The emergence of plastic electronics satisfies the increasing demand for flexible electronics. However, it has caused severe ecological problems. Flexible electronics based on natural materials are increasing to hopefully realize the “green” and eco‐friendly concept. Herein, recent advances in the design and fabrication of green flexible electronics are reviewed. First, this review comprehensively introduces various natural materials and derivatives, focusing particularly on fibroin and silk, wood and paper, plants, and biomass. Second, fabrication techniques for modifying natural materials, including physical and chemical methods, are presented, after which their merits and demerits are thoroughly discussed. Green flexible electronics based on natural materials, comprising electrical wires/electrodes, antennas, thermal management devices, transistors, memristors, sensors, energy‐harvesting devices, energy‐storage devices, displays, actuators, electromagnetic shielding, and integration systems, are described in detail. Finally, perspectives on the existing challenges and opportunities to employ natural materials in green flexible electronics are presented.

Green Flexible Electronics: Natural Materials, Fabrication, and Applications

3D-printed PEDOT:PSS for soft robotics

Artificial intelligence-powered electronic skin

Recent advances in wearable electromechanical sensors—Moving towards machine learning-assisted wearable sensing systems

Human skin sensing of mechanical stimuli originates from transduction of mechanoreceptors that converts external forces into electrical signals. Although imitating the spatial distribution of those mechanoreceptors can enable developments of electronic skins capable of decoupled sensing of normal/shear forces and strains, it remains elusive. We report a three-dimensionally (3D) architected electronic skin (denoted as 3DAE-Skin) with force and strain sensing components arranged in a 3D layout that mimics that of Merkel cells and Ruffini endings in human skin. This 3DAE-Skin shows excellent decoupled sensing performances of normal force, shear force, and strain and enables development of a tactile system for simultaneous modulus/curvature measurements of an object through touch. Demonstrations include rapid modulus measurements of fruits, bread, and cake with various shapes and degrees of freshness. 

A three-dimensionally architected electronic skin mimicking human mechanosensation

Encoding of tactile information in hand via skin-integrated wireless haptic interface

Underneath the ear skin there are rich vascular network and sensory nerve branches. Hence, the 3D mapping of auricular electrophysiological signals can provide new biomedical perspectives. However, it is still extremely challenging for current sensing techniques to cover the entire ultra‐curved auricle. Here, a 3D graphene‐based ear‐conformable sensing device with embedded and distributed 3D electrodes for full‐auricle physiological monitoring is reported. As a proof‐of‐concept, spatiotemporal auricular electrical skin resistance (AESR) mapping is demonstrated for the first time, and human subject‐specific AESR distributions are observed. From the data of more than 30 ears (both right and left ears), the auricular region‐specific AESR changes after cycling exercise are observed in 98% of the tests and are clustered into four groups via machine learning‐based data analyses. Correlations of AESR with heart rate and blood pressure are also studied. This 3D electronic platform and AESR‐based biometrical findings show promising biomedical applications.

https://onlinelibrary.wiley.com/doi/pdfdirect/10.1002/adhm.202201404

Mapping of Spatiotemporal Auricular Electrophysiological Signals Reveals Human Biometric Clusters

With the advancement of Digital Chinese Medicine (DCM) using micro/nano sensors and AI technologies, researchers have invested significant efforts in acquiring pulse wave information and relating them to disease symptoms. Many flexible sensors have been developed in the past decade attempting to improve and digitize Traditional Chinese Medicine Pulse Sphygmopalpation (TCMPS) data. Typically, TCM doctors use their fingers to obtain physiological information from the wrist radial artery and determine a patient’s physical condition by sensing temporal and spatial information of the pulse waves. However, an important element in the TCMPS method is that the fingertips must apply three different pressure levels onto a patient’s wrist in order to properly obtain the wave patterns described by TCM doctors. Unfortunately, past work failed at addressing this fundamental problem for TCMPS – existing flexible sensors have not reported the capability of measuring the truly applied pressure onto a patient’s skin while measuring temporal variations of the pulse waves. We present here a method that allows TCM doctors to record a patient’s pulse in real-time using a tactile sensor array while the fingertip pressure applied to the skin can also be recorded. We believe our work will significantly advance TCM by providing a repeatable and stable pulse wave collection methodology incorporating the vital concept of sphygmopalpation with variable applied fingertip pressures. 

Sphygmopalpation of Pulse Waves with Variable Applied Fingertip Pressure using a MEMS Flexible Tactile Sensor Array

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Encoding of tactile information in hand via skin-integrated wireless haptic interface

Mapping of Spatiotemporal Auricular Electrophysiological Signals Reveals Human Biometric Clusters

Sphygmopalpation of Pulse Waves with Variable Applied Fingertip Pressure using a MEMS Flexible Tactile Sensor Array