Optoacoustic blood ‘stethoscope’ holds promise for CV monitoring

Researchers from Nanyang Technological University in Singapore report on a flexible optoacoustic blood ‘stethoscope’ (OBS) for noninvasive, multiparametric, and continuous cardiovascular (CV) monitoring without the need for complex procedures.

“Although there are various methods to extract blood information, these are often invasive, lack continuity, involve bulky instruments, or have complicated testing procedures,” said the researchers. “Thus, continuous and noninvasive monitoring of clinically significant physiological signatures from deeply embedded blood vessels … remains a challenge.”

A device that ‘listens’ to blood

“[The OBS is a lightweight and flexible device] that can adhere to the skin … and detect multiple physiological vital signs such as hypoxia, exogenous agent decay, vascular compliance, and endothelial dysfunction, and reconstruct a 3D blood vessel model with our tailored algorithm, providing remarkable insight into CV functions from various aspects,” they said.

Like a conventional stethoscope, the OBS does ‘listen’ to light-induced acoustic waves to characterize blood properties, hence its name. [Nat Commun 2023;14:4692]

The OBS can be attached to the hand, wrist, or any other body part, with soft and pliable layers. A polydimethylsiloxane-fabricated convex micro-lens array works as optical focusing element for delivering light, while a piezoelectric polyvinylidene fluoride film functions as acoustic receiver.

The team tested the device on three human volunteers to check for perfusion, venous distensibility, and flow-mediated dilation, as well as in mice. Ultrasound imaging was also employed for cross-validation.

Caveats with current sensors

Some sensors can only go as far as the superficial vasculature, while those that can facilitate deep vascular measurements may have limited capabilities in capturing variations in blood physiological markers. [Sci Adv 2015;1:e1500701; Nat Biomed Eng 2018;2:687-695; Nat Biomed Eng 2021;5:737-748; Nat Biomed Eng 2021;5:749-758]

“Due to the rapid photon diffusion and scattering inside human tissues, [current] optical sensors can only statistically read out the light radiation region’s gross physiological information but cannot resolve any specific blood vessel in deep tissue,” the researchers explained.

“In short, current blood sensors are unable to locate blood vessels underneath the skin to provide direct information on distributed blood properties. Beyond that, they can only offer a single physiological vital measurement, [thus limiting their capability to provide a] comprehensive and multidimensional assessment,” they added.

A new dimension for sensors

The device provides an alternative for measuring venous distensibility and arterial endothelial function. It can also recognize blood vessels in deep tissues and record their corresponding hypoxic states. Moreover, its flexibility defies the limits and complexities of conventional setups and offers more versatility and accuracy for monitoring CV disease.

“[The OBS] opens a new sensing dimension for existing flexible electronics. The detection range is extended from areas directly below the sensor patch to a much wider window, and the detection mode evolves from 1D sensing to 3D imaging,” they said.

Other uses

Furthermore, by choosing a suitable wavelength, the device may even extend its capabilities by measuring pH values, blood glucose, and blood temperature; it may even have potential to monitor tumours. [Adv Mater 2015;27:6820-6827; Sci Rep 2018;8:1059; J Biomed Opt 2009;14:054024; Trends Biotechnol 2011;29:213-221]

However, there are still technical issues to iron out before deploying this preliminary prototype to other general settings, hence the need for more in-depth investigations, the researchers noted.

“Future studies will focus on further improvement by integrating denser acoustic and optical elements, incorporating LED light sources, and compacting acquisition circuits and wireless transmission systems … for distributed, mobile, and real-time monitoring,” they said.

“Further clinical validation may also broaden the application of the OBS to accelerate the development of mobile medical [devices and] to advance current state-of-the-art clinical practices for the prevention and investigation of CV-related diseases,” they concluded.