1. Introduction

Recent years have witnessed a rapid growth in the development of microfluidics. Owing to their capability of manipulating tiny volumes of liquids with high precision, microfluidic systems have attracted significant attention [1,2]. To date, fabrication of microchannels heavily relies on photolithography which is inherently a two-dimensional (2D) planar fabrication technology [3,4]. Fabrication of three-dimensional microfluidic structures by photolithography-based techniques therefore requires stacking and bonding, leading to increased complexity and cost. One elegant solution for achieving 3D microfluidic structures in transparent substrates is to use femtosecond laser direct writing, as demonstrated by many groups [5–10]. As a direct and maskless fabrication technique, the microfluidic structures fabricated by femtosecond laser have indeed found a broad spectrum of applications, such as optofluidic sensors with various functions including refractive index monitoring [11], nanoaquariums for observing living organisms [12,13], and microfluidic lasers [14], single-cell detection and manipulation [15,16], and rapid screening of algae populations [17], etc. Generally, microfluidic channels can be formed in glass by two strategies: either by femtosecond laser direct writing followed by chemical etching [18–21] or by water-assisted [22–25] femtosecond laser 3D drilling. More recently, we have developed a technique which allows fabrication of microfluidic channels of arbitrary configurations and lengths in a porous glass material immersed in water followed by postannealing [26,27]. Using this technique, we can rapidly fabricate complex 3D micro-total analysis systems (μ-TAS) and Lab-on-a-chip (LOC) systems for biological and chemical analysis applications.

In this article, we show, for the first time to the best of our knowledge, that 3D microfluidic channels can also be fabricated in porous glass using a Q-switched, frequency-doubled Nd:YAG laser. The processing mainly consists of two steps: (1) formation of hollow microfluidic channels in porous glass immersed in Rhodamine 6G (Rh6G) dissolved in water by nanosecond laser ablation; and (2) postannealing of the fabricated porous glass sample at 1120 °C for consolidation of the sample. Here, Rh6G plays an important role as an absorber for promoting the ablation by the nanosecond laser within the porous glass sample, e. g., near the focal volume. The mechanism behind this technique is somewhat similar to that of laser induced backside wet etching (LIBWE) [28–30], in which glass surface in contact with organic solution can be ablated by nanosecond laser to form surface microstructures. Due to the fact that the porous glass is immersed in a solution doped with Rh6G, the liquid can penetrate into the substrate through the nanopores. Since we use a 532 nm laser whose wavelength is close to the absorption peak of Rh6G (~520 nm), we choose the Rh6G in water for achieving an efficient absorption at the focus. Therefore, ablation inside glass is possible. There is no doubt that replacing the femtosecond laser by the nanosecond laser will greatly reduce the cost for fabricating 3D microfluidic devices whereas in the meantime enhance the stability of the laser source.

2. Experiment

In the experiment, we use a high-silica porous glass as the substrate material. The porous glass samples were obtained by removing the borate phase from phase-separated alkali-borosilicate glass in hot acid solution [31]. The phase-separated alkali-borosilicate glasses were cut to 10 mm × 10 mm × 2 mm coupons and polished before treated in hot acid. The composition of the porous glass is 95.5SiO2-4B2O3-0.5Na2O (wt. %). The pores with a mean size of ~10 nm occupy 40% volume of the whole glass and are distributed uniformly in the glass to form a 3D connective network, allowing liquid to infiltrate into the glass.

The 3D microstructure inside the porous glass was formed using nanosecond laser direct writing. The laser pulses are operated at 532 nm wavelength, with a duration (full-width at half-maximum (FWHM)) of ~8 ns and a repetition rate of 10 Hz. We stress that a YAG laser operated at higher repetition rates should be better than the 10 Hz system used in our experiment, which is able to offer higher scanning speeds. The average power of the laser beam is controlled through a combination of polarizer and wave plate and a set of neutral density filters. In addition, in order to ensure a high quality beam, a circular aperture is used to clip the initial 8.8 mm beam to 5 mm. The incident pulse energy was chosen to be ~100 μJ. The beam was focused into the porous glass through a 50 × microscope objective (BX51, Olympus, Tokyo, Japan) with a numerical aperture of 0.80. The samples can be arbitrarily translated three dimensionally by a computer -controlled X-Y-Z stage with a resolution of 0.1 μm. A charge couple device (CCD) connected to a personal computer is used for monitoring the whole direct-writing process in real time, and the procedures of the fabricated structures could be captured by the CCD camera.

Figure 1(a) shows a schematic illustration of the system for carrying out nanosecond laser direct writing in porous glass immersed in Rh6G diluted with water at a concentration of C = 6.0 × 10 ⁻³ mol/L. The refractive index of the porous glass is 1.48. Before being immersed in the liquid, the porous glass appears opaque due to the strong scattering of light from the nanopores; however, after soaked in the liquid, because of the smaller refractive-index difference between the glass (n≅1.46) and the absorbing liquid (n≅1.33) as compared to the refractive-index difference between the glass and air (n≅1), the scattering of light can be significantly reduced, which facilitates formation of high-quality focal spot inside glass. The nanosecond laser beam was focused into the porous glass through the microscope objective. Some straight microfluidic channels were fabricated inside the porous sample at different depth as illustrated in Fig. 1(b). The microchannels were fabricated by single scan at a translation speed of 10 μm/s until the desired length is achieved, followed by several back-and-forth scans at a translation speed of 100 μm/s for completely removing the debris remaining in the channel with the assistance of the bubbles formed in the Rh6G water solution. Uniform channels of a total length up to ~1 cm were achieved after the multiple scan. The total process for fabricating the 1 cm-long channel took ~1 hrs, limited by the low repetition rate of our YAG laser. In our previous experiment with femtosecond laser direct writing, the translation speed was set at 100 μm/s because of its relatively higher repetition rate (i. e., 250 kHz). After the laser direct writing, a post-annealing process was applied to the sample for collapsing all the nano-pores in the glass, so that concealed microfluidic channels are formed. The temperature inside the furnace was ramped up to 1120 °C at a rate of 1°C/min and held at 1120°C for 2 hours and then naturally cooled down to room temperature.After the postannealing, the porous glass was turned into a compact solid glass with high transparency. Before the post-annealing, the sample was soaked in acetone to take an ultrasonic bath for 30 min for removing the Rh6G molecules. A small amount of residual Rh6G dye left behind by the ultrasonic bath was further completely removed during the postannealing.

