A miniature piezo motor positions an autofocus lens integrated with the driver on a small PCB.
Courtesy of Physik Instrumente L.P.

Minimally invasive surgery (MIS) aims to cause patients as little discomfort as possible, and endoscopy plays an important role. Traditional chip-on-the-tip endoscopes use fixed-focus optics which cannot display objects at different distances with optimum resolution. A miniature drive integrated into the optics could allow variable focusing, so objects could be displayed in sharp focus.

Endoscopes that allow minimally invasive surgery are used in larynx diagnostics or laparoscopy. Optics and special instruments (scissors, hooks, grasping forceps, ultrasound scissors) are then introduced into the abdominal cavity through working trocars. Following surgery, the patient experiences less post-operative pain and can be discharged earlier, due to the fast healing process. The minimized risk of wound infections and wound healing disorders allows treatment of high-risk patients.

Imaging sensor placement

Traditional endoscope technology has reached a high degree of sophistication, however, image transmission through classical optics is a drawback. With the evolution of microelectronics and compact imaging sensors, progress allows integrating a small camera into the endoscope tip (chip-on-the-tip), improving image quality.

Attaching a manual zoom lens or focus lens between the eyepiece and camera sets the focus to different object distances and zoom details. The digital zoom function of the chip-on-the-tip technology only allows a detail to be enlarged, decreasing quality. The absence of a focus function forces a compromise between depth of focus and image brightness.

Optical zoom function for chip-on-the-tip cameras

Actuators for optical focus and zoom functions could be installed between the optics and the imaging chip. With 10mm diameters, the potential installation space is small, making it possible to find the right drives for the zoom and focus lens.

Miniaturized ceramic piezo motors and voice coil drives could open a new area of application in the field. Miniature precision drives offer a solid base for use in modern, high-resolution microscopes.

Miniature piezo motors

Miniaturized PIMag voice coil linear drive provides high speed and motion up to 25mm, enough for the requirements in endoscopes.
Courtesy of Physik Instrumente L.P.

Piezo motors’ direct-drive eliminates converting rotation into linear motion, removing the need for gears and other mechanical components that can wear and reduce reliability. Piezo motors are small, lightweight, and consume minimal energy, and can hold a position without consuming power.

A number of different piezo motor principles are feasible – mini-rod inertia piezo motor, ultrasonic piezo ceramic motor, flexure-type inertia motor, and ratchet-type piezo motor – and all provide the necessary precision for focus and zoom control in endoscopy. Ultrasonic motors and inertia motors are compact and cost effective.

Voice coil drives

In addition to piezoelectric motors, traditional electromagnetic linear motors (voice coils) are also an option.

Voice coil actuators are related to the transducers used in loudspeakers. They can be manufactured with small dimensions and provide motion ranges in the millimeter to centimeter range, just enough for endoscopy. Given these drive options, it should be interesting which of them will contribute to better image quality and depth of focus in the next generation of chip-on-the-tip endoscopes.

Miniaturized controller for a voice coil drive with integrated position sensor.
Courtesy of Physik Instrumente L.P.

Scanner tubes

Compact piezo XY scanners can be used for high-resolution, two-photon endoscopy. Recently, a spiral fiber scanner based on a miniature piezo scanner tube (1.5mm OD) was used for biological imaging of parts of the eye. The 2mm diameter instrument provides lateral and axial resolution as low as 1.5µm.

Design options

The field of piezo motion devices and voice coil drives as applied to endoscopy has expanded rapidly in recent years, and today’s medical device engineers have a wealth of piezo and voice coil drive technologies at their disposal.

Physik Instrumente L.P. (PI)

About the author: Birgit Bauer is a business development manager for healthcare at PI.