Preparation and application in drug controlled delivery of pH‐sensitive P(CE‐co‐DMAEMA‐co‐MEG) hydrogel

A pH‐sensitive hydrogel [P(CE‐co‐DMAEMA‐co‐MEG)] was synthesized by the free‐radical crosslinking polymerization of N,N‐dimethylaminoethyl methacrylate (DMAEMA), poly(ethylene glycol) methyl ether methacrylate(MPEG‐Mac) and methoxyl poly(ethylene glycol)‐poly(caprolactone)‐methacryloyl methchloride (PCE‐Mac). The effects of pH and monomer content on swelling property, swelling and deswelling kinetics of the hydrogels were examined and hydrogel microstructures were investigated by SEM. Sodium salicylate was chosen as a model drug and the controlled‐release properties of hydrogels were pilot studied. The results indicated that the swelling ratios of the gels in stimulated gastric fluids (SGF, pH = 1.4) were higher than those in stimulated intestinal fluids (SIF, pH = 7.4), and followed a non‐Fickian and a Fickian diffusion mechanism, respectively. In vitro release studies showed that its release rate depends on different swelling of the network as a function of the environmental pH and DMAEMA content. SEM micrographs showed homogenous pore structure of the hydrogel with open pores at pH 1.4. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40737.     

Electroactive polyacrylamide/chitosan/polypyrrole hydrogel for captopril release controlled by electricity

The demand for the development of new therapies and devices for controlled drug release has been continuously increasing, especially those based on materials sensitives to external stimuli, such as electricity. Therefore, in this work, acrylamide was polymerized in the presence of chitosan (CS), using N,N′-methylenebisacrylamide as cross-linking, followed by immersion in pyrrole aqueous solution and chemical polymerization to obtain an electroactive hydrogel of polyacrylamide/CS/polypyrrole (PA/CS/PPy) (67.5/7.5/25% wt.); this electroactive hydrogel was later used in drug delivery controlled by electricity studies. The synthesized PA/CS/PPy hydrogel was characterized by scanning electron microscopy, FTIR spectroscopy, and thermogravimetric analysis. It was observed that the hydrogel presented pores in the range of 50–200 μm with CS and PPy well incorporated to the cross-linked PA. The hydrogel swelling percentage (S) was determined at different pHs. It was observed that S was independent of pH, with S = 700% and a swelling kinetics described by the Fickian diffusion mechanism at alkaline pH. PA/CS/PPy hydrogel was used to absorb captopril (a drug for hypertension control), and its kinetics release at different applied potentials and pH was studied. Release kinetics were described by the Korsmeyer–Peppas model, while release mechanism was a Case-II transport without current at alkaline pH; under electrical stimuli, the mechanism presented an anomalous transport with ON–OFF profile, increasing the release rate with the applied voltage showing its electroactivity in the captopril release.     

Controlled release of aminophylline from poly (N‐isopropylacrylamide‐co‐itaconic acid) hydrogels

N‐Isopropylacrylamide (NIPA) has been copolymerized with itaconic acid (IA) in the presence of N,N‐methylenebisacrylamide (BIS) as crosslinker. The swelling capacity and the release rate of aminophylline at 37 °C are reported. Maximum equilibrium swelling increases as the itaconic acid content in the hydrogel increases. The experimental data suggest clearly that the swelling process obeys second‐order kinetics. According to this, the kinetic constant, k, and the maximum equilibrium swelling, W_∞, have been calculated. Drug release from fully swollen hydrogels follows Fick's law closely, but deviates from it for xerogels. © 2001 Society of Chemical Industry     

Swelling behaviour and paracetamol release from poly(<Emphasis Type="Italic">N</Emphasis>-isopropylacrylamide-itaconic acid) hydrogels

Copolymer hydrogels of N-isopropylacrylamide and itaconic acid (IA), crosslinked with N,N′-methylenebisacrylamide, were prepared by radical copolymerization. These hydrogels were investigated with regard to their composition to find materials with satisfactory swelling and drug release properties. A paracetamol is used as a model drug to investigate drug release profile of the hydrogels. It was found that the investigated hydrogels exhibited pH- and temperature-dependent swelling behaviour with restricted swelling and lower equilibrium degree of swelling at lower pH values and temperatures above the LCST value of PNIPAM (around 34 °C). The diffusion exponent for paracetamol release indicate that the mechanism of paracetamol release are governed by Fickian diffusion, while in all release media initial diffusion coefficient was lower than late time diffusion coefficient. Furthermore, the paracetamol release rate depends on the hydrogel degree of swelling and it increased in the first stage of diffusion process, whereas was no significant difference thereafter. The presence of the IA moieties incorporated into the network weakened the shear resistance of the hydrogels. In order to calculate the pore size the characteristic ratio for PNIPAM, C _n  = 11.7, was calculated. Based on the pore size, the investigated hydrogels can be regarded as microporous. According to the obtained results swelling behaviour, mechanical properties, drug-loading capacity and the drug release rate could be controlled by hydrogel composition and crosslinking density, which is important for application of the investigated hydrogels as drug delivery systems.     

