Ultrathin and Wearable Microtubular Epidermal Sensor for Real‐Time Physiological Pulse Monitoring

Abstract

high performance of these sensors, their complicated structures, in many cases, require more expensive and complex production routes, limiting its scalability and reproducibility. [33] Conductive liquids belong to another class of materials suitable for deployment in soft and stretchable sensing platforms. These liquids assume the shape of the microchannels within the soft silicone elastomer due to the weak intermolecular forces of attraction between particles. As such, they are highly patternable and reconfigurable. [34][35][36] Recently, several liquid metals that exist in a fluid state in room temperatures are gaining popularity. Among which, Gallistan and eutectic gallium-indium (eGaIn) are two widely reported electrically conductive liquid metallic alloys that are nontoxic and highly deformable. [37] Microfluidic devices based on the liquid metals have therefore been increasingly employed as wearable pressure sensors, [38] strain sensors, [39] temperature sensors, [40] and even more recently as a thermotherapy platform. [41] However, the macroscopic monolithic film format of microfluidics may not fully meet the requirements of imperceptibility and affect conformability due to many subtle but nonflat topologies on the surface of human skin. Thus, an ultrasensitive tactile sensor that is ultrathin, ultralight, and shape configurable is highly desirable.Here, we report an ultrathin microtubular resistive sensor that is soft, flexible, stretchable, and simple to manufacture. The microtube facilitates the deployment of liquid metallic alloy eGaIn which serves as a thin flexible conduit with excellent electrical conductivity and mechanical deformability. Specifically, by considering the radius and thickness of the microtube, we realize an ultrasensitive liquid-based tactile sensor with high flexibility and durability. The self-sustaining fiber-like shape of the sensor is entirely conformal to human interfaces due to its ability to twist around 3D curvatures and objects. In addition, its tiny footprint of 120 µm in diameter makes it almost imperceptible when worn on bare skin. The results of this work establish an attractive option for imperceptible epidermal healthcare diagnostics and monitoring platform.To achieve sensitive force measurements, we utilize liquid metallic alloy within a tubular structure in a submillimeter regime. Liquid metal core fiber of ≈400 µm inner diameter and ≈100 µm in wall thickness has been previously demonstrated as a strain sensor. [42] However, in our current study, we developed a facile manufacturing strategy to achieve an ultrathin, soft core fiber that was previously unattainable. The fabrication process A flexible, stretchable, soft, and ultrathin wearable microtubular sensor that is highly sensitive to mechanical perturbations is developed. The sensor comprises a unique architecture consisting of a liquid-state conductive element core within a soft silicone elastomer microtube. The microtubular sensor can distinguish forces as small as 5 mN and possesses a high force sensitivity ...

Materials and Structures toward Soft Electronics

“…This attribute enables various methods of shaping and patterning the liquid metals. These patterning methods can be divided into four categories, as have been well reviewed in other references: i) lithography‐based processes, either directly or indirectly; ii) molding by the use of pneumatic pressure or other forces to inject the metal into predefined templates; iii) additive deposition by printing or coating the liquid metal only in desired regions; and iv) subtractive etching by selective removal of the liquid metals from a substrate, e.g., by laser ablation . Recent advances of these methods can achieve complex 3D structures and high resolutions (minimal line width of 2 µm) of the liquid metals, as shown in Figure g,h, respectively.…”

Flexible Hybrid Sensors for Health Monitoring: Materials and Mechanisms to Render Wearability

“…In recent years, we have witnessed an explosive growth in the use of wearable physical sensors that can be woven onto garments, [148][149][150] adhered on garments [1,8,41,[151][152][153][154][155][156][157][158][159] or directly adhered to skin. [10][11][12][13][14][15][16]88,90,117,141,[159][160][161][162][163][164][165][166][167][168][169][170] Here, we first provide a brief overview of the physical sensing parameters possible on our body and the significance of them to health.…”

“…Similarly, mechanophysiological signals can be derived from our skin when forces are transduced to the skin surfaces. Examples include heart rate, [6,7] respiration rate, [8,9] pulse rate, [10][11][12][13] and blood pressure. Additionally, our skin interfaces with the environment, allowing us to measure forces and environmental variations in our daily activities.…”

Conformal Devices for Thermal Sensing and Heating in Biomedical and Human–Machine Interaction Applications

“…Flexible electronics for health monitoring have been one of the leading directions in the development of healthcare devices . Ultrathin devices with a few micrometer thickness can offer excellent conformality and promising performances in measuring physiological signals . Compared with traditional electronics with rigid substrates, flexible devices can minimize the mechanical mismatch, irritation, and discomfort on the subject under test.…”