Towards intervertebral disc engineering: Bio-mimetics of form and function of the annulus fibrosus lamellae

The aging western society is heavily afflicted with intervertebral disc (IVD) degeneration. Replacement or repair of the degenerated IVD with an artificial bio-mimetic construct is one of the challenges of future research due to its complex structure and unique biomechanical function. Herein, biocomposite laminates made of long collagen fibers in unidirectional (-1.3 ± 2.1°) and angle-plied ± 30° orientations (30.4 ± 6.4 and -29.8 ± 4.5), embedded in alginate hydrogel, were fabricated to mimic the form of single annulus fibrosus (AF) lamella and the circumferential AF, respectively. The mechanical behavior of the composites was measured and compared with in vitro existing data of the human native AF as well as with new data obtained from ovine and bovine specimens. The mechanical behavior was found to reproduce the full stress- strain behavior of the human AF single lamella in several regions of the AF and the Young's modulus was 28.3 ± 8.6 MPa. Moreover, the modulus of the angle-plied laminates was 16.8 ± 2.9 MPa, which is approximately 5% less than the in vitro data. The full stress-strain behavior was also compared with bovine and ovine circumferential AF samples and found to be very similar, with a difference in the modulus of 4.1% and 19.7%, respectively. Moreover, an FE model of the L3-L4 functional spinal unit (FSU) was developed and calibrated to evaluate the mechanical ability of the biocomposite to be used as an AF substitute under physiological IVD loading modes. The biocomposite demonstrated a good ability to mimic the stiffness of the native tissue under physiologic loading modes as flexion, extension, lateral bending and compression, but was too flexible under torsion. It was found that the proposed biomimetics AF design resulted in a compatible function in several mechanical levels, which holds great potential to be used as a viable AF replacement towards full IVD engineering.