Step-Index Optical Fiber Made of Biocompatible Hydrogels

Fabrication of Cellulose-Based Biopolymer Optical Fibers and Their Theoretical Attenuation Limit

Currently, almost all polymer optical materials are derived from fossil resources with known consequences for the environment. In this work, a processing route to obtain cellulose-based biopolymer optical fibers is presented. For this purpose, the optical properties such as the transmission and the refractive index dispersion of regenerated cellulose, cellulose diacetate, cellulose acetate propionate, and cellulose acetate butyrate were determined from planar films. Cellulose fibers were produced using a simple wet-spinning setup. They were examined pure and also coated with the cellulose derivatives to obtain core-cladding-structured optical fibers. The cellulose-based optical fibers exhibit minimum attenuations between 56 and 82 dB m^-1 at around 860 nm. The ultimate transmission loss limit of the cellulose-based optical fibers was simulated to characterize the attenuation progression. By reducing extrinsic losses, cellulose-based biopolymer optical fibers could attain theoretical attenuation minima of 84.6 × 10^-3 dB m^-1 (507 nm), 320 × 10^-3 dB m^-1 (674 nm), and 745.2 × 10^-3 dB m^-1 (837 nm) and might substitute fossil-based polymer optical fibers in the future.

Luminescent Gold Nanocluster-Methylcellulose Composite Optical Fibers with Low Attenuation Coefficient and High Photostability

Because of their lightweight structure, flexibility, and immunity to electromagnetic interference, polymer optical fibers (POFs) are used in numerous short-distance applications. Notably, the incorporation of luminescent nanomaterials in POFs offers optical amplification and sensing for advanced nanophotonics. However, conventional POFs suffer from nonsustainable components and processes. Furthermore, the traditionally used luminescent nanomaterials undergo photobleaching, oxidation, and they can be cytotoxic. Therefore, biopolymer-based optical fibers containing nontoxic luminescent nanomaterials are needed, with efficient and environmentally acceptable extrusion methods. Here, such an approach for fibers wet-spun from aqueous methylcellulose (MC) dispersions under ambient conditions is demonstrated. Further, the addition of either luminescent gold nanoclusters, rod-like cellulose nanocrystals or gold nanocluster-cellulose nanocrystal hybrids into the MC matrix furnishes strong and ductile composite fibers. Using cutback attenuation measurement, it is shown that the resulting fibers can act as short-distance optical fibers with a propagation loss as low as 1.47 dB cm^-1 . The optical performance is on par with or even better than some of the previously reported biopolymeric optical fibers. The combination of excellent mechanical properties (Young's modulus and maximum strain values up to 8.4 GPa and 52%, respectively), low attenuation coefficient, and high photostability makes the MC-based composite fibers excellent candidates for multifunctional optical fibers and sensors.

Adaptive and multifunctional hydrogel hybrid probes for long-term sensing and modulation of neural activity

To understand the underlying mechanisms of progressive neurophysiological phenomena, neural interfaces should interact bi-directionally with brain circuits over extended periods of time. However, such interfaces remain limited by the foreign body response that stems from the chemo-mechanical mismatch between the probes and the neural tissues. To address this challenge, we developed a multifunctional sensing and actuation platform consisting of multimaterial fibers intimately integrated within a soft hydrogel matrix mimicking the brain tissue. These hybrid devices possess adaptive bending stiffness determined by the hydration states of the hydrogel matrix. This enables their direct insertion into the deep brain regions, while minimizing tissue damage associated with the brain micromotion after implantation. The hydrogel hybrid devices permit electrophysiological, optogenetic, and behavioral studies of neural circuits with minimal foreign body responses and tracking of stable isolated single neuron potentials in freely moving mice over 6 months following implantation.

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.

Optical interferometry based micropipette aspiration provides real-time sub-nanometer spatial resolution

Micropipette aspiration (MPA) is an essential tool in mechanobiology; however, its potential is far from fully exploited. The traditional MPA technique has limited temporal and spatial resolution and requires extensive post processing to obtain the mechanical fingerprints of samples. Here, we develop a MPA system that measures pressure and displacement in real time with sub-nanometer resolution thanks to an interferometric readout. This highly sensitive MPA system enables studying the nanoscale behavior of soft biomaterials under tension and their frequency-dependent viscoelastic response.

The bright side of sound: perspectives on the biomedical application of sonoluminescence

Light is a physical phenomenon that is very important to human life, and has been investigated in its nature, behaviour and properties throughout human history although the most impressive improvements in the use of light in human activities, and of course in medicine, began just two centuries ago. However, despite the enormous progress in diagnosis, therapy and surgery to assess health and treat diseases, the delivery of light sources in vivo remains a challenge. In this regard, several strategies have been developed to overcome this drawback, the most interesting of which is the involvement of ultrasound. In this review, the authors examine how ultrasound may improve light delivery in vivo with a special emphasis on one of the most intriguing ultrasound-mediated phenomena called sonoluminescence, which is the conversion of mechanical ultrasound energy into light.

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.

