Using a single femtosecond laser to fabricate both microfluidic channels and optical waveguides on the same fused silica substrate, a group of researchers has come one step closer to creating a complete three-dimensional lab-on-a-chip device for in situ optical biosensing. The laser technology avoids complicated multilayer processing, while the integration of both structures on the same substrate provides enhanced sensitivity and the possibility of multiple excitation sites in the channels.The researchers, from Istituto di Fotonica e Nanotecnologie-CNR at Politecnico di Milano in Italy, created a device comprising a 2.2-mm-long microfluidic channel and three optical waveguides to investigate the single-laser production technique. They generated the microchannel with a Ti:sapphire laser that delivered 150-fs pulses of 800-nm light through a 50× objective, focusing the beam 100 to 300 μm below the surface of the substrate. A translation stage moved the substrate perpendicularly to the beam at 20 μm/s to create a sensitized channel-shaped region from one side of the glass to the other. Subsequent etching in a 20 percent aqueous solution of hydrogen fluoride selectively removed material from the irradiated region, creating a void microchannel.Figure 1. Microchannels are fabricated by femtosecond laser pulses through one side of a glass substrate (end view). Overlapping channels from opposite sides of the substrate create nearly uniform channels (top view). A scanning electron microscopy image shows the resulting circular channels.By varying the length of the irradiated region, the researchers achieved different extents of etching overlap from opposite sides of the substrate and created different contours. They created 2.2-mm channels that were nearly uniform, with 110-μm diameters at the ends and a 90-μm diameter in the center. Less overlap produced channels as long as 3 mm, with entrance diameters of 90 and 50-μm diameters in the center. The method demonstrated good reproducibility. They employed the same laser, this time focusing it with a 20× objective to create three optical waveguides with 10-μm diameters. The control of the beam’s focal volume is essential for precise determination of the location and shape of the waveguides with respect to the microfluidic laser channels. Spaced 200 μm apart and intersecting perpendicularly with the microfluidic channel, the waveguides continued into the channel without damaging the walls, thus avoiding any beam divergence and ensuring spatial selectivity. The researchers manufactured the waveguides to support traditional optical sensing in the UV-visible range, including ultraviolet absorption and laser-induced fluorescence.To test a sample in the device, they filled a channel with a rhodamine 6G and ethylene glycol solution and coupled 543-nm light into the waveguides via an optical fiber. A Schott long-pass filter revealed yellow fluorescence at three points in the microfluidic channel corresponding to the locations of each optical waveguide (Figure 2). No scattered light from the waveguides reached the microfluidic channel, indicating precise excitation sites and the good quality of the waveguides. An intensity profile obtained by a Sony CCD camera across the fluorescent filament in the microchannel demonstrated a signal-to-noise ratio of ~20 dB.Figure 2. A schematic (top) shows the three optical waveguides intersecting perpendicularly with a microchannel inside the substrate. Microscope images (bottom) reveal laser-induced fluorescence at the points of intersection after 543-nm light is coupled into the waveguides through an optical fiber. Image reprinted with permission from Applied Physics Letters.Although the femtosecond laser technique is limited to fabricating channels of only a few millimeters, it can produce three-dimensional devices in a single step. The integration of optical waveguides allows photonic sensing applications to function directly on lab-on-a-chip devices. Coupling multiple waveguides demonstrates the possibility of parallel sensing at different points along the microchannel, which might be achieved by using the femtosecond laser writing method to integrate a waveguide splitter onto the chip.Applied Physics Letters, June 4, 2007, 231118.