High Speed Three-Dimensional Scanner for In Vivo Non-Invasive Optical Biopsy Using Two-Photon Microscopy

We have recently demonstrated the modeling, design, and micro-fabrication process of a millimeter-scale, high-speed endoscopic scanner that is to be integrated at the distal end of an endomicroscope. The scanner system consists of (1) an active Silicon op¬tical bench (SOB), which constrains, aligns, and thermally actuates (1) mm-size optics (GRIN lens and prism) at 5 Hz and (2) a slim fiber resonator that excites the double-clad photonic bandgap fiber at ~1 kHz. The scanner system has a 7-millimeter device envelope with a range of 100 micrometers in X, Y and Z. The design of a two-photon endoscope requires scanning of focused light to create tissue images, and scanning actuator technology still proves to be a bottleneck for practical endoscope design. The performance (force-speed-stroke) criteria for the prototype endomicroscope design are generated based on clinical needs. The strict force, speed, and stroke requirements (~10 mN, 1 kHz, and 100 µm) call for a new method for actuation. The low voltage requirement for future in vivo examination/operation makes a new class of thermomechanical actuators (TMAs) a suitable candidate among other micro-actuation technologies.

The two-photon imaging technique requires scanning of focused light to create tissue images. The endoscopic scanner may enable the design and construction of a miniaturized two-photon microscopic system to image the surface and sub-surface cells (up to 200 microns depth) of internal tissues with sub-cellular resolution. The two-photon endomicroscope is designed to perform non-invasive, in vivo, optical biopsy, which has numerous benefits over excisional biopsy. For example, non-invasive optical screening may decrease the number of excision biopsies required, and optical biopsy can provide more informed selection of excisional biopsy sites, minimizing incorrect diagnosis due to random sampling. This is useful for detecting cancer at an early stage among other diseases.

The chevron TMAs on the SOB are optimized through the geometric contouring method to provide enhanced force, displacement and reduced power consumption compared to common chevron actuators. This also allows the TMAs to be operated at lower temperature and thus makes the TMAs more suitable for precision actuation. Early models and experiments of the contour shaping method have confirmed that the maximum achievable thermal strain of a driving beam may be increased by 29%, the actuator stroke may be increased by a factor of 3 or more, and identical force or displacement characteristics may be achieved with 90% reduction in power. A new high-speed pulsing technique has also been investigated recently; it enhances the dynamic performance of the contoured TMAs. Preliminary simulation results indicate a 12% bandwidth increase, 30% stroke enhancement, and 70% power reduction. This technique, together with the geometric contouring method for TMAs, may potentially increase the bandwidth of the endoscopic scanner by a factor of 10 and therefore meet the functional requirements for a two-photon scanning endomicroscope.