04Feb

Silicon Photonics Device for Arterial Stiffness: Interview with Roel Baets and Patrick Segers, Imec and Ghent University

Imec, a research and innovation hub for nanoelectronics and digital technologies based in Belgium, in collaboration with numerous partners, has developed a new medical device to screen for arterial stiffness, a risk-factor for cardiovascular disease. Routine screening of arterial stiffness in a doctor’s office is currently impossible, meaning that this risk-factor is underused in the fight against cardiovascular disease. The new device is based on silicon photonics and could enable minimally skilled medical staff to easily and non-invasively assess patients for arterial stiffness at the point of care.

The silicon photonic chips to perform laser doppler vibrometry on a patient’s skin to deduce metrics for arterial stiffness and to diagnose cardiovascular diseases

The device functions using a silicon photonics chip to perform laser Doppler vibrometry, whereby it targets the pulsing skin over an artery with a low power laser. The beams from the laser are reflected back to the device and it can analyze changes in the Doppler shift to assess the frequency and vibration amplitude of the artery, from which one can derive a measurement of arterial stiffness.

The device has recently been tested in a clinical feasibility study with 100 patients, and showed results that are comparable with more complex and expensive measurement techniques. The researchers are in the process of producing more of the devices with a view to conducting a larger clinical feasibility study, over a longer period of time, and with a greater number of patients.

Medgadget had the opportunity to ask Roel Baets and Patrick Segers some questions about the device and the concept. Roel Baets is a program leader at imec and professor of photonics at Ghent University. Patrick Segers is a professor of biomedical engineering, also at Ghent University.

Conn Hastings, Medgadget: How did you get involved in this area? How did this collaboration arise?

Prof. Baets: My field of research is photonics, and more specifically silicon photonics. To put it simply, my team develops optical chips, in close collaboration with imec. Everybody is very well acquainted with electronic chips, as used in laptops and smartphones, but our chips are optical chips that process optical signals (light signals) rather than electronic ones. This is a field of work that, so far, was mostly relevant for the optical fiber networks that form the backbone of the internet. At both ends of a telecommunication optical fiber you find optical chips that convert optical signals into electronic signals and vice versa. About 10 years ago, I started to realize that the same chip technology would have enormous potential for sensors, in particular, sensors for medical and environmental applications. The fact is that light is a key enabler for a very diverse range of sensors. Soon after, I started a collaboration with Patrick Segers, a biomedical engineering professor in a neighboring department, to explore optical techniques for the detection of vibrations in the body resulting from the periodic heartbeat. It took almost 5 years for PhD-student Yanlu Li to develop the basic proof of concept that the technology was working. Then we moved to another gear and set up an international consortium, involving, amongst others, medical device company Medtronic as well as clinical partners, to jointly develop a first prototype for clinical feasibility studies on the measurement of arterial stiffness with our new technology. This consortium secured additional funding from the European Commission under the CARDIS grant.

Medgadget: Please give us some background on cardiovascular disease and the burden it places on society.

Prof. Segers: Cardiovascular disease is an encompassing term, referring to all diseases affecting the cardiovascular system. Worldwide, cardiovascular disease is the cause of death in more than 30 percent of the population, including acute lethal disorders such as myocardial infarction (heart attack) caused by obstruction of the vasculature supplying the heart, and stroke, an obstruction of the blood supply to the brain. Besides death, myocardial infarction and stroke often lead to temporary or permanent disabilities, creating a huge socio-economic cost and burden for the affected people, their family and society in general. Cardiovascular disease most often develops slowly, without any alarming symptoms, and hits by surprise. It is therefore essential to develop ways to prevent such incidents, and to identify people at risk of such incidents before an incident has happened. This is now done on the basis of known risk factors such as a subject’s age, gender, blood cholesterol levels and blood pressure. However, it still frequently occurs that people at low risk develop disease and suffer from stroke or myocardial infarction.

Medgadget: So, how does arterial stiffness influence cardiovascular disease? How does it help clinicians to diagnose cardiovascular diseases?

