1 Introduction
Motivation
A major branch of bioengineering involves building Biomedical Devices. Biomedical devices are engineered systems that interface with the body to acquire physiological information or to deliver therapy.
Measuring a cell or organ requires a transducer that converts a physical or chemical property of tissue into an electrical signal, followed by a chain that amplifies, filters, and digitizes it.
The transducer that is needed depends on what is being measured, but the end measurement is typically an analog signal that requires digitization. For example, voltage (an EKG), light (a smartwatch heart-rate sensor), and chemical concentration (a wearable glucose monitor) use different transducers to measure human physiology, but the signal is ultimately converted into ones and zeros on a computer.
A common feature of physiological signals is that they are small relative to noise. Surface electrocardiography signals are roughly 1 mV, but electrical noise from nearby power lines, and the voltage shifts produced when an electrode moves against the skin, are routinely larger than that. Therefore, people working on biomedical devices often also specialize in electrical engineering and signal processing.
The heart
We will use the heart to explore several important ways that medical devices work. The heart is sealed inside the chest, so a clinician will always try non-invasive methods first. What can a doctor measure about the heart without poking anything into the body?
The heart is a pump, and pumping makes sound. Recording those sounds at the chest wall gives a phonocardiogram: what a doctor hears through a stethoscope.
Sound can travel in the other direction too. A device can send sound waves into the chest and record the echoes, building an image of the beating heart. This is ultrasound.
The heart coordinates heart chamber contraction using specialized cells that produce electrical signals. These signals spread to the surface of the skin and can be recorded with an electrocardiogram (ECG, or EKG).
This module discusses what pressure and electrical signals reveal about heart function, and how they are measured.