Towards smart tattoos: implantable biosensors for continuous glucose monitoring

Recent advances in materials and manufacturing of implantable devices for continuous health monitoring

Implantable devices are vital in healthcare, enabling continuous monitoring, early disease detection, informed decision-making, enhanced outcomes, cost reduction, and chronic condition management. These devices provide real-time data, allowing proactive healthcare interventions, and contribute to overall improvements in patient care and quality of life. The success of implantable devices relies on the careful selection of materials and manufacturing methods. Recent materials research and manufacturing advancements have yielded implantable devices with enhanced biocompatibility, reliability, and functionality, benefiting human healthcare. This paper provides a comprehensive overview of the latest developments in implantable medical devices, emphasizing the importance of material selection and manufacturing methods, including biocompatibility, self-healing capabilities, corrosion resistance, mechanical properties, and conductivity. It explores various manufacturing techniques such as microfabrication, 3D printing, laser micromachining, electrospinning, screen printing, inkjet printing, and nanofabrication. The paper also discusses challenges and limitations in the field, including biocompatibility concerns, privacy and data security issues, and regulatory hurdles for implantable devices.

FT-IR, FT-raman and UV spectra and ab initio HF and DFT study of conformational analysis, molecular structure and properties of ortho- meta- and para-chlorophenylboronic acid isomers

In this study, the FT-IR, FT-Raman, and UV-Vis spectroscopic properties of three monosubstituted phenylboronic acid derivatives: ortho-chlorophenylboronic acid (o-ClPhBA), meta-chlorophenylboronic acid (m-ClPhBA) and para-chlorophenylboronic acid (p-ClPhBA) molecules are investigated both experimentally and theoretically using Density Functional Theory (B3LYP) and Hartree Fock (HF). In order to find the stable possible conformations of the compounds, the conformational analysis was carried out by running potential energy surface (PES) scan by means of rotation of two structural parameters, the dihedral angles indicated as φ2 (C6-B-O1-H1A) and φ3 (C6-B-O2-H2A), varying from -180° to 180° with an increment of 10° using B3LYP/6-31G level of theory. Also, to determinate the most stable conformer for all the molecules, potential energy curve (PEC) the stable possible conformations on PES scan were investigated as a function of φ1 (C1-C6-B-O1) dihedral angle from 0° up to 180° with an increment of 10° using B3LYP/6-311++G(d,p) and HF/6-311++G(d,p) level of theory. For all the studied compounds, two conformational structures (conformer anti-anti, syn-syn) that did not have imaginary frequency values outside the equilibrium state (conformer anti-syn) were detected theoretically at the both methods. Due to their conformational flexibility, the relative stabilities of the anti-syn, anti-anti, and syn-syn conformers of o-ClPhBA, m-ClPhBA, and p-ClPhBA are 0.0, 4.66, and 6.76 kcal/mol, respectively, at the B3LYP/6-311++G(d,p) level of theory. For the HF/6-311++G(d,p) level of theory, the relative stabilities are 0.00, 4.54, and 6.11 kcal/mol for o-ClPhBA; 0.00, 3.98, and 1.51 kcal/mol for m-ClPhBA; and 0.00, 4.10, and 1.44 kcal/mol for p-ClPhBA, respectively. Some of the determined stable conformations of these molecules are different in symmetry groups. It was observed that the increase in the symmetry was effective in the of molecular properties, especially for vibrational frequencies. The structural parameter, dipole moments (μ), vibrational frequencies, polarizability (α), hyperpolarizability (β), the highest occupied molecular orbital (HOMO), and the lowest unoccupied molecular orbital (LUMO) of the stable conformers were calculated by using Ab initio HF/6-311++G(d,p) and DFT/B3LYP/6-311++G(d,p) level of theory. The assignments of fundamental vibrational modes of the studied molecule were performed based on total energy distribution (TED) analysis.

A long lifetime and highly sensitive wearable microneedle sensor for the continuous real-time monitoring of glucose in interstitial fluid

The short lifetime and low sensitivity of the current glucose electrochemical sensors are two major issues for implementing continuous real-time monitoring of glucose in vivo. Here we show that a unique microneedle-based glucose monitoring skin patch (termed here MGMSP) can continuously measure glucose in real time in live animals with micromolar sensitivity and over 14 days of service life. This MGMSP employs a glucose oxidase (GOD) and carbon nanotube (CNT) modified hollow syringe as electrochemical sensor for glucose monitoring, an integrated circuit for signal processing and transmission, and the real-time glucose levels are displayed on smartphone via Bluetooth. The designed microneedle device protects the stability of the sensing molecules immobilized within the inner surface of hollow syringe and simultaneously the interior space of hollow syringe substantially increases the amount of immobilized sensing molecules. This microneedle design thus extends the lifetime as well as improves the detection sensitivity. The final MGMSP enables the continuous real-time monitoring of glucose in the interstitial fluid of live rats. This innovative microneedle-based MGMSP could potentially provide the public with high-accuracy continual glucose monitoring.

Implantable microfluidics: methods and applications

Implantable microfluidics involves integrating microfluidic functionalities into implantable devices, such as medical implants or bioelectronic devices, revolutionizing healthcare by enabling personalized and precise diagnostics, targeted drug delivery, and regeneration of targeted tissues or organs. The impact of implantable microfluidics depends heavily on advancements in both methods and applications. Despite significant progress in the past two decades, continuous advancements are still required in fluidic control and manipulation, device miniaturization and integration, biosafety considerations, as well as the development of various application scenarios to address a wide range of healthcare issues. In this review, we discuss advancements in implantable microfluidics, focusing on methods and applications. Regarding methods, we discuss progress made in fluid manipulation, device fabrication, and biosafety considerations in implantable microfluidics. In terms of applications, we review advancements in using implantable microfluidics for drug delivery, diagnostics, tissue engineering, and energy harvesting. The purpose of this review is to expand research ideas for the development of novel implantable microfluidic devices for various healthcare applications.

