Insulin delivery governed by covalently modified lectin-glycogen gels sensitive to glucose

Recent Insights into Glucose-Responsive Concanavalin A-Based Smart Hydrogels for Controlled Insulin Delivery

Many efforts are continuously undertaken to develop glucose-sensitive biomaterials able of controlling glucose levels in the body and self-regulating insulin delivery. Hydrogels that swell or shrink as a function of the environmental free glucose content are suitable systems for monitoring blood glucose, delivering insulin doses adapted to the glucose concentration. In this context, the development of sensors based on reversible binding to glucose molecules represents a continuous challenge. Concanavalin A (Con A) is a bioactive protein isolated from sword bean plants (Canavalia ensiformis) and contains four sugar-binding sites. The high affinity for reversibly and specifically binding glucose and mannose makes Con A as a suitable natural receptor for the development of smart glucose-responsive materials. During the last few years, Con A was used to develop smart materials, such as hydrogels, microgels, nanoparticles and films, for producing glucose biosensors or drug delivery devices. This review is focused on Con A-based materials suitable in the diagnosis and therapeutics of diabetes. A brief outlook on glucose-derived theranostics of cancer is also presented.

Nature-Derived Polysaccharide-Based Composite Hydrogels for Promoting Wound Healing

Numerous innovative advancements in dressing technology for wound healing have emerged. Among the various types of wound dressings available, hydrogel dressings, structured with a three-dimensional network and composed of predominantly hydrophilic components, are widely used for wound care due to their remarkable capacity to absorb abundant wound exudate, maintain a moisture environment, provide soothing and cooling effects, and mimic the extracellular matrix. Composite hydrogel dressings, one of the evolved dressings, address the limitations of traditional hydrogel dressings by incorporating additional components, including particles, fibers, fabrics, or foams, within the hydrogels, effectively promoting wound treatment and healing. The added elements enhance the features or add specific functionalities of the dressings, such as sensitivity to external factors, adhesiveness, mechanical strength, control over the release of therapeutic agents, antioxidant and antimicrobial properties, and tissue regeneration behavior. They can be categorized as natural or synthetic based on the origin of the main components of the hydrogel network. This review focuses on recent research on developing natural polysaccharide-based composite hydrogel wound dressings. It explores their preparation and composition, the reinforcement materials integrated into hydrogels, and therapeutic agents. Furthermore, it discusses their features and the specific types of wounds where applied.

Glucose-Responsive Insulin and Delivery Systems: Innovation and Translation

Type 1 and advanced type 2 diabetes treatment involves daily injections or continuous infusion of exogenous insulin aimed at regulating blood glucose levels in the normoglycemic range. However, current options for insulin therapy are limited by the risk of hypoglycemia and are associated with suboptimal glycemic control outcomes. Therefore, a range of glucose-responsive components that can undergo changes in conformation or show alterations in intermolecular binding capability in response to glucose stimulation has been studied for ultimate integration into closed-loop insulin delivery or "smart insulin" systems. Here, an overview of the evolution and recent progress in the development of molecular approaches for glucose-responsive insulin delivery systems, a rapidly growing subfield of precision medicine, is presented. Three central glucose-responsive moieties, including glucose oxidase, phenylboronic acid, and glucose-binding molecules are examined in detail. Future opportunities and challenges regarding translation are also discussed.

Smart approaches to glucose-responsive drug delivery

A grand challenge in the field of "smart" drug delivery has been the quest to create formulations that can sense glucose and respond by delivering an appropriate dose of insulin. This approach, referred to as the "fully synthetic pancreas", envisions closed-loop insulin therapy. The strategies for incorporating glucose sensing into formulations can be broadly categorized into three subsets: enzymatic sensing, natural glucose-binding proteins and synthetic molecular recognition. Here, we highlight some examples of each of these approaches. The challenges remaining en route to the realization of closed-loop insulin therapy are substantial, and include improved response time, more authentic fidelity in glycemic control, improved biocompatibility for delivery materials and assurance of both safety and efficacy. The ubiquitous existence of glucose, combined with the unstable and toxic properties of insulin, further compound efforts towards the generation of a fully synthetic pancreas. However, given the growing incidence of both type-1 and type-2 diabetes, there is significant potential impact from the realization of such an approach on improving therapeutic management of the disease.

