Structuring Hydrogel Surfaces for Tribology

Gombert, Yvonne; Simič, Rok; Roncoroni, Fabrice; Dübner, Matthias; Geue, Thomas; Spencer, Nicholas D.

doi:10.1002/admi.201901320

Cited by 48 publications

(55 citation statements)

References 33 publications

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“…4. The result is qualitatively also very similar to the results with 7.5 wt.% PAAm hydrogels obtained using the same reciprocating setup in our previous work [18,24]. For the self-mated, brushy hydrogels in pin-on-disc contact, the coefficient of friction varied between 0.02 and 0.03 and was almost speed independent, as shown in Fig.…”

Section: Frictionsupporting

confidence: 90%

“…The curves obtained on the surfaces of the glass-molded hydrogels followed a Hertzian contact model, indicating a homogeneous structure with an elastic modulus of 29 kPa ± 1 kPa. On the other hand, the PS-molded hydrogel showed a much softer initial response, with a progressive increase in the slope of the indentation curve beyond the Hertzian regime, as also observed in our recent work [18,24]. Fitting the Hertzian contact model to the initial 1 µm of the indentation depth gives an elastic modulus below 0.1 kPa.…”

Section: Infrared Spectroscopysupporting

confidence: 84%

“…The solution was carefully poured into piranha-cleaned glass petri dishes or into sterilized polystyrene (PS) petri dishes to a height of approximately 3-4 mm. The two different molding materials were utilized in order to produce hydrogels with two distinct surfaces, being either dense and crosslinked in the case of the glass mold, or sparse and brushy in the case of the PS mold [18,23,24]. The molds, filled with the solution, were polymerized for 20 min under UV light with an intensity of about 1.4 mW/cm 2 at a wavelength of 365 nm (Stratalinker UV Crosslinker 2400, Stratagene Corp., La Jolla, CA, USA).…”

Section: Methodsmentioning

confidence: 99%

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Importance of Hydration and Surface Structure for Friction of Acrylamide Hydrogels

et al. 2020

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To understand the dissipative mechanisms in soft hydrogel lubrication, polyacrylamide (PAAm) hydrogels with two distinct surface structures were examined under various contact conditions. The characteristic speed-dependent friction of the selfmated, crosslinked hydrogel surfaces could be explained by hydrodynamic shearing of a thin water layer between two rather impermeable bodies. On the other hand, the frictional response of brushy hydrogel surfaces is dependent on the contact conditions and the level of surface hydration. In a migrating contact, brushy hydrogels showed low, speed-independent friction (µ ~ 0.01) likely due to a thick layer of shearing liquid trapped within the sparse surface network. In stationary contact, however, brushy hydrogel surfaces can partially exude water from the near-surface region over time, as shown by timeresolved Fourier-transform infrared (FTIR) spectroscopy. This is assumed to be reflected in a friction increase over time. Interfacial shearing appears to shorten the characteristic exudation times compared to those observed under static loading. Once fluid has been exuded, brushy surfaces were shown to reach similar friction values as their crosslinked analogs. The results thus indicate that the dominating dissipation mechanism during sliding at low contact pressures is shearing of the interfacial liquid film, rather than poro-elastic dissipation within the bulk. Maintenance of surface hydration is therefore crucial, in order to take advantage of the low friction of such systems.

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Section: Frictionsupporting

confidence: 90%

Section: Infrared Spectroscopysupporting

confidence: 84%

Section: Methodsmentioning

confidence: 99%

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Importance of Hydration and Surface Structure for Friction of Acrylamide Hydrogels

et al. 2020

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“…This assumption has recently been scrutinized due to the variety of mold materials available, as well as an early discovery of a less-dense surface layer [84]. For a single chemistry of polyacrylamide prepared in an identical protocol with the exception of the mold material on a spectrum between hydrophilic glass and hydrophobic bulk polymers, a softer surface extending down micrometers was discovered to be associated with the more hydrophobic molds [85,86]. The softer surface was hypothesized to be sparse at its outer extent and increase in density toward the bulk, where the "bulk" being some effective screening distance from the mold (Fig.…”

