Hydrogel optical fibers for continuous glucose monitoring

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.

Dual-Function Wearable Hydrogel Optical Fiber for Monitoring Posture and Sweat pH

In recent years, wearable devices have been widely used for human health monitoring. Such monitoring predominantly relies on the principles of optics and electronics. However, electronic detection is susceptible to electromagnetic interference, and traditional optical fiber detection is limited in functionality and unable to simultaneously detect both physical and chemical signals. Hence, a wearable, embedded asymmetric color-blocked optical fiber sensor based on a hydrogel has been developed. Its sensing principle is grounded in the total internal reflection within the optical fiber. The method for posture sensing involves changes in the light path due to fiber bending with color blocks providing wavelength-selective modulation by absorption changes. Sweat pH sensing is facilitated by variations in fluorescence intensity triggered by sweat-induced conformational changes in Rhodamine B. With just one fiber, it achieves both physical and chemical signal detection. Fabricated using a molding technique, this fiber boasts excellent biocompatibility and can accurately discern single and multiple bending points, with a recognition range of 0-90° for a single segment, a detection limit of 0.02 mm-1 and a sweat pH sensing linear regression R2 of 0.993, alongside great light propagation properties (-0.6 dB·cm-1). With its extensive capabilities, it holds promise for applications in medical monitoring.

Advancements in optical fiber-based wearable sensors for smart health monitoring

Healthcare system is undergoing a significant transformation from a traditional hospital-centered to an individual-centered one, as a result of escalating chronic diseases, ageing populations, and ever-increasing healthcare costs,. Wearable sensors have become widely used in health monitoring systems since the COVID-19 pandemic. They enable continuous measurement of important health indicators like body temperature, wrist pulse, respiration rate, and non-invasive bio fluids like saliva and perspiration. Over the last few decades, the development has mostly concentrated on electrochemical and electrical wearable sensors. However, due to the drawbacks of such sensors, such as electronic waste, electromagnetic interference, non-electrical security, and poor performance, researchers are exhibiting a strong interest in optical principle-based systems. Fiber-based optical wearables are among the most promising healthcare systems because of advancements in high-sensitivity, durable, multiplexed sensing, and simple integration with flexible materials to improve wearability and simplicity. We present an overview of recent developments in optical fiber-based wearable sensors, focusing on two mechanisms: wavelength interrogation and intensity modulation for the detection of body temperature, pulse rate, respiration rate, body movements, and biomedical noninvasive fluids, with a thorough examination of their benefits and drawbacks. This review also focuses on improving working performance and application techniques for healthcare systems, including the integration of nanomaterials and the usage of the Internet of Things (IoT) with signal processing. Finally, the review concludes with a discussion of the future possibilities and problems for optical fiber-based wearables.

Portable optical fiber biosensors integrated with smartphone: technologies, applications, and challenges [Invited]

The increasing demand for individualized health monitoring and diagnostics has prompted considerable research into the integration of portable optical fiber biosensors integrated with smartphones. By capitalizing on the benefits offered by optical fibers, these biosensors enable qualitative and quantitative biosensing across a wide range of applications. The integration of these sensors with smartphones, which possess advanced computational power and versatile sensing capabilities, addresses the increasing need for portable and rapid sensing solutions. This extensive evaluation thoroughly examines the domain of optical fiber biosensors in conjunction with smartphones, including hardware complexities, sensing approaches, and integration methods. Additionally, it explores a wide range of applications, including physiological and chemical biosensing. Furthermore, the review provides an analysis of the challenges that have been identified in this rapidly evolving area of research and concludes with relevant suggestions for the progression of the field.

Microneedles coated with composites of phenylboronic acid-containing polymer and carbon nanotubes for glucose measurements in interstitial fluids

A microneedle (MN) sensor coated with a sensing composite material was proposed for measuring glucose concentrations in interstitial fluid (ISF). The sensing composite material was prepared by blending a polymer containing glucose-responsive phenylboronic acid (PBA) moieties (i.e., polystyrene-block-poly(acrylic acid-co-acrylamidophenylboronic acid)) with conductive carbon nanotubes (CNTs). The polymer exhibited reversible swelling behavior in response to glucose concentrations, which influenced the distribution of the embedded CNTs, resulting in sensitive variations in electrical percolation, even when coated onto a confined surface of the MN in the sensor. We varied the ratio of PBA moieties and the loading amount of CNTs in the sensing composite material of the MN sensor and tested them in vitro using an ISF-mimicking gel with physiological glucose concentrations to determine the optimal sensitivity conditions. When tested in animal models with varying blood glucose concentrations, the MN sensor coated with the selected sensing material exhibited a strong correlation between the measured electrical currents and blood glucose concentrations, showing accuracy comparable to that of a glucometer in clinical use.

