Preclinical Evaluation of Poly(HEMA-co-acrylamide) Hydrogels Encapsulating Glucose Oxidase and Palladium Benzoporphyrin as Fully Implantable Glucose Sensors

Insertable Glucose Sensor Using a Compact and Cost-Effective Phosphorescence Lifetime Imager and Machine Learning

Optical continuous glucose monitoring (CGM) systems are emerging for personalized glucose management owing to their lower cost and prolonged durability compared to conventional electrochemical CGMs. Here, we report a computational CGM system, which integrates a biocompatible phosphorescence-based insertable biosensor and a custom-designed phosphorescence lifetime imager (PLI). This compact and cost-effective PLI is designed to capture phosphorescence lifetime images of an insertable sensor through the skin, where the lifetime of the emitted phosphorescence signal is modulated by the local concentration of glucose. Because this phosphorescence signal has a very long lifetime compared to tissue autofluorescence or excitation leakage processes, it completely bypasses these noise sources by measuring the sensor emission over several tens of microseconds after the excitation light is turned off. The lifetime images acquired through the skin are processed by neural network-based models for misalignment-tolerant inference of glucose levels, accurately revealing normal, low (hypoglycemia) and high (hyperglycemia) concentration ranges. Using a 1 mm thick skin phantom mimicking the optical properties of human skin, we performed in vitro testing of the PLI using glucose-spiked samples, yielding 88.8% inference accuracy, also showing resilience to random and unknown misalignments within a lateral distance of ∼4.7 mm with respect to the position of the insertable sensor underneath the skin phantom. Furthermore, the PLI accurately identified larger lateral misalignments beyond 5 mm, prompting user intervention for realignment. The misalignment-resilient glucose concentration inference capability of this compact and cost-effective PLI makes it an appealing wearable diagnostics tool for real-time tracking of glucose and other biomarkers.

Insertable Biosensors: Combining Implanted Sensing Materials with Wearable Monitors

Insertable biosensor systems are medical diagnostic devices with two primary components: an implantable biosensor within the body and a wearable monitor that can remotely interrogate the biosensor from outside the body. Because the biosensor does not require a physical connection to the electronic monitor, insertable biosensor systems promise improved patient comfort, reduced inflammation and infection risk, and extended operational lifetimes relative to established percutaneous biosensor systems. However, the lack of physical connection also presents technical challenges that have necessitated new innovations in developing sensing chemistries, transduction methods, and communication modalities. In this review, we discuss the key developments that have made insertables a promising option for longitudinal biometric monitoring and highlight the essential needs and existing development challenges to realizing the next generation of insertables for extended-use diagnostic and prognostic devices.

Chain-Extendable Crosslinked Hydrogels Using Branching RAFT Modification

Functional crosslinked hydrogels were prepared from 2-hydroxyethyl methacrylate (HEMA) and acrylic acid (AA). The acid monomer was incorporated both via copolymerization and chain extension of a branching, reversible addition–fragmentation chain-transfer agent incorporated into the crosslinked polymer gel. The hydrogels were intolerant to high levels of acidic copolymerization as the acrylic acid weakened the ethylene glycol dimethacrylate (EGDMA) crosslinked network. Hydrogels made from HEMA, EGDMA and a branching RAFT agent provide the network with loose-chain end functionality that can be retained for subsequent chain extension. Traditional methods of surface functionalization have the downside of potentially creating a high volume of homopolymerization in the solution. Branching RAFT comonomers act as versatile anchor sites by which additional polymerization chain extension reactions can be carried out. Acrylic acid grafted onto HEMA–EGDMA hydrogels showed higher mechanical strength than the equivalent statistical copolymer networks and was shown to have functionality as an electrostatic binder of cationic flocculants.

