Soft and Stretchable Optical Waveguide: Light Delivery and Manipulation at Complex Biointerfaces Creating Unique Windows for On-Body Sensing

Next-Generation Wearable/Implanted Sensors Based on Fiber Optic and Its Application: From in Vitro to in Vivo

Wearable sensors are significant for health status, diagnosing diseases, and adjusting postoperative interventions to monitor the physiological information on humans continuously. The first generation of wearable sensors has gained rapid growth in medical health for monitoring physical parameters. Recently, emerging fiber optics (FOs) with small diameters have been attached to desired locations of the human epidermis or fabrics for monitoring physiological change activity. Because of its strong soft tissue affinity and excellent biocompatibility, FO has been injected into human skin, blood vessels, and the brain for sensing of biological parameters. The detection of FO has been extended, ranging from physical parameters to chemical and biological parameters. Also, the application of FO has shifted from wearable sensors in vitro to implanted sensors in vivo. Thus, FO is expected to launch a milestone contribution to next-generation wearable/implanted sensors. Based on the success, this review focuses on wearable and implantable FO-based sensors. The three main design strategies of single point, distributed, and FO array were profiled. The significant application of the detection of the physical, chemical, and biological parameters was discussed. The opportunities and challenges of wearable/implantable FO-based sensors were highlighted to promote their development for commercial applications.

Support-Free Implantable Photoelectrochemical Hydrogel Fiber Enables Long-Term Monitoring in Free-Behaving Organisms

The development of long-term and in situ in vivo monitoring techniques is critical for environmental biology, life sciences, and analytical chemistry. However, existing in vivo analysis methods are limited by the complex and large instruments or adverse impacts of rigid implanted substrates on living organisms, making it difficult to achieve continuous in situ detection. Herein, taking advantage of the flexibility and biocompatibility of the hydrogel fiber and solving its instability or opacity problems caused by ionic or polymer conduction for hydrogel fibers, a photoelectrochemical (PEC) hydrogel fiber free of conventional rigid substrate support is successfully prepared and achieves long-term tracking of persistent organic pollutants in free-behaving fish, timely identifying their environmental ecological risks. This support-free PEC fiber exhibits fascinating properties of electrical and light conductivity, flexibility, antifouling ability, and biocompatibility, allowing it to be implanted in vivo for 70 days without experiencing significant loss of sensing performance and causing apparent inflammation and immune responses. Moreover, the fabricated fiber not only achieves in vitro pentachlorophenol detection with high selectivity, low detection limit, good reproducibility, and dual-mode sensing but also realizes in vivo monitoring of pentachlorophenol enriched in fish brain for up to 70 days with satisfactory reliability, unraveling its tempting potential for various in vivo application.

Hydrogel-Based Biointerfaces: Recent Advances, Challenges, and Future Directions in Human-Machine Integration

Human-machine interfacing (HMI) has emerged as a critical technology in healthcare, robotics, and wearable electronics, with hydrogels offering unique advantages as multifunctional materials that seamlessly connect biological systems with electronic devices. This review provides a detailed examination of recent advancements in hydrogel design, focusing on their properties and potential applications in HMI. We explore the key characteristics such as biocompatibility, mechanical flexibility, and responsiveness, which are essential for effective and long-term integration with biological tissues. Additionally, we highlight innovations in conductive hydrogels, hybrid and composite materials, and fabrication techniques such as 3D/4D printing, which allow for the customization of hydrogel properties to meet the demands of specific HMI applications. Further, we discuss the diverse classes of polymers that contribute to hydrogel conductivity, including conducting, natural, synthetic, and hybrid polymers, emphasizing their role in enhancing electrical performance and mechanical adaptability. In addition to material design, we examine the regulatory landscape governing hydrogel-based biointerfaces for HMI applications, addressing the key considerations for clinical translation and commercialization. An analysis of the patent landscape provides insights into emerging trends and innovations shaping the future of hydrogel technologies in human-machine interactions. The review also covers a range of applications, including wearable electronics, neural interfaces, soft robotics, and haptic systems, where hydrogels play a transformative role in enhancing human-machine interactions. Thereafter, the review addresses the challenges hydrogels face in HMI applications, including issues related to stability, biocompatibility, and scalability, while offering future perspectives on the continued evolution of hydrogel-based systems for HMI technologies.

