Background

Articular cartilage is a load-bearing tissue that covers the surface of bones in joints and functions as a low-friction surface and a mechanical damper. Even in a harsh mechanical environment, it demonstrates excellent resiliency. Articular cartilage is a hydrogel-like, matrix-rich tissue without vasculature and neuronal component that contains only 5 10 % of specialised cells, so called chondrocytes, which maintain the structural and functional integrity of the matrix. This matrix is organised into characteristic depth zones, each with distinct physicochemical and biological properties and functions, that work together to impart low-friction, wear-resistant behaviour to diarthrodial joints.

Articular cartilage lacks a sufficient intrinsic repair response and cannot heal. Hence the zonal structure and function are commonly and irreversibly lost following trauma and in disease. As a result, cartilage defects are prone to develop into OA, the predominant cartilage disease. OA is characterised by a loss of cartilage, typically progressing from superficial fibrillation to complete erosion down to the underlying subchondral bone.

There are several technologies available for treatment of articular cartilage defects but none of these provide a consistent and durable long-term solution. The current status quo of cartilage tissue engineering can produce cartilaginous structures that lack the specific organisation of the native tissue. Hence, at the moment, this newly formed tissue merely delays full joint replacement by prosthesis.

As cartilage is a hydrogel-like matrix rich tissue, one promising regenerative approach is the use of (cell-laden) hydrogel matrices to fill cartilage defects. Hydrogels are water swollen three-dimensional (3D) networks of hydrophilic polymers that allow encapsulation of cells and diffusion of nutrients. They are thus especially attractive for cartilage repair as they recapitulate several features of the natural cartilage ECM. Additionally, hydrogels allow for efficient and homogeneous cell seeding, can provide biologically-relevant chemical and physical signals and can be formed in various shapes.