Challenges in the Fabrication of Biodegradable and Implantable Optical Fibers for Biomedical Applications

The limited penetration depth of visible light in biological tissues has encouraged researchers to develop novel implantable light-guiding devices. Optical fibers and waveguides that are made from biocompatible and biodegradable materials offer a straightforward but effective approach to overcome this issue. In the last decade, various optically transparent biomaterials, as well as different fabrication techniques, have been investigated for this purpose, and in view of obtaining fully fledged optical fibers. This article reviews the state-of-the-art in the development of biocompatible and biodegradable optical fibers. Whilst several reviews that focus on the chemical properties of the biomaterials from which these optical waveguides can be made have been published, a systematic review about the actual optical fibers made from these materials and the different fabrication processes is not available yet. This prompted us to investigate the essential properties of these biomaterials, in view of fabricating optical fibers, and in particular to look into the issues related to fabrication techniques, and also to discuss the challenges in the use and operation of these optical fibers. We close our review with a summary and an outline of the applications that may benefit from these novel optical waveguides.

Soft Materials by Design: Unconventional Polymer Networks Give Extreme Properties

Hydrogels are polymer networks infiltrated with water. Many biological hydrogels in animal bodies such as muscles, heart valves, cartilages, and tendons possess extreme mechanical properties including being extremely tough, strong, resilient, adhesive, and fatigue-resistant. These mechanical properties are also critical for hydrogels' diverse applications ranging from drug delivery, tissue engineering, medical implants, wound dressings, and contact lenses to sensors, actuators, electronic devices, optical devices, batteries, water harvesters, and soft robots. Whereas numerous hydrogels have been developed over the last few decades, a set of general principles that can rationally guide the design of hydrogels using different materials and fabrication methods for various applications remain a central need in the field of soft materials. This review is aimed at synergistically reporting: (i) general design principles for hydrogels to achieve extreme mechanical and physical properties, (ii) implementation strategies for the design principles using unconventional polymer networks, and (iii) future directions for the orthogonal design of hydrogels to achieve multiple combined mechanical, physical, chemical, and biological properties. Because these design principles and implementation strategies are based on generic polymer networks, they are also applicable to other soft materials including elastomers and organogels. Overall, the review will not only provide comprehensive and systematic guidelines on the rational design of soft materials, but also provoke interdisciplinary discussions on a fundamental question: why does nature select soft materials with unconventional polymer networks to constitute the major parts of animal bodies?

Biocompatible diameter-oscillating fiber with microlens endface

Optical fibers have been widely applied to life science, such as laser delivering, fluorescence collection, biosensing, bioimaging, etc. To resolve the challenges of advanced multiphoton biophotonic applications utilizing ultrashort laser pulses, here we report a flexible diameter-oscillating fiber (DOF) with microlens endface fabricated by using Polydimethylsiloxane (PDMS) elastomers. The diameter of the DOF is designed to longitudinally vary for providing accurate dispersion management, which is important for near-infrared multiphoton biophotonics that usually involves ultrashort laser pulses. The variation range and period of the DOF's diameter can be flexibly adjusted by controlling the parameters during the fabrication, such that dispersion curves with different oscillation landscapes can be obtained. The dispersion oscillating around the zero-dispersion baseline gives rise to a minimized net dispersion as the ultrashort laser pulse passes through the DOF - reducing the temporal broadening effect and resulting in transform-limited pulsewidth. In addition, the endface of the DOF is fabricated with a microlens, which is especially useful for laser scanning/focusing and fluorescence excitation. It is anticipated that this new biocompatible DOF is of great interest for biophotonic applications, particularly multiphoton microscopy deep inside biological tissues.

Technological Challenges in the Development of Optogenetic Closed-Loop Therapy Approaches in Epilepsy and Related Network Disorders of the Brain

Epilepsy is a chronic, neurological disorder affecting millions of people every year. The current available pharmacological and surgical treatments are lacking in overall efficacy and cause side-effects like cognitive impairment, depression, tremor, abnormal liver and kidney function. In recent years, the application of optogenetic implants have shown promise to target aberrant neuronal circuits in epilepsy with the advantage of both high spatial and temporal resolution and high cell-specificity, a feature that could tackle both the efficacy and side-effect problems in epilepsy treatment. Optrodes consist of electrodes to record local field potentials and an optical component to modulate neurons via activation of opsin expressed by these neurons. The goal of optogenetics in epilepsy is to interrupt seizure activity in its earliest state, providing a so-called closed-loop therapeutic intervention. The chronic implantation in vivo poses specific demands for the engineering of therapeutic optrodes. Enzymatic degradation and glial encapsulation of implants may compromise long-term recording and sufficient illumination of the opsin-expressing neural tissue. Engineering efforts for optimal optrode design have to be directed towards limitation of the foreign body reaction by reducing the implant's elastic modulus and overall size, while still providing stable long-term recording and large-area illumination, and guaranteeing successful intracerebral implantation. This paper presents an overview of the challenges and recent advances in the field of electrode design, neural-tissue illumination, and neural-probe implantation, with the goal of identifying a suitable candidate to be incorporated in a therapeutic approach for long-term treatment of epilepsy patients.

Fabrication of Sub-Micron Polymer Waveguides through Two-Photon Polymerization in Polydimethylsiloxane

Flexible ultra-compact low-loss optical waveguides play a vital role in the development of soft photonics. The search for suitable materials and innovative fabrication techniques to achieve low loss long polymer optical waveguides and interconnects has proven to be challenging. In this paper, we demonstrate the fabrication of submicron optical waveguides in polydimethylsiloxane (PDMS) using divinylbenzene (DVB) as the photopolymerizable monomer through two-photon polymerization (2PP). We show that the commercial oxime ester photoinitiator Irgacure OXE02 is suitable for triggering the DVB photopolymerization, resulting in a stable and controllable fabrication process for the fabrication of defect-free, 5-cm long waveguides. We further explore a multi-track fabrication strategy to enlarge the waveguide core size up to ~3 μm for better light confinement and reduced cross-talk. In these waveguides, we measured a refractive index contrast on the order of 0.005 and a transmission loss of 0.1 dB/cm at 710 nm wavelength.

