Glucose-sensitivity of glucose oxidase-containing cationic copolymer hydrogels having poly(ethylene glycol) grafts
Introduction
Glucose-sensitive membranes and hydrogels can be used effectively to achieve feedback-controlled release of insulin. These polymeric gels demonstrate a reversible swelling behavior in response to glucose concentrations in the surrounding medium. The resulting changes in the mesh size modulate release of insulin physically imbibed in the gels. Since insulin diffusion through these gels is dependent on the changes in mesh size caused by the varying concentration of glucose in the medium, these gels are effective in bringing about feedback-responsive release.
Glucose oxidase (GOD) immobilization on polymers has been investigated in detail in attempts to produce materials for the construction of new types of glucose sensors [1]. Work has also been published on the use of GOD for stimulation of pH-responsive release from polymer matrices and membranes [2]. GOD has been successfully immobilized on a wide variety of polymers, such as polyacrylates [3], polymethacrylates [4], polyethylene [5], polypyrroles [6], silica [7], and poly(vinyl alcohol) [8], [9]. The activity of immobilized enzymes is an important parameter that determines the performance of the hydrogel membrane. In most cases, there is a significant loss of enzyme activity due to harsh methods employed during immobilization and polymerization. Stability of these enzymes is also a major concern as irreversible degradation might occur with time [9].
The next important criterion determining the feasibility of these gels in insulin delivery is their optimal performance under physiological conditions. Heller and co-workers [10], [11] observed that the glucose-sensitivity of poly(ortho esters) could be improved by incorporating tertiary amines into the structure. Albin et al. [12] studied GOD-immobilized polyacrylamide gels and poly(dimethylaminoethyl methacrylate) (PDMAEM) gels in the form of macroporous and microporous matrices. The kinetics of solute transport in these membranes was found to be limited by the solubility of oxygen in the surrounding medium.
The studies conducted by Goldraich and Kost [13] on GOD-immobilized copolymers of 2-hydroxyethyl methacrylate (HEMA) and dimethylaminoethyl methacrylate (DEAEM) crosslinked with tetra(ethylene glycol) dimethacrylate (TEGDMA) indicated very slow deswelling rates. These gave rise to significant gel irreversibilities as the gels were unable to recover their original conformation when transferred between two solutions containing varying concentrations of glucose. This was further confirmed by Ishihara and co-workers using GOD-immobilized polyamides [14], [15] and polymethacrylates [16]. The slow collapse was attributed to the small range of pH variation (between 6.2 and 6.6) attained in the matrix.
In our previous work [17], [18], we studied the pH sensitivity of poly(diethylaminoethyl-g-ethylene glycol) (P(DEAEM-g-EG)) hydrogels. It was shown that these hydrogels had a distinct transition behavior, characterized by a steep change in swollen volume at pH 7.0. Below this pH, the hydrogels exhibited high swelling ratios. Above the transition pH, the hydrogels were relatively collapsed with much smaller swelling ratios. Both of these states were related to their corresponding mesh sizes The ionization of the tertiary amine side groups under these conditions resulted in the swelling of the polymer through two mechanisms: (i) increase in the hydrophilicity of the polymer and (ii) electrostatic repulsion between the positively charged groups. In the absence of glucose, the pH in the microenvironment of the gels was found to remain constant at 7.4 and the gel remained collapsed. For applications, two different geometries were studied, namely, discs and microparticles. Of course, the dynamics of the pH-sensitive swelling/deswelling behavior was considerably faster in the case of microparticles.
In our research, GOD was immobilized in the hydrogel to impart glucose-sensitivity to the matrix. To improve the rate of reaction and, therefore, the swelling rates within the gel matrix, one option was to immobilize catalase in the hydrogel along with GOD. Low conversions and reduced rates of reactions might be due to the inhibitory effect of hydrogen peroxide that is formed as a byproduct of the glucose oxidation reaction. Catalase reduces hydrogen peroxide and eventually removes it from the system. By this reaction, some oxygen is recovered and made available for the formation of acid. Thus, an efficient redox system is set up within the gel which operates to minimize unwanted products and provide maximum availability of reactants for the formation of hydrogen ions [12].
Incorporation of poly(ethylene glycol) grafts on the main chains of the hydrogel was an innovative approach based on previous work done in our laboratory [19]. These grafts were expected to retard the degradation of enzymes and proteins within the gel. For an internally implantable device, the presence of certain ‘stealth’ groups helps to minimize the immunoreaction and subsequent rejection by the body. PEG is known to have such stealth properties, which can help maximize the lifetime of the hydrogel in the body [20].
