Foreign Body Reaction to a Subcutaneously Implanted Self-Cleaning, Thermoresponsive Hydrogel Membrane for Glucose Biosensors

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.

Impact of PEG sensitization on the efficacy of PEG hydrogel-mediated tissue engineering

While poly(ethylene glycol) (PEG) hydrogels are generally regarded as biologically inert blank slates, concerns over PEG immunogenicity are growing, and the implications for tissue engineering are unknown. Here, we investigate these implications by immunizing mice against PEG to stimulate anti-PEG antibody production and evaluating bone defect regeneration after treatment with bone morphogenetic protein-2-loaded PEG hydrogels. Quantitative analysis reveals that PEG sensitization increases bone formation compared to naive controls, whereas histological analysis shows that PEG sensitization induces an abnormally porous bone morphology at the defect site, particularly in males. Furthermore, immune cell recruitment is higher in PEG-sensitized mice administered the PEG-based treatment than their naive counterparts. Interestingly, naive controls that were administered a PEG-based treatment also develop anti-PEG antibodies. Sex differences in bone formation and immune cell recruitment are also apparent. Overall, these findings indicate that anti-PEG immune responses can impact tissue engineering efficacy and highlight the need for further investigation.

Colloidal Phenol-Amine Coating on Implants for Improved Anti-Inflammation and Osteogenesis

Phenol-amine coatings have attracted significant attention in recent years owing to their adjustable composition and multifaceted biological functionalities. The current preparation of phenol-amine coatings, however, involves a chemical reaction within the solution or interface, resulting in lengthy preparation times and necessitating specific reaction conditions, such as alkaline environments and oxygen presence. The facile, rapid, and eco-friendly preparation of phenol-amine coatings under mild conditions continues to pose a challenge. In this study, we use a macromolecular phenol-amine, Tanfloc, to form a stable colloid under neutral conditions, which was then rapidly adsorbed on the titanium surface by electrostatic action and then spread and fused to form a continuous coating within several minutes. This nonchemical preparation process was rapid, mild, and free of chemical additives. The in vitro and in vivo results showed that the Tanfloc colloid fusion coating inhibited destructive inflammation, promoted osteogenesis, and enhanced osteointegration. These remarkable advantages of the colloidal phenol-amine fusion coating highlight the suitability of its future application in clinical practice.

Hydrogels for Chemical Sensing and Biosensing

The development and synthesis of hydrogels for chemical and biosensing are of great value. Hydrogels can be tailored to its own physical structure, chemical properties, biocompatibility, and sensitivity to external stimuli when being used in a specific environment. Herein, hydrogels and their applications in chemical and biosensing are mainly covered. In particular, it is focused on the manner in which hydrogels serve as sensing materials to a specific analyte. Different types of responsive hydrogels are hence introduced and summarized. Researchers can modify different chemical groups on the skeleton of the hydrogels, which make them as good chemical and biosensing materials. Hydrogels have great application potential for chemical and biosensing in the biomedical field and some emerging fields, such as wearable devices.

Electrochemical Wearable Biosensors and Bioelectronic Devices Based on Hydrogels: Mechanical Properties and Electrochemical Behavior

Hydrogel-based wearable electrochemical biosensors (HWEBs) are emerging biomedical devices that have recently received immense interest. The exceptional properties of HWEBs include excellent biocompatibility with hydrophilic nature, high porosity, tailorable permeability, the capability of reliable and accurate detection of disease biomarkers, suitable device-human interface, facile adjustability, and stimuli responsive to the nanofiller materials. Although the biomimetic three-dimensional hydrogels can immobilize bioreceptors, such as enzymes and aptamers, without any loss in their activities. However, most HWEBs suffer from low mechanical strength and electrical conductivity. Many studies have been performed on emerging electroactive nanofillers, including biomacromolecules, carbon-based materials, and inorganic and organic nanomaterials, to tackle these issues. Non-conductive hydrogels and even conductive hydrogels may be modified by nanofillers, as well as redox species. All these modifications have led to the design and development of efficient nanocomposites as electrochemical biosensors. In this review, both conductive-based and non-conductive-based hydrogels derived from natural and synthetic polymers are systematically reviewed. The main synthesis methods and characterization techniques are addressed. The mechanical properties and electrochemical behavior of HWEBs are discussed in detail. Finally, the prospects and potential applications of HWEBs in biosensing, healthcare monitoring, and clinical diagnostics are highlighted.