Lighting the Path to Precision Healthcare: Advances and Applications of Wearable Photonic Sensors

Recent advancements in wearable photonic sensors have marked a transformative era in healthcare, enabling non-invasive, real-time, portable, and personalized medical monitoring. These sensors leverage the unique properties of light toward high-performance sensing in form factors optimized for real-world use. Their ability to offer solutions to a broad spectrum of medical challenges - from routine health monitoring to managing chronic conditions, inspires a rapidly growing translational market. This review explores the design and development of wearable photonic sensors toward various healthcare applications. The photonic sensing strategies that power these technologies are first presented, alongside a discussion of the factors that define optimal use-cases for each approach. The means by which these mechanisms are integrated into wearable formats are then discussed, with considerations toward material selection for comfort and functionality, component fabrication, and power management. Recent developments in the space are detailed, accounting for both physical and chemical stimuli detection through various non-invasive biofluids. Finally, a comprehensive situational overview identifies critical challenges toward translation, alongside promising solutions. Associated future outlooks detail emerging trends and mechanisms that stand to enable the integration of these technologies into mainstream healthcare practice, toward advancing personalized medicine and improving patient outcomes.

Wearable multifunctional optical sensor based on Er3+/Yb3+ co-doped Gd2O3 nanoparticles and tapered U-shaped fiber

Wearable sensors with multiple functions are attracting significant attention due to their broad applications in health monitoring and human-computer interaction. Despite significant progress in wearable sensors, it is a significant challenge to monitor temperature and stress simultaneously with a single sensor. A wearable multifunctional optical sensor based on Er3+/Yb3+ co-doped Gd2O3 nanoparticles and a tapered U-shaped fiber is proposed to monitor both temperature and stress in this paper. Temperature resolution of about 0.16℃ is achieved by monitoring the fluorescence intensity ratio (FIR) around 562 nm and 522 nm emitted by Er3+/Yb3+ co-doped Gd2O3 phosphors, which are integrated in a single-mode fiber (SMF). The stress measurement is obtained by monitoring the fluorescence intensity change around 522 nm, which is insensitive to temperature. The results show that the pressure sensitivity and low detection limit are 7% kPa-1 and 127 Pa, respectively. In addition, the response time of 20 ms are achieved for stress sensing. As a proof-of-concept, human skin temperature and heart and respiratory rates are detected before and after exercise by positioning the sensing probe on the wrist. Furthermore, heart and respiratory rates in different parts of the body are also monitored, which are in good agreement with one another. The results demonstrate that the proposed wearable multifunctional optical sensor has huge potential for health monitoring.

Flexible Pressure, Humidity, and Temperature Sensors for Human Health Monitoring

The rapid advancements in artificial intelligence, micro-nano manufacturing, and flexible electronics technology have unleashed unprecedented innovation and opportunities for applying flexible sensors in healthcare, wearable devices, and human-computer interaction. The human body's tactile perception involves physical parameters such as pressure, temperature, and humidity, all of which play an essential role in maintaining human health. Inspired by the sensory function of human skin, many bionic sensors have been developed to simulate human skin's perception to various stimuli and are widely applied in health monitoring. Given the urgent requirements for sensing performance and integration of flexible sensors in the field of wearable devices and health monitoring, here is a timely overview of recent advances in pressure, humidity, temperature, and multi-functional sensors for human health monitoring. It covers the fundamental components of flexible sensors and categorizes them based on different response mechanisms, including resistive, capacitive, voltage, and other types. Specifically, the application of these flexible tactile sensors in the area of human health monitoring is highlighted. Based on this, an extended overview of recent advances in dual/triple-mode flexible sensors integrating pressure, humidity, and temperature tactile sensing is presented. Finally, the challenges and opportunities of flexible sensors are discussed.

Hydrogel for light delivery in biomedical applications

Traditional optical waveguides or mediums are often silica-based materials, but their applications in biomedicine and healthcare are limited due to the poor biocompatibility and unsuitable mechanical properties. In term of the applications in human body, a biocompatible hydrogel system with excellent optical transparency and mechanical flexibility could be beneficial. In this review, we explore the different designs of hydrogel-based optical waveguides derived from natural and synthetic sources. We highlighted key developments such as light emitting contact lenses, implantable optical fibres, biosensing systems, luminating and fluorescent materials. Finally, we expand further on the challenges and perspectives for hydrogel waveguides to achieve clinical applications.