 

Fig. 1 (a) Schematic view of experimental setup. (b) Flow chart for the whole fabrication process.

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3. Results and discussion

Top view optical micrograph of 3D microfluidic channels embedded in porous glass before post-annealing are shown in Fig. 2(a) , and microchannels partially filled with water are shown by the closed-up view in Fig. 2(b). The diameter of the microfluidic channels is approximately 20 μm. After the postannealing process, fluorescence microscopy image of the microchannels filled with a solution of fluorescein is shown in Fig. 2(c). The confined fluorescent solution gives a clear evidence that the nanopores have all collapsed to form the consolidated substrate. (No leakage is observed in this experiment even if the liquid is stocked in the microfluidic channels for 7 days). It should be specifically mentioned that, since all the pores have collapsed, the total volume of the annealed substrate will decrease to 60% ~70% of that of the unannealed sample. As a result, the diameter of the microchannels will decrease from ~20 μm to ~17 μm. However, the shapes of the microchannels remain almost unchanged.

 

Fig. 2 (a) Top view optical micrograph of 3D microfluidic channels embedded in porous glass before post-annealing. (b) Closed-up view of microchannel partially filled with water before post-annealing. (c) Fluorescence microscopy image of the microchannels filled with a solution of fluorescein. The confined fluorescent solution gives a proof that the nanopores have all collapsed to form the consolidated substrate.

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For investigating the cross sections of micro-channels, six microchannels are fabricated 210 μm beneath the porous glass surface with pulse energies of 60, 120, 180, 160, 170, and 180 μJ, respectively, and three more microchannels are fabricated 420 μm beneath the glass surface with pulse energies of 120, 160, and 180 μJ, respectively. We then broke up the microchannels by mechanical cutting and subsequent cleavage. Next, we polished the sample from the end of the channel until the microchannels were exposed. Figure 3 shows the cross sections of the microchannels fabricated in porous glass at the different positions after the postannealing. We can see that the higher the pulse energy the larger the width of the channel. Furthermore, for channels deeper in the sample, their widths are smaller than those embedded shallower in the sample. This is most probably due to the absorption of light by the Rh6G solution. The consolidation of the porous glass also results in up-shift of the microchannel, leading to a distance shortened by ~15% between the glass surface and the channels.

 

Fig. 3 SEM image of the cross-sectional view of the cleaved microchannels after the postanneanling. Small cracks can be found near the microchannel. The pulse energy chosen for fabricating each channel is indicated in the SEM image.

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In order to examine the morphology of the innerwall of microchannel, we polished the sample from the top surface until the microchannel was exposed. A scanning electron microscope (SEM) image of the surface morphology of the sample after the post-annealing is shown in Fig. 4 . It is clear that the innerwalls of the channel after the post-annealing process show relatively high surface roughness, which can be attributed to the debris redeposited onto the inner wall of the microchannel. The debris possibly could be reduced or even totally removed by an additional chemical etching in diluted HF acid, as we will try in the future.

 

Fig. 4 SEM image of the innerwall of the microfluidic channel after post-annealing.

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With the nanosecond laser operated at the 10 Hz repetition rate, we can obtain the microchannel with a total length of ~1 cm and a diameter of approximately 20 μm, as shown in Figs. 2(a) and 2(b). It is noteworthy that for achieving such thin channels, the pulse energy is chosen to be ~100 μJ, and the scanning speed is 10 μm/s. We expect that microfluidic channels of higher uniformity can be achieved with multiple scans at higher translation speeds, for which use of a high-repetition-rate YAG laser should be more appropriate.

The mechanism of fabrication of microfluidic channel in the porous glass by nanosecond laser direct writing should be similar to that by femtosecond laser writing as described in Refs [26,27], except that in the case of femtosecond laser writing, ablation is initiated by multiphoton absorption of glass or water; whereas in the nanosecond laser writing case, it is the Rh6G molecules which strongly absorb the laser energy and generate seed electrons. The free seed electrons are then driven by the laser field so that they strike nearby atoms, causing them to be ionized to make new free electrons, which in turn are driven by the laser field, resulting in an avalanche ionization to produce a plasma. This mechanism is similar to that of LIBWE [28–30]. However, with solid glass materials, the traditional LIBWE does not allow fabrication of large scale 3D hollow structures inside a transparent substrate by direct writing. Since bubbles can be formed in the liquid also around the focus, the bubbles can carry the debris when they are driven out of the channel as observed in our previous experiment [27]. This process repeats itself during the laser direct writing, allowing formation of long channels when the scanning of focal spot progresses.

4. Conclusions

To summarize, we demonstrate the fabrication of 3D microfluidic channels by nanosecond laser direct writing in the porous glass, providing an alternative solution for fabrication of 3D microfluidic systems in glass which are usually achieved with femtosecond laser pulses.