Microneedle-based glucose monitoring: a review from sampling methods to wearable biosensors

Blood glucose (BG) monitoring is critical for diabetes management. In recent years, microneedle (MN)-based technology has attracted emerging attention in glucose sensing and detection. In this review, we summarized MN-based sampling for glucose collection and glucose analysis in detail. First, different principles of MN-based biofluid extraction were elaborated, including external negative pressure, capillary force, swelling force and iontophoresis, which would guide the shape design and material optimization of MNs. Second, MNs coupled with different analysis approaches, including Raman methods, colorimetry, fluorescence, and electrochemical sensing, were emphasized to exhibit the trend towards highly integrated wearable sensors. Finally, the future development prospects of MN-based devices were discussed.

Hydrogel fibers for wearable sensors and soft actuators

Owing to superior softness, wetness, responsiveness, and biocompatibility, bulk hydrogels are being intensively investigated for versatile functions in devices and machines including sensors, actuators, optics, and coatings. The one-dimensional (1D) hydrogel fibers possess the metrics from both the hydrogel materials and structural topology, endowing them with extraordinary mechanical, sensing, breathable and weavable properties. As no comprehensive review has been reported for this nascent field, this article aims to provide an overview of hydrogel fibers for soft electronics and actuators. We first introduce the basic properties and measurement methods of hydrogel fibers, including mechanical, electrical, adhesive, and biocompatible properties. Then, typical manufacturing methods for 1D hydrogel fibers and fibrous films are discussed. Next, the recent progress of wearable sensors (e.g., strain, temperature, pH, and humidity) and actuators made from hydrogel fibers is discussed. We conclude with future perspectives on next-generation hydrogel fibers and the remaining challenges. The development of hydrogel fibers will not only provide an unparalleled one-dimensional characteristic, but also translate fundamental understanding of hydrogels into new application boundaries.

An ionic/thermal-responsive agar/alginate wet-spun microfiber-shaped hydrogel combined with grooved/wrinkled surface patterns and multi-functions

A dual stimuli-responsive wet-spun microfiber-shaped hydrogel is prepared by injecting a hot blend of two stimuli biopolymers alginate (i.e., ionic-responsive) and agar (i.e., temperature-responsive) into a pre-cooling and metal cation containing coagulation bath. Experimental results indicate the fiber microstructure could be manipulated by the extrusion rate and cooling temperature, achieving an anisotropic shrinkage characteristic and novel grooved/wrinkled surface patterns. Importantly, the integration of metal cations (e.g., Ca²⁺and/or Zn²⁺) was confirmed to significantly improve the hydrogel mechanical properties (i.e., double networks) and enhanced blue fluorescent intensity as a typical metal-polymer complexation formed within the agar gel matrix. Moreover, the functionality-independent double networks enabled typical pH-shape memory and sustainable antibacterial properties have also been demonstrated. Therefore, combing the facile fabricating approach and multifunctionality, this study would advance the development of stimuli-responsive hydrogel microfiber for complex biomedical systems.

A High-Linearity Glucose Sensor Based on Silver-Doped Con A Hydrogel and Laser Direct Writing

A continuous glucose monitoring (CGM) system is an ideal monitoring system for the blood glucose control of diabetic patients. The development of flexible glucose sensors with good glucose-responsive ability and high linearity within a large detection range is still challenging in the field of continuous glucose detection. A silver-doped Concanavalin A (Con A)-based hydrogel sensor is proposed to address the above issues. The proposed flexible enzyme-free glucose sensor was prepared by combining Con-A-based glucose-responsive hydrogels with green-synthetic silver particles on laser direct-writing graphene electrodes. The experimental results showed that in a glucose concentration range of 0–30 mM, the proposed sensor is capable of measuring the glucose level in a repeatable and reversible manner, showing a sensitivity of 150.12 Ω/mM with high linearity of R2 = 0.97. Due to its high performance and simple manufacturing process, the proposed glucose sensor is excellent among existing enzyme-free glucose sensors. It has good potential in the development of CGM devices.

