A Medical Application of Physics - Blood Pressure

I’m sure many of us in class are familiar with the inflatable cuff put around our bicep at the doctor's office that is used to measure blood pressure, also known as a sphygmomanometer. This device helps to distinguish between diastolic (low pressure) and systolic (high pressure) within a blood pulse, which make up the blood-pressure measurement in units of millimeters of mercury. However, this measurement can be confounded in certain situations and is often not taken frequently enough for individuals with certain conditions, even with all the smart technology many people wear on their wrists daily. In order to fill this blood-pressure data gap, Roozbeh Jafari, Deji Akinwande, and their colleagues have created a seemingly weightless and unobtrusive blood-pressure sensor that can be carried around everywhere. How did they do this you might ask? Well, they designed a temporary tattoo that is made of graphene and protected by an ultrathin polymer film shown in the image below. 

V = IR (Ohm’s Law) 

The 6 graphene electrodes, that stick to the skin through Van der Waals forces alone, are placed over the radial artery (and 6 on the ulnar artery) and measure the bioimpedance in the wrist which is converted blood pressure measurement through a machine learning algorithm, where bioimpedance is the resistance of tissue to an alternating electrical current. It takes the blood pressure of someone without even directly measuring pressure! Blood is rich in ions and serves as a great electrical conductor. As it pulses through a particular spot, the impedance drops which can be seen in the graph. The electrodes introduce a tiny current into the wrist and the induced potential measurement is proportional to the impedance. Additionally, the amount of time it takes for the pulse to propagate is correlated with the blood pressure of an individual (faster = higher BP), however, this isn’t so simple and involves a specifically curated algorithm. 

While we haven’t fully covered these topics in class, forces are definitely at play within this product, including the forces of the blood pushing against an artery or vein and the equal and opposite force suggested by Newton’s third law. This device takes into account many aspects of biophysics like biochemistry, fluid flow, pressure, and electrical currents, and I find this interconnectedness fascinating and pertinent to my personal study of physics.  

https://physicstoday.scitation.org/doi/10.1063/PT.3.5076