Ultrathin hydrogel films for rapid optical biosensing

Liquid crystalline composite hydrogels with large pH-triggered anisotropic swelling for embolotherapy

Inspired by the anisotropic structure of biological tissues, anisotropic hydrogels have been developed using various nanofillers, however, it remains a big challenge to synthesize hydrogels with large swelling anisotropy. Herein a single molecule filler, α-helical polypeptide, instead of nanoscale fillers, was used to synthesize anisotropic hydrogels. First nematic liquid crystal of poly(γ-benzyl l-glutamate) (PBLG) was prepared by shearing and stabilized by embedding in a crosslinked polymer matrix. The resulting PBLG composite gels were then converted to poly(L-glutamic acid) (PLGA) composite gels by debenzylation. The rigid rod-like structure of α-helical PBLG chains makes them easy to be orientated. The pH-sensitivity of PLGA makes the resulting composite gels pH-sensitive without the need to couple with a stimuli-responsive hydrogel matrix. In response to pH change PLGA composite gels swell anisotropically with a much larger swelling degree in the radial direction than in the axial direction. The swelling anisotropy (3.43) is much higher than most anisotropic hydrogels, particularly the stimuli-responsive ones reported previously. The composite gel also exhibits anisotropic mechanical properties with a larger Young's modulus in the axial direction than that in the radial direction. Preliminary test demonstrated that the composite gels have potential in embolotherapy thanks to its large pH-triggered anisotropic swelling. STATEMENT OF SIGNIFICANCE: Anisotropic hydrogels have important biomedical applications. Introduction of oriented nanofillers has been demonstrated a popular and versatile method for their synthesis, however, it remains a big challenge to achieve large swelling anisotropy. Herein a single molecule filler, α-helical polypeptide, instead of nanoscale fillers, was used to synthesize anisotropic hydrogels. This filler can be easily oriented by shearing. More importantly, as single molecule filler, it can constrain the swelling of hydrogel matrix more effectively. Using this filler, a pH-sensitive hydrogel with large swelling anisotropy (3.43) was successfully synthesized. Thanks to its large pH-triggered anisotropic swelling the hydrogel was successfully used as embolic agent to occlude vessels.

Multiple Responsive Hydrogel Films Based on Dynamic Phenylboronate Bond Linkages with Simple but Practical Linear Response Mode and Excellent Glucose/Fructose Response Speed

Multiple responsive hydrogels are usually constructed by the addition of many different functional groups. Generally, these groups have different responsive behaviors which lead to interleaved and complex modes of the multi-response system. It is difficult to get a practical application. In this study, we show that multi-response hydrogels can also be constructed using dynamic bonds as crosslinks. The multiple responsive hydrogel films with thicknesses on the sub-micrometer or micrometer scale can be fabricated from P(DMAA-3-AAPBA), a copolymer of N,N-dimethylacrylamide, 3-(acrylamido)phenylboronic acid, and poly(vinylalcohol) (PVA) though a simple layer-by-layer (LbL) technique. The driving force for the film build up is the in situ-formed phenylboronate ester bonds between the two polymers. The films exhibit Fabry-Perot fringes on their reflection spectra which can be used to calculate the equilibrium swelling degree (SD_e) of the film so as to characterize its responsive behaviors. The results show that the films are responsive to temperature, glucose, and fructose with simple and practical linear response modes. More importantly, the speed of which the films respond to glucose or fructose is quite fast, with characteristic response times of 45 s and 7 s, respectively. These quick response films may have potential for real-time, continuous glucose or fructose monitoring. With the ability to bind with these biologically important molecules, one can expect that hydrogels may find more applications in biomedical areas in the future.

Modeling the Phase Transition in Hydrophobic Weak Polyelectrolyte Gels under Compression

One of the emerging water desalination techniques relies on the compression of a polyelectrolyte gel. The pressures needed reach tens of bars, which are too high for many applications, damage the gel and prevent its reuse. Here, we study the process by means of coarse-grained simulations of hydrophobic weak polyelectrolyte gels and show that the necessary pressures can be lowered to only a few bars. We show that the dependence of applied pressure on the gel density contains a plateau indicating a phase separation. The phase separation was also confirmed by an analytical mean-field theory. The results of our study show that changes in the pH or salinity can induce the phase transition in the gel. We also found that ionization of the gel enhances its ion capacity, whereas increasing the gel hydrophobicity lowers the pressure required for gel compression. Therefore, combining both strategies enables the optimization of polyelectrolyte gel compression for water desalination purposes.