Optical Glucose Sensors Based on Chitosan-Capped ZnS-Doped Mn Nanomaterials

The primary goal of glucose sensing at the point of care is to identify glucose concentrations within the diabetes range. However, lower glucose levels also pose a severe health risk. In this paper, we propose quick, simple, and reliable glucose sensors based on the absorption and photoluminescence spectra of chitosan-capped ZnS-doped Mn nanomaterials in the range of 0.125 to 0.636 mM glucose corresponding to 2.3 mg/dL to 11.4 mg/dL. The detection limit was 0.125 mM (or 2.3 mg/dL), much lower than the hypoglycemia level of 70 mg/dL (or 3.9 mM). Chitosan-capped ZnS-doped Mn nanomaterials retain their optical properties while improving sensor stability. This study reports for the first time how the sensors' efficacy was affected by chitosan content from 0.75 to 1.5 wt.%. The results showed that 1 %wt chitosan-capped ZnS-doped Mn is the most-sensitive, -selective, and -stable material. We also put the biosensor through its paces with glucose in phosphate-buffered saline. In the same range of 0.125 to 0.636 mM, the sensors-based chitosan-coated ZnS-doped Mn had a better sensitivity than the working water environment.

A review of Optical Point-of-Care devices to Estimate the Technology Transfer of These Cutting-Edge Technologies

Despite the remarkable development related to Point-of-Care devices based on optical technology, their difficulties when used outside of research laboratories are notable. In this sense, it would be interesting to ask ourselves what the degree of transferability of the research work to the market is, for example, by analysing the relation between the scientific work developed and the registered one, through patent. In this work, we provide an overview of the state-of-the-art in the sector of optical Point-of-Care devices, not only in the research area but also regarding their transfer to market. To this end, we explored a methodology for searching articles and patents to obtain an indicator that relates to both. This figure of merit to estimate this transfer is based on classifying the relevant research articles in the area and the patents that have been generated from these ones. To delimit the scope of this study, we researched the results of a large enough number of publications in the period from 2015 to 2020, by using keywords "biosensor", "optic", and "device" to obtain the most representative articles from Web of Science and Scopus. Then, we classified them according to a particular classification of the optical PoC devices. Once we had this sampling frame, we defined a patent search strategy to cross-link the article with a registered patent (by surfing Google Patents) and classified them accordingly to the categories described. Finally, we proposed a relative figure called Index of Technology Transference (IoTT), which estimates to what extent our findings in science materialized in published articles are protected by patent.

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.

Fluorescent sensor based on pyrroloquinoline quinone (PQQ)-glucose dehydrogenase for glucose detection with smartphone-adapted signal analysis

A new nano-structured platform for fluorescent analysis using PQQ-dependent glucose dehydrogenase (PQQ-GDH) was developed, particularly using a smartphone for transduction and quantification of optical signals. The PQQ-GDH enzyme was immobilized on SiO₂ nanoparticles deposited on glass microfiber filter paper, providing a high load of the biocatalytic enzyme. The platform was tested and optimized for glucose determination using a wild type of the PQQ-GDH enzyme. The analysis was based on the fluorescence generated by the reduced form of phenazine methosulfate produced stoichiometrically to the glucose concentration. The fluorescent signals were generated at separate analytical spots on the paper support under wavelength (365 nm) UV excitation. The images of the analytical spots, dependent on the glucose concentration, were obtained using a photo camera of a standard smartphone. Then, the images were processed and quantified using software installed in a smartphone. The developed biocatalytic platform is the first step to assembling a large variety of biosensors using the same platform functionalized with artificial allosteric chimeric PQQ-dependent glucose dehydrogenase activated with different analytes. The future combination of the artificial enzymes, the presently developed analytical platform, and signal processing with a smartphone will lead to novel point-of-care and end-user biosensors applicable to virtually all possible analytes.

Ramírez M, Zezza P, Lucío MI, Bañuls MJ, Maquieira Á, Morales-Vidal M, Beléndez A, Pascual I. Processing of Holographic Hydrogels in Liquid Media: A Study by High-Performance Liquid Chromatography and Diffraction Efficiency

The storage of time-stable holographic gratings in hydrogel matrices when the material is immersed in aqueous media is a real challenge at present. The optimization of the storage stages of the holograms must be properly investigated to identify the most suitable development processes. For this reason, this work is focused on the study of the optimization of the washing stages of the hydrogels based on acrylamide and N,N’-methylenebis(acrylamide) once unslanted transmission holograms have been stored. High-performance liquid chromatography and UV-visible measurements have been employed in our system to analyze the composition of the washing solutions. PBST and DMSO:H₂O are used as solvents in the washing stages. The diffraction efficiencies are measured during the washing stages and after the storing of the holograms during several days in PBST. Maximum diffraction efficiencies of 38 and 27.6% are reached when PBST and DMSO:H₂O are employed, respectively, for the washing process. Holograms show temporal stability after being stored immersed in PBST at 4 °C for 4 days.