In this work, the glucose sensitivity of P(DEAEM-g-EG) hydrogels was investigated. The effect of a glucose stimulus and the eventual decrease in the pH of the hydrogel on swelling were studied. The change in the swelling ratio and the mesh size under a glucose stimulus were determined.
Section snippets
Experimental
The preparation of discs and microparticles of glucose-sensitive P(DEAEM-g-EG) hydrogels involves the copolymerization of functionalized enzymes, methacrylate comonomers and crosslinking agent. GOD and catalase were functionalized before they were added to the monomer mixture. This was done by dissolving 0.01 g of GOD and 175 μl of catalase (both obtained from Sigma, St. Louis, MO, USA) in 5 ml of pH 7.4 phosphate buffer solution. To this solution, 2 μl of acryloyl chloride (Aldrich, Milwaukee,
Results and discussions
The behavior of glucose-sensitive P(DEAEM-g-EG) hydrogels in a glucose environment was studied. As these hydrogels would operate under feedback conditions, the swelling properties were expected to depend on the glucose concentration. In our earlier studies, the swelling rates of hydrogel discs were found to be considerably slow. Microparticles of the same materials proved to be not only effective but also sufficiently controllable, so that insulin release from them could be easily modulated.
Conclusions
The glucose-sensitive characteristics of glucose oxidase and catalase containing P(DEAEM-g-EG) gels were investigated. It was established that the enzymes in the gels were active and could produce an acidic environment within the hydrogel causing the network to swell. Catalase was found to increase the rate of swelling of the gel significantly. The effects of the parameters, such as crosslinking ratio and enzyme loading, on the kinetics of the reaction were established. Higher enzyme loading
Acknowledgements
This work was supported in part by grants from the Showalter Foundation (Indianapolis, IN) and the Purdue Research Foundation.
References (22)
- et al.
Second generation biosensors
Biosens. Bioelect.
(1991) - et al.
Modified PMMA monosize microbeads for glucose oxidase immobilization
Chem. Eng. J.
(1997) Chemically self-regulated drug delivery systems
J. Control. Release
(1988)- et al.
Release of insulin from pH-sensitive poly(ortho esters)
J. Control. Release
(1990) - et al.
Glucose sensitive membranes for controlled delivery of insulin: insulin transport studies
J. Control. Release
(1985) - et al.
Glucose-sensitive polymeric matrices for controlled drug delivery
Clin. Mater.
(1993) - et al.
Glucose-sensitive membranes for controlled release of insulin
- et al.
Radiation grafting of acrylic acid onto polytetrafluoroethylene films for glucose oxidase immobilization and its application in membrane biosensor
J. Membr. Sci.
(1993) - et al.
Glucose oxidase immobilized polyethylene-g-acrylic acid membrane for glucose oxidase sensor
Biotech. Bioeng.
(1990) - et al.
Immobilization of glucose oxidase in thin polypyrrole films: influence of polymerization conditions and film thickness on the activity and stability of the immobilized enzyme
Biotech. Bioeng.
(1993)
Characterization and application of glucose oxidase immobilized in silica hydrogel
J. Chem. Tech. Biotech.
Cited by (201)
Biomedical applications of stimuli-responsive “smart” interpenetrating polymer network hydrogels
2024, Materials Today BioSmart stimuli-responsive chitosan hydrogel for drug delivery: A review
2023, International Journal of Biological MacromoleculesComplementing the circular economy with a life cycle assessment of sustainable hydrogels and their future prospects
2023, Sustainable Hydrogels: Synthesis, Properties, and Applications3D bioprinting of emulating homeostasis regulation for regenerative medicine applications
2023, Journal of Controlled ReleaseResponsive biomaterials for 3D bioprinting: A review
2022, Materials TodayCitation Excerpt :For instance, glucose oxidase and catalase were incorporated into pH-responsive cationic poly(diethylaminomethyl methacrylate-g-ethylene) glycol gels. The activated glucose oxidase converted glucose into gluconic acid, lowered the pH of the environment, and induced sol–gel transitions [93]. Nanomaterials such as gold and SiO2-gold nanoshells have been incorporated into temperature-responsive interpenetrating polymer networks to synthesize thermally responsive nanocomposite systems.
Recent advances in the smart insulin delivery systems for the treatment of diabetes
2021, European Polymer Journal
- 1
Present address: BioArray Solutions, 120 Centennial Avenue, Piscataway, NJ 08854, USA.
- 2
Present address: Department of Chemical Engineering, University of Delaware, Newark, DE 19716, USA.