A silicone guide seeded with stem cells could be implanted at a spinal cord injury site to help restore some function to paralysed patients

3D bio-printing – fabricating a scaffold onto which living cells can grow – has undergone significant advances in recent years, going from a technique to create structures whose function derives only from their shape, such as ears, to bones and even parts of organs. Researchers from the University Minnesota have now developed a 3D-printed implant which, they believe, may be able to restore some function to patients with long-term spinal-cord injuries.

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Neuronal sten cells, engineered from any cell in the recipient's body, would be printed directly onto the silicone implant from the samne equipment which printed it

The team, led by Michael McAlpine, a mechanical engineer, and Ann Parr, from the Department of neurosurgery and Stem Cell Institute, describes in the journal Advanced Functional Materials how they 3D printed a guide made of silicone and then printed neuronal stem cells directly on top of it. The concept is that this guide would be implanted into the injured area of the spinal cord to serve as a bridge between living nerve cells above and below the area of injury. This may alleviate pain and help patients regain some functions such as control of muscles, bowel and bladder.

"This is the first time anyone has been able to directly 3D print neuronal stem cells derived from adult human cells on a 3D-printed guide and have the cells differentiate into active nerve cells in the lab," McAlpine said.

Previous animal research has used electronic devices to transmit signals across the damaged area of a spinal cord, but this technique would avoid having to implant electronics and depend upon batteries.

The research involved using a number of new techniques, including bio engineering developed at Minnesota. The stem cells are derived from any kind of cell from an adult, such as a skin cell or a blood cell. The medical researchers on the team reprogrammed the cells to turn them into neuronal stem cells. Meanwhile, the mechanical engineers developed a printer that can print both the guide and the living cells. The guide is multifunctional: it both provides a structure onto which the cells can be printed, and keeps them alive while allowing them to change into neurons.

"3D printing such delicate cells was very difficult," McAlpine said. "The hard part is keeping the cells happy and alive. We tested several different recipes in the printing process. The fact that we were able to keep about 75 percent of the cells alive during the 3D-printing process and then have them turn into healthy neurons is pretty amazing."

In laboratory tests, the cells extended fibres through 150µm wide channels incorporated into the guide structure, and monitoring calcium ion levels in these fibres confirmed that the cells were functional. "This is a very exciting first step in developing a treatment to help people with spinal cord injuries," said Parr. "Currently, there aren't any good, precise treatments for those with long-term spinal cord injuries." The research was result of advances in bio engineering and 3D printing occurring at the same time, she added.

Parr cautioned that this research would be unlikely to present a complete cure for paralysis resulting from spinal cord injury. "We've found that relaying any signals across the injury could improve functions for the patients," she said. "There's a perception that people with spinal cord injuries will only be happy if they can walk again. In reality, most want simple things like bladder control or to be able to stop uncontrollable movements of their legs. These simple improvements in function could greatly improve their lives."

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