Miniature 3D-printed rod-like refractive objective for endoscopic applications

The three-dimensional (3D)-printed refractive rod objective for endoscopy preserves the compact, gradient index (GRIN)-like format while delivering a wide field of view (FOV) comparable to its diameter and larger than that of a commercial GRIN lens.

We focus on the design, fabrication, and experimental validation of a proof-of-concept refractive rod objective for fluorescence imaging of mouse colon tissue, with performance compared with a commercial GRIN lens.

A 1× magnification refractive rod-like objective was designed in Zemax OpticStudio and fabricated using two-photon polymerization additive manufacturing. The objective consists of a sequence of convex refractive surfaces printed in contact at their vertices, with refractive index contrast provided by partially or non-polymerized resin contained within an enclosing wall. The lens has a diameter of $500\mu m$ with a clear aperture of $470\mu m$ , a total length of 2.06 mm, and a working distance of 1.6 mm and was optimized for a numerical aperture of 0.075 and a $500\text{-}\mu m$ design field at 525-nm light. Three photopolymer resins (IP-S, IP-Visio, and IPX-Clear) were evaluated through excitation-emission matrix measurements to assess autofluorescence. By imaging group 7, element 6 of a 1951 United States Air Force (USAF) resolution target, the field was assessed by plot profile and modulation transfer function measurements.

The fabricated objective resolved group 7, element 6 of a USAF resolution target ( $4.38\mu m$ ), closely matching the theoretical diffraction-limited resolution of $4.27\mu m$ . Compared with a commercial $600\text{-}\mu m$ -diameter GRIN lens, the 3D-printed objective achieved a substantially larger FOV (498 versus $188\mu m$ ). Spectral characterization showed that IP-Visio exhibited the lowest autofluorescence under 455-nm excitation when unpolymerized resin was present. Using an IP-Visio objective, fluorescence images of proflavine-stained mouse colon tissue were successfully acquired.

The demonstrated refractive rod-like objective combines the compact geometry of a GRIN lens with the aberration correction capability of multi-element refractive optics, enabling uniform resolution across a large FOV. The approach also allows material selection tailored to fluorescence imaging requirements. Future work will focus on integration with fiber bundles and on fully polymerized designs with spatially tuned refractive index for improved long-term stability.