Sensitive and specific detection of saccharide species based on fluorescence: update from 2016

Increasing evidence supports the critical role of saccharides in various pathophysiological steps of tumor progression, where they regulate tumor proliferation, invasion, hematogenic metastasis, and angiogenesis. The identification and recognition of these saccharides provide a solid foundation for the development of targeted drug preparations, which are however not fully understood due to their complex and similar structures. In order to achieve fluorescence sensing of saccharides, extensive research has been conducted to design molecular probes and nanoparticles made of different materials. This paper aims to provide in-depth discussion of three main topics that cover the current status of the carbohydrate sensing based on the fluorescence sensing mechanism, including a phenylboronic acid-based sensing platform, non-boronic acid entities, as well as an enzyme-based sensing platform. It also highlights efforts made to understand the recognition mechanisms and improve the sensing properties of these systems. Finally, we present the challenge of achieving high selectivity and sensitivity recognition of saccharides, and suggest possible future avenues for exploration.

Insight into continuous glucose monitoring: from medical basics to commercialized devices

According to the latest statistics, more than 537 million people around the world struggle with diabetes and its adverse consequences. As well as acute risks of hypo- or hyper- glycemia, long-term vascular complications may occur, including coronary heart disease or stroke, as well as diabetic nephropathy leading to end-stage disease, neuropathy or retinopathy. Therefore, there is an urgent need to improve diabetes management to reduce the risk of complications but also to improve patient's quality life. The impact of continuous glucose monitoring (CGM) is well recognized, in this regard. The current review aims at introducing the basic principles of glucose sensing, including electrochemical and optical detection, summarizing CGM technology, its requirements, advantages, and disadvantages. The role of CGM systems in the clinical diagnostics/personal testing, difficulties in their utilization, and recommendations are also discussed. In the end, challenges and prospects in future CGM systems are discussed and non-invasive, wearable glucose biosensors are introduced. Though the scope of this review is CGMs and provides information about medical issues and analytical principles, consideration of broader use will be critical in future if the right systems are to be selected for effective diabetes management.

Conformational-switch biosensors as novel tools to support continuous, real-time molecular monitoring in lab-on-a-chip devices

Recent years have seen continued expansion of the functionality of lab on a chip (LOC) devices. Indeed LOCs now provide scientists and developers with useful and versatile platforms across a myriad of chemical and biological applications. The field still fails, however, to integrate an often important element of bench-top analytics: real-time molecular measurements that can be used to "guide" a chemical response. Here we describe the analytical techniques that could provide LOCs with such real-time molecular monitoring capabilities. It appears to us that, among the approaches that are general (i.e., that are independent of the reactive or optical properties of their targets), sensing strategies relying on binding-induced conformational change of bioreceptors are most likely to succeed in such applications.

Continuous Glucose Monitoring Enabled by Fluorescent Nanodiamond Boronic Hydrogel

Continuous monitoring of glucose allows diabetic patients to better maintain blood glucose level by altering insulin dosage or diet according to prevailing glucose values and thus to prevent potential hyperglycemia and hypoglycemia. However, current continuous glucose monitoring (CGM) relies mostly on enzyme electrodes or micro-dialysis probes, which suffer from insufficient stability, susceptibility to corrosion of electrodes, weak or inconsistent correlation, and inevitable interference. A fluorescence-based glucose sensor in the skin will likely be more stable, have improved sensitivity, and can resolve the issues of electrochemical interference from the tissue. This study develops a fluorescent nanodiamond boronic hydrogel system in porous microneedles for CGM. Fluorescent nanodiamond is one of the most photostable fluorophores with superior biocompatibility. When surface functionalized, the fluorescent nanodiamond can integrate with boronic polymer and form a hydrogel, which can produce fluorescent signals in response to environmental glucose concentration. In this proof-of-concept study, the strategy for building a miniatured device with fluorescent nanodiamond hydrogel is developed. The device demonstrates remarkable long-term photo and signal stability in vivo with both small and large animal models. This study presents a new strategy of fluorescence based CGM toward treatment and control of diabetes.

Closed Bipolar Electrode-Enabled Electrochromic Sensing of Multiple Metabolites in Whole Blood

We report a closed bipolar electrode (CBE)-based sensing platform for the detection of diagnostic metabolites in undiluted whole human blood. The sensor is enabled by electrode chemistry based on: (1) a mixed layer of blood-compatible adsorption-resistant phosphorylcholine (PPC) and phenylbutyric acid (PBA), (2) ferrocene (Fc) redox mediators, and (3) immobilized redox-active enzymes. This scheme is designed to overcome nonspecific protein adsorption and amplify sensing currents in whole human fluids. The scheme also incorporates a diffusing mediator to increase electronic communication between the immobilized redox enzyme and the working electrode. The use of both bound and freely diffusing mediators is synergistic in producing the electrochemical response. The sensor is realized by linking the analyte cell, containing the specific electrode surface architecture, through a CBE to a reporter cell containing the electrochromic reporter, methyl viologen (MV). The colorless-to-purple color change accompanying the 1e^- reduction of MV²⁺ is captured using a smartphone camera. Subsequent red-green-blue analysis is performed on the acquired images to determine cholesterol, glucose, and lactate concentrations in whole blood. The CBE blood metabolite sensor produces a linear color change at clinically relevant concentration ranges for all metabolites with good reproducibility (∼5% or better) and with limits of detection of 79 μM for cholesterol, 59 μM for glucose, and 86 μM for lactate. Finally, metabolite concentration measurements from the CBE blood metabolite sensor are compared with results from commercially available FDA-approved blood cholesterol, glucose, and lactate meters, with an average difference of ∼3.5% across all three metabolites in the ranges studied.

