Biomimetic apposition compound eye fabricated using microfluidic-assisted 3D printing

Arthropods' compound eyes, honed over half a billion years of evolution, possess exceptional visual capabilities. These eyes inspire the development of artificial counterparts, but traditional 2D fabrication methods struggle to replicate their intricate 3D structure. This research addresses this limitation by introducing a novel biomimetic apposition compound eye (BAC-eye) fabricated through a microfluidic-assisted 3D printing process. The significance lies in creating a device that accurately mirrors the functionality of a natural compound eye, offering advantages in image acquisition and processing compared to existing artificial systems. The improved design facilitates seamless integration with existing planar image sensors, reducing the complexity and cost associated with image processing and sensor requirements. This technology holds immense potential for enhancing diverse fields, from advanced endoscopy to sophisticated machine vision applications fostering more intuitive human-robot interactions. The study aims to demonstrate the fabrication and performance of this novel BAC-eye, detailing its structural design, fabrication methodology, optical characteristics, and imaging capabilities.

Existing artificial compound eyes often rely on conventional 2D microfabrication techniques, such as transferring planar microlens arrays onto a spherical surface. While these techniques are relatively straightforward, they can compromise uniformity and overall performance. Advanced techniques like 3D laser writing or two-photon polymerization address the 3D fabrication challenges but often create incompatibility with commercial planar image sensors. Deformable optoelectronics offer a potential solution by curving the photodetector array to match the compound eye but limitations arise due to challenges in scaling up the size. The current study aims to overcome these limitations by introducing a biomimetic approach that uses a unique fabrication method for compatibility with existing planar imaging sensors.

The BAC-eye design mimics the anatomical structure of an apposition compound eye. Each microlens functions like a corneal facet lens, with a cylindrical post and silicone-elastomer waveguide acting as a crystalline cone and rhabdom, respectively. The internal structure minimizes crosstalk. The fabrication process begins with 3D printing a mold with a hemispherical pit patterned with 522 cylindrical microholes. A microfluidic-assisted molding technique, leveraging surface tension and centrifugal force, shapes the acrylate resin within each microhole into a concave lens. A complementary hemispherical substrate with hollow pipelines is 3D printed. The substrate and mold are combined, and the pipelines and microholes are filled with RTV silicone. After curing, separating the hemisphere from the mold yields the BAC-eye. The optical properties are optimized by using a photosensitive polymer dyed with Sudan Black 3 to absorb stray light and eliminate crosstalk. The optical performance is characterized using simulations and experiments focusing on light transmission, crosstalk, angular sensitivity, and image formation. Panoramic imaging and 3D point source tracking experiments are conducted to demonstrate the BAC-eye's functionality.

The microfluidic-assisted 3D printing successfully generated a BAC-eye with 522 microlenses arranged omnidirectionally across a hemisphere. The microlenses exhibited a uniform curvature of 91.9 ± 0.8 μm, matching theoretical predictions. Optical characterization confirmed efficient light transmission and minimal crosstalk between waveguides (extinction ratios of 16.1 and 15.2 dB). Ray tracing simulations and experiments showed efficient light coupling and propagation, with a total optical loss of 5.37 dB attributed mainly to waveguide bending and narrowing. The ommatidia demonstrated a wide acceptance angle of approximately 44°. Panoramic imaging experiments successfully reconstructed full-color, wide-angle images. 3D point source tracking was demonstrated, with a root-mean-square deviation of <0.16 mm. The system's ability to map 3D images onto a 2D sensor was effectively demonstrated.

The successful fabrication and characterization of the BAC-eye demonstrate a significant advance in biomimetic compound eye technology. The use of microfluidic-assisted 3D printing allows for precise control over the microstructure and eliminates the limitations of traditional 2D fabrication methods. The seamless integration with planar image sensors simplifies image processing and reduces the system's complexity and cost. The results show the BAC-eye's potential for applications requiring wide-field-of-view imaging and 3D position tracking. The high accuracy of 3D position tracking opens up possibilities for advanced applications in robotics and medical imaging. The scalability of the fabrication method suggests that further improvements in resolution and sensitivity can be achieved by increasing the number of ommatidia and improving the filling factor. This technology shows substantial promise in improving various fields.

This study successfully demonstrated a biomimetic apposition compound eye fabricated using a novel microfluidic-assisted 3D printing technique. The BAC-eye exhibits high optical fidelity, efficient light transmission, minimal crosstalk, and a wide acceptance angle. Its capability to generate full-color panoramic images and accurately track 3D point sources underscores its potential for diverse applications in imaging, sensing, and robotics. Future work could focus on increasing the number of ommatidia to enhance resolution and exploring the use of imaging-over-fiber technology to further improve image quality and achieve ultra-high resolution. The potential for an emitting mode using a 2D display also warrants investigation.

The current design of the BAC-eye has a limited field of view of 170°, which is slightly less than the full 180° field of view in many natural compound eyes. The precision of 3D position tracking could be further enhanced by increasing the number of ommatidia and the bit depth of the CMOS camera. The experiments were conducted using specific light sources and targets, and the performance in more complex environments may differ. Further research is needed to assess the BAC-eye's robustness and adaptability to various real-world conditions.