Sliding Friction of Zwitterionic Hydrogel and Its Electrostatic Origin

Abstract

Polyzwitterionic materials, which have both cationic and anionic groups in each repeating unit of polymer, show excellent antibiofouling properties. In this study, the surface friction of carboxybetaine type zwitterionic hydrogels, poly(N-(carboxymethyl)-N,N-dimethyl-2-(methacryloyloxy)-ethanaminium, inner salt) (PCDME), against glass substrates were investigated in aqueous solutions. The friction measurement was performed using a rheometer with parallel plate geometry and the sliding interface was monitored during the measurement. The frictional stress on glass was high in water and it showed weak dependence on pressure as long as the two sliding surfaces were in complete contact. The results performed in solutions with varied ionic strength revealed that the high friction on glass substrates has an electrostatic origin. The electrostatic potential measurement revealed that the PCDME gels have an isoelectric point at pH 8.5. Since the glass substrates carrying negative charges in pure water, the gel and the glass have electrostatic attraction in water. Study on the effect of pH has shown that below pH 8.5, attraction between the positively charged gels and negatively charged glass gives high friction, while above pH 8.5, the electrical double layer repulsion between two negatively charged surfaces gives low friction. From these results, it is concluded that although the PCDME gels behave like neutral gels in the bulk properties, their surface properties sensitively change with pH and ionic strength of the medium. ■ INTRODUCTIONPolyzwitterionic materials, which have both cationic and anionic groups in each repeating unit of polymer, are drawing great attention as biomaterials due to their excellent antibiofouling properties and biocompatibility. 1−10 The antibiofouling materials that prevent the attachment of bioorganisms in wet environment are in a great demand for advanced technologies. Recently a tough double network (DN) hydrogel has been developed by using poly(2-acrylamido-2-methylpropanesulfonic acid) (PAMPS) as the first network and a carboxybetaine type zwitterion poly(N-(carboxymethyl)-N,Ndimethyl-2-(methacryloyloxy)ethanaminium, inner salt) (PCDME) as the second network. 11 Comparing to the conventional double network (DN) hydrogels that use neutral polyacrylamide or poly(dimethyl acrylamide) as the second network, 12 the DN hydrogel using the zwitterionic polymer as the second network exhibits excellent antibiofouling properties in addition to high mechanical strength and toughness. 11 This promises a great potential of the zwitterion-based hydrogels as better candidate for bioapplication, for example, as coating materials of low-friction biomedical devices and implants. Understanding the surface frictional properties of the zwitterion hydrogels is indispensable for these potential applications.As the water-swollen hydrogels have many common features to that of internal organs in our body, the sliding friction of hydrogels in aqueous solution has recently been drawing great scientific attention. 13−24...

Nanoporous Substrate‐Infiltrated Hydrogels: a Bioinspired Regenerable Surface for High Load Bearing and Tunable Friction

“…In Figure d, we also report a hydrogel surface potential (PCDME), which we use to approximately describe our AA‐hydrogel ζ ‐potential. The pH‐regulated electrostatic interaction (proportional to the electrostatic repulsion p r ) between gel and PDMS surface is proportional to the product of their surface potential . The latter is shown in Figure d as a function of the solution pH.…”

“…Copyright 2013, Elsevier), for plasma‐oxidized PDMS (Reproduced with permission . Copyright 2013, Elsevier), and for a PCDME hydrogel (Reproduced with permission . Copyright 2014.…”

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