Prof Segers: This question has a dual answer. In the first place, increased arterial stiffness has a direct impact on the functioning of the cardiovascular system. If we consider the heart as a pump that has to supply the body with a given blood flow with oxygen and nutrients, it requires more energy for the heart as the arterial system gets stiffer. It also reduces the cushioning effect that the healthy arterial system has, which ensures that our systolic blood pressure (typically 120 mmHg) does not become too high when the heart contracts and empties. It also ensures that the diastolic blood pressure (typically 80 mmHg) does not become too low while the heart is filling. With arterial stiffening, this cushioning effect is diminished, leading to a higher stress exerted on the arterial system, and especially on the organs that are supplied by that arterial system such as the heart, kidneys and brain. There is more and more evidence that there is an association between arterial stiffening and cognitive function.

The second part is that arterial stiffness turns out to be a very good way to quantify the cumulative effect of the exposure of the arterial tree to the above cited risk factors (the effect of aging, but also its exposure to high blood pressure, life style factors, etc.). As it is a measure that integrates exposure to risk, it may be more accurate in predicting someone’s risk on a cardiovascular incident (myocardial infarction, stroke) than a measurement of blood pressure, which only provides a snapshot of the person’s blood pressure at one instant in time, rather than throughout the day. Additionally, there is some uncertainty on the measurement of the arm cuff blood pressure measurement.

Medgadget: How does the new device work? Is it simple to use?

Prof. Baets: A very low power laser beam is focused on the skin area above a major artery such as the carotid artery in the neck. The reflected light is subject to the well-known Doppler effect, meaning the color of light is shifted by a very small amount when the skin moves as a result of the heartbeat. This shift is measured in our optical chip. When multiple beams are used, one can extract the velocity of the pressure wave that propagates in the arteries upon each heartbeat. This velocity depends on the stiffness of the artery walls. In summary, what we measure is simply the arterial stiffness.

The technique is non-invasive and completely safe, without any side-effects. It is also easy to perform in roughly one minute. It is simpler than taking somebody’s blood pressure.

Medgadget: Please tell us about the recent clinical feasibility study you performed to test the device.

Prof. Baets: In the CARDIS project, we involved several clinical teams with expertise in cardiovascular research. One of them was the team of Dr. Pierre Boutouyrie at Georges Pompidou European Hospital in Paris. With the prototype device developed in CARDIS, which looks a bit like a hair dryer, a clinical feasibility study was conducted on 100 patients. The purpose was two-fold: on one hand we wanted to see whether good-quality heart-induced skin movement signals can be obtained from a broad variety of patients. On the other hand, we wanted to get a feel for the acceptance of the procedure by these patients. Overall, we can say the results are highly positive on both aspects, even if it is true to say that there is still a large amount of data analysis to be done to find the most robust algorithm to extract a reliable estimate of stiffness from the measured data. This is work in progress.

Medgadget: What are your future plans for the system? Do you envisage that this device would eventually be a standard piece of equipment for community doctors?

Prof. Baets: There is a major chicken-and-egg problem behind the large-scale introduction of a new medical procedure based on a sophisticated device which can become very cheap when produced in sufficient volume. You may compare it to the introduction of pulse oximetry—for the monitoring of a person’s oxygen saturation–in the 1980s and 1990s. Today, pulse oximetry is ubiquitous in medical practice all around the world. It is cheap and life-saving, but the chip technologies inside were not as cheap thirty years ago. With that, some sort of positive feedback mechanism is needed in the evolution from low-volume trials with expensive devices to high-volume usage with cheap devices. In this evolution, there is a responsibility for all stakeholders: the clinicians, the engineers, the governments and the investors.

Our future needs a similar evolution. On the technological side, we need to evolve from expensive prototypes to much lower cost second generation devices. With that, we can service many more medical centers to experiment with the technique and develop best practice medical approaches and protocols. The final target is a device with a cost similar to a smartphone that provides relevant medical data about a person’s cardiovascular risk profile. Such a device can be used by general practitioners and more generally in any point-of-care setting.

It is worth emphasizing that the key enabler in this, from a technological point of view, is the optical chip. The key strength of silicon photonics is that an optical chip becomes very cheap when produced in reasonable volume. Think of a production cost-per-chip of a few Euros as soon as the manufacturing volume is of the order of tens of thousands. But when there is an arterial stiffness measuring device next to every blood pressure measuring device it is trivial to reach such manufacturing volumes.

So yes, our ambition is that arterial stiffness measurement based on silicon photonics will become ubiquitous in medical practice. It may bring big societal value at low cost.

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