A planar micro rotary actuator for endoscopic optical scanning

Highlights

• Electromagnetic micro rotary actuator developed for distal side-viewing endoscopy.

• Planar architecture and ferrofluid bearing of the actuator allow for downsizing.

• The actuator offers a wide range of rotation speeds to enable multimodal imaging.

• Scanner’s hollow design with micro mirror achieves 360° viewing with no blind spot.

• Prototype rotating a laser shows its feasibility as a distal endoscopic scanner.

Abstract

Optical endoscopy is an essential technique for diagnosing disease in modern medicine. Side-viewing endoscopic probes with an embedded distal actuator that circumferentially scans an imaging light within an organ’s lumen are expected to significantly enhance the imaging ability of the probes. To this end, this work develops and experimentally analyzes an electromagnetic micro rotary actuator with a planar and hollow architecture. The actuator uses a liquid-phase bearing based on ferrofluid that is self-sustained on the magnetic rotor to eliminate complex bearing structures and enable low-friction actuation. Compared to preceding tubular designs, the planar topology allows for 83% downsizing in the actuator’s axial size, which helps preserve probe flexibility. Furthermore, the hollow design provides an unobstructed optical path through the actuator body, which removes imaging blind spots. A proof-of-concept scanner device is prototyped using flex-circuit microfabrication and 3D printing techniques for its electromechanical characterization. The prototype is successfully driven to revolve the rotor with 45° steps on a ring-shaped planar stator, which are then cycled at high frequencies to continuously spin the rotor up to a maximum rate of 2000 rpm with a device temperature below 45 °C, a safety threshold for tissue damage. The demonstrated stepwise and high-speed modes of laser scanning suggests the feasibility of the scanner design for multimodal imaging application toward advancing optical endoscope technology.

Introduction

Endoscopic imaging is becoming a more common way to detect diseases early because it allows physicians to directly observe the hard-to-reach areas inside the body, ranging from the arteries to the esophagus [1], [2], [3], [4]. Optical endoscopes use various imaging techniques, such as white light endoscopy, optical coherence tomography (OCT), Raman spectroscopy, and others [1]. Endoscopic OCT renders high resolution (<3 µm) images of tissue cross-sections in real time [5], while Raman spectroscopy analyzes chemical composition to differentiate between normal and diseased tissues [6], [7]. Both OCT and Raman endoscopic methods direct a laser beam to the area of interest using either a forward- or side-viewing optic fiber probe, and then analyze the reflected light along the same optical path [1], [8]. The forward-viewing configuration sees directly in front of the probe and is useful for performing medical procedures like biopsies; however, scanning the imaging beam is extremely difficult [9]. The side-viewing configuration bends the light 90° with an angled mirror at the distal tip of the probe, which, when continuously revolved for 360°, can image the entire sidewall of a lumen [8].

There have been a wide range of studies on improving image quality in side-viewing probes by using distal optical scanners to reduce the image distortion caused by rotating the entire probe equipped with a fixed distal mirror [4], [8], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24]. The distal scanners reported to use small commercial motors are, however, limited in both functionality and cost by scanning at only high speeds, being axially long, and being prohibitively expensive for disposable use ($1000-$2000) [8], [25]. While distal scanners based on micro-electro-mechanical systems (MEMS) have been developed to address some of these issues [26], [27], [28] they have inherent limitations in their scanning range (e.g., ± 20° [26]) due to the 2-dimensional (2D) nature of silicon-based devices. These shortcomings of reported distal scanners pose a barrier for increasing the imaging modalities and capabilities of endoscopes. For example, multimodal imaging is of clinical interest to provide physicians with more information through 360° side-viewing endoscopy, but requires a scanner with a large range of rotational speeds (e.g., precision stepping for Raman Spectroscopy and high speed for OCT) [18].

Micro rotary actuators using ferrofluid bearings have been investigated to address this need, allowing for both stepwise and continuous modes of operation, while reducing the structural design complexity [17], [18], [19], [22], [23]. The working principle of this ferrofluid-assisted actuator is discussed elsewhere [29]. Briefly, ferrofluids are magnetic liquids constructed by suspending ferromagnetic nanoparticles in a carrier fluid, which concentrates in areas with higher magnetic flux density, e.g., the poles of a permanent magnet [30]. The accumulated ferrofluid in these areas can lift the magnet against its substrate, creating a low friction bearing. This principle was used in previous tubular micro actuators to both center and lubricate the rotor without needing micromachined bearings [17], [18], [19], [22], [23]. However, these actuators use rotors with relatively long axial sizes (e.g., 3 mm), which reduces the overall flexibility of the catheter when integrated [18]. Additionally, the electrical wiring of these actuators and others [20] blocks parts of the imaging beam during rotation, resulting in blind spots in a large portion of the visual field [15], [16].

The current work leverages the advantages of the ferrofluid bearing mechanism to study a novel planar micro rotary actuator for its potential application to side-viewing endoscopic probes (Fig. 1(a)). The primary aim of adopting the planar architecture is to drastically reduce the axial length of the actuator, which in turn enhances the flexibility (and thus maneuverability) of the integrated probe. Additionally, the new actuator is designed to have a hollow structure to eliminate the shadowing effects. To achieve the hollow design, a mirror is attached to the distal end of a ring magnetic rotor that is spun by the proximally placed hollow planar stator. This creates an unobstructed optical path through the entire actuator, into the rotating mirror, and out of the scanner. The design and prototyping for the above planar actuator and optical scanner are discussed in the next section. This is followed by the experimental results from the electromechanical characterization of the fabricated prototype, and a demonstration of circumferential laser scanning to verify its viability as an endoscopic optical scanner.