Providers of wireless connectivity solutions have tread a long, hard path to develop sensor-based medical devices that dispense with cords and cables. Along the way, they have had to confront myriad challenges, from power and usability issues to data-transmission and security concerns. But now that designers and manufacturers of sensor-based medical devices--in collaboration with sensor and semiconductor chip suppliers--have succeeded in overcoming many of these challenges, a new generation of wireless medical devices is appearing on the market.

Medical Devices Cut Loose
The main advantage of adapting wireless technologies to medical device applications is the ease of use they afford doctors and patients alike. However, despite their obvious benefits, the use of wireless sensing technology in medical applications is relatively new, remarks Stephen J. Swift, senior vice president/general manager in the communications and medical products group at Microsemi Corp. (Aliso Viejo, CA).

For example, while wireless sensors are beginning to be used in ECG and pulse-oximetry equipment, wired devices are still ubiquitous features in most hospital suites. "Patients still have ECG patches attached to their chests so that their cardiac function can be monitored during surgery, recovery, and--in some cases--in the ICU," Swift comments. "However, the wires connecting these sensors to bedside monitors are very inconvenient. They can be pulled off when patients roll over, or they make patients immobile."

Pulse oximeters, or SpO₂ monitors, pose similar challenges. Traditionally worn on a finger or clipped to an earlobe, these devices incorporate sensors to measure how much oxygen a patient is receiving into the bloodstream--known as partial pressure of oxygen. Like ECG systems, pulse oximeters traditionally use wires to connect patients to bedside monitors, resulting in the same types of issues associated with wired ECG patches. "Being able to eliminate the wires used in ECG, pulse oximetry, and other medical device applications is important," Swift notes. "Developing sensors with wireless capability for medical device applications improves the practice of medicine while increasing patient comfort."

Providing radio-frequency integrated circuits that interface directly with sensors to pick up physiological signals and measure a variety of bodily functions, Microsemi offers technologies that make wireless connectivity possible for a range of medical devices. Based on RF CMOS chips, the company's connectivity technologies are integrated into medical device sensors to enable wireless functionality. The chips interface with the sensors in a variety of ways, depending on the type of sensor. However, all such chips contain a converter that picks up a sensor signal, converts it into a digital signal, and then transmits this signal wirelessly to the medical device.

"Our chips can enable a range of different sensor applications that measure parameters used in human clinical data collection, such as temperature, pressure, pulse oximetry, and ECG," Swift says. "In addition, a couple of companies are working on blood pressure, developing a wrist cuff that measures the patient's blood pressure and telemeters it to a cell phone or other electronic device." Just about any bodily function that can be picked up or measured can be made wireless, he adds.

Easing into Low-Power Sensors
Despite its potential, wireless technology hasn't been used more readily in the medical device sphere for several reasons, according to David Niewolny, medical business development manager for microcontrollers, microprocessors, and wireless devices at Freescale Semiconductor (Austin, TX). Two of the most important hurdles include meeting the ease-of-use requirements common to many medical devices and achieving low power consumption.

One of the keys to developing handheld devices is to make them easier to use, Niewolny remarks. But that may be easier said than done. "For example, while Bluetooth has been used in some medical device technologies, it is not easy for this communication technology to pair medical devices with whatever they need to connect to," Niewolny remarks. To overcome such usability issues, Freescale has developed chips for wireless medical device sensors based on ZigBee, a specification for a suite of high-level communication protocols that mandates the use of small, low-power digital radios based on the IEEE 802 standard for personal area networks. ZigBee, according to Niewolny, has substantial ease-of-use advantages, especially its meshing capability.

ZigBee's ability to be integrated into wireless medical sensors is attributable to data rate, Niewolny explains. While transmitting larger data rates requires higher-power technologies such as Bluetooth and streaming or imaging data requires Wi-Fi, many medical devices such as blood pressure or pulse-oximetry devices do not transmit a significant amount of data. A growing feature of the medical device marketplace, ZigBee is just as suitable for low-data-rate applications as is the up-and-coming Bluetooth Low Energy, or Bluetooth 4.0, technology, Niewolny adds.

"The question we ask our customers all the time is, 'What do you want to connect with?'" Niewolny says. "If you're just looking to establish basic point-to-point-type communication or you can use a mesh network for things such as asset tracking, that's where ZigBee comes in. But for applications involving smart mobile devices such as tablets or phones, Bluetooth Low Energy is preferable."

