Softsonics is a spin-off from the University of California, San Diego
Most people have been to a physician’s office and had their blood pressure taken with an inflatable cuff around their arm. The test, which measures the maximum and minimum pressure on the brachial artery as the heart beats, is an important predictor of health, but dates back to the nineteenth century and has several limitations. A company spun off from the University of California, San Diego, is hoping its device will provide a deeper and more accurate measurement of blood pressure, both for people in intensive care and for those going about their daily lives.
Softsonics is developing a soft, flexible patch that can be worn on the skin over the carotid artery or jugular vein, and which uses pulses of ultrasound to measure blood pressure1. Such a device, the developers say, could provide continuous readings in deep tissue, which are not possible with current technologies. “When we talk to doctors who deal with patients day to day, they tell us they care about an easy way to standardize blood-pressure measurement and get a 24-hour continuous measurement,” says Shu Xiang, an engineer and co-founder of the company, which is based in San Diego.
For most people, blood pressure is measured once or twice during a visit to a physician, and visits are often months apart. That makes it difficult to know how pressure varies over the course of a day, or during sleep, and what that might mean for health. It can also lead to white-coat syndrome, in which people have elevated pressure in clinical settings. Even self-monitoring at home is sporadic, can’t be done during sleep and can happen only when the person sits still.
In cardiac intensive-care units, blood pressure is monitored continuously by catheters inserted deep inside the arteries. This provides more information about the pressure that affects the brain, heart and kidneys, but carries all the normal risks of an invasive procedure and is impossible to do in people who are mobile. “We don’t want to poke a hole in every patient just for normal blood-pressure monitors,” Xiang says.
Softsonics’s approach relies on bendable electronics developed by co-founder Sheng Xu, a nanoengineering researcher at the University of California, San Diego. The electronics are based on what Xu calls a “bridge and island” design. Thin, serpentine copper wires connect small islands of components, with the whole device encapsulated in silicone. Because even the rigid parts are small and spread out — the biggest component measures 1.2 square millimetres — the device as a whole remains supple. “Those rigid components are far apart,” Xu says, “so that the overall system is soft and stretchy.”
The electronics include piezoelectric devices, which change shape in response to electricity and produce an electrical output in response to mechanical pressure. A voltage applied to the actuators causes them to emit an ultrasonic pulse, which penetrates several centimetres into the body (see ‘A soft touch’). When the ultrasonic waves bounce off an artery and return to the patch, the waves squeeze the piezoelectrics and produce a voltage. Measuring the time difference between when waves return from the near wall of the artery and from the far wall provides the diameter of the artery, and watching how that diameter changes as the blood pulses through gives the blood pressure.
The technology is still in the very early stages of development. Softsonics was founded only in February, and the coronavirus pandemic has disrupted some of its work. Housed in the University of California’s Center for Accelerated Innovation, it has filed two US and international patent applications. It has three employees and is supported by funding from the founders and by research grants from the US National Institute of Biomedical Imaging and Bioengineering.
So far, the company has demonstrated the concept in laboratory prototypes connected by wires to a power source and readout electronics. However, a practical device will have to incorporate a battery and a wireless transmitter. The company is still doing market research to decide whether to develop the technology for in-patient or out-patient use, but Xiang thinks that it could prove useful as a non-invasive replacement for arterial catheters in intensive care. Steven Steinhubl, a cardiologist who heads the digital medicine department at Scripps Research Translational Institute in La Jolla, California, says the big advantage of the approach in the intensive-care setting is that it’s non-invasive, so the risk of infection is essentially eliminated. First, however, the company will have to validate the device by using it alongside catheters and comparing its performance to the current standard.
If the researchers can demonstrate the clinical value of the ultrasonic patch, the technology might also make it possible for people admitted to hospital for heart, liver or kidney failure to be intensively monitored at home, providing new levels of diagnostic information.
It’s not yet clear whether continuous readings — as opposed to the separate points of systolic and diastolic pressure obtained with a cuff — and the measurement of deep rather than secondary arteries provides benefits over existing devices. But that’s mostly because such data haven’t been available, Xiang says.
Steinhubl agrees that such a personal monitor has potential. “Any accurate measures of continuous blood pressure in a real-world setting will substantially change our knowledge of what is normal and abnormal blood pressure for individuals and lead to its more precise treatment,” he says. He thinks that the biggest barrier to using this or any sensor technology that produces reams of data is working out the significance. “Too often sensors generate tons of useful data and then expect already over-stretched physicians and nurses to be the ones to make sense of it all,” he says. The data will probably need to be fed to an artificial-intelligence system that provides the monitoring and alerts carers if it detects a problem.
Softsonics will face a competitive consumer market. In 2018, the US Food and Drug Administration approved a blood-pressure monitoring watch from Japanese electronics company Omron that uses an inflatable wrist band in place of a cuff. Since then, other makers of wearable fitness devices have come up with their own approaches.
Xu says he wants to go beyond what other wearable devices can measure, such as pulse or activity levels. Whether they use accelerometers, chemical sensors or light-emitting diodes, those devices don’t penetrate more than about a centimetre below the skin, he says. His ultrasonic sensors reach four times that depth. “My group had a vision of using a wearable device to non-invasively and continuously monitor those biological processes in central organs and deep tissue,” he says. “So instead of scratching the surface, we are scratching the depths.
Wang, C. et al. Nature Biomed. Engineer. 2, 687–695 (2018).