Lab on a Chip Blog

From space shuttles to military equipment, or even the kitchenware that we use on daily basis, lasers have found use in more places than we often realise. Interestingly, the solid-state lasers were believed to be a solution to an unknown problem after their invention in 1950s. In that time nobody—including their developer Charles Townes—noticed that the lasers were one of the game changer inventions in the world’s history. After tens of years, in 1964, the laser technology was awarded with a Nobel prize when its potential for diverse applications were realized.

So far we have only discussed solid-state lasers, but there is definitely capacity in the laser world to improve the performance and versatility with the help of little twists, such as optofluidic lasers. “Optofluidics” is a synergic combination of optical systems and microfluidics. In other words, optical systems are built or synthesized from liquids, aiming to serve as good as their solid-state equivalents. For example, two immiscible liquids form a smooth surface in their interface, leading to a laser cavity or an optical resonator with a very high Q-factor that allows for operating at low energy levels. This emerging field gains importance when considering most of the biochemical reactions that occur in aqueous environments. Optofluidic laser systems are flexible to change their optical properties by just replacing the liquid media; and with this twist, lasers have new application areas including diagnosis of genetic disorders at the cellular level and in vivo biosensing.

What is more exciting about the optofluidic lasers is that they can be biodegradable and easily tunable in microenvironments. Researchers in University of Michigan recently showed that one of the most abundant pigments on earth, chlorophylls, can maintain both biodegradability and tunability in optical systems owing to their fluorescence capabilities. Chlorophylls have a high Q-factor, dual-absorption bands in the visible spectrum, and a large shift between absorption and emission bands, suggesting that chlorophylls can be used as donors in fluorescence resonance energy transfer (FRET) laser (Figure 1). In this study, chlorophyll a was isolated from spinach leaf and used as the gain medium and the donor to develop a novel optofluidic laser. Two lasing bands of chlorophyll a was investigated by both theoretical and experimental means. Concentration-dependent studies enabled more insight for the mechanism determining when, where, and why the laser emission band appears. This new technique seems to gain increasing attention for applications in in vivo and in vitro biosensing, solar lighting and energy harvesting.

Since its inception over 50 years ago, inkjet printing has become the most widely adopted method for the reproduction of images and text on paper. Inherently a microfluidic technology, inkjet printing is a process in which individual ink droplets, between 10 to 100 µm diameters, are placed at software-configurable locations on a substrate. The resulting dots can combine to create photo-quality images with resolutions of up to several thousand dots per inch. Such exquisite precision and flexibility make inkjet printing particularly attractive for the scalable fabrication of fine features. Indeed, in addition to coloured inks, this technology has also been adapted to deposit a wide range of materials for manufacturing, including metals, ceramics, polymers, and even biological cells.¹

Not surprisingly, the lab on a chip community is also beginning to leverage printing technologies for the fabrication of microfluidic components.² However, this is not straightforward because the quality of the printed features often depends on the interaction of the ink droplets with the substrate material. Since microfluidic devices are predominantly prototyped using polydimethylsiloxane (PDMS),³ recent research has focused on the development of PDMS-compatible methods for inkjet printing.

In one example, Profs Dachao Li at the Tianjin University, China and Robert C. Roberts at the University of Hong Kong used inkjet printing to form robust silver microelectrodes on PDMS-based microfluidics for glucose sensing.⁴ In forming these electrodes, the PDMS material posed two challenges: 1) poor adhesion of silver to PDMS inhibited robust electrode formation and, 2) the inherent hydrophobicity of PDMS promoted coalescence of neighboring ink droplets, which impaired the precision of printed features.

To overcome these challenges, the researchers modified the substrate using (3-Mercaptopropyl)trimethoxysilane, a coupling reagent that presents thiol groups on the PDMS surface. The thiol groups not only served as covalent anchoring sites for the inkjet-printed silver but also increased the wettability of the ink droplet, which improved the precision of the printed features.

Remarkably, the printed electrodes formed on these substrates tolerated very harsh stress tests, including immersion in water, exposure to high pressure air stream, and even ultrasonication. This new fabrication strategy enabled the team to create an all-in-one PDMS system with fluid handling and a three-electrode electrochemical cell for transdermal detection of glucose (Picture 1).

