pH‐thermoreversible hydrogels. I. Synthesis and swelling behaviors of the (N‐isopropylacrylamide‐co‐acrylamide‐co‐2‐hydroxyethyl methacrylate) copolymeric hydrogels

A series of pH‐thermoreversible hydrogels that exhibited volume phase transition was synthesized by various molar ratios of N‐isopropylacrylamide (NIPAAm), acrylamide (AAm), and 2‐hydroxyethyl methacrylate (HEMA). The influence of environmental conditions such as temperature and pH value on the swelling behavior of these copolymeric gels was investigated. Results showed that the hydrogels exhibited different equilibrium swelling ratios in different pH solutions. Amide groups could be hydrolyzed to form negatively charged carboxylate ion groups in their hydrophilic polymeric network in response to an external pH variation. The pH sensitivities of these gels also depended on the AAm content in the copolymeric gels; thus the greater the AAm content, the higher the pH sensitivity. These hydrogels, based on a temperature‐sensitive hydrogel, demonstrated a significant change of equilibrium swelling in aqueous media between a highly solvated, swollen gel state and a dehydrated network response to small variations of temperature. pH‐thermoreversible hydrogels were used for a study of the release of a model drug, caffeine, with changes in temperature. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 221–231, 1999     

Photo‐cross‐linked biodegradable thermo‐ and pH‐responsive hydrogels for controlled drug release

A new strategy was developed to prepare thermo‐ and pH‐sensitive hydrogels by the crosslinking of poly(N‐isopropylacrylamide) with a biodegradable crosslinker derived from poly(L ‐glutamic acid). Hydrogels were fabricated by exposing aqueous solutions of precursor containing photoinitiator to UV light irradiation. The swelling behaviors of hydrogels at different temperatures, pHs, and ionic strengths were examined. The hydrogels shrank under acidic condition or at temperature above their collapse temperature and would swell in neutral or basic media or at lower temperature. These processes were reversible as the pH or temperature changed. All hydrogels exhibited no weight loss in the simulated gastric fluid but degraded rapidly in the simulated intestinal condition. Bovine serum albumin were used as a model protein drug and loaded into the hydrogels. The in vitro drug release experiment was carried out at different pH values and temperatures. The pH and temperature dependent release behaviors indicated the promising application of these materials as stimuli‐responsive drug delivery vehicles. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Swelling characterizations of chitosan and polyacrylonitrile semi‐interpenetrating polymer network hydrogels

Temperature‐ and pH‐responsive semi‐interpenetrating polymer network (semi‐IPN) hydrogels constructed with chitosan and polyacrylonitrile (PAN) were studied. The characterizations of semi‐IPN hydrogels were investigated using Fourier transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC). IPN hydrogels exhibited a relatively high swelling ratio, 23.31%–145.20% at room temperature. The swelling ratio of hydrogels depends on pH and temperature. DSC was used to determine the amount of free water in IPN hydrogels. The amount of free water increased with increasing chitosan content in the semi‐IPN hydrogels. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 87: 2011–2015, 2003     

Preparation of pH‐sensitive poly(vinyl alcohol‐g‐methacrylic acid) and poly(vinyl alcohol‐g‐acrylic acid) hydrogels by gamma ray irradiation and their insulin release behavior

A series of pH‐responsive hydrogels were studied as potential drug carriers for the protection of insulin from the acidic environment of the stomach before releasing in the small intestine. Hydrogels based on poly(vinyl alcohol) networks grafted with acrylic acid or methacrylic acid were prepared by a two‐step process. Poly(vinyl alcohol) hydrogels were prepared by gamma ray irradiation (50 kGy) and then followed by grafting either acrylic acid or methacrylic acid onto these poly(vinyl alcohol) hydrogels with subsequent irradiation (5–20 kGy). These graft hydrogels showed pH‐sensitive swelling behavior and were used as carriers for the controlled release of insulin. The in vitro release of insulin was observed for the insulin‐loaded hydrogels in a simulated intestinal fluid (pH 6.8) but not in a simulated gastric fluid (pH 1.2). The release behavior of insulin in vivo in a rat model confirmed the effectiveness of the oral delivery of insulin to control the level of glucose. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 636–643, 2004     

