Botryoidal assembly of cholesteryl-pullulan/poly(N-isopropylacrylamide) nanogels

Hybrid nanogels consisting of cholesteryl-modified pullulan (CHP) and poly(N-isopropylacrylamide) (PNIPAM) were synthesized by graft free-radical copolymerization of N-isopropylacrylamide (NIPAM) onto methacryloyl-substituted CHP nanogels (CHPMA) in water at 50 degrees C in the presence of a water-soluble free radical initiator. Depending on the initial NIPAM/CHPMA ratio, CHP-PNIPAM (CN) nanogels containing 30.8-84.8 wt % PNIPAM were obtained in the form of self-assembled nanoparticles with a hydrodynamic radius (Rh) of 69.0-116.0 nm in water kept at 20 degrees C. Hybrid nanogels of sufficiently high NIPAM content, such as the sample CN90, which contains 79.6 wt % NIPAM, exhibited a two-step response to changes in solution (3 mg/mL) temperature: a decrease in Rh from 93 to 57 nm as the temperature increased from 20 to 35 degrees C, followed by a sharp increase in Rh from 57 nm to 90 nm at 55 degrees C. Both steps in this temperature response were reversible. The multistep response to temperature of the CN nanogels was attributed to the morphology of the nanogels, which are seen as consisting of grape-like (botryoidal) clusters of associated native nanogels held together via cholesteryl cross-linking points and held together by the grafted PNIPAM chains.     

Thermo‐Responsive Hydrogels with Nanodomains: Rapid Shrinking of a Nanogel‐Crosslinking Hydrogel of Poly(N‐isopropyl acrylamide)

Rapidly shrinking poly(N‐isopropyl acrylamide) (PNIPAM) hydrogels are prepared by crosslinking with self‐assembled nanogels that consist of cholesteryl‐ and methacryloyl‐substituted pullulan (CHPMA). The CHPMA nanogel (R_h = 26.4 nm) was used as a crosslinker for a hydrophilic nanodomain. Transmission electron microscopy images of the nanogel‐crosslinked PNIPAM hydrogel reveal a well‐defined nanoporous structure. The nanogel‐crosslinked PNIPAM hydrogel shows rapid shrinking based on its structure. The shrinking half‐time was ≈2 min, which is about 3 400 times faster than that of a PNIPAM hydrogel crosslinked by methylene(bisacrylamide).

     

Sparsely cross-linked "nanogels" for microchannel DNA sequencing

We have developed sparsely cross-linked "nanogels", sub-colloidal polymer structures composed of covalently linked, linear polyacrylamide chains, as novel DNA sequencing matrices for capillary electrophoresis. The presence of covalent cross-links affords nanogel matrices with enhanced network stability relative to standard, linear polyacrylamide (LPA), improving the separation of large DNA fragments. Nanogels were synthesized via inverse emulsion (water-in-oil) copolymerization of acrylamide and N,N-methylenebisacrylamide (Bis). In order to retain the fluidity necessary in a replaceable polymer matrix for capillary array electrophoresis (CAE), a low percentage of the Bis cross-linker (< 10(-4) mol%) was used. Nanogels were characterized by multiangle laser light scattering and rheometry, and were tested for DNA sequencing by CAE with four-color laser-induced fluorescence (LIF) detection. The properties and performance of nanogel matrices were compared to those of a commercially available LPA network, which was matched for both weight-average molar mass (Mw) and extent of interchain entanglements (c/c*). Nanogels presented in this work have an average radius of gyration of 226 nm and a weight-average molar mass of 8.8 x 10(6) g/mol. At concentrations above the overlap threshold, nanogels form a clear, viscous solution, similar to the LPA matrix (Mw approximately 8.9 x 10(6) g/mol). The two matrices have similar flow and viscosity characteristics. However, because of the physical network stability provided by the internally cross-linked structure of the nanogels, a substantially longer read length ( approximately 63 bases, a 10.4% improvement) is obtained with the nanogel matrix at 98.5% accuracy of base-calling. The nanogel network provides higher-selectivity separation of ssDNA sequencing fragments longer than 375 bases. Moreover, nanogel matrices require 30% less polymer per unit volume than LPA. This is the first report of a sequencing matrix that provides better performance than LPA, in a side-by-side comparison of polymer matrices matched for Mw and extent of interchain entanglements.     