Responsive materials for self-regulated insulin delivery

With diabetes mellitus becoming an important public health concern, insulin-delivery systems are attracting increasing interest from both scientific and technological researchers. This feature article covers the present state-of-the-art glucose-responsive insulin-delivery system (denoted as GRIDS), based on responsive polymer materials, a promising system for self-regulated insulin delivery. Three types of GRIDS are discussed, based on different fundamental mechanisms of glucose-recognition, with: a) glucose enzyme, b) glucose binding protein, and c) synthetic boronic acid as the glucose-sensitive component. At the end, a personal perspective on the major issues yet to be worked out in future research is provided.

Development of a reversed-phase high-performance liquid chromatography method for the analysis of components from a closed-loop insulin delivery system

A reversed-phase HPLC method has been developed which enables separation of the three components of a closed-loop insulin delivery system, namely concanavalin A methacrylamide (Con A-MA), dextran methacrylate (Dex-MA) and bovine insulin. The analysis of Con A-MA represents a significant challenge due to the formation of multiple conformations on contact with the chromatographic surface and the mobile phase. The extent of conformational change is shown to be dependent on a number of parameters: column temperature, mobile phase pH, contact time with the chromatographic surface, salt type and concentration and the organic modifier. By manipulation of these variables, protein denaturation can be minimised and recovery improved.

Use of glucose-responsive material to regulate insulin release from constitutively secreting cells

Genetically-engineered cells offer a solution to the cell availability problem in tissue engineering a pancreatic substitute for the treatment of insulin-dependent diabetes. These cells can be non-beta cells, such as hepatocytes or myoblasts, retrieved as a biopsy from the same patient and genetically engineered to secrete recombinant insulin constitutively or under transcriptional regulation. However, the continuous or slowly responsive insulin secretion dynamics from these cells cannot provide physiologic glucose regulation in patients. Our objective consists of using such cells as an insulin source and of regulating insulin release by incorporating a glucose-responsive material, which acts as a control barrier for insulin in a cell-material hybrid device. Experiments were performed with insulinoma betaTC3 cells, HepG2 hepatomas, and C2C12 myoblasts, the latter two genetically-modified to constitutively secrete insulin. The control barrier consisted of concanavalin A (con A)-based glucose-responsive material, which forms a gel at low and a sol at high glucose concentrations. Results demonstrated that the device released insulin at a higher rate in response to glucose challenges. In contrast, a device containing an inert hydrogel instead of glucose-responsive material released insulin at an essentially constant rate, irrespective of the surrounding glucose concentration. Necessary material improvements include increased sensitivity to glucose, so that the material responds to physiologically relevant glucose concentrations, and increased stability. The prospects of developing a properly functional, implantable substitute based on engineered non-beta cells and glucose-responsive material, and the material and device improvements that need to be made prior to in vivo experiments, are discussed.

NAD-sensitive hydrogel for the release of macromolecules

A hydrogel membrane containing immobilized ligands and receptors was synthesized and investigated for the controlled diffusion of test proteins (cytochrome C and hemoglobin). Both Cibacron blue (ligand) and lysozyme (receptor) were covalently linked to dextran molecules that were subsequently crosslinked to form a gel. The resulting stable hydrogels contained both covalent and affinity crosslinks such that their intrinsic porosities were sensitive to competitive displacers of the affinity interaction between lysozyme and Cibacron blue. Transport experiments in a twin chamber diffusion cell showed that as NAD was added to the donor side, the dissociation of the binding sites between the Cibacron blue and the lysozyme led to an increase in protein diffusion through the hydrogel. The results showed that addition of NAD caused a saturable concentration-dependent increase in the transport of both cytochrome C and hemoglobin. This effect was shown to be both specific and reversible.