Wearable Pressure Sensors for Pulse Wave Monitoring

Cardiovascular diseases remain the leading cause of death worldwide. The rapid development of flexible sensing technologies and wearable pressure sensors have attracted keen research interest and have been widely used for long-term and real-time cardiovascular status monitoring. Owing to compelling characteristics, including light weight, wearing comfort, and high sensitivity to pulse pressures, physiological pulse waveforms can be precisely and continuously monitored by flexible pressure sensors for wearable health monitoring. Herein, an overview of wearable pressure sensors for human pulse wave monitoring is presented, with a focus on the transduction mechanism, microengineering structures, and related applications in pulse wave monitoring and cardiovascular condition assessment. The conceptualizations and methods for the acquisition of physiological and pathological information related to the cardiovascular system are outlined. The biomechanics of arterial pulse waves and the working mechanism of various wearable pressure sensors, including triboelectric, piezoelectric, magnetoelastic, piezoresistive, capacitive, and optical sensors, are also subject to systematic debate. Exemple applications of pulse wave measurement based on microengineering structured devices are then summarized. Finally, a discussion of the opportunities and challenges that wearable pressure sensors face, as well as their potential as a wearable intelligent system for personalized healthcare is given in conclusion.

Long-Term In Vivo Glucose Monitoring by Polymer-Dot Transducer in an Injectable Hydrogel Implant

Optical sensors have attracted a great deal of interest for glucose detection. However, their practical applications for continuous glucose monitoring are still constrained by operational reliability in subcutaneous tissues. Here, we show an implantable hydrogel platform embedded with luminescent polymer dots (Pdots) for sensitive and long-term glucose monitoring. We use Pdot transducer in a polyacrylamide hydrogel matrix to construct an implantable platform. The hydrogel-Pdot transducer showed bright luminescence with ratiometric response to glucose changes. The in vitro and in vivo sensitivities of the hydrogel implant were enhanced by varying the enzyme concentration and injection volume. After implantation, the hydrogel with Pdot transducer remained at the implanted site without migration for 1 month and can be removed from the subcutaneous tissue for further analysis. Our results indicate that the hydrogel-Pdot platform maintains the intrinsic sensing property with excellent stability during 1 month implantation, while fibrous capsule formation on the implant in some cases needs to be solved for long-term continuous glucose monitoring.

Optical Hydrogel Detector for pH Measurements

Measuring pH has become a major key for determining health conditions, and food safety. The traditional pH assessment approaches are costly and offer low sensitivity. Here, a novel pH sensor based on a pH-responsive hydrogel has been developed. A Fresnel lens pattern was replicated on the surface of the pH-responsive hydrogel using the replica mould method. The pH sensors were tested in a pH range of 4-7. Introducing various pH solutions to the pH sensor led to volumetric shifts as the hydrogel swelled with pH. Consequently, the dimensions of the replicated Fresnel lens changed, modifying the focal length and the focus efficiency of the optical sensor. As a result, the measured optical power at a fixed distance from the sensor changed with pH. The optical sensor showed the best performance in the acidic region when pH changed from 4.5 to 5.5, in which the recorded power increased by 13%. The sensor exhibited high sensitivity to pH changes with a short respond time in a reversible manner. The developed pH optical sensor may have applications in medical point-of-care diagnostics and wearable continuous pH detection devices.

Applications of Advanced Materials for Non-Enzymatic Glucose Monitoring: From Invasive to the Wearable Device

Diabetes mellitus (DM) is a global health problem leading to many complications and disabilities in life adjusting activities and even dead. Monitoring glucose levels is a key factor in diagnosis and management of DM. Conventional glucose sensors consisted of immobilized enzymes, are so susceptible to environmental conditions. In this way, nonenzymatic biosensors have attracted extensive attentions in many clinical diagnostics applications. To date, the finger pricking test is a common enzyme-based glucometer that is an invasive and inconvenient and may lead to infections in the injection sites. So, working on the possibility of cutaneous or subcutaneous insertion of devices as a noninvasive or minimally-invasive systems for continuous glucose controlling approaches through human biofluids (blood, perspiration, tears, saliva, etc.) have stimulated growing interest. This review summarizes recent nonenzymatic and noninvasive biofluids glucose monitoring systems which are highly resilience and stretchable to continuously adapt to body movements during common physical activity. Sensors are based on their constituent materials including carbon-based, metal nanoparticles, polymer, and hydrogel systems are classified for electrochemical, and optical glucose detection. Finally, we address the drawbacks and challenges of enzyme-free sensors which are aroused sustaining research passion to be used in point-of-care medical diagnostics applications.

