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.     

Elucidation of the Mechanism of Enhanced Insulin Uptake and Release from pH Responsive Hydrogels

Summary: Environmentally responsive hydrogels composed of poly(methacrylic acid-g-ethylene glycol) (P(MAA-g-EG)) have shown promise for oral insulin delivery due to their pH responsive complexation behavior. A series of hydrogel formulations were polymerized with varying amounts of crosslinker and varying monomer volume fraction. The mesh size of the network depended primarily on pH, varying from 8.0 to 27.2 nm. Insulin loading efficiency varied directly with crosslink density, ranging from 42.7 to 84.9% of available insulin loaded into the hydrogels. The release of insulin was performed with each polymer formulation at 5 pH levels ranging from 2.7 to 6.8. Insulin release was less than 20% for all formulations tested with insulin for the duration of the 3 hour release study for all pH levels considered except when the pH was 6.8, at which point the release occurred as a burst. Loading studies performed with insulin glargine, an insulin analog with an increased pI, showed the same trends as native insulin. However, the release of insulin glargine only occurred at a pH level above that of the pI of the protein. These results indicate that hydrogen bonds and ionic interactions between the protein and P(MAA-g-EG) may strongly influence its loading and release behavior in vitro.     

A Comprehensive Outlook of Synthetic Strategies and Applications of Redox‐Responsive Nanogels in Drug Delivery

Increasing recognition of the role of oxidative stress in the pathogenesis of many clinical conditions and the existence of defined redox potential in healthy tissues has led to extensive research in the development of redox‐responsive materials for biomedical applications. Especially, considerable growth has been seen in the fabrication of polymeric nanogel–based drug delivery carriers utilizing redox‐responsive cross‐linkers bearing a variety of functional groups via various synthetic strategies. Redox‐responsive polymeric nanogels provide an advantage of facile chemical modification post synthesis and exhibit a remarkable response to biological redox stimuli. Due to the interdisciplinary nature of the subject, a more profound combined conceptual knowledge from a chemical and biological point of view is imperative for the rational design of redox‐responsive nanogels. The present review provides an insight into the design and fabrication of redox‐responsive nanogels with particular emphasis on synthetic strategies utilized for the development of redox‐responsive cross‐linkers, polymerization techniques being followed for nanogel development and biomedical applications. Cooperative effect of redox trigger with other stimuli such as pH and temperature in the evolution of dual and triple stimuli‐responsive nanogels is also discussed.     

Controlled Release of Insulin Based on Temperature and Glucose Dual Responsive Biomicrocapsules

The treatment of diabetes lies in developing novel functional carriers, which are expected to have the unique capability of monitoring blood glucose levels continuously and dispensing insulin correctly and timely. Hence, this study is proposing to create a smart self-regulated insulin delivery system according to changes in glucose concentration. Temperature and glucose dual responsive copolymer microcapsules bearing N-isopropylacrylamide and 3-acrylamidophenylboronic acid as main components were developed by bottom-spray coating technology and template method. The insulinoma β-TC6 cells were trapped in the copolymer microcapsules by use of temperature sensitivity, and then growth, proliferation, and glucose-responsive insulin secretion of microencapsulated cells were successively monitored. The copolymer microcapsules showed favorable structural stability and good biocompatibility against β-TC6 cells. Compared with free cells, the biomicrocapsules presented a more effective and safer glucose-dependent insulin release behavior. The bioactivity of secreted and released insulin did not differ between free and encapsulated β-TC6 cells. The results demonstrated that the copolymer microcapsules had a positive effect on real-time sensing of glucose and precise controlled release of insulin. The intelligent drug delivery system is supposed to mimic insulin secretion in a physiological manner, and further provide new perspectives and technical support for the development of artificial pancreas.     