Hybrid pancreatic tissue substitute consisting of recombinant insulin-secreting cells and glucose-responsive material

Insulin-dependent diabetes is a serious pathological condition, currently treated by blood glucose monitoring and daily insulin injections, which, however, do not prevent long-term complications. A tissue-engineered pancreatic substitute has the potential to provide a more physiologic, less invasive, and potentially less costly treatment of the disease. A major issue in developing such a substitute is the cells being used. Nonpancreatic cells, retrieved from the same patient and genetically engineered to secrete insulin constitutively or with some glucose responsiveness, offer the significant advantages of being immune-acceptable and relaxing the tissue availability limitations, which exist with islets from cadaveric donors. These cells, however, do not have insulin secretion dynamics appropriate for restoration of euglycemia in higher animals and, eventually, humans. In this study, we present the concept of a hybrid pancreatic substitute consisting of such cells sequestered in a material exhibiting glucose-dependent changes of its permeability to insulin. A Concanavalin A-glycogen material sandwiched between two polycarbonate membranes and exhibiting glucose-dependent sol-gel transformations was used. Rates of insulin transport through this material in gel and sol forms were characterized for both FITC-labeled insulin in solution and insulin secreted by betaTC3 mouse insulinoma cells. Effective diffusivities through sol were found to be up to 3.5-fold higher than through the gel state of the material. A mathematical model of a hybrid construct was formulated and analyzed to simulate the secretory behavior in response to step ups and downs in the surrounding glucose concentration. The experimental and modeling studies indicate that a hybrid pancreatic substitute consisting of constitutively secreting cells and glucose-responsive material has the potential to provide a more physiologic regulation of insulin release than the cells by themselves or in an inert material.

Glucose-sensitive nanoassemblies comprising affinity-binding complexes trapped in fuzzy microshells

A new design for glucose monitoring with "smart" materials based on self assembly, competitive binding, and resonance energy transfer (RET) is presented. The basic transduction principle is changing RET efficiency from fluorescein isothiocyanate (FITC) to tetramethylrhodamine isothiocyanate (TRITC), as FITC-dextran is displaced from TRITC-Concanavalin A (Con A) with the addition of glucose. Nanoscale fabrication by self-assembly of Con A/dextran into multilayer films, followed by polymer multilayers. The advantages of this approach include physical localization and separation of sensing molecules from the environment via entrapment of the biosensorelements in a semi-permeable polymeric shell, and only functional molecules are included in the sensors. To realize these nanostructures, dissolvable resin microparticles were coated with FITC-dextran+TRITC-Con A multilayers, followed by polyelectrolyte multilayers, and the core particles were then dissolved to yield hollow capsules. The nanoassembly process was studied using microbalance mass measurements, fluorescence spectroscopy, confocal fluorescence microscopy, and zeta-potential measurements. The key findings are that the specific binding between Con A and dextran can be used to deposit ultrathin multilayer films, and these exhibit changing RET in response to glucose. Fluorescence spectra of a microcapsules exhibited a linear, glucose-specific, 27% increase in the relative fluorescence of FITC over the 0-1800 mg/dL range. These findings demonstrate the feasibility of using self-assembled microcapsules as optical glucose sensors, and serve as a basis for work toward better understanding the properties of these novel materials.

