Fabrication of bilayer nerve conduits with sustained drug release using a multi-material, embedded 3D printing strategy

Nerve conduits are commonly used in peripheral nerve repair, but clinically available conduits offer limited functional recovery. While three-dimensional (3D) printing has emerged as a promising technique for nerve conduit fabrication, the fabrication of conduits with both geometrical complexity and biochemical guidance remains challenging. Here, we introduce a multi-material, embedded 3D printing approach to fabricate bilayer nerve conduits capable of sustained drug release. In this approach, bilayer conduits are formed by sequentially extruding two crosslinker-containing inks - a biomaterial ink and a sacrificial ink - into a photocrosslinkable gel precursor support bath. As a demonstration, we fabricated conduits with a gelatin methacryloyl (GelMA)/poly(ethylene glycol) diacrylate (PEGDA)-based outer layer and fibrin-based inner layer. A decoupling of drug delivery and mechanical support is uniquely enabled by the bilayer design, where the outer layer provides mechanical strength and stability, while the inner layer enables the sustained release of nerve growth factor (NGF). The mechanical properties of bilayer conduits with varying diameters were characterized by compressive testing, and drug delivery from bilayer conduits with NGF loaded in the inner layer was quantified using in vitro NGF release and bioactivity assays. Finally, we demonstrated the fabrication of bilayer conduits with branched and multi-lumen geometries, which are challenging to fabricate with existing strategies. Altogether, these results highlight the promise of 3D printed nerve conduits leveraging tunable biomaterials to both physically guide and biochemically promote nerve regeneration.