When a severe injury slices through a peripheral nerve, the communication cable linking the brain to the limbs snaps entirely. Without direct structural guidance, regrowing nerve fibres quickly lose their direction, stranding downstream muscles without electrical signals and causing them to permanently wither.
To bridge such critical gaps, surgeons typically harvest a healthy sensory nerve from another part of the patient's body to serve as a patch. While clinically reliable, this strategy robs one part of the body to mend another, leaving patients with donor-site numbness, chronic pain, and limited tissue options. Hollow plastic tubes offer an off-the-shelf alternative, but across wider voids they fail because they lack the physical cues and biochemical nourishment that infant axons require to bridge the divide.
In the International Journal of Extreme Manufacturing , Prof. Jun Yin at Zhejiang University and his co-workers have engineered a triple-layered artificial nerve conduit embedded with living cells that steers nerve regrowth across deep gaps, achieving functional recovery in rats that rivals a natural graft.
Creating this living bridge required cracking an old bioprinting dilemma. Electro-assisted bioprinting relies on high-voltage electrical fields to pull ink through tiny nozzles into ultra-fine filaments. However, printing living cells at high resolution traditionally forces a fatal trade-off: thick, viscous inks maintain crisp shapes after deposition, but squeezing them through fine nozzles exerts intense shearing friction that tears cell membranes apart.
Thinning the ink relieves this pressure and keeps cells alive, but the printed threads immediately slump into shapeless puddles upon landing. Conventional chemical hardening agents can poison cellular cargo, while the high electrical voltages required to draw out fine fibres can destroy delicate living tissue.
Prof. Yin's team resolved this conflict by using temperature to control bioink behaviour at each stage of fabrication. Using a bioink formulated from alginate, polyethylene oxide (PEO), and thermosensitive gelatin methacryloyl (GelMA) carrying living Schwann cells, the helper cells that naturally insulate and guide nerves, the team managed the ink's flow state through exact thermal zones. The nozzle barrel is warmed to 25 °C, softening the gel just enough to flow freely without generating lethal shear stress. The instant the jet lands onto a chilled 4 °C collection plate, it solidifies into a stable microfilament before it can spread. Guided by a gentle 1.0-kilovolt electric field, the platform spins continuous fibres measuring just 36 µm across while preserving cell survival above 95%.
These microfilaments act as microscopic railway tracks for regenerating tissue. In laboratory cultures, nerve cells seeded along the tracks organized into dense, directional ribbons rather than scattering aimlessly. Genetic sequencing revealed the underlying molecular switch: contact with the printed tracks and chemical signals from the living Schwann cells activated axon-guidance genes such as Ephb1 and Gnai1, switching on cytoskeletal steering programmes. At the same time, genes linked to programmed cell death and routine cell multiplication were quieted, directing the cells' energy away from cell division and entirely toward extending elongated signalling cables.
Prof. Yin's team combined this technique with complementary fabrication methods inside a single, unified direct-writing machine to produce a three-layer conduit that mimics natural nerve architecture.
The outer layer is an open mesh of polycaprolactone (PCL) that acts like an exoskeleton, withstanding repeated compression and surgical suture forces up to 1.64 N. The middle layer is a porous nanofibre membrane with microscopic pores averaging 0.55 μm wide, which lets oxygen and nutrients circulate while blocking scar-forming connective tissue from invading the tube. The innermost layer contains the temperature-printed Schwann cell microfilaments, forming an active biological trackway.
Implanted across 10 mm sciatic nerve gaps in rats, the composite conduits restored severed connections over 16 weeks. Regenerated nerve bundles formed thick myelin insulation sheaths averaging 1.02 μm, closely approaching the 1.15 μm thickness observed in natural autografts. Repaired nerves conducted electrical impulses at 5.67 metres per second, matching the autograft control rate of 5.88 metres per second and nearly doubling the speed of acellular conduits.
This rapid reconnection rescued distal calf muscles from denervation atrophy, preserving approximately 49% of their wet weight compared to under 24% for empty conduits, while restoring normal hindpaw coordination during walking trials.
Integrating melt writing, electrospinning, and delicate cell patterning onto a single platform demonstrates that multi-layered living implants can be built without shifting fragile constructs across separate machines. For the public, the platform provides proof of concept that artificial devices can perform on par with autologous tissue grafts without sacrificing a second healthy nerve. Before human trials can begin, researchers must validate the conduits across multi-centimetre nerve gaps in large animal models, identify low-temperature sterilization methods that preserve embedded living cells, and tune the degradation rate of the outer shell to match the slower timeline of human nerve healing.
International Journal of Extreme Manufacturing (IJEM, IF: 25.1) is devoted to publishing articles of the highest quality and significance to pushing the limits of scales, precision, performance and environments in manufacturing.
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