THE INFLUENCE OF SWELLING ON LOCAL ELASTIC PROPERTIES OF POLYACRYLAMIDE HYDROGELS

Abstract

Polyacrylamide (PAM)hydrogelsarecommonly usedas soft cell culture substrates for cell mechanical and mechanobiological studiesbecause oftheir tunable stiffness,easeof handling, transparent natureand surface functionalization to promote cell adhesion. The dependence of bulk rheological and local elastic properties (for example, as assessed by means of Atomic Force Microscopyor AFM) of PAM hydrogels onmonomer and cross-linkerconcentrationsand on polymerization temperature havebeen extensively investigated.PAM hydrogels are known to swell, which may affect their elastic properties and therefore may complicate the interpretation ofcell culturingexperiments because of a lack of control of substrate stiffness. Direct measurements of the effect of swelling on PAM elastic properties are scarce. We report here, for the first time, the direct observation of swelling (by measuring the volumetric swelling ratio) and its influence onlocal elastic properties, as measuredby AFM. Bulk rheological measurements were performed as well to enable the comparison between local and global elastic properties during and after hydrogel polymerization. Four PAM hydrogel compositions were considered,with corresponding storage shear moduliof 4530 Pa(termed stiffest), 2900 Pa(stiff), 538 Pa(soft)and 260 Pa(softest) as measured immediately after polymerization. These values as well as values obtained during hydrogel polymerization were found to be in good agreement with the local elastic moduli derived from AFM and assuming hydrogel incompressibility. After polymerization, the hydrogels were subjected to swelling conditions over six days in phosphate buffered salineand swelling ratios and local elastic moduli were measured each day.Additional short term measurements (at 0, 3, 6, 9, 12 and 24 hours) were performed for the soft and softest gelsto study their swelling kinetics in more detail. Swelling ratio and local elastic modulus did not change with time for the stiffest and stiff gels, while for the soft and softest gels substantialchanges between Day 0 and Day 1were found for both swelling ratio (21.6%and 133% increase for soft and softest gel respectively) and local elastic modulus (33.7%and 33.3% decrease for soft and softest gel respectively). Experimental data werefurther analysed theoretically by combining models of ideal elastomeric gels with a poroelastic swelling kinetics model, which confirmed the validity of observed trends with respect to literature data. The results demonstrate that swelling can have an important effect on PAM elastic properties and must be taken into account when using PAM as a cell culture substrate, particularly for PAM gels with low monomer and cross-linker concentrations