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.

Agarose and poly(ethylene-glycol) (PEG) are commonly used as scaffolds for cell and tissue engineering applications [1]. Agarose is a natural biomaterial that is thought to be inert [2] and permits growing cells and tissues in a three-dimensional suspension [3]. Gels synthesized from photoreactive poly(ethylene glycol) (PEG) macromonomers are well suited as cell carriers because they can be rapidly photopolymerized in vivo by a chain radical polymerization that is not toxic to cells, including chondrocytes. This paper explores the differences in mechanical behavior between agarose, a physically cross-linked hydrogel, and PEG, a chemically cross-linked hydrogel, to set the foundation for choosing hydrogel properties and chemistries for a desired tissue engineering application.