Responsive Aqueous Foams

Remarkable properties have emerged recently for aqueous foams, including ultrastability and responsiveness. Responsive aqueous foams refer to foams for which the stability can be switched between stable and unstable states with a change in environment or with external stimuli. Responsive foams have been obtained from various foam stabilizers, such as surfactants, proteins, polymers, and particles, and with various stimuli. Different strategies have been developed to design this type of soft material. We briefly review the two main approaches used to obtain responsive foams. The first approach is based on the responsiveness of the interfacial layer surrounding the gas bubbles, which leads to responsive foams. The second approach is based on modifications that occur in the aqueous phase inside the foam liquid channels to tune the foam stability. We will highlight the most sophisticated approaches, which use light, temperature, and magnetic fields and lead to switchable foam stability.     

pH‐ and Thiol‐Responsive BODIPY‐Based Photosensitizers for Targeted Photodynamic Therapy

A diiodo distyryl boron dipyrromethene (BODIPY) core was conjugated to two ferrocenyl quenchers through acid‐labile ketal and/or thiol‐cleavable disulfide linkers, of which the fluorescence and photosensitizing properties were significantly quenched through a photoinduced electron‐transfer process. The two symmetrical analogues that contained either the ketal or disulfide linkers could only be activated by a single stimulus, whereas the unsymmetrical analogue was responsive to dual stimuli. Upon interaction with acid and/or dithiothreitol (DTT), these linkers were cleaved selectively. The separation of the BODIPY core and the ferrocenyl moieties restored the photoactivities of the former in phosphate buffered saline and inside the MCF‐7 breast cancer cells, rendering these compounds as potential activable photosensitizers for targeted photodynamic therapy. The dual activable analogue exhibited the greatest enhancement in intracellular fluorescence intensity in both an acidic environment (pH 5) and the presence of DTT (4 mm ). Its photocytotoxicity against MCF‐7 cells also increased by about twofold upon preincubation with 4 mm of DTT. The activation of this compound was also demonstrated in nude mice bearing a HT29 human colorectal carcinoma. A significant increase in fluorescence intensity in the tumor was observed over 9 h after intratumoral injection.     

Degradable Cationic Nanohydrogel Particles for Stimuli‐Responsive Release of siRNA

Well‐defined nanogels have become quite attractive as safe and stable carriers for siRNA delivery. However, to avoid nanoparticle accumulation, they need to provide a stimuli‐responsive degradation mechanism that can be activated at the payload's site of action. In this work, the synthetic concept for generating well‐defined nanohydrogel particles is extended to incorporate disulfide cross‐linkers into a cationic nanonetwork for redox‐triggered release of oligonucleotide payload as well as nanoparticle degradation under reductive conditions of the cytoplasm. Therefore, a novel disulfide‐modified spermine cross‐linker is designed that both allows disassembly of the nanogel as well as removal of cationic charge from residual polymer fragments. The degradation process is monitored by scanning electron microscopy (SEM) and fluorescence correlation spectroscopy (FCS). Moreover, siRNA release is analyzed by agarose gel electrophoresis and a fluorescent RNA detection assay. The results exemplify the versatility of the applied nanogel manufacturing process, which allows alternative stimuli‐responsive core cross‐linkers to be integrated for triggered oligonucleotide release as well as effective biodegradation for reduced nanotoxicity.

     

Disassembly‐Driven Fluorescence Turn‐on of Polymerized Micelles by Reductive Stimuli in Living Cells

Stimuli‐response nanoparticles have emerged as powerful tools for imaging and therapeutic applications. Ideally, they should be assembled from biodegradable materials featuring small size and cooperative response to biological stimuli that trigger particle disassembly and release of an active molecule that could be readily monitored in situ. A concept is developed that consists of organic nanoparticles, assembled from fluorescent amphiphiles and polymerized with a redox‐cleavable cross‐linker. We obtained 20 nm nanoparticles bearing self‐quenched Nile Red dye residues, which can disassemble in living cells into highly fluorescent molecular units owing to an external or internal reductive stimulus. The obtained results pave the way to new stimuli‐responsive nanomaterials for applications in background‐free imaging as well as in drug delivery, as the concept can be further extended to other active molecules including drugs and to cross‐linkers cleavable by other biological stimuli.     