Engineering Polysaccharide-Based Hydrogel Photonic Constructs: From Multiscale Detection to the Biofabrication of Living Optical Fibers

Solid-state optics has been the pillar of modern digital age. Integrating soft hydrogel materials with micro/nanooptics could expand the horizons of photonics for bioengineering. Here, wet-spun multilayer hydrogel fibers are engineered through ionic-crosslinked natural polysaccharides that serve as multifunctional platforms. The resulting flexible hydrogel structure and reversible crosslinking provide tunable design properties such as adjustable refractive index and fusion splicing. Modulation of the optical readout via physical stimuli, including shape, compression, and multiple optical inputs/outputs is demonstrated. The unique permeability of the hydrogels is also combined with plasmonic nanoparticles for molecular detection of SARS-CoV-2 in fiber-coupled biomedical swabs. A tricoaxial 3D printing nozzle is then employed for the continuous fabrication of living optical fibers. Light interaction with living cells enables the quantification and digitalization of complex biological phenomena such as 3D cancer progression and drug susceptibility. These fibers pave the way for advances in biomaterial-based photonics and biosensing platforms.

Label-free detection of C-Reactive protein using bioresponsive hydrogel-based surface relief diffraction gratings

Responsive hydrogel-based surface relief gratings have demonstrated great performances as transducers in optical sensing. However, novel and smart designs of hydrogels are needed for the appropriate detection of analytes and biomolecules since the existing materials are very limited to specific molecules. In this work, a biosensing system based on surface relief gratings made of bioresponsive hydrogels has been developed. In particular, the hydrogel contains phosphocholine moieties to specifically recognize C-Reactive protein (CRP). The CRP-Sensing hydrogel capacity to selectively detect CRP was fully demonstrated. Using Direct Laser Interference Patterning, micro-gratings were created on CRP-Sensing hydrogel substrates and applied for the label-free sensing of CRP using a simple laser-based homemade optical setup. Limits of detection (LOD) and quantification (LOQ) in human serum dilutions of 1.07 and 8.92 mg L^-1, respectively, were reached. These results demonstrate that the biosensing system allows the selective label-free detection of CRP within concentration ranges around those related to risks of cardiovascular diseases and sepsis. Besides, amplification strategies have been carried out improving the sensitivity, widening the linear range, and reaching better LOD and LOQ (0.30 mg L^-1 and 4.36 mg L^-1). Finally, all the approaches were tested for the quantification of CRP in certified human serum with recoveries of around 100%.

Wearable Bifocal Contact Lens for Continual Glucose Monitoring Integrated with Smartphone Readers

Commercial implantable continuous glucose monitoring devices are invasive and discomfort. Here, a minimally-invasive glucose detection system is developed to provide quantitative glucose measurements continually based on bifocal contact lenses. A glucose-sensitive phenylboronic acid derivative is immobilized in a hydrogel matrix and the surface of the hydrogel is imprinted with a Fresnel lens. The glucose-responsive hydrogel is attached to a commercial soft contact lens to be transformed into a bifocal contact lens. The contact lens showed bifocal lengths; far-field focal length originated from the contact lens' curvature, and near-field focal length resulting from the Fresnel lens. When tear glucose increased, the refractive index and groove depth of the Fresnel lens changed, shifting the near-field focal length and the light focusing efficiency. The recorded optical signals are detected at an identical distance far from the contact lens change. The bifocal contact lens allowed for detecting the tear glucose concentration within the physiological range of healthy individuals and diabetics (0.0-3.3 mm). The contact lens rapidly responded to glucose concentration changes and reached 90% of equilibrium within 40 min. The bifocal contact lens is a wearable diagnostic platform for continual biomarker detection at point-of-care settings.