Besides its meshing ability, ZigBee is also easy to integrate into medical device sensors to provide them with wireless functionality, remarks Steven Dean, Freescale's global healthcare segment lead. Thus, while ZigBee can be supplied in chip form, it is also offered as a system-in-package that incorporates an antenna, a wireless balloon LAN, and a microcontroller. "In consumer medical applications, pieces of end devices are designed, developed, and rolled out into the market, and then a manufacturer may want to add wireless connectivity," Dean comments. "But they don't want to go in and mess with the guts of what they've already developed. They really want something that they can just bolt on." Because ZigBee can meet this need, system-in-package technology is helping to enable manufacturers of medical sensing applications to go wireless.

In addition to ease-of-use issues, ensuring low power consumption has also been a prime challenge with wireless medical device sensors. "Power consumption products such as Wi-Fi have not been very conducive to devices using coin-cell batteries, a standard configuration in many portable medical devices for both consumer and clinical applications," Niewolny remarks. "For both point-to-point communication or for aggregating data into the Cloud for future retrieval of electronic medical records, the power-consumption targets of some wireless technologies have not quite been there." However, because of its ability to offer low power consumption, ZigBee has become the most readily available technology for use in wireless consumer, or home, medical device applications, Niewolny notes.

Despite its many benefits, ZigBee might soon be outflanked by Bluetooth Low Energy, however. "Many of the features now available in Bluetooth Low Energy have been found in ZigBee for quite a while, including its low-power-consumption capability," Niewolny says. "But the fact that Bluetooth has caught on more than ZigBee, at least in the medical device market, had everyone waiting for the arrival of Bluetooth Low Energy." And now that it is already being used in smart mobile devices or devices that will soon be marketed, I think there is going to be a huge surge in Bluetooth Low Energy-enabled devices."

Securing Data Transmissions
Achieving this ideal of low power consumption has been the main issue driving the development of wireless sensor technologies, remarks Jay Brown, sensor unit vice president of NVE Corp. (Eden Prairie, MN). Used in such battery-powered implantable devices as pacemakers, ICDs, neurostimulators, and drug pumps, the company's sensors must accommodate low power budgets. "For example, a typical implantable medical device has a power, or current, budget of 10 ?A," Brown says. "The device has got to run on that 10 ?A for 15 years to ensure long battery life."

Unlike the sensor applications for which Microsemi and Freescale are developing wireless chips, NVE's wireless sensors are used to authenticate incoming data transmissions to implantable devices. "Typically, our sensors are used as a kind of safety interlock in wireless medical device applications," Brown says. "The problem that manufacturers of implantable medical devices face is that the patient and the implantable device are constantly bombarded with RF communication links from cell phones and many other electronic devices. Despite the security protocols that device makers may build into their devices, it's difficult for them to completely rule out the possibility that some spurious RF signal will reprogram their devices. In order to avoid this issue, such manufacturers use our sensors."

NVE's GMR-based sensors ensure that wireless RF information sent by the physician to the implanted medical device is secure. The physician accomplishes this function by holding a wand containing an RF transmitter and a magnetic field against the patient's chest before sending programming information to the device. "Our sensor detects the presence of the magnetic field, enabling it to validate the good information while ignoring all other RF signals," Brown says. "The valid information is used to reprogram the medical device."

The objective of NVE's sensors is to ensure that the implantable device is not misprogrammed inadvertently, according to Brown. "When you send a magnetic-field pulse containing high and low magnetic-field values to the medical device, this pulse looks like a data stream to our sensors. The sensors transmit this pulse to the medical device and confirm that the incoming information is secure and valid before the device's software and microcontroller are modified." In short, the sensors receive magnetic information and translate it into electrical information that the microcontroller can read.

While current-generation wireless medical device sensors are used in a range of devices for monitoring or controlling such bodily functions as partial pressure of oxygen or the electrical activity of the heart, tomorrow's wireless technologies face steeper challenges. For example, one of the holy grails for such companies as Medtronic is to develop an artificial pancreas. Such technology would be based on an insulin pump and a glucose sensor. "Ideally, such technology would employ an implanted glucose sensor that uses either rechargeable batteries or energy-harvesting technologies to telemeter the glucose signal wirelessly through the skin to the pump," Microsemi's Swift comments. "This capability is not available today, but we are providing the wireless radio technology to enable it to happen."