Extraordinary Modulation of Disulfide Redox‐Responsiveness by Cooperativity of Twin‐Disulfide Bonds

Disulfide bonds have frequently been incorporated into synthetic materials to promote sensitivity of the systems towards different redox environments. Although many strategies have been developed to rationally tune the stability of disulfide linkers, methods to tune their responsiveness towards different redox environments remain elusive. In this work we have developed and explored a disulfide linker bearing two independent disulfide bonds, referred to as a twin‐disulfide linker. We have demonstrated that the twin‐disulfide linker displays an ultrahigh stability at lower concentrations of reducing agent or in weakly reducing environments without a significant compromise in the sensitivity of its response to highly reducing environments such as cytoplasm, a feature that is in remarkable contrast to the traditional single disulfide bonds. Such an extraordinary responsiveness arises from the cooperativity of the twin‐disulfide bonds, which should be of particular interest for applications such as controlled drug delivery and sensing, because relatively large differences in disulfide stability in different redox environments is desired in these applications.     

Rapid,On‐Command Debonding of Stimuli‐Responsive Cross‐Linked Adhesives by Continuous,Sequential Quinone Methide Elimination Reactions

Adhesives that selectively debond from a surface by stimuli‐induced head‐to‐tail continuous depolymerization of poly(benzyl ether) macro‐cross‐linkers within a poly(norbornene) matrix are described. Continuous head‐to‐tail depolymerization provides faster rates of response than can be achieved using a small‐molecule cross‐linker, as well as responses to lower stimulus concentrations. Shear‐stress values for glass held together by the adhesive reach 0.51±0.10 MPa, whereas signal‐induced depolymerization via quinone methide intermediates reduces the shear stress values to 0.05±0.02 MPa. Changing the length of the macro‐cross‐linkers alters the time required for debonding, and thus enables the programmed sequential release of specific layers in a glass composite material.     

Enzyme‐Responsive Multifunctional Magnetic Nanoparticles for Tumor Intracellular Drug Delivery and Imaging

Enzyme‐responsive, hybrid, magnetic silica nanoparticles have been employed for multifunctional applications in selective drug delivery and intracellular tumor imaging. In this study, doxorubicin (Dox)‐conjugated, enzyme‐cleavable peptide precursors were covalently tethered onto the surface of uniform silica‐coated magnetic nanoparticles through click chemistry. This enzyme‐responsive nanoparticle conjugate demonstrated highly efficient Dox release upon specific enzyme interactions in vitro. It also exhibits multiple functions in selective tumor intracellular drug delivery and imaging in the tumor cells with high cathepsin B expression, whereas it exhibited lower cytotoxicity towards other cells without enzyme expression.     

Anion Responsive Imidazolium‐Based Polymers

Stimuli responsiveness in polymer design is providing basis for diversely new and advanced materials that exhibit switchable porosity in membranes and coatings, switchable particle formation and thermodynamically stable nanoparticle dispersions, polymers that provide directed mechanical stress in response to intensive fields, and switchable compatibility of nanomaterials in changing environments. The incorporation of ionic liquid monomers has resulted in many new polymers based on the imidazolium group. These polymers exhibit all of the above‐articulated material properties. Some insight into how these anion responsive polymers function has become empirically available. Much opportunity remains for extending our understanding as well as for designing more refined stimuli‐responsive materials.     

Glucose and pH Dual‐Responsive Nanogels for Efficient Protein Delivery

Direct delivery of protein suffers from their in vitro and in vivo instability, immunogenicity, and a relatively short half‐life within the body. To overcome these challenges, pH and glucose dual‐responsive biodegradable nanogels comprised of dextran and poly(L‐glutamic acid)‐g‐methoxy poly‐(ethylene glycol)/phenyl boronic acid (PLG‐g‐mPEG/PBA) are designed. The cross‐linked network imparted drug‐loading efficacy of α‐amylase up to 55.6% and hyaluronidase up to 29.1%. In vitro protein release profiles reveal that the release of protein is highly dependent on the pH or glucose concentrations, that is, less amount of protein is released at pH 7.4 or healthy blood glucose level (1 mg mL^?1 glucose), while quicker release of protein occurs at pH 5.5 or diabetic blood glucose level (above 3 mg mL^?1 glucose). Circular dichroism spectra show that the secondary structure of released protein is maintained compared to naive protein. Overall, the nanogels have provided a simple and effective strategy to deliver protein.     