Cellular infiltration in an injectable sulfated cellulose nanocrystal hydrogel and efficient angiogenesis by VEGF loading

BACKGROUND

Cellular infiltration and angiogenesis into implanted biomaterial scaffolds are crucial for successful host tissue integration and tissue regeneration. Cellulose nanocrystal (CNC) is a nano-sized cellulose derivative, which can form an injectable physical gel with salts. Sulfate groups of sulfated CNC (CNC-S) can act as a binding domain to various growth factors and cytokines with a heparin-binding domain for sustained release of them. Vascular endothelial growth factor (VEGF) can promote the proliferation of endothelial cells and angiogenesis. In this study, VEGF-loaded CNC-S hydrogel was evaluated as an injectable scaffold that can induce cellular infiltration and angiogenesis.

METHODS

CNC-S was hydrolyzed to get desulfated CNC (CNC-DS), which was used as a negative control group against CNC-S. Both CNC-S and CNC-DS hydrogels were prepared and compared in terms of biocompatibility and VEGF release. The hydrogels with or without VEGF loading were subcutaneously injected into mice to evaluate the biocompatibility, cellular infiltration, and angiogenesis induction of the hydrogels.

RESULTS

Both hydrogels possessed similar stability and shear-thinning behavior to be applicable as injectable hydrogels. However, CNC-S hydrogel showed sustained release (until 8 weeks) of VEGF whereas CNC-DS showed a very fast release of VEGF with a large burst. Subcutaneously injected CNC-S hydrogel showed much enhanced cellular infiltration as well as better biocompatibility with milder foreign body response than CNC-DS hydrogel. Furthermore, VEGF-loaded CNC-S hydrogel induced significant angiogenesis inside the hydrogel whereas VEGF-loaded CNC-DS did not.

CONCLUSION

CNC-S possesses good properties as a biomaterial including injectability, biocompatibility, and allowing cellular infiltration and sustained release of growth factors. VEGF-loaded CNC-S hydrogel exhibited efficient angiogenesis inside the hydrogel. The sulfate group of CNC-S was a key for good biocompatibility and the biological activities of VEGF-loaded CNC hydrogel.

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.

Translational Applications of Hydrogels

Advances in hydrogel technology have unlocked unique and valuable capabilities that are being applied to a diverse set of translational applications. Hydrogels perform functions relevant to a range of biomedical purposes-they can deliver drugs or cells, regenerate hard and soft tissues, adhere to wet tissues, prevent bleeding, provide contrast during imaging, protect tissues or organs during radiotherapy, and improve the biocompatibility of medical implants. These capabilities make hydrogels useful for many distinct and pressing diseases and medical conditions and even for less conventional areas such as environmental engineering. In this review, we cover the major capabilities of hydrogels, with a focus on the novel benefits of injectable hydrogels, and how they relate to translational applications in medicine and the environment. We pay close attention to how the development of contemporary hydrogels requires extensive interdisciplinary collaboration to accomplish highly specific and complex biological tasks that range from cancer immunotherapy to tissue engineering to vaccination. We complement our discussion of preclinical and clinical development of hydrogels with mechanical design considerations needed for scaling injectable hydrogel technologies for clinical application. We anticipate that readers will gain a more complete picture of the expansive possibilities for hydrogels to make practical and impactful differences across numerous fields and biomedical applications.

Analytical Parameters of a Novel Glucose Biosensor Based on Grafted PFM as a Covalent Immobilization Technique

Bioanalytical methods, in particular electrochemical biosensors, are increasingly used in different industrial sectors due to their simplicity, low cost, and fast response. However, to be able to reliably use this type of device, it is necessary to undertake in-depth evaluation of their fundamental analytical parameters. In this work, analytical parameters of an amperometric biosensor based on covalent immobilization of glucose oxidase (GOx) were evaluated. GOx was immobilized using plasma-grafted pentafluorophenyl methacrylate (pgPFM) as an anchor onto a tailored HEMA-co-EGDA hydrogel that coats a titanium dioxide nanotubes array (TiO2NTAs). Finally, chitosan was used to protect the enzyme molecules. The biosensor offered outstanding analytical parameters: repeatability (RSD = 1.7%), reproducibility (RSD = 1.3%), accuracy (deviation = 4.8%), and robustness (RSD = 2.4%). In addition, the Ti/TiO2NTAs/ppHEMA-co-EGDA/pgPFM/GOx/Chitosan biosensor showed good long-term stability; after 20 days, it retained 89% of its initial sensitivity. Finally, glucose concentrations of different food samples were measured and compared using an official standard method (HPLC). Deviation was lower than 10% in all measured samples. Therefore, the developed biosensor can be considered to be a reliable analytical tool for quantification measurements.

