DOI: 10.1002/admt.201800628 applications (e.g., soft robotics, medical devices).[1–6] Despite state-of-the-art bulkbased planar integrated-circuit devices, their rigid and brittle nature gives rise to the incompatibility with curvilinear and soft human bodies, restricting the development of newborn human-friendly interactive electronics. In contrast, the bendable and flexible wearable electronics could be conformally attached onto human bodies almost without discomfort and succeed in performing a great deal of sensing functionalities. Realization of such promising goals requires the flexible sensor platforms provided with crucial characteristics of light weight, ultrathinness, superior flexibility, stretchability, high sensitivity as well as rapid response.[7–10] Inspired by the perceptive features of human skins, the wearable sensor systems are capable of acquiring abundant information from the external environment with the assistance of sensing modules, such as pressure sensors, strain sensors, temperature sensors, etc.[11] A typical example is their application in prosthetics that could afford the capacity to perceive touch or temperature for the disabled.[12] Additionally, the wearable sensor systems are able to identify physical or chemical signals produced by the human body, providing promising opportunities to evaluate health states.[5,13–15] Conventional skin-like sensor platforms primarily comprise one or two sensing modules, data processing units, and power supplies. Their unitary functionality, however, cannot satisfy the increasing demands of IoTs. Recently, the rapid advances in novel sensing materials, fabrication strategies, and innovative electronic constitution contribute to the development of versatile integration of multimodal sensors, which could synchronously distinguish diverse stimuli from the complex environment and monitor multiple vital signs from the human body.[16,17] In spite of several attempts done in terms of such multimodal sensor systems, one of the cumbersome issues originates from the crosscoupling effect among different categories of signals simultaneously generated by various sensors. Furthermore, the skin-like multiple sensor systems usually suffer from the limited number of repeated use, resulting in their high use-cost. The development of separable versatile devices may address this issue with one layer realized by costeffective materials and fabrication manners for disposable use and the other composed of relatively expensive components for repeatable applications.[18] Additionally, the multiple bending or Skin-inspired wearable devices hold great potentials in the next generation of smart portable electronics owing to their intriguing applications in healthcare monitoring, soft robotics, artificial intelligence, and human–machine interfaces. Despite tremendous research efforts dedicated to judiciously tailoring wearable devices in terms of their thickness, portability, flexibility, bendability as well as stretchability, the emerging Internet of Things demand the skininterfaced flexible systems to be endowed with additional functionalities with the capability of mimicking skin-like perception and beyond. This review covers and highlights the latest advances of burgeoning multifunctional wearable electronics, primarily including versatile multimodal sensor systems, self-healing material-based devices, and self-powered flexible sensors. To render the penetration of human-interactive devices into global markets and households, economical manufacturing techniques are crucial to achieve large-scale flexible systems with high-throughput capability. The booming innovations in this research field will push the scientific community forward and benefit human beings in the near future.

Multifunctional Skin-Inspired Flexible Sensor Systems for Wearable Electronics

Surface-enhanced Raman scattering (SERS) spectroscopy provides a noninvasive and highly sensitive route for fingerprint and label-free detection of a wide range of molecules. Recently, flexible SERS has attracted increasingly tremendous research interest due to its unique advantages compared to rigid substrate-based SERS. Here, the latest advances in flexible substrate-based SERS diagnostic devices are investigated in-depth. First, the intriguing prospect of point-of-care diagnostics is briefly described, followed by an introduction to the cutting-edge SERS technique. Then, the focus is moved from conventional rigid substrate-based SERS to the emerging flexible SERS technique. The main part of this report highlights the recent three categories of flexible SERS substrates, including actively tunable SERS, swab-sampling strategy, and the in situ SERS detection approach. Furthermore, other promising means of flexible SERS are also introduced. The flexible SERS substrates with low-cost, batch-fabrication, and easy-to-operate characteristics can be integrated into portable Raman spectroscopes for point-of-care diagnostics, which are conceivable to penetrate global markets and households as next-generation wearable sensors in the near future.

