Engineers aren’t the only ones with motion control on the brain. In fact, most people walk around every day with the help of the world’s finest motion system, the one found in our inner ears.
“The inner ear is the crowning achievement of biological motion systems,” says Jeffrey R. Holt, a Ph.D. neuroscientist who studies hearing at the University of Virginia. The inner ear contains hair cells that act as mechanotransducers to convert mechanical displacements from sounds and head movements into electrical signals for the brain. As part of our built-in balance system, these hair cells, which are usually no more than 30 micrometers long and 5 micrometers wide, can pick up linear and rotational head movements as small as 0.1 nm, according to Holt.
“And they’re incredibly fast,” he says. Human hearing covers a frequency range from roughly 20 Hz to 20 kHz and healthy hair cells can respond to sound-induced displacements across that entire frequency range. Some mammals have even higher-speed auditory systems — with frequency ranges that top out at 150 kHz.
The speed and resolution offered by hair cells is good news for anyone who’s trying to remain upright or have a conversation. But for the researchers interested in the workings of the inner ear, the tiny hair cells pose a huge engineering problem as they try to design lab equipment capable of studying the cells’ mechanotransduction process. “Many aspects of hair cell function are poorly understood because of limitations in our ability to create test and measurement equipment that’s fast enough and capable of working with nanoscale displacements,” says Holt.
Recently, though, Holt and his fellow researchers at the Holt-Géléoc Lab of Sensory Neurophysiology have made some breakthroughs in the design of the test equipment that allows them to study the auditory role of the hair cells.
They’ve built a new test system using nano-positioning components from Physik Instrumente (PI). The system, which has earned the lab the $25,000 NANO Innovation Grant from PI, will help the lab meet its broader research goals — developing a better understanding of the ear’s physiology as a first step toward developing therapies that can cure inherited deafness and balance dysfunction. “That’s really our ultimate goal,” says Holt.
The design of the lab’s newest test and measurement equipment also has implications for engineers in high-tech industries that increasingly have to contend with nano- and micro-scale motion problems. “Nano-positioning systems are definitely becoming more mainstream. The big driver is the semiconductor industry where the metrology requirements increasingly run up against the limitations of classical micro-positioning systems based on ball screws and motors,” says Stefan Vorndran, director of marketing for PI in North America.
Tiny Test Rig
Holt and his researchers characterize the inner ear’s mechanotransduction process through experiments that first mechanically stimulate living hair cells — from mice — and then measure the resulting electrical signal. To do that, they designed a nanostimulator that can mimic the deflections and frequencies associated with hearing (see diagrams). The system can today handle frequencies to about 10 kHz and trigger defections from 0.1 nm to a couple of microns — which is actually a huge range at this scale.
Built by researchers Andrea Lelli and Eric Stauffer, the nanostimulator is built around a PI PL033 piezo actuator. They picked it for its high-resonant frequency, small step size, low capacitance and compact size. To drive the actuator at frequencies relevant to human hearing, they use a rapid piezo driver, PI’s E-505.
According to Lelli, the nanostimulator has been designed to limit resonance by keeping the load on the actuator itself to a minimum. “The only object moved by the piezo is a small glass micropipette which is bent at the tip and rounded to fit in the hair cell bundle,” he says. With a mass of just 60 to 80 mg, the pipette is glued to the front face of the actuator and supported by a Teflon guide block to minimize lateral movement and oscillations. The actuator’s back face in the axis of motion attaches to a rigid steel rod mounted on a PI “Nanocube,” a positioning device with a nanometer step size. The nano-positioning device aligns the nanostimulator tip with the hair bundle.
The system lastly includes components to measure the millivolt electrical signals generated by the deflected hair cells. A second pipette connected to another Nanocube positioning stage and an analog recording device — based on a simple RC circuit — collects the electrical data. The experiments take place under a Zeiss Axioskop FS Plus upright microscope equipped with differential interference contrast optics and 63X water immersion objective.
The system on conceptual level resembles test equipment that the lab’s researchers have designed and built themselves in past years. “We already had experience with piezo actuators and nano-positioning,” says Holt. But the system that uses the PI equipment “works better than anything we’ve found so far,” he says. And the improvements have helped broaden the lab’s inner ear inquiries.
Holt says the lab’s previous systems could reliably deflect the hair cells only at frequencies up to a few hundred Hertz. “The PI components allowed us to put together equipment that’s a couple orders of magnitude faster,” he says. That speed has let Holt and his researchers perform what they couldn’t perform in the past. “Up to now, our focus has been on the vestibular, or balance, functions of the hair cells,” Holt says.
With the high-frequency capabilities offered by its new nanostimulator, Holt and his researchers have already started to conduct experiments that could reveal new details about the physiology of hearing. “We hadn’t been able to look at the auditory functions of hair cells before because our equipment was too slow,” Holt says. “Now, we’re in a position to confirm some of our theories and suspicions about how these cells work.”
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