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Material extrusion (MEX) of soft, multifunctional composites consisting of liquid metal (LM) droplets can enable highly tailored properties for a range of applications from soft robotics to stretchable electronics. However, an understanding of how LM ink rheology and print process parameters can reconfigure LM droplet shape during MEX processing for in-situ control of properties and function is currently limited. Herein, the material (ink viscosity, and LM droplet size) and process (nozzle velocity, height from print bed, and extrusion rate) parameters are determined which control LM microstructure during MEX of these composites. The interplay and interdependence of these parameters is evaluated and nearly spherical LM droplets are transformed into highly elongated ellipsoidal shapes with an average aspect ratio of 12.4 by systematically tuning each individual parameter. Material and process relationships are established for the LM ink which show that an ink viscosity threshold should be fulfilled to program the LM microstructure from spherical to an ellipsoidal shape during MEX. Additionally, the thin oxide layer on the LM droplets is found to play a unique and critical role in the reconfiguration and retention of droplet shape. Finally, two quantitative design maps based on material and process parameters are presented to guide MEX additive manufacturing strategies for tuning liquid droplet architecture in LM-polymer inks. The insights gained from this study enable informed design of materials and manufacturing to control microstructure of emerging multifunctional soft composites. 

Ultrasound is a safe, noninvasive diagnostic technique used to measure internal structures such as blood vessels and the velocity of blood flow in the human body. The ability to continuously measure blood flow in major cerebral arteries would enable the early detection of medical problems such as stroke. However, current ultrasound technology consists of rigid, hand-held probes that are arduous to use, sensitive to movement, and are primarily designed for intermittent, instead of continuous use. Here, we describe the design of a wearable ultrasound patch for continuously measuring blood flow velocity through the middle cerebral artery (MCA) that can be assessed from the temple region of the head. The wearable ultrasound patch is composed of an array of piezoelectric elements that are wired together using flexible electrical conductors and encapsulated in an elastic substrate. To improve ultrasound energy transfer, a soft and conformal composite matching layer is introduced. The matching layer consists of gallium-based liquid metal (LM) microdroplets dispersed in a silicone elastomer. The acoustic impedance of the matching layer can be tuned by varying the volume loading of LM. The wearable ultrasound patch will provide new opportunities to continuously measure blood flow velocity and ultimately enable early detection of medical problems such as stroke.

 

Octopus-inspired switchable adhesives are integrated with sensing, processing, and control for robust underwater manipulation. 

Soft, elastically deformable composites with liquid metal (LM) droplets can enable new generations of soft electronics, robotics, and reconfigurable structures. However, techniques to control local composite microstructure, which ultimately governs material properties and performance, is lacking. Here a direct ink writing technique is developed to program the LM microstructure (i.e., shape, orientation, and connectivity) on demand throughout elastomer composites. In contrast to inks with rigid particles that have fixed shape and size, it is shown that emulsion inks with LM fillers enable in situ control of microstructure. This enables filaments, films, and 3D structures with unique LM microstructures that are generated on demand and locked in during printing. This includes smooth and discrete transitions from spherical to needle‐like droplets, curvilinear microstructures, geometrically complex embedded inclusion patterns, and connected LM networks. The printed materials are soft (modulus < 200 kPa), highly deformable (>600 % strain), and can be made locally insulating or electrically conductive using a single ink by controlling the process conditions. These capabilities are demonstrated by embedding elongated LM droplets in a soft heat sink, which rapidly dissipates heat from high‐power LEDs. These programmable microstructures can enable new composite paradigms for emerging technologies that demand mechanical compliance with multifunctional response.

 

Lightweight and elastically deformable soft materials that are thermally conductive are critical for emerging applications in wearable computing, soft robotics, and thermoregulatory garments. To overcome the fundamental heat transport limitations in soft materials, room temperature liquid metal (LM) has been dispersed in elastomer that results in soft and deformable materials with unprecedented thermal conductivity. However, the high density of LMs (>6 g cm⁻³) and the typically high loading (⩾85 wt%) required to achieve the desired properties contribute to the high density of these elastomer composites, which can be problematic for large‐area, weight‐sensitive applications. Here, the relationship between the properties of the LM filler and elastomer composite is systematically studied. Experiments reveal that a multiphase LM inclusion with a low‐density phase can achieve independent control of the density and thermal conductivity of the elastomer composite. Quantitative design maps of composite density and thermal conductivity are constructed to rationally guide the selection of filler properties and material composition. This new multiphase material architecture provides a method to fine‐tune material composition to independently control material and functional properties of soft materials for large‐area and weight‐sensitive applications.

 

Elastomers embedded with droplets of liquid metal (LM) alloy represent an emerging class of soft multifunctional composites that have potentially transformative impact in wearable electronics, biocompatible machines, and soft robotics. However, for these applications it is crucial for LM alloys to remain liquid during the entire service temperature range in order to maintain high mechanical compliance throughout the duration of operation. Here, LM‐based functional composites that do not freeze and remain soft and stretchable at extremely low temperatures are introduced. It is shown that the confinement of LM droplets to micro‐/nanometer length scales significantly suppresses their freezing temperature (down to −84.1 from −5.9 °C) and melting point (down to −25.6 from +17.8 °C) independent of the choice of matrix material and processing conditions. Such a supercooling effect allows the LM inclusions to preserve their fluidic nature at low temperatures and stretch with the surrounding polymer matrix without introducing significant mechanical resistance. These results indicate that LM composites with highly stabilized droplets can operate over a wide temperature range and open up new possibilities for these emerging materials, which are demonstrated with self‐powered wearable thermoelectric devices for bio‐sensing and personal health monitoring at low temperatures.

 

Stretchable high‐dielectric‐constant materials are crucial for electronic applications in emerging domains such as wearable computing and soft robotics. While previous efforts have shown promising materials architectures in the form of dielectric nano‐/microinclusions embedded in stretchable matrices, the limited mechanical compliance of these materials significantly limits their practical application as soft energy‐harvesting/storage transducers and actuators. Here, a class of liquid metal (LM)–elastomer nanocomposites is presented with elastic and dielectric properties that make them uniquely suited for applications in soft‐matter engineering. In particular, the role of droplet size is examined and it is found that embedding an elastomer with a polydisperse distribution of nanoscale LM inclusions can enhance its electrical permittivity without significantly degrading its elastic compliance, stretchability, or dielectric breakdown strength. In contrast, elastomers embedded with microscale droplets exhibit similar improvements in permittivity but a dramatic reduction in breakdown strength. The unique enabling properties and practicality of LM–elastomer nanocomposites for use in soft machines and electronics is demonstrated through enhancements in performance of a dielectric elastomer actuator and energy‐harvesting transducer.