Tethering zwitterionic polymer coatings to mediated glucose biosensor enzyme electrodes can decrease sensor foreign body response yet retain sensor sensitivity to glucose

Foreign body response (FBR) is a major challenge that affects implantable biosensors and medical devices, including glucose biosensors, leading to a deterioration in device response over time. Polymer shields are often used to mitigate this issue. Zwitterionic polymers (ZPs) are a promising class of materials that reduce biofouling of implanted devices. A series of ZPs each containing tetherable epoxide functional groups was synthesised for application as a polymer shield for eventual application as implantable glucose biosensors. The polymer shields were initially tested for the ability to resist fibrinogen adsorption and fibroblast adhesion. All synthesised ZPs showed comparable behaviour to a commercial Lipidure ZP in resisting fibrinogen adsorption. Nafion, a common anionic shield used against electrochemical interferents, showed higher protein adsorption and comparable cell adhesion resistance as uncoated control surfaces. However, a poly(2-methacryloyloxyethyl phosphorylcholine-co-glycidyl methacrylate) (MPC)-type ZP showed similar behaviour to Lipidure, with approximately 50% reduced fibrinogen adsorption and 80% decrease in fibroblast adhesion compared to uncoated controls. An MPC-coated amperometric glucose biosensor showed comparable current density and a 1.5-fold increase in sensitivity over an uncoated control biosensor, whereas all other polymer shields tested, including Lipidure, Nafion and a poly(ethyleneglycol) polymer, resulted in lower sensitivity and current density. Collectively, these characteristics make MPC-polymer shield coatings an appealing possibility for use in implantable glucose sensors and other implanted devices with the aim of reducing FBR while maintaining sensor performance.

Catalytic Biosensors Operating under Quasi-Equilibrium Conditions for Mitigating the Changes in Substrate Diffusion

Despite the success of continuous glucose measuring systems operating through the skin for about 14 days, long-term implantable biosensors are facing challenges caused by the foreign-body reaction. We present a conceptually new strategy using catalytic enzyme-based biosensors based on a measuring sequence leading to minimum disturbance of the substrate equilibrium concentration by controlling the sensor between "on" and "off" state combined with short potentiometric data acquisition. It is required that the enzyme activity can be completely switched off and no parasitic side reactions allow substrate turnover. This is achieved by using an O₂ -independent FAD-dependent glucose dehydrogenase embedded within a crosslinked redox polymer. A short measuring interval allows the glucose concentration equilibrium to be restored quickly which enables the biosensor to operate under quasi-equilibrium conditions.

[Smart skin-A new technology in the area of digital dermatology]

Numerous developments in the field of digital medicine have helped to improve the treatment and management of diseases. Smart skin is one promising technology. Through sensors that are attached to the skin, a wide variety of physiological parameters can be measured, e.g., concentration of hormones, presence of inflammation markers, or the glucose level. As this technology can be applied to different parts of the body, information about various organ systems can be obtained. In the case of diabetes, research is already very advanced due to its endemic relevance and the need for long-term treatment. For example, invasive blood measurement can be replaced by implantable tattoos which react to a change in the glucose level by changing its color. In the context of type 1 diabetes, a closed-loop control circuit can be created with so-called microneedling, which results in independent insulin delivery when blood glucose levels are too high. Moreover, there are also smart skin innovations for the management of chronic wounds. With the continuous measurement of physiological indicators such as pH, temperature, or bacterial milieu, the condition of the wound can be observed. The basic principles of the smart skin technology can be transferred into many areas in the field of dermatological care and, therefore, also represent a relevant aspect for dermatologists in the care of their patients. Continuous developments in the field of smart skin technologies show high potential for further research in a wide range of specialties with the aim to facilitate everyday clinical life for patients and physicians.

Bioengineering a glucose oxidase nanosensor for near-infrared continuous glucose monitoring

Single-walled carbon nanotubes (SWCNTs) emit photostable near-infrared (NIR) fluorescence that is conducive for optical glucose monitoring. Such SWCNT-based optical sensors often require the immobilization of proteins that can confer glucose selectivity and reactivity. In this work, we immobilize a glucose-reactive enzyme, glucose oxidase (GOx), onto SWCNTs using a N-(1-pyrenyl)maleimide (PM) crosslinker via thiol bioconjugation of engineered cysteine residues. We compare the conjugation of several glucose oxidase variants containing rationally-engineered cysteines and identify a D70C variant that shows effective bioconjugation. The bioconjugation was characterized through both absorption and fluorescence spectroscopy. Furthermore, we demonstrate an application for continuous glucose monitoring in the NIR-II optical region using the bioconjugated reaction solution, which shows a reversible response to physiological concentrations of glucose. Finally, we develop a miniaturized NIR-II reader to be used for cell cultures that require continuous glucose monitoring.

