Mini motor could crush kidney stones

A tiny, inexpensive motor with simple circuitry may become the motive force in micromedical applications in the near future.

A tiny, inexpensive motor with simple circuitry may become the motive force in micromedical applications in the near future, according to a Penn State engineer.

The smallest of these ultrasonic, piezoelectric motors developed by researchers at Penn State’s Materials Research Institute is about the size of a grain of rice. They are 1.8 millimetres in diameter and 4 millimetres long.

‘Initially, our applications for these motors are aimed at medical uses,’ says Dr. Kenji Uchino, professor of electrical engineering. ‘Because the motors are so small, and can be manufactured so cheaply, they appear ideal for applications where small diameter, disposable instruments are required.’

Some of these applications include specialised urinary catheters and endoscopic instruments. Currently, catheters with instruments to break up kidney stones, must be about 3 millimetres in diameter to accommodate the instruments. The 1.8 millimetre diameter motor would allow the catheter to become smaller and consequently more comfortable. The motor itself has sufficient power to break up kidney stone materials.

Another application, although one not requiring such a small motor, would be on the end of an endoscope. A mirror could be controlled by the motor to allow the light delivered by the fibre optic filament to illuminate a larger area of the upper digestive tract and physicians to view larger areas.

‘Because these motors are not electromagnetic, but electromechanical or piezoelectric, they are inherently non magnetic, which, with proper choice of materials, would allow them to be used in surgery performed using Magnetic Resonance Imaging, ‘ says Uchino. ‘Brain surgery is sometimes done this way.’

The researchers have fabricated the prototype motors from readily available materials, because they want to be able to mass produce the motors inexpensively. Each motor consists of a hollow metal tube, two sides flattened at 90 or more degrees. Two strips of PZT, a lead zirconate titanate that is piezoelectric, are fastened to the flattened areas. This tube becomes the motor’s stator. Inside the tube, the rotor consists of a rod held down with a spring or just a spring.

Piezoelectric materials deform when an electrical voltage is applied to them. By deforming the strips on the outside of the stator, the tube wobbles. This wobble causes the rotor to spin and the motor to move.

Electromagnetic motors have not been shrunk below 10 millimetres in size. They are very high speed and only about 2 percent efficient. The efficiency of the piezoelectric motors is about 28 percent.

Initially, piezoelectric motors were made with tubes of piezoelectric materials, but they proved too expensive. Now, Uchino chooses aluminium, stainless steel, plastic or brass tubes that are readily available off the shelf. With the proper materials, the motors can be inexpensively manufactured and tailored to a variety of applications. Besides medical applications, they could function in appliances, computers and even wristwatches.

Uchino, Burhanettin Koc, Kirkkale University, Turkey; and Serra Cagatay, graduate student in Penn States intercollege program in materials, have applied for a provisional patent on the smaller motor. Uchino and IMRE – Singapore National Institute hold the patent for an earlier piezoelectric motor design.