Simple surface tension sensors may allow field, home diagnosis
By Mark Dwortzan
A surfactant is a substance that reduces the surface tension of the liquid in which it is dissolved, thus enabling the liquid to disperse more easily when it comes in contact with a wettable material. For instance, laundry detergents help water penetrate through fabric and break up stains. Milkfat also acts as a surfactant, causing droplets of whole milk to wick into a certain class of materials, unlike low-fat or skim, which would bead up like water on a duck’s back.
What makes this more than an intriguing factoid is that one can use it to design a material to evaluate the fat content in breast milk, a critical factor in neonatal health. If the milk fails to penetrate the material, then it contains an inadequate concentration of fat (and calories). Caloric deficiency affects the nearly 10 percent of newborns who fail to thrive due to malnutrition, and the more than 80 percent of mothers who choose formula largely for this reason.
Over the past two years in Professor Mark Grinstaff’s (BME, MSE, Chemistry) lab, BME PhD student Eric Falde has been engineering a polymer sensor that indicates if breast milk has sufficient calories for nursing newborns. He tuned the polymer to switch from non-wetted (the milk beads up) to wetted (the milk wicks through) when there’s an inadequate level of milkfat (too much surface tension in the milk), and to release a purple dye to show when this switch occurs. Intended for home or field use, the sensor provides a far more rapid, affordable, portable, and simple test than today’s standard of care, which relies on a bulky centrifuge or high-pressure liquid chromatography to separate and analyze milk components.
Grinstaff, Falde and Stephan Yohe (BME, PhD’13) describe their results in the online edition of Advanced Healthcare Materials.
“A facile and rapid measurement of the fat content of breast milk may provide a means for mothers to ensure that their infants are receiving sufficient nutrition and help more than 400,000 infants a year,” said Grinstaff.
Falde created the sensor using a technique called electrospinning which applies a high electric field to a polymer solution and spins the solution into a mesh of fine fibers. The mesh consists of a top layer that responds to small changes in liquid surface tension to resist or absorb a test droplet, and a bottom layer that reveals a color change when wetted.
“When a small droplet of test liquid is deposited on the sensor, it either remains or gets rapidly absorbed and changes color, depending on whether the liquid is above or below the surface tension threshold,“ said Falde, who adjusts the threshold value by tuning how hydrophobic (water-resistant) the polymer will be.
Because it’s designed to switch between wetted and non-wetted states with liquids of a particular surface tension, the sensor could be tuned to detect surface tension changes (corresponding to abnormal levels of fats and proteins) in other biological fluids—blood, urine, saliva and more—that may serve as indicators of a wide range of medical conditions. User-friendly and power-free, such sensors could be deployed at the point of care, including in the home.
“It’s difficult to create a simple test that screens for kidney disease, but a urine sample should show surface tension differences so that we can design a less invasive, faster and more comfortable diagnostic,” said Falde. In the paper, he described two prototype sensor meshes, each tuned to a different surface tension detection range, that he designed, evaluated and tested with human breast milk and urine samples. The latter sensor tested for high bile acid levels, an indicator of kidney disease.