Biocompatible and Biodegradable Polymer Optical Fiber for Biomedical Application: A Review

This article discusses recent advances in biocompatible and biodegradable polymer optical fiber (POF) for medical applications. First, the POF material and its optical properties are summarized. Then, several common optical fiber fabrication methods are thoroughly discussed. Following that, clinical applications of biocompatible and biodegradable POFs are discussed, including optogenetics, biosensing, drug delivery, and neural recording. Following that, biomedical applications expanded the specific functionalization of the material or fiber design. Different research or clinical applications necessitate the use of different equipment to achieve the desired results. Finally, the difficulty of implanting flexible fiber varies with its flexibility. We present our article in a clear and logical manner that will be useful to researchers seeking a broad perspective on the proposed topic. Overall, the content provides a comprehensive overview of biocompatible and biodegradable POFs, including previous breakthroughs, as well as recent advancements. Biodegradable optical fibers have numerous applications, opening up new avenues in biomedicine.

Advances in Biosensors for Continuous Glucose Monitoring Towards Wearables

Continuous glucose monitors (CGMs) for the non-invasive monitoring of diabetes are constantly being developed and improved. Although there are multiple biosensing platforms for monitoring glucose available on the market, there is still a strong need to enhance their precision, repeatability, wearability, and accessibility to end-users. Biosensing technologies are being increasingly explored that use different bodily fluids such as sweat and tear fluid, etc., that can be calibrated to and therefore used to measure blood glucose concentrations accurately. To improve the wearability of these devices, exploring different fluids as testing mediums is essential and opens the door to various implants and wearables that in turn have the potential to be less inhibiting to the wearer. Recent developments have surfaced in the form of contact lenses or mouthguards for instance. Challenges still present themselves in the form of sensitivity, especially at very high or low glucose concentrations, which is critical for a diabetic person to monitor. This review summarises advances in wearable glucose biosensors over the past 5 years, comparing the different types as well as the fluid they use to detect glucose, including the CGMs currently available on the market. Perspectives on the development of wearables for glucose biosensing are discussed.

Engineering Hydrogel-Based Biomedical Photonics: Design, Fabrication, and Applications

Light guiding and manipulation in photonics have become ubiquitous in events ranging from everyday communications to complex robotics and nanomedicine. The speed and sensitivity of light-matter interactions offer unprecedented advantages in biomedical optics, data transmission, photomedicine, and detection of multi-scale phenomena. Recently, hydrogels have emerged as a promising candidate for interfacing photonics and bioengineering by combining their light-guiding properties with live tissue compatibility in optical, chemical, physiological, and mechanical dimensions. Herein, the latest progress over hydrogel photonics and its applications in guidance and manipulation of light is reviewed. Physics of guiding light through hydrogels and living tissues, and existing technical challenges in translating these tools into biomedical settings are discussed. A comprehensive and thorough overview of materials, fabrication protocols, and design architectures used in hydrogel photonics is provided. Finally, recent examples of applying structures such as hydrogel optical fibers, living photonic constructs, and their use as light-driven hydrogel robots, photomedicine tools, and organ-on-a-chip models are described. By providing a critical and selective evaluation of the field's status, this work sets a foundation for the next generation of hydrogel photonic research.

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

Over the past few decades, optical waveguides have been increasingly used in wearable/implantable devices for on-body sensing. However, conventional optical waveguides are stiff, rigid, and brittle. A mismatch between conventional optical waveguides and complex biointerfaces makes wearable/implantable devices uncomfortable to wear and potentially unsafe. Soft and stretchable polymer optical waveguides not only inherit many advantages of conventional optical waveguides (e.g., immunity to electromagnetic interference and without electrical hazards) but also provide a new perspective for solving the mismatch between conventional optical waveguides and complex biointerfaces, which is essential for the development of light-based wearable/implantable sensors. In this review, polymer optical waveguides' unique properties, including flexibility, biocompatibility and biodegradability, porosity, and stimulus responsiveness, and their applications in the wearable/implantable field in recent years are summarized. Then, we briefly discuss the current challenges of high optical loss, unstable signal transmission, low manufacturing efficiency, and difficulty in deployment during implantation of flexible polymer optical waveguides, and propose some possible solutions to these problems.

