pH-Responsive Carbopol Electrospun Fiber Membranes for Biomedical Applications

Electrospun fiber membranes are versatile materials with significant potential for biomedical and chemical industries. They exhibit very high surface area and porosity, which facilitates interaction with ambient environments and response to external stimuli. Electrospun fibers can be formed in complex shapes, in homogenous and multi-layer (core-sheath) structures, and can include various functional agents released in a controlled manner.¹ One important consideration is the pH of the surrounding media due to key role of pH in many biological functions. pH-responsive materials can be used for various sensing and treatment modalities. For example, fiber membranes have been developed² containing a mixture of pH-sensitive Eudragit polymers that dissolve in specific pH conditions and release drug payloads in targeted locations of the GI system.
We investigated the properties of pH-responsive electrospun nanofibers incorporated with biocompatible/degradable Carbopol^®, commonly used in pharmaceuticals and personal care products. Carbopol® 974P NF is a polyacrylic acid polymer (CH₂-CHCO₂H)n highly cross-linked with allyl pentaerythritol. Because of the high viscosity, it has been used as a liquid thickening, extended release, bioadhesion or stabilizing agent. Additionally, the low toxicity of Carbopol® makes it a very versatile mucosal- and skin-compatible material for use in oral health and topical applications. Carbopol^® is an interesting polymer for use in electrospun membranes due to its ability to modulate viscosity as a function of pH.³ Carbopol^® was dispersed by sonication into an electrospinning solution used to form fibers with even distribution throughout the host polyvinylpyrrolidone (PVP) fiber. The resultant Carbopol^®/PVP membranes show rapid hydration into a composite hydrogel within 15 min. Increased absorption of buffer solution was observed as pH became more basic. At pH 6 the membrane mass was ~30x the initial dry mass, reaching a 40x increase at pH 8. Pore size of the membrane was assessed using capillary flow porometry, indicating that as the Carbopol^® fraction in the fibers increased from 0 to 50%, the pore size increased from 6 to 8µm. Porosity was measured by comparative densities and was found to vary minimally (88-90%). Regarding pH buffering capacity of the gel, as the Carbopol^® fraction of the membrane is increased, the pH of the gel trends towards pH 4. Spreadability is a combination of viscoelastic properties and the structural properties of polymer network. The spreadability of the resultant hydrogel was dependent on the concentration of Carbopol and the starting pH of the buffer solution. At low starting pH of 4, Carbopol^® concentration has a minimal effect on spreadability while maintaining solution pH. At high starting pH of 8 increasing Carbopol^® concentration results in significant reduction of the gel pH, which in turn increases its spreadability. When applying a basic solution of pH 8 to the membrane, high Carbopol^® loadings result in extreme quenching of pH towards pH4, resulting in high-spreadable (low viscosity) gels. Conversely, low Carbopol^® loadings result in minimal pH change with a low spreadability gel (high viscosity). These findings provide guidelines for rational designs of pH responsive Carbopol^® fibers for various applications, including drug delivery, wound dressing, contraceptive devices, and prevention of sexually transmitted diseases.

1. Han, D.; Steckl, A. J., Coaxial Electrospinning Formation of Complex Polymer Fibers and their Applications. ChemPlusChem 2019, 84 (10), 1453-1497.
2. Han, D.; Steckl, A. J., Selective pH-Responsive Core–Sheath Nanofiber Membranes for Chem/Bio/Med Applications: Targeted Delivery of Functional Molecules. ACS Appl. Mater. Interfaces 2017, 9 (49), 42653-42660.
3. Curran, S. J.; Hayes, R. E.; Afacan, A.; Williams, M. C.; Tanguy, P. A., Properties of Carbopol Solutions as Models for Yield-Stress Fluids. Journal of Food Science 2006, 67 (1), 176-180.