Warp-knitted fabric structures for a novel biomimetic artificial intervertebral disc for the cervical spine

As an attempt to better replicate the complex kinematics of a natural disc, a novel biomimetic artificial intervertebral disc replacement (bioAID) has been developed containing a swelling hydrogel core as nucleus pulposus, a fiber jacket as annulus fibrosus and metal endplates to connect the device to the adjacent vertebrae. The first prototype consisted of a weft-knitted fiber jacket, in which only a single fiber was used to create the jacket structure. This can endanger the structural integrity of the complete device upon yarn damage. Therefore, in this study, several warp-knitted textile structures were assessed to (1) ensure structural integrity, (2) while allowing for swelling constraint of the hydrogel and (3) behaving as one integrated unit similar to the natural IVD. Moreover, the fiber jacket should (4) act as a scaffold that allows bone ingrowth to ensure long-term stability and (5) have a good durability, (6) be wear resistant and (7) have good manufacturing feasibility with good quality control. In this study, 4 different stitch patterns, including 2 x 1 and 1 x 1 lapping with and without a pillar stitch, were produced. The effect of the stitch pattern and stitch density on the fabric mechanical properties and device swelling and compressive strength was assessed. As a next step, the effect of using multiple layers of fabrics, mimicking the layered structure of annulus fibrosus, on the functional capacity of the bioAID was characterized. All textile structures were capable of limiting the swelling of the hydrogel while withstanding its internal pressure and showing sufficient wear resistance. However, only the 2 x 1 and 2 x 1 with pillar stitch had a pore size range that was suitable for cell infiltration to facilitate osseointegration as well as having the highest strength of the complete device to ensure safety under compression loading. Incorporating different number of jacket layers of these two stitch patterns did not show any significant effect. When also taking the structural parameters into consideration, the 2 x 1 lapping design with 4 layers was able to constrain hydrogel swelling, provide a high compressive strength, could facilitate cell infiltration and had dimensions within the range of a natural intervertebral disc.