The Effect of Hyper-Physiological Mechanical Loading on an Osteochondral Microfluidic Organ-on-a-Chip Model and Shear Modulus of Cartilage Organoids

Abstract

Osteoarthritis (OA) is a degenerative multi-tissue disease of the articular joint, with abnormal loading considered as one of the risk factors. Current 3D in vitro models are limited by their inability to deposit neo-bone and neo-cartilage in interaction, while simultaneously allowing the study of mechanical loading on the osteochondral compartment. In this work, a microfluidic osteochondral organ-on-a-chip (OoC) model was redesigned and fabricated to study the effect of mechanical loading on the osteochondral compartment inside the microfluidic chip. Additionally, the effect of hyper-physiological loading on the shear modulus of chondrocyte pellets and its relation with anabolic cartilage markers ACAN and COL2A1 was investigated.
After successful fabrication of the microfluidic OoC model that enables application of consistent mechanical load onto the osteochondral construct inside the chip, an injurious hyper-physiological loading regime was applied. To accomplish this, a computational model was developed to correlate the elastic strain in the cartilage tissue construct with the indentation applied to the chip. Gene expression analysis via quantitative reverse transcription polymerase chain reaction (RT-qPCR) was used to assess the effects of this hyper-physiological loading regime on the chip and compared to mechanically loaded chondrocytes pellets. The presence of anabolic genes, ACAN and COL2A1, in the microfluidic chip, along with comparable levels of the cartilage homeostasis marker FRZB relative to the chondrocyte pellet model, indicated successful chondrogenesis inside the chip. Furthermore, significant upregulation of mechanical loading marker PTGS2 confirmed stress levels were achieved in the chondrocytes inside the chip model. Upon hyper-physiological mechanical loading, the chip model showed significant upregulation of ACAN, COL2A1 and FRZB 12 hours post-mechanical loading. However, in the pellet model, ACAN and FRZB were downregulated as a result of mechanical loading. Hyper-physiological mechanical loading also increased the shear modulus of chondrocytes pellets, with a significant effect observed 7 days post-mechanical loading. Furthermore, a positive correlation was found between COL2A1 expression and the shear modulus. This study demonstrates that the developed microfluidic OoC model can serve as a platform to test the effect of hyper-physiological mechanical loading of neo-bone and neo-cartilage in interaction. Future studies should enhance the model by improving the integration of the luers and bonding the microfluidic model to an incompressible material while exploring the effects of a more intense loading regime.