Wafer-Level, High-Performance, Flexible Sensors Based on Organic Nanoforests for Human-Machine Interactions

Wearable flexible sensors based on dual-network ionic hydrogels with xanthan gum/sodium alginate/polyacrylamide/gallium indium alloy

With the rapid development of wearable electronic devices and smart sensors, flexible sensors have received much attention due to their excellent mechanical properties and good adaptability. However, developing a simple method to produce conductive hydrogels with excellent electrical conductivity, mechanical properties, environmental stability, and durability is still a major challenge. In this study, a novel dual-network composite flexible sensor was developed, which was mainly composed of xanthan gum (XG), sodium alginate (SA), polyacrylamide (PAAm), and gallium‑indium alloy (Ga-In). The sensor combined the good biocompatibility and thickening properties of natural polysaccharides, the flexibility of polymers, and the excellent electrical conductivity of conductive metal alloys. The sensors exhibited good mechanical properties (stress ≈ 400 KPa, strain ≈ 540 %), high fatigue resistance, recoverability and excellent environmental adaptability. In addition, the addition of liquid metal could increase the conductivity (1.83 S m-1) of the hydrogel while maintaining high transparency, and the flexible sensor device constructed from it had high sensitivity to strain (GF = 2.75). Therefore, the hydrogel as a flexible sensor showed promising applications in detecting human movement, which could monitor the movement of human joints, micro-expressions, and handwriting. This will provide new ideas for scientific management of sports and health monitoring.

Sodium Alginate/MXene-Based Flexible Humidity Sensors with High-Humidity Durability and Application Potentials in Breath Monitoring and Non-Contact Human-Machine Interfaces

Flexible humidity sensors (FHSs) with fast response times and durability to high-humidity environments are highly desirable for practical applications. Herein, an FHS based on crosslinked sodium alginate (SA) and MXene was fabricated, which exhibited high sensitivity (impedance varied from 107 to 105 Ω between 10% and 90% RH), good selectivity, prompt response times (response/recover time of 4 s/11 s), high sensing linearity (R2 = 0.992) on a semi-logarithmic scale, relatively small hysteresis (~5% RH), good repeatability, and good resistance to highly humid environments (negligible changes in sensing properties after being placed in 98% RH over 24 h). It is proposed that the formation of the crosslinking structure of SA and the introduction of MXene with good conductivity and a high specific surface area contributed to the high performance of the composite FHS. Moreover, the FHS could promptly differentiate the respiration status, recognize speech, and measure fingertip movement, indicating potential in breath monitoring and non-contact human-machine interactions. This work provides guidance for developing advanced flexible sensors with a wide application scope in wearable electronics.

Oriented bouncing of droplets with a small Weber number on inclined one-dimensional nanoforests

Asymmetric superhydrophobic structures with anisotropic wettability can achieve directional bouncing of droplets and thus can have applications in directional self-cleaning, liquid transportation, and heat transfer. To achieve convenient large-scale preparation of asymmetric superhydrophobic surfaces, inclined nanoforests are prepared in this work using a technique of competitive ablation polymerization, which allows the control of the inclined angles, diameters, and heights of the nanostructures. In this study, such asymmetric structures with the smallest dimension (230 nm diameter) known are achieved by a simple etching method to guide droplet unidirectional bouncing. With such nanoforests, the mechanism of droplet bouncing on their surface is investigated, and controllable droplet bouncing over a long distance is achieved using droplets with a low Weber number. The proposed structure has a promising future in directional self-cleaning, liquid transportation and heat transfer.