This paper provides an overview of recent research in the use of microelectromechanical systems (MEMS) actuators for beam steering applications, including optical coherence tomography (OCT). Prototype scanning devices have been fabricated out of polyimide substrates using conventional integrated circuit technology. The devices utilize piezoelectric bimorphs to mechanically actuate the torsion mirror structure made of polyimide. The material properties of the polyimide allow very large scan angles to be realized in the devices while using low voltages. Prototype devices have demonstrated optical scan angles of over 80 degrees with applied voltages of only 40V. Different device sizes have also been demonstrated with resonant frequencies between 15-60Hz (appropriate for real-time imaging). Analytical models have been developed that predict resonant frequency of the device as well as the angular displacement of the mirror. Further finite element modeling (FEM) has been done using ANSYS. These models closely reflect measured scan angles of the prototype devices. Based upon these models, further refinements can be made to the design to produce specific resonant frequencies for use in a multitude of applications. These models are currently being used to design and fabricate multiple devices on a single wafer with minimal post processing requirements. The ability to fabricate these devices in bulk will reduce their cost and potentially make them disposable to reduce the cost of their use in numerous applications, including patient care when used in biomedical imaging applications.
We have modeled, fabricated, and tested polyimide amplified piezoelectric bimorph scanning mirrors for application in optical coherence tomography (OCT). These scanning mirrors are fabricated using photolithography using polyimide as a substrate. These devices use bimorph actuators to drive polyimide micromechanical structures at resonance. The forced vibration of these micromechanical structures causes polysilicon gold plated mirrors attached to two torsion hinges to tilt. Operating the device at resonance allows us to achieve very large displacements of the mirror at real-time imaging speeds. The large scan angles and fast imaging speeds give these novel scanning devices the potential to be used to image larger areas of tissue to search for diseases such as mucosal cancers and to guide interventional procedures such as laser ablations and biopsies in real time. The mirror and support structures were modeled using one-dimensional beam theory and fundamental vibration mechanics. The structures were also modeled and simulated using ANSYS, a finite element analysis package. The finite element modeling has also lead to the development of new methods to fabricate the entire devices on a single silicon wafer. Prototype scanning devices have demonstrated optical scan angles up to 97 degrees with applied voltages from 15-60 V at a resonant frequencies ranging from 12-60 Hz, appropriate for real time imaging. These amplified bimorph imaging probes have been integrated into the scanning arm of a Spectral Domain OCT (SD-OCT) imaging system and have been used to generate preliminary in vivo human skin images at frame rates of 25 frames per second.
This paper provides an overview of several years of research in the use of polyimide MEMS actuators for medical imaging applications, including high frequency ultrasound and optical coherence tomography (OCT). These scanning devices are microfabricated out of polyimide substrates using conventional integrated circuit technology. The material properties of the polyimide allow very large scan angles to be realized and also allow the resonant frequencies of the structures to be in the appropriate ranges for real-time imaging. The primary application of these probes is endoscopic and catheter-based imaging procedures. The microfabrication enables the creation of very small devices essential for compact imaging probes. In addition, they can be fabricated in bulk, reducing their cost and potentially making them disposable to reduce the cost of patient care and minimize the potential for patient cross-contamination. Several different scanning geometries and actuators have been investigated for imaging applications, including both forward-viewing and side-scanning configurations. Probes that utilize both electrostatic polyimide actuators and piezoelectric bimorphs to mechanically scan the ultrasound or OCT imaging beams will be presented. These probes have been developed for both use in both ultrasound and OCT imaging systems. Medical applications of these probes include the early detection of cancerous and precancerous conditions in the bladder and other mucosal tissues. These imaging probes will allow the physician to visualize the subsurface microstructure of the tissues and detect abnormalities not visible through the use of conventional endoscopic imaging techniques. Prototype devices have been used to image geometric wire phantoms, <i>in vitro</i> porcine tissue, and <i>in vivo</i> subjects. The progress made over the last several years in the development of these polyimide scanning probes will be presented.