Simple,convenient, low‐cost,and solventless greener way to pH‐responsive polymeric hydrogels: Synthesis and characterization

Utilization of hydrogels in a simple, low‐cost, solventless. and greener approach toward the pH‐responsive hydrogels which comprise of citric acid (CA) with varying glycol unit viz., ethylene glycol (EG), diethylene glycol (DEG), and triethylene glycol (TEG) were prepared along with methacrylic acid (MAA). The formations of pre‐polymer and hydrogels were confirmed using ¹³C‐NMR and FT‐IR spectral techniques. Thermal studies (TGA, DTA, and DSC) and morphology (SEM) of various hydrogels have been investigated. Swelling studies of hydrogels at different pH ranging from 4.0 to 10.0 have also been performed. The results of swelling studies imply that the percentage of swelling is comparatively higher at neutral pH than acidic and alkaline pH. The reciprocal relationship was identified among thermal stability and swelling behavior of hydrogels while increasing the chain length from EG to TEG. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41921.     

Synthesis and characterization of fast responsive thermo‐ and pH‐sensitive poly[(N,N‐diethylacrylamide)‐co‐(acrylic acid)] hydrogels

BACKGROUND: A considerable amount of research has been focused on smart hydrogels that can respond to external environmental stimuli, especially temperature and pH. In this study, fast responsive thermo‐ and pH‐sensitive poly[(N,N‐diethylacrylamide)‐co‐(acrylic acid)] hydrogels were prepared by free radical copolymerization in aqueous solution using poly(ethylene glycol) (PEG) as a pore‐forming agent. RESULTS: Swelling studies showed that the hydrogels produced had both temperature and pH sensitivity. The deswelling kinetics at high temperature demonstrated that the shrinking rates were influenced by the addition of the pore‐forming agent and the amount of acrylic acid in the initial total monomers. The deswelling curves in low‐buffer solutions had two stages. Pulsatile swelling studies indicated that the PEG‐modified hydrogels were superior to the normal ones. These different swelling properties were further confirmed by the results of scanning electron microscopy. CONCLUSION: Such fast responsive thermo‐ and pH‐sensitive hydrogels are expected to be useful in biomedical fields for stimuli‐responsive drug delivery systems. Copyright © 2008 Society of Chemical Industry     

Synthesis and network parameters of hydrophobic poly(N‐[3‐(dimethylaminopropyl)]methacrylamide‐co‐lauryl acrylate) hydrogels

Hydrophobic poly(N‐[3‐(dimethylaminopropyl)]methacrylamide‐co‐lauryl acrylate) [P(DMAPMA‐co‐LA)] hydrogels with different LA content were synthesized by free‐radical crosslinking copolymerization of corresponding monomers in water by using N,N‐methylenebis(acrylamide) as the crosslinker, ammonium persulfate as the initiator, and N,N,N′,N′‐tetramethylethylenediamine as the activator. The swelling equilibrium of the hydrogels was investigated as a function of temperature and hydrophobic comonomer content in pure water. An interesting feature of the swelling behavior of the P(DMAPMA‐co‐LA) hydrogels with low LA content was the reshrinking phase transition where the hydrogels swell once and collapse as temperature was varied in the range of 30–40°C. The average molecular mass between crosslinks (M?_c) and polymer–solvent interaction parameter (χ) of the hydrogels were calculated from equilibrium swelling values. The enthalpy (ΔH) and entropy (ΔS) changes appearing in the χ parameter for the hydrogels were determined by using the Flory–Rehner theory based on the phantom network model of swelling equilibrium. The positive values for ΔH and ΔS indicated that the hydrogels had a positive temperature‐sensitive property in water, that is, swelling at a higher temperature and shrinking at a lower temperature. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 4159–4166, 2006     

Synthesis and characterization of poly{N‐[3‐(dimethylamino) propyl] methacrylamide‐co‐itaconic acid} hydrogels for drug delivery

