“Unlike conventional cuff-based blood pressure monitors that use an inflatable air bladder to apply pressure to the arm, this continuously measures blood pressure with a compact flexible electronic patch,” according to the university.
It infers blood pressure based on time-of-arrival difference between the electrical signal caused by the heart as it contracts and the mechanical ‘pulse’ traditionally felt at the wrist.
“Electrical signals are transmitted rapidly throughout the body as soon as the heart beats, so they are detected almost immediately at the wrist,” said the university. “On the other hand, mechanical signals are delayed in transmission as blood is pushed out during heart contraction, so it takes some time for the wrist skin to move after the heart beats.”
High blood pressure speeds the progress of the mechanical wave, while low blood pressure slows it.
The electrical pulse is detected as a voltage difference between two contact patches (see diagram), while the mechanical pulse is detected through resistance variation in a nearby serpentine conductive track.
Developing a suitable high-performance electrode was a large part of the project, with the team settling on one based on the low melting point metal gallium.
Unfortunately, liquid gallium resists being made into tracks as its naturally high surface tension self-assembles small balls instead.
To get over this, the team deposited a thin layer of these balls onto a rigid substrate coated with the silicone polymer PDMS, and then drew the required track pattern with a 532nm laser.
This energy breaks up the metal’s native amorphous surface oxide, allowing the balls to merge into a track – which can be as narrow as a few microns using this process.
High temperature during irradiation favours the formation of crystalline beta-gallium oxide instead of amorphous oxide, which modifies surface behavour and allows the non-modified gallium balls to be etched away leaving just the tracks.
By a simple physical contact, the track pattern was then transferred onto ‘Tegaderm’, a thin bio-compatible adhesive film made by 3M, and it is this be-tracked Tegaderm that is stuck to the wrist.
Once transferred to the Tegaderm, these laser-created gallium tracks can be stretched to seven times their deposited length without breaking, and will also not break after 10,000 stretching cycles at more modest elongations.
The resulting patch “possesses excellent electrical conductivity and is easily deformable, without the need for additional chemicals”, said the University.
Results show both systolic and diastolic pressures can be recovered, said the researchers, and their values agree well with cuff-type measurements even through exercise. Breathing rate has also been measured from the available signals.
“Our system proposes a new healthcare interface capable of detecting and analysing physiological signals non-invasively and in real time,” said project leader Professor Seung Hwan Ko.
Seoul National University worked with Carnegie Mellon University.
The research is described in the paper ‘Highly sensitive cuffless blood pressure monitoring with selective laser-sintered liquid metal conductors‘ in Advanced Functional Materials (available in full without payment). For more on the timing theory, scroll down that page to reach the ‘supplementary information’ pdf.
The above diagram was provided on the Seoul National University website, and appears similar to one in the Advanced Functional Materials paper. Electronics Weekly is checking that its use in this article is legitimate.
