The U.S. Food and Drug Administration (FDA) noted that home health care is one of the fastest-growing segments of the medical device industry. Driven by increasing life expectancy, a growing population of chronic-disease patients, and rising healthcare costs, more intelligent and user-friendly medical devices are entering the consumer market.
These health products include blood glucose meters, digital blood pressure monitors, blood-gas analyzers, digital pulse and heart-rate monitors, digital thermometers, pregnancy tests, transdermal drug-delivery systems, dialysis systems, and oxygen concentrators. Many of these instruments can connect wirelessly via the Internet to clinicians’ offices to enable continuous online monitoring and diagnosis of seriously ill patients.
As medical electronics technologies become more sophisticated, designers must ensure that clinicians, patients, and especially home users can safely and effectively operate these products. Design requirements have become more stringent, and many requirements can conflict. For designers, this means integrating more functionality into a given chip or circuit board while minimizing power consumption.
“When selecting ICs for home healthcare electronics, designers face the challenge of balancing size, low power, low cost, high reliability, long life, and safety,” said Steve Kennelly, senior manager of the medical products division at Microchip. “The required processing capability depends on who will use the medical device.”
Many medical devices are produced in relatively small volumes, making it difficult to achieve low market costs through automated manufacturing alone. A positive trend is the falling prices of individual electronic components used in these products, such as sensors, MCUs, displays, and memory.
Another challenge for medical electronics is achieving the much higher level of sealing required compared with nonmedical products, while aggressive miniaturization makes sealing more difficult.
Requirements for Home Medical Devices
Healthcare professionals are generally trained to use medical devices, but simplicity is essential for patients at home. High-integration chips, advanced DSPs and microcontrollers, high-density flash memory, and MEMS sensors all help achieve ease of use for nonprofessional users.
“We welcome these seemingly conflicting requirements because they present opportunities for innovation,” said Todd Schneider, vice president of medical business at AMI Semiconductor. Many of the company’s medical designs use application-specific standard products (ASSPs) and application-specific integrated circuits (ASICs). “We have over 20 years of experience in medical electronics, so we understand the technical challenges these devices face.”
Performance priorities vary by application. For example, low cost is the top priority for portable single-use blood glucose meters, which often rely on disposable chemical test strips. Portable home dialysis systems must prioritize reliability and long life, with cost secondary. Implantable devices such as pacemakers must deliver high reliability, small size, and long life with minimal power consumption; cost is a lesser concern in these cases.
Size Matters
Engineers must make trade-offs among sensors, analog-to-digital converters, amplification and filtering, control and data processing, power supplies, displays, and wireless transceiver circuits. Size is typically a primary constraint, especially for implantable electronics where minimal tissue intrusion is essential. Implantable devices often contain a sensor, signal-processing circuits, or a transmitter that must fit within a tiny catheter or probe for insertion into tissue. Smaller dimensions also make implantation easier for clinicians.
For example, smart pills that include sensors, cameras, and RF transmitters enable noninvasive internal imaging. DexCon’s implantable glucose monitor uses an ultra-low-power ASIC from AMI Semiconductor and continuously monitors patients via RF transmission in the 402–405 MHz band.
Single-use blood glucose meters are shrinking; current units are similar in size to handheld PDAs, and some are as small as wristwatches while still housing sensors, microcontrollers, LCDs, and batteries. These instruments typically use optical or electrochemical sensors to measure blood glucose. A patient pricks a fingertip and places a drop of blood on a disposable test strip, and the device reads the glucose level. Both the device and test strips must be designed for low cost.
The wireless ECG Holter monitor is a good example of miniaturization. Using existing IC designs from ADI, the monitor is small enough to be mounted on the back of an ECG electrode. Its lower noise and substantially reduced interference provide a more accurate signal than traditional designs.
Reducing Power Consumption
Lower power consumption is a key objective for battery-powered and portable home medical devices. Lower power translates to longer battery life and allows designers to use smaller batteries by taking advantage of modern MCUs with power-management features.