https://www.onlinelibrary.wiley.com/doi/epdf/10.1002/advs.201900925

Toward Flexible Surface-Enhanced Raman Scattering (SERS) Sensors for Point-of-Care Diagnostics

Thanks to some technical progress in interferencefilter design based on different technologies, we can finally successfully implement the concept of multispectral filter array-based sensors. This article provides the relevant state-of-the-art for multispectral imaging systems and presents the characteristics of the elements of our multispectral sensor as a case study. The spectral characteristics are based on two different spatial arrangements that distribute eight different bandpass filters in the visible and near-infrared area of the spectrum. We demonstrate that the system is viable and evaluate its performance through sensor spectral simulation.

Multispectral Filter Arrays: Recent Advances and Practical Implementation

In wearable electronics, to acquire stability and simultaneously preserve stretchability, sensitivity, and scalability is of high significance yet challenging for practical device applications. In this work, a kirigami-structured graphene–polymer hybrid nanocomposite is proposed for strain sensors by a laser direct writing technique on a polyimide sheet. Using such kirigami structures, the strain in the sensor material can be drastically reduced in the circumstances of high stretching conditions. To protect the device, ecoflex polymer is applied as the passivation layer. Depending on the applications, ecoflex grid-wrapped and film-encapsulated devices are realized with high stretchability (>200% strain) and sensitivity (>80% resistance change at 60% strain), respectively. Significantly, this sensor platform presents almost no performance degradation even after >60 000 stretching cycle tests due to less strain within the sensor. Furthermore, as proof-of-concept for human-interactive applications, motion detection and respiration monitoring with a breathing period of 3–4 s for a comparatively long duration (>2 hours) are successfully demonstrated. These facile, scalable, highly stable and stretchable strain sensors afford a new route towards next-generation reliable skin-inspired wearable electronics.

Highly stable kirigami-structured stretchable strain sensors for perdurable wearable electronics

Wettability transition of laser textured brass surfaces inside different mediums

Surface enhanced Raman spectroscopy (SERS) has drawn much research interest in the past decades as an efficient technique to detect low-concentration molecules. Among many technologies, which can be used to fabricate SERS substrates, laser ablation is a simple and high-speed method to produce large-area SERS substrates. This work investigates the angular texturing effect by dynamic laser ablation and its influence on SERS signals. By tuning the angle between the Si surface and laser irradiation, the distributions and sizes of laser induced hybrid micro/nano-structures are studied. By decorating with a silver film, plenty of hot spots can be created among these structures for SERS. It is found that when the incident laser angle is 15° at the laser fluence of 16.0 J/cm2, the SERS performance is well optimized. This work realizes antisymmetric distribution of nanoparticles deposited on Si surface, which provides a flexible tuning of the hybrid micro/nano-structures’ fabrication with high controllability for practical applications.

Hybrid micro/nano-structure formation by angular laser texturing of Si surface for surface enhanced Raman scattering.

Surface-enhanced Raman scattering (SERS) has been extensively investigated as an effective approach for trace species detection. Silver nanostructures are high-sensitivity SERS substrates in common use, but their poor chemical stability impedes practical applications. Herein, a stable and sensitive SERS substrate based on the hybrid structures of graphene/silver film/laser-textured Si (G/Ag/LTSi) was developed, and a simple, rapid, and low-cost fabrication approach was explored. Abundant nanoparticles were directly created and deposited on the Si surface via laser ablation. These aggregated nanoparticles functioned as hotspots after a 30 nm Ag film coating. A monolayer graphene was transferred to the Ag film surface to prevent the Ag from oxidation. The SERS behavior was investigated by detecting R6G and 4-MBT molecules. The experimental results indicate that the maximum enhancement factor achieved by the G/Ag/LTSi substrate is over 107 and less than 23% SERS signals lost when the substrate was exposed to ambient conditions for 50 days. The covering graphene layer played crucial roles in both the Raman signals enhancement and the Ag nanostructure protection. The stable and sensitive SERS performance of G/Ag/LTSi substrate evince that the present strategy is a useful and convenient route to fabricate large-area graphene-silver plasmonic hybrids for SERS applications.

