In-channel integration of designable microoptical devices using flat scaffold-supported femtosecond-laser microfabrication for coupling-free optofluidic cell counting

Abstract

The high-precision integration of three-dimensional (3D) microoptical components into microfluidics in a customizable manner is crucial for optical sensing, fluorescence analysis, and cell detection in optofluidic applications; however, it remains challenging for current microfabrication technologies. This paper reports the in-channel integration of flexible two-dimensional (2D) and 3D polymer microoptical devices into glass microfluidics by developing a novel technique: flat scaffold-supported hybrid femtosecond laser microfabrication (FSS-HFLM). The scaffold with an optimal thickness of 1–5 µm is fabricated on the lower internal surface of a microfluidic channel to improve the integration of high-precision microoptical devices on the scaffold by eliminating any undulated internal channel surface caused by wet etching. As a proof of demonstration, two types of typical microoptical devices, namely, 2D Fresnel zone plates (FZPs) and 3D refractive microlens arrays (MLAs), are integrated. These devices exhibit multicolor focal spots, elongated (>three times) focal length and imaging of the characters ‘RIKEN’ in a liquid channel. The resulting optofluidic chips are further used for coupling-free white-light cell counting with a success rate as high as 93%. An optofluidic system with two MLAs and a W-filter is also designed and fabricated for more advanced cell filtering/counting applications. By fabricating microscale polymer lenses in hollow glass microchannels, scientists realize an optofluidic chip that detects and counts cells. This scheme works by employing a femtosecond laser to ‘grow’ by two-photon polymerization various three-dimensional optical structures, such as microlens arrays, on a flat polymer scaffold on the internal surface of a microfluidic channel. White light is then shone through the lenses and the presence of cells in the optical path is detected by a drop in light transmission. Dong Wu and co-workers from RIKEN’s Laser Technology Laboratory in Japan and Jilin University in China say that this fabrication approach could also prove useful for constructing a variety of active and passive micro-optic devices, such as optical switches, on-chip microlasers and amplifiers for lab-on-a-chip applications.