A novel pH‐sensitive hydrogel system composed of itaconic acid (IA) and N‐[3‐(dimethylamino) propyl] methacrylamide was designed. This system was prepared by aqueous copolymerization with N,N‐methylene bisacrylamide as a chemical crosslinker. The chemical structure of the hydrogels was characterized by Fourier transform infrared (FTIR) spectroscopy. The microstructure and morphology of the hydrogels were evaluated by X‐ray diffraction (XRD) and scanning electron microscopy (SEM). The SEM study of hydrogels on higher magnification revealed a highly porous morphology with uniformly arranged pores ranging from 40 to 200 μm in size. XRD analysis revealed the amorphous nature of the hydrogels, and it was found that an increase in the IA content in the monomer feed greatly reduced the crystallinity of the hydrogels. Swelling experiments were carried out in buffer solutions at different pH values (1.2–10) at 37°C ± 1°C to investigate their pH‐dependent swelling behavior and dimensional stability. An increase in the acid part (IA) increased the swelling ratio of the hydrogels. Temperature‐sensitive swelling of the hydrogels was investigated at 20–70°C in simulated intestinal fluid. The hydrogels swelled at higher temperatures and shrank at lower temperatures. 5‐Aminosalicylic acid (5‐ASA) was selected as a model drug, and release experiments were carried out under simulated intestinal and gastric conditions. 5‐ASA release from the poly N‐[3‐(dimethylamino) propyl] methacrylamide‐co‐itaconic acid‐80 (PDMAPMAIA‐80) hydrogel was found to follow non‐Fickian diffusion mechanism under gastric conditions, and a super case II transport mechanism was found under intestinal conditions. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Swelling behavior of semi‐interpenetrating polymer network hydrogels composed of poly(vinyl alcohol) and poly(acrylamide‐co‐sodium methacrylate)

Interpenetrating polymer network (IPN) hydrogels based on poly(vinyl alcohol) (PVA) and poly(acrylamide‐co‐sodium methacrylate) poly(AAm‐co‐SMA) were prepared by the semi IPN method. These IPN hydrogels were prepared by polymerizing aqueous solution of acrylamide and sodium methacrylate, using ammonium persulphate/N,N,N¹,N¹‐tetramethylethylenediamine (APS/TMEDA) initiating system and N,N¹‐methylene‐bisacrylamide (MBA) as a crosslinker in the presence of a host polymer, poly(vinyl alcohol). The influence of reaction conditions, such as the concentration of PVA, sodium methacrylate, crosslinker, initiator, and reaction temperature, on the swelling behavior of these IPNs was investigated in detail. The results showed that the IPN hydrogels exhibited different swelling behavior as the reaction conditions varied. To verify the structural difference in the IPN hydrogels, scanning electron microscopy (SEM) was used to identify the morphological changes in the IPN as the concentration of crosslinker varied. In addition to MBA, two other crosslinkers were also employed in the preparation of IPNs to illustrate the difference in their swelling phenomena. The swelling kinetics, equilibrium water content, and water transport mechanism of all the IPN hydrogels were investigated. IPN hydrogels being ionic in nature, the swelling behavior was significantly affected by environmental conditions, such as temperature, ionic strength, and pH of the swelling medium. Further, their swelling behavior was also examined in different physiological bio‐fluids. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 302–314, 2005     

Characterization of poly(N‐(hydroxymethyl)methacrylamide‐ATU) hydrogels synthesized by γ radiation

Poly(N‐(hydroxymethyl)methacrylamide‐1‐allyl‐2‐thiourea), (poly(NHMMA‐ATU)) hydrogels were synthesized by γ radiation, using ⁶⁰Co γ source at different radiation doses, to change the porosity and crosslinking density of the hydrogels. The percent of 1‐allyl‐2‐thiourea (ATU) in the monomer mixture before the irradiation was varied between 2.5% and 10.0%, to increase the content of ATU, which was involved in some different applications in the hydrogels. Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), elemental analysis, and the swelling experiments were used to characterize the poly(NHMMA‐ATU) hydrogels synthesized in this study. Characterization results of hydrogels showed that crosslinking density of the hydrogels was increased by the increasing radiation dose and ATU content in the irradiated mixture. Swellability of these hydrogels was found to be high enough to allow the metal ions and biomolecules getting inside the hydrogels to interact with all active groups on/in the hydrogels in the adsorption applications. Equilibrium swelling ratio of the hydrogels at pH 0.5 is at least half of the equilibrium swelling ratio of the hydrogels at pH 7.0. Oscillatory swelling behavior of poly(NHMMA‐ATU) hydrogels between pH 0.5 and pH 7.0 showed that the hydrogels are quite stable at different pH conditions. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 1657–1664, 2006     

