Elastomer engineering finally gets deep images from skin patch ultrasound

Engineers at MIT have found a way to get sub-mm resolution and deep penetration from a stick-on ultrasound patch that can be worn for at least two days and peels off at the end of monitoring without leaving residue.

In a proof-of-concept, they demonstrated sub-mm resolution images in the first centimetres of tissue, allowing blood vessels to be monitored live, for example, and also few-mm resolution down to 20cm or so, allowing deeper organs including the heart to be imaged.

In one demonstration, detailed blood flow patterns within the bicep muscle of a volunteer were recorded every half hour as they exercised and rested over 48 hours.

Neck blood vessels imaged at MIT using a stick-on patch

“With imaging, we might be able to capture the moment in a workout before overuse, and stop before muscles become sore,” said MIT research engineer Xiaoyu Chen. “We do not know when that moment might be yet, but now we can provide imaging data that experts can interpret.”

An issue with wearable ultrasound patches, said MIT, is that something has to flex with the skin: either the transducer array has to flex, locking the developer into low-resolution arrays with current technology, or a high-resolution rigid array has to be coupled to the skin in a way that is acoustically sound, durable and healthy for the user.

The MIT team has taken the latter approach, fabricating it own rigid high-resolution phased arrays, operating at 3, 7 and 10MHz, for use with external beam-forming electronics.

The 3MHz 40 x 40 element (20 x 20mm) device was operated as a phased array, and the other two were 40 x 20 (20 x 10mm) and operated as linear arrays. Along-the-beam resolution was 0.77mm at 3MHz, and better than 0.2mm at 10MHz.

For imaging down to 60mm, a plane wave compounding algorithm was used then, below this, phased array harmonic imaging was matched with spatial compounding algorithms.

To couple a solid ultrasound transducer to the skin, according to MIT, the conventional options are liquids or gels which work well initially but dry out and fail before two days are reached, or elastomers made from silicone, acrylic or polyurethane which cannot match the liquids or gels for acoustic performance.

The MIT team’s answer was to custom-design a tough flexible and stretchy hydrogel with high water content to give it good acoustic performance, and then to encapsulate this between two thin elastomer membranes to keep the water in.

Even these sealing membranes use bespoke chemistry: with a hand-crafted ‘bioadhesive’ layer sealing the skin-side and sticking the assembly to the user, and a triple-layer (hydrophobic-hydrophilic-adhesive) polyurethane elastomer membrane sealing the other side and bonding it to the hard sensing array.

Lastly, a tungsten-loaded coating at the rear of the ultrasonic array acts as an acoustic back-stop to improve forward acoustic performance.

The whole patch: membranes, hydrogel, array and backstop, is 3mm thick and has been dubbed a ‘Baus’ (bioadhesive ultrasound) device.

“The researchers ran the ultrasound sticker through a battery of tests with volunteers, who wore the stickers on various parts of their bodies, including the neck, chest, abdomen, and arms,” said MIT. “The stickers stayed attached to their skin, and produced clear images of underlying structures for up to 48 hours. During this time, volunteers performed a variety of activities in the lab, from sitting and standing, to jogging, biking, and lifting weights.”

At the moment, the system needs a ribbon cable connection to external signal processing and storage, but a wireless demonstrator is in the pipeline.

“We imagine we could have a box of stickers, each designed to image a different location of the body,” said project researcher Professor Xuanhe Zhao. “The patches would communicate with your cellphone, where AI algorithms would analyse the images on demand. With a few patches on your body, you could see your internal organs.”

Even in their current form “the devices could be applied to patients in the hospital, similar to heart-monitoring EKG stickers, and could continuously image internal organs without requiring a technician to hold a probe in place for long periods of time,” said the university.

MIT worked with the Mayo Clinic in Rochester Minnesota.

The work is described in ‘Bioadhesive ultrasound for long-term continuous imaging of diverse organs‘, published in Science. Only the paper’s abstract can be viewed without payment, although the freely-available supplementary information has much to please the engineering eye, and reveals an impressive match between simulation results and real-world measurement.