Bridging the Sciences
Researchers in Ohio State’s Smart Materials and Structures Laboratory turned to biology to develop acoustic sensors that could be used in medical, automotive or aviation applications.
Marcelo Dapino, associate professor of mechanical engineering, and Derek Hansford, associate professor of biomedical engineering and materials science and engineering, are using a traditional material, polyvinylidene fluoride (PVDF), to create a highly sensitive microphone.
Instead of using the PVDF in its usual form of film sheets, Dapino and Hansford, with Jian Xu, who received his doctorate this spring, and Daniel Gallego-Perez, a doctoral student, developed a better design: They fabricated the PVDF into an array of micron-sized pillars shaped like cilia, the short hair-like outgrowths on cells of certain tissues. They then placed a glass coverslip on top of the array; the glass layer is directly exposed to the acoustic pressure.
The area ratio creates pressure amplification, thus increasing the electrical response of the PVDF. A patterned micro-fabricated electrode is placed between the PVDF and glass layer to reduce the electrical capacitance (the ability to hold charge) and further increase the sensitivity. A continuous, flat electrode is placed underneath the array.
The overall designhas superior sensitivity, enabling it to measure weaker signals than existing microphones can detect, and can be packaged in a sub-millimeter enclosure.
PVDF is a piezoelectric polymer that can convert mechanical energy to electric energy, or vice versa. This polymer has been widely used for sensor development in a range of military, industrial and biomedical applications. Sensors based on PVDF have many advantages, such as wide frequency range (0.001-109 Hz), vast dynamic range (10−8-106 psi), high compliance, high mechanical strength and impact resistance.
The sensor developed at Ohio State could be used in a microphone array to detect sound intensity and direction with unprecedented sensitivity and accuracy. Applications include miniature stethoscopes for real-time monitoring of lung sounds, especially for infants and small children; echo-location detectors for vehicle navigation; and sensor arrays for aeroacoustic measurements. Currently, these applications utilize microphones that are too large, too expensive or not sufficiently sensitive in the high frequency (ultrasonic) range.
The spacing, size and configuration of the micro-pillar array were determined through computer modeling of the PVDF material’s properties in combination with finite element modeling of the coupled electro-mechanical structure. The optimized geometry was used to guide the fabrication through MEMS-based casting, photolithography and poling of the micro-sensor.
The sensor has a sound pressure level range of 35-180 decibels, frequency bandwidth of at least 100 kHz, and sensitivity 60 times greater than commercial PVDF film. The researchers expect that addressing the existing limitations of the micro-fabrication process could lead to sensitivities 100 times greater than commercial PVDF.
The team has fabricated prototypes of the sensor, but in advance of full development, the researchers also need to address challenges such as proper packaging and testing under lab and environmentally-relevant conditions.
The research is supported by $120,000 from the National Science Foundation and from Advanced Numerical Solutions LLC, a Hilliard, Ohio-based Small Business Innovation Research company supported by the NSF, that specializes in creating simulation and analysis software for complex acoustic environments such as automotive powertrains.