Closed-Loop Diabetes Minipatch Based on a Biosensor and an Electroosmotic Pump on Hollow Biodegradable Microneedles

Developing a miniaturized, low-cost, and smart closed-loop system for diabetes could significantly improve life quality and benefit millions of people. Conventional closed-loop devices are large in size and exorbitant. Here, we unprecedentedly demonstrate an electrically controlled flexible closed-loop patch for continuous diabetes management by integrating hollow biodegradable microneedles with a biosensing device and an electroosmotic pump. The hollow microneedles were fabricated using a combination of soft lithography and micromachining. The outer layer of the microneedles was functionalized to serve as a biosensing device for the in situ sensitive and accurate monitoring of interstitial glucose. The inner layer of the microneedles was integrated with a flexible electroosmotic pump to deliver insulin, and the delivery rate was electrically controlled by the glucose level from the biosensing device. The closed-loop system successfully stabilized the blood glucose levels of diabetic rats in a normal and safe range. The system is painless, miniaturized, cost-effective, and flexible. It is anticipated that it could open up exciting new avenues for fundamental studies of new closed-loop devices as well as practical applications for diabetes management.

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.

Tattoo Inks for Optical Biosensing in Interstitial Fluid

The persistence of traditional tattoo inks presents an advantage for continuous and long-term health monitoring in point of care devices. The replacement of tattoo pigments with optical biosensors aims a promising alternative for monitoring blood biomarkers. Tattoo inks functionalization enables the control of interstitial biomarkers with correlated concentrations in plasma, to diagnose diseases, evaluate progression, and prevent complications associated with physio pathological disorders or medication mismatches. The specific biomarkers in interstitial fluid provide a new source of information, especially for skin diseases. The study of tattoo inks displays insufficient regulation in their composition, a lack of reports of the related complications, and a need for further studies on their degradation kinetics. This review focuses on tattoo optical biosensors for monitoring dermal interstitial biomarkers and discusses the clinical advantages and main challenges for in vivo implantation. Tattoo functionalization provides a minimally invasive, reversible, biocompatible, real-time sensing with long-term permanence and multiplexing capabilities for the control, diagnosis, and prevention of illness; it enables self-controlling management by the patient, but also the possibility of sending the records to the doctor.

Low dimensional materials for glucose sensing

Biosensors are essential components for effective healthcare management. Since biological processes occur on molecular scales, nanomaterials and nanosensors intrinsically provide the most appropriate landscapes for developing biosensors. Low-dimensional materials have the advantage of offering high surface areas, increased reactivity and unique physicochemical properties for efficient and selective biosensing. So far, nanomaterials and nanodevices have offered significant prospects for glucose sensing. Targeted glucose biosensing using such low-dimensional materials enables much more effective monitoring of blood glucose levels, thus providing significantly better predictive diabetes diagnostics and management. In this review, recent advances in using low dimensional materials for sensing glucose are summarized. Sensing fundamentals are discussed, as well as invasive, minimally-invasive and non-invasive sensing methods. The effects of morphological characteristics and size-dependent properties of low dimensional materials are explored for glucose sensing, and the key performance parameters such as selectivity, stability and sensitivity are also discussed. Finally, the challenges and future opportunities that low dimensional materials can offer for glucose sensing are outlined.

Integration of interstitial fluid extraction and glucose detection in one device for wearable non-invasive blood glucose sensors

Wearable non-invasive glucose sensors that can provide human a painless and portable means to monitor their blood glucose and manage their health condition draw great attentions, recently. Non-invasive human glucose sensors by detecting glucose in interstitial fluid (ISF) extracted through a reverse iontophoresis (RI) approach have been widely investigated, but the current challenges are their complex structure and instability for continuous monitor. Herein, we demonstrate a simple two-electrode non-invasive blood glucose sensor, which is fabricated by using graphene/carbon nanotubes/glucose oxidase composite textile and graphene/carbon nanotube/silver/silver chloride composite textile as the working electrode and counter electrode, respectively. By using one single device, extraction of ISF through RI process is firstly conducted by loading a certain electric current between two electrodes, then the glucose concentration in the ISF is detected through an amperometric approach by using the same two electrodes. The feasibility of these non-invasive glucose sensors is validated on porcine skin, nude mice and human. The blood glucose concentration calculated according to the response currents of the two-electrode sensors is highly consistent with that measured by commercial glucose meter. Furthermore, the used textile-like electrodes provide the non-invasive blood glucose sensors with excellent flexible and wearable properties, which make them promising to be integrated with other electronic units for monitor and management of human health.

A D-shaped fiber SPR sensor with a composite nanostructure of MoS2-graphene for glucose detection

Fiber-based techniques make it possible to implant a miniaturized and flexible surface plasmon resonance (SPR) sensor into the human body for glucose detection. However, the miniaturization of fiber SPR sensors results in low sensitivity compared with traditional prism-type SPR sensors due to limited sensing area. In this paper, we proposed a D-shaped fiber SPR sensor with a composite nanostructure of molybdenum disulfide (MoS₂)-graphene to improve the sensor sensitivity. Compared with the traditional cylindrical fiber, the planar sensing area on the side-polished fiber makes it easier to modify two-dimensional materials. Chemical vapor deposition (CVD) graphene and CVD MoS₂ were modified on the sensor surface to obtain the MoS₂-graphene composite nanostructure. π-π stacking interactions were used to modify pyrene-1-boronic acid (PBA) on the graphene. The excellent photoelectric properties of the MoS₂-graphene composite nanostructure and the ability of PBA to specifically bind glucose molecules improved the glucose detection performance of the SPR sensor. The results show that specific detection of glucose was realized and that the highest sensitivity was achieved with three-layer MoS₂ and monolayer graphene.