However, low power does not always mean smaller batteries. Devices that require significant computation, such as cochlear implants, may have batteries larger than the circuit volume. Cochlear implants must operate dynamically, so static sleep modes are difficult to use. These implants are often powered by an inductive power source worn externally, and they must run at high clock rates over a wide dynamic range, consuming substantial power.
Another factor affecting power consumption is IC manufacturing process. ICs made in 0.13 μm processes tend to have higher leakage and static power than earlier, larger processes. “We reduce power by optimizing the wafer chemistry in the manufacturing process,” said Todd Schneider.
Lowering operating voltage and carefully managing capacitance effects help reduce leakage currents. This is why manufacturers increasingly use chip-stacking approaches in 3D packaging rather than squeezing every device onto a single planar area.
Certain techniques help manage power: lowering clock rates and reducing active time can reduce power consumption. “The key is fast power-up,” Schneider noted. Waking a chip rapidly into active mode and keeping it in sleep mode as long as possible minimizes overall power.
Understanding application-specific IC function requirements is valuable because designers can implement necessary functions with gate logic in hardware. Although less flexible, this approach can significantly remove unneeded features from a chip and thus reduce power consumption.
Microchip’s dsPIC33F family offers three operating modes—idle, sleep, and low-power sleep—with multiple options in each mode, allowing designers to tune power consumption to the application (see Figure 1).
TI recently introduced an ultra-low-power MCU with a complete signal chain for portable medical diagnostics such as personal blood-pressure monitors, spirometers, pulse oximeters, and heart-rate monitors. The 16-bit RISC SoC MSP430FG4270 integrates a full functional chain to help designers develop low-cost portable medical devices.
Figure 2: TI’s MSP430FG4270 16-bit RISC MCU integrates the key modules required for low-power, low-cost portable medical products such as blood glucose meters.
The device offers five low-power modes to extend battery life. In standby mode, current is only 1.1 μA with an operating voltage of 1.8 to 3.6 V. At 1 MHz and 2.2 V, the device consumes about 250 μA. At a 10,000-unit quantity, the unit price is $3.78, according to TI.
NEC Electronics offers low-cost 8-bit MCUs such as the 78k0/Lx3 series, with many features tailored for portable healthcare applications. These full-flash devices integrate on-chip LCD controllers/drivers and consume very little power, drawing only 2.3 μA in standby.
There have also been advances in achieving high audio quality for ultra-low-power hearing applications such as hearing aids. AMI Semiconductor’s Eziro 5910 ASSP integrates a flexible filtering engine called a hearing enhancer that delivers high-quality audio at extremely low power. This hearing enhancer consumes less than 1 mA while providing full 24-bit processing for high-quality sound and long battery life.
DSPs in Home Medical Devices
DSPs are increasingly used in medical electronics to handle complex computation and reduce power consumption. They play a significant role in portable medical ultrasound imaging, enabling more accurate and clearer 3D imaging compared with earlier 2D systems.
An award-winning subband electronic stethoscope designed by ATM Semiconductor uses an ultra-low-power DSP that employs oversampled filter-bank signal processing. The DSP provides 21 dB of gain and greatly improves performance compared with traditional acoustic stethoscopes. It operates at 1.8 V and consumes 4.1 mW. The entire electronic stethoscope consumes 47 mW, most of which (43 mW) is used by the LCD display.
Cochlear, a cochlear implant manufacturer, recently collaborated with AMI Semiconductor to design and manufacture a next-generation DSP-based SoC for implantable products. The DSP-based design provides greater processing power in a smaller package, enabling lower power consumption (longer battery life) and higher audio quality than non-DSP methods.
Whether selecting DSPs, MCUs, displays, sensors, or other components, IC choices for medical applications require careful trade-offs. For example, compact flash (CF) memory is widely used in devices such as Holter monitors to record ECG data. Although CF memory is common in consumer electronics, not all CF memory meets the performance, wear balancing, error correction, and data-protection standards required for medical devices. “We were the first company to design CF cards specifically to meet the strict requirements and advanced performance needs of medical devices,” said Mark Downey, strategy development director at White Electronic Designs. Low-cost CF memory designed for consumer applications cannot always satisfy medical-device standards for performance and data integrity.