Integrated dynamic wet spinning of core-sheath hydrogel fibers for optical-to-brain/tissue communications

Hydrogel optical light-guides have received substantial interest for applications such as deep-tissue biosensors, optogenetic stimulation and photomedicine due to their biocompatibility, (micro)structure control and tissue-like Young's modulus. However, despite recent developments, large-scale fabrication with a continuous synthetic methodology, which could produce core-sheath hydrogel fibers with the desired optical and mechanical properties suitable for deep-tissue applications, has yet to be achieved. In this study, we report a versatile concept of integrated light-triggered dynamic wet spinning capable of continuously producing core-sheath hydrogel optical fibers with tunable fiber diameters, and mechanical and optical propagation properties. Furthermore, this concept also exhibited versatility for various kinds of core-sheath functional fibers. The wet spinning synthetic procedure and fabrication process were optimized with the rational design of the core/sheath material interface compatibility [core = poly(ethylene glycol diacrylate-co-acrylamide); sheath = Ca-alginate], optical transparency, refractive index and spinning solution viscosity. The resulting hydrogel optical fibers exhibited desirable low optical attenuation (0.18 ± 0.01 dB cm⁻¹ with 650 nm laser light), excellent biocompatibility and tissue-like Young's modulus (<2.60 MPa). The optical waveguide hydrogel fibers were successfully employed for deep-tissue cancer therapy and brain optogenetic stimulation, confirming that they could serve as an efficient versatile tool for diverse deep-tissue therapy and brain optogenetic applications.

Optical Waveguides and Integrated Optical Devices for Medical Diagnosis, Health Monitoring and Light Therapies

Optical waveguides and integrated optical devices are promising solutions for many applications, such as medical diagnosis, health monitoring and light therapies. Despite the many existing reviews focusing on the materials that these devices are made from, a systematic review that relates these devices to the various materials, fabrication processes, sensing methods and medical applications is still seldom seen. This work is intended to link these multidisciplinary fields, and to provide a comprehensive review of the recent advances of these devices. Firstly, the optical and mechanical properties of optical waveguides based on glass, polymers and heterogeneous materials and fabricated via various processes are thoroughly discussed, together with their applications for medical purposes. Then, the fabrication processes and medical implementations of integrated passive and active optical devices with sensing modules are introduced, which can be used in many medical fields such as drug delivery and cardiovascular healthcare. Thirdly, wearable optical sensing devices based on light sensing methods such as colorimetry, fluorescence and luminescence are discussed. Additionally, the wearable optical devices for light therapies are introduced. The review concludes with a comprehensive summary of these optical devices, in terms of their forms, materials, light sources and applications.

Structural dimensions depending on light intensity in a 3D printing method that utilizes in situ light as a guide

Conventional 3D printing methods require the addition of a supporting layer in order to accurately and reliably fabricate the desired final product. However, the use of supporting material is not economically viable, and during the process of removing the supporting material, the shape or the properties of the final product may be distorted. In our previous work, we proposed and demonstrated the concept of a new 3D printing method that utilizes the in situ light as a guide for the fabrication of freestanding overhanging structures without the need for supporting material. In this study, the influence of the light intensity on the diameter of the structure and the thickness of the layer produced per droplet is analyzed in order to identify the geometric range of structures that can be fabricated by the new 3D printing method. As the intensity of the light increased, the diameter of the structure also increased and the thickness of the layer per droplet decreased. This result is determined by a combination of factors; (1) the rebound motion of the photocurable droplet and (2) the surface area of the structure that needs to be covered.

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.

Agarose-based structured optical fibre

Biocompatible and resorbable optical fibres emerge as promising technologies for in vivo applications like imaging, light delivery for phototherapy and optogenetics, and localised drug-delivery, as well as for biochemical sensing, wherein the probe can be implanted and then completely absorbed by the organism. Biodegradable waveguides based on glasses, hydrogels, and silk have been reported, but most of these devices rely on complex fabrication procedures. In this sense, this paper proposes a novel structured optical fibre made of agarose, a transparent, edible material used in culture media and tissue engineering. The fibre is obtained by pouring food-grade agar into a mould with stacked rods, forming a solid core surrounded by air holes in which the refractive index and fibre geometry can be tailored by choosing the agarose solution composition and mould design, respectively. Besides exhibiting practical transmittance at 633 nm in relation to other hydrogel waveguides, the fibre is also validated for chemical sensing either by detecting volume changes due to agar swelling/dehydration or modulating the transmitted light by inserting fluids into the air holes. Therefore, the proposed agarose-based structured optical fibre is an easy-to-fabricate, versatile technology with possible applications for medical imaging and in vivo biochemical sensing.