NIR Luminescent Oxygen-Sensing Nanoparticles for Continuous Glucose and Lactate Monitoring

A highly sensitive, biocompatible, and scalable phosphorescent oxygen sensor formulation is designed and evaluated for use in continuous metabolite sensors for biological systems. Ethyl cellulose (EC) and polystyrene (PS) nanoparticles (NPs) stabilized with Pluronic F68 (PF 68), Polydimethylsiloxane-b-polyethyleneglycol methyl ether (PDMS-PEG), sodium dodecylsulfate (SDS), and cetyltimethylammonium bromide (CTAB) were prepared and studied. The resulting NPs with eight different surfactant−polymer matrix combinations were evaluated for physical properties, oxygen sensitivity, effect of changes in dispersion matrix, and cytotoxicity. The EC NPs exhibited a narrower size distribution and 40% higher sensitivity than PS, with Stern−Volmer constants (Ksv) 0.041−0.052 µM−1 for EC, compared to 0.029−0.034 µM−1 for PS. Notably, ethyl cellulose NPs protected with PF68 were selected as the preferred formulation, as they were not cytotoxic towards 3T3 fibroblasts and exhibited a wide phosphorescence lifetime response of >211.1 µs over 258−0 µM and ~100 µs over 2.58−0 µM oxygen, with a limit of detection (LoD) of oxygen in aqueous phase of 0.0016 µM. The EC-PF68 NPs were then efficiently encapsulated in alginate microparticles along with glucose oxidase (GOx) and catalase (CAT) to form phosphorescent nanoparticles-in-microparticle (NIMs) glucose sensing microdomains. The fabricated glucose sensors showed a sensitivity of 0.40 µs dL mg−1 with a dynamic phosphorescence lifetime range of 46.6−197.1 µs over 0−150 mg dL−1 glucose, with a glucose LoD of 18.3 mg dL−1 and maximum distinguishable concentration of 111.1 mg dL−1. Similarly, lactate sensors were prepared with NIMs microdomains containing lactate oxidase (LOx) and found to have a detection range of 0−14 mg dL−1 with LoD of 1.8 mg dL−1 and maximum concentration of 13.7 mg dL−1 with lactate sensitivity of 10.7 µs dL mg−1. Owing to its versatility, the proposed NIMs-based design can be extended to a wide range of metabolites and different oxygen-sensing dyes with different excitation wavelengths based on specific application.

A wavelength-induced frequency filtering method for fluorescent nanosensors in vivo

Fluorescent nanosensors hold the potential to revolutionize life sciences and medicine. However, their adaptation and translation into the in vivo environment is fundamentally hampered by unfavourable tissue scattering and intrinsic autofluorescence. Here we develop wavelength-induced frequency filtering (WIFF) whereby the fluorescence excitation wavelength is modulated across the absorption peak of a nanosensor, allowing the emission signal to be separated from the autofluorescence background, increasing the desired signal relative to noise, and internally referencing it to protect against artefacts. Using highly scattering phantom tissues, an SKH1-E mouse model and other complex tissue types, we show that WIFF improves the nanosensor signal-to-noise ratio across the visible and near-infrared spectra up to 52-fold. This improvement enables the ability to track fluorescent carbon nanotube sensor responses to riboflavin, ascorbic acid, hydrogen peroxide and a chemotherapeutic drug metabolite for depths up to 5.5 ± 0.1 cm when excited at 730 nm and emitting between 1,100 and 1,300 nm, even allowing the monitoring of riboflavin diffusion in thick tissue. As an application, nanosensors aided by WIFF detect the chemotherapeutic activity of temozolomide transcranially at 2.4 ± 0.1 cm through the porcine brain without the use of fibre optic or cranial window insertion. The ability of nanosensors to monitor previously inaccessible in vivo environments will be important for life-sciences research, therapeutics and medical diagnostics.

Monte Carlo method for assessment of a multimodal insertable biosensor

SIGNIFICANCE

Continuous glucose monitors (CGMs) are increasingly utilized as a way to provide healthcare to the over 10% of Americans that have diabetes. Fully insertable and optically transduced biosensors are poised to further improve CGMs by extending the device lifetime and reducing cost. However, optical modeling of light propagation in tissue is necessary to ascertain device performance.

