According to the U.S. Food and Drug Administration (FDA), home healthcare is the fastest-growing segment of the medical device industry. Longer life spans, an increasing number of patients with chronic medical conditions, and rising health costs are the main forces behind the trend of immersing the consumer home market with “smarter” and “friendlier” medical devices (Fig. 1).
What kinds of medical care products are turning up in the home? Blood glucose meters, digital blood-pressure meters, blood gas meters, digital pulse and heart-rate monitors, digital thermometers, pregnancy testers, iontophoresis transdermal drug-delivery systems, dialysis systems, and oxygen concentrators. Many can be interconnected via the Internet, often wirelessly, to the offices of healthcare providers for constant on-the-spot monitoring of vital signs and diagnostics.
As technological medical product developments become more complicated, so do the requirements for their design—many of which can be conflicting—to ensure that they can be used safely and effectively by everyone, including healthcare providers and patients, and most importantly, the home user. For the design engineering community, all of this has meant doing more on the chip or circuit board in a given amount of space while dissipating the least amount of power.
“The biggest problem for a designer of ICs for home healthcare products is knowing how to properly balance issues like small size, lower power, costs, reliability, lifetime, processing power, and safety,” explains Steve Kennelly, senior manger of the Medical Products Group of Microchip Technology. “The amount of processing power needed is influenced by who the user of the medical device is.”
Many medical devices aren’t produced in very large quantities, making it difficult to automate their manufacture and achieve low market costs. One positive note on this front is that prices for individual electronic components within these products (sensors, MCUs, displays, memory, etc.) are sliding.
Yet another roadblock is achieving hermetic levels well beyond those required for non-medical applications. Across-the-board miniaturization makes hermeticity even harder to attain.
Get To Know Your Device
Operating medical equipment is more intuitive for doctors and clinicians since they’re trained to use such tools. For a home patient, however, operational simplicity is much more important. Fortunately, the latest highly integrated ICs, sophisticated DSPs and microcontrollers, high-density flash memories, and advanced microelectromechanical systems (MEMS) sensors help to achieve that goal.
“We welcome conflicting requirements since they represent an opportunity for us to innovate,” explains Todd Schneider, vice president in the Medical Business Unit of AMI Semiconductor, which largely uses application-specific standard products (ASSPs) and application-specific ICs (ASICs) in its medical designs. “We’ve been in the medical device business for over 20 years and understand the technical challenges posed.”
Every healthcare medical product requires a different set of performance priorities, depending on the application. For example, low cost is one of the top parameters for something like portable glucose meters, which often use disposable chemical strips. A portable home dialysis system, on the other hand, must have reliability and long life as top-of-the-list requirements in a design, with cost being a secondary factor. Implantable devices like automated pacemakers primarily must be highly reliable and small with a long life and as little power dissipation as possible. Cost isn’t a factor in this case.
Size Does Matter
As a result of the large number of performance requirements for medical products, engineers face various design tradeoffs. This means they must carefully balance what kind of sensor, analog-to-digital conversion, amplification and filtering, control and data-processing, power supply, display, and wireless transceiver circuits to use.
Size often is a major design constraint, particularly for medical implants, where minimal invasiveness is an absolute must. Such implants typically contain a sensor, some signal-processing circuitry, and possibly a transmitter, all designed to fit into a tiny catheter or probe that’s inserted into the human body. Small size also makes it easier for the physician or healthcare provider to place the implant in the body.
For example, some smart pills contain sensors, cameras, and RF transmitters that provide a clear and non-invasive view of internal organs. Similarly, DexCon Inc. uses an ultra-low-power ASIC system-on-achip (SoC) from AMI Semiconductor in its implantable glucose monitor, which continuously monitors diabetics using RF transmissions in the 402- to 405-MHz range.
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Glucose meters that are disposable also look to get as small as possible. Typically, they’re the size of handheld personal digital assistants (PDAs). Some meters are even about the size of a small wristwatch, yet they contain a sensor, microcontroller, liquid-crystal display (LCD), and batteries. These meters generally use optical or electrochemical sensors that measure glucose levels. Patients prick themselves to obtain a drop of blood for a disposable test strip, which the meter reads to determine glucose levels. The fact that the machines and the test strips are disposable necessitates a low-cost design, too.
A good example of miniaturization can be seen in a wireless electrocardiogram (ECG) Holter monitor (Fig. 2). Using off-the-shelf Analog Devices ICs, this monitor is small enough to fit on the backside of an ECG electrode, yet it provides more accurate signals than traditional designs thanks to lower noise and greatly reduced interference levels.
Cut The Power
Lower power consumption is critical and much desired in medical products, particularly battery-operated and portable home devices. Simply put, power reduction means a longer battery life. It also gives designers the flexibility to use smaller batteries and take advantage of the latest generation of MCU ICs, which offer powermanagement features.