Two-Dimensional and Three-Dimensional Ultrathin Multilayer Hydrogels through Layer-by-Layer Assembly

Stimuli-responsive multilayer hydrogels have opened new opportunities to design hierarchically organized networks with properties controlled at the nanoscale. These multilayer materials integrate structural, morphological, and compositional versatility provided by alternating layer-by-layer polymer deposition with the capability for dramatic and reversible changes in volumes upon environmental triggers, a characteristic of chemically cross-linked responsive networks. Despite their intriguing potential, there has been limited knowledge about the structure-property relationships of multilayer hydrogels, partly because of the challenges in regulating network structural organization and the limited set of the instrumental pool to resolve structure and properties at nanometer spatial resolution. This Feature Article highlights our recent studies on advancing assembly technologies, fundamentals, and applications of multilayer hydrogels. The fundamental relationships among synthetic strategies, chemical compositions, and hydrogel architectures are discussed, and their impacts on stimuli-induced volume changes, morphology, and mechanical responses are presented. We present an overview of our studies on thin multilayer hydrogel coatings, focusing on controlling and quantifying the degree of layer intermixing, which are crucial issues in the design of hydrogels with predictable properties. We also uncover the behavior of stratified "multicompartment" hydrogels in response to changes in pH and temperature. We summarize the mechanical responses of free-standing multilayer hydrogels, including planar thin coatings and films with closed geometries such as hollow microcapsules and nonhollow hydrogel microparticles with spherical and nonspherical shapes. Finally, we will showcase potential applications of pH- and temperature-sensitive multilayer hydrogels in sensing and drug delivery. The knowledge about multilayer hydrogels can advance the rational design of polymer networks with predictable and well-tunable properties, contributing to modern polymer science and broadening hydrogel applications.

Highly Elastic, Sensitive, Stretchable, and Skin-Inspired Conductive Sodium Alginate/Polyacrylamide/Gallium Composite Hydrogel with Toughness as a Flexible Strain Sensor

As a classic flexible material, hydrogels show great potential in wearable electronic devices. The application of strain sensors prepared using them in human health monitoring and humanoid robotics is developing rapidly. However, it is still a challenge to fabricate a high-toughness, large-tensile-deformation, strain-sensitive. and human-skin-fit hydrogel with the integration of excellent mechanical properties and high electrical conductivity. In this study, a flexible sensor using a highly strain-sensitive skin-like hydrogel with acrylamide and sodium alginate was designed using liquid metallic gallium as a "reactive" conductive filler. The sensor had a low elastic modulus (30 kPa) similar to that of skin, a high-toughness (2.25 MJ m^-3), self-stiffness, a large tensile deformation (1400%), recoverability, and excellent fatigue resistance. Moreover, the addition of gallium might enhance the electrical conductivity (1.9 S m^-1) of the hydrogel while maintaining high transparency, and the flexible sensor device constructed from it showed high sensitivity to strain (gauge factor = 4.08) and pressure (gauge factor = 0.455 kPa^-1). As a result, the hydrogel sensor could monitor various human motions, including large-scale joint bending and tiny facial expression, breathing, voice recognition, and handwriting. Furthermore, it might even be used for human-computer communication.

Glycopolymer based LbL Multilayer Thin Films with Embedded Liposomes

Layer-by-layer (LbL) self-assembly emerged as an efficient technique for fabricating coating systems for, e.g., drug delivery systems with great versatility and control. In this work, we describe protecting group free and aqueous-based syntheses of bioinspired glycopolymer electrolytes. Thin films of the glycopolymers are fabricated by LbL self-assembly and function as scaffolds for liposomes, which potentially can encapsulate active substances. We investigate the adsorbed mass, pH stability and integrity of glycopolymer coatings as well as the embedded liposomes via whispering gallery mode (WGM) technology and quartz crystal microbalance with dissipation (QCM-D) monitoring, which enable label-free characterization. Glycopolymer thin films, with and without liposomes, are stable in the physiological pH range. QCM-D measurements verify the integrity of lipid vesicles. Thus, we present the fabrication of glycopolymer-based surface coatings with embedded and intact liposomes. This article is protected by copyright. All rights reserved.

Free-standing conductive hydrogel electrode for potentiometric glucose sensing

Flexible conductive polymer hydrogels are attracting attention as an electrode material. Electrochemical biosensors with conductive polymer hydrogels have been developed because they have some advantages such as biocompatibility, high conductivity, 3D nanostructure, solvated surface, and enlarged interface. Conductive polymer hydrogels bearing receptor molecules such as enzymes in its 3D nanostructure enable the detection of target analytes with high sensitivity. However, because such hydrogels are fragile, they cannot stand on their own and a supporting substrate is required to fabricate them. This means that the loss of mechanical toughness is detrimental for their application to flexible biosensors. In this study, we have proposed a free-standing conductive hydrogel electrode with no coating on a substrate, which is composed of polyaniline with phenyl boronic acid including polyvinyl alcohol, for potentiometric glucose sensing. In addition, its electrical responsivity to glucose has been confirmed by investigating its mechanical properties at various glucose concentrations, considering the hydrogel compositions.

A free-standing conductive hydrogel electrode with no coating on a substrate is proposed for potentiometric glucose sensing.