Gum Acacia‐cl‐poly(acrylamide)@carbon nitride Nanocomposite Hydrogel for Adsorption of Ciprofloxacin and its Sustained Release in Artificial Ocular Solution

In the present work, a nanocomposite hydrogel is designed consisting of gum acacia, poly(acrylamide) and carbon nitride by facile microwave approach. This nanocomposite hydrogel is sensitive to environmental stimuli which is essential for its application in environmental remediation and as a drug delivery system. The effects of carbon nitride percentage and microwave Watt variation on swelling capacity of gum acacia‐cl‐poly(acrylamide)@carbon nitride (Ga‐cl‐PAM@C₃N₄) nanocomposite hydrogel are analyzed. The structural characterizations are considered by numerous techniques such as FTIR (Fourier transform infra‐red spectroscopy), X‐ray diffraction, transmission electron microscopy, scanning electron microscopy, and elemental mapping. Batch experiment is performed for remediation of ciprofloxacin (CIP) drug from water. Various parameters such as effect of ciprofloxacin doses, Ga‐cl‐PAM@C₃N₄ nanocomposite hydrogel dosage, pH, time and temperature for adsorption of CIP on gum acacia‐cl‐poly(acrylamide)@carbon nitride nanocomposite hydrogel is examined. Maximum adsorption capacity of Ga‐cl‐PAM@C₃N₄ nanocomposite hydrogel observed is 169.49 mg g^?1 at pH 6.4. The drug loading and drug release capacity of Ga‐cl‐PAM@C₃N₄ nanocomposite hydrogel is investigated for ciprofloxacin. Drug release is monitored in artificial ocular solution (pH 8), saline (pH 5.5), acetate buffer (pH 2.2), and distilled water. Maximum drug release is observed in artificial ocular solution.     

pH–temperature responsive poly(HPA‐Co‐AMHS) hydrogel as a potential drug‐release carrier

In this study, a novel pH–temperature‐responsive copolymer was first synthesized by the radical copolymerization between HPA (2‐hydroxypropyl acrylate and 2‐hydroxyisopropyl acrylate) and AMHS (aminoethyl methacrylate hydrochloric salt). The molecular structure of the corresponding copolymer has been confirmed by ¹H‐NMR and FTIR. The lower critical solution temperature of the resulting copolymer exhibited a considerable dependence upon the ratio of monomers and pH value in the medium. On the basis of the copolymer, a hydrogel as drug release carrier was prepared via the introduction of a crosslinker, N,N′‐methylenebisacrylamide. The swelling behaviors of hydrogel in the different pH value, temperature, and NaCl concentration have indicated that the hydrogel showed a remarkable phase transition at 31.5°C. The swelling ratio was increased with an increasing of pH value, especially in the greater pH values. By the use of caffeine as a model drug, we investigated the caffeine‐controlled release from hydrogel systematically as a function of pH value, temperature, and crosslinker content. The caffeine release was sensitive to the temperature. Only 55% caffeine was released from the hydrogel at room temperature, whereas ～ 92% caffeine diffused into the medium at 37°C. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Effect of network mesh size and swelling to the drug delivery from pH responsive hydrogels

pH responsive hydrogels are ideal platforms for numerous therapeutic delivery applications, including oral delivery, as they are capable of overcoming the many barriers that must be considered when creating an effective drug delivery system. Understanding of the innate hydrogel network structure and its swelling behavior at environmentally relevant conditions is vital for designing hydrogel network capable of effective controlled drug release. Herein, we explored how to expand traditional techniques of swelling and pore characterization to gain better insight into the performance of anionic microparticles composed of the poly(methyl methacrylate-co-acrylic acid) with varying molar percentage of 10, 20, and 30 mol% of MMA, for controlled release of low-molecular-weight drugs. By evaluating these carrier systems at environmental conditions, we can observe changes in swelling and pore size of the anionic hydrogel networks as a function of MMA, which was then correlated with the release profiles of the small-molecular-weight drug sodium nitrate. With the correlation of the swelling behavior of the networks and the release profiles, we demonstrated how the expansion of swelling parameters at relevant pH values provides further incite for evaluating for the optimal blend for controlled release. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48767.     