Glucose‐Responsive Trehalose Hydrogel for Insulin Stabilization and Delivery

Effective delivery of therapeutic proteins is important for many biomedical applications. Yet, the stabilization of proteins during delivery and long‐term storage remains a significant challenge. Herein, a trehalose‐based hydrogel is reported that stabilizes insulin to elevated temperatures prior to glucose‐triggered release. The hydrogel is synthesized using a polymer with trehalose side chains and a phenylboronic acid end‐functionalized 8‐arm poly(ethylene glycol) (PEG). The hydroxyls of the trehalose side chains form boronate ester linkages with the PEG boronic acid cross‐linker to yield hydrogels without any further modification of the original trehalose polymer. Dissolution of the hydrogel is triggered upon addition of glucose as a stronger binder to boronic acid (K_b = 2.57 vs 0.48 m ⁻¹ for trehalose), allowing the insulin that is entrapped during gelation to be released in a glucose‐responsive manner. Moreover, the trehalose hydrogel stabilizes the insulin as determined by immunobinding after heating up to 90 °C. After 30 min heating, 74% of insulin is detected by enzyme‐linked immunosorbent assay in the presence of the trehalose hydrogel, whereas only 2% is detected without any additives.     

Nanofibrillar Stimulus‐Responsive Cholesteric Microgels with Catalytic Properties

We report composite stimulus‐responsive cholesteric catalytically active microgels derived from filamentous supramolecular building blocks: cellulose nanocrystals (CNCs). The variation in the microgel dimensions and pitch in response to the change in ambient conditions was governed by the polymer component. The cholesteric morphology of the microgels resulted from the self‐organization of CNCs in spherical confinement. The microgels exhibited excellent structural integrity and functioned as microreactors in catalytic hydrolysis reactions and in the synthesis of metal nanoparticles. Because of these collective properties, the reported microgels show much promise for application in the design of functional responsive materials.     

A biuret‐derived,MS‐cleavable cross‐linking reagent for protein structural analysis: A proof‐of‐principle study

Chemical cross‐linking combined with mass spectrometry (XL‐MS) and computational modeling has evolved as an alternative method to derive protein 3D structures and to map protein interaction networks. Special focus has been laid recently on the development and application of cross‐linkers that are cleavable by collisional activation as they yield distinct signatures in tandem mass spectra. Building on our experiences with cross‐linkers containing an MS‐labile urea group, we now present the biuret‐based, CID‐MS/MS‐cleavable cross‐linker imidodicarbonyl diimidazole (IDDI) and demonstrate its applicability for protein cross‐linking studies based on the four model peptides angiotensin II, MRFA, substance P, and thymopentin.     

Reversible pH‐Responsive Coacervate Formation in Lipid Vesicles Activates Dormant Enzymatic Reactions

In situ, reversible coacervate formation within lipid vesicles represents a key step in the development of responsive synthetic cellular models. Herein, we exploit the pH responsiveness of a polycation above and below its pK_a, to drive liquid–liquid phase separation, to form single coacervate droplets within lipid vesicles. The process is completely reversible as coacervate droplets can be disassembled by increasing the pH above the pK_a. We further show that pH‐triggered coacervation in the presence of low concentrations of enzymes activates dormant enzyme reactions by increasing the local concentration within the coacervate droplets and changing the local environment around the enzyme. In conclusion, this work establishes a tunable, pH responsive, enzymatically active multi‐compartment synthetic cell. The system is readily transferred into microfluidics, making it a robust model for addressing general questions in biology, such as the role of phase separation and its effect on enzymatic reactions using a bottom‐up synthetic biology approach.     

