Spinal cord injury microrobots developed by researchers at ETH Zurich and the University of Zurich (UZH) have restored normal movement in mice whose spinal cords were completely severed, with treated animals showing improved gait, stride length, coordination and exploratory behaviour within 28 days. The results, published in Nature Materials, also include successful trials in zebrafish, where the approach produced quick, substantial and lasting movement gains.
How the spinal cord injury microrobots work
The treatment begins with a skin sample taken from the patient, which researchers convert into induced pluripotent stem cells. Those cells are then directed to become neuro progenitor cells (NPCs): the building blocks of nerve tissue. Separately, the team produces nanoparticles with an inner layer that responds to magnetic fields and an outer layer that converts that magnetic response into electrical signals.
The two components are combined in a culture medium just one square centimetre in area, developed by team member and study co-author Professor Salvador Pané i Vidal of ETH Zurich’s Multi-Scale Robotics Lab. Within about 30 minutes, the cells and nanoparticles merge to form what the researchers call ‘NPCbots’. Once several million of these have been extracted, the therapy is ready to administer. A magnetic field then guides the NPCbots precisely to the injury site, where the electrical stimulation they generate encourages the stem cells to accelerate tissue repair.
The approach is a direct response to the limitations of existing techniques. Implantable electrode nerve stimulation can restore some lost movement, but it requires placing electrodes into an extremely sensitive area, and transplanted cells do not always survive or integrate properly into surrounding tissue. The NPCbot method sidesteps the need for implanted electrodes entirely and delivers stimulation at the cellular level.
What the animal trials showed
In the mouse model, where the spinal cord does not repair itself naturally, nerve cells at each end of the severed spinal column had reconnected by day 28 of the trial. The treated animals’ movement patterns became increasingly normal over that period. Gait, stride length, coordination and exploratory behaviour all improved substantially, and the treatment was well tolerated, with no evidence of adverse effects or immune reactions.
The zebrafish results, while less directly comparable to human physiology, reinforced the picture. Those animals showed rapid and lasting improvement in movement following treatment. Zebrafish spinal cords do regenerate naturally, so the trials were designed to confirm the technology’s compatibility with repair processes rather than to demonstrate repair where none would otherwise occur.
The UZH collaboration, confirmed by ETH Zurich, broadens the research base behind the project and connects it to clinical and biological expertise beyond the engineering focus of the ETH team. That combination of disciplines is reflected in the breadth of the study’s methodology, from the molecular biology of stem cell conversion to the magnetic engineering of the nanoparticle core.
The road to human trials
Translating the results to people will require several more stages of testing. The researchers expect the nanoparticles to be stable and minimally reactive, in part because of a barium-titanate coating, and the particles may eventually dissolve in muscle tissue. The team still wants to establish whether the particles are excreted in some way, and to determine which magnetic field parameters work best for human anatomy alongside the optimal duration of stimulation.
‘In addition to many clinical aspects, we first need to test which magnetic fields work best in humans and determine the optimal stimulation duration,’ said Hao Ye, senior scientist and the study’s first author, in a news release.
There is currently no reliable way to repair nerve damage in the human spinal cord. Nerve cells in the cord rarely regenerate on their own, and scar tissue actively blocks the regrowth of nerve fibres, which is why injuries at this level so often result in permanent loss of function. The ETH Zurich and UZH team is not claiming a cure, but the 28-day reconnection of completely severed nerve tissue in a mouse model, with no observed side effects, is a result that warrants continued investigation. The next phase is further animal studies focused on side effects and stimulation parameters before any human application can be considered.