Preparation and characterization of novel cationic copolymer hydrogels with pH sensitivity and thermosensitivity

Cationic hydrogels were synthesized through the copolymerization of N‐isopropylacrylamide and dimethylaminoethylmethacrylate. N,N′‐Methylenebisacrylamide was used as a crosslinking agent, and sodium bisulfite/ammonium persulfate was used as an initiator. The equilibrium and dynamic swelling properties were investigated to reveal the pH sensitivity and thermosensitivity of the hydrogels. The conclusion was drawn that the prepared cationic hydrogels demonstrated critical sensitivity at 37°C and pH 7.0–8.0 and that the stronger the acidity was of the buffered solution, the shorter the equilibrium swelling time was of the hydrogels. Drug‐release experiments in vitro were carried out at 37°C (close to body temperature), at pH 1.4 (close to the pH of the stomach), and at pH 7.4 (close to the pH of the intestine). The release results indicated that the drug (chloramphenicol) was released more rapidly from the prepared hydrogel in a pH 1.4 buffered solution than in a pH 7.4 one, and this was consistent with the results predicted from the experiments of the swelling kinetics. Moreover, the drug‐release process was confirmed by scanning electron micrographs of the hydrogels embedded with chloramphenicol. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 3602–3608, 2006     

Synthesis of hydrogel for drug delivery studies utilizing photoinitiator‐free photopolymerization based on the donor/acceptor pair,N‐vinylpyrrolidinone and hydroxypentyl maleimide

Photo‐differential scanning calorimetry (photo‐DSC) was used to study the rate of photoinitiator‐free copolymerization of a donor/acceptor pair involving N‐vinylpyrrolidinone (NVP), and a water‐soluble N‐substituted maleimide, hydroxypentyl maleimide (HPMI). Glucose, 1,1‐diethoxy ethane (DEE) and isopropyl alcohol (IPA) were included for the evaluation of their efficiencies as hydrogen donors, and glucose was shown to be the most efficient in enhancing the rate of polymerization. This photoinitiator (PI)‐free system was extended to hydrogel preparation using the radiation method with UV as the radiation source based on the same donor/acceptor pair, ie NVP/HPMI, in the presence of glucose as the hydrogen donor. Swelling and drug release tests showed that this hydrogel exhibited high swelling ability and a rather fast drug release rate when using theophylline as the model drug. These tests also revealed that the drug release kinetics and the water diffusion into this hydrogel did not adhere to the Fickian model. Cytotoxicity testing showed no evidence of this hydrogel being cytotoxic. © 2002 Society of Chemical Industry     

The effects of the pH value of the swelling medium on the kinetics of the swelling of a poly(acrylic acid) hydrogel

The isothermal kinetics curves of the swelling of a poly(acrylic acid) hydrogel in buffer solutions of different pH values (5, 7, and 9) at temperatures ranging from 30 to 40°C were determined. The possibilities of applying the Fick kinetics model and the Peppas equation were examined. It was found that the applicability of these models were limited. The kinetics model of a first‐order chemical reaction was found to describe the swelling kinetics of the PAA hydrogel in all the investigated buffer solution at all the investigated temperatures. Swelling kinetics is determined by the rate of expansion of the network. The kinetic parameters (E_a, ln A) of the swelling of the PAA hydrogel in buffer solutions of different pH values were determined. The activation energy and the pre‐exponential factor of the swelling of the PAA hydrogel in buffer medium decreased with increasing pH value of the swelling medium. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Swelling mechanism of porous P(VP‐co‐MAA)/PNIPAM semi‐IPN hydrogels with various pore sizes prepared by a freeze treatment