Electrochemical glucose sensors in diabetes management: an updated review (2010-2020)

While over half a century has passed since the introduction of enzyme glucose biosensors by Clark and Lyons, this important field has continued to be the focus of immense research activity. Extensive efforts during the past decade have led to major scientific and technological innovations towards tight monitoring of diabetes. Such continued progress toward advanced continuous glucose monitoring platforms, either minimal- or non-invasive, holds considerable promise for addressing the limitations of finger-prick blood testing toward tracking glucose trends over time, optimal therapeutic interventions, and improving the life of diabetes patients. However, despite these major developments, the field of glucose biosensors is still facing major challenges. The scope of this review is to present the key scientific and technological advances in electrochemical glucose biosensing over the past decade (2010-present), along with current obstacles and prospects towards the ultimate goal of highly stable and reliable real-time minimally-invasive or non-invasive glucose monitoring. After an introduction to electrochemical glucose biosensors, we highlight recent progress based on using advanced nanomaterials at the electrode-enzyme interface of three generations of glucose sensors. Subsequently, we cover recent activity and challenges towards next-generation wearable non-invasive glucose monitoring devices based on innovative sensing principles, alternative body fluids, advanced flexible materials, and novel platforms. This is followed by highlighting the latest progress in the field of minimally-invasive continuous glucose monitoring (CGM) which offers real-time information about interstitial glucose levels, by focusing on the challenges toward developing biocompatible membrane coatings to protect electrochemical glucose sensors against surface biofouling. Subsequent sections cover new analytical concepts of self-powered glucose sensors, paper-based glucose sensing and multiplexed detection of diabetes-related biomarkers. Finally, we will cover the latest advances in commercially available devices along with the upcoming future technologies.

Engineered Niches to Analyze Mechanisms of Metastasis and Guide Precision Medicine

Cancer metastasis poses a challenging problem both clinically and scientifically, as the stochastic nature of metastatic lesion formation introduces complexity for both early detection and the study of metastasis in preclinical models. Engineered metastatic niches represent an emerging approach to address this stochasticity by creating bioengineered sites where cancer can preferentially metastasize. As the engineered niche captures the earliest metastatic cells at a nonvital location, both noninvasive and biopsy-based monitoring of these sites can be performed routinely to detect metastasis early and monitor alterations in the forming metastatic niche. The engineered metastatic niche also provides a new platform technology that serves as a tunable site to molecularly dissect metastatic disease mechanisms. Ultimately, linking the engineered niches with advances in sensor development and synthetic biology can provide enabling tools for preclinical cancer models and fosters the potential to impact the future of clinical cancer care.

Anti-Biofouling Strategies for Long-Term Continuous Use of Implantable Biosensors

The growing trend for personalized medicine calls for more reliable implantable biosensors that are capable of continuously monitoring target analytes for extended periods (i.e., >30 d). While promising biosensors for various applications are constantly being developed in the laboratories across the world, many struggle to maintain reliable functionality in complex in vivo environments over time. In this review, we explore the impact of various biotic and abiotic failure modes on the reliability of implantable biosensors. We discuss various design considerations for the development of chronically reliable implantable biosensors with a specific focus on strategies to combat biofouling, which is a fundamental challenge for many implantable devices. Briefly, we introduce the process of the foreign body response and compare the in vitro and the in vivo performances of state-of-the-art implantable biosensors. We then discuss the latest development in material science to minimize and delay biofouling including the usage of various hydrophilic, biomimetic, drug-eluting, zwitterionic, and other smart polymer materials. We also explore a number of active anti-biofouling approaches including stimuli-responsive materials and mechanical actuation. Finally, we conclude this topical review with a discussion on future research opportunities towards more reliable implantable biosensors.

Antimicrobial Honey-Inspired Glucose-Responsive Nanoreactors by Polymerization-Induced Self-Assembly

The rise of antimicrobial resistance is at the forefront of global healthcare challenges, with antimicrobial infections on track to overtake cancer as a leading cause of death by 2050. The high effectiveness of antimicrobial enzymes used in combination with the protective, inert nature of polymer materials represents a highly novel approach toward tackling microbial infections. Herein, we have developed biohybrid glucose oxidase-loaded semipermeable polymersome nanoreactors, formed using polymerization-induced self-assembly, and demonstrate for the first time their ability to "switch on" their antimicrobial activity in response to glucose, a ubiquitous environmental stimulus. Using colony-counting assays, it was demonstrated that the nanoreactors facilitate up to a seven-log reduction in bacterial growth at high glucose concentrations against a range of Gram-negative and Gram-positive bacterial pathogens, including a methicillin-resistant Staphylococcus aureus clinical isolate. After demonstrating the antimicrobial properties of these materials, their toxicity against human fibroblasts was assessed and the dosage of the nanoreactors further optimized for use as nontoxic agents against Gram-positive bacteria under physiological blood glucose concentrations. It is envisaged that such biohybrid nanomaterials will become an important new class of antimicrobial biomaterials for the treatment of bacterial infections.

