Low-Power Laser Etching May Speed Lab-On-A-Chip Fabrication

A novel method for fabricating poly-dimethylsiloxane (PDMS) waveguides using low-power laser etching could make it easier and less expensive to incorporate optical sensing onto lab-on-a-chip devices. The PDMS waveguides can be easily integrated into lab-on-a-chip devices made of the same material.

Beam splitter validation test design. Separation between the legs with respect to the central input channel is controlled to vary the output efficiency as it is getting closer to the critical angle. Courtesy of Daniel Pérez Calixto, Diego Zamarrón Hernández.

Researchers from Universidad National Autonomous University of Mexico created a mold for the waveguides using the laser beam of a CD/DVD burner to etch a clear sheet of acrylic. Because low-power laser sources like the ones in CD/DVD burners typically aren’t absorbed by transparent materials, the researchers coated the acrylic with highly absorbent nanocarbon. This created pinpoint areas of intense heat that could be used to etch the material with microscale resolution.

The researchers next created PDMS with two different indices of refraction by carefully modifying the mixing and curing conditions of the material. They filled the etched micromold with PDMS of one refractive index, cured the material, then placed a layer of PDMS with a different refractive index on top. After another curing step, the researchers removed the PDMS from the mold, flipped it and added another layer of PDMS to create a waveguide completely embedded into two slabs of PDMS.

To verify the reproducibility of the mixing and curing recipe used to control the optical properties of PDMS, the researchers measured the refractive index of their fabricated PDMS layers several times. They also showed that the optical losses of waveguides made with this technique matched those reported for more complicated fabrication techniques.

“In addition to being low-cost, our technique accomplishes rapid prototyping of waveguides that can make it possible to integrate light-based capabilities such as interferometric devices into lab-on-a-chip devices,” said researcher Mathieu Hautefeuille. “It is also possible to fabricate long waveguides with our method, which can be a great advantage in lab-on-chip devices.”

Using their novel technique, the researchers fabricated an 8-mm long, Y-shaped PDMS beam splitter. Experiments showed that the beam splitter could separate a laser beam into two output arms, and that the light could be switched between each arm by changing the position and angle of the optical fiber delivering the light.

The researchers believe that their fabrication technique could be useful for several applications, including those requiring precision microstructuring. They say that the technique could be used to etch other polymer materials in addition to PDMS.

“Our new method is compatible with the development of lab-on-chip platforms where integrated optical waveguides can be a great tool for light-based diagnostics or monitoring applications,” said Hautefeuille.

The researchers are now working to demonstrate that their method can be used to fabricate more complex integrated optical devices such as an interferometer that could serve as an all-PDMS platform for sensing applications.

“Our study shows that short-pulsed lasers are not strictly necessary to etch transparent polymers and plastics with a micron-scale resolution,” said Hautefeuille. “The use of a recycled CD/DVD unit further shows that you might be able to stretch the usage of equipment that could be starting to look out of date.”

The most common material used to make lab-on-a-chip devices today is the silicone PDMS because of its optical, mechanical and chemical properties, its low cost and the ease with which it can be structured at the microscale. As these devices become more common and complex, there may be a need for less expensive ways to incorporate all-PDMS optical components such as waveguides to direct light onto and within the chip.

“To the best of our knowledge, this is the first time that low-power laser etching has been used to microstructure polymers for optical waveguide fabrication,” said Hautefeuille. “This study shows that a very inexpensive laser platform, based on a CD/DVD unit in our case, can compete with high-power lasers for such applications.”

The research was published in Optical Materials Express, a publicaton of OSA, The Optical Society of America (doi: 10.1364/OME.7.001343).

Using a new simple and inexpensive fabrication approach, the researchers fabricated an 8-millimeter long, Y-shaped all-PDMS beamsplitter. Shown here is the top view. Courtesy of Daniel Calixto Pérez, Diego Zamarrón Hernández.