Optimal design and fabrication of novel devices for high-performance optofluidic applications is a key issue for the development of advanced lab-on-a-chip systems. Parallel cell counting with a high success rate and simple mode of operation is a challenging goal. Current cell-counting methods, using optical waveguides or flow cytometry, typically require a precise coupling of the probe light and involve complex operations. In the present paper, a novel multifunctional cell counting microdevice is designed. It uses a center-pass optofluidic microlens array (MLA) consisting of seven microlenses and an M-shaped confining wall with 9 μm-diameter apertures. The device can be fabricated in a three-dimensional microchannel by ship-in-a-bottle femtosecond laser integration based on two-photon polymerization with optimized experimental parameters. Each microlens produces approximately the same intensity at the focal positions (within ±5%) under white-light illumination, while the confining wall restricts 6∼8 μm-width cells to passing through the edges of two adjacent microlenses because the aperture opens toward their centers. The device demonstrates coupling-free parallel cell counting with a 100% success rate by monitoring the optical intensity variations at each spot. As a result, this method features both easy operation and high performance. Furthermore, the confining wall can filter deformed cells having 15 μm width.

A simple technique for the preparation of vertical electrodes on the sidewalls of three-dimensional (3D) glass microfluidic structures using water-assisted femtosecond-laser ablation followed by electroless plating is presented. The introduction of water during the laser direct-write ablation process greatly enhances the removal of the debris generated inside the glass, leading to an improvement in the quality of the ablated micropatterns. As a result, high-quality deposition of metal structures onto the ablated patterns can be realized by subsequent electroless plating processes. This new technique successfully performs sidewall metal patterning in 3D microfluidic structures with high flexibility. It is used to write the logo “LOC” in different sizes and form vertical electrodes 500 μm in height with an aspect ratio of ∼50. 3D glass microfluidic structures monolithically integrated with vertical electrodes, which are a kind of electrofluidic device, enable us to flexibly control the movement of C. elegans in channels based on electrotaxis.

Correction for ‘Vertical sidewall electrodes monolithically integrated into 3D glass microfluidic chips using water-assisted femtosecond-laser fabrication for in situ control of electrotaxis’ by Jian Xu et al., RSC Adv., 2015, 5, 24072–24080.

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.

Femtosecond lasers have unique characteristics of ultrashort pulse width and extremely high peak intensity; however, one of the most important features of femtosecond laser processing is that strong absorption can be induced only at the focus position inside transparent materials due to nonlinear multiphoton absorption. This exclusive feature makes it possible to directly fabricate three-dimensional (3D) microfluidic devices in glass microchips by two methods: 3D internal modification using direct femtosecond laser writing followed by chemical wet etching (femtosecond laser-assisted etching, FLAE) and direct ablation of glass in water (water-assisted femtosecond laser drilling, WAFLD). Direct femtosecond laser writing also enables the integration of micromechanical, microelectronic, and microoptical components into the 3D microfluidic devices without stacking or bonding substrates. This paper gives a comprehensive review on the state-of-the-art femtosecond laser 3D micromachining for the fabrication of microfluidic, optofluidic, and electrofluidic devices. A new strategy (hybrid femtosecond laser processing) is also presented, in which FLAE is combined with femtosecond laser two-photon polymerization to realize a new type of biochip termed the ship-in-a-bottle biochip.

Space-selective metallization of the inside of glass microfluidic structures using femtosecond laser direct-write ablation followed by electroless plating is demonstrated. Femtosecond laser direct writing followed by thermal treatment and successive chemical etching allows us to fabricate three-dimensional microfluidic structures inside photosensitive glass. Then, femtosecond laser ablation followed by electroless metal plating enables flexible deposition of patterned metal films on desired locations of not only the top and bottom walls but also the sidewalls of fabricated microfluidic structures. A volume writing scheme for femtosecond laser irradiation inducing homogeneous ablation on the sidewalls of microfluidic structures is proposed for sidewall metallization. The developed technique is used to fabricate electrofluidics in which microelectric components are integrated into glass microchannels. The fabricated electrofluidics are applied to control the temperature of liquid samples in the microchannels for the enhancement of chemical reactions and to manipulate the movement of biological samples in the microscale space.

The demand for increased sensitivity in the concentration analysis of biochemical liquids is a crucial issue in the development of lab on a chip and optofluidic devices. We propose a new design for optofluidic devices for performing highly sensitive biochemical liquid assays. This design consists of a microfluidic channel whose internal walls are coated with a polymer and an optical waveguide embedded in photostructurable glass. The microfluidic channel is first formed by three-dimensional femtosecond laser micromachining. The internal walls of the channel are then coated by the dipping method with a polymer that has a lower refractive index than water. Subsequently, the optical waveguide is integrated with the microfluidic channel. The polymer coating on the internal walls permits the probe light, which is introduced by the optical waveguide, to propagate along the inside of the microfluidic channel. This results in a sufficiently long interaction length between the probe light and a liquid sample in the channel and thus significantly improves the sensitivity of absorption measurements. Using the fabricated optofluidic chips, we analyzed protein in bovine serum albumin to concentrations down to 7.5 mM as well as 200 nM glucose-D.