Effect of pericellular matrix formation by chondrocytes cultured in agarose gel on the viscoelastic properties of agarose gel matrix

The viscoelasticity of chondrocyte-seeded agarose gel (AGC0) and that of chondrocyte-seeded agarose gel after 21 days of cultivation (AGC3) were investigated. In AGC3, pericellular matrix (PCM)-like material around each chondrocyte was found to be constructed, which was confirmed by an optical micrograph in conjunction with toluidine blue staining. The relaxation modulus of each of the chondrocyte-agarose gel composite systems was measured by a non-constrained indentation method. Stress-strain curves for all of the specimens examined had a toe region followed by a linear region terminated by specimen fracture. The slope of the linear region of AGC0 was smaller than that of AG, while the SS curve of AGC0 was indistinguishable from that of AGC3. All of the relaxation curves studied were typical of gels, having a fast relaxation process up to 10³ s followed by a plateau. The relaxation modulus of AGC0 was smaller than that of agarose gel (AG), the decrement in relaxation modulus from AG to AGC0 being attributed to the seeding of chondrocytes that have a smaller modulus than that of agarose gels. However, the relaxation modulus of AGC3 was increased at the early viscoelastic region in particular, as compared with that of AGC0. The increments in the relaxation modulus in AGC3 were attributed to the PCM-like material produced by chondrocytes, where the produced material may provide crosslink points and reinforce the agarose gel.