Liquid lenses that can fit in a microchip are now possible, say researchers in the US.

Sindy Tang and Claudiu Stan in the Whitesides group at the University of Harvard in the United States explained why they chose liquids: 'Conventional solid optical components are too bulky to be placed directly on a chip.'

The role of a lens is to focus light. They have achieved this with streams of liquid flowing in a microfluidic channel. 'We can vary the shape, and therefore the focus, of the lens by changing the flow of the liquids inside the microchannels,' said Tang. 'Liquids have some unique properties that are useful for optics. The interface between the two liquids in microfluidic systems is intrinsically smooth. Fluidic optical compounds, therefore, do not require the polishing that solid optical parts need.'

Tang explained their motivation: 'An example of a biochemical application is a microfluidic flow cytometer, where localized excitation of fluorescent labels in cells or beads in essential in counting them accurately.'

Albert van den Berg, a microfluidics expert at the University of Twente, describing the work as 'an original and creative way to make an optical lens', said that it had the advantage of relatively simple construction and being able to be integrated into a lab-on-a-chip system. He cautioned that it may not have the optical precision of other, already commercialized, technologies: 'Electrowetting-based lenses are much faster and have better optical quality.'

Tang said they are building a 'toolbox' of optical components. 'We have developed waveguides, evanescent couplers, splitters and lenses. We are now working on prisms and sensors based on similar principles,' she said. However, Tang noted there are still limitations to what you can fit onto a chip. 'We have made a microfluidic dye laser as an on-chip light source, but it still requires an external laser. It is even harder to integrate a detector directly on a chip.'

Colin Batchelor