Injured knee? Send in the microbots

Researchers have invented magnetic microbots that deliver stem cells to damaged cartilage in the knee, a discovery that could lead to treatments that prevent debilitating osteoarthritis.

The device was tested in animals, but the authors, led by Eunpyo Choi at the Korea Institute of Medical Microrobotics in Gwangju, Korea, are aiming to get approval for clinical trials in humans.

The knee is lined with a glossy coat of hyaline cartilage that smooths and lubricates the hinge joint. But the cartilage is easily damaged in sporting injuries and through general wear and tear.

The result can be bone scraping on bone and painful osteoarthritis, eventually leading to a joint replacement – the incidence of knee cartilage damage nearly tripled between 1996 and 2011, with the number of knee replacements projected to grow 85% by 2030.

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Close up images of the magnetic microrobots showing the porous structure and pore morphology. Credit: Go et al., Sci. Robot. 5, eaay6626 (2020)

There are treatments that try to repair the defect and slow the progression to arthritis, although none comes up to snuff.

One involves drilling into the bone to stimulate the bone marrow to make more cartilage. But it produces an inferior grade called fibrocartilage that doesn’t stand up to the mechanical stresses.

Scientists are also experimenting with mesenchymal stem cells, sourced from the person’s fat or bone marrow and put into the cartilage defect. The problem is when you inject the cells into the knee, they mostly float away in the fluid that fills the joint during surgery.

The current research, published in Science Robotics, offers an ingenious way of targeting the stem cells at the injured bit of cartilage and keeping them there.

The team used a polymer called PLGA to manufacture spherical microbots that look a bit like a soccer ball full of holes, just a lot smaller. The bots are around 350 micrometres in diameter, half the width of a grain of sugar.  

The next challenge was to make them magnetic, and to do it with something biodegradable and non-toxic.

The researchers coated the bot with a combination of ferumoxytol, an injectable iron supplement approved for human use by the US Food and Drug Administration, and chitosan, a sugar found in the exoskeleton of shellfish that’s used to treat high blood pressure and cholesterol. 

The newly magnetic mini-footies were then impregnated with stem cells ready to be injected into a rabbit’s knee.

The bunny had a small divot in the cartilage on the upper side of the knee joint. It was prepped for treatment while the knee was encased in a Star Wars-like contraption of electromagnetic coils housed in gleaming white cylinders.

Then a hundred of the bots were injected while a video recorded the unfolding drama.

With the magnets turned off, the microbots just fall away with gravity – hardly any stay in the cartilage defect. With the magnet turned on, however, almost all the bots attach to the injured zone, staying put for three weeks with the help of another magnet attached to the outside of the knee.

A series of images shows significant healing of the defect at the three-week mark.

The team is now intent on transforming its invention for use in people.

“We will build a microrobot system for human knee cartilage regeneration, where targeting and fixation devices will be made to keep their size small while maintaining their current functionality,” they write.

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