AIM

Monte Carlo modeling of photon transport through tissue was used to assess the luminescent output of a fully insertable glucose biosensor that uses a multimodal Förster resonance energy transfer competitive binding assay and a phosphorescence lifetime decay enzymatic assay.

APPROACH

A Monte Carlo simulation framework of biosensor luminescence and tissue autofluorescence was built using MCmatlab. Simulations were first validated against previous research and then applied to predict the response of a biosensor in development.

RESULTS

Our results suggest that a diode within the safety standards for light illumination on the skin, with far-red excitation, allows the luminescent biosensor to yield emission strong enough to be detectable by a common photodiode.

CONCLUSIONS

The computational model showed that the expected fluorescent power output of a near-infrared light actuated barcode was five orders of magnitude greater than a visible spectrum excited counterpart biosensor.

Inorganic-Organic Interpenetrating Network Hydrogels as Tissue-Integrating Luminescent Implants: Physicochemical Characterization and Preclinical Evaluation

Sensors capable of accurate, continuous monitoring of biochemistry are crucial to the realization of personalized medicine on a large scale. Great strides have been made to enhance tissue compatibility of long-term in vivo biosensors using biomaterials strategies such as tissue-integrating hydrogels. However, the low level of oxygen in tissue presents a challenge for implanted devices, especially when the biosensing function relies on oxygen as a measure-either as a primary analyte or as an indirect marker to transduce levels of other biomolecules. This work presents a method of fabricating inorganic-organic interpenetrating network (IPN) hydrogels to optimize the oxygen transport through injectable biosensors. Capitalizing on the synergy between the two networks, various physicochemical properties (e.g., swelling, glass transition temperature, and mechanical properties) are shown to be independently adjustable while maintaining a 250% increase in oxygen permeability relative to poly(2-hydroxyethyl methacrylate) controls. Finally, these gels, when functionalized with a Pd(II) benzoporphyrin phosphor, track tissue oxygen in real time for 76 days as subcutaneous implants in a porcine model while promoting tissue ingrowth and minimizing fibrosis around the implant. These findings support IPN networks for fine-tuned design of implantable biomaterials in personalized medicine and other biomedical applications.

Review: Nanomaterials for Reactive Oxygen Species Detection and Monitoring in Biological Environments

Reactive oxygen species (ROS) and dissolved oxygen play key roles across many biological processes, and fluorescent stains and dyes are the primary tools used to quantify these species in vitro. However, spatio-temporal monitoring of ROS and dissolved oxygen in biological systems are challenging due to issues including poor photostability, lack of reversibility, and rapid off-site diffusion. In particular, ROS monitoring is hindered by the short lifetime of ROS molecules and their low abundance. The combination of nanomaterials and fluorescent detection has led to new opportunities for development of imaging probes, sensors, and theranostic products, because the scaffolds lead to improved optical properties, tuneable interactions with cells and media, and ratiometric sensing robust to environmental drift. In this review, we aim to critically assess and highlight recent development in nanosensors and nanomaterials used for the detection of oxygen and ROS in biological systems, and their future potential use as diagnosis tools.

A Versatile Multichannel Instrument for Measurement of Ratiometric Fluorescence Intensity and Phosphorescence Lifetime

Optical biosensing is being actively investigated for minimally-invasive monitoring of key biomarkers both in vitro and in vivo. However, typical benchtop instruments are not portable and are not well suited to high-throughput, real-time analysis. This paper presents a versatile multichannel instrument for measurement of emission intensity and lifetime values arising from luminescent biosensor materials. A detailed design description of the opto-electronic hardware as well as the control software is provided, elaborating a flexible, user-configurable system that may be customized or duplicated for a wide range of applications. This article presents experimental measurements that prove the in vitro and in vivo functionality of the system. Such tools may be adopted for many research and development purposes, including evaluation of new biosensor materials, and may also serve as prototypes for future miniaturized handheld or wearable devices.