Vibrant reflective sensors with percolation film Fabry-Pérot nanocavities

Dynamically reconfigurable structural colors are promising materials for new smart optical systems. However, improved reflected color quality (e.g., saturation, optical contrast, angular invariance) and larger tuning range/sensitivity are needed. Here, we demonstrate a vibrant, actively tunable system which meets these needs via coupling broadband plasmonic resonators to a responsive polymer film. Our structure consists of near-percolation gold nanoislands deposited on a poly[methyl methacrylate] (PMMA) spacer above a gold mirror, forming a Fabry-Pérot nanocavity. Broadband absorption in this system creates vivid reflected colors, while the polymer spacer enables continuous tuning over a wide color space. By exploiting swelling effects in PMMA, we show fast, reversible color switching in response to organic vapors. Our sensitive optical structure amplifies small vapor-induced changes in the spacer thickness, enabling naked-eye detection of changes as small as 10 nm. Additionally, optical absorption >99% yields modulation contrasts up to 80:1, opening the door to ultra-sensitive on-chip signal measurements, complementing the visual colorimetric readout. This structure has immediate implications for colorimetric bio/chemical sensing and may also find application to reflective displays and flexible/adaptive optical coatings.

Smart and Functionalized Development of Nucleic Acid-Based Hydrogels: Assembly Strategies, Recent Advances, and Challenges

Nucleic acid-based hydrogels that integrate intrinsic biological properties of nucleic acids and mechanical behavior of their advanced assemblies are appealing bioanalysis and biomedical studies for the development of new-generation smart biomaterials. It is inseparable from development and incorporation of novel structural and functional units. This review highlights different functional units of nucleic acids, polymers, and novel nanomaterials in the order of structures, properties, and functions, and their assembly strategies for the fabrication of nucleic acid-based hydrogels. Also, recent advances in the design of multifunctional and stimuli-responsive nucleic acid-based hydrogels in bioanalysis and biomedical science are discussed, focusing on the applications of customized hydrogels for emerging directions, including 3D cell cultivation and 3D bioprinting. Finally, the key challenge and future perspectives are outlined.

Programmable Color in a Free-Standing Photonic Microgel Film with Ultra-Fast Response

In this work, a free-standing microgel film with programmable and angle-independent structural color is prepared via a simple but effective method. Dried poly(styrene-N-isopropylacrylamide-acrylic acid) (pStNIPAAmAA) microgels were stabilized by inter-microgel crosslinking, and thus, only microgels were used to build the optical hydrogel. The free-standing microgel film displayed tunable structural color by the swelling/deswelling of the microgels under external stimuli, such as temperature, pH, ionic strength, and organic solvent. Moreover, the structural color of the film is angle-independent for the disordered microgel arrays. It is worth noting that programmable color stripes which have the panther chameleon's ability to change skin color are successfully fabricated by patterning microgels with different thermoresponsivities. More importantly, the microgel film has an ultrafast response to temperature (1.41 s from 20 to 40 °C) and pH (2.24 s from pH 8.3 to pH 2.0), much faster than that of most optical materials reported in previous studies.

Engineering Hydrogel-Based Biomedical Photonics: Design, Fabrication, and Applications

Light guiding and manipulation in photonics have become ubiquitous in events ranging from everyday communications to complex robotics and nanomedicine. The speed and sensitivity of light-matter interactions offer unprecedented advantages in biomedical optics, data transmission, photomedicine, and detection of multi-scale phenomena. Recently, hydrogels have emerged as a promising candidate for interfacing photonics and bioengineering by combining their light-guiding properties with live tissue compatibility in optical, chemical, physiological, and mechanical dimensions. Herein, the latest progress over hydrogel photonics and its applications in guidance and manipulation of light is reviewed. Physics of guiding light through hydrogels and living tissues, and existing technical challenges in translating these tools into biomedical settings are discussed. A comprehensive and thorough overview of materials, fabrication protocols, and design architectures used in hydrogel photonics is provided. Finally, recent examples of applying structures such as hydrogel optical fibers, living photonic constructs, and their use as light-driven hydrogel robots, photomedicine tools, and organ-on-a-chip models are described. By providing a critical and selective evaluation of the field's status, this work sets a foundation for the next generation of hydrogel photonic research.

Molecule-Specific Terahertz Biosensors Based on an Aptamer Hydrogel-Functionalized Metamaterial for Sensitive Assays in Aqueous Environments

Metamaterial-inspired terahertz (THz) biosensors are devoted to developing high-sensitivity and label-free biosensing strategies. However, most meaningful molecular signals are obscured by the strong THz absorption of solvent water. Most reported THz biosensors require the tested samples to be tediously dried or replaced with a low-absorption medium, which impairs the original bioactivity and the distribution homogeneity of targets. As described in this proposed strategy, a molecule-specific THz biosensor was fabricated from an aptamer hydrogel-functionalized THz metamaterial. Benefitting from the strong interaction with the localized electric field of the metamaterial, trace thrombin-induced variations in the hydration state of the hydrogel can be sensitively probed, which was investigated experimentally and theoretically. The optimized THz biosensor exhibited remarkable specificity for actual serum sample assays and excellent sensitivity, with a relatively low detection limit of 0.40 pM in the human serum matrix. The proposed strategy could serve as a model system to develop various molecule-specific THz biosensors for aqueous molecule sensing.

