# How to weigh tiny objects using sound

11 Apr 2017

A $12 device that can measure the mass of microgram-sized objects in fluid has been developed by researchers in the US. The sensor is driven by a piezoelectric speaker and measures the change in the resonant frequency of a glass tube as the object passes through it. The team used the device to measure mass changes in several biological samples and says that the sensor has applications in a wide range of fields, such as developmental biology, toxicology, materials science and plant science. Mass is an important physical measurement that can provide crucial information about the nature of an object. However, weighing microgram-sized biological samples such as embryos in liquid, can be very tricky indeed. While mass measurements can offer valuable insights into the biological state and health of such specimens, they cannot be easily made with standard laboratory equipment. ### Nanogram resolution To tackle this shortcoming, William Grover and colleagues at the University of California, Riverside, have created a simple mass sensor from off-the-shelf electronics and a short length of glass tubing bent into a “U” shape. The glass tube is attached to a small speaker and the bottom of the “U” passes through a photointerrupter – a device that uses an LED and a light sensor to detect the presence, or not, of an object. This simple set-up cost around US$12, yet can determine the mass of a microgram-sized object with a resolution of a few hundred nanograms.

It provides a pretty complete picture of the physical properties of a sample
William Grover, University of California, Riverside

The speaker keeps the glass tube vibrating at its resonant frequency using a simple feedback circuit from the photointerrupter, which detects the oscillation rate. As the object being weighed is pumped through the tube it changes the tube’s resonance frequency. This change is detected by the photointerrupter and can be used to calculate the objects mass, volume and density.

“If the object has a different density than the fluid, then it will change the sensor’s mass when it flows through,” explains Grover. “If the object is denser than the fluid around it, it’ll make the sensor slightly heavier and that makes the sensor’s frequency go down. If the object is less dense than the fluid, it makes the sensor slightly lighter and that makes the sensor’s frequency go up. By measuring these frequency changes, we measure the buoyant mass of the object.”

### Germinating seeds

The sensor was calibrated with microbeads of known mass. The team then demonstrated that it can measure changes in the mass of zebra-fish embryos – a common model for embryological development studies – as they react to toxins. The device was also used to measure the degradation rates of nano-sized biomaterials used in medical implants, as well as mass and density changes in germinating seeds.

“It’s fundamentally a mass sensor, so at the most basic level it can weigh tiny objects in fluid,” says Grover. “But by using a method that Archimedes first described over 2000 years ago, we can also use it to measure the volume and density of the objects. So it provides a pretty complete picture of the physical properties of a sample.”

Grover expects the sensor to have many applications, but he is especially interested in using it to “study the development of organisms, measure biodegradable materials, and monitor the environment”. The new device is described in PLOS ONE.