Soft Polymer Optical Fiber Sensors for Intelligent Recognition of Elastomer Deformations and Wearable Applications

In recent years, soft robotic sensors have rapidly advanced to endow robots with the ability to interact with the external environment. Here, we propose a polymer optical fiber (POF) sensor with sensitive and stable detection performance for strain, bending, twisting, and pressing. Thus, we can map the real-time output light intensity of POF sensors to the spatial morphology of the elastomer. By leveraging the intrinsic correlations of neighboring sensors and machine learning algorithms, we realize the spatially resolved detection of the pressing and multi-dimensional deformation of elastomers. Specifically, the developed intelligent sensing system can effectively recognize the two-dimensional indentation position with a prediction accuracy as large as ~99.17%. The average prediction accuracy of combined strain and twist is ~98.4% using the random forest algorithm. In addition, we demonstrate an integrated intelligent glove for the recognition of hand gestures with a high recognition accuracy of 99.38%. Our work holds promise for applications in soft robots for interactive tasks in complex environments, providing robots with multidimensional proprioceptive perception. And it also can be applied in smart wearable sensing, human prosthetics, and human-machine interaction interfaces.

Soft touchless sensors and touchless sensing for soft robots

Soft robots are characterized by their mechanical compliance, making them well-suited for various bio-inspired applications. However, the challenge of preserving their flexibility during deployment has necessitated using soft sensors which can enhance their mobility, energy efficiency, and spatial adaptability. Through emulating the structure, strategies, and working principles of human senses, soft robots can detect stimuli without direct contact with soft touchless sensors and tactile stimuli. This has resulted in noteworthy progress within the field of soft robotics. Nevertheless, soft, touchless sensors offer the advantage of non-invasive sensing and gripping without the drawbacks linked to physical contact. Consequently, the popularity of soft touchless sensors has grown in recent years, as they facilitate intuitive and safe interactions with humans, other robots, and the surrounding environment. This review explores the emerging confluence of touchless sensing and soft robotics, outlining a roadmap for deployable soft robots to achieve human-level dexterity.

Light-Emitting Microfibers from Lotus Root for Eco-Friendly Optical Waveguides and Biosensing

Optical biosensors based on micro/nanofibers are highly valuable for probing and monitoring liquid environments and bioactivity. Most current optical biosensors, however, are still based on glass, semiconductors, or metallic materials, which might not be fully suitable for biologically relevant environments. Here, we introduce biocompatible and flexible microfibers from lotus silk as microenvironmental monitors that exhibit waveguiding of intrinsic fluorescence as well as of coupled light. These features make single-filament monitors excellent building blocks for a variety of sensing functions, including pH probing and detection of bacterial activity. These results pave the way for the development of new and entirely eco-friendly, potentially multiplexed biosensing platforms.

Advanced Self-Powered Biofuel Cells with Capacitor and Nanogenerator for Biomarker Sensing

Self-powered biofuel cells (BFCs) have evolved for highly sensitive detection of biomarkers such as noncodon micro ribonucleic acids (miRNAs) in the presence of interfering substrates. Self-charging supercapacitive BFCs for in vivo and in vitro cellular microenvironments represent the most prevalent sensing mechanism for diagnosis. Therefore, self-powered biosensing (SPB) with a capacitor and contact separation with a triboelectric nanogenerator (TENG) offers electrochemical and colorimetric dual-mode detection via improved electrical signal intensity. In this review, we discuss three major components: stretchable self-powered BFC design, miRNA sensing, and impedance spectroscopy. A specific focus is given to 1) assembling of sensors for biomarkers, 2) electrical output signal intensification, and 3) role of supercapacitors and nanogenerators in SPBs. We outline the key features of stretchable SPBs and the sequence of miRNA sensing by SPBs. We have emphasized the need of a supercapacitor and nanogenerator for SPBs in the context of advanced assembly of the sensing unit. Finally, we outline the role of impedance spectroscopy in the detection and estimation of biomarkers. We highlight key challenges in SPBs for biomarker sensing, which needs improved sensing accuracy, integration strategies of electrochemical biosensing for in vitro and in vivo microenvironments, and the impact of miRNA sensing on cancer diagnostics. This article attempts a specific focus on the accuracy and limitations of sensing unit for miRNA biomarkers and associated tool for boosting electrical signal intensity for a potential big step further.