Bifunctional Aryloxylate Esters as potential oxidatively cleavable linkers

Selectively cleavable linkers are essential parts in environmentally responsive materials. Here, we introduce aryl oxalate esters (AOE) as one of the first examples for oxidatively cleavable linkers. To this end a series of novel AOEs was synthesized and explored regarding the H₂O₂-dependent degradation. All AOEs were cleaved selectively at the oxalate group. The degradation rate was clearly dependent on the substituents. Further, it was found that the H₂O₂ based degradation undergoes an autocatalysis mechanism.     

Visualizing the Dynamics of Temperature‐ and Solvent‐Responsive Soft Crystals

We demonstrate that three flexible MOFs termed FJI‐H11‐R (FJI‐H=Hong's group in Fujian Institute of Research on the Structure of Matter, R=Me, Et, ⁱPr) can reversibly respond to temperature and solvents via structural transformations, which can be visualized by in situ single‐crystal X‐ray snapshot analyses. FJI‐H11‐R exhibit colossal anisotropic thermal expansion, with a record‐high uniaxial positive thermal‐expansion coefficient of 653.2×10^?6 K^?1 observed in FJI‐H11‐Me. Additionally, large c‐axial shrinkage of 32.4 % is also observed during desolvation. The stimuli‐responsive mechanism reveals the structural evolutions are related to the rotations and deformations of the organic linkers.     

Directing the breathing behavior of pillared-layered metal-organic frameworks via a systematic library of functionalized linkers bearing flexible substituents

Flexible metal-organic frameworks (MOFs), also referred to as soft porous crystals (SPCs), show reversible structural transitions dependent on the nature and quantity of adsorbed guest molecules. In recent studies it has been reported that covalent functionalization of the organic linker can influence or even integrate framework flexibility ("breathing") in MOFs. However, rational fine-tuning of such responsive properties is very desirable but challenging as well. Here we present a powerful approach for the targeted manipulation of responsiveness and framework flexibility of an important family of pillared-layered MOFs based on the parent structure [Zn(2)(bdc)(2)(dabco)](n) (bdc = 1,4-benzenedicarboxylate; dabco = 1,4-diazabicyclo[2.2.2]octane). A library of functionalized bdc-type linkers (fu-bdc), which bear additional dangling side groups at different positions of the benzene core (alkoxy groups of varying chain length with diverse functionalities and polarity), was generated. Synthesis of the materials [Zn(2)(fu-bdc)(2)(dabco)](n) yields the respective collection of highly responsive MOFs. The parent MOF is only weakly flexible; however, the substituted frameworks of [Zn(2)(fu-bdc)(2)(dabco)](n) contract drastically upon guest removal and expand again upon adsorption of DMF (N,N-dimethylformamide), EtOH, or CO(2), etc., while N(2) is hardly adsorbed and does not open the narrow-pored form. These "breathing" dynamics are attributed to the dangling side chains that act as immobilized "guests", which interact with mobile guest molecules as well as with themselves and with the framework backbone. The structural details of the guest-free, contracted form and the gas sorption behavior (phase transition pressure, hysteresis loop) are highly dependent on the nature of the substituent at the linker and can therefore be adjusted using our approach. Combining our library of functionalized linkers with the concept of mixed-component MOFs (solid solutions) offers very rich additional dimensions of tailoring the structural dynamics and responsiveness. Implementation of two differently functionalized linkers in varying ratios yields multicomponent single-phased [Zn(2)(fu-bdc')(2x)(fu-bdc″)(2-2x)(dabco)](n) MOFs (0 < x < 1) of increased inherent complexity, which feature a non-linear dependence of their gas sorption properties on the applied ratio of components. Hence, the responsive behavior of such pillared-layered MOFs can be extensively tuned via an intelligent combination of functionalized linkers.