Mechanically and Electronically Robust Transparent Organohydrogel Fibers

Stretchable conductive fibers are key elements for next-generation flexible electronics. Most existing conductive fibers are electron-based, opaque, relatively rigid, and show a significant increase in resistance during stretching. Accordingly, soft, stretchable, and transparent ion-conductive hydrogel fibers have attracted significant attention. However, hydrogel fibers are difficult to manufacture and easy to dry and freeze, which significantly hinders their wide range of applications. Herein, organohydrogel fibers are designed to address these challenges. First, a newly designed hybrid crosslinking strategy continuously wet-spins hydrogel fibers, which are transformed into organohydrogel fibers by simple solvent replacement. The organohydrogel fibers show excellent antifreezing (< -80 °C) capability, stability (>5 months), transparency, and stretchability. The predominantly covalently crosslinked network ensures the fibers have a high dynamic mechanical stability with negligible hysteresis and creep, from which previous conductive fibers usually suffer. Accordingly, strain sensors made from the organohydrogel fibers accurately capture high-frequency (4 Hz) and high-speed (24 cm s^-1 ) motion and exhibit little drift for 1000 stretch-release cycles, and are powerful for detecting rapid cyclic motions such as engine valves and are difficult to reach by previously reported conductive fibers. The organohydrogel fibers also demonstrate potential as wearable anisotropic sensors, data gloves, soft electrodes, and optical fibers.

Fabrication of a Monolithic Lab-on-a-Chip Platform with Integrated Hydrogel Waveguides for Chemical Sensing

Hydrogel waveguides have found increased use for variety of applications where biocompatibility and flexibility are important. In this work, we demonstrate the use of polyethylene glycol diacrylate (PEGDA) waveguides to realize a monolithic lab-on-a-chip device. We performed a comprehensive study on the swelling and optical properties for different chain lengths and concentrations in order to realize an integrated biocompatible waveguide in a microfluidic device for chemical sensing. Waveguiding properties of PEGDA hydrogel were used to guide excitation light into a microfluidic channel to measure the fluorescence emission profile of rhodamine 6G as well as collect the fluorescence signal from the same device. Overall, this work shows the potential of hydrogel waveguides to facilitate delivery and collection of optical signals for potential use in wearable and implantable lab-on-a-chip devices.

Soft and Stretchable Polymeric Optical Waveguide-Based Sensors for Wearable and Biomedical Applications

The past decades have witnessed the rapid development in soft, stretchable, and biocompatible devices for applications in biomedical monitoring, personal healthcare, and human-machine interfaces. In particular, the design of soft devices in optics has attracted tremendous interests attributed to their distinct advantages such as inherent electrical safety, high stability in long-term operation, potential to be miniaturized, and free of electromagnetic interferences. As the alternatives to conventional rigid optical waveguides, considerable efforts have been made to develop light-guiding devices by using various transparent and elastic polymers, which offer desired physiomechanical properties and enable wearable/implantable applications in optical sensing, diagnostics, and therapy. Here, we review recent progress in soft and stretchable optical waveguides and sensors, including advanced structural design, fabrication strategies, and functionalities. Furthermore, the potential applications of those optical devices for various wearable and biomedical applications are discussed. It is expected that the newly emerged soft and stretchable optical technologies will provide a safe and reliable alternative to next-generation, smart wearables and healthcare devices.

Multifunctional Fibers to Shape Future Biomedical Devices

Fiber‐based configurations are highly desirable for wearable and implantable biomedical devices due to their unique properties, such as ultra‐flexibility, weavability, minimal invasiveness, and tissue adaptability. Recent developments have focused on the fabrication of fibrous devices with multiple biomedical functions, such as noninvasively or minimally invasively monitoring of physiological signals, delivering drugs, transplanting cells, and recording and stimulating nerves. In this Review, the recent progress of these multifunctional fiber‐based devices in terms of their composite materials, fabrication techniques, structural designs, device‐tissue interfaces, and biomedical applications is carefully described. The remaining challenges and future directions in this emerging and exciting research field are also highlighted.

Bioresorbable optical sensor systems for monitoring of intracranial pressure and temperature

Continuous measurements of pressure and temperature within the intracranial, intraocular, and intravascular spaces provide essential diagnostic information for the treatment of traumatic brain injury, glaucoma, and cardiovascular diseases, respectively. Optical sensors are attractive because of their inherent compatibility with magnetic resonance imaging (MRI). Existing implantable optical components use permanent, nonresorbable materials that must be surgically extracted after use. Bioresorbable alternatives, introduced here, bypass this requirement, thereby eliminating the costs and risks of surgeries. Here, millimeter-scale bioresorbable Fabry-Perot interferometers and two dimensional photonic crystal structures enable precise, continuous measurements of pressure and temperature. Combined mechanical and optical simulations reveal the fundamental sensing mechanisms. In vitro studies and histopathological evaluations quantify the measurement accuracies, operational lifetimes, and biocompatibility of these systems. In vivo demonstrations establish clinically relevant performance attributes. The materials, device designs, and fabrication approaches outlined here establish broad foundational capabilities for diverse classes of bioresorbable optical sensors.

Muscle-like fatigue-resistant hydrogels by mechanical training

Skeletal muscles possess the combinational properties of high fatigue resistance (1,000 J/m2), high strength (1 MPa), low Young's modulus (100 kPa), and high water content (70 to 80 wt %), which have not been achieved in synthetic hydrogels. The muscle-like properties are highly desirable for hydrogels' nascent applications in load-bearing artificial tissues and soft devices. Here, we propose a strategy of mechanical training to achieve the aligned nanofibrillar architectures of skeletal muscles in synthetic hydrogels, resulting in the combinational muscle-like properties. These properties are obtained through the training-induced alignment of nanofibrils, without additional chemical modifications or additives. In situ confocal microscopy of the hydrogels' fracturing processes reveals that the fatigue resistance results from the crack pinning by the aligned nanofibrils, which require much higher energy to fracture than the corresponding amorphous polymer chains. This strategy is particularly applicable for 3D-printed microstructures of hydrogels, in which we can achieve isotropically fatigue-resistant, strong yet compliant properties.