Section: Mold Materials and Imparted Propertiesmentioning

confidence: 99%

Review: Friction and Lubrication with High Water Content Crosslinked Hydrogels

et al. 2020

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As soft aqueous hydrogels have moved from new materials to the basis for real engineered devices in the last 20 years, their surface friction and lubrication are emerging as critical aspects of their function. The flexibility to alter and augment their mechanical and surface properties through control of the crosslinked 3D polymer networks has produced materials with diverse surface behaviors, even with the relatively simple composition of a single monomer and crosslink chemistry. Correspondingly with new understandings of the bulk behavior of hydrogels has been the identification of the mechanisms that govern the lubricity and frictional response under dynamic sliding conditions. Here we review these efforts, closely examining and identifying the internal and external influences that drive tribological response in high water content crosslinked hydrogels. The roles of surface structure, elasticity, contact response, charge, water interaction and water flow are addressed here as well as current synthesis and testing methods. We also collect open questions as well as the future needs to fully understand and exploit the surface properties of hydrogels for sliding performance.

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“…In hydrogels, as the water content increases, the mesh size (ξ) of the hydrogel network also increases [2,21,32]. The increase in mesh size causes a dramatic decrease in the elastic modulus (E ∝ ξ −3 ) [33,34] of the gel and also causes a reduction in the friction coefficient (µ ∝ ξ −1 ) [21]. Increasing mesh size thereby reduces the frictional shear stress (τ ∝ ξ −3 ) by increasing the contact area, decreasing the contact pressure, and dropping the friction coefficient [2].…”

Section: Introductionmentioning

confidence: 99%

Surface Gel Layers Reduce Shear Stress and Damage of Corneal Epithelial Cells

et al. 2020

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Soft contact lenses are medical devices made from aqueous polymeric gels that are worn on the eye to correct refractive errors. These devices interrupt the natural contact pairing between the cornea and the eyelid and create two interfaces comprised of a synthetic material and the epithelia-contact lens surfaces versus (1) the cornea and (2) the eyelid conjunctiva. The cellular responses to friction and shear stress are thought to contribute to contact lens discomfort. This study performs direct contact shear experiments using in vitro biotribological experiments using a microtribometer equipped with a hydrogel membrane probe. Sections from commercial contact lenses are held in place on a spherically capped membrane probe during reciprocating sliding experiments against confluent monolayers of living human telomerase-immortalized corneal epithelial cells (hTCEpi). The contact lenses were loaded against the cell monolayers to physiological contact pressures between 400 and 1300 Pa under an applied load of 200 µN. The reciprocating distance was 3 mm, at a sliding speed of 1 mm/s, and the maximum duration of sliding was 1000 cycles. Five commercially available lenses (somofilcon A, stenfilcon A, etafilcon A, verofilcon A, and delefilcon A) were used to evaluate the cell layer responses to aqueous gels of differing composition, surface modulus, and lubricity. Cell damage was measured via propidium iodide staining and in situ fluorescence microscopy. The shear stresses varied from 16 ± 2 Pa (delefilcon A and verofilcon A) to 86 ± 12 Pa (stenfilcon A), and cell damage increased with increasing shear stress and increasing sliding duration. The two lens materials that have high water content surface gel layers (delefilcon A and verofilcon A) showed distinctly lower measures of cell damage as compared to the other lenses. Surface gel layers with a large polymer mesh size and high water content are shown to be an effective approach to lower the contact pressure, lower the friction coefficient, and thereby lower the shear stress and cell damage.

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Structuring Hydrogel Surfaces for Tribology

Cited by 48 publications

References 33 publications

Importance of Hydration and Surface Structure for Friction of Acrylamide Hydrogels

Importance of Hydration and Surface Structure for Friction of Acrylamide Hydrogels

Review: Friction and Lubrication with High Water Content Crosslinked Hydrogels

Surface Gel Layers Reduce Shear Stress and Damage of Corneal Epithelial Cells

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