In situ formation of nanocomposite double-network hydrogels with shear-thinning and self-healing properties

Nanocomposite double-network hydrogels (ncDN hydrogels) are recently introduced to address the limitations of traditional DN hydrogels, such as the lack of diversity in the network structure and the restricted functionalities. However, two challenges remain, including the time-consuming preparation and the lack of shear-thinning and self-healing properties. Here, our approach to developing versatile ncDN hydrogels is through the use of multiple interfacial crosslinking chemistries (i.e., noncovalent interactions of electrostatic interaction and hydrogen bonds as well as dynamic covalent interactions of imine bonds and boronate ester bonds) and surface functionalized nanomaterials (i.e. phenylboronic acid modified reduced graphene oxide (PBA-rGO)). PBA-rGO was used as a multivalent gelator to further crosslink the two polymer chains (i.e. triethylene glycol-grafted chitosan (TEG-CS) and polydextran aldehyde (PDA)) in DN hydrogels, forming the TEG-CS/PDA/PBA-rGO ncDN hydrogels in seconds. The microstructures (i.e. pore size) and properties (i.e. rheological, mechanical, and swelling properties) of the ncDN hydrogels can be simply modulated by changing the amount of PBA-rGO. The dynamic bonds in the polymeric network provided the shear-thinning and self-healing properties to the ncDN hydrogels, allowing the hydrogels to be injected and molded into varied shapes as well as self-repair the damaged structure. Besides, the designed TEG-CS/PDA/PBA-rGO ncDN hydrogels were cytocompatible and also exhibited antibacterial activity. Taken together, we hereby provide a nanomaterial approach to fabricate a new class of ncDN hydrogels with tailorable networks and favorite properties for specific applications.

An enzyme-free capacitive glucose sensor based on dual-network glucose-responsive hydrogel and coplanar electrode

Glucose sensors are vital devices for blood glucose detection in the diabetes care. Different from traditional electrochemical devices based on glucose oxidase, the glucose sensor based on the glucose-responsive hydrogel is more robust owing to its enzyme-free principle. However, integrating the high sensitivity, fast response, wide measuring range and low-cost fabrication into a hydrogel sensor is still challenging. In this study, we present a physical capacitive sensor, which consists of interdigital carbon electrodes (ICEs) fabricated by a direct laser writing technology and glucose-responsive hydrogel (DexG-Con A hydrogel) built by UV curing in situ. The dielectric property of DexG-Con A hydrogel changes accordingly with the change in environmental glucose concentration. Experimental results demonstrate that in a glucose concentration range of 0-30 mM, the proposed hydrogel sensor is capable of measuring the glucose level in a repeatable and reversible manner, showing a short responsive time of less than 2 min and a high sensitivity of 8.81 pF mM-1 at a glucose range of 0-6 mM. Owing to its simple fabrication process, low-cost and high performance, the proposed glucose sensor shows great potential on batch production for continuous glucose monitoring application.

Passive and wireless, implantable glucose sensing with phenylboronic acid hydrogel-interlayer RF resonators

A phenylboronic acid-based, hydrogel-interlayer Radio-Frequency (RF) resonator is demonstrated as a highly-responsive, passive and wireless sensor for glucose monitoring. Constructs are composed of unanchored, capacitively-coupled split rings interceded by glucose-responsive hydrogels. Phenylboronic acid-hydrogels exhibit volumetric and dielectric variations in response to environmental glucose concentrations-these are efficiently converted to large shifts in the resonant response of interlayer-RF sensors. These tiny, stretchable and scalable sensors (5 mm × 5 mm x 250 μm) require no microelectronics or power at the sensing node and can be read-out remotely via near-field coupling. Sensors exhibit high sensitivities (~10% shift in resonant frequency-corresponding to 50 MHz-per 150 mg/dL of glucose), possess a limit of detection of 10 mg/dL, and a step response time of approximately 1 h to abrupt shifts in carbohydrate concentration. Notably, these sensors exhibited no signal drift or hysteresis over the time periods characterized herein (45 days at room temperature). We transform sensors into bioelectronic RF reporter-tags via the attachment of a single LED-these remotely report on glucose concentration via emitted light. We anticipate the non-degradative, long-term nature of both RF read-out and phenylboronic acid-based hydrogels will enable biosensors capable of long-term, remote read-out of glucose.

