Fertile Area for Low-Power Devices

Rapid advances in portable elec­tronics are bringing hope for cou­ples that have fertility problems. For example, a compact sensing system measures body temperature changes in fractions of degrees over a month-long span to determine fertility levels, using efficient power conservation techniques to let one sensor last for a full monthly cycle.

Societal changes such as older marriage ages have brought fertility problems for as many as one in six couples. In response, startup Cambridge Tempera­ture Concepts has leveraged extensive research into fertility to create its DuoFertility sensing system.

It consists of a small temperature-sensing module that is attached to the woman's body and a handheld reader that includes lights which predict the best dates for conception. This reader has a USB connection so data can be downloaded to PCs and sent to the company for further analysis by CTC's staff of specialists.

The sensor continuously measures body basal temperature to determine when a woman is ovulating, offering 99-percent accuracy. It uses this informa­tion to predict the days in the month when she is most likely to be fertile, providing this data up to six days in advance. Constantly monitoring minute temperature changes eliminates the many variations that occur when women have to take their temperature manually.

"The patient doesn't have to wake up at a specific time. That eliminates issues from things like whether their partner stole the blankets or the temperature change that occurs just by getting out of bed and getting a thermometer," says Dr. Shamus Husheer, CTO of CTC. His studies in particle accelera­tion that culminated with a Ph.D. from Cambridge University were "all about instrumentation." He teamed up with professors doing fertility research to form the company in 2006.

The heart of the system is a coin-sized module that attaches to the user's body, looking for changes that are only a small fraction of degree over the course of a month. These sensors must last more than a month, since changing the sensor during that cycle could induce variations that are difficult to factor in.

"Body-worn electronics have to be very small with long lifetimes. This sensor has an average power consumption of less than 1 microamp so it can work for months on the smallest coin battery," Husheer says.

The sensor's microcontroller plays a major role in extending battery life. Hush­eer and his design team opted for an 8-bit Microchip PIC16 886. A key factor was the chip's ultra-low-power wake-up.

When it's time to take a reading, it powers up quickly, then swiftly returns to a sleep mode. "The measurement is over and done within a millisecond," Husheer says.

A combination of sensors and packag­ing make it possible to take precise tem­peratures regardless of whether the sensor is open on one side or covered by the user's arm. A pair of matched thermistors takes the temperature. They measure the temperature and the heat flow from one side of the coin to the other. "We not only measure the temperature on the body, we can determine whether the arm is open or closed," Husheer says.

Temperature flows from the body side to the external side, letting the sensor consistently measure heat flow. To ensure the sensor is in tight contact with the skin, Cambridge offers six different adhesives that adhere to different skin types.

This compact sensor is housed in an in­jection-molded component that includes thermally conductive plastic polymers that surround the sensor. The patented technique includes a non-conductive over­molded plastic that protects the unit.

The other major portion of the sensor module is the communication sys­tem that sends data to the reader. The body-worn module sends data using a modified RFID chip that has a coil an­tenna that's roughly the size of the coin. Communication is initiated by holding the receiver near the sensor. That transfer requires higher power consumption than measuring data, so engineers wanted to minimize usage. To do that, readings are held in a couple megabytes of stand-alone Flash that stores data so downloads can be spaced a few days apart.

The receiver that collects this data is a palm-sized reader powered by a 48-MHz, 16-bit processor. The 106-pin device from Microchip has a number of peripherals including counters, timers and a USB port. While many companies that opt for standard components end up with unused functions, CTC engineers found a version that was almost a perfect fit.

"We use every peripheral except one," Husheer says. "The on-chip USB is criti­cal because it lets us make a device that looks like a Flash disk when it's plugged into a PC."

Developers used different programming strategies for their two components. Soft­ware in the sensor is written in assembly language to minimize size. Software in the reader is written primarily in C, speeding development and shortening development time compared to the more time-consum­ing task of writing assembly code.