Geometrical characteristics of polyelectrolyte nanogel particles and their polyelectrolyte complexes studied by dynamic and static light scattering

The geometric characteristics of nanogel particles in aqueous solutions were studied by determining their ratios of radius of gyration (mean-square radius; Rg) to hydrodynamic radius (Rh), Rg/Rh, derived from static light scattering and dynamic light scattering experiments, respectively. The various nanogel samples studied included ones composed of lightly cross-linked N-isopropylacrylamide (NIPA) polymer, NIPA-based anionic or cationic copolymers, and amphoteric terpolymers. Polyelectrolyte complexes between anionic or cationic nanogels and oppositely charged polyions or nanogels having opposite charges were also studied. Most NIPA and NIPA-based polyelectrolyte nanogels in a swollen state had Rg/Rh values >0.775, which is the theoretically predicted value for a solid sphere. In a collapsed state, one may expect nanogel particles to be spherical in shape; however, this was not the case for a variety of nanogel samples, either with or without charges. These data were consistent with the idea that the surfaces of these nanogel particles were decorated with attached dangling chains. The Rg/Rh data from polyelectrolyte-nanogel complexes, however, indicated different structures from this. It was found that most of the polyelectrolyte-nanogel complex particles had Rg/Rh approximately 0.775. This suggested that the complexed nanogel particles were spherical in shape and that there were no dangling surface chains.     

Biodegradable nanogels prepared by atom transfer radical polymerization as potential drug delivery carriers: synthesis, biodegradation, in vitro release, and bioconjugation

Stable biodegradable nanogels cross-linked with disulfide linkages were prepared by inverse miniemulsion atom transfer radical polymerization (ATRP). These nanogels could be used for targeted drug delivery scaffolds for biomedical applications. The nanogels had a uniformly cross-linked network, which can improve control over the release of encapsulated agents, and the nanogels biodegraded into water-soluble polymers in the presence of a biocompatible glutathione tripeptide, which is commonly found in cells. The biodegradation of nanogels can trigger the release of encapsulated molecules including rhodamine 6G, a fluorescent dye, and Doxorubicin (Dox), an anticancer drug, as well as facilitate the removal of empty vehicles. Results obtained from optical fluorescence microscope images and live/dead cytotoxicity assays of HeLa cancer cells suggested that the released Dox molecules penetrated cell membranes and therefore could suppress the growth of cancer cells. Further, OH-functionalized nanogels were prepared to demonstrate facile applicability toward bioconjugation with biotin. The number of biotin molecules in each nanogel was determined to be 142,000, and the formation of bioconjugates of nanogels with avidin was confirmed using optical fluorescence microscopy.     

Kinetic properties of aryldialkylphosphatase immobilised on chitosan myristic acid nanogel

Organophosphorus (OP) compounds are extensively used in agricultural practice for pest management. However, their residues have a long half-life in the ecosystem as well as in the agro-products, posing a serious threat to human and animal health. Aryldialkylphosphatase (EC 3.1.8.1) is widely used in detoxification procedures. In the present study, aryldialkylphosphatase was immobilised on synthesised cross-linked nano-sized gel particles, also known as nanogels, in order to enhance the enzyme’s physicochemical properties. Accordingly, a new nanogel consisting of chitosan and myristic acid (CMA nanogel) was synthesised and characterised by way of Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The aryldialkylphosphatase-CMA nanogel conjugate was then assayed by FTIR, and its physicochemical characteristics were also investigated. The data obtained from SEM and TEM showed the nanogels to be homogenous spherical particles less than 50 nm in diameter. The proper formation of the nanogel and nanobioconjugate was also confirmed by FTIR spectra. In comparison with the free enzyme, the pH and thermal stability of the aryldialkylphosphatase were enhanced by the covalent immobilisation. Moreover, the immobilised enzyme could maintain approximately half of its activity over more than one month. The kinetic parameters of the aryldialkylphosphatase-CMA nanogel conjugate were also shown to undergo remarkable improvements, hence the synthesised CMA-nanogel could act as a promising support for aryldialkylphosphatase immobilisation. It is suggested that the aryldialkylphosphatase-CMA nanogel could be used for detoxifying paraoxon; a nerve agent. Further clinical experiments are underway.     

