Two challenging issues in biological testing are devices’ portability and to using as little sample as possible. With this framework in mind, lab-on-a-chip devices play a major role, as they aim to integrate most of the facilities of a biology or chemistry laboratory in just a few millimeters device. They are basically made of micrometer size channels, where samples are separated, selected and mixed, and they also integrate some kind of testing system to analyze them. Optofluidic devices are a particular type of lab-on-a-chip device where light is used to test and handle samples. The use of light to analyze biological samples is widely spread as it is fast, highly accurate and, in some particular cases, allows for the analysis of samples after undergoing some pre-treatment which might change its properties.
Among the several technologies used to build optofluidic devices micromachining with femtosecond lasers is quite promising, as it allows to produce both the micrometer channels for fluid sample travelling and the optical networks needed for analysis. Thanks to its unique 3D capabilities, the channels can be integrated with the optical networks using the same laser. Although this technology is not suitable for mass-producing labs-on-a-chip, it is quite interesting in the fast prototyping of new devices as you have an idea and you can make and test your device in a few hours. If you are satisfied with the outcome, you can then think about mass producing the device using a different technique, such as photolithography.
A femtosecond laser emits light pulses that last for 10-13 seconds so when the laser beam is focused by a microscope objective it gives strong high peak intensities. If it is focused inside a glass, there will be some non-linear effects, which will permanently modify the material within the focal region: you can modify a small region of some micrometer size without touching the surrounding material!
Three different kinds of modification might take place depending on the power used:
- a) a region where the refractive index of the glass is higher and as a result the light is confined inside it, what is called a waveguide (channel for light)
- b) an area where acid etching is twenty times faster and, as a consequence, you can create micrometer holes after putting the sample in an acid solution;
- c) ablation/disruption takes place destroying the material.
With the combination of modifications a) and b) it is possible to create perfectly aligned microfluidic and optical networks during the same irradiation process. After the etching step, we will have an embedded optofluidic device almost ready for experiments, with no need to close it as everything will have been manufactured inside the glass. In the next paragraph are presented two specific examples where femtosecond laser micromachining played a fundamental role in the manufacture of an optofluidic device.
In a lab-on-a-chip, the final device’s geometry is based on the specific experiment it is intended for. For example, in the case of DNA molecules’ separation and detection, the microfluidic network has a cross geometry, with different waveguides bringing light to the central channel at specific positions. DNA molecules travel through the microfluidic channels and, when they are excited by the light as they pass in front of the waveguide, a signal is emitted, which is then detected and will translate into the presence, or not, of a disease’s genetic trace.
Optofluidic devices are essential when handling and sorting single cells. An integrated optical sorter and stretcher is an example of a chip fabricated by femtosecond lasers. This chip is based on a microfluidic network with an X geometry where two waveguides cross the central channel and perpendicularly face it on both sides. The infrared laser light is coupled inside the waveguides so when it arrives to the channel there are two perfectly aligned counter propagating beams. Once the cell sample is loaded, and a cell passes through the two beams, it is trapped by optical forces and stops there. At this point, if the power inside the waveguides is increased, the optical forces will deform the cell and this is how we will be able to study its mechanical properties, without physically touching it. Afterwards, depending on the outcome of the stretching, the cell can be pushed with another waveguide to one of the output arms. In this device the cells are manipulated without touching them.
It is amazing to realize that we are using light to fabricate lab-on-chip and for biological testing inside the same lab-on-chip, isn’t it?
1 – R. Gattass and E. Mazur, Femtosecond laser micromachining in transparent materials, Nat. Photon. 2, pp. 219–225, 2008.
2 – X. Fan and M. White, Optofluidic microsystems for chemical and biological analysis, Nature Photonics 5, 591-597 (2011).
3 – R. Martínez Vázquez, R. Osellame, M. Cretich, M. Chiari, C. Dongre, H. Hoekstra, M. Pollnau, H. van der Vlekkert, R. Ramponi, and G. Cerullo, Optical sensing in microfluidic lab-on-a-chip by femtosecond-laser-written waveguides, Anal.Bioanal. Chem. 393, pp. 1209–1216, 2009.
4 – T. Yang, P. Paiè, G. Nava, F. Bragheri, R. Martinez Vazquez, P. Minzioni, M. Veglione, M. Di Tano, C. Mondello, R. Osellame and I. Cristiani, An integrated optofluidic device for single-cell sorting driven by mechanical properties, Lab on Chip 15, 1262-1266 (2015).
Rebeca Martínez Vázquez obtained her Ph.D. in Physics in 2005 at UAM, Madrid, Spain. Since 2010 she is a staff researcher at Istituto di Fotonica e Nanotecnologie (IFN-CNR) in Milan, Italy. She started her research in femtosecond laser micromachining in 2005 applied to integrated optics. Since 2007 most of her research activity is focused in the fabrication of plastic and glass optofluidic devices.