However, lower power dissipation may not always correlate with a smaller battery size. When computational power requirements are large, as in cochlear implantable hearing devices, the battery can be larger than the circuit it powers. In cochlear implants, dynamic-mode operation is mandatory, and static “sleep” modes are difficult to use. These implants, powered inductively from a source worn outside the ear, must operate continuously using a fast clock rate and at a wide dynamic range that burns up a lot of power.
Another issue affecting consumption is the process on which an IC is made. ICs manufactured on 0.13-µm processes feature more leakage and static power consumption than earlier-generation ICs made with wider line geometries. “We optimize an IC wafer’s chemistry during manufacture for lower power consumption,” says AMI Semiconductor’s Todd Schneider.
Reducing an IC’s operating voltage and carefully managing capacitance effects can go a long way toward reducing current drains. This is one reason why medical system manufacturers tend to stack chips atop one another (in 3D packaging) instead of squeezing everything on a limited-area planar surface.
Fortunately, certain techniques help manage power consumption. For example, a slower clock rate and less time operating in the dynamic mode can minimize power dissipation. “The key is to power up quickly,” says Schneider. Waking a chip up into the dynamic mode and putting it back to sleep for as long as possible ensures lower power consumption.
A thorough understanding of an application’s need for IC functions is quite valuable, enabling designers to hard-code all necessary functions in gates. Though this approach isn’t very flexible, it can significantly cut down on any unneeded functions on a chip and, therefore, minimize power dissipation.
Designers can choose from three operating modes on Microchip Technology’s dsPIC33F series microcontrollers, each with multiple options: idle, sleep, and doze. This arms designers with more flexibility to scale power consumption to fit the application (Fig. 3).
Texas Instruments recently introduced an ultra-low-power MCU with a complete signal chain for portable medical diagnostics like personal blood-pressure monitors, spirometers, pulse oximeters, and heart-rate monitors (Fig. 4). The 16-bit reduced instruction- set computer (RISC) MSP430FG4270 SoC integrates a comprehensive chain of functions needed for the design of low-cost portable medical equipment.
Five low-power modes are available to extend battery lifetime. In the standby mode, power consumption is a mere 1.1 µA, operating from a supply of 1.8 to 3.6 V. At 1 MHz and 2.2 V, the device dissipates roughly 250 µA, which is exceptionally low. The unit’s price of $3.78 each in quantities of 10,000 units is the lowest in the industry for equivalent ICs, according to Texas Instruments.
Inexpensive 8-bit MCUs like the 78k0/Lx3 family from NEC Microelectronics are available with many features designed for portable healthcare applications. The all-flash-memory units feature on-chip LCD controllers/drivers and dissipate very little power. They only draw 2.3 µA in standby mode.
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Remarkable progress has been made in creating very highquality sounds for audiology applications (e.g., hearing aids) that use extremely low power. The Ezairo 5910 ASSP from AMI Semiconductor includes a flexible filtering engine called the Hear accelerator, which features extremely low-power operation at very high-quality sounds (Fig. 5). The accelerator draws less than 1 mA while providing full 24-bit processing for high-quality sounds with long battery life.
DSPs Chipping In
DSPs are now finding more homes in medical products, helping to handle complex computations and lowering power dissipation. They’ve had a large impact in portable medical ultrasonic imaging equipment, where they’ve enabled more accurate and clearer 3D imaging than previous 2D representations.
An ultra-low-power DSP lies at the heart of an award-winning sub-band electronic stethoscope, which was designed by AMI Semiconductor with a signal-processing approach using an oversampled filterbank (Fig. 6). The unit provides 21 dB of amplification— a significant improvement over conventional stethoscopes that attenuate sound signals. Yet it operates from just 1.8 V and consumes 4.1 mW. Total power dissipation is 47 mW, most of which (43 mW) is consumed by an LCD readout.
Cochlear Ltd., a manufacturer of cochlear implants, recently selected AMI Semiconductor to co-design and manufacture future-generation DSP-based SoCs for the implants. The DSPbased designs will provide greater computational power in a smaller package. They also will enable lower-power (longer battery life) and higher-quality sounds, which are tough to cost-effectively create when using conventional non-DSP approaches.
Whether it’s for a DSP, MCU, display, sensor, or other function, the choice of an IC for medical applications must be weighed carefully. Take compact flash memory, which is widely used in such medical equipment as the aforementioned Holter monitors that record heart ECG data. This same type of memory permeates consumer electronics devices. But, according to Mark Downey, director of strategic development of White Electronic Designs Corp., not all compact flash memories are created equal.
“We’ve been the first company to offer compact flash-memory cards designed specifically to meet the stringent requirements and advanced performance needs of medical devices,” Downey says. He points out that low-cost compact flash memories designed for consumer applications do not meet the performance, wear-leveling, error-correction, and data-protection specifications required by medical devices.