Thermo‐ and pH‐sensitive poly(vinylmethyl ether)/carboxymethylchitosan hydrogels crosslinked using electron beam irradiation or using glutaraldehyde as a crosslinker

BACKGROUND: Stimuli‐sensitive or intelligent hydrogels have been investigated for many biomedical and pharmaceutical applications. Those hydrogels with dual sensitivity will have more extensive potential applications. The aim of the work presented was to prepare a series of thermo‐ and pH‐sensitive hydrogels based on poly(vinylmethyl ether) (PVME) and carboxymethylchitosan (CMCS). The hydrogels were crosslinked using electron beam irradiation (EB) or using glutaraldehyde (GA) as a crosslinker at room temperature. RESULTS: The structures of the PVME/CMCS hydrogels obtained using the two crosslinking methods are proposed. The effects of component polymer ratio, GA content, irradiation dose, temperature and pH on the swelling behavior of the PVME/CMCS hydrogels were studied. There is a sharp decrease in the swelling ratios when the temperature increases from 25 to 37 °C. At low pH and also at high pH, the hydrogels have a higher swelling ratio; however, deswelling occurs evidently at a pH of around 3. CONCLUSION: The study shows that both EB and GA crosslinked hydrogels are thermo‐ and pH‐ sensitive, simultaneously. Thus, they may be potential candidates for both thermo‐ and pH‐sensitive applications. Copyright © 2009 Society of Chemical Industry     

Copolymers of acrylic acid with N‐vinylpyrrolidinone and 2‐hydroxyethyl methacrylate as pH‐responsive hydrogels synthesized through a photoinitiator‐free photopolymerization technique

pH‐sensitive hydrogels for biomedical applications were synthesized using a photoinitiator‐free technique involving the initiation of photopolymerization by donor/acceptor pairs. The differential photocalorimetric technique indicated a high polymerization rate for the N‐vinylpyrrolidinone (NVP, donor)/acrylic acid (AA, acceptor) pair at a 1:1 molar ratio. However, photopolymerization of larger quantities of these monomers (1:1 molar ratio) produced a water‐soluble polymer. Nevertheless, an anionic hydrogel was successfully formed when a small quantity of 2‐hydroxyethyl methacrylate (HEMA) was included in the NVP/AA formulation. A mixture of HEMA and AA, although both are classified as acceptors, photopolymerized to produce a copolymer which functioned as an anionic hydrogel. The swelling and drug release of these hydrogels were investigated in acidic, neutral and basic pH environments. Their biocompatibility with HaCaT human epidermal keratinocyte cells was tested and a positive cell growth as evidenced by the 3‐[4,5‐dimethylthiazol‐2‐yl]‐2,5‐diphenyl tetrazolium bromide (MTT) cell proliferation assay indicated that these hydrogels have no toxic effect on HaCaT. Copyright © 2006 Society of Chemical Industry     

pH/temperature‐responsive semi‐IPN hydrogels composed of alginate and poly(N‐isopropylacrylamide)

Amino semitelechelic poly(N‐isopropylacrylamide) (PNIPAAm) was prepared by radical polymerization with aminoethanethiol hydrochloride as a chain‐transfer agent. Semi‐interpenetrating polymer network (semi‐IPN) hydrogels, composed of alginate and amine‐terminated PNIPAAm, were prepared by crosslinking with calcium chloride. From the swelling behaviors of semi‐IPNs at various pH's and Fourier transform infrared spectra at high temperatures, the formation of a polyelectrolyte complex was confirmed from the reaction between carboxyl groups in alginate and amino groups in modified PNIPAAm. Semi‐IPN hydrogels reached an equilibrium swelling state within 24 h. The water state in hydrogels, investigated by differential scanning calorimetry, showed that sample CAN55 [alginate/PNIPAAm (w/w) = 50/50] exhibited the lowest equilibrium water content and free water content among the hydrogels tested, which was attributed to its more compact structure compared to other samples and the high content of interchain bonding within the hydrogels. Alginate/PNIPAAm semi‐IPN hydrogels exhibited a reasonable sensitivity to the temperature, pH, and ionic strength of swelling medium. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 1128–1139, 2002     

