3D printed implants are soft bridges that guide new nerve cells to grow across a tear or break in an injured spinal cord and restored movement. The work has so far shown promise in rats with severe spinal cord injury.
The implants are hydrogel structures that can be rapidly 3D printed into different sizes and shapes, making them easily customizable to fit the precise anatomy of a patient’s spinal cord injury. Researchers fill the implants with neural stem cells and then they are fitted, like missing puzzle pieces, into sites of spinal cord injury. New nerve cells grow and axons—long, hair-like extensions through which nerve cells pass signals to other nerve cells—regenerate, allowing new nerve cells to connect with each other and the host spinal cord tissue.
“Using our rapid 3D printing technology, we’ve created a scaffold that mimics central nervous system structures. Like a bridge, it aligns regenerating axons from one end of the spinal cord injury to the other. Axons by themselves can diffuse and regrow in any direction, but the scaffold keeps axons in order, guiding them to grow in the right direction to complete the spinal cord connection,” said co-senior author Shaochen Chen, professor of nanoengineering at the UC San Diego Jacobs School of Engineering and faculty member of the Institute of Engineering in Medicine at UC San Diego.
“In recent years and papers, we’ve progressively moved closer to the goal of abundant, long-distance regeneration of injured axons in spinal cord injury, which is fundamental to any true restoration of physical function,” said Tuszynski.
“The new work puts us even closer to the real thing because the 3D scaffolding recapitulates the slender, bundled arrays of axons in the spinal cord. It helps organize regenerating axons to replicate the anatomy of the pre-injured spinal cord,” added co-first author Kobi Koffler, assistant project scientist in Tuszynski’s group.
A four-centimeter implant modeled to fit an actual human spinal cord injury.
Faster, More Precise Printing
The implants contain dozens of tiny, 200-micrometer-wide channels (twice the width of a human hair) that guide neural stem cell and axon growth along the length of the spinal cord injury. The rapid 3D printing technology that Chen’s team developed produces two-millimeter-sized implants in just 1.6 seconds. Traditional nozzle printers take several hours to produce much simpler structures.
And the process is scalable to human spinal cord sizes. As a proof of concept, researchers printed four-centimeter-sized implants modeled from MRI scans of actual human spinal cord injuries. These were printed within 10 minutes.
Restoring Lost Connections
Researchers grafted two-millimeter implants, loaded with neural stem cells, into sites of severe spinal cord injury in rats. After a few months, new spinal cord tissue had regrown completely across the injury and connected the severed ends of the host spinal cord. Treated rats regained significant functional motor improvement in their hind legs.
“This marks another key step toward conducting clinical trials to repair spinal cord injuries in people,” Koffler said. “The scaffolding provides a stable, physical structure that supports consistent engraftment and survival of neural stem cells. It seems to shield grafted stem cells from the often toxic, inflammatory environment of a spinal cord injury and helps guide axons through the lesion site completely.”
Earlier this year, his team created a new line of spinal cord neural stem cells capable of dispersing throughout the spinal cord. The cells can be maintained for long periods of time, constituting a potentially improved, clinically translatable cell source for future replacement strategies.
Meanwhile on the engineering side, Chen’s team has been developing a next generation 3D bioprinting technology capable of producing detailed microstructures that mimic the sophisticated designs and functions of biological tissues. Chen’s lab has used this technology in the past to create life-like liver tissue and intricate blood vessel networks. One of their ongoing projects involves printing heart tissue for people who have suffered a heart attack and for treating other cardiac diseases.
The new spinal cord implant is one of the latest in this line of 3D bioprinted tissues.
The researchers are currently scaling up the technology and testing on larger animal models in preparation for potential human testing. Next steps also include incorporation of proteins within the spinal cord scaffolds that further stimulate stem cell survival and axon outgrowth.
Chen and Zhu have co-founded the startup, Allegro 3D, to commercialize their rapid bioprinting technology.
Current methods for bioprinting functional tissue lack appropriate biofabrication techniques to build complex 3D microarchitectures essential for guiding cell growth and promoting tissue maturation1. 3D printing of central nervous system (CNS) structures has not been accomplished, possibly owing to the complexity of CNS architecture. Here, we report the use of a microscale continuous projection printing method (μCPP) to create a complex CNS structure for regenerative medicine applications in the spinal cord. μCPP can print 3D biomimetic hydrogel scaffolds tailored to the dimensions of the rodent spinal cord in 1.6 s and is scalable to human spinal cord sizes and lesion geometries. We tested the ability of µCPP 3D-printed scaffolds loaded with neural progenitor cells (NPCs) to support axon regeneration and form new ‘neural relays’ across sites of complete spinal cord injury in vivo in rodents1,2. We find that injured host axons regenerate into 3D biomimetic scaffolds and synapse onto NPCs implanted into the device and that implanted NPCs in turn extend axons out of the scaffold and into the host spinal cord below the injury to restore synaptic transmission and significantly improve functional outcomes. Thus, 3D biomimetic scaffolds offer a means of enhancing CNS regeneration through precision medicine.