A team of engineers and neuroscientists has built a spinal implant that may one day be used to heal a broken spinal cord in humans.
A team of engineers and neuroscientists has built a spinal implant that may one day be used to heal a broken spinal cord in humans. The implant consists of a piezoelectric scaffold loaded with Schwann cells, which can repair injured axons—nerve parts that propagate impulses between cells.
Roughly 291,000 people in the Unites States suffer from spinal cord injuries with approximately 17,730 new cases added each year, according to 2019 data from National Spinal Cord Injury Statistical Center. That may underestimate the problem. In 2013, the Christopher and Dana Reeve Foundation estimated that nearly 1.5 million people in the U.S. are paralyzed due to spinal cord injuries. Most spinal cord injuries are caused by vehicle accidents and falls, but some result from sports, recreational activities, and military actions, such as combat or projectile wounds.
In a healthy spinal cord, axons protruding from the brainstem are wrapped in a sheath of protein and fat called myelin that facilitates the transmission of nerve impulses through the central nervous system. When a spinal column is injured, axons lose their myelin sheaths, triggering a cascade of events that impede nerve cell regeneration.
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“The spinal cord is fragile,” said Treena Arinzeh, director of Tissue Engineering and Applied Biomaterials Lab at New Jersey Institute of Technology, who worked on designing the piezoelectric scaffold. “Any form of traumatic injury that hits the cord causes an inflammatory response and damage to the neural tissue, which, unlike other tissue in the body, doesn't have a regenerative capacity and is unable to repair itself.”
To solve this problem, Arinzeh, together with neuroscience professor Mary Bartlett Bunge from the Miami Project to Cure Paralysis, created a piezoelectric scaffold loaded with Schwann cells, which can generate a myelin sheath around neuronal axons. When implanted into the spinal cord, Schwann cells wrap themselves around the injured axons and gradually mature into myelin coating. They also secrete neurotrophins—a family of proteins—that encourage nerve growth.
“They help protect the injured spinal tissue and promote axon regrowth,” Bunge said. “Once axons regrow, Schwann cells would form myelin around them.”
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Although Schwann cells don't naturally exist inside the spinal cord column, they are present in the peripheral nervous system and can be easily harvested from the patient’s limbs. Researchers found that combining them with a piezoelectric scaffold, which generates electrical pulses in response to mechanical stimuli, improves recovery.
Researchers built the scaffold by electrospinning a polymer material called piezoelectric polyvinylidene fluoride trifluoroethylene (PVDF-TrFE). Cells, and particularly neuronal cells, respond to this polymer’s electrical activity by increasing their uptake of ions, charged particles present in the environment and important for cellular health.
“Electrical signaling changes the ion flow into the cells and the cells will grow from that,” Arinzeh explained. “It will stimulate axonal growth.”
The resulting scaffold is designed to bridge the gap between the two severed ends of a spinal cord.
“It’s like a drinking straw, and the cut ends of the cord are inserted into the ends of that drinking straw,” she said. “And the Schwann cells are put into the gap between the damaged ends of the spinal cord.”
On the inside, this scaffold is composed of fibers to “guide” axons to attach and grow alongside them. Axons grab and hold onto those fibers, Arinzeh said, while Schwann cells engage in axon repair.
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In recent studies, bioengineers tested the piezoelectric scaffold loaded with Schwann cells on rats whose spinal cord had been severed. They found that rodents started to regrow axons from the area above the injury and through the implanted conduit.
“Now we are trying to show whether they will grow beyond the implanted fibers, into the host spinal cord,” Arinzeh said.
Schwann cells alone have proven to be a promising approach to repair spinal cords in human trials, but they were not able to heal the injury completely. Researchers hope that combining Schwann cells with piezoelectric scaffolds and, perhaps, growth hormones will produce better results in humans as it did in rats.
Lina Zeldovich is an independent writer.
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