Implantable MEMS sensor gets jiggy with self-powering design

January 27, 2012 — Straight outta Purdue University, a miniaturized implantable medical micro electro mechanical (MEMS) pressure sensor chip can be recharged with the block-rockin beats — acoustic waves — of rap music. The device could help regulate conditions like aneurysms, or incontinence due to paralysis.

Figure 1. The principles behind the operation of a miniature medical sensor powered by acoustic waves, notably from rap music. SOURCE: Birck Nanotechnology Center, Purdue University.

The sensor uses a cantilever that vibrates to the rhythm of the boogie at 200-500 hertz. Acoustic energy from a strong bass component reaches this frequency, and can pass through popping & locking body tissue. These vibrations generate electricity, storing a charge in a capacitor, said Babak Ziaie, a Purdue University professor of electrical and computer engineering and biomedical engineering. When the frequency falls outside of the proper range, the cantilever stops vibrating, automatically sending the electrical charge to the sensor, which takes a pressure reading and transmits data as radio signals. As the bass line drops, the frequency is continually changing, inducing the sensor to repeatedly alternate intervals of storing charge and transmitting data.

Figure 2. The miniature pressure sensor designed to be implanted in the human body. Good vibrations from music or plain tones drive a vibrating cantilever, generating a charge to power the sensor. SOURCE: Birck Nanotechnology Center, Purdue University.

The cantilever beam is lead zirconate titanate PZT by nature, a piezoelectric ceramic material that generates electricity when compressed. The sensor is about 2cm long.

A receiver that picks up the data from the sensor could be placed several inches from the patient. "You would only need to do this for a couple of minutes every hour or so to monitor either blood pressure or pressure of urine in the bladder," Ziaie said. "It doesn’t take long to do the measurement."

Playing tones within a certain frequency range also can be used instead of music. "A plain tone is a very annoying sound," Ziaie said. "We thought it would be novel and also more aesthetically pleasing to use music." Researchers experimented with four types of music: rap, blues, jazz and rock. "Rap is the best because it contains a lot of low frequency sound, notably the bass," Ziaie said.

Researchers tested the device in a water-filled balloon. The sensor is capable of monitoring pressure in the urinary bladder and in the sack of a blood vessel damaged by an aneurism. Such a technology could be used in a system for treating incontinence in people with paralysis by checking bladder pressure and stimulating the spinal cord to close the sphincter that controls urine flow from the bladder. More immediately, it could be used to diagnose incontinence. The conventional diagnostic method now is to insert a probe with a catheter, which must be in place for several hours while the patient remains at the hospital.

"A wireless implantable device could be inserted and left in place," Ziaie said, allowing patients to be monitored with fewer restrictions. Conventional implantable devices are powered by batteries or inductance coils and transmitters that must be precisely and closely aligned..

The MEMS device was created in the Birck Nanotechnology Center at Purdue University’s Discovery Park. A patent application has been filed for the design.

Findings are detailed in a paper to be presented during the IEEE MEMS conference, which will be Jan. 29 to Feb. 2 in Paris. The paper was written by doctoral student Albert Kim, research scientist Teimour Maleki and Ziaie.

Learn more at www.purdue.edu.

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