pH‐ and temperature‐responsive IPN hydrogels based on soy protein and poly(N‐isopropylacrylamide‐co‐sodium acrylate)

pH‐ and temperature‐responsive interpenetrating polymer network (IPN) hydrogels based on soy protein and poly(N‐isopropylacrylamide‐co‐sodium acrylate) were successfully prepared. The structure and properties of the hydrogels were characterized by Fourier transform infrared spectroscopy, scanning electron microscopy, differential scanning calorimetry, and thermogravimetric analyzer. The equilibrium and dynamic swelling/deswelling behaviors and the drug release properties of the hydrogels responding to pH and/or temperature were also studied in detail. The hydrogels have the porous honeycomb structures, good miscibility and thermal stability, and good pH‐ and temperature‐responsivity. The volume phase transition temperature of the hydrogels is ca. 40°C. Changing the soy protein or crosslinker content could be used to control the swelling behavior and water retention, and the hydrogels have the fastest deswelling rate in pH 1.2 buffer solutions at 45°C. Bovine serum albumin release from the hydrogels has the good pH and temperature dependence. The results show that the proposed IPN hydrogels may have potential applications in the field of biomedical materials such as in drug delivery systems. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39781.     

Swelling behavior of poly(acrylamide‐co‐N‐vinylimidazole) hydrogels under different environment conditions

A series of random copolymers of acrylamide and N‐vinylimidazole, poly(AAm‐co‐NVI), with various compositions were prepared using redox copolymerization. The influence of environmental conditions such as pH, temperature, and ionic strength on the swelling behavior of the copolymeric hydrogels was investigated. The hydrogels exhibited the highest equilibrium swelling in basic medium at high temperature. Equilibrium swelling decreased with rising ionic strength at pH 5.0. As pH increased, equilibrium swelling of the hydrogels increased at pH 11.0 and I = 0.20 M. Swelling kinetics of the hydrogels was found to be non‐Fickian at 25°C. The process tended to be Fickian at higher pH and temperature. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 1783–1788, 2005     

Preparation and characterization of a novel pH‐, thermo‐, and ionic strength‐responsive hydrogels based on xanthan gum–poly(aspartic acid)

A series of interpenetrating polymer networks (IPN) hydrogels with different compositions that based on xanthan gum (XG) and poly(aspartic acid) (PASP) were synthesized. The effects of various external surrounding stimuli, including pH, temperature, and ionic strength on XG–PASP hydrogels swelling properties were investigated. Chemical structural changes of the IPN hydrogels were characterized by Fourier transform infrared spectroscopy (FT‐IR), differential scanning calorimetry (DSC), transmission electron microscopy (TEM), and swelling ratio measurement. The swelling process was found to be a Fickian diffusion and reached swelling equilibrium quickly. It was found that the feed composition of PASP was an important factor that affected the properties of IPN hydrogels. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Swelling and hydrolytic degradation behaviour of pH‐responsive hydrogels of poly[(N‐isopropylacrylamide)‐co‐(methacrylic acid)] crosslinked by biodegradable polycaprolactone chains

