pH-Dependent friction of Polyacrylamide hydrogels

Polyacrylamide hydrogels are widely used in biomedical applications due to their tunable mechanical properties and charge neutrality. Our recent tribological investigations of polyacrylamide gels have revealed tunable and pH-dependent friction behavior. To determine the origins of this pH-responsiveness, we prepared polyacrylamide hydrogels with two different initiating chemistries: a reduction-oxidation (redox)-initiated system using ammonium persulfate (APS) and N,N,N’N’-tetramethylethylenediamine (TEMED) and a UV-initiated system with 2-hydroxy-4’-(2-hydroxyethoxy)-2-methylpropiophenone (Irgacure 2959). Hydrogel swelling, mechanical properties, and tribological behavior were investigated in response to solution pH (ranging from ~ 0.34 to 13.5). For polyacrylamide hydrogels in sliding contact with glass hemispherical probes, friction coefficients decreased from µ = 0.07 ± 0.02 to µ = 0.002 ± 0.002 (redox-initiated) and from µ = 0.05 ± 0.03 to µ = 0.003 ± 0.003 (UV-initiated) with increasing solution pH. With hemispherical polytetrafluoroethylene (PTFE) probes, friction coefficients of redox-initiated hydrogels similarly decreased from µ = 0.06 ± 0.01 to µ = 0.002 ± 0.001 with increasing pH. Raman spectroscopy measurements demonstrated hydrolysis and the conversion of amide groups to carboxylic acid in basic conditions. We therefore propose that the mechanism for pH-responsive friction in polyacrylamide hydrogels may be credited to hydrolysis-driven swelling through the conversion of side chain amide groups into carboxylic groups and/or crosslinker degradation. Our results could assist in the rational design of hydrogel-based tribological pairs for biomedical applications from acidic to alkaline conditions.

Methods
Microindentations and sliding experiments were conducted with a custom-built linear reciprocating tribometer. Data was analyzed and processed with the following Matlab codes (Tribometer_Indent_Analysis_v5 and Compiling_Friction_Data_v7). 

Raman spectroscopy was conducted with a confocal Raman microscope (Horiba Jobin Yvon T640000) with an excitation wavelength of 488 nm and laser power of 400 mW. A 50x objective lens was used to focus the laser light 10 µm below the sample surface. Spectra data was obtained with a resolution of 0.66 cm-1.  

X-ray photoelectron spectroscopy was conducted with a Thermo Scientific ESCALABTM Xi+. A monochromatic Al/Kα x-ray radiation with a pass energy of 100 eV, dwell time of 50 ms, and spot size of 650 µm was used. Spectra were obtained after 105 s of etching using a 6 kV Ar1000+ cluster ion source.