Rapid advances in portable elecÂtronics are
bringing hope for couÂples 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 TemperaÂture 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 informaÂtion 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
acceleraÂtion 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. HushÂeer 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 packagÂing make it possible to take
precise temÂperatures 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 inÂjection-molded component
that includes thermally conductive plastic polymers that surround the sensor.
The patented technique includes a non-conductive overÂmolded plastic that
protects the unit.
The other major portion of the sensor module is the communication
sysÂtem that sends data to the reader. The body-worn module sends data using a
modified RFID chip that has a coil anÂtenna 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
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 critiÂcal 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. SoftÂware 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-consumÂing task of
writing assembly code.