Multifunctional Smart Optical Fibers: Materials, Fabrication, and Sensing Applications

This paper presents a review of the development of optical fibers made of multiple materials, particularly including silica glass, soft glass, polymers, hydrogels, biomaterials, Polydimethylsiloxane (PDMS), and Polyperfluoro-Butenylvinyleth (CYTOP). The properties of the materials are discussed according to their various applications. Typical fabrication techniques for specialty optical fibers based on these materials are introduced, which are mainly focused on extrusion, drilling, and stacking methods depending on the materials’ thermal properties. Microstructures render multiple functions of optical fibers and bring more flexibility in fiber design and device fabrication. In particular, micro-structured optical fibers made from different types of materials are reviewed. The sensing capability of optical fibers enables smart monitoring. Widely used techniques to develop fiber sensors, i.e., fiber Bragg grating and interferometry, are discussed in terms of sensing principles and fabrication methods. Lastly, sensing applications in oil/gas, optofluidics, and particularly healthcare monitoring using specialty optical fibers are demonstrated. In comparison with conventional silica-glass single-mode fiber, state-of-the-art specialty optical fibers provide promising prospects in sensing applications due to flexible choices in materials and microstructures.

Hydrogel optical fibers for continuous glucose monitoring

Continuous glucose monitoring facilitates the stringent control of blood glucose concentration in diabetic and intensive care patients. Optical fibers have emerged as an attractive platform; however, their practical applications are hindered due to lack of biocompatible fiber materials, complex and non-practical readout approaches, slow response, and time-consuming fabrication processes. Here, we demonstrate the quantification of glucose by smartphone-integrated fiber optics that overcomes existing technical limitations. Simultaneously, a glucose-responsive hydrogel was imprinted with an asymmetric microlens array and was attached to a multimode silica fiber's tip during photopolymerization, and subsequent interrogated for glucose sensing under physiological conditions. A smartphone and an optical power meter were employed to record the output signals. The functionalized fiber showed a high sensitivity (2.6 μW mM^-1), rapid response, and a high glucose selectivity in the physiological glucose range. In addition, the fiber attained the glucose complexation equilibrium within 15 min. The lactate interference was also examined and it was found minimal ∼0.1% in the physiological range. A biocompatible hydrogel made of polyethylene glycol diacrylate was utilized to fabricate a flexible hydrogel fiber to replace the silica fiber, and the fiber's tip was functionalized with the glucose-sensitive hydrogel during the ultraviolet light curing process. The biocompatible fiber was quickly fabricated by the molding, the readout approach was facile and practical, and the response to glucose was comparable to the functionalized silica fiber. The fabricated optical fiber sensors may have applications in wearable and implantable point-of-care and intensive-care continuous monitoring systems.

Flexible fiber-based optoelectronics for neural interfaces

Neurological and psychiatric conditions pose an increasing socioeconomic burden on our aging society. Our ability to understand and treat these conditions relies on the development of reliable tools to study the dynamics of the underlying neural circuits. Despite significant progress in approaches and devices to sense and modulate neural activity, further refinement is required on the spatiotemporal resolution, cell-type selectivity, and long-term stability of neural interfaces. Guided by the principles of neural transduction and by the materials properties of the neural tissue, recent advances in neural interrogation approaches rely on flexible and multifunctional devices. Among these approaches, multimaterial fibers have emerged as integrated tools for sensing and delivering of multiple signals to and from the neural tissue. Fiber-based neural probes are produced by thermal drawing process, which is the manufacturing approach used in optical fiber fabrication. This technology allows straightforward incorporation of multiple functional components into microstructured fibers at the level of their macroscale models, preforms, with a wide range of geometries. Here we will introduce the multimaterial fiber technology, its applications in engineering fields, and its adoption for the design of multifunctional and flexible neural interfaces. We will discuss examples of fiber-based neural probes tailored to the electrophysiological recording, optical neuromodulation, and delivery of drugs and genes into the rodent brain and spinal cord, as well as their emerging use for studies of nerve growth and repair.

Highly Compressible Cross-Linked Polyacrylamide Hydrogel-Enabled Compressible Zn-MnO2 Battery and a Flexible Battery-Sensor System

The fast advancement in flexible and wearable electronics has put up with new requirements on the energy storage device with improved tolerance to deformation apart from offering power output. Despite the tremendous progress in stretchable energy storage devices, the compressional energy storage devices have indeed received limited research attention. In this work, an intrinsically compressible rechargeable battery was proposed using the Zn-MnO₂ chemistry and a cross-linked polyacrylamide hydrogel electrolyte. Interestingly, the battery exhibited not only good energy storage performances but also excellent tolerance against large compressional strain without sacrificing the energy storage capability. It was also found that the ionic conductivities of the hydrogel increased with the values of the compressional strain, leading to an enhanced electrochemical performance. More importantly, upon dynamic compression, the voltage output of the battery can be very stable and reliable. Consequently, the battery assembled using the hydrogel electrolyte can be used to power a luminescent panel even with a 3 kg load on top of it. It was also demonstrated that the flexible sensor powered by our compressible battery exhibited similar and stable sensory signals compared with the same sensor powered by two commercial alkaline batteries. Furthermore, because of the excellent mechanical property of our battery, a smart wristband fabricated by integrating two battery packs and the flexible piezoresistive sensor could be powered and used to monitor the pressure exerted, demonstrating the battery's potential as the wearable power source for the flexible and wearable devices.

