Scientists have developed a battery-free, easily implantable device that fools the brain into thinking that the stomach is full after just a few nibbles of food - an advance that could help combat the obesity epidemic. In laboratory testing, the devices developed by researchers at the University of Wisconsin-Madison in the US helped rats shed almost 40 per cent of their body weight. More than 700 million adults and children worldwide are obese, say researchers who dubbed the growing weight-related health problems a "rising pandemic." The battery-free, easily implantable weight-loss devices could offer a promising new weapon for battling the bulge, according to the study published in the journal Nature Communications. Measuring less than one centimetre across, the tiny devices - which are safe for use in the body and implantable via a minimally invasive procedure - generate gentle electric pulses from the stomach's natural churning motions and deliver them to the vagus nerve, which links the brain and the stomach. That gentle stimulation dupes the brain into thinking that the stomach is full after only a few nibbles of food. "The pulses correlate with the stomach's motions, enhancing a natural response to help control food intake," said Xudong Wang, a UW-Madison professor. Unlike gastric bypass, which permanently alters the capacity of the stomach, the effects of the new devices also are reversible. When researchers removed the devices after 12 weeks, the study's rats resumed their normal eating patterns and weight bounced right back on. The device has several advantages over an existing unit that stimulates the vagus nerve for weight loss. That existing unit, "Maestro," approved by the Food and Drug Administration in 2015, administers high-frequency zaps to the vagus nerve to shut down all communication between the brain and stomach. It requires a complicated control unit and bulky batteries which frequently must be recharged. That ongoing maintenance can be a big barrier to use, said Luke Funk, a surgery professor at UW-Madison. "One potential advantage of the new device over existing vagus nerve stimulators is that it does not require external battery charging, which is a significant advantage when you consider the inconvenience that patients experience when having to charge a battery multiple times a week for an hour or so," said Funk. Wang's device contains no batteries, no electronics, and no complicated wiring. It relies instead on the undulations of the stomach walls to power its internal generators. That means the device only stimulates the vagus nerve when the stomach moves. "It's automatically responsive to our body function, producing stimulation when needed," said Wang.