The inventor of the the 'fidget spinner' that is taking playgrounds by storm is not making a penny because she could not afford US$400 ($580) to renew the patent.

Catherine Hettinger originally came up with the concept 20 years ago as a way of occupying her daughter and helping ADHD sufferers, but they have only recently become a craze and tens of millions of units have now been sold.

The 62-year-old had a patent from 1997 until 2005, but was not able to afford the fee to renew her exclusivity to the gadgets, which was US$400, the Daily Mail reported.

This means that Mrs Hettinger, from Florida, has to sit back and watch the companies developing the modern version of her invention, without getting any money.

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When she first started to sell them, she had reasonable success, selling enough units to break even, but interest from major toy companies dipped and she was forced to relinquish the patent.

Two decades on, they are now the must-have item for children - who can now customise and trade the devices.

She told the Guardian: "Several people have asked me: 'Aren't you really mad?' But for me I'm just pleased that something I designed is something that people understand and really works for them.

"There's just a lot of circumstances in modern life when you're boxed in, you're cramped in, and we need this kind of thing to de-stress. It's also fun. That's the thing about culture, once everybody starts doing it, it's kind of OK."

A JustGiving page has even been set up, attempting to raise £20,000 ($37,600) for her, to make up for her profits that never came to be.

But they have not come without controversy - an Oxfordshire school banned fidget spinners amid claims that the controversial new 'craze' is a scam.

Pupils at Heyford Park Free School near Bicester are no longer allowed to bring in the gadgets.

At least one expert has debunked claims that they are a health aid.

Dr Mark Rapport, MD and Director of the Children's Learning Clinic at the University of Central Florida's Department of Psychology, doubts that they help ADHD and autism sufferers.

He said that while his current and past research indicates that many children with ADHD benefit from some forms of movement when engaged in challenging cognitive tasks, he has not come across any studies examining the potential benefits or adverse effects of fidget spinners.

His study found that children with ADHD who participated in activities involving 'gross body movement,' which is movement of the limbs or large parts of the body, performed better than those who sat still during memory tasks.

But he said fidget spinners don't require the user to engage in gross body movement, which appears to increase brain arousal necessary to engage in many cognitive tasks.

"Using a spinner like gadget is more likely to serve as a distraction than a benefit for individuals with ADHD," he said.