A Review of Research Progress on the Performance of Intelligent Polymer Gel

Intelligent polymer gel, as a popular polymer material, has been attracting much attention for its application. An intelligent polymer gel will make corresponding changes to adapt to the environment after receiving stimuli; therefore, an intelligent polymer gel can play its role in many fields. With the research on intelligent polymer gels, there is great potential for applications in the fields of drug engineering, molecular devices, and biomedicine in particular. The strength and responsiveness of the gels can be improved under different configurations in different technologies to meet the needs in these fields. There is no discussion on the application of intelligent polymer gels in these fields; therefore, this paper reviews the research progress of intelligent polymer gel, describes the important research of some intelligent polymer gel, summarizes the research progress and current situation of intelligent polymer gel in the environment of external stimulation, and discusses the performance and future development direction of intelligent polymer gel.

Chitosan Hydrogels with Embedded Thermo- and pH-Responsive Microgels as a Potential Carrier for Controlled Release of Drugs

We report a promising strategy based on chitosan (CS) hydrogels and dual temperature- and pH-responsive poly(N-isopropylacrylamide-co-methacrylic acid) (PNIPAM-co-MAA) microgels to facilitate release of a model drug, moxifloxacin (MFX). In this protocol, first, the microgels were prepared using a free radical copolymerization method, and subsequently, these carboxyl-group-rich soft particles were incorporated inside the hydrogel matrix using an EDC-NHS amidation method. Interestingly, the resulting microgel-embedded hydrogel composites (MG-HG) acting as a double barrier system largely reduced the drug release rate and prolonged the delivery time for up to 68 h, which was significantly longer than that obtained using microgels or hydrogels alone (20 h). On account of the dual-responsive features of the embedded microgels and the variation of water-solubility of drug molecules as a function of pH, MFX could be released in a controllable manner by regulating the temperature and pH of the delivery medium. The release kinetics followed a Korsmeyer-Peppas model, and the drug delivery mechanism was described by Fickian diffusion. Both the gel precursors and the hydrogel composites exhibited low cytotoxicity against mammalian cell lines (HeLa and HEK-293) and no deleterious hemolytic activity up to a certain higher concentration, indicating excellent biocompatibility of the materials. Thus, the unprecedented combination of modularity of physical properties caused by soft particle entrapment, unique macromolecular architecture, biocompatibility, and the general utility of the stimuli-responsive polymers offers a great promise to use these composite materials in drug delivery applications.

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.

Temperature- and pH-responsive poly(N-isopropylacrylamide-co-methacrylic acid) microgels as a carrier for controlled protein adsorption and release

Herein, we report controlled protein adsorption and delivery of thermo- and pH-responsive poly(N-isopropylacrylamide-co-methacrylic acid) (PNIPAM-co-MAA) microgels at different temperatures, pH values and ionic strengths by employing bovine serum albumin (BSA) as a model protein. For these dual-responsive microgels, we found that the BSA adsorption was driven by several of six competing contributions, viz., physical diffusion (PD), hydrophobic interactions (HI), electrostatic attraction (EA), hydrogen bonding (HB) and temperature or pH-induced seizing action (SA_T or SA_pH), depending on the temperature and pH of the solution. Compared to the pure PNIPAM microgels, the higher swelling degree of the PNIPAM-co-MAA microgels allowed a large amount of BSA loading under any experimental conditions. A largest BSA adsorption of 45.1 μg mg^-1 was achieved at 40 °C and pH 4 due to the presence of all six contributions. The BSA adsorption and delivery could be further tuned by changing the crosslinking density within the microgels. The BSA binding onto the microgels was found to be ionic strength dependent, which could be attributed to the charge shielding of Na⁺ ions, salting out of BSA and aggregate formation of the microgels. The adsorbed BSA could be controllably released by adjusting the temperature and pH of the experiment, and with the help of sodium dodecyl sulphate (SDS) addition so as to eliminate each interaction between BSA and the microgels. Thus, this study can be useful to design a stimuli-responsive microgel-based carrier for controlled release of proteins.

Microgel encapsulated nanoparticles for glucose-responsive insulin delivery

An insulin delivery system that self-regulates blood glucose levels has the potential to limit hypoglycemic events and improve glycemic control. Glucose-responsive insulin delivery systems have been developed by coupling glucose oxidase with a stimuli-responsive biomaterial. However, the challenge of achieving desirable release kinetics (i.e., insulin release within minutes after glucose elevation and duration of release on the order of weeks) still remains. Here, we develop a glucose-responsive delivery system using encapsulated glucose-responsive, acetalated-dextran nanoparticles in porous alginate microgels. The nanoparticles respond rapidly to changes in glucose concentrations while the microgels provide them with protection and stability, allowing for extended glucose-responsive insulin release. This system reduces blood sugar in a diabetic mouse model at a rate similar to naked insulin and responds to a glucose challenge 3 days after administration similarly to a healthy animal. With 2 doses of microgels containing 60 IU/kg insulin each, we are able to achieve extended glycemic control in diabetic mice for 22 days.

