Hydrogel properties influence ECM production by chondrocytes photoencapsulated in poly(ethylene glycol) hydrogels

Abstract

When using hydrogel scaffolds for cartilage tissue engineering, two gel properties are particularly important: the equilibrium water content (q, equilibrium swelling ratio) and the compressive modulus, K. In this work, chondrocytes were photoencapsulated in degrading and nondegrading poly(ethylene glycol)-based hydrogels to assess extracellular matrix (ECM) formation as a function of these gel properties. In nondegrading gels, the glycosaminoglycan (GAG) content was not significantly different in gels when q was varied from 4.2 to 9.3 after 2 and 4 weeks in vitro. However, gels with a q of 9.3 allowed GAGs to diffuse throughout the gels homogenously, but a q < or = 5.2 resulted in localization of GAGs pericellularly. Interestingly, in the moderately crosslinked gels with a K of 360 kPa, an increase in type II collagen synthesis was observed compared with gels with a higher (960 kPa) and lower (30 kPa) K after 4 weeks. With the incorporation of degradable linkages into the network, gel properties with an initially high K (350 kPa) and final high q (7.9) were obtained, which allowed for increased type II collagen synthesis coupled with a homogenous distribution of GAGs. Thus, a critical balance exists between gel swelling, mechanics, and degradation in forming a functional ECM.

Mechanical confinement regulates cartilage matrix formation by chondrocytes

“…In faster relaxing gels, however, elastic stresses are dissipated, and chondrocytes form wide regions of cartilage matrix that become interconnected. A similar trend is observed in elastic hydrogels that are engineered to be degradable 12,49 . While it is difficult to compare these approaches, as degradation causes changes of other physical properties, the similarities broadly suggest that engineered degradation and faster relaxation represent two complementary approaches to improving cartilage matrix formation in hydrogels.…”

“…The elastic moduli of the hydrogels used in these studies ranges from 2 kPa to 100 kPa 12,14,20,21 , which is comparable to the moduli of chondrocyte PCM, ranging from 10 kPa to 75 kPa 29 . Enhanced stiffness in covalently crosslinked PEG based hydrogels, found to exhibit minimal stress relaxation here, restricted production of sGAG and collagens, distribution of cartilage matrix, and proliferation of chondrocytes 20,21 .…”

“…While there have been many mechanical property measurements of viscoelasticity in cartilage 36 , chondrons 37,38 , and chondrocytes 39,40 , this mechanotransduction study examined how the matrix viscoelasticity impacts chondrocyte behavior. Previous studies have found limited proliferation and deposition of cartilage matrix by chondrocytes encapsulated in non-degradable or slowly-degrading hydrogels of PEG 12 , agarose 13 , alginate 41 and hyaluronic acid 22 . Similar behaviors were observed for chondrocytes encapsulated in slow relaxing alginate hydrogels.…”

“…Various hydrogels, such as poly-ethylene glycol 12 , agarose 13 , alginate 9 , hyaluronic acid 14 , and collagen 15 have been used for 3D culture of chondrocytes, and some aspects of cartilage tissue have been replicated in these hydrogels 15 . However, proliferation of chondrocytes can be limited 16 and cartilage matrix deposition often occurs only immediately adjacent to the cells leading to islands of cartilage matrix within a sea of hydrogel 15,17 .…”

“…However, proliferation of chondrocytes can be limited 16 and cartilage matrix deposition often occurs only immediately adjacent to the cells leading to islands of cartilage matrix within a sea of hydrogel 15,17 . The source of limited proliferation and cartilage matrix formation may be the elastic nature of hydrogels typically used, as elastic stresses in these hydrogels would be expected to resist cell proliferation and the formation of cartilage matrix 12,15 . Recent studies have found that viscoelastic hydrogels, in which stresses are relaxed over time, promote spreading and proliferation of adherent cells, and formation of an interconnected bone matrix by osteogenically differentiated MSCs in 3D culture 18,19 .…”