Optical glucose biosensor built-in disposable strips and wearable electronic devices

On-demand screening, real-time monitoring and rapid diagnosis of ubiquitous diseases, such as diabetes, at early stages are indispensable in personalised treatment. Emerging impacts of nano/microscale materials on optical and portable biosensor strips and devices have become increasingly important in the remarkable development of sensitive visualisation (i.e. visible inspection by the human eye) assays, low-cost analyses and personalised home testing of patients with diabetes. With the increasing public attention regarding the self-monitoring of diabetes, the development of visual readout, easy-to-use and wearable biosensors has gained considerable interest. Our comprehensive review bridges the practical assessment gap between optical bio-visualisation assays, disposable test strips, sensor array designs and full integration into flexible skin-based or contact lens devices with the on-site wireless signal transmission of glucose detection in physiological fluids. To date, the fully modulated integration of nano/microscale optical biosensors into wearable electronic devices, such as smartphones, is critical to prolong periods of indoor and outdoor clinical diagnostics. Focus should be given to the improvements of invasive, wireless and portable sensing technologies to improve the applicability and reliability of screen display, continuous monitoring, dynamic data visualisation, online acquisition and self and in-home healthcare management of patients with diabetes.

Biomedical optical fibers

Optical fibers with the ability to propagate and transfer data via optical signals have been used for decades in medicine. Biomaterials featuring the properties of softness, biocompatibility, and biodegradability enable the introduction of optical fibers' uses in biomedical engineering applications such as medical implants and health monitoring systems. Here, we review the emerging medical and health-field applications of optical fibers, illustrating the new wave for the fabrication of implantable devices, wearable sensors, and photodetection and therapy setups. A glimpse of fabrication methods is also provided, with the introduction of 3D printing as an emerging fabrication technology. The use of artificial intelligence for solving issues such as data analysis and outcome prediction is also discussed, paving the way for the new optical treatments for human health.

Rational Design of Smart Hydrogels for Biomedical Applications

Hydrogels are polymeric three-dimensional network structures with high water content. Due to their superior biocompatibility and low toxicity, hydrogels play a significant role in the biomedical fields. Hydrogels are categorized by the composition from natural polymers to synthetic polymers. To meet the complicated situation in the biomedical applications, suitable host–guest supramolecular interactions are rationally selected. This review will have an introduction of hydrogel classification based on the formulation molecules, and then a discussion over the rational design of the intelligent hydrogel to the environmental stimuli such as temperature, irradiation, pH, and targeted biomolecules. Further, the applications of rationally designed smart hydrogels in the biomedical field will be presented, such as tissue repair, drug delivery, and cancer therapy. Finally, the perspectives and the challenges of smart hydrogels will be outlined.

An enzyme-free capacitive glucose sensor based on dual-network glucose-responsive hydrogel and coplanar electrode

Glucose sensors are vital devices for blood glucose detection in the diabetes care. Different from traditional electrochemical devices based on glucose oxidase, the glucose sensor based on the glucose-responsive hydrogel is more robust owing to its enzyme-free principle. However, integrating the high sensitivity, fast response, wide measuring range and low-cost fabrication into a hydrogel sensor is still challenging. In this study, we present a physical capacitive sensor, which consists of interdigital carbon electrodes (ICEs) fabricated by a direct laser writing technology and glucose-responsive hydrogel (DexG-Con A hydrogel) built by UV curing in situ. The dielectric property of DexG-Con A hydrogel changes accordingly with the change in environmental glucose concentration. Experimental results demonstrate that in a glucose concentration range of 0-30 mM, the proposed hydrogel sensor is capable of measuring the glucose level in a repeatable and reversible manner, showing a short responsive time of less than 2 min and a high sensitivity of 8.81 pF mM-1 at a glucose range of 0-6 mM. Owing to its simple fabrication process, low-cost and high performance, the proposed glucose sensor shows great potential on batch production for continuous glucose monitoring application.

pH-Sensitive Biomaterials for Drug Delivery

The development of precise and personalized medicine requires novel formulation strategies to deliver the therapeutic payloads to the pathological tissues, producing enhanced therapeutic outcome and reduced side effects. As many diseased tissues are feathered with acidic characteristics microenvironment, pH-sensitive biomaterials for drug delivery present great promise for the purpose, which could protect the therapeutic payloads from metabolism and degradation during in vivo circulation and exhibit responsive release of the therapeutics triggered by the acidic pathological tissues, especially for cancer treatment. In the past decades, many methodologies, such as acidic cleavage linkage, have been applied for fabrication of pH-responsive materials for both in vitro and in vivo applications. In this review, we will summarize some pH-sensitive drug delivery system for medical application, mainly focusing on the pH-sensitive linkage bonds and pH-sensitive biomaterials.