In another example, Profs Yanlin Song and Fengyu Li at the Chinese Academy of Sciences, China developed an elegant inkjet-printed method ⁵ for the fabrication of enclosed PDMS microchannels. This process conventionally requires at least 4 steps: 1) create a solid template via photolithography, 2) cast PDMS prepolymer mixture in the template, 3) cure and separate the PDMS from the template, and 4) bond the molded PDMS to a substrate.

To simplify this procedure, the researchers implemented a liquid template formed by inkjet-printing (Picture 2). Here, an immiscible liquid pattern is printed directly on a pool of PDMS prepolymer solution. Within seconds, the liquid spontaneously submerges below the prepolymer solution and acts as a template. Upon heating, the polymer solidifies while the liquid template evaporates, leaving behind an enclosed PDMS microchannel device.

In the development of this method, one major challenge was maintaining the stability of the liquid template in the prepolymer solution. Due to surface tension, printed liquid templates tend to break up and relax into discontinuous spherical droplets.  Although this instability can be inhibited by increasing the liquid viscosity, high viscosities are not compatible with inkjet printing. To address this challenge, the researchers used an ink whose viscosity is tunable by temperature. At room temperature, the ink has relatively low viscosity (10 centipoise), which is compatible with inkjet printing. When the ink is printed on a cooled (3-4^oC) pool of prepolymer solution, the viscosity of the liquid increases dramatically (>40 centipoise), which enabled the formation of a stable liquid template with no breakages.

This inkjet-printed approach enabled the formation of remarkably versatile PDMS microchannels. The diameters of the fabricated channels can range from 100 to 900 µm just by changing the template design. In addition, the interior surface of the channel can be modified by including vinyl-terminated functional molecules in the ink composition, which covalently incorporates in the PDMS matrix during thermal curing. Applying this technique with vinyl-terminated poly(ethylene glycol)methacrylate molecules, the researchers effortlessly imbue their devices with protein fouling resistance.

In summary, the lab on a chip community is beginning to leverage the benefits of inkjet printing in the fabrication of microfluidic components. By engineering the interaction of the ink droplets with the substrate material, researchers are devising innovative ways to fabricate robust electrodes and versatile microchannels without the need for cleanroom facilities and complicated procedures.

1.    I. M. Hutchings and G. D. Martin, Inkjet technology for digital fabrication, John Wiley & Sons, 2012.
2.    P. Tseng, C. Murray, D. Kim and D. Di Carlo, Lab on a Chip, 2014, 14, 1491-1495.
3.    E. Berthier, E. W. K. Young and D. Beebe, Lab on a Chip, 2012, 12, 1224-1237.
4.    J. Wu, R. Wang, H. Yu, G. Li, K. Xu, N. C. Tien, R. C. Roberts and D. Li, Lab on a Chip, 2015, 15, 690-695.
5.    Y. Guo, L. Li, F. Li, H. Zhou and Y. Song, Lab on a Chip, 2015, 15, 1759-1764.

These HOT articles published in February 2015 were recommended by our referees and are free* to access for 4 weeks

On-chip surface acoustic wave lysis and ion-exchange nanomembrane detection of exosomal RNA for pancreatic cancer study and diagnosis
Daniel Taller, Katherine Richards, Zdenek Slouka, Satyajyoti Senapati, Reginald Hill, David B. Go and Hsueh-Chia Chang
Lab Chip, 2015,15, 1656-1666
DOI: 10.1039/C5LC00036J, Paper

Three-dimensional heterogeneous assembly of coded microgels using an untethered mobile microgripper
Su Eun Chung, Xiaoguang Dong and Metin Sitti
Lab Chip, 2015,15, 1667-1676
DOI: 10.1039/C5LC00009B, Paper

A flexible lab-on-a-chip for the synthesis and magnetic separation of magnetite decorated with gold nanoparticles
Flávio C. Cabrera, Antonio F. A. A. Melo, João C. P. de Souza, Aldo E. Job and Frank N. Crespilho
Lab Chip, 2015, Advance Article
DOI: 10.1039/C4LC01483A, Paper

Take a look at our Lab on a Chip 2015 HOT Articles Collection!

*Access is free until 30.04.15 through a publishing personal account. It’s quick, easy and free to register!