BACKGROUND: The objective of the work reported was to investigate the effect of gel microstructure on the swelling mechanism. A series of porous gels with various pore sizes were prepared by freeze‐treating a conventional hydrogel that contained various amounts of water at ? 20 °C. Scanning electron microscopy and differential scanning calorimetry were used to characterize the microstructure of the porous gels. RESULTS: The experimental results showed that the water content during freezing was the key factor controlling the microstructure. Measurement of swelling kinetics showed that a greater amount of water during the freezing process would lead to a rapid swelling rate. The apparent diffusion coefficients (D_p) at all times during the swelling process were obtained by fitting the experimental data to the diffusion equation. The values of D_p suggested that the swelling mechanism of the gels depends on the pore size. The diffusion exponent (n) obtained by fitting the fractional mass change (M_t/M_∞) to equation of Fick's law further confirmed that the swelling mechanism of the gels is determined by the pore size. CONCLUSION: The swelling mechanism of the gels is determined by the microstructure related to the pore size and the thickness of struts. The microstructure can be controlled by adjusting the water content of the hydrogels by changing the pH of the swelling medium prior to freezing. Copyright © 2008 Society of Chemical Industry     

Synthesis,Characterization, and swelling behaviors of acrylic acid/carboxymethyl cellulose superabsorbent hydrogel by glow‐discharge electrolysis plasma

This article exploits a new approach for synthesis of acrylic acid/carboxymethyl cellulose (AA/CMC) superabsorbent hydrogel in aqueous solution by a simple one‐step using glow‐discharge electrolysis plasma, in which N,N′‐methylenebisacrylamide (MBA) was used as a crosslinking agent. The reaction parameters affecting the equilibrium swelling, that is, discharge voltage, discharge time, mass ratio of AA to CMC, content of crosslinker, and degree of neutralization, were systematically optimized to achieve a superabsorbent hydrogel with a maximum equilibrium swelling. The structure, thermal stability, and morphology of AA/CMC superabsorbent hydrogel were characterized by Fourier transform infrared spectroscopy, X‐ray diffraction analysis, thermogravimetric analysis, and scanning electron microscopy. The swelling kinetics in distilled water and swelling behaviors in various pH solutions and salts solutions (NaCl, KCl, MgCl₂, CaCl₂, AlCl₃, and FeCl₃) were investigated in detail. The effect of six cationic salt solutions on the equilibrium swelling had the following order K⁺ > Na⁺ > Mg²⁺ > Ca²⁺ > Al³⁺ > Fe³⁺. In addition, the pH‐reversibility was preliminarily investigated with alternating pH between 6.5 and 2.0. The results showed that the equilibrium swelling of AA/CMC was achieved in 90 min. The hydrogel was responsive to the pH and salts, and was reversible swelling and deswelling behavior. POLYM. ENG. SCI., 54:2310–2320, 2014. © 2013 Society of Plastics Engineers     

Modified carrageenan. 4. Synthesis and swelling behavior of crosslinked κC‐g‐AMPS superabsorbent hydrogel with antisalt and pH‐responsiveness properties

To synthesize a novel biopolymer‐based superabsorbent hydrogel, 2‐acrylamido‐2‐methylpropanesulfonic acid (AMPS) was grafted onto kappa‐carrageenan (κC) backbones. The graft copolymerization reaction was carried out in a homogeneous medium and in the presence of ammonium persulfate (APS) as an initiator, N,N,N′,N′‐tetramethyl ethylenediamine (TMEDA) as an accelerator, and N,N′‐methylene bisacrylamide (MBA) as a crosslinker. A proposed mechanism for κC‐g‐AMPS formation was suggested and the hydrogel structure was confirmed using FTIR spectroscopy. The affecting variables on swelling capacity, i.e., the initiator, the crosslinker, and the monomer concentration, as well as reaction temperature, were systematically optimized. The swelling measurements of the hydrogels were conducted in aqueous solutions of LiCl, NaCl, KCl, MgCl₂, CaCl₂, SrCl₂, BaCl₂, and AlCl₃. Due to the high swelling capacity in salt solutions, the hydrogels may be referred to as antisalt superabsorbents. The swelling of superabsorbing hydrogels was measured in solutions with pH ranging 1 to 13. The κC‐g‐AMPS hydrogel exhibited a pH‐responsiveness character so that a swelling–deswelling pulsatile behavior was recorded at pH 2 and 8. The overall activation energy for the graft copolymerization reaction was found to be 14.6 kJ/mol. The swelling kinetics of the hydrogels was preliminarily investigated as well. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 255–263, 2005     

pH-sensitive hydrogel based on carboxymethyl chitosan/sodium alginate and its application for drug delivery