BACKGROUND: Stimuli‐responsive hydrogels are typically obtained from non‐biodegradable monomers. The use of biodegradable crosslinkers can overcome this limitation. In this context, the main aim of this work was to use modified polycaprolactone as a crosslinker in the preparation of pH‐responsive hydrogels based on N‐isopropylacrylamide and methacrylic acid to give poly[(N‐isopropylacrylamide)‐co‐(methacrylic acid)] (P(N‐iPAAm‐co‐MAA)). RESULTS: Poly(caprolactone) dimethacrylate macromonomer was synthesized and successfully employed as crosslinker with various ratios in the synthesis of well‐known pH‐responsive hydrogels of P(N‐iPAAm‐co‐MAA). The swelling properties of these degradable hydrogels were investigated. They practically do not swell at pH = 2, but exhibit a very high swelling capacity in distilled water and in solutions of pH = 7. In addition, degradation studies at pH = 12 showed that the hydrolysis of the ester groups in the polycaprolactone chains produces, after a relatively short time, the total solubilization of the polymer chains. CONCLUSION: The hydrogels under study have certain characteristics that could make them good candidates for use as matrices in controlled drug delivery. On the one hand, they do not swell in acid pH solution (stomach conditions) but they swell extensively at neutral pH. On the other hand, they became rapidly water soluble following degradation. Copyright © 2009 Society of Chemical Industry     

Electrical actuation of electroresponsive hydrogels based on poly(acrylamide‐co‐acrylic acid)/graphite suitable for biomedical applications

A pH‐sensitive composite hydrogel based on poly(acrylamide‐co‐acrylic acid)/graphite was prepared by solution polymerization process in presence of redox initiator potassium persulfate/N,N,N′,N′‐tetramethylethylenediamine and cross‐linker (ethylene glycol dimethylacrylate). The structures of the hydrogels were confirmed using Fourier transform infrared, X‐ray diffraction, and scanning electron microscopy (SEM) study. Tensile strengths of the hydrogels were determined by using a universal tensile machine, whereas the electrical conductivities of the hydrogels were evaluated using Four‐probe method. The influence of cross‐linker, graphite content, and temperature on the conductivity of the hydrogel was also investigated. The bending behavior of the conducting hydrogels was investigated by exposing the hydrogels under electric field in aqueous medium. By studying the swelling ratio of the polymer synthesized under different conditions, optimization conditions were found for a polymer with the highest swelling ratio. Also, the hemolytic potentiality test revealed that prepared hydrogels are biocompatible in nature. POLYM. COMPOS., 35:27–36, 2014. © 2013 Society of Plastics Engineers     

Swelling of pH‐sensitive chitosan–poly(vinyl alcohol) hydrogels

The swelling properties of different chitosan–poly(vinyl alcohol) (PVA) hydrogels were investigated as functions of the medium pH and salt concentration. The maximum swelling ability of the hydrogels was at a buffer pH of approximately 3, regardless of the PVA content in the hydrogels. The maximum mass of the swollen hydrogels was about 13 times that of their contracted counterparts. The cyclical swelling and contraction between pH 3 and pH 7 buffers and pH 3 and pH 3 buffers with salt confirmed the Donnan swelling mechanism of these hydrogels. The swelling mechanism was considered the transfer of water molecules driven by a concentration gradient. This was represented by a simplified mass‐balance model, which neglected the effect of the ionization reaction, for the initial swelling period. The effective mass‐transfer coefficient of water molecules during swelling, estimated with this model, gradually decreased with increasing PVA content in the hydrogels. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 4665–4671, 2006     

Release of vitamin B12 from poly(N‐vinyl‐2‐pyrrolidone)‐crosslinked polyacrylamide hydrogels: A kinetic study

Hydrogels, composed of poly(N‐vinyl‐2‐pyrrolidone) and crosslinked polyacrylamide, were synthesized and the release of vitamin B₁₂ from these hydrogels was studied as a function of the degree of crosslinking and pH of the external swelling media. The three drug‐loaded hydrogel samples synthesized with different crosslinking ratios of 0.3, 0.7, and 1.2 (in mol %) follow different drug‐release mechanisms, that is, chain relaxation with zero‐order, non‐Fickian and Fickian, or diffusion‐controlled mechanisms. To establish a correlation between their swelling behavior and drug‐release mechanism, the former was studied by the weight‐gain method and, at the same time, the concentration of the drug released was studied colorimetrically. Various swelling parameters such as the swelling exponent n, gel‐characteristic constant k, penetration velocity v, and diffusion coefficient D were evaluated to reflect the quantitative aspect of the swelling behavior of these hydrogels. Finally, the drug‐release behavior of the hydrogels was explained by proposing the swelling‐dependent mechanism. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 1706–1714, 2000