Patterned Photonic Array Based on an Intertwined Polymer Network Functionalized with a Nonenzymatic Moiety for the Visual Detection of Glucose

A patterned photonic array chip based on an intertwined polymer network (IPN) is proposed for the visual detection of glucose. The IPN networks are composed of photonic and poly(acrylic acid) (PAA) networks. Aminophenylboronic acid, as a nonenzymatic glucose-responsive moiety that can covalently bond to glucose at alkaline pH, forming tetragonal complexes, is immobilized in the PAA network; in hydrogels, this bonding generates Donnan osmotic pressure, resulting in a volumetric increase of the photonic IPN and reflected color change. The photonic band gap wavelength linearly increases with the glucose concentration (in the 1-12 mM range), with a limit of detection of 0.35 mM. The dots of the photonic IPN array respond independently, with high sensitivity and stability, to glucose via color changes; different glucose levels, from hypo- to hyperglycemia, can be visually detected in this way. Serum samples spiked with different glucose concentrations were tested for practical evaluation of the chip. The proposed chip could be utilized as a new biosensor platform for cost-effective and easy visual detection in remote areas, without the need of advanced instruments or technology.

A CO2-responsive hydrogel film for optical sensing of dissolved CO2

CO₂-monitoring plays an important role in medicine, environmental sciences, and food industries. Here, a new CO₂-responsive hydrogel film was fabricated from branched polyethyleneimine (BPEI) and partially oxidized dextran (PO-Dex) via layer-by-layer (LBL) assembly, based on the in situ Schiff base reaction between the two polymers. The swelling behaviours of the films were studied using Fabry-Perot fringes on their reflection spectra. Like ordinary hydrogels, the BPEI/PO-Dex films swell in water. In addition, they swell to a larger degree when CO₂ is introduced, due to the reaction between CO₂ and the amino groups in BPEI. The CO₂-induced swelling can be reported by the shift of the Fabry-Perot fringes on their reflection spectra; therefore, the BPEI/PO-Dex film can be used as an optical sensor for dissolved CO₂. In the new sensor, the BPEI/PO-Dex film acts as a CO₂-sensing material and Fabry-Perot cavity simultaneously. The introduction of ordered structure is no longer required. The response of the sensor to CO₂ is linear and reversible. Unlike other hydrogel-based sensors that suffer from a slow response, the new sensor can respond to CO₂ quickly, making it applicable for real-time, continuous monitoring of CO₂ levels in solution.

Ultrathin Photonic Polymer Gel Films Templated by Non-Close-Packed Monolayer Colloidal Crystals to Enhance Colorimetric Sensing

Responsive polymer-based sensors have attracted considerable attention due to their ability to detect the presence of analytes and convert the detected signal into a physical and/or chemical change. High responsiveness, fast response speed, good linearity, strong stability, and small hysteresis are ideal, but to gain these properties at the same time remains challenging. This paper presents a facile and efficient method to improve the photonic sensing properties of polymeric gels by using non-close-packed monolayer colloidal crystals (ncp MCCs) as the template. Poly-(2-vinyl pyridine) (P2VP), a weak electrolyte, was selected to form the pH-responsive gel material, which was deposited onto ncp MCCs obtained by controlled O₂ plasma etching of close-packed (cp) MCCs. The resultant ultrathin photonic polymer gel film (UPPGF) exhibited significant improvement in responsiveness and linearity towards pH sensing compared to those prepared using cp MCCs template, achieving fast visualized monitoring of pH changes with excellent cyclic stability and small hysteresis loop. The responsiveness and linearity were found to depend on the volume and filling fraction of the polymer gel. Based on a simple geometric model, we established that the volume increased first and then decreased with the decrease of template size, but the filling fraction increased all the time, which was verified by microscopy observations. Therefore, the responsiveness and linearity of UPPGF to pH can be improved by simply adjusting the etching time of oxygen plasma. The well-designed UPPGF is reliable for visualized monitoring of analytes and their concentrations, and can easily be combined in sensor arrays for more accurate detection.

Hydrogel Interferometry for Ultrasensitive and Highly Selective Chemical Detection

Developing ultrasensitive chemical sensors with small scale and fast response through simple design and low-cost fabrication is highly desired but still challenging. Herein, a simple and universal sensing platform based on a hydrogel interferometer with femtomol-level sensitivity in detecting (bio)chemical molecules is demonstrated. A unique local concentrating effect (up to 10⁹ folds) in the hydrogel induced by the strong analyte binding and large amount of ligands, combined with the signal amplification effect by optical interference, endows this platform with an ultrahigh sensitivity, specifically 10^-14 m for copper ions and 1.0 × 10^-11 mg mL^-1 for glycoprotein with 2-4 order-of-magnitude enhancement. The specific chemical reactions between selected ligands and target analytes provide high selectivity in detecting complex fluids. This universal principle with broad chemistry, simple physics, and modular design allows for high performance in detecting wide customer choices of analytes, including metal ions and proteins. The scale of the sensor can be down to micrometer size. The nature of the soft gel makes this platform transparent, flexible, stretchable, and compatible with a variety of substrates, showing high sensing stability and robustness after 200 cycles of bending or stretching. The outstanding sensing performance grants this platform great promise in broad practical applications.

Dynamic layer-by-layer films on nanofiber membrane: a platform for ultra-sensitive bacterial concentration detection

Herein, a new platform was established for ultra-sensitive bacterial concentration detection. The sensing system had a linear relationship with the logarithm of bacterial concentration from 1 × 101 to 1 × 105 CFU mL-1 within 5 min. Moreover, the platform showed excellent consistency compared with the traditional standard method against practical samples.