Polymeric biomaterials for biophotonic applications

With the growing importance of optical techniques in medical diagnosis and treatment, there exists a pressing need to develop and optimize materials platform for biophotonic applications. Particularly, the design of biocompatible and biodegradable materials with desired optical, mechanical, chemical, and biological properties is required to enable clinically relevant biophotonic devices for translating in vitro optical techniques into in situ and in vivo use. This technological trend propels the development of natural and synthetic polymeric biomaterials to replace traditional brittle, nondegradable silica glass based optical materials. In this review, we present an overview of the advances in polymeric optical material development, optical device design and fabrication techniques, and the accompanying applications to imaging, sensing and phototherapy.

Leeches-Inspired Hydrogel-Elastomer Integration Materials

Inspired by the functions of leeches, for the first time homogeneous materials integrating hydrogels and elastomers were achieved by free radical polymerization. 2-Methoxyethyl acrylate (MEA) was used as elastomer monomer and Pluronics functionalized with vinyl groups acted as cross-linkers to impart the hydrogel property to the materials. The resulting Pluronic/PMEA gels possess a swelling ratio of about 210% and good water-retaining ability. Compression tests of Pluronic/PMEA gels at swelling equilibrium state show a stress up to 1.6 MPa under 85% strain. The gels act as elastomer after dehydration. Uniaxial tensile fracture stress and the elongation reached 1200 kPa and 500%, respectively, and compression stress was above 22 MPa. Furthermore, the Pluronic/PMEA gels also show self-healing properties. Owing to the excellent mechanical performance in both wet and dry conditions, this hydrogel-elastomer integrated material may have potential applications in tissue engineering, soft robotics, and biomedical devices.

Quantum Dots-Doped Tapered Hydrogel Waveguide for Ratiometric Sensing of Metal Ions

Advances in fluorescent nanomaterials and photonics have led to a new generation of photonic devices for applications in biosensing, diagnostics, and therapy. However, for clinical utility, biocompatibility and limited light guiding in tissues pose significant challenges. Here, we report a new type of soft, biocompatible, and tapered optical waveguide with capability of delivering light in deep tissues and demonstrate it as a ratiometric probe for rapid point-of-care detection of metal ions. The waveguide was made from quantum dots (QDs)-incorporated biocompatible hydrogels and coated with a thin sensing film to ensure fast exchanges with the surrounding analytes. The tapered design of the waveguide allows more light extraction for efficient excitation of the coating film. To achieve ratiometric measurements, two types of QDs with well-resolved emission bands are synthesized and immobilized in the waveguide and the coating film, respectively. We show that the ratiometric readout of the waveguide sensor is free of environmental disturbances and exhibits negligible drifts when applied in various environments such as being immersed in water or embedded in tissues. The waveguide device provides a new photonic-sensing platform that may allow being engineered to sense a wide range of metal ions and analytes.

Optomechanical time-domain reflectometry

Optical fibres constitute an exceptional sensing platform. However, standard fibres present an inherent sensing challenge: they confine light to an inner core. Consequently, distributed fibre sensors are restricted to the measurement of conditions that prevail within the core. This work presents distributed analysis of media outside unmodified, standard fibre. Measurements are based on stimulated scattering by guided acoustic modes, which allow us to listen where we cannot look. The protocol overcomes a major difficulty: guided acoustic waves induce forward scattering, which cannot be mapped using time-of-flight. The solution relies on mapping the Rayleigh backscatter contributions of two optical tones, which are coupled by the acoustic wave. Analysis is demonstrated over 3 km of fibre with 100 m resolution. Measurements distinguish between air, ethanol and water outside the cladding, and between air and water outside polyimide-coated fibres. The results establish a new sensor configuration: optomechanical time-domain reflectometry, with several potential applications.

Biocompatible and Implantable Optical Fibers and Waveguides for Biomedicine

Optical fibers and waveguides in general effectively control and modulate light propagation, and these tools have been extensively used in communication, lighting and sensing. Recently, they have received increasing attention in biomedical applications. By delivering light into deep tissue via these devices, novel applications including biological sensing, stimulation and therapy can be realized. Therefore, implantable fibers and waveguides in biocompatible formats with versatile functionalities are highly desirable. In this review, we provide an overview of recent progress in the exploration of advanced optical fibers and waveguides for biomedical applications. Specifically, we highlight novel materials design and fabrication strategies to form implantable fibers and waveguides. Furthermore, their applications in various biomedical fields such as light therapy, optogenetics, fluorescence sensing and imaging are discussed. We believe that these newly developed fiber and waveguide based devices play a crucial role in advanced optical biointerfaces.

Light-Guiding Biomaterials for Biomedical Applications

Optical techniques used in medical diagnosis, surgery, and therapy require efficient and flexible delivery of light from light sources to target tissues. While this need is currently fulfilled by glass and plastic optical fibers, recent emergence of biointegrated approaches, such as optogenetics and implanted devices, call for novel waveguides with certain biophysical and biocompatible properties and desirable shapes beyond what the conventional optical fibers can offer. To this end, exploratory efforts have begun to harness various transparent biomaterials to develop waveguides that can serve existing applications better and enable new applications in future photomedicine. Here, we review the recent progress in this new area of research for developing biomaterial-based optical waveguides. We begin with a survey of biological light-guiding structures found in plants and animals, a source of inspiration for biomaterial photonics engineering. We describe natural and synthetic polymers and hydrogels that offer appropriate optical properties, biocompatibility, biodegradability, and mechanical flexibility have been exploited for light-guiding applications. Finally, we briefly discuss perspectives on biomedical applications that may benefit from the unique properties and functionalities of light-guiding biomaterials.