Preparation of monodisperse HPMC/PAA hybrid nanogels via surfactant-free seed polymerization

Herein we report a facile surfactant-free two-step method to prepare hydroxypropyl methylcellulose/poly(acrylic acid) (HPMC/PAA) hybrid nanogels. The HPMC/poly(N,N′-methylenebisacrylamide) (PMBA) nanoparticles were firstly prepared by free radical polymerization of N,N′-methylenebisacrylamide (MBA) in HPMC aqueous solution. In the second step, HPMC/PAA nanogels were synthesized by polymerization using the as-prepared HPMC/PMBA nanoparticles as the seeds and acrylic acid (AA) and MBA as the monomer and cross-linker, respectively. Dynamic light scattering (DLS) experiments indicated the nanogels were monodispersed with the nanogel sizes ranging from 95 to 310 nm and the polydispersity index (PDI) values ranging from 0.043 to 0.122. Transmission electron microscope (TEM) experiments demonstrated that nanogels have a core/shell structure. Furthermore, the monodisperse nanogels have a good temperature and pH sensibility, and the nanogel diameter was decreased with increasing temperature while increased with rising pH. This method provides a new way of preparation of monodisperse polymer nanogels with a core/shell structure.     

Novel nanogels as drug delivery systems for poorly soluble anticancer drugs

Two types of novel nanogels were prepared using shell cross-linking of Pluronic F127 micelles with polyethylenimine (PEI) (F127/PEI nanogel), and penetrating network of poly(butylcyanoacrylate) (PBCA) in Pluronic F127 micelles (F127/PBCA nanogel). Poorly soluble anticancer drug, paclitaxel (PTX) and 10-hydroxycamptothecin (HCPT), were used as model drugs and incorporated into nanogels. The results obtained from FT-IR spectroscopy confirmed that the drugs were molecularly dispersed in the nanogels. DLS measurements demonstrated that the nanogel size distribution was narrow with average diameter less than 200 nm. TEM images indicated that the nanogels were spherical in shape and had smooth surfaces. The drug-loaded nanogels showed sustained release profiles compared with the free drugs as revealed by in vitro release experiments. Cytotoxicity tests showed that the cytotoxicity of drug-loaded nanogels against cancer cell in vitro was much higher than that of the free drug. The data demonstrate that these novel nanogels improved stability towards dilution, increased solubility and showed better cellular uptake by cells compared with free drug.     

Interpenetrating polymer network (IPN) nanogels based on gelatin and poly(acrylic acid) by inverse miniemulsion technique: synthesis and characterization

Novel interpenetrating polymer network (IPN) nanogels composed of poly(acrylic acid) and gelatin were synthesised by one pot inverse miniemulsion (IME) technique. This is based on the concept of nanoreactor and cross-checked from template polymerization technique. Acrylic acid (AA) monomer stabilized around the gelatin macromolecules in each droplet was polymerized using ammonium persulfate (APS) and tetramethyl ethylene diamine (TEMED) in 1:5 molar ratio and cross-linked with N,N-methylene bisacrylamide (BIS) to form semi-IPN (sIPN) nanogels, which were sequentially cross-linked using glutaraldehyde (Glu) to form IPNs. Span 20, an FDA approved surfactant was employed for the formation of homopolymer, sIPN and IPN nanogels. Formation of stable gelatin-AA droplets were observed at 2% surfactant concentration. Dynamic light scattering (DLS) and scanning electron microscopy (SEM) studies of purified nanogels showed small, spherical IPN nanogels with an average diameter of 255 nm. In contrast, sIPN prepared using the same method gave nanogels of larger size. Fourier-transform infrared (FT-IR) spectroscopy, SEM, DLS, X-ray photoelectron spectroscopy (XPS) and zeta potential studies confirm the interpenetration of the two networks. Leaching of free PAA chains in sIPN upon dialysis against distilled water leads to porous nanogels. The non-uniform surface of IPN nanogels seen in transmission electron microscopy (TEM) images suggests the phase separation of two polymer networks. An increase of N/C ratio from 0.07 to 0.17 (from PAA gel to IPN) and O/C ratio from 0.22 to 0.37 (from gelatin gel to IPN) of the nanogels by XPS measurements showed that both polymer components at the nanogel surface are interpenetrated. These nanogels have tailoring properties in order to use them as high potential drug delivery vehicles for cancer targeting.     

