Wearable blood pressure sensor edges closer to reality

Blumio wearable protoype
Wearable blood pressure monitors based on Blumio's new type of sensor, which uses Infineon’s XENSIV radar chip, are expected to emerge in 2022.

Silicon Valley startup Blumio is closing in on the Holy Grail of blood pressure monitoring technology: A wearable sensor that provides continuous measurement. The use of a contact-less, pressure-less sensor can produce accurate blood pressure measurements to tonometer-based approaches, which are more difficult to use and apply.

FierceElectronics spoke with Blumio Co-Founder and CEO Catherine Liao about the significance of a wearable blood pressure monitoring device, its basic operating principles, and her company’s go-to-market strategy.

FE: From what I’ve read, developing a technology for continuous measurement of blood pressure is a notoriously hard problem to solve. Why do you think you will succeed when others have not?

Liao: Continuous blood pressure monitoring is currently possible with arterial line, applanation tonometry, and volume clamping. I think your question refers to the challenges with cuffless blood pressure monitoring.

Past attempts mainly focused on using a combination of off-the-shelf sensors including electrocardiogram (ECG), photoplethysmography (PPG), and ballistocardiogram (BCG) to determine blood pressure using pulse transit time or pulse wave analysis. Unfortunately, none of the aforementioned sensors provides a pressure measurement, and thus introduce additional variables that cause blood pressure predictions to deviate, resulting in inaccurate measurement. Dr. Jay Pandit of Northwestern recently published a great paper that summarizes these approaches and their challenges.

We are fundamentally designing a new type of sensor using radar. While there are FDA-approved blood pressure monitors based on the tonometer, they tend to be difficult to use as it requires precise placement and constant pressure exerted onto the measurement site that causes discomfort and bruising. In our IEEE paper we demonstrated that our sensor can produce a pressure signal that is comparable to that of a tonometer.  This gives us high confidence that we can produce accurate BP measurements equivalent to tonometer-based approaches, with all the advantages expected from a cuffless wearable.

FE: How much does blood pressure fluctuate throughout the day and why is it an important breakthrough to be able to measure it continuously?

Liao: There is a natural fluctuation to blood pressure — from minute to minute and hour to hour. These fluctuations generally happen within a normal range. But when blood pressure regularly spikes higher than normal, it’s a sign that something isn’t right in our body and merits investigation. Getting an overview of how one’s blood pressure fluctuates throughout the day will make it possible to identify the cause behind the fluctuation, thus enabling targeted treatment / action plan to address it. 

A 10-year  study following 63,910 adults found that ambulatory blood-pressure measurements were a stronger predictor of all-cause and cardiovascular mortality than clinic blood-pressure measurements. White-coat hypertension was not benign, and masked hypertension was associated with a greater risk of death than sustained hypertension.

FE: Blumio had expected to have a product on the market in 2017, which obviously didn’t happen. What challenges are you currently working to solve before the product is ready for market?

Liao: Back in 2016 we successfully developed a prototype blood pressure monitoring system using home-grown radars that could detect pulsation from the artery at the wrist and the upper arm.  In human studies we found that interactions with the skin surface introduced unwanted noise into the signal. This caused us to explore alternative radar systems which is how we ended up working with Infineon’s XENSIV radar chip, a frequency-modulated continuous wave (FMCW) system operating at 60 Ghz. 

Blumio blood pressure sensor on prototype board

Blumio's sub-gigahertz prototype, which also has a smart phone app.

FE: What issues does radar solve and what issues does it introduce in this application?

Liao: The biggest benefit with radar is that it does not apply pressure to the site of measurement and does not require contact with the skin. In our current prototype system, the radar chip is positioned a few millimeters from the surface of the skin.

The disadvantage of radar is motion noise. Because the motion we’re measuring is so small (as small as 0.05mm of skin distention), it can get buried in larger motions from bodily movements. That being said, blood pressure doesn’t need to be measured heartbeat-by-heartbeat in everyday settings. In fact, health experts recommend that a person’s blood pressure be taken in a seated position after 5 minutes of rest. This is where we can leverage a motion sensor to tell us whether a person has been still (and for how long) to capture a blood pressure measurement that is clinically relevant.  