Fatigue fracture of nearly elastic hydrogels

Polyacrylamide hydrogels are highly stretchable and nearly elastic. Their stress-stretch curves exhibit small hysteresis, and change negligibly after many loading cycles. Polyacrylamide is used extensively in applications, and is the primary network for many types of tough hydrogels. Recent experiments have shown that polyacrylamide hydrogels are susceptible to fatigue fracture, but available data are limited. Here we study fatigue fracture of polyacrylamide hydrogels of various water contents. We form polymer networks in all samples under the same conditions, and then obtain hydrogels of 96, 87, 78, and 69 wt% of water by solvent exchange. We measure the crack extension under cyclic loads, and the fracture energy under monotonic loading. For the hydrogels of the four water contents, the fatigue thresholds are 4.3, 8.4, 20.5, and 64.5 J m-2, and the fracture energies are 18.9, 71.2, 289, and 611 J m-2. The measured thresholds agree well with the predictions of the Lake-Thomas model for hydrogels of high water content, but not in the case of low water content. It is hoped that further basic studies will soon follow to aid the development of fatigue-resistant hydrogels.

Fatigue Fracture of Self-Recovery Hydrogels

Hydrogels of superior mechanical behavior are under intense development for many applications. Some of these hydrogels can recover their stress-stretch curves after many loading cycles. These hydrogels are called self-recovery hydrogels or even fatigue-free hydrogels. Such a hydrogel typically contains a covalent polymer network, together with some noncovalent, reversible interactions. Here we show that self-recovery hydrogels are still susceptible to fatigue fracture. We study a hydrogel containing both covalently cross-linked polyacrylamide and un-cross-linked poly(vinyl alcohol). For a sample without precut crack, the stress-stretch curve recovers after thousands of loading cycles. For a sample with a precut crack, however, the crack extends cycle by cycle. The threshold for fatigue fracture depends on the covalent network but negligibly on noncovalent interactions. Above the threshold, the noncovalent interactions slow down the extension of the crack under cyclic loads.

Bonding dissimilar polymer networks in various manufacturing processes

Recently developed devices mimic neuromuscular and neurosensory systems by integrating hydrogels and hydrophobic elastomers. While different methods are developed to bond hydrogels with hydrophobic elastomers, it remains a challenge to coat and print various hydrogels and elastomers of arbitrary shapes, in arbitrary sequences, with strong adhesion. Here we report an approach to meet this challenge. We mix silane coupling agents into the precursors of the networks, and tune the kinetics such that, when the networks form, the coupling agents incorporate into the polymer chains, but do not condensate. After a manufacturing step, the coupling agents condensate, add crosslinks inside the networks, and form bonds between the networks. This approach enables independent bonding and manufacturing. We formulate oxygen-tolerant hydrogel resins for spinning, printing, and coating in the open air. We find that thin elastomer coatings enable hydrogels to sustain high temperatures without boiling.

Hydrogels and hydrophobic elastomers of various shapes are difficult to bond in arbitrary sequences. Here the authors mix silane coupling agents into a precursor to form hydrogel resins with robust properties that can be spun, coated and printed in air.

Bioresorbable optical fiber Bragg gratings

We demonstrate, for the first time, an inscription and wet dissolution study of Bragg gratings in a bioresorbable calcium-phosphate glass optical fiber. Bragg gratings, with average refractive index changes of 5.8×10^-4, were inscribed using 193 nm excimer laser radiation. Results on the dissolution of the irradiated fiber in simulated physiological conditions are presented after immersing a tilted Bragg grating in a phosphate buffered saline solution for 56 h; selective chemical etching effects are also reported. The investigations performed pave the way toward the use of such phosphate glass fiber Bragg gratings for the development of soluble photonic sensing probes for the efficient in vivo monitoring of vital mechanical or chemical parameters.

Flexible biodegradable citrate-based polymeric step-index optical fiber

Implanting fiber optical waveguides into tissue or organs for light delivery and collection is among the most effective ways to overcome the issue of tissue turbidity, a long-standing obstacle for biomedical optical technologies. Here, we report a citrate-based material platform with engineerable opto-mechano-biological properties and demonstrate a new type of biodegradable, biocompatible, and low-loss step-index optical fiber for organ-scale light delivery and collection. By leveraging the rich designability and processibility of citrate-based biodegradable polymers, two exemplary biodegradable elastomers with a fine refractive index difference and yet matched mechanical properties and biodegradation profiles were developed. Furthermore, we developed a two-step fabrication method to fabricate flexible and low-loss (0.4 db/cm) optical fibers, and performed systematic characterizations to study optical, spectroscopic, mechanical, and biodegradable properties. In addition, we demonstrated the proof of concept of image transmission through the citrate-based polymeric optical fibers and conducted in vivo deep tissue light delivery and fluorescence sensing in a Sprague-Dawley (SD) rat, laying the groundwork for realizing future implantable devices for long-term implantation where deep-tissue light delivery, sensing and imaging are desired, such as cell, tissue, and scaffold imaging in regenerative medicine and in vivo optogenetic stimulation.