Carboxymethyl chitosan sodium salt (CMCS)/sodium alginate (SA), a pH-sensitive hydrogel composed of CMCS and SA crosslinked by 1-ethyl-3-(3-dimethyl aminopropyl) carbodiimide (EDC) and N-hydroxysuccinimide, has been evaluated in vitro as a potential carrier for protein drug delivery of bovine serum albumin (BSA). The crosslinked structures, pore morphologies, and mechanical properties of the composite CMCS/SA hydrogel at different pH have been characterized by Fourier-transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), and dynamic mechanical analysis (DMA). The swelling behavior of the prepared hydrogel was assessed at different pH values, 1.2, 4.0, 6.86, 7.4, and 9.0. The in vitro slow release ability of the CMCS/SA hydrogel was assessed at 37°C and pH 1.2 or pH 7.4 to simulate gastrointestinal and mouth environments in vivo. The efficiency was found to be greater than 90% at pH 7.4. The composite CMCS/SA hydrogel showed no cytotoxic effect toward L-929 cells according to the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide test. These findings demonstrate that the composite hydrogel has promising potential for drug delivery. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 46911.     

Delivery of bupivacaine included in poly(acrylamide‐co‐monomethyl itaconate) hydrogels as a function of the pH swelling medium

Copolymeric hydrogels of poly(acrylamide‐co‐monomethyl itaconate) (A/MMI) crosslinked with N,N′‐methylenbisacrylamide (NBA) were synthesized as devices for the controlled release of bupivacaine (Bp). Two compositions of the copolymer, 60A/40MMI and 75A/25MMI, were studied. A local anesthetic was included in the feed mixture of polymerization (2–8 mg Bp/tablet) and by immersion of the copolymeric tablets in an aqueous solution of the drug. A very large amount of Bp (36–38 mg Bp/tablet) was included in the gels by sorption due to interactions between the drug and the side groups of the hydrogels. Swelling and drug release were in accordance with the second Fick's law at the first stages of the processes. The swelling behavior of these copolymers depended on the pH of the medium. The equilibrium swelling degree (W_∞) was larger at pH 7.5 (W_∞ ≈ 90 wt %) than at pH 1.5 (W_∞ ≈ 52–64 wt %) due to the ionization of the side groups of the copolymer. Release of the drug also depended on the pH of the swelling medium; at pH 7.5, about 60% of the included drug was released, and at pH 1.5, about 80% was released. Bp release was controlled by the comonomer composition of the gels, their drug‐load, and the pH of the swelling medium. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 327–334, 2002     

Photopolymerised thermo-responsive poly(N,N-diethylacrylamide)-based copolymer hydrogels for potential drug delivery applications

Novel thermo-sensitive N,N-diethylacrylamide (DEAAm) based copolymer hydrogels were prepared via UV-induced free radical bulk polymerisation. UV polymerisation was employed to avoid the use of potentially toxic solvents; solution polymerisation has been the most common means for the preparation of PDEAAm-based hydrogels in the literature to date. The resultant hydrogels were analysed using nuclear magnetic resonance, Fourier transform infrared spectroscopy and modulated differential scanning calorimetry. Parameters such as the crosslinking degree and the nature of the incorporated hydrophilic component, N-vinyl-2-pyrrolidone (NVP) or N,N-dimethylacrylamide (DMAAm) were found to impact hydrogel structure, mechanical properties and swelling kinetics. Pulsatile swelling studies indicated that the hydrogels had thermo-reversible properties which were greatly affected by test temperature, nature of hydrophilic monomer used and crosslinker content. Aminophylline was selected as a model solute for drug loading and release studies by thermal deswelling in HCl buffer (pH 1.4) and phosphate buffer media (pH 6.8). The observed lag time prior to significant drug release from the more crosslinked P(DEAAm-NVP) hydrogels could make them suitable for delayed specific release in the intestine and potential alternatives to layers or membranes in time-specific and site-specific swelling-controlled drug delivery systems.     

Sustained release of potassium diclofenac from a pH‐responsive hydrogel based on gum arabic conjugates into simulated intestinal fluid

This work describes the preparation, the swelling properties and the potassium diclofenac (KDF) release profile of hydrogels of gum arabic (GA), N′,N′‐dimethylacrylamide, and methacrylic acid. In order to convert GA into a hydrogel, the polysaccharide was vinyl‐modified with glycidyl methacrylate. The hydrogels showed pH‐responsive swelling changes, which were more expressive in the basic environment. Release data of KDF were adjusted to a diffusion‐based kinetic model that provides an important insight on affinity of the drug for hydrogel and solvent, which may be the leading parameter for release of guest molecules from polymers. The KDF release from the hydrogels into simulated intestinal fluid decreases when the amount of modified GA increases. This was demonstrated to be due to the higher affinity of KDF for GA‐richer hydrogel, which makes the anti‐inflammatory release less favorable. The analysis of released drug half‐time (t_1/2 = 16.10 and 21.51 h) indicated sustained release characteristics. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43319.     

