Off-board power

17 July 2012

Three main sections of the prototype wirelessly powered capsule

An inside view

CE is an important medical diagnostic tool in which a capsule containing a camera is swallowed by a patient, allowing an internal view of their gastrointestinal tract. Particularly used to examine parts of the small intestine beyond the reach of conventional endoscopy, images from the capsule can be used to identify problems like bleeding and cancers.

Current clinical applications of CE have two main limitations, especially in comparison to conventional endoscopy methods – limited image frame rate and resolution, and longevity. Typically, a capsule endoscope may provide 256 x 256 pixel images at 2 fps. Relying on an internal battery, capsules operate for around 8 hours, however this may not be long enough for the capsule to pass through the digestive tract in some patients. The limited operation period, low resolution images and slow frame rate all increase the chances of missing a bleed or cancer. Both limitations relate to the packaging challenges inherent with such a small capsule. Conventional endoscopy methods, where a camera is inserted into the body via a flexible tube, can place bulkier components outside the body and plug straight into a constantly operating (and backed-up) mains power supply.

A thirst for power

Constant efforts to miniaturise electronics mean the real root of both limitations is the energy budget of the necessarily tiny capsule battery. A lot of work goes into reducing the power requirements of capsule systems and a typical value may now be only 10 mW, however, increasing the image resolution and frame rate will increase power requirements. This has led to interest in WPT technology as a way to provide more power, consistently, for longer. With their system, reported in this issue, the team at Tsinghua University’s Institute of Microelectronics have achieved transfer power in a model human body environment in the range 20 to 150 mW, more than sufficient to power their capsule, designed to take 12 fps at 480 x 480 pixels.

Two key limitations on WPT CE systems are capsule positioning in the digestive tract and power transfer efficiency. The efficiency is typically in the range of 1–33.1% when transmission distance from a power transmitter outside the body is a few tens of centimetres. The Tsinghua team have a particular focus on improving the efficiency, team member Tianjia Sun explained: “Increasing the transfer efficiency has two benefits. First, the possibly harmful electromagnetic radiation level to the patient can be reduced. Then, if the power efficiency is high enough, the power transmitter outside the body can use batteries instead of mains power, allowing the patient to walk freely without the need to plug in.”

The team’s system has given power transfer efficiency improvements of more than 16% on average, compared to previous works. The key to the improvement lies in the receiving circuit. The receiver in this kind of application must be omnidirectional, as capsule orientation varies freely in the body. In previous designs, 3D coil structures are connected to a group of rectifiers. The outputs of the rectifiers connect together to combine the power, but the voltage outputs of the rectifiers are different, reducing the power combining efficiency. “This phenomenon is similar to when new and old batteries are mixed together. Our specially designed receiving circuit can keep an identical voltage output over all channels,” said Sun.

A new foundation

Moving from research laboratory to clinical use is always challenging, and the team are clear on their key challenge. Although significantly improved over previous work, the power transfer efficiency still needs to be increased in order to meet ICNIRP guidelines for radiation levels. As part of this effort they are also working on a very high efficiency switch-mode full-NMOS rectifier. In conventional applications using 110–220 V mains power, dropout voltage of 0.35–0.7 V of rectifier diodes can be neglected. However, in this application the input and output voltages of the rectifier are very low (~ 3 V) so dropout voltage significantly degrades the overall efficiency.

Power transfer efficiency is even more critical as the team look beyond the current limitations of CE to including extra capabilities. The team envision future self-propelled capsules that can be controlled to navigate to a desired location and even take biopsy samples. As Sun said, “Besides CE, there are other implants, like artificial retinal prosthesis, implantable ECG recorders, artificial hearts and electrical stimulators that could rely on WPT. But all these functions rely on more energy, and WPT seems the perfect technology to satisfy these demands. We believe WPT will act as a basic but important supporting technology in future healthcare. It’s likely that many future medical devices will be wirelessly powered for life-long operation and ultra-small size.”

The Letter presenting the results on which this article is based can be found on the IET Digital Library.

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