pH‐responsive squeezing polysaccharidic nanogels for efficient docetaxel delivery

In this study, we report pH‐responsive polysaccharidic nanogels comprising starch grafted with 3‐(diethylamino)propylamine (DEAP, as an inner soft nanogel core) and poly(ethylene glycol) (PEG, as an outer hydrophilic nanogel shell). Here, the DEAP moieties (pK_b ~ pH 7.0) enhance the lipophilicity of the nanogel core at pH 7.4, improving the loading efficiency of an antitumor model drug (docetaxel [DTX]) in the core. However, the DEAP moieties could be protonated below pH 7.0, resulting in the mediation of ion‐dipole interactions with hydroxyl groups abundant in starch backbone. This event causes the electrostatic condensation of the nanogel core and enables the acceleration of drug release by squeezing of the core. We demonstrated that the nanogels selectively release the drug given a weakly acidic pH stimulus. These drug release trends are reversible with changes in pH. As a result, the nanogels are able to efficiently reduce MDA‐MB‐231 tumor cell population in acidic pH environments.     

Nanogel engineering for new nanobiomaterials: from chaperoning engineering to biomedical applications

Nanosize hydrogels (nanogels) are polymer nanoparticles with three‐dimensional networks, formed by chemical and/or physical cross‐linking of polymer chains. Recently, various nanogels have been designed, with a particular focus on biomedical applications. In this review, we describe recent progress in the synthesis of nanogels and nanogel‐integrated hydrogels (nanogel cross‐linked gels) for drug‐delivery systems (DDS), regenerative medicine, and bioimaging. We also discuss chaperone‐like functions of physical cross‐linking nanogel (chaperoning engineering) and organic‐inorganic hybrid nanogels. © 2010 The Japan Chemical Journal Forum and Wiley Periodicals, Inc. Published online in Wiley InterScience ( www.interscience.wiley.com ) DOI 10.1002/tcr.201000008     

Thermoresponsive drug controlled release from chitosan‐based hydrogel embedded with poly(N‐isopropylacrylamide) nanogels

A facile synthetic strategy was developed for the preparation of thermoresponsive nanocomposite hydrogels comprising crosslinked chitosan (CS) networks and poly(N‐isopropylacrylamide) [p(NIPAAm)] nanogels. First, thermoresponsive p(NIPAAm) nanogels were synthesized via emulsion polymerization. The p(NIPAAm) nanogels were introduced into methacrylamide CS (MC) solution and the free‐radical initiated crosslinking reaction of MC produced nanogel‐embedded hydrogels. The last step involves the loading of the antibacterial model drug levofloxacin (LFX) into the prepared nanocomposite hydrogels by allowing the preformed hydrogels to swell to equilibrium in the drug's aqueous solution. The integration of p(NIPAAm) nanogel into CS networks facilitates thermoresponsive release of LFX with an enhancement of the drug‐loading capacity within the hydrogel. Notably, thermoresponsive drug‐release was achieved without unwarranted modification of the hydrogel's dimension and shape, although an increase in temperature caused the collapse of the p(NIPAAm) nanogels. The thermoresponsive property of the investigated nanocomposite hydrogel is beneficial and may offer broad opportunities for drug temperature‐triggered release for clinical applications. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 1907–1914     

3-Diethylaminopropyl-bearing glycol chitosan as a protein drug carrier

Polysaccharidic nanogels were fabricated with bovine serum albumin (BSA) and a glycol chitosan (GCS) grafted with functional 3-diethylaminopropyl (DEAP) groups. These nanogels were investigated to evaluate their cellular uptake in HeLa cells and in vivo fate in nude mice tumor model. Unlike free BSA, GCS-g-DEAP/BSA nanogels improved cellular uptake of BSA. Furthermore, this system led to an enhanced blood circulation and a high accumulation of BSA in the tumor site. Our collective results strongly support that GCS-g-DEAP/BSA nanogel is a potential carrier system for high molecular weight proteins.     

