Comparative study of the viscoelastic mechanical behavior of agarose and poly(ethylene glycol) hydrogels

Abstract

This study presents a comparative investigation into differences in the mechanical properties between two hydrogels commonly used in cartilage tissue engineering [agarose vs. poly(ethylene glycol) (PEG)], but which are formed through distinctly different crosslinking mechanisms (physical vs. covalent, respectively). The effects of hydrogel chemistry, precursor concentration, platen type (nonporous vs. porous) used in compression bioreactors, and degradation (for PEG) on the swelling properties and static and dynamic mechanical properties were examined. An increase in precursor concentration resulted in decreased equilibrium mass swelling ratios but increased equilibrium moduli and storage moduli for both hydrogels (p < 0.05). Agarose displayed large stress relaxations and a frequency dependence indicating its viscoelastic properties. Contrarily, PEG hydrogels displayed largely elastic behavior with minimal stress relaxation and frequency dependence. In biodegradable PEG hydrogels, the largely elastic behavior was retained during degradation. The type of platen did not affect static mechanical properties, but porous platens led to a reduced storage modulus for both hydrogels implicating fluid flow. In summary, agarose and PEG exhibit vastly different mechanical behaviors; a finding largely attributed to differences in their chemistries and fluid movement. Taken together, these design choices (hydrogel chemistry/structure, loading conditions) will likely have a profound effect on the tissue engineering outcome.

Mechanical characterization of oligo(ethylene glycol)-based hydrogels by dynamic nanoindentation experiments

“…This is in good agreement with recent results reported by Shapiro and Oyen (Shapiro and Oyen, 2014) and Roberts et al (Roberts et al, 2011), who showed that the elastic modulus of PEG hydrogels increases with increasing the total polymer concentration.…”

“…n agreement with other reports (Nayar et al, 2012;Oyen, 2013;Roberts et al, , the storage modulus of the hydrogel samples increased with increasing polymer concentration. This is evidenced by the lower ‫'ܧ‬ obtained for H30/700 in comparison to those obtained for H50/750 and H50/1000 (see Table 2).…”

“…This observation is consistent with measurements performed by Roberts et al (Roberts et al, 2011) on similar poly(ethylene glycol) (PEG) hydrogels using dynamic unconfined compression in the frequency range of 0.01 to 10 Hz. PEGDA (~1700 kPa) (Kohn and Ebenstein, 2013) indentation.…”

“…The error bars span one standard deviation about the mean and can the (Nayar et al, 2012;Oyen, 2013;Roberts et al, , the storage modulus of the hydrogel samples increased with ained for sample 750 and H50/1000 (see Table 2). In ) and Loss Factor ‫) ߜ݊ܽݐ(‬ (b) show the values of loss modulus and loss factor measured for all samples analyzed in this work.…”

“…Dynamic Mechanical Analysis (DMA) was used by Cauich-Rodriguez et al (Cauich-Rodriguez et al, 1996) to characterize the viscoelastic properties of hydrogel blends over the frequency range of 0.1 to 50 Hz. Additionally, uniaxial confined or unconfined compression has been extensively used due to the ease of sample preparation and simple test methodology (Iza et al, 1998;Roberts et al, 2011;Thomas et al, 2004). While these test methods are among the types needed to investigate the hydrogels constitutive behavior in the time and frequency domains, the wide variety also reflects the fact that each technique has limitations.…”