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Build blood pressure monitors using sensor modules, ready-to-use algorithms

Blood pressure monitoring, now part of daily life, has mostly been achieved with bulky and mechanical cuff-based medical devices. That’s changing with the availability of solid-state blood-pressure monitoring solutions that are non-invasive and don’t require cuffs.

More importantly, various design kits featuring basic hardware and software building blocks allow developers to jumpstart 24/7 cuffless blood-pressure measurement designs. The basic premise of these reference designs and evaluation kits is to reduce the number of design steps in the development process.

Figure 1 The highly-integrated sensor modules and ready-to-use algorithms accelerate the development of blood pressure monitors in various form factors. Source: Infineon

Blood pressure measurement, which requires specific expertise areas, such as filtering functions for extracting the waveforms required for measurement, is extremely time-consuming. On the other hand, with an evaluation kit, engineers can take the calculations extracted from pulse waveforms and simulate them using graphical user interface (GUI) development tools. Next, these simulations can be copied from the GUI tool to the firmware and verified in the actual application being developed.

Highly-integrated hardware

In these reference designs and evaluation kits, single-chip solutions are most prominent. For a start, they are crucial in miniaturization, taking blood pressure monitors to even wearable form factors. Second, blood pressure monitors are typically battery-powered, so low power consumption is a key design consideration and that favors single-chip solutions.

Not surprisingly, therefore, reference designs for blood-pressure monitoring are increasingly built around single-chip solutions that incorporate rich analog functions. That includes high-resolution A/D converters, programmable gain instrumentation amplifiers, D/A converters, operational amplifiers, and other circuits required for blood pressure measurement.

Take the example of the blood pressure monitoring evaluation kit from Renesas, built around the company’s RL78 microcontroller-based ASSP, which includes analog functions required for blood pressure measurement. The evaluation kit comprises a pressure sensor, arm cuff, pump, electronically-controlled valve, LCD panel, and a reference board built around the RL78/H1D chip. The kit also includes reference software and a GUI development tool.

The European chipmaker ams has also built a reference design around its AS7024 sensor module in collaboration with a partner. The sensor module includes three LEDs, photodiodes, an optical front end and sequencer for heart-rate measurement (HRM), and an analog front end for electrocardiograms (ECG).

Figure 2 The AS7024 sensor module has proven to be accurate to <± 7.5 mmHg for systolic pressure and <± 5.5 mmHg for diastolic pressure. Source: ams

The design solution analyzes synchronized HRM and ECG measurements to calculate blood pressure; these blood pressure measurements have been validated in a clinical trial carried out at the Medical University of Graz in Austria. The reference design and evaluation board for the AS7024 is complemented with an ams Simblee Bluetooth wireless module for connectivity, wireless charging capability, and a smartphone app that displays heart rate measurements, blood pressure, vagal tone and arterial elasticity.

The software complement

Besides highly-integrated hardware solutions, software and accompanying algorithms are also critical in accelerating the blood pressure monitor designs. The accompanying software in reference designs is crucial in implementing blood pressure measurement, HRM, heart rate variability (HRV), and ECG.

The software encompasses the algorithms required for blood pressure measurement, and these algorithms can be easily modified using GUI development tools. Algorithms usually run on an MCU, eliminating the need to integrate software code on the application processor. The MCU or sensor hub IC includes the firmware that performs the blood pressure measurements while executing algorithms for these measurements.

The reference design solutions—both from Renesas and ams—come with software components. For instance, with Renesas evaluation kit, pressure sensor, pump, electronically controlled valve components and pulse width modulation (PWM) control can be set from the GUI tool.

Figure 3 Design kits aim to reduce the barriers to entry for developers who are new to the healthcare device ecosystem and may not have application-specific design expertise. Source: Renesas

The critical importance of the software component in blood pressure monitoring solutions is underscored by chipmaker Infineon’s collaboration with Blumio, the cardiovascular health monitoring expert that develops software and algorithms for consumer and medical wearable devices. While creating a development board for a non-invasive blood pressure sensor, based on its XENSIV radar chipset, Infineon joined hands with Blumio for the startup’s system understanding of blood pressure monitor designs.

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