Modulating the foreign body response of implants for diabetes treatment

Diabetes Mellitus is a group of diseases characterized by high blood glucose levels due to patients' inability to produce sufficient insulin. Current interventions often require implants that can detect and correct high blood glucose levels with minimal patient intervention. However, these implantable technologies have not reached their full potential in vivo due to the foreign body response and subsequent development of fibrosis. Therefore, for long-term function of implants, modulating the initial immune response is crucial in preventing the activation and progression of the immune cascade. This review discusses the different molecular mechanisms and cellular interactions involved in the activation and progression of foreign body response (FBR) and fibrosis, specifically for implants used in diabetes. We also highlight the various strategies and techniques that have been used for immunomodulation and prevention of fibrosis. We investigate how these general strategies have been applied to implants used for the treatment of diabetes, offering insights on how these devices can be further modified to circumvent FBR and fibrosis.

Hydrogel-Based Optical Ion Sensors: Principles and Challenges for Point-of-Care Testing and Environmental Monitoring

Hydrogel is a unique family of biocompatible materials with growing applications in chemical and biological sensors. During the past few decades, various hydrogel-based optical ion sensors have been developed aiming at point-of-care testing and environmental monitoring. In this Perspective, we provide an overview of the research field including topics such as photonic crystals, DNAzyme cross-linked hydrogels, ionophore-based ion sensing hydrogels, and fluoroionophore-based optodes. As the different sensing principles are summarized, each strategy offers its advantages and limitations. In a nutshell, developing optical ion sensing hydrogels is still in the early stage with many opportunities lying ahead, especially with challenges in selectivity, assay time, detection limit, and usability.

Nonspecific Binding-Fundamental Concepts and Consequences for Biosensing Applications

Nature achieves differentiation of specific and nonspecific binding in molecular interactions through precise control of biomolecules in space and time. Artificial systems such as biosensors that rely on distinguishing specific molecular binding events in a sea of nonspecific interactions have struggled to overcome this issue. Despite the numerous technological advancements in biosensor technologies, nonspecific binding has remained a critical bottleneck due to the lack of a fundamental understanding of the phenomenon. To date, the identity, cause, and influence of nonspecific binding remain topics of debate within the scientific community. In this review, we discuss the evolution of the concept of nonspecific binding over the past five decades based upon the thermodynamic, intermolecular, and structural perspectives to provide classification frameworks for biomolecular interactions. Further, we introduce various theoretical models that predict the expected behavior of biosensors in physiologically relevant environments to calculate the theoretical detection limit and to optimize sensor performance. We conclude by discussing existing practical approaches to tackle the nonspecific binding challenge in vitro for biosensing platforms and how we can both address and harness nonspecific interactions for in vivo systems.

Implantable Sensors Based on Gold Nanoparticles for Continuous Long-Term Concentration Monitoring in the Body

Implantable sensors continuously transmit information on vital values or biomarker concentrations in bodily fluids, enabling physicians to survey disease progression and monitor therapeutic success. However, currently available technologies still face difficulties with long-term operation and transferability to different analytes. We show the potential of a generalizable platform based on gold nanoparticles embedded in a hydrogel for long-term implanted biosensing. Using optical imaging and an intelligent sensor/reference-design, we assess the tissue concentration of kanamycin in anesthetized rats by interrogating our implanted sensor noninvasively through the skin. Combining a tissue-integrating matrix, robust aptamer receptors, and photostable gold nanoparticles, our technology has strong potential to extend the lifetime of implanted sensors. Because of the easy adaptability of gold nanoparticles toward different analytes, our concept will find versatile applications in personalized medicine or pharmaceutical development.

