylsiloxane (PDMS). The liquid metal microparticles were initially electrically isolated, but applying mechanical loading caused the permanent formation of highly localized conductive traces. Bonding films of LMEE onto a PDMS dielectric layer resulted in a stretchable microstrip structure that is capable of radio frequency (RF) transmission. Scattering parameter (S‐parameter) measurements for reflection and transmission are presented for these microstrip lines as the electrical length increased up to 19%. A customized clamp was utilized to isolate the mechanical strain on the material from the electrical connectors and allow for transmission line characterization under applied strain and dielectric characterization of the LMEE material was performed. The stretchable microstrip lines show remarkable consistency in transmission response at 0.5–5 GHz when mechanically loaded to 40% strain for 1000 loading cycles.

Advances in materials, designs, and controls are propelling the field of soft robotics at an incredible rate; however, current methods for prototyping soft robots remain cumbersome and struggle to incorporate desirable geometric complexity. Herein, a vat photopolymerizable self-healing elastomer system capable of extreme elongations up to 1000% is presented. The material is formed from a combination of thiol/acrylate mixed chain/step-growth polymerizations and uses a combination of physical processes and dynamic-bond exchange via thioethers to achieve full self-healing capacity over multiple damage/healing cycles. These elastomers can be three dimensional (3D) printed with modular designs capable of healing together to form highly complex and large functional soft robots. Additionally, these materials show reprogrammable resting shapes and compatibility with self-healing liquid metal electronics. Using these capabilities, subcomponents with multiple internal channel systems were printed, healed together, and combined with functional liquid metals to form a high-wattage pneumatic switch and a humanoid-scale soft robotic gripper. The combination of 3D printing and self-healing elastomeric materials allows for facile production of support-free parts with extreme complexity, resulting in a paradigm shift for the construction of modular soft robotics.

Inorganic Ga–In alloy nanoparticles suspended in a traditional thermal transport fluid simultaneously increase the overall thermal diffusivity of the fluid and serve as a cyclable solid–liquid PCM slurry, providing a thermal sink definable over a wide temperature range.

Gallium alloys with other low melting point metals, such as indium or tin, to form room-temperature liquid eutectic systems. The gallium in the alloys rapidly forms a thin surface oxide when exposed to ambient oxygen. This surface oxide has been previously exploited for self-stabilization of liquid metal nanoparticles, retention of metastable shapes, and imparting stimuli-responsive behavior to the alloy surface. In this work, we study the effect of water as an oxidant and its role in defining the alloy surface chemistry. We identify several pathways that can lead to the formation of gallium oxide hydroxide (GaOOH) crystallites, which may be undesirable in many applications. Furthermore, we find that some crystallite formation pathways can be reinforced by typical top-down particle synthesis techniques like sonication. This improved understanding of interfacial interactions provides critical insight for process design and implementation of advanced devices that utilize the unique coupling of flexibility and conductivity offered by these gallium-based liquid metal alloys.

Graphene oxide encapsulation of liquid metal particles creates a physical barrier that protects the particles against chemically etching environments and mechanical input.