Cell Adhesive Character of Phenylboronic Acid-Modified Insulin and Its Potential as Long-Acting Insulin

Phenylboronic acid (PBA) derivatives have attracted substantial attention owing to their unique character of forming dynamic covalent bonds with polyol compounds. Recent studies have shown interactions between PBA and sugar chains on the cell surface; they have interesting applications for sensors and drug delivery systems. In this study, we prepared phenylboronic acid-modified insulin (PBA-Ins) to evaluate its glucose-lowering activity and cell adhesiveness. In the case of intravenous injection, PBA-Ins showed longer glucose-lowering activity than native insulin. We hypothesized that this prolonged effect was the result of the interaction between the PBA moiety and sugar chains on the cell surface. Red blood cells (RBCs) were used as a cell model, and we confirmed PBA-Ins's affinity for RBCs, which induced RBC agglutination. Interestingly, using an alternative PBA-Ins administration route markedly changed its glucose-lowering activity. Unlike the intravenous injection of PBA-Ins, the subcutaneous injection showed a small effect on glucose level, which indicated that a small amount of PBA-Ins was absorbed into the bloodstream. This suggested the importance of investigating the interaction between the PBA moiety and many types of cells, such as adipocytes, in subcutaneous tissues.

Toward Long-Term Implantable Glucose Biosensors for Clinical Use

Continuous glucose monitoring (CGM) sensors have led a paradigm shift to painless, continuous, zero-finger pricking measurement in blood glucose monitoring. Recent electrochemical CGM sensors have reached two-week lifespans and no calibration with clinically acceptable accuracy. The system with the recent CGM sensors is identified as an “integrated glucose monitoring system,” which can replace finger-pricking glucose-testing for diabetes treatment decisions. Although such innovation has brought CGM technology closer to realizing the artificial pancreas, discomfort and infection problems have arisen from short lifespans and open wounds. A fully implantable sensor with a longer-term lifespan (90 days) is considered as an alternative CGM sensor with high comfort and low running cost. However, it still has barriers, including surgery for applying and replacing and frequent calibration. If technical refinement is conducted (e.g., stability and reproducibility of sensor fabrication), fully implantable, long-term CGM sensors can open the new era of continuous glucose monitoring.

Soft and flexible material-based affinity sensors

Recent advances in biosensors and point-of-care (PoC) devices are poised to change and expand the delivery of diagnostics from conventional lateral-flow assays and test strips that dominate the market currently, to newly emerging wearable and implantable devices that can provide continuous monitoring. Soft and flexible materials are playing a key role in propelling these trends towards real-time and remote health monitoring. Affinity biosensors have the capability to provide for diagnosis and monitoring of cancerous, cardiovascular, infectious and genetic diseases by the detection of biomarkers using affinity interactions. This review tracks the evolution of affinity sensors from conventional lateral-flow test strips to wearable/implantable devices enabled by soft and flexible materials. Initially, we highlight conventional affinity sensors exploiting membrane and paper materials which have been so successfully applied in point-of-care tests, such as lateral-flow immunoassay strips and emerging microfluidic paper-based devices. We then turn our attention to the multifarious polymer designs that provide both the base materials for sensor designs, such as PDMS, and more advanced functionalised materials that are capable of both recognition and transduction, such as conducting and molecularly imprinted polymers. The subsequent content discusses wearable soft and flexible material-based affinity sensors, classified as flexible and skin-mountable, textile materials-based and contact lens-based affinity sensors. In the final sections, we explore the possibilities for implantable/injectable soft and flexible material-based affinity sensors, including hydrogels, microencapsulated sensors and optical fibers. This area is truly a work in progress and we trust that this review will help pull together the many technological streams that are contributing to the field.

New Directions for Artificial Cells Using Prototyped Biosystems

Microfluidics has has enabled the generation of  a range of single compartment and multicompartment vesicles and bilayer-delineated droplets that can be assembled in 2D and 3D. These model systems are becoming increasingly used as artificial cell chassis and as biomimetic constructs for assembling tissue models, engineering therapeutic delivery systems, and screening drugs. One bottleneck in developing this technology is the time, expertise, and equipment required for device fabrication. This has led to interest across the microfluidics community in using rapid prototyping to engineer microfluidic devices from computer-aided-design (CAD) drawings. We highlight how this rapid-prototyping revolution is transforming the fabrication of microfluidic devices for artificial cell construction in bottom-up synthetic biology. We provide an outline of the current landscape and present how advances in the field may give rise to the next generation of multifunctional biodevices, particularly with Industry 4.0 on the horizon. Successfully developing this technology and making it open-source could pave the way for a new generation of citizen-led science, fueling the possibility that the next multibillion-dollar start-up could emerge from an attic or a basement.

Scalable High-Performance Ultraminiature Graphene Micro-Supercapacitors by a Hybrid Technique Combining Direct Writing and Controllable Microdroplet Transfer

Miniaturization of energy storage devices can significantly decrease the overall size of electronic systems. However, this miniaturization is limited by the reduction of electrode dimensions and the reproducible transfer of small electrolyte drops. This paper reports first a simple scalable direct writing method for the production of ultraminiature microsupercapacitor (MSC) electrodes, based on femtosecond laser reduced graphene oxide (fsrGO) interlaced pads. These pads, separated by 2 μm spacing, are 100 μm long and 8 μm wide. A second stage involves the accurate transfer of an electrolyte microdroplet on top of each individual electrode, which can avoid any interference of the electrolyte with other electronic components. Abundant in-plane mesopores in fsrGO induced by a fs laser together with ultrashort interelectrode spacing enables MSCs to exhibit a high specific capacitance (6.3 mF cm^-2 and 105 F cm^-3) and ∼100% retention after 1000 cycles. An all graphene resistor-capacitor (RC) filter is also constructed by combining the MSC and a fsrGO resistor, which is confirmed to exhibit highly enhanced performance characteristics. This new hybrid technique combining fs laser direct writing and precise microdroplet transfer easily enables scalable production of ultraminiature MSCs, which is believed to be significant for practical application of micro-supercapacitor microelectronic systems.