FE: How does radar technology work for blood pressure measurement and why use 60 GHz?

Liao: The high frequency of the 60 Ghz eliminated the noise that we observed with our home-grown system. This increased frequency signal has a much smaller wavelength and thus is very well suited to measuring the small perturbations of the skin surface that would be expected from an arterial surface pulse. Plus, the small size (5mm x 6mm) made it suitable for a wearable form factor. Earlier this year we published a paper in IEEE with more details on how the 60 Ghz radar can be leveraged for arterial pulse detection, and a comparison with signals from a tonometer - a tool used for continuous measurement of arterial pressure.

FE: You’re using the XENSIV radar IC from Infineon. Is the sensor ready to go as is? If not, what further developments are needed?

Liao: The XENSIV radar IC from Infineon is released and available. In fact, Google uses it in its Pixel phones for gesture sensing. However, the radar IC on its own is not enough for health sensing. We are finalizing our algorithms  that transform the radar signal into an arterial pressure waveform and the subsequent calculation of blood pressure. You can learn more about gesture sensing here. And here’s a picture of Infineon’s XENSIV IC used in our latest prototype device.

Infineon radar sensor

FE: Can you talk specifically about the algorithm you are developing and whether you are using machine learning?

Liao: There are 2 sets of algorithms that we are developing:

  1. Radar → Pressure Waveform. This algorithm takes radar signal from the XENSIV radar chip and produces a high -fidelity arterial pressure waveform that can be used to assess cardiovascular metrics. We use signal processing techniques to perform this conversion.
  2. Pressure Waveform → Blood Pressure. This algorithm takes in the arterial pressure waveform that’s generated from the radar sensor, and translates it into discrete blood pressure values (systolic, diastolic, and mean blood pressure).

We use machine learning to help us refine the algorithm but the algorithm itself is based on signal processing techniques. Machine learning allows us to quickly sift through hundreds of characteristics within the radar signal and pinpoint those that are most relevant to blood pressure. Without it this could potentially take years to accomplish.

FE: What is the product that you will be delivering to the market?

Liao: The vision for our sensor is to be the enabler of health wearables in the future. So early on we decided that we should make the sensor we are developing widely available to device makers, not unlike what InvenSense did with its combined three-axis accelerometer and gyroscope that has made its way into every smartphone and wearable out there.

To accomplish that vision, our first priority is to deliver a sensor that can be used by medical device makers to build their own health wearables. Our sensor outputs a high-fidelity arterial pressure waveform that enables different types of health sensing algorithms to be developed. We actually have an example of a medical device maker accomplishing just that. CardieX, a 26-year old global health technology company and an investor in Blumio, that focuses on hypertension and cardiovascular disease recently ported their central blood pressure algorithm to work with our sensor. Experts believe that central blood pressure is more accurate and useful than peripheral blood pressure. Our sensor will make such metrics accessible outside of clinical settings.

Our longer-term vision is a clinical use case for ambulatory blood pressure monitoring, in which our sensor would be built into a service prescribed by physicians. Potentially hypertensive patients would be prescribed 24-hour ambulatory blood pressure monitoring to capture their BP fluctuations. Existing devices used for such monitoring rely on inflatable cuffs that are intrusive to one’s daily routines. Frequent compression of the cuff causes pain, often bruising, and interrupts sleep. When you consider that nearly half of American adults  are hypertensive, we have an opportunity to deliver an innovation that provides a positive impact on people’s lives.

FE: What is your new timetable for launch?

Liao: We are targeting the launch and availability of our sensor in 2021. Prior to the launch, we will make a sensor development kit available to joint development partners with a radar development board, compute unit and sample code for arterial waveform recording and analysis. We will also work with Infineon to provide a reference design with guidelines and tips on how to  integrate Infineon’s XENSIV radar chip into a circuit board. We expect to see blood pressure monitors based on our sensor to emerge starting in 2022.

Editor’s Note: Blumio’s Catherine Liao will be speaking about her journey working for the startup she co-founded at MedTech Innovation Week, a digital event series taking place October 19-22, 2021. For more information and to register for your free pass click here.