Order-Disorder Transition in Doped Microgel Colloidal Crystals and Its Application for Optical Sensing

Hydrogel photonic crystal-based optical sensors usually can only be used as free-standing films. Here, a doped microgel colloidal crystal film was developed as glucose sensor, which exploits structural order-disorder transition, instead of change in lattice constant, to report an analyte. Changing glucose concentration induces a change in structural order degree in the crystal and hence a change in the intensity of the stop band, and thus reports glucose concentration in the media. The response is fast and reversible. As the overall swelling degree of the gel does not change, it can be used as substrate-attached film.

Glucose Sensing with Phenylboronic Acid Functionalized Hydrogel-Based Optical Diffusers

Phenylboronic acids have emerged as synthetic receptors that can reversibly bind to cis-diols of glucose molecules. The incorporation of phenylboronic acids in hydrogels offers exclusive attributes; for example, the binding process with glucose induces Donnan osmotic pressure resulting in volumetric changes in the matrix. However, their practical applications are hindered because of complex readout approaches and their time-consuming fabrication processes. Here, we demonstrate a microimprinting method to fabricate densely packed concavities in phenylboronic acid functionalized hydrogel films. A microengineered optical diffuser structure was imprinted on a phenylboronic acid based cis-diol recognizing motif prepositioned in a hydrogel film. The diffuser structure engineered on the hydrogel was based on laser-inscribed arrays of imperfect microlenses that focused the incoming light at different focal lengths and direction resulting in a diffused profile of light in transmission and reflection readout modes. The signature of the dimensional modulation was detected in terms of changing focal lengths of the microlenses due to the volumetric expansion of the hydrogel that altered the diffusion spectra and transmitted beam profile. The transmitted optical light spread and intensity through the sensor was measured to determine variation in glucose concentrations at physiological conditions. The sensor was integrated in a contact lens and placed over an artificial eye. Artificial stimulation of variation in glucose concentration allowed quantitative measurements using a smartphone's photodiode. A smartphone app was utilized to convert the received light intensity to quantitative glucose concentration values. The developed sensing platform offers low cost, rapid fabrication, and easy detection scheme as compared to other optical sensing counterparts. The presented detection scheme may have applications in wearable real-time biomarker monitoring devices at point-of-care settings.

The synthesis of a contraction-type glucose-sensitive microgel working at physiological temperature guided by a new glucose-sensing mechanism

A new glucose-sensing mechanism was proposed, guided by which a contraction-type glucose-sensitive microgel working at physiological temperature was synthesized successfully.

Biomolecule-Responsive Hydrogels in Medicine

Recent advances and applications of biomolecule-responsive hydrogels, namely, glucose-responsive hydrogels, protein-responsive hydrogels, and nucleic-acid-responsive hydrogels are highlighted. However, achieving the ultimate purpose of using biomolecule-responsive hydrogels in preclinical and clinical areas is still at the very early stage and calls for more novel designing concepts and advance ideas. On the way toward the real/clinical application of biomolecule-responsive hydrogels, plenty of factors should be extensively studied and examined under both in vitro and in vivo conditions. For example, biocompatibility, biointegration, and toxicity of biomolecule-responsive hydrogels should be carefully evaluated. From the living body's point of view, biocompatibility is seriously depended on the interactions at the tissue/polymer interface. These interactions are influenced by physical nature, chemical structure, surface properties, and degradation of the materials. In addition, the developments of advanced hydrogels with tunable biological and mechanical properties which cause no/low side effects are of great importance.

Supersensitive Oxidation-Responsive Biodegradable PEG Hydrogels for Glucose-Triggered Insulin Delivery

Reactive oxygen species (ROS)-responsive polymers and hydrogels represent an emerging family of intelligent materials owing to the key functions of ROS in physiological processes or pathological diseases. Nonetheless, the weaknesses such as low sensitivity, slow response, instability, and low mechanical strength are associated with the limited ROS-responsive polymeric or supramolecular hydrogels. In this study, a novel type of oxidation-responsive degradable hydrogels was fabricated by the redox-initiated radical polymerization of a 4-arm-poly(ethylene glycol) (PEG) acrylic macromonomer that possesses a H₂O₂-cleavable phenylboronic acid linker in each of the arms. The macroscopic hydrogels have the features of good cytocompatibility, moderate mechanical strength, and fast response toward H₂O₂ of low concentration, owing to the covalently cross-linked hydrophilic PEG network and high sensitivity of the linker. They could encapsulate biomacromolecules, such as insulin and glucose oxidase (GOx), with high efficacy, affording a new glucose-responsive insulin-delivery platform on the basis of enzymatic transformation of a biochemical signal (glucose) into an oxidative stimulus (H₂O₂). Interestingly, in vitro results demonstrate that the same GOx-loaded hydrogel exhibited disparate degradation modes under different triggering molecules, that is, bulk degradation by H₂O₂ and surface erosion by glucose. Moreover, compared to the macroscopic hydrogel, the nanogel with a diameter of ∼160 nm prepared by inverse emulsion polymerization showed a much higher degradation rate even under triggering of 20 μM H₂O₂, a pathologically available concentration in vivo.

