By Koji Sugioka

The rapid development of the femtosecond laser has revolutionized materials processing due to its unique characteristics of ultrashort pulse width and extremely high peak intensity. In particular, the high peak intensity allows nonlinear interactions such as multiphoton absorption and tunneling ionization to be induced in transparent materials, which provides versatility in terms of the materials that can be processed. More interestingly, irradiation with tightly focused femtosecond laser pulses inside transparent materials makes three-dimensional (3D) micro- and nanofabrication available due to efficient confinement of the nonlinear interactions within the focal volume.

Using this feature, subtractive manufacturing based on internal processing can realize the direct fabrication of 3D microfluidics, micromechanics, microelectronics and photonic microcomponents in glass. These microcomponents can be easily integrated in a single glass microchip by a simple procedure using a femtosecond laser to realize more functional microdevices, such as integrated biochips and photonic microdevices. Additive manufacturing based on multiphoton absorption (two-photon polymerization: TPP) enables the fabrication of 3D polymer micro- and nanostructures for photonic devices, micro- and nanomachines, and microfluidic devices, and has applications for biomedical and tissue engineering.

Although biochip fabrication based on the internal processing in glass with femtosecond laser pulses has achieved great successes, one significant drawback is its relatively lower fabrication resolution compared with that of TPP. To overcome this difficulty, a group at RIKEN Center for Advanced Photonics proposed a new method termed hybrid femtosecond laser microfabrication (HFLM). The technique involves successive subtractive (femtosecond internal processing of glass) and additive (TPP) 3D microprocessing to realize highly functional biochips, enabling fabrication of novel biochips by the integration of various 3D polymer micro/nanostructures into flexible 3D glass microfluidic channels.

Figure 1 shows a schematic illustration of the fabrication procedure for a functional biochip by HFLM. It involves two main steps. The first step is to fabricate 3D microfluidic structure by femtosecond laser 3D direct writing of photosensitive Foturan glass (Fig. 1a) followed by thermal treatment (Fig. 1b) and successive chemical wet etching in a diluted hydrofluoric acid solution (Fig. 1c). The surface smoothness is improved by post thermal treatment after the etching. The second step is to integrate functional microcomponents into the resulting glass microfluidic structure for chip functionalization by the TPP procedure (Fig. 1d) filling the closed microfluidic structure with the epoxy-based negative-type resin SU-8, (Fig. 1e) femtosecond 3D direct writing in SU-8 after the prebaking, (Fig. 1f) creation of polymer 3D microstructure inside the microfluidic structure after the developing). The fabricated microchip is referred to as a “ship-in-a-bottle” biochip, since the polymer 3D microstructure is created in the closed 3D glass microfluidic structure after the microfluidics fabrication.

Fig. 1. Schematic illustration of the fabrication procedure for a 3D ship-in-a-bottle biochip by HFLM. It mainly consists of (a) fs laser 3D direct writing of photosensitive Foturan glass followed by (b) a thermal treatment, (c) HF etching, (d) polymer filling, (e) TPP and (f) developing.

Fig. 1. Schematic illustration of the fabrication procedure for a 3D ship-in-a-bottle biochip by HFLM. It mainly consists of (a) fs laser 3D direct writing of photosensitive Foturan glass followed by (b) a thermal treatment, (c) HF etching, (d) polymer filling, (e) TPP and (f) developing.