Current and Emerging Technology for Continuous Glucose Monitoring

Diabetes has become a leading cause of death worldwide. Although there is no cure for diabetes, blood glucose monitoring combined with appropriate medication can enhance treatment efficiency, alleviate the symptoms, as well as diminish the complications. For point-of-care purposes, continuous glucose monitoring (CGM) devices are considered to be the best candidates for diabetes therapy. This review focuses on current growth areas of CGM technologies, specifically focusing on subcutaneous implantable electrochemical glucose sensors. The superiority of CGM systems is introduced firstly, and then the strategies for fabrication of minimally-invasive and non-invasive CGM biosensors are discussed, respectively. Finally, we briefly outline the current status and future perspective for CGM systems.

Self and directed assembly: people and molecules

Self-assembly and directed-assembly are two very important aspects of supramolecular chemistry. As a young postgraduate student working in Canada with Tom Fyles my introduction to Supramolecular Chemistry was through the self-assembly of phospholipid membranes to form vesicles for which we were developing unimolecular and self-assembling transporter molecules. The next stage of my development as a scientist was in Japan with Seiji Shinkai where in a “Eureka” moment, the boronic acid templating unit (directed-assembly) of Wulff was combined with photoinduced electron transfer systems pioneered by De Silva. The result was a turn-on fluorescence sensor for saccharides; this simple result has continued to fuel my research to the present day. Throughout my career as well as assembling molecules, I have enjoyed bringing together researchers in order to develop collaborative networks. This is where molecules meet people resulting in assemblies worth more than the individual “molecule” or “researcher”. My role in developing networks with Japan was rewarded by the award of a Daiwa-Adrian Prize in 2013 and I was recently rewarded for developing networks with China with an Inaugural CASE Prize in 2015.

Surface-Enhanced Raman Spectroscopy Biosensing: In Vivo Diagnostics and Multimodal Imaging

This perspective presents recent developments in the application of surface-enhanced Raman spectroscopy (SERS) to biosensing, with a focus on in vivo diagnostics. We describe the concepts and methodologies developed to date and the target analytes that can be detected. We also discuss how SERS has evolved from a "point-and-shoot" stand-alone technique in an analytical chemistry laboratory to an integrated quantitative analytical tool for multimodal imaging diagnostics. Finally, we offer a guide to the future of SERS in the context of clinical diagnostics.

A continuous glucose monitoring device by graphene modified electrochemical sensor in microfluidic system

This paper presents a continuous glucose monitoring microsystem consisting of a three-electrode electrochemical sensor integrated into a microfluidic chip. The microfluidic chip, which was used to transdermally extract and collect subcutaneous interstitial fluid, was fabricated from five polydimethylsiloxane layers using micromolding techniques. The electrochemical sensor was integrated into the chip for continuous detection of glucose. Specifically, a single-layer graphene and gold nanoparticles (AuNPs) were decorated onto the working electrode (WE) of the sensor to construct a composite nanostructured surface and improve the resolution of the glucose measurements. Graphene was transferred onto the WE surface to improve the electroactive nature of the electrode to enable measurements of low levels of glucose. The AuNPs were directly electrodeposited onto the graphene layer to improve the electron transfer rate from the activity center of the enzyme to the electrode to enhance the sensitivity of the sensor. Glucose oxidase (GOx) was immobilized onto the composite nanostructured surface to specifically detect glucose. The factors required for AuNPs deposition and GOx immobilization were also investigated, and the optimized parameters were obtained. The experimental results displayed that the proposed sensor could precisely measure glucose in the linear range from 0 to 162 mg/dl with a detection limit of 1.44 mg/dl (S/N = 3). The proposed sensor exhibited the potential to detect hypoglycemia which is still a major challenge for continuous glucose monitoring in clinics. Unlike implantable glucose sensors, the wearable device enabled external continuous monitoring of glucose without interference from foreign body reaction and bioelectricity.

Porous, Dexamethasone-loaded polyurethane coatings extend performance window of implantable glucose sensors in vivo

Continuous glucose sensors offer the promise of tight glycemic control for insulin dependent diabetics; however, utilization of such systems has been hindered by issues of tissue compatibility. Here we report on the in vivo performance of implanted glucose sensors coated with Dexamethasone-loaded (Dex-loaded) porous coatings employed to mediate the tissue-sensor interface. Two animal studies were conducted to (1) characterize the tissue modifying effects of the porous Dex-loaded coatings deployed on sensor surrogate implants and (2) investigate the effects of the same coatings on the in vivo performance of Medtronic MiniMed SOF-SENSOR™ glucose sensors. The tissue response to implants was evaluated by quantifying macrophage infiltration, blood vessel formation, and collagen density around implants. Sensor function was assessed by measuring changes in sensor sensitivity and time lag, calculating the Mean Absolute Relative Difference (MARD) for each sensor treatment, and performing functional glucose challenge test at relevant time points. Implants treated with porous Dex-loaded coatings diminished inflammation and enhanced vascularization of the tissue surrounding the implants. Functional sensors with Dex-loaded porous coatings showed enhanced sensor sensitivity over a 21-day period when compared to controls. Enhanced sensor sensitivity was accompanied with an increase in sensor signal lag and MARD score. These results indicate that Dex-loaded porous coatings were able to elicit an attenuated tissue response, and that such tissue microenvironment could be conducive towards extending the performance window of glucose sensors in vivo.