Phenylboronic Acid-Functionalized Layer-by-Layer Assemblies for Biomedical Applications

Recent progress in the development of phenylboronic acid (PBA)-functionalized layer-by-layer (LbL) assemblies and their biomedical applications was reviewed. Stimuli-sensitive LbL films and microcapsules that exhibit permeability changes or decompose in response to sugars and hydrogen peroxide (H₂O₂) have been developed using PBA-bearing polymers. The responses of PBA-modified LbL assemblies arise from the competitive binding of sugars to PBA in the films or oxidative decomposition of PBA by H₂O₂. Electrochemical glucose sensors have been fabricated by coating the surfaces of electrodes by PBA-modified LbL films, while colorimetric and fluorescence sensors can be prepared by modifying LbL films with boronic acid-modified dyes. In addition, PBA-modified LbL films and microcapsules have successfully been used in the construction of drug delivery systems (DDS). Among them, much effort has been devoted to the glucose-triggered insulin delivery systems, which are constructed by encapsulating insulin in PBA-modified LbL films and microcapsules. Insulin is released from the PBA-modified LbL assemblies upon the addition of glucose resulting from changes in the permeability of the films or decomposition of the film entity. Research into insulin DDS is currently focused on the development of high-performance devices that release insulin in response to diabetic levels of glucose (>10 mM) but remain stable at normal levels (~5 mM) under physiological conditions.

Glucose-Sensitive Hydrogel Optical Fibers Functionalized with Phenylboronic Acid

Hydrogel optical fibers are utilized for continuous glucose sensing in real time. The hydrogel fibers consist of poly(acrylamide-co-poly(ethylene glycol) diacrylate) cores functionalized with phenylboronic acid. The complexation of the phenylboronic acid and cis-diol groups of glucose enables reversible changes of the hydrogel fiber diameter. The analyses of light propagation loss allow for quantitative glucose measurements within the physiological range.

Renal Function Replacement by Hemodialysis: Forty-Year Anniversary and a Glimpse into the Future at Hand

From its introduction in 1943 and until the late 1970s, hemodialysis (HD) has been a lengthy and cumbersome treatment administered by a few skilled physicians and technicians to a very limited number of terminal kidney patients. The technological innovations introduced over the years made HD a treatment administered and supervised by nursing personnel to a very large numbers of kidney patients, hopefully until recovery of kidney functions or kidney transplantation. In 2013, it is estimated that 2.250.00 kidney patients were treated worldwide, and their number is steadily increasing. Shortage of transplant kidneys and quality of current treatments has contributed to increasing the survival of HD patients. Today, it is not unusual to find patients who have been on HD for longer than twenty years. All this generated the feeling that performance of membranes and dialysis technology has reached its limit. Recently, the increasing economic burden of healthcare caused by people ageing and the increasing incidence of degenerative diseases (e.g. diabetes and cardiovascular diseases), and the economic crisis has pushed many governments and health insurances to cut resources for healthcare. The main consequence is that investments in research and development in HD have been significantly reduced. The question is whether there is indeed no need for innovation in HD.

In this paper, it is discussed how the paradigm of HD has changed and what possibly are now the drivers for innovation in HD. A few ideas are proposed that could be developed by adapting existing technologies to the future needs of HD.

Reverse thermo-responsive hydrogels prepared from Pluronic F127 and gelatin composite materials

A series of F127–gelatin composite hydrogels with reverse thermo-responsive and tunable mechanical properties were developed.

Two-step volume phase transition mechanism of poly(N-vinylcaprolactam) hydrogel online-tracked by two-dimensional correlation spectroscopy

The volume phase transition of PVCL hydrogel was online-tracked by 2D correlation FTIR spectroscopy, and a two-step mechanism was proposed.

Controlling swelling/deswelling of stimuli-responsive hydrogel nanofilms in electric fields

The swelling/deswelling transition of pH-sensitive, electrode-grafted, hydrogel nanofilms when exposed to electric fields is studied by theoretical analysis. In acidic conditions, the response of these films to changes in pH is dominated by network-surface interactions, while intra-network electrostatic repulsions, which are highly modulated by the adsorption of salt ions, determine material response at a higher pH. Film thickness is a non-monotonic function of solution pH and displays a local maximum, a local minimum or both, depending on the salt concentration and the applied voltage. We suggest the use of these materials in the development of biosensors and control of enzyme activity.