Photoluminescent Peptide-Based Nanostructures as FRET Donor for Fluorophore Dye

A great interest has been recently generated by the discovery that peptide-based nanostructures (NSs) endowed with cross-β structure may show interesting photoluminescent (PL) properties. It was shown that NSs formed by PEGylated hexaphenylalanine (PEG₈ -F6, PEG=polyethylene glycol) are able to emit at 460 nm when excited at 370 or 410 nm. Here, the possibility to transfer the fluorescence of these PEG₈ -F6-based NSs by foster resonance electron transfer (FRET) phenomenon to a fluorescent dye was explored. To achieve this aim, the 4-chloro-7-nitrobenzofurazan (NBD) dye was encapsulated in these NSs. Structural data in solution and in solid state, obtained by a variety of techniques (circular dichroism, Fourier-transform infrared spectroscopy, wide-angle X-ray scattering, and small-angle X-ray scattering), indicated that the organization of the peptide spine of PEG₈ -F6 NS, which consists of anti-parallel β-sheets separated by a dry interface made of interacting phenylalanine side chains, was maintained upon NBD encapsulation. The spectroscopic characterization of these NSs clearly showed a red-shift of the emission fluorescence peak both in solution and in solid state. This shift from 460 to 530 nm indicated that a FRET phenomenon from the peptide-based to the fluorophore-encapsulated NS occurred. FRET could also be detected in the PEG₈ -F6 conjugate, in which the NBD was covalently bound to the amine of the compound. On the basis of these results, it is suggested that the red-shift of the intrinsic PL of NSs may be exploited in the bio-imaging field.

Synthetic Engineering of Spider Silk Fiber as Implantable Optical Waveguides for Low-Loss Light Guiding

A variety of devices used for biomedical engineering have been fabricated using protein polymer because of their excellent properties, such as strength, toughness, biocompatibility, and biodegradability. In this study, we fabricated an optical waveguide using genetically engineered spider silk protein. This method has two significant advantages: (1) recombinant spider silk optical waveguide exhibits excellent optical and biological properties and (2) biosynthesis of spider silk protein can overcome the limitation to the research on spider silk optical waveguide due to the low yield of natural spider silk. In detail, two kinds of protein-based optical waveguides made from recombinant spider silk protein and regenerative silkworm silk protein were successfully prepared. Results suggested that the recombinant spider silk optical waveguide showed a smoother surface and a higher refractive index when compared with regenerative silkworm silk protein. The optical loss of recombinant spider silk optical waveguide was 0.8 ± 0.1 dB/cm in air and 1.9 ± 0.3 dB/cm in mouse muscles, which were significantly lower than those of regenerative silkworm silk optical waveguide. Moreover, recombinant spider silk optical waveguide can meet the demand to guide and efficiently deliver light through biological tissue. In addition, recombinant spider silk optical waveguide showed low toxicity to cells in vitro and low-level inflammatory reaction with surrounding tissue in vivo. Therefore, recombinant spider silk optical waveguide is a promising implantable device to guide and deliver light with low loss.

Glucose-Sensitive Hydrogel Optical Fibers Functionalized with Phenylboronic Acid

Hydrogel optical fibers are utilized for continuous glucose sensing in real time. The hydrogel fibers consist of poly(acrylamide-co-poly(ethylene glycol) diacrylate) cores functionalized with phenylboronic acid. The complexation of the phenylboronic acid and cis-diol groups of glucose enables reversible changes of the hydrogel fiber diameter. The analyses of light propagation loss allow for quantitative glucose measurements within the physiological range.

Flexible and stretchable nanowire-coated fibers for optoelectronic probing of spinal cord circuits

Studies of neural pathways that contribute to loss and recovery of function following paralyzing spinal cord injury require devices for modulating and recording electrophysiological activity in specific neurons. These devices must be sufficiently flexible to match the low elastic modulus of neural tissue and to withstand repeated strains experienced by the spinal cord during normal movement. We report flexible, stretchable probes consisting of thermally drawn polymer fibers coated with micrometer-thick conductive meshes of silver nanowires. These hybrid probes maintain low optical transmission losses in the visible range and impedance suitable for extracellular recording under strains exceeding those occurring in mammalian spinal cords. Evaluation in freely moving mice confirms the ability of these probes to record endogenous electrophysiological activity in the spinal cord. Simultaneous stimulation and recording is demonstrated in transgenic mice expressing channelrhodopsin 2, where optical excitation evokes electromyographic activity and hindlimb movement correlated to local field potentials measured in the spinal cord.

Light in diagnosis, therapy and surgery

Light and optical techniques have made profound impacts on modern medicine, with numerous lasers and optical devices being currently used in clinical practice to assess health and treat disease. Recent advances in biomedical optics have enabled increasingly sophisticated technologies — in particular those that integrate photonics with nanotechnology, biomaterials and genetic engineering. In this Review, we revisit the fundamentals of light–matter interactions, describe the applications of light in imaging, diagnosis, therapy and surgery, overview their clinical use, and discuss the promise of emerging light-based technologies.

Highly Stretchable, Strain Sensing Hydrogel Optical Fibers

A core-clad fiber made of elastic, tough hydrogels is highly stretchable while guiding light. Fluorescent dyes are easily doped into the hydrogel fiber by diffusion. When stretched, the transmission spectrum of the fiber is altered, enabling the strain to be measured and also its location.