The Squeeze Pressure Bioreactor: Design and Modelling of a Non-Contact Device for Mechanical Stimulation of Tissue Engineered Constructs

Diseases of hyaline cartilage represent one of the major health problems, especially in industrialized countries with high life expectancy. The erosion of the articulating surfaces of joints, known as osteoarthritis, currently affects more than 200 million citizens worldwide, and more than 50% of the patients need or will need a surgical treatment. Articular cartilage is a three dimensional avascular tissue, which covers the ends of all synovial joints. During normal daily function, articular cartilage can be repeatedly subjected to forces up to several time body weight, but it is able to provide articulating joints with a nearly frictionless motion. Despite its tremendously important function, articular cartilage has limited capacity for auto regeneration after degenerative and rheumatic diseases, like arthritis, as well as traumatic injuries. Cartilage problems are a huge and still unsolved medical issue, which therefore represents one of the most important tissue engineering targets requiring high quality products as fast as possible. For this reason, the possibility to recreate in vitro cartilage substitutes as a real alternative to total joint replacement represents an increasing and hopeful market, in which many research groups are still working. At the moment, one of the main findings in invitro cartilage studies is the importance of the role of mechanical stimuli and dynamic loads for the chondrocytes growth and differentiation. Several studies using cartilage explants or chondrocytes seeded in 3D scaffolds have shown that mechanical compressive loads affect the cells metabolic activity and their matrix production. In order to simulate the in vivo environment, the use of bioreactors is becoming fundamental: bioreactors can provide the chemical and mechanical signals that optimize tissue development. Furthermore, bioreactors could be an important instrument to reduce the cost of clinical studies, used as in vitro predictors of in vivo performance. In this way,the use of bioreactors can reduce animal studies, helping the scientists to focus their attention in the right direction before starting pre-clinical studies, which are usually more expensive than preliminary research. In the past few years, several systems for the application of different mechanical stimuli to chondrocytes have been developed. Most of these can generate biomechanical-like forces such as the direct compression, tensile and shear forces, or hydrostatic pressure, in order to stimulate the articular chondrocyctes to increase their matrix production. Generally, the most important requirements that a culture system has to satisfy are high reliability and usability, perfect sterility, easy control of all the important culture parameters and low cost. In this work, a new system, inspired by the synovial environment of mobile joints and able to apply an innovative type of stimulation on articular chondrocytes is described and modeled. The SQPR (SQueeze PRessure) bioreactor chamber is designed to impose a cyclic hydrodynamic pressure on cell cultures, constructs or tissues slices. The basic principle of this new system is the generation of a localized contact less overpressure on articular chondrocytes, using a simple vertical piston movement. This kind of stimulation is particularly useful for neo-tissue or fresh-constructs, in which cells require a dynamic environment to maintain their differentiate state, but at the same time do not tolerate direct compression or high shear stress. When the piston moves down, a controlled hydrodynamic overpressure and a shear stress is generated over the cell surface, stimulating the chondrocytes to improve their matrix production. The fluid dynamics inside the SQPR bioreactor is illustrated from an analytical and numerical point of view. We show how these models can predict the pressure, velocity field and wall shear stress generated on the cell surface of the construct. The bioreactor design is presented in detail and validation tests on chondrocytes are described.