Tunable Dual‐Thermoresponsive Core–Shell Nanogels Exhibiting UCST and LCST Behavior

Thermoresponsive polymers change their physical properties as the temperature is changed and have found extensive use in a number of fields, especially in tissue engineering and in the development of drug delivery systems. The synthesis of a novel core–shell nanogel composed of N‐isopropylacrylamide and sulfobetaine by reversible addition fragmentation chain transfer polymerization is reported. The core–shell architecture of the nanogels is confirmed using energy dispersive X‐ray spectroscopy in scanning transmission electron microscopy. These nanogels exhibit dual thermoresponsive behavior, i.e., the core of the nanogel exhibits lower critical solution temperature, while the shell displays upper critical solution temperature behavior. Transition temperatures can be easily tuned by changing the molecular weight of the constituent polymer. These nanogels can be efficiently used in temperature‐triggered delivery of therapeutic proteins and drugs.     

Stable and pH-sensitive nanogels prepared by self-assembly of chitosan and ovalbumin

Two natural macromolecules, chitosan and ovalbumin, were used to produce nanogels by a new, green, and convenient method. Chitosan and ovalbumin solutions were mixed; the pH of the resulting solution was adjusted; and the solution was successively stirred and heated. After that, ovalbumin gelled forming nanospheres. The chitosan chains are supposed to be partly trapped in the nanogel core upon heating because of the electrostatic attractions between chitosan and ovalbumin, while the rest of the chitosan chains should form the shell of the nanogels. The nanogels did not change the size distribution after long-time storage and did not dissociate in the pH range of 2-10.5. The dispersibility, size, and hydrophobicity/hydrophilicity of the nanogels are pH-dependent. The nanogels are good candidates for cosmetic and pharmaceutical applications.     

Gradient polymer networks formed by photopolymerization with self‐floating polysiloxane‐containing nanogel

Polysiloxane‐containing nanogels can be used as a fast, convenient and environmentally friendly method to control gradient photopolymerization and to obtain gradient polymer network because of its self‐floating feature. The chain length of polysiloxane is a key factor that influences the self‐floating capability of the polysiloxane‐containing nanogel. This paper reports a series of nanogels compositions synthesized with methacrylate‐modified polysiloxanes with different chain lengths, urethane dimethacrylate (UDMA) and isobornyl methacrylate (IBMA) at a molar ratio of 10:20:70 in the presence of a thiol chain transfer agent. The effect of polysiloxane chain length on self‐floating capability of the nanogel and gradient polymer network was researched. The results show that polysiloxane chain length is the main driving force for the self‐floating capability of the nanogels. The nanogel with long polysiloxane chain length exhibits good self‐floating capability in the monomer–polymer matrix because of the lower surface tension of polysiloxane. Furthermore, the gradient polymer network containing the nanogel with long polysiloxane chain length presents lower dispersion surface energy and greater hardness and thermostability. Copyright © 2016 John Wiley & Sons, Ltd.     

Lysozyme-dextran core-shell nanogels prepared via a green process

A novel method has been developed for preparing nanogels with a lysozyme core and dextran shell. The method involves the Maillard dry-heat process and heat-gelation process. First, lysozyme-dextran conjugates were produced through the Maillard reaction. Then, the conjugate solution was heated above the denaturation temperature of lysozyme to produce nanogels. The nanogels are of spherical shape having a hydrodynamic diameter of about 200 nm and swelling ratio of about 30. The nanogel solutions are stable against long-term storage as well as changes in pH and ionic strength. Ibuprofen has been used as a drug model to study the electrostatic and hydrophobic interactions with these nanogels at different pH values. The study reveals that the nanogels are more suitable for loading protonated ibuprofen. We have verified that the knowledge of the formation mechanism of lysozyme-dextran nanogels can be applied to prepare other globular protein-dextran nanogels.     