Galactose-based polymer-containing phenylboronic acid as carriers for insulin delivery

The galactose-based polymer is a promising drug delivery material. Herein, a new galactose-based block copolymer, termed as 6-O-vinyl sebacic acid-D-galactopyranosyl ester block 3-acrylamide phenylboric acid p(OVNG-b-AAPBA) was successfully synthesized by 'block copolymer' method. The structure of p(OVNG-b-AAPBA) was proved by nuclear magnetic hydrogen spectrum (1 HNMR) and infrared (IR), the thermal stability was observed by thermogravimetric analyzer, and the molecular weights (Mw and Mn) were demonstrated by Gel permeation chromatography (GPC). The above test results suggested that the polymer of p(OVNG-b-AAPBA) was successfully synthesized, and it had optimal molecular weight and thermal stability, which could be used for investigating the drug delivery system. Then, this block copolymer was prepared to the nanoparticle (NP), these NPs had a satisfactory morphology, and their safety was verified by MTT and chronic animal toxicology test. In addition, insulin was encapsulated by the p(OVNG-b-AAPBA) NPs, the drug loading rate and encapsulation efficiency increased with that of AAPBA in the polymer. Finally, this study confirmed that these NPs can effectively maintain the blood sugar of diabetic mice at 96 h. In conclusion, the current study suggested that the insulin-loaded galactose-based polymer-block-3-acrylamide phenylboric acid NPs had slow-release/glucose-responsive drug release performance, which might play an active role in the diabetes therapy.

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.

Nanogels and Microgels: From Model Colloids to Applications, Recent Developments, and Future Trends

Nanogels and microgels are soft, deformable, and penetrable objects with an internal gel-like structure that is swollen by the dispersing solvent. Their softness and the potential to respond to external stimuli like temperature, pressure, pH, ionic strength, and different analytes make them interesting as soft model systems in fundamental research as well as for a broad range of applications, in particular in the field of biological applications. Recent tremendous developments in their synthesis open access to systems with complex architectures and compositions allowing for tailoring microgels with specific properties. At the same time state-of-the-art theoretical and simulation approaches offer deeper understanding of the behavior and structure of nano- and microgels under external influences and confinement at interfaces or at high volume fractions. Developments in the experimental analysis of nano- and microgels have become particularly important for structural investigations covering a broad range of length scales relevant to the internal structure, the overall size and shape, and interparticle interactions in concentrated samples. Here we provide an overview of the state-of-the-art, recent developments as well as emerging trends in the field of nano- and microgels. The following aspects build the focus of our discussion: tailoring (multi)functionality through synthesis; the role in biological and biomedical applications; the structure and properties as a model system, e.g., for densely packed arrangements in bulk and at interfaces; as well as the theory and computer simulation.

Layer-by-Layer Assembly of Microgel Colloidal Crystals via Photoinitiated Alkyne-Azide Click Reaction

Layer-by-layer (LBL) assembly of colloidal crystals (CCs) allows for the fine control of the thickness and architecture of the resulting crystals. Various methods have been developed for the LBL assembly of CCs of hard spheres. However, these methods are inapplicable for microgel CCs owing to the softness and deformability of microgel spheres. In this study, a method was proposed for the LBL assembly of microgel CCs. To build the first monolayer, azide-modified microgel spheres were assembled into a three-dimensional (3D) CC. The first 111 plane of the 3D CC close to the substrate was then fixed in situ onto the substrate via photoinitiated alkyne-azide click reaction between the azide groups on the microgels and the alkyne groups on the substrate surface. The removal of unbonded particles resulted in a microgel monolayer with a high degree of order. The second monolayer was assembled in a similar manner, i.e., a 3D microgel CC was initially assembled followed by in situ fixation of the first 111 plane of the 3D crystal with the underlying microgel monolayer by photoinitiated alkyne-azide click reaction. For this purpose, instead of azide-modified microgel spheres, alkyne-modified microgel spheres were used for the assembly of the second layer. Confocal studies confirmed that the second monolayer was located on top of the first layer. When the lattice constant of the 3D CC approximated that of the underlying microgel monolayer, the second monolayer exhibited a high degree of order. Repeating this process led to alternating deposition of highly ordered monolayers of azide-modified and alkyne-modified microgels onto the substrate. Similar to the microgel CCs obtained by the self-assembly of microgel spheres in bulky dispersions, face-centered cubic and hexagonal-close-packed structures also coexisted in the LBL-assembled microgel CCs.