In Vitro and In Vivo Studies of Biodegradability and Biocompatibility of Poly(εCL)-b-Poly(EtOEP)-Based Films

The control of surface bioadhesive properties of the subcutaneous implants is essential for the development of biosensors and controlled drug release devices. Poly(alkyl ethylene phosphate)-based (co)polymers are structurally versatile, biocompatible and biodegradable, and may be regarded as an alternative to poly(ethylene glycol) (PEG) copolymers in the creation of antiadhesive materials. The present work reports the synthesis of block copolymers of ε-caprolactone (εCL) and 2-ethoxy-1,3,2-dioxaphospholane-2-oxide (ethyl ethylene phosphate, EtOEP) with different content of EtOEP fragments, preparation of polymer films, and the results of the study of the impact of EtOEP/εCL ratio on the hydrophilicity (contact angle of wetting), hydrolytic stability, cytotoxicity, protein and cell adhesion, and cell proliferation using umbilical cord multipotent stem cells. It was found that the increase of EtOEP/εCL ratio results in increase of hydrophilicity of the polymer films with lowering of the protein and cell adhesion. MTT cytotoxicity test showed no significant deviations in toxicity of poly(εCL) and poly(εCL)-b-poly(EtOEP)-based films. The influence of the length of poly(EtOEP)chain in block-copolymers on fibrotic reactions was analyzed using subcutaneous implantation experiments (Wistar line rats), the increase of the width of the fibrous capsule correlated with higher EtOEP/εCL ratio. However, the copolymer-based film with highest content of polyphosphate had been subjected to faster degradation with a formation of developed contact surface of poly(εCL). The rate of the degradation of polyphosphate in vivo was significantly higher than the rate of the degradation of polyphosphate in vitro, which only confirms an objective value of in vivo experiments in the development of polymer materials for biomedical applications.

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.

A metal-free radical technique for cross-linking of polymethylhydrosiloxane or polymethylvinylsiloxane using AIBN

A new method was developed for the metal-free cross-linking of silicone rubbers. This process uses azobisisobutyronitrile (AIBN) to selectively react with Si-H and vinyl groups as a free-radical initiator for the thermal curing of polymethylhydrosiloxane (PMHS) and polymethylvinylsiloxane (PMVS). The AIBN-initiated curing reaction between the Si-H groups of PMHS generated Si-O-Si and Si-Si cross-links. In contrast, PMVS was cured via the formation of C-C bonds through "methyl-vinyl" and "vinyl-vinyl" mechanisms. Curing reactions were performed at 80-120 °C in air and confirmed by 13C and 29Si solid state NMR analyses and swelling trials.

Hydrogel membranes: A review

Hydrogel membranes (HMs) are defined and applied as hydrated porous media constructed of hydrophilic polymers for a broad range of applications. Fascinating physiochemical properties, unique porous architecture, water-swollen features, biocompatibility, and special water content dependent transport phenomena in semi-permeable HMs make them appealing constructs for various applications from wastewater treatment to biomedical fields. Water absorption, mechanical properties, and viscoelastic features of three-dimensional (3D) HM networks evoke the extracellular matrix (ECM). On the other hand, the porous structure with controlled/uniform pore-size distribution, permeability/selectivity features, and structural/chemical tunability of HMs recall membrane separation processes such as desalination, wastewater treatment, and gas separation. Furthermore, supreme physiochemical stability and high ion conductivity make them promising to be utilised in the structure of accumulators such as batteries and supercapacitors. In this review, after summarising the general concepts and production processes for HMs, a comprehensive overview of their applications in medicine, environmental engineering, sensing usage, and energy storage/conservation is well-featured. The present review concludes with existing restrictions, possible potentials, and future directions of HMs.

