On the Lubrication Mechanism of Surfaces Covered with Surface‐Attached Hydrogels

Abstract

The compressive and tribological behavior of chemically crosslinked, surface-attached hydrogel layers has been investigated by indentation and friction tests using an atomic force microscope provided with a colloidal probe, where the probe is covered with a chemically identical hydrogel layer. The hydrogel layers are composed of a polydimethyl acrylamide copoly mer containing methacryloyl benzophenone units which is photochemically crosslinked and bound to substrates carrying self-assembled monolayers of a benzophenone group containing silane. The compression and friction behavior of the thus generated surface-attached hydrogel samples, which due to the surface attachment can only swell in one dimension, are studied as a function of fi lm thickness and crosslink density. It is found that the pressure-induced deswelling in the contact region dominates the friction between two surfaces coated with surface-attached hydrogels and that the rate of loading and the fi lm thickness determine the tribological properties, especially when the layer thicknesses are lower than 1 μm. properties of materials has evolved from more empiric, macroscopic investigations in the direction of molecularly oriented studies at the micro-or even nanoscale. [ 1,2 ] A very attractive research topic in bio-tribology is to elucidate how natural joints obtain their unique friction an wear properties. Joints like human hips or knees are designed to last (hopefully) for a life time and provide low friction values and good wear properties despite that under certain circumstances the forces, both normal and shear forces, acting on the joints are very high. This is especially the case during sports activities such as long-distance running, weight lifting, or skiing or upon impact such as landing with a parachute to name just a few examples. In all these cases, strong forces act upon the joints and occasional peak loads occur, putting strong demands on the stability of the assembly and onto a reduction of the friction. The coeffi cient of friction (COF) between cartilage surfaces can be as low as μ = 0.001, which, despite strong advances in the development of lubricants, can be hardly duplicated even by the most sophisticated technological means on such a macroscopic level. Breakdown of the lubrication process can lead to wear of the cartilage, and more than 50% of the population may eventually suffer osteoarthritic pains

Probing the frictional properties of soft materials at the nanoscale

“…hydrogel particle) to an AFM cantilever, friction between ultra-soft systems at the nanoscale was not measured until 2016. It was only recently, when Li et al 33 were able to perform FFM experiments between a hydrogel probe and hydrogel surface. They used an elegant approach where a polystyrene colloidal probe was coated with a poly(dimethyl acrylamide-co-methacryloyl oxybenzophenone) (PDMAA-co-MABP)-based hydrogel layer to elucidate lubrication on a hydrogel substrate.…”

“…Subsequently, Matzelle et al 30 studied a hard probe/soft surface, while Bogdanovic et al 31 used materials other than glass to create a relatively soft cellulose-based colloidal probe. Kim et al 32 used a soft colloidal probe/soft surface system for the first time to study friction on hydrogel contact lenses, while Li et al 33 used a hydrogel probe/hydrogel surface to understand the lubrication mechanism of crosslinked hydrogel layers.…”

“…The left side of the figure illustrates FFM studies using a sharp tip, either silica, [58][59][60][61][62][63][64][65][66][67][68][69][70] or silicon nitride. 32,42,68, On the right side of the figure are shown the FFM studies using a colloidal probe made from: collagen; 96 latex; 97 polyethylene (PE); 25,28,45,98 polyethylene glycol (PEG); 99 poly(methyl methacrylate) PMMA; 26,27,95,100 polystyrene (PS); 33,71,101 cellulose; 31,[102][103][104][105][106][107][108][109] and silica. 35,36,98,[110][111][112][113][114][115][116][117] The volume of the symbol corresponds to the number of studies using the specific syste...…”

“…5b). 33 However, on hydrated hydrogels, friction is significantly lower than that in the dry hydrogels and progressively increases with the sliding velocity due to increasing polymer concentration in the contact area during shearing. This suggests that in hydrated hydrogels the viscous drag of water within the hydrogel at the interface is the main cause of hydration lubrication.…”

Review: Friction and Lubrication with High Water Content Crosslinked Hydrogels

“…The Gemini contact (hydrogel-hydrogel contact) of the surface-attached hydrogel (Fig. 4a, b) at nanoscale (thickness can be as low as 260 nm) largely depends on the load, loading rate, polymer to crosslink density, and the local solvent deswelling dynamics through the porous polymer network [67]. In a static compression test, thinner layers have higher local relaxation zone compared to the total layer thickness and thus, the surface behaves stiffer (Fig.…”