Glucose oxidase@Cu-hemin metal-organic framework for colorimetric analysis of glucose

The work presents a novel glucose oxidase@Cu-hemin metal-organic frameworks (GOD@ Cu-hemin MOFs) with a ball-flower structure as bienzymatic catalysts for detection of glucose. The GOD@Cu-hemin MOFs exhibits great stability as compared with free horseradish peroxidase and GOD toward harsh conditions because the ball-flower-like shell of Cu-hemin MOF effectively protects from GOD. Thus, the GOD@Cu-Hemin MOFs can be used in external harsh conditions such as high temperature and acid/base. The GOD@Cu-hemin MOFs is capable of sensitive and selective detection of glucose via peroxidase-like of Cu-hemin MOFs and GOD by using 3,3',5,5'-tetramethylbenzidine (TMB) as a substrate. Under the existence of glucose, O₂ is reduced into H₂O₂ via GOD@Cu-hemin MOFs. The produced H₂O₂ as well as Cu-hemin MOFs oxidize TMB into blue oxTMB which shows UV-Vis absorbance at 652. The absorption intensity of oxTMB linearly increases with the increasing concentration of glucose from 0.01 to 1.0 mM with detection limit of 2.8 μM. An integrated agarose hydrogel film (Aga/GOD@Cu-hemin MOF/TMB) sensor is rationally designed for colorimetric detection of glucose. The sensor displays a response range of 30 μM-0.8 mM with a detection limit of 0.01 mM. The result indicates that the Cu-hemin MOFs are an ideal carrier for the encapsulation of enzymes.

Healthcare Applications of pH-Sensitive Hydrogel-Based Devices: A Review

pH-sensitive hydrogels have been developed greatly over the past few years. This has been possible due to the synthesis of new hydrogel systems with increased sensitivity – a sensitivity of up to 10⁻⁵ pH units has already been established. Recently, pH-sensitive hydrogels have shown to be very useful in biomedical applications, such as targeted cancer treatment and treatment of skin lesions. Prolonged drug release has been made available through the use of such hydrogels. The synthesis of pH-sensitive hydrogels is also quick and cost-effective. This review presents a background on the properties of pH-sensitive hydrogels and discusses some of the hydrogels with different sensitivity ranges and their possible applications. A range of synthesis processes have also been briefly introduced along with the fabrication of different structures such as microcantilevers and contact lenses.

Recent advances of electrochemical and optical enzyme-free glucose sensors operating at physiological conditions

Diabetes is a pathological condition that requires the continuous monitoring of glucose level in the blood. Its control has been tremendously improved by the application of point-of-care devices. Conventional enzyme-based sensors with electrochemical and optical transduction systems can successfully measure the glucose concentration in human blood, but they suffer from the low stability of the enzyme. Non-enzymatic wearable electrochemical and optical sensors, with low-cost, high stability, point-of-care testing and online monitoring of glucose levels in biological fluids, have recently been developed and can help to manage and control diabetes worldwide. Advances in nanoscience and nanotechnology have enabled the development of novel nanomaterials that can be implemented for the use in enzyme-free systems to detect glucose. This review summarizes recent developments of enzyme-free electrochemical and optical glucose sensors, as well as their respective wearable and commercially available devices, capable of detecting glucose at physiological pH conditions without the need to pretreat the biological fluids. Additionally, the evolution of electrochemical glucose sensor technology and a couple of widely used optical detection systems along with the glucose detection mechanism is also discussed. Finally, this review addresses limitations and challenges of current non-enzymatic electrochemical, optical, and wearable glucose sensor technologies and highlights opportunities for future research directions.

Printed Soft Optical Waveguides of PLA Copolymers for Guiding Light into Tissue

The application of optical technologies in treating pathologies and monitoring disease states requires the development of soft, minimal invasive and implantable devices to deliver light to tissues inside the body. Here, we present soft and degradable optical waveguides from poly(d,l-lactide) and derived copolymers fabricated by extrusion printing in the desired dimensions and shapes. The obtained optical waveguides propagate VIS to NIR light in air and in tissue at penetration depths of tens of centimeters. Besides, the printed waveguides have elastomeric properties at body temperature and show softness and flexibility in the range relevant for implantable devices in soft organs. Printed waveguides were able to guide light across 8 cm tissue and activate photocleavage chemical reactions in a photoresponsive hydrogel (in vitro). The simplicity and flexibility of the fiber processing method and the optical and mechanical performance of the obtained waveguides exemplify how rational study of medically approved biomaterials can lead to useful inks for printing cost-effective and flexible optical components for potential use in medical contexts.