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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.

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.