Functional Microgels: Recent Advances in Their Biomedical Applications

Here, a spotlight is shown on aqueous microgel particles which exhibit a great potential for various biomedical applications such as drug delivery, cell imaging, and tissue engineering. Herein, different synthetic methods to develop microgels with desirable functionality and properties along with degradable strategies to ensure their renal clearance are briefly presented. A special focus is given on the ability of microgels to respond to various stimuli such as temperature, pH, redox potential, magnetic field, light, etc., which helps not only to adjust their physical and chemical properties, and degradability on demand, but also the release of encapsulated bioactive molecules and thus making them suitable for drug delivery. Furthermore, recent developments in using the functional microgels for cell imaging and tissue regeneration are reviewed. The results reviewed here encourage the development of a new class of microgels which are able to intelligently perform in a complex biological environment. Finally, various challenges and possibilities are discussed in order to achieve their successful clinical use in future.

Polyamidoamine Dendrimer Microgels: Hierarchical Arrangement of Dendrimers into Micrometer Domains with Expanded Structural Features for Programmable Drug Delivery and Release

In this paper, we report on the fabrication of micron-sized dendrimer hydrogels (μDHs) using the water-in-oil (w/o) inverse microemulsion method coupled with the highly efficient aza-Michael addition. EDA core polyamidoamine (PAMAM) dendrimer G5 (10 w%) and polyethylene glycol diacrylate (PEG-DA, M n = 575 g/mol) (the molar ratio of amine/acrylate = 1/1) were dissolved in the water phase and added to hexane in the presence of surfactants span 80/tween 80 (5/1, w/w) (volume ratio of hexane to surfactants: 70:1) to form w/o microemulsions, in which PAMAM G5 cross-links with PEG-DA via the aza-Michael addition reaction. The resulting microgels are within 3-5 μm with relatively narrow size distribution. μDHs are pH-responsive degradable. They show good cytocompatibility and do not cause acute toxicity in vivo. Furthermore, they can realize a high loading of the hydrophobic drug CPT and enter the cells in the form of particles. The CPT and CPT/dendrimer complex can be slowly released following the zero-order release kinetics. Taken together, μDHs possessing hierarchically ordered dendrimers in micron domains represent a new class of microparticles with expanded structural features for programmable drug delivery and release.

Glucose-Induced Transition among Three States of a Doped Microgel Colloidal Crystal

For the first time here, we report a colloid crystal capable of undergoing transition among three states in response to external stimuli. The colloidal crystal was assembled from poly( N-isopropylacrylamide) (PNIPAM) microgel and doped with poly( N-isopropylacrylamide- co-2-acrylamido-phenylboronic acid) (P(NIPAM-2-AAPBA)) microgel. The ordered structure was locked by in situ photopolymerization. Taking advantage of the different responses of the two microgels to external stimuli, defect state can be induced and erased reversibly. Particularly, because the dopant, that is, P(NIPAM-2-AAPBA) microgel sphere, shrinks with increasing glucose concentration, its size changes from larger than the host, that is, PNIPAM microgel sphere, to equal to the host, and finally smaller than the host. Therefore, upon addition of glucose, the crystal undergoes transition from a state with acceptor-type defect, to no defect state, and then to a state with donor-type defect. The transition among the three states is fully reversible. In addition, the response of the doped crystal to glucose is relatively fast.

Adjustable Tribological Behavior of Glucose-Sensitive Hydrogels

Stimuli-responsive hydrogels have been considered to have various applications in numerous fields. In the present work, a double-network (DN) hydrogel has been synthesized. The copolymers of 2-acrylamide-2-methylpropane sulfonic acid (AMPS) and acrylamide (AM) [P(AMPS- co-AM)] are prepared as the 1st network and poly(acrylic acid) as the 2nd network. This DN hydrogel is sensitive to glucose by introducing the glucose-sensitive group phenylboronic acid to the network. The tribological properties of this glucose-sensitive DN hydrogel have been investigated using a universal mechanical tester (UMT-5). The tribological results show that the friction coefficient varied with the glucose solution. The friction coefficient increased to a maximum of 0.06, and finally decreased to 0.025 with the increase in the glucose concentration. An adjustable friction coefficient of the hydrogel, between 0.025 and 0.056, was achieved along with the change of lubricant. According to the tribological experimental results and the analysis of the DN structure, it can be deduced that a hydrated layer exists in the interface of the hydrogel. The hydrated layers consisting of water molecules are bounded with the hydrophilic group of the hydrogel network by hydrogen bonds. The change in the number of water molecules leads to the difference in the water content of the hydrogel, which further resulted in the various tribological properties. In addition, the hydrogel's mesh size also has an impact on the change in friction coefficient. In general, the adjustable friction of the hydrogel in a glucose environment is achieved.