Swelling of glucose-responsive gels functionalized with boronic acid

A model is developed for the elastic response of a glucose-sensitive gel functionalized with boronic acid under swelling in aqueous solutions of glucose with various pH. A gel is treated as a three-phase medium composed of a solid phase (partially ionized polymer network), solvent (water), and solute (mobile glucose molecules and ions). Constitutive equations are derived by means of the free energy imbalance inequality for three-dimensional deformation with finite strains. Numerical analysis demonstrates the ability of the model to describe the effects of pH, molar fraction of glucose, and concentration of functional groups on equilibrium water uptake diagrams under unconstrained and constrained swelling.

Biothermal sensing of a torsional artificial muscle

Biomolecule responsive materials have been studied intensively for use in biomedical applications as smart systems because of their unique property of responding to specific biomolecules under mild conditions. However, these materials have some challenging drawbacks that limit further practical application, including their speed of response and mechanical properties, because most are based on hydrogels. Here, we present a fast, mechanically robust biscrolled twist-spun carbon nanotube yarn as a torsional artificial muscle through entrapping an enzyme linked to a thermally sensitive hydrogel, poly(N-isopropylacrylamide), utilizing the exothermic catalytic reaction of the enzyme. The induced rotation reached an equilibrated angle in less than 2 min under mild temperature conditions (25-37 °C) while maintaining the mechanical properties originating from the carbon nanotubes. This biothermal sensing of a torsional artificial muscle offers a versatile platform for the recognition of various types of biomolecules by replacing the enzyme, because an exothermic reaction is a general property accompanying a biochemical transformation.

Ultrathin polymer gel-infiltrated monolayer colloidal crystal films for rapid colorimetric chemical sensing

The ultrathin polymer gel-infiltrated monolayer colloidal crystal film shows rapid, linear, reversible, and colorimetric responses to pH variations.

Substrate-Independent Robust and Heparin-Mimetic Hydrogel Thin Film Coating via Combined LbL Self-Assembly and Mussel-Inspired Post-Cross-linking

In this work, we designed a robust and heparin-mimetic hydrogel thin film coating via combined layer-by-layer (LbL) self-assembly and mussel-inspired post-cross-linking. Dopamine-grafted heparin-like/-mimetic polymers (DA-g-HepLP) with abundant carboxylic and sulfonic groups were synthesized by the conjugation of adhesive molecule, DA, which exhibited substrate-independent adhesive affinity to various solid surfaces because of the formation of irreversible covalent bonds. The hydrogel thin film coated substrates were prepared by a three-step reaction: First, the substrates were coated with DA-g-HepLP to generate negatively charged surfaces. Then, multilayers were obtained via LbL coating of chitosan and the DA-g-HepLP. Finally, the noncovalent multilayers were oxidatively cross-linked by NaIO4. Surface ATR-FTIR and XPS spectra confirmed the successful fabrication of the hydrogel thin film coatings onto membrane substrates; SEM images revealed that the substrate-independent coatings owned 3D porous morphology. The soaking tests in highly alkaline, acid, and concentrated salt solutions indicated that the cross-linked hydrogel thin film coatings owned high chemical resistance. In comparison, the soaking tests in physiological solution indicated that the cross-linked hydrogel coatings owned excellent long-term stability. The live/dead cell staining and morphology observations of the adhered cells revealed that the heparin-mimetic hydrogel thin film coated substrates had low cell toxicity and high promotion ability for cell proliferation. Furthermore, systematic in vitro investigations of protein adsorption, platelet adhesion, blood clotting, and blood-related complement activation confirmed that the hydrogel film coated substrates showed excellent hemocompatibility. Both the results of inhibition zone and bactericidal activity indicated that the gentamycin sulfate loaded hydrogel thin films had significant inhibition capability toward both Escherichia coli and Staphylococcus aureus bacteria. Combined the above advantages, it is believed that the designed heparin-mimetic hydrogel thin films may show high potential for applications in various biological and clinical fields, such as long-term hemocompatible and drug-loading materials for implants.

Swelling-induced surface instability patterns guided by pre-introduced structures

Swelling-induced, spontaneously generated surface instability patterns in substrate-attached hydrogel films can be harnessed for advanced applications, however, methods to control their formation and morphology are missing. Here we propose that their generation may be guided by intentionally pre-introduced line structures. While uniform gel films produce irregular polygonal instability patterns, instability patterns generated in pre-patterned films with hexagonal line structures are regular hexagons with long-range order. The pre-introduced line structures act as defects in the generation of the surface instability patterns, which determine the position of the creases, regulate their rearrangement and determine their final morphology. The contrast between the pre-introduced structures and the surrounding area should be high enough for the pre-introduced structures to act as defects. Only when the characteristic wavelength of the pre-introduced pattern matches with the one of the gel film, perfect hexagonal patterns can be obtained. The gel films with uniform topographic features may find various advanced applications.

Glucose-induced decomposition of layer-by-layer films composed of phenylboronic acid-bearing poly(allylamine) and poly(vinyl alcohol) under physiological conditions

Phenylboronic acid-bearing poly(allylamine)/poly(vinyl alcohol) layer-by-layer films coupled with glucose oxidase decomposed in the presence of glucose under physiological conditions.