Ageing populations, increased homecare and political implications associated with reducing healthcare budgets all contribute to the stability of the long-term growth market in the medical field for semiconductor suppliers. Largely due to these and other contributing factors, the industrial/medical market for semiconductors is expected to grow to $33bn by 2013, according to industry analysts.
This topic was discussed with a broad audience interested in product manufacturing and services in the personal healthcare field at October's GSA Emerging Opportunities Expo & Conference held in Santa Clara, California, US, giving conference attendees examples of how chip vendors have seized the opportunity for low-cost systems-on-chips (SoC) in portable medical devices.
Medical electronics original equipment manufacturers (OEMs) are developing more sophisticated personal healthcare solutions for treating and monitoring common illnesses. These products now greatly improve the quality of healthcare at an affordable cost.
Microcontrollers (MCUs) play a significant role in a variety of portable medical instrumentation products such as personal blood pressure monitors, spirometers, pulse oximeters and heart rate monitors. In most of these products, the actual physiological signals are analogue and need signal conditioning techniques such as amplification and filtering before they can be measured, monitored or displayed.
MCUs in portable medical products
Typical chip requirements in portable medical applications are embedded high-performance analogue peripherals within an MCU operating with ultra-low power. Today's modern chip making technology brings this to a reality in a single SoC device. It is realistic to find an off-the-shelf SoC suitable for use with portable medical electronics applications operating with long battery life.
Analogue front-end design in portable battery operated medical electronics can be simplified by using embedded high-performance peripherals such as opamps, analogue to digital converter and digital to analogue converter integrated with a low-power MCU. The MCU not only offers digital filtering and processing, but also displays parametric results of physiological data such as blood pressure, lung capacity, heart rate and blood oxygen.
Communication is possible with serial wired or wireless technology. These features can all be added while meeting demanding power consumption requirements, by turning off peripherals for a standby current in the fractions of micro amperes.
Blood pressure monitors
In this application a bridge type pressure transducer is typically used as a sensor attached to an inflatable cuff. The transducer can be energised via port pins only during pressure measurement thereby saving power. The output from the sensor is proportional to the pressure and is in the microvolt range.
This signal needs to be amplified before it can be digitised for measurement by the analogue to digital converter. Amplification can be achieved with the integrated opamps.
Spirometers, also known as pulmonary function testing equipment, are used in medical diagnostics for measuring a patient's lung capacity. In this application, the measured parameter is air flow rate during inhalation and exhalation in litres/min.
The sensor used for this application is typically a pneumotach transducer, essentially a differential pressure transducer. This application design is similar to the blood pressure monitor except that an inflation motor is not required.
The rest of the application is straightforward, measuring the flow using the integrated sigma-delta analogue to digital converter and comparing the measured values against stored standardised values. The Flash memory is useful for storing a variety of standardised values, making the design suitable for use with a variety of situations. Similar to the blood pressure monitor application, the low power operation of the MCU offers long battery life and high-integration reduces cost with increased system reliability.
Pulse oximeters and heart rate monitors
Pulse oximeters are devices that measure blood oxygen saturation and heart rate of a patient. In the commonly used non-invasive optical plethysmography technique, oximeters consist of a peripheral probe combined with the MCU unit displaying the oxygen saturation and pulse rate. The same optical sensor is used for heart rate detection and oxygen saturation measurement in this application.
This technology provides an easy, accurate and non-invasive way to estimate arterial blood oxygen saturation and heart rate levels. The probe is placed on a peripheral point of the body such as a fingertip, ear lobe or the nose. The probe includes two light-emitting diodes (LEDs), one in the visible red spectrum (660nm) and the other in the infrared spectrum (940nm). Figure 2 shows this probe placed on a finger.
The light beams pass through the tissues to a photo detector. During passage through the tissues, the light is partially absorbed by haemoglobin in the red blood cells in differing amounts depending on the oxygen saturation level. First, by measuring the absorption at the two wavelengths, the MCU can precisely compute the proportion of haemoglobin that is oxygenated. Second, the light signal following transmission through the tissues has a pulse component resulting from the changing volume of arterial blood with each heart beat.
The two LEDs must be driven with constant current sources to guarantee a stable brightness condition during measurement.
The constant current source with automatic gain control (AGC) feedback can be derived using the internal digital to analogue converter and a simple algorithm running in the MCU. The MCU can select out the absorbance of the pulsatile fraction of blood – due to arterial blood, from non-pulsatile venous or capillary blood and other tissue pigments constant absorbance.
Recent measurement techniques have reduced the interference effects on oxygen saturation calculation. Time division multiplexing, where the LEDs are cycled many times per second, helps to eliminate background noise. Quadrature division multiplexing is a further advance where the red and infrared signals are separated in phase rather than time and then recombined in phase later.
Saturation values are averaged out over several seconds. Depending on the particular monitor, the pulse rate is also calculated from the number of LED cycles between successive pulsatile signals and averaged out over a similar variable period of time.
From the proportions of light absorbed at each frequency, the MCU calculates the ratio of the two parameters. Stored within the MCU's Flash memory is a series of oxygen saturation values obtained from experiments where volunteers were given increasingly hypoxic mixtures of gases to breath. The MCU compares the absorption ratio at the two light wavelengths measured with these stored values, and then digitally displays the oxygen saturation as a percentage. Typically, the values in the 70% to 100% range are accurate. Below 70% the data is extrapolated because it is not possible to have data from humans below this oxygenation level.
The complete pulse oximeter circuit realisation can be made with a MSP430FG479 based portable medical system. The integrated op-amps, 16-bit sigma-delta analogue to digital converter and dual digital to analogue converters offer a complete analogue front-end solution. The digital to analogue converter combined with the on-chip reference helps generate a constant current source for the LEDs. One of the op-amps is used as an I to V converter for the sensor photodiode. AGC is provided by
adjusting the LED brightness using the digital to analogue converter output and a software algorithm executed by the MCU.
The amplified and filtered output is digitised by the sigma-delta analogue to digital converter and averaged out by the software. This data for both the red and infrared sources and their ratio is collected and calculated. This ratio is compared against the stored standard data and the oxygen saturation is accurately determined. The computed oxygen value is displayed on the LCD as a percentage.
The A/D conversion values also carry the pulsating heart beat information, which is averaged by software for about five seconds and the heart rate is computed. This is displayed on the LCD as well. Additionally, the PWM output of the MCU drives a piezo beeper briefly for every heart beat. The periodic beep serves as an indicator for proper sensor positioning and signal pick-up.
Benefits of MCUs
Now that the challenge of choosing the right MCU for these applications is addressed, the next step for designers of these systems is software development. Several compilers and debuggers are available today and the debugger hardware is very inexpensive.
The debugger hardware required is a simple interface unit connected to a PC via USB. The full feature real-time emulation allows break-points to be set in hardware inside the chip and provides real-time operation while debugging.
The device offers great value for the money in system design because of its high integration and ease of code development. The Flash programme memory allows instant code refreshing during application development. With this technology quick time-to-market is a reality for the designers using an MCU in their applications. The 60KB size memory in this device also serves as a data logger due to in-system programmability of the Flash memory.