Abstract
Improving the safety efficacy ratio of existing drugs is a current challenge to be addressed rather than the development of novel drugs which involve much expense and time. The efficacy of drugs is affected by a number of factors such as their low aqueous solubility, unequal absorption along the gastrointestinal (GI) tract, risk of degradation in the acidic milieu of the stomach, low permeation of the drugs in the upper GI tract, systematic side effects, etc. This review aims to enlighten readers on the role of pH sensitive hydrogels in drug delivery, their mechanism of action, swelling, and drug release as a function of pH change along the GI tract. The basis for the selection of materials, their structural features, physical and chemical properties, the presence of ionic pendant groups, and the influence of their pKa and pKb values on the ionization, consequent swelling, and targeted drug release are also highlighted.
Highlights
Aside from the use of pH sensitive hydrogels in biomedical applications, they are being effectively exploited in the engineering field as microfluidic valves [122] to control the flow of liquids owing to their swelling and deswelling in response to the pH
In the case of sustained release drug delivery systems, the drug is entrapped in the carrier matrix and a small portion of it is released in the stomach and the remaining portion is released in a controlled manner in the intestine
Biocompatibility, biodegradability, and non-toxicity are the main attributes of any material to be used for biomedical applications
Summary
The term hydrogel dates back to 1894 when this term was used by Lee et al [1] for colloidal gels of certain inorganic salts. The first ever hydrogel reported in 1949 for biomedical implant was poly(vinyl alcohol) cross-linked with formaldehyde and marketed with the trade name Ivalon [3]. Later in 1958, Danno prepared poly(vinyl alcohol) hydrogels cross-linked by passing gamma irradiation through an aqueous solution [4]. The synthesis of poly(2-hydroxyethyl methacrylate) (pHEMA) gels for contact lens application by Wichterle and Lim [5] in 1960 was the revolution for present day hydrogels. The development of hydrogels for biomedical applications gained particular attention in the 1970s especially in the field of stimuli sensitive hydrogels, the so called smart hydrogels of modern times. Hydrogels especially gained much attention for organ and tissue reparative and regenerative matrices in the field of tissue engineering [1]
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