Liquid-Core Hydrogel Optical Fiber Fluorescence Probes

This paper first reports a liquid-core hydrogel optical fiber fluorescence probe. It is composed of a liquid core, a high-refractive-index hydrogel fiber core, and a low-refractive-index hydrogel fiber cladding, which is completely different from many existing optical fiber fluorescence probes. The sensing solution with sensitive materials is sealed as a liquid core, and it can sufficiently react with small-molecule targets penetrating through the hydrogel fiber cladding and core, thus inducing variations in the fluorescence signals. These fluorescence signals can be localized and transmitted within the probe and finally collected and quantified for target detection. This proposed probe can be simply and rapidly fabricated and reused, and it was proven to have high sensitivity, accuracy, and selectivity in practical applications. Therefore, this liquid-core hydrogel optical fiber fluorescence probe will enable a novel sensing platform for small-molecule analyte detection that faces on-site detection challenges.

Stretchable reflective coating for soft optical waveguides and sensors

Soft robots must embody mechanosensing capabilities to merge with and act in the environment. Stretchable waveguides are making a mark in soft mechanical sensing since they are built from pristine elastomers. Therefore, they are insensitive to electromagnetic fields and weakly affect the deformations of the robot. However, issues in light-shielding, signal decoupling, and core-cladding interfaces are still open challenges. In this work, titanium oxide particles (TiO₂) are dispersed in silicone elastomers to develop a soft optical shield coating. Results show that the added particles do not harden the matrix and reduce light transmission. Almost full NIR shielding is achieved by adding 1.0 vol% of TiO₂ in 150 μm thick films. These properties make the proposed shielding coating an excellent candidate for soft mechanosensing. An open-access tool is developed to design soft optical devices by programming light transmittance at desired wavelengths by tuning, both, TiO₂ concentration and film thickness. Finally, two proof-of-concepts are demonstrated, a soft waveguide and a soft strain sensor, by integrating the developed material to shield a transparent PDMS resin and a semi-transparent Ecoflex00-10 matrix, respectively. The soft waveguide can stretch up to 40% with very low optical loss, while the optical strain sensor can detect strain up to 90%. In both cases, bending, folding, and indentation of the devices have a significantly low impact on light transmission. These results can pave the way to design new optical transmission devices and sensors that exploit light reflection and that allow for discriminating different types of mechanical stimuli in soft robots.

End-to-end design of wearable sensors

Wearable devices provide an alternative pathway to clinical diagnostics by exploiting various physical, chemical and biological sensors to mine physiological (biophysical and/or biochemical) information in real time (preferably, continuously) and in a non-invasive or minimally invasive manner. These sensors can be worn in the form of glasses, jewellery, face masks, wristwatches, fitness bands, tattoo-like devices, bandages or other patches, and textiles. Wearables such as smartwatches have already proved their capability for the early detection and monitoring of the progression and treatment of various diseases, such as COVID-19 and Parkinson disease, through biophysical signals. Next-generation wearable sensors that enable the multimodal and/or multiplexed measurement of physical parameters and biochemical markers in real time and continuously could be a transformative technology for diagnostics, allowing for high-resolution and time-resolved historical recording of the health status of an individual. In this Review, we examine the building blocks of such wearable sensors, including the substrate materials, sensing mechanisms, power modules and decision-making units, by reflecting on the recent developments in the materials, engineering and data science of these components. Finally, we synthesize current trends in the field to provide predictions for the future trajectory of wearable sensors.

Metal-Nanostructure-Decorated Spider Silk for Highly Sensitive Refractive Index Sensing

Highly sensitive detection of refractive index (RI) is essential for the analysis of the bio-microenvironment and basic cellular reactions. To achieve this, optic-fiber RI sensors based on localized surface plasmon resonance (LSPR) have been widely used for their flexibility and high sensitivity. However, the current optic-fiber RI sensors are mainly fabricated using glass, which makes them face the challenges in biocompatibility and biosafety. In this work, a RI sensor with high sensitivity is fabricated using metal-nanostructure-decorated spider silk. The spider silk, which is directly dragged from Araneus ventricosus, is natural protein-based biopolymer with low attenuation, good biocompatibility and biodegradability, large RI, great flexibility, and easy functionalization. Hence, the spider silk can be an ideal alternative to glass for sensing in biological environments with a wide RI range. Different kinds of metal nanostructures, such as gold nanorods (GNRs), gold nanobipyramids (GNBP), and Ag@GNRs, are decorated on the surface of the spider silk utilizing the surface viscidity of the silk. By directing a beam of white light into the spider silk, the LSPR of the metal nanostructures was excited and a highly sensitive RI sensing (the highest sensitivity of 1746 nm per refractive index was achieved on the GNBP-decorated spider silk) was obtained. This work may pave a new way to precise and sensitive biosensing and bioanalysis.

