Blood Pressure Monitor (A3051)
© 2024 Nathan Sayer, Open Source Instruments Inc.© 2024 Kevan Hashemi, Open Source Instruments Inc.
Blood Pressure Monitor (A3051)
[22-OCT-24] The Blood Pressure Monitor (A3051, BPM) is a new implantable telemetry sensor currently in development at OSI. We have a prototype circuit, a physical sample, and a specification. We hope to produce the first working sensors for trials at collaborating institudes in the Spring of 2025.
The A3051 is equipped with an LPS28DFW pressure sensor. The pressure sensor device is only 2.8 mm square and 1.9 mm high. Nevertheless, it provides a flange into which we can glue a short steel tube. Within the flange is a silicone barrier that protects the sensor from water, while at the same time conveying water pressure to the sensor mechanism. We encapsulate the circuit and its battery with epoxy, then coat with silicone, and leave the 5 mm of the steel tube protruting. Onto this we slide a silicone sleeve. The sleeve forms a tight fit over the steel, but also a tight fit around a 640-μm diameter catheter, such as BB31695-PE/1. The surgeon implants the device in the animal, with the tube running directly to the artery in which she wishes to measure blood pressure.
The median arterial blood pressure in a conscious mouse is roughly 103 mmHg (14 kPa) above atmospheric. When under anethesia, median pressure can drop to 64 mmHg (8.5 kPa) above atmospheric. With certain stimulants, the median pressure can reach 150 mmHg (20 kPa) above atmospheric. Variation in pressure during heartbeat is ±30 mmHg (±4.0 kPa). The absolute blood pressure in a mouse in the Earth's atmosphere will never exceed 101 + 20 + 4 = 125 kPa. If we perform experiments on mice in vacuum chambers, we might well see absolute blood pressure drop to 10 kPa. If we perform our experiments in a pressurized chamber, we might see two or three atmosphere pressure. The LPS28DFW pressure sensor deployed on the A3051 circuit provides a pressure measurement covering the absolute pressure range 2.6 kPa to 406 kPa. With sixteen-bit digitization, our resolution is 6 Pa, or 0.1% of the variation in a mouse's artery. The absolute accuracy of the sensor is ±50 Pa, and relative accuracy is ±2 Pa. Expansion of the catheter plastic and compression of trapped air within the catheter tube will, we suspect, generate systematic errors in our blood pressure measurement of order ±10% of the diastolic-systolic variation, so the errors introduce by the pressure sensor appear to be more than ten times smaller than those generated by the catheter.
The A3051A provides 128 SPS of sixteen-bit resolution absolute pressure measurement, with output 0 counts representing pressure zero and output 65535 representing pressure 406 kPa. Current consumption is roughly 300 μA at 37°C. Its CR2330 battery has capacity 11 mA-days, so the device will run continuously while implanted for 37 days. Future mouse versions will provide 64 SPS with current consumption 100 μA. When equipped with a 2000 mA-day CR1225 battery, its mass will be around 2.2 g and operating life will be 30 days.
[23-OCT-24] The first version we plan to build is the A3051A, 128 SPS blood pressure sensor for rats, mass 7.2 g. This prototype version is larger and heavier than it needs to be because we have layed out the circuit board for ease of develoment, so that the circuit itself contributes 3 g to the mass when it will ultimately contribute only 1 g. We will reduce the mass to 5.8 g as soon as we have the prototype working.
[22-NOV-24] All our designs are free and open-source, with copyright protection presented in the GNU Public License, Version 3.0.
S3051A_1: Schematic for mouse-sized sensor with WLCSP-25 logic chip and LPS28 pressure sensor.[23-OCT-24] The BPM turns on and off with a magnet. The A3051A broadcasts its blood pressure measurement at 128 SPS using our micro-power telemetry transmission protocol. See our Telemetry Manual for a description of the telemetry system. The blood pressure signal will then appear in your telemetry recordings along with any other biopotential, acceleration, and temperature signals you may be receiving from other telemetry devices. You record all these signals with the Neurorecorder and play them back or export them to another file format with the Neuroplayer.
[22-NOV-24] For details of the development and production of the A3051, see its Developement page.