A reversible hydrogel membrane for controlling the delivery of macromolecules

Glucose-sensitive hydrogel membranes have been synthesized and characterized for their rate-of-delivery of macromolecules. The mechanism for changing this rate is based on variable displacement of the affinity interaction between dextran and concanavalin A (con A). Our main objective was to characterize the diffusion of model proteins (insulin, lysozyme, and BSA) through the membrane, in response to changes in environmental glucose concentrations. Membranes were constructed from crosslinked dextrans to which con A was coupled via a spacer arm. Changes in the porosity of the resulting hydrogel in the presence of glucose led to changes in the diffusion rate observed for a range of proteins. Gels of specified thickness were cast around to nylon gauze support (pore size, 0.1 mm) to improve mechanical strength. Diffusion of proteins through the gel membrane was determined using a twin-chamber diffusion cell with the concentrations being continuously monitored using a UV-spectrophotometer. Changes in the transport properties of the membranes in response to glucose were explored and it was found that, while 0.1M D-glucose caused a substantial, but saturateable, increase in the rates of diffusion of both insulin and lysozyme, controls using glycerol or L-glucose (0.1M) had no significant effect. Sequential addition and removal of external glucose in a stepwise manner showed that permeability changes were reversible. As expected, diffusion rates were inversely proportional to membrane thickness. A maximum increase in permeability was observed at pH 7.4 and at 37 degrees C. The results demonstrate that this hydrogel membrane functions as a smart material allowing control of solute delivery in response to specific changes in its external environment.

Covalent coupling of concanavalin A to a Carbopol 934P and 941P carrier in glucose-sensitive gels for delivery of insulin

A novel glucose-sensitive gel formulation, containing concanavalin A and specific polysaccharides, was stabilised via covalent coupling to two structurally different carbomers. The bonding was done to minimise leaching of gel components thereby preventing toxicity and preserving the working mechanism of the gel. Increased gel stability was introduced by covalently bonding amine groups present on the lysine residues of concanavalin A to carboxylic moieties on Carbopol 934P NF and 941P NF using carbodiimide chemistry. The introduction of dextran then produced a glucose-sensitive formulation that transformed from gel to sol in the presence of free glucose. Rheological examination of glucose-sensitive gels stabilised in this way and containing varying concentrations of glucose was conducted with a cone and plate viscometer used in continual rotation mode. A decrease in viscosity over the chosen glucose concentration range was exhibited by both carbomer-stabilised formulations. The subsequent testing of such formulations in in-vitro diffusion experiments revealed that the leaching of concanavalin A from the covalently coupled gels is restricted significantly with respect to non-coupled formulations. In addition, insulin delivery in response to glucose in the physiologically relevant glucose concentration range has been demonstrated using the carbomer-stabilised gels at 37 degrees C. The performance of this self-regulating drug delivery system has been improved in terms of increased gel stability with reduced component leaching.

Manufacturing factors affecting the drug delivery function of thermo-responsive membrane prepared by adsorption of binary liquid crystals

A binary mixture of cholesteric oleyl carbonate (COC) and cholesteryl nonanoate (CN) with a weight ratio of 36% and 64% was adsorbed onto the cellulose nitrate membrane to prepare a thermo-responsive membrane. The thermo-responsive function of these COC/CN-adsorbed membranes was evaluated by an in vitro penetration study via repeatedly exchanging the temperature cycle. The manufacturing factors for preparing these thermo-responsive membranes, such as the binary COC/CN concentration used, immersion temperature of binary COC/CN chloroform solution, immersion temperature and time period of distilled water, vacuum-drying or air-drying process, and vacuum drying temperature, have been explored. The results indicated that the immersion temperature at 23 degrees C for membrane in binary COC/CN chloroform solution with 10 min and in distilled water with 12 h was the optimal temperature for fabricating the on-off thermo-responsive membranes prepared either by 22% or 25% concentration of binary COC/CN mixture. A good thermo-responsive function was further evidenced for these COC/CN-adsorbed membranes via vacuum-drying at 65 degrees C for 1 h. A possible mechanism concerning the thermal-dependent molecular motion, orientation and/or alignment of binary COC/CN mixture entrapped into the membrane may play an important role for the thermo-responsive behavior.