If you felt full after only a few bites of food, you'd probably eat a lot less. It could happen. Researchers at the University of Madison-Wisconsin have built a device that makes people feel full faster. When it was tested in rats, the rats lost about 40 percent of their body weight.

It's a good start on an implantable device that could help people who are seriously overweight curb their eating.

Unlike gastric bypass surgery, which permanently alters the stomach, the effects of the new device are reversible.

The tiny device works by sending a tiny electrical signal from the stomach to the vagus nerve, which in turn tells the nerve to signal the brain that you are full. It only sends the signal when the stomach muscles are moving as they are when food enters the stomach. So while you may be hungry as usual, a few bites later you aren't. There are no batteries or complicated wiring. The device uses the undulations of the stomach walls to generate its own power. The stomach's movement creates a buildup of charge inside the device that ultimately generates the electrical current powering the device.

Led by Xudong Wang, a professor in the College of Engineering at the University of Wisconsin-Madison, the group that invented the device has extensive experience with wearable and implantable devices that generate their own electricity, having previously created several, including one that harvests energy from people's beating hearts and, more recently, a motion-powered bandage that helps heal wounds with electrical impulses.

The vagus nerve stimulator or VNS is about the size of a potato chip. It is implanted in the stomach in what the researchers describe as a minimally invasive procedure. Unlike gastric bypass surgery, which permanently alters the stomach, the effects of the new device are reversible. When Wang and his colleagues removed the device after 12 weeks, rats resumed their normal eating patterns and regained their lost weight.

In the study, rats that had the device implanted lost 35 percent of their body weight within 18 days, a loss that stabilized at around 38 percent for the remaining 75 days of the study. They also became leaner than the other rats in the study, with less body fat.

The new device has no control unit or battery and is roughly the size of a penny. Maestro, a similar device already on the market, sends high-frequency electrical impulses to the vagus nerve that shut down all communication between the brain and stomach. It requires a complicated control unit and bulky batteries that need to be recharged frequently.

Wang's group will be testing the device on larger animals next, before hopefully moving on to human trials.

An open access article on the VNS device appears in Nature Communications.