Exploration of a Modular Microcapsule Scaffold System for Cartilage Tissue Engineering

Shallow defects in human articular cartilage defects do not heal, and defects that reach the subchondral bone are replaced with fibrous tissue that lacks desired mechanical properties. Autologous cartilage grafts cause donor site morbidity, and joint replacements require complex surgeries. This study investigated a modular tissue engineering approach for cartilage repair. Rat bone marrow mesenchymal stem cells (MSCs) were encapsulated in microcapsules made of complexes of chitosan with blends of hyaluronan (HA) and chondroitin-4-sulfate (CSA), and an optional interior of collagen type I gel. Chitosan is biocompatible and biodegradable, and HA and CSA are glycosaminoglycans (GAGs) found in cartilage extracellular matrix (ECM) that have been shown to enhance the chondrogenic differentiation of MSCs in terms of the ECM the chondrocytes produce. The capsules are formed by dripping a suspension of MSCs in GAG solution into a stirring chitosan solution. An ionic complex of the cationic chitosan and the anionic GAGs forms at the droplet boundaries, trapping the cells inside spherical membranes with diameters on the order of hundreds of micrometers. These capsules can be fused together in molds to potentially form large tissue constructs. Capsules made with a blend of HA and CSA (HA/CSA) had spherical walls, and encapsulated MSCs formed multiple aggregates within 24 hours. Capsules made of HA had thick, wrinkled walls, and the encapsulated MSCs remained dispersed. MSCs in collagen-containing capsules formed single, large aggregates. By day 7, the HA-collagen capsules exhibited a 38% reduction in diameter, likely due to contraction of the collagen gel by MSCs, which may be desirable for creating dense tissue cultures. When cultured in chondrogenic medium, the MSC aggregates in HA/CSA capsules became smooth but remained as distinct aggregates, while the HA capsules filled in with opaque material, and underwent a 39% diameter reduction over 28 days. At 21 days, the capsules contained 82±4, 81±3, 69±6, and 106±16 pg collagen/cell seeded in the HA growth, HA chondrogenic, HA/CSA growth, and the HA/CSA chondrogenic cultures, respectively. Histological staining with safranin-O and toluidine blue showed the production of proteoglycans in all culture conditions. The contraction of the HA capsules and the high collagen production in HA/CSA capsules make both capsule types promising for further study.