Localised light delivery on melanoma cells using optical microneedles

Light-based therapy is an emerging treatment for skin cancer, which has received increased attention due to its drug-free and non-invasive approach. However, the limitation of current light therapy methods is the inability for light to penetrate the skin and reach deep lesions. As such, we have developed a polylactic acid (PLA) microneedles array as a novel light transmission platform to perform in vitro evaluation regarding the effect of light therapy on skin cancer. For the first time, we designed and fabricated a microneedle array system with a height fixation device that can be installed in a cell culture dish and an LED array for blue light irradiation. The effect of the blue light combined with the microneedles on cell apoptosis was evaluated using B16F10 melanoma cells and analyzed by Hoechst staining. Our results demonstrate that blue light can be transmitted by microneedles to skin cells and effectively affect cell viability.

Smart Materials Enabled with Artificial Intelligence for Healthcare Wearables

Contemporary medicine suffers from many shortcomings in terms of successful disease diagnosis and treatment, both of which rely on detection capacity and timing. The lack of effective, reliable, and affordable detection and real‐time monitoring limits the affordability of timely diagnosis and treatment. A new frontier that overcomes these challenges relies on smart health monitoring systems that combine wearable sensors and an analytical modulus. This review presents the latest advances in smart materials for the development of multifunctional wearable sensors while providing a bird's eye‐view of their characteristics, functions, and applications. The review also presents the state‐of‐the‐art on wearables fitted with artificial intelligence (AI) and support systems for clinical decision in early detection and accurate diagnosis of disorders. The ongoing challenges and future prospects for providing personal healthcare with AI‐assisted support systems relating to clinical decisions are presented and discussed.

Bioresorbable Photonics: Materials, Devices and Applications

Bio-photonic devices that utilize the interaction between light and biological substances have been emerging as an important tool for clinical diagnosis and/or therapy. At the same time, implanted biodegradable photonic devices can be disintegrated and resorbed after a predefined operational period, thus avoiding the risk and cost associated with the secondary surgical extraction. In this paper, the recent progress on biodegradable photonics is reviewed, with a focus on material strategies, device architectures and their biomedical applications. We begin with a brief introduction of biodegradable photonics, followed by the material strategies for constructing biodegradable photonic devices. Then, various types of biodegradable photonic devices with different functionalities are described. After that, several demonstration examples for applications in intracranial pressure monitoring, biochemical sensing and drug delivery are presented, revealing the great potential of biodegradable photonics in the monitoring of human health status and the treatment of human diseases. We then conclude with the summary of this field, as well as current challenges and possible future directions.

Soft Inductive Coil Spring Strain Sensor Integrated with SMA Spring Bundle Actuator

This study proposes a soft inductive coil spring (SICS) strain sensor that can measure the strain of soft actuators. The SICS sensor, produced by transforming a shape memory alloy (SMA) wire with the same materials as that of an SMA spring bundle actuator (SSBA) into a coil spring shape, measures inductance changes according to length changes. This study also proposes a manufacturing method, output characteristics of the SICS sensor applicable to the SSBA among soft actuators, and the structure of the SICS sensor-integrated SSBA (SI-SSBA). In the SI-SSBA, the SMA spring bundle and SICS sensor have structures corresponding to the muscle fiber and spindle of the skeletal muscle, respectively. It is demonstrated that when a robotic arm with one degree of freedom is operated by attaching two SI-SSBAs in an antagonistic structure, the displacement of the SSBA can be measured using the proposed strain sensor. The output characteristics of the SICS sensor for the driving speed of the robotic arm were evaluated, and it was experimentally proven that the strain of the SSBA can be stably measured in water under a temperature change of 54 °C from 36 to 90 °C.