A study of shrinkage stress reduction and mechanical properties of nanogel-modified resin systems

A series of nanogel compositions were prepared from urethane dimethacrylate (UDMA) and isobornyl methacrylate (IBMA) in the presence of a thiol chain transfer agent. The linear oligomer of IBMA was synthesized by a similar solution polymerization technique. The nanogels were prepared with different crosslinker concentrations to achieve varied branching densities and molecular weights. The prepolymers were dispersed in triethylene glycol dimethacrylate at loading levels ranging from 10 wt.% to 50 wt.%. Photopolymerization reaction kinetics of all prepolymer modified systems were enhanced relative to the nanogel-free control during early stage polymerization while limiting conversion was similar for most samples. Volumetric polymerization shrinkage was reduced proportionally with the prepolymer content while the corresponding decrease in polymerization stress was potentially greater than an additive linear behavior. Flexural strength for inert linear polymer-modified systems decreased significantly with the increase in the prepolymer content; however, with an increase in the crosslinker concentration within the nanogel additives, and an increase in the concentration of residual pendant reactive sites, flexural strength was maintained or improved regardless of the nanogel loading level. This demonstrates that covalent attachment rather than just physical entanglement with the polymer matrix is important for effective polymer mechanical reinforcement by nanogel additives. Reactive nanogel additives can be considered as a practical, generic means to achieve substantial reductions in polymerization shrinkage and shrinkage stress in common polymers.     

Microrheology of polysaccharide nanogel-integrated system

The viscoelastic behavior of a cholesterol-modified pullulan (CHP) nanogel at various concentrations was measured using passive particle-tracking microrheology. Microrheology measures stress–strain relationships in small volumes of material by monitoring the response of probes embedded in the medium. Although microrheology is a useful way to overcome sample volume limitations, the application of the method to CHP nanogel systems has not been reported. The viscoelastic spectra of the CHP nanogels obtained from the microrheological measurements were in good agreement with the bulk rheological measurements for each sample, demonstrating that microrheological measurement is effective in CHP nanogel systems. The gelation behavior of CHP nanogel dispersions containing pullulans of different molecular weights was also investigated by microrheology. CHP nanogels made from 1.0 or 4.0?×?10⁵ molecular weight pullulans formed a macrogel at around 3.0 wt%, whereas the CHP nanogel consisting of 0.55?×?10⁵ molecular weight pullulan did not form a macrogel. This suggests that the mechanical properties of the system can be controlled by the molecular weight of the pullulan used. These insights into gelation behavior should be useful in predicting the most favorable conditions for developing novel materials.     

Role of parallel reformable bonds in the self-healing of cross-linked nanogel particles

We develop a hybrid computational approach to examine the mechanical properties and self-healing behavior of nanogel particles that are cross-linked by both stable and labile bonds. The individual nanogels are modeled via the lattice spring model (LSM), which is an effective method for probing the response of materials to mechanical deformation. The cross-links between the nanogels are simulated via the hierarchical Bell model (HBM), which allows us to capture the rupturing of multiple parallel bonds as the result of an applied force. Because the labile bonds are relatively reactive, they can reform after they have been ruptured. To incorporate the possibility of bonds reforming, we modify the HBM formalism and validate the modified HBM by considering a system of two surfaces, which are connected by multiple parallel bonds. We then use our hybrid HBM/LSM to simulate the behavior of the cross-linked nanogels under a tensile deformation. In these simulations, each labile linkage between the nanogels contains at most N parallel bonds. We vary the fraction of labile linkages and the value of N in these linkages to determine the optimal conditions for improving the robustness of the material. Although numerous parallel bonds within a linkage enhance the strength of the material, these bonds diminish the ductility and the ability of the material to undergo the structural rearrangements that are necessary for self-repair. For a relatively low fraction of labile bonds and N ≤ 4, however, we can significantly improve the strength of the material and preserve the self-healing properties. For instance, a sample with 30% labile linkages and N = 4 per linkage is roughly 200% stronger than a sample that is cross-linked solely by stable bonds and can still undergo self-repair in response to the tensile deformation. The results reveal how mechanical stress can lead not only to the appearance of cavities within the material but also to bond formation that "heals" these cavities and thus prevents the catastrophic failure of the material.