Filtering and mixing are key functions for biochip applications, and have been well studied recently with most efforts concentrated on the fabrication of microcomponents with a single function of either a filter or a mixer. If one microcomponent possesses multifunctions, it will be more useful and attractive. As shown in Fig. 2a, a novel multifunctional filter-mixer device was designed in which two filters were combined with the inlet and outlet of one passive-type mixer. The mixer has a configuration of layered crossing tubes to guide and rearrange fluids effectively and can realize fast mixing in a short channel length. For example, the left-side fluid in the 2nd and 4th layers (indicated by green color numbers and arrows) was realigned from left to right while the right-side fluid in the 1st and 3rd layers (indicated by red color numbers and arrows), was realigned from right to left. Both fluids were rearranged alternately in the vertical direction. In addition to the effect of layer-rearrangement, momentum would also contribute to effective mixing, because the fluids passing inside the tubes have left/right momentum and the fluids discharging to the outside of the tubes have up/down momentum. To realize higher mixing in a shorter distance, the tilted angle was designed as much as 45o (Fig. 2b). Figures 2b-2d show SEM images of the fabricated device made of a polymer on a flat glass surface according to the designed model shown in Fig. 2a. The center part of the mixer is sandwiched between two filters with a hole size of 8 mm, as shown in the schematic image of Fig. 2a and a magnified SEM image of Fig. 2e.

Fig. 2. (a) Schematic design principle of a novel multifunctional filter-mixer device in which two filters were formed at the inlet and outlet of a passive-type mixer. (b) Top-view SEM image of the fabricated multifunctional device by TPP and its mixing mechanism. (c-e) 30o, 45o tilted view and magnified SEM images observed from different directions clearly showing the 2 filters and 4 layer-microstructures of the mixer.

Fig. 2. (a) Schematic design principle of a novel multifunctional filter-mixer device in which two filters were formed at the inlet and outlet of a passive-type mixer. (b) Top-view SEM image of the fabricated multifunctional device by TPP and its mixing mechanism. (c-e) 30o, 45o tilted view and magnified SEM images observed from different directions clearly showing the 2 filters and 4 layer-microstructures of the mixer.

The multifunctional device with the same structure was thus integrated in a Y-shaped microchannel embedded in the glass substrate as schematically illustrated in Fig. 3a. By pouring water and Rhodamine B (dissolved in water, 20-50 ppm, flow speed ~4 mm/s) as two different kinds of solvents, the two were effectively mixed in the microfluidic channel integrated with the microdevice (Fig. 3b). On the other hand, in the simple microfluidic channel without the microdevice (Fig. 3c), no mixing occurred and laminar flow was produced. The device also successfully filtered some dust particles (~10 mm) in the solvents (enlarged inset in Fig. 3b). The mixing efficiency was quantitatively estimated to be as high as 87.2% by extracting the grayscale intensity from the optical microscopic images (insets in Figs. 3b and 3c). It is worth noting that the distance for mixing corresponding to the distance between A and B is about 270 mm, which is almost same as the channel width. This means that this device can realize high mixing performance even in such a short distance.

Fig. 3. (a) Schematic illustration of a 3D ship-in-a-bottle biochip fabricated by HFLM. (b, c) Comparison of mixing efficiency in the microchannels integrated (b) with and (c) without a device.

Fig. 3. (a) Schematic illustration of a 3D ship-in-a-bottle biochip fabricated by HFLM. (b, c) Comparison of mixing efficiency in the microchannels integrated (b) with and (c) without a device.

In conclusion, a new method referred to as HFLM consisting of femtosecond laser internal processing of glass and TPP demonstrated fabrication of glass/polymer composite true 3D biochips. Such distinct microchips have been termed ship-in-a-bottle biochips. As a proof-of-concept of high performance functions and applications, the ship-in-a-bottle biochips were applied to the multifunctionality of filtering and mixing. Thus, the synergetic combination of subtractive and additive microfabrication into a hybrid approach will open up a new door to enhance the flexibility and/or capability of 3D femtosecond micro and nanofabrication by taking the advantages of complementary characteristics of each individual approach. Future advances of this technology will lead to the development of smart manufacturing platforms for innovative applications in a variety of fields including integrated photonic devices, functional microfluidics, optofluidics, medical devices, MEMS and MEMS packaging.

Dr. Koji Sugioka is a leader of RIKEN-SIOM Joint Research Unit in RIKEN Center for Advanced Photonics.