An Optical Urate Biosensor Based on Urate Oxidase and Long-Lifetime Metalloporphyrins

Gout is a condition that affects over 8 million Americans. This condition is characterized by severe pain, and in more advanced cases, bone erosion and joint destruction. This study explores the fabrication and characterization of an optical, enzymatic urate biosensor for gout management, and the optimization of the biosensor response through the tuning of hydrogel matrix properties. Sensors were fabricated through the co-immobilization of oxygen-quenched phosphorescent probes with an oxidoreductase within a biocompatible copolymer hydrogel matrix. Characterization of the spectral properties and hydrogel swelling was conducted, as well as evaluation of the response sensitivity and long-term stability of the urate biosensor. The findings indicate that increased acrylamide concentration improved the biosensor response by yielding an increased sensitivity and reduced lower limit of detection. However, the repeatability and stability tests highlighted some possible areas of improvement, with a consistent response drift observed during repeatability testing and a reduction in response seen after long-term storage tests. Overall, this study demonstrates the potential of an on-demand, patient-friendly gout management tool, while paving the way for a future multi-analyte biosensor based on this sensing platform.

In Vivo Biosensing Using Resonance Energy Transfer

Solution-phase and intracellular biosensing has substantially enhanced our understanding of molecular processes foundational to biology and pathology. Optical methods are favored because of the low cost of probes and instrumentation. While chromatographic methods are helpful, fluorescent biosensing further increases sensitivity and can be more effective in complex media. Resonance energy transfer (RET)-based sensors have been developed to use fluorescence, bioluminescence, or chemiluminescence (FRET, BRET, or CRET, respectively) as an energy donor, yielding changes in emission spectra, lifetime, or intensity in response to a molecular or environmental change. These methods hold great promise for expanding our understanding of molecular processes not just in solution and in vitro studies, but also in vivo, generating information about complex activities in a natural, organismal setting. In this review, we focus on dyes, fluorescent proteins, and nanoparticles used as energy transfer-based optical transducers in vivo in mice; there are examples of optical sensing using FRET, BRET, and in this mammalian model system. After a description of the energy transfer mechanisms and their contribution to in vivo imaging, we give a short perspective of RET-based in vivo sensors and the importance of imaging in the infrared for reduced tissue autofluorescence and improved sensitivity.

In Vivo Biosensing: Progress and Perspectives

In vivo biosensors are emerging as powerful tools in biomedical research and diagnostic medicine. Distinct from "labels" or "imaging", in vivo biosensors are designed for continuous and long-term monitoring of target analytes in real biological systems and should be selective, sensitive, reversible and biocompatible. Due to the challenges associated with meeting all of the analytical requirements, we found relatively few reports of research groups demonstrating devices that meet the strict definition in vivo. However, we identified several case studies and a range of emerging materials likely to lead to significant developments in the field.

Composite Hydrogels with Engineered Microdomains for Optical Glucose Sensing at Low Oxygen Conditions

There is a growing need for advanced tools that enable frequent monitoring of biomarkers for precision medicine. In this work, we present a composite hydrogel-based system providing real-time optical bioanalyte monitoring. The responsive material, alginate-in-alginate (AnA), is comprised of an alginate hydrogel with embedded bioactive, nanofilm-coated phosphorescent microdomains; palladium tetracarboxyphenylporphyrin serves as an optical indicator, glucose oxidase as a model enzyme, and layer-by-layer deposited polyelectrolyte multilayers (PEMs) as the diffusion barrier. Glutaraldehyde crosslinking of the nanofilms resulted in a dramatic reduction in glucose diffusion (179%) while oxygen transport was not significantly affected. The responses of the AnA hydrogels to step changes of glucose at both ambient and physiological oxygen levels were evaluated, revealing controlled tuning of sensitivity and dynamic range. Stability, assessed by alternately exposing the responsive AnA hydrogels to extremely high and zero glucose concentrations, resulted in no significant difference in the response over 20 cycles. These AnA hydrogels represent an attractive approach to biosensing based on biocompatible materials that may be used as minimally-invasive, implantable devices capable of optical interrogation. The model glucose-responsive composite material studied in this work will serve as a template that can be translated for sensing additional analytes (e.g., lactate, urea, pyruvate, cholesterol) and can be used for monitoring other chronic conditions.

Carbon-dot–hydrogel for enzyme-mediated bacterial detection

A hybrid carbon-dot (C-dot)–hydrogel matrix was constructed and employed for detection of bacteria.