STATEMENT OF SIGNIFICANCE

In the present article, a coating to extend the functionality of implantable glucose sensors in vivo was developed. Our study showed that the delivery of an anti-inflammatory agent with the presentation of micro-sized topographical cues from coatings may lead to improved long-term glucose sensor function in vivo. We believe that improved function of sensors treated with the novel coatings was a result of the observed decreases in inflammatory cell density and increases in vessel density of the tissue adjacent to the devices. Furthermore, extending the in vivo functionality of implantable glucose sensors may lead to greater adoption of these devices by diabetic patients.

Boronic acids for fluorescence imaging of carbohydrates

Carbohydrate biomarkers are particularly important targets for fluorescence imaging given their pivotal role in numerous important biological events. This review highlights the development of fluorescence imaging agents based on boronic acids.

Ex vivo optical measurements of glucose diffusion kinetics in native and diabetic mouse skin

The aim of this study was to estimate the glucose diffusion coefficients ex vivo in skin of mice with diabetes induced in vivo by alloxan in comparison to non-diabetic mice. The temporal dependences of collimated transmittance of tissue samples immersed in glucose solutions were measured in the VIS-NIR spectral range to quantify the glucose diffusion/permeability coefficients and optical clearing efficiency of mouse skin. The average thickness of intact healthy and diabetic skin was 0.023 ± 0.006 cm and 0.019 ± 0.005 cm, respectively. Considerable differences in optical and kinetic properties of diabetic and non-diabetic skin were found: clearing efficiency was 1.5-fold better and glucose diffusivity was 2-fold slower for diabetic skin. Experimental Setup for measuring collimated transmittance spectra of mouse skin samples.

Novel silica surface charge density mediated control of the optical properties of embedded optically active materials and its application for fiber optic pH sensing at elevated temperatures

Silica and silica incorporated nanocomposite materials have been extensively studied for a wide range of applications. Here we demonstrate an intriguing optical effect of silica that, depending on the solution pH, amplifies or attenuates the optical absorption of a variety of embedded optically active materials with very distinct properties, such as plasmonic Au nanoparticles, non-plasmonic Pt nanoparticles, and the organic dye rhodamine B (not a pH indicator), coated on an optical fiber. Interestingly, the observed optical response to varying pH appears to follow the surface charge density of the silica matrix for all the three different optically active materials. To the best of our knowledge, this optical effect has not been previously reported and it appears universal in that it is likely that any optically active material can be incorporated into the silica matrix to respond to solution pH or surface charge density variations. A direct application of this effect is for optical pH sensing which has very attractive features that can enable minimally invasive, remote, real time and continuous distributed pH monitoring. Particularly, as demonstrated here, using highly stable metal nanoparticles embedded in an inorganic silica matrix can significantly improve the capability of pH sensing in extremely harsh environments which is of increasing importance for applications in unconventional oil and gas resource recovery, carbon sequestration, water quality monitoring, etc. Our approach opens a pathway towards possible future development of robust optical pH sensors for the most demanding environmental conditions. The newly discovered optical effect of silica also offers the potential for control of the optical properties of optically active materials for a range of other potential applications such as electrochromic devices.

Boronic acids for sensing and other applications - a mini-review of papers published in 2013

Boronic acids are increasingly utilised in diverse areas of research. Including the interactions of boronic acids with diols and strong Lewis bases as fluoride or cyanide anions, which leads to their utility in various sensing applications. The sensing applications can be homogeneous assays or heterogeneous detection. Detection can be at the interface of the sensing material or within the bulk sample. Furthermore, the key interaction of boronic acids with diols allows utilisation in various areas ranging from biological labelling, protein manipulation and modification, separation and the development of therapeutics. All the above uses and applications are covered by this mini-review of papers published during 2013.

The future of laboratory medicine - a 2014 perspective

Predicting the future is a difficult task. Not surprisingly, there are many examples and assumptions that have proved to be wrong. This review surveys the many predictions, beginning in 1887, about the future of laboratory medicine and its sub-specialties such as clinical chemistry and molecular pathology. It provides a commentary on the accuracy of the predictions and offers opinions on emerging technologies, economic factors and social developments that may play a role in shaping the future of laboratory medicine.

Using personal sensors to assess the exposome and acute health effects

Introduction: The exposome encompasses the totality of human environmental exposures. Recent developments in sensor technology have made it possible to better measure personal exposure to environmental pollutants and other factors. We aimed to discuss and demonstrate the recent developments in personal sensors to measure multiple exposures and possible acute health responses, and discuss the main challenges ahead. Methods: We searched for a range of sensors to measure air pollution, noise, temperature, UV, physical activity, location, blood pressure, heart rate and lung function and to obtain information on green space and emotional status/mood and put it on a person. Results and Conclusions: We discussed the recent developments and main challenges for personal sensors to measure multiple exposures. We found and put together a personal sensor set that measures a comprehensive set of personal exposures continuously over 24 h to assess part of the current exposome and acute health responses. We obtained data for a whole range of exposures and some acute health responses, but many challenges remain to apply the methodology for extended time periods and larger populations including improving the ease of wear, e.g., through miniaturization and extending battery life, and the reduction of costs. However, the technology is moving fast and opportunities will come closer for further wide spread use to assess, at least part of the exposome.