Inhibition of biofilm formation by rough shark skin-patterned surfaces

In this study, we investigate the microscale structure of shark skin denticles at abdomen (A) and fin (F) locations, analyze the roughness and wetting properties related to their microstructures, and evaluate the effect of the surface properties on early bacterial attachment and biofilm formation. Microstructural analysis by scanning electron microscopy and confocal laser scanning microscopy confirmed the length (A: 165-180 μm vs. F: 145-165 μm), width (A: 86-100 μm vs. F: 64-70 μm), height (A: 10.5-13.5 μm vs. F: 6.2-8.8 μm), and density (A: 110-130 denticles/mm²vs. F: 80-130 denticles/mm²) of the denticles. The results showed that the roughness and hydrophobicity properties were affected with slight differences in the microscale architecture. The denticles with a larger width, higher ridge, and denser overlap provided a rougher and more hydrophobic surface. The microscale structure not only affected surface properties but also the biological attachment process. The microscale topography of shark skin slightly promoted bacterial attachment at an early stage, but prevented bacteria from developing biofilms. This systematic investigation provides insights into the effects of the surface topography of shark skin on its anti-fouling mechanism, which will enable the future development of various products related to human activity, such as healthcare products, underwater devices and applications, and water treatment applications.

A self-cleaning, mechanically robust membrane for minimizing the foreign body reaction: towards extending the lifetime of sub-Q glucose biosensors

Long-term, subcutaneously implanted continuous glucose biosensors have the potential to improve diabetes management and reduce associated complications. However, the innate foreign body reaction (FBR) both alters the local glucose concentrations in the surrounding tissues and compromises glucose diffusion to the biosensor due to the recruitment of high-metabolizing inflammatory cells and the formation of a dense, collagenous fibrous capsule. Minimizing the FBR has mainly focused on "passively antifouling" materials that reduce initial cellular attachment, including poly(ethylene glycol) (PEG). Instead, the membrane reported herein utilizes an "actively antifouling" or "self-cleaning" mechanism to inhibit cellular attachment through continuous, cyclic deswelling/reswelling in response to normal temperature fluctuations of the subcutaneous tissue. This thermoresponsive double network (DN) membrane is based on N-isopropylacrylamide (NIPAAm) and 2-acrylamido-2-methylpropane sulfonic acid (AMPS) (75:25 and 100:0 NIPAAm:AMPS in the 1st and 2nd networks, respectively; "DN-25%"). The extent of the FBR reaction of a subcutaneously implanted DN-25% cylindrical membrane was evaluated in rodents in parallel with a PEG-diacrylate (PEG-DA) hydrogel as an established benchmark biocompatible control. Notably, the DN-25% implants were more than 25× stronger and tougher than the PEG-DA implants while maintaining a modulus near that of subcutaneous tissue. From examining the FBR at 7, 30 and 90 days after implantation, the thermoresponsive DN-25% implants demonstrated a rapid healing response and a minimal fibrous capsule (~20-25 µm), similar to the PEG-DA implants. Thus, the dynamic self-cleaning mechanism of the DN-25% membranes represents a new approach to limit the FBR while achieving the durability necessary for long-term implantable glucose biosensors.

Modern Strategies To Achieve Tissue-Mimetic, Mechanically Robust Hydrogels

Hydrogels are frequently used biomaterials due to their similarity in hydration and structure to biological tissues. However, their utility is limited by poor mechanical properties, namely, a lack of strength and stiffness that mimic that of tissues, particularly load-bearing tissues. Thus, numerous recent strategies have sought to enhance and tune these properties in hydrogels, including interpenetrating networks (IPNs), macromolecular cross-linking, composites, thermal conditioning, polyampholytes, and dual cross-linking. Individually, these approaches have achieved hydrogels with either high strength (σ f > 10 MPa), high stiffness (E > 1 MPa), or, less commonly, both high strength and stiffness (σ f > 10 MPa and E > 1 MPa). However, only certain unique combinations of these approaches have been able to synergistically achieve retention of a high, tissuelike water content as well as high strength and stiffness. Applying such methods to stimuli-responsive hydrogels has also produced robust, smart biomaterials. Overall, methods to achieve hydrogels that simultaneously mimic the hydration, strength, and stiffness of soft and load-bearing tissues have the potential to be used in a much broader range of biomedical applications.