PMUT: Re‑engineering ultrasound for wearables and implantables

Much of today’s health monitoring still relies on episodic data: measurements taken in controlled settings, at specific moments in time. But for many conditions, particularly cardiovascular and chronic diseases, this offers only a partial picture.

Continuous monitoring enables gaining deeper insight into human physiology and developing more sophisticated AI models, but only if it can be implemented as a wearable or implantable device. This requires new kind of ultrasonic transducers.

VTT's PMUT technology (Piezoelectric Micromachined Ultrasonic Transducer) answers this challenge. It can be used to measure, for example, blood pressure, body composition or the small movements of major arteries reliably, unobtrusively and continuously.

"PMUT is an enabling technology for wearables and implantable systems. While some competing technologies share individual advantages, PMUT uniquely combines the necessary performance characteristics and scalable fabrication in a way that most alternatives cannot," says Cyril Karuthedath, who works as a Senior Scientist on edical microsystems at VTT.
 

PMUT devices are very small ultrasound transducers fabricated using MEMS (Micro Electro Mechanical Systems) technologies, with dimensions ranging from millimetres to centimetres. Compared with conventional ultrasound technologies, their key advantage lies in low operating voltage, which directly translates into low power consumption.

Another practical benefit of PMUT‑based solutions is their suitability for conformable system designs, which is particularly important for wearables and implantable applications where rigid structures do not accommodate natural body movement or enable reliable long‑term measurements on or within tissue. Signal quality and user comfort are essential in applications such as continuous cardiovascular monitoring. 

The main alternative to PMUTs is CMUT (Capacitive Micromachined Ultrasonic Transducer) technology. While they both offer distinct advantages in miniaturisation and integration, CMUTs typically require high voltages. This increases power consumption and system complexity, making it less ideal for wearable and implantable devices.

"For many resource-constrained applications, PMUT is the only sensible choice. Additionally, PMUTs are manufactured through MEMS semiconductor processes, allowing large numbers of devices to be produced in a single batch, making them far more scalable and cost-effective than traditional bulk piezoelectric transducers," Karuthedath points out.

Due to their MEMS-based nature, PMUTs offer significant design flexibility. They can be easily fabricated in various configurations, from single elements to 1D and 2D arrays, with customisable sizes, shapes and dimensions.

One example highlighting PMUTs advantages is the recent licensing deal VTT made with Canary Medical to utilise VTT's PMUT and MEMS-based sensors in their implantable cardiovascular products. Providing continuous data to patients and their clinicians helps them to better manage chronic cardiovascular disease conditions.

"In the future, PMUTs could prove valuable in other areas, such as therapeutic ultrasound, which has been researched a lot. That way, it might be possible to alleviate symptoms for chronic illnesses, such as Parkinson’s disease, or allow medication pass more freely by opening the blood-brain barrier. Maybe one day, PMUTs could be used for treating cancer," Karuthedath ponders future possibilities.
 

VTT's offering approaches PMUTs as part of a system development continuum rather than standalone sensors. VTT helps companies develop functional prototypes for their PMUT applications.

Partnership typically starts with a feasibility study, where VTT assesses the requirements of the customer’s application and determines whether the concept is technically viable. That way, customers can identify key risks, performance limits and design constraints before making significant investments.

Additional benefit lies in leveraging existing PMUT and ultrasound transducer platforms when possible, which reduces the fabrication costs significantly. Once feasibility is confirmed, VTT moves into the design, fabrication and characterisation of application‑specific PMUT devices – either based on customer requirements or specifications jointly defined during the feasibility phase.

VTT uses aluminium nitride (AlN) and scandium doped aluminium nitride (ScAlN) in its PMUTs to enable low‑power operation and enhanced sensitivity. AlN provides a stable base and ScAlN boosts performance for demanding applications.

"Both materials are also lead-free, ensuring compliance with EU’s RoHS Directive. The new legislation is set to phase out lead allowances in electronic and ceramic components, including piezoelectric ceramics, by 2027," Karuthedath notes.

VTT offers complete system‑level and software development, integrating PMUT devices with electronics and turning raw ultrasound signals into usable metrics through algorithms, signal processing and AI. VTT also develops the supporting electronics needed to operate the transducers and the full system.

"Depending on the application, this may extend to ASIC (Application‑Specific Integrated Circuit) and CMOS‑compatible (Complementary Metal‑Oxide‑Semiconductor) design, ensuring that the PMUT integrates cleanly with the surrounding electronics and functions as part of a reliable, low‑power system,” Karuthedath explains.

To validate the prototype's performance, VTT utilises practical phantom studies and laboratory validation using tissue‑mimicking models. If the project moves towards commercial production, VTT transfers IP, fabrication know‑how and documentation to manufacturing partners to enable scalable production.

Together with its customers, VTT is redefining what ultrasound can become with advanced materials, smart engineering and modern system design.