Graphene-Ag Hybrids on Laser-Textured Si Surface for SERS Detection.

The invention relates to a comprehensive diagnostic apparatus for xerophthalmia The comprehensive diagnostic apparatus comprises an upper computer, a controller, a Placido disc, a near-infrared light illuminating system, a white light illuminating system, and an imaging system, wherein the white light illuminating system comprises a diffuse reflection plate and a white light LED array, the imaging system comprises an industrial camera, an automatic focusing lens, an optical filter wheel disc, a motor and a gear, the diagnostic apparatus respectively measures the tear meniscus height, the area of a tiny blood vessel of a conjunctiva, the structural and morphological parameters of meibomian glands, the breakup time of a tear film, and the thickness of a lipid layer of the tear film according to the principles of visible light/near-infrared light imaging measurement, structured light imaging measurement and white light interferometry, so as to provide objective diagnostic evidences and preliminary diagnostic result for the diagnosis of the xerophthalmia The diagnostic apparatus can replace a series of separated diagnosis items during the clinical diagnosis of the xerophthalmia, the diagnosis flow of the xerophthalmia is simplified, the diagnosis time is shortened, and the objectivity and the repeatability of the diagnosis result are improved

Comprehensive diagnostic apparatus for xerophthalmia

Herein we design a fiber sensor able to simultaneously measure the temperature and the pressure under harsh conditions, such as strong electromagnetic interference and high pressure. It is built on the basis of the fiber-optic Fabry–Perot (F–P) interference and the temperature sensitive mechanism of fluorescent materials. Both halogen lamps and light-emitting diodes (LED) are employed as the excitation light source. The reflected light from the sensor contains the low coherent information of interference cavity and the fluorescent lifetime. This information is independent due to the separate optical path and the different demodulation device. It delivers the messages of pressure and temperature, respectively. It is demonstrated that the sensor achieved pressure measurement at the range of 120–400 KPa at room temperature with a sensitivity of 1.5 nm/KPa. Moreover, the linearity of pressure against the cavity length variation was over 99.9%. In the meantime, a temperature measurement in the range of 25–80 °C, with a sensitivity of 0.0048 ms/°C, was obtained. These experimental results evince that this kind of sensor has a simple configuration, low-cost, and easy fabrication. As such, it can be particularly applied to many fields.

https://www.mdpi.com/1424-8220/19/5/1097/pdf

Hybrid Fiber Optic Sensor, Based on the Fabry⁻Perot Interference, Assisted with Fluorescent Material for the Simultaneous Measurement of Temperature and Pressure.

Surface enhanced Raman scattering (SERS) is an efficient technique to detect low concentration molecules. In this work, periodical silicon nanowires (SiNWs) integrated with metal-insulator-metal (MIM) layers are employed as SERS substrates. Laser interference lithography (LIL) combined with reactive ion etching (RIE) is used to fabricate large-area periodic nanostructures, followed by decorating the MIM layers. Compared to MIM disks array on Si surface, the SERS enhancement factor (EF) of the MIM structures on the SiNWs array can be increased up to 5 times, which is attributed to the enhanced electric field at the boundary of the MIM disks. Fur-thermore, high density of nanoparticles and nanogaps serving as hot spots on sidewall surfaces also contribute to the enhanced SERS signals. Via changing the thickness of the insulator layer, the plasmonic resonance can be tuned, which provides a new localized surface plasmon resonance (LSPR) characteristic for SERS applications.

Chentao Zhang

Papers

Hybrid micro/nano-structure formation by angular laser texturing of Si surface for surface enhanced Raman scattering.

Graphene-Ag Hybrids on Laser-Textured Si Surface for SERS Detection.

Comprehensive diagnostic apparatus for xerophthalmia

Hybrid Fiber Optic Sensor, Based on the Fabry⁻Perot Interference, Assisted with Fluorescent Material for the Simultaneous Measurement of Temperature and Pressure.

Hybrid metal-insulator-metal structures on Si nanowires array for surface enhanced Raman scattering