Preparation,characterization, and water‐sorption study of polyvinyl alcohol based hydrogels with grafted hydrophilic and hydrophobic segments

The interpenetrating polymer network hydrogels based on poly(vinyl alcohol) were obtained by graft copolymerization of acrylamide and styrene onto polyvinyl alcohol in the presence of N,N′‐methylene bisacrylamide as a crosslinking agent. The hydrogels were characterized by optical microscopy, scanning electron microscopy, infrared spectral analysis, differential scanning calorimeter, and thermogravimetric analysis. The hydrogels showed enormous swelling in aqueous medium and displayed swelling characteristics, which were highly dependent on the chemical composition of the hydrogels and pH of the swelling medium. The kinetics of water uptake and the mechanisms of water transport were studied as a function of composition of the hydrogel and pH of the swelling medium. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 95: 1129–1142, 2005     

β‐cyclodextrin/poly(vinyl alcohol) hydrogels containing phenylpropionic acid and naphthylamine: dual pH‐sensitive release

Hydrogel composed of β‐cyclodextrin (β‐CD) and poly(vinyl alcohol) was prepared in a strong alkaline condition using epichlorohydrin as a crosslinker. Phenylpropionic acid (PPA) and naphthylamine (NA) were loaded in the cavities of β‐CD residues to endow the hydrogel with a dual pH‐sensitive characteristic. In release experiments using fluorescein isothiocyanate‐dextran (FITC‐dextran) as a dye, PPA/NA‐loaded hydrogel exhibited an extensive release not only in acidic conditions (e.g. pH 3.0) but also in alkaline conditions (e.g. pH 9.0). PPA and NA will be highly ionized at the alkaline and the acidic pH and they could promote swelling of the hydrogel, causing an extensive release at those pH values. However, the release was suppressed at mid pH values (e.g. pH 5.0 and pH 7.4), possibly due to the formation of salt bridges between PPA^? and NA⁺. In fact, the degree of swelling at mid pH was lower than that observed at strong acidic pH and alkaline pH. According to SEM images, the pore size and the texture compactness of hydrogels which had been subjected to swelling at different pH values could also account for the dual pH‐sensitive releases. The hydrogels exhibited dual pH sensitivities in terms of FITC‐dextran release and swelling. These hydrogels might be used as a pH‐sensitive vehicle for water‐soluble drugs. © 2013 Society of Chemical Industry     

Influence of the swelling history on the swelling kinetics of stimuli-responsive poly[(N-isopropylacrylamide)-co-(methacrylic acid)] hydrogels

The influence of the swelling history on the swelling behavior of poly[(N-isopropylacrylamide)-co-(methacrylic acid)] P[(N-iPAAm)-co-(MAA)] random copolymers hydrogels synthesized by free radical polymerization in solution of N-iPAAm and MAA comonomers crosslinked with tetraethylene glycol dimethyl acrylate (TEGDMA) has been studied. The swelling behavior under pH 7 at 18, 29, 39 and 49 °C of this series of copolymers, previously soaked either at pH 2 or 7 has been investigated. The swelling kinetics of these two series of samples displays different behavior as function of the composition and temperature. However, the equilibrium swelling values only show slight dependences on the previous soaking pH and temperature. When samples are soaked at pH 7, then the swelling at pH 7 follows a first order kinetics, irrespective of the copolymer composition or the temperature at which the experiment has been carried out. In this case, the swelling process is very fast and depends only slightly on temperature. The first order rate constant increases with the MAA content in the hydrogel. Furthermore, the swelling rate of copolymer hydrogels soaked at pH 2, show strong dependence on composition and temperature. They follow an autocatalytic swelling kinetics due to the disruption of hydrogen bond arrangements. An initial slow water uptake is followed by an acceleration process, in which water molecules inside the gel help the next water molecules to come in. Two rate constants, a first-order rate constant and an autocatalytic one have been obtained from the kinetics analysis. They have revealed different temperature dependence which may be due to a balance between hydrophobic and hydrogen bond interactions. The temperature dependence of the swelling kinetics is stronger and more complex for copolymers treated under pH 2 than for copolymers soaked under pH 7.