Opto-Smart Systems in Microfluidics

Introduction

The intrinsic features of microfluidic devices ensure two main characteristics during an analytical process: low consumption of reagents and sample as well as rapid and repeatable analysis protocols (Martinez, Phillips, Whitesides, & Carrilho, 2010). However, to date, extensive chemical and biological tasks need to be carried out outside the microfluidic device to prepare and pre-process samples prior microfluidic manipulation as conventional lab-on-a-chip devices are not yet suitable for non-ideal sample analysis (Gonzalez Crevillen, Hervas, Angel Lopez, Cristina Gonzalez, & Escarpa, 2007). These tasks include sampling, pre-concentration, fluorescence labelling, filtration, mixing, sample analysis, as well as many other techniques that require manpower and are very time-consuming (Argentiere, Gigli, Mortato, Gerges, & Blasi, 2012).

Therefore, the main aim is to realise downscaling of these conventional tasks within the microfluidic devices and consequently develop innovative systems capable of preparing and/or analysis of samples “on-chip”. If these functions could be integrated within the microfluidic device, then time and costs would be reduced and high throughput and high degree of automation would be realised (R. Byrne, Benito-Lopez, & Diamond, 2010). For this purpose two main ways of investigation are considered: an “evolutionary” and a “revolutionary” approach.

The “evolutionary” approach involves tremendous downscale of conventional units, and their assembly level by level to achieve the desired functionalities (Abgrall & Gue, 2007; Laser & Santiago, 2004; Psaltis, Quake, & Yang, 2006). However, microfluidic systems developed to date according to this approach, generally require complicated control systems and are by far from being trivial and widely accessible.

The “revolutionary” approach in the area of microfluidics is recently emerging and consists in the incorporation of stimuli-responsive materials into microfluidic devices, to obtain smart, functional, highly controllable components integrated in the microfluidic device (F. Benito-Lopez et al., 2010; M. Czugala, Ziolkowski, Byrne, Diamond, & Benito-Lopez, 2011; Florea, Diamond, & Benito-Lopez; Florea, Hennart, Diamond, & Benito-Lopez, 2012; Ziółkowski, Czugala, & Diamond, 2012). For instance, the use of a light stimulus in microfluidic systems is appealing because it can be applied precisely in different sections within a microdevice, in a non-invasive manner.

The possibility of controlling flow in microfluidics using opto-stimulus will offer new platforms with unprecedented flexibility and improved versatility. Some recent reviews have already been devoted to specific aspects of this field (R. Byrne et al., 2010; M. Chen et al., 2011; M. Czugala et al., 2011; Padgett & Di Leonardo, 2011). The use of holographic optical tweezers for optical manipulation, actuation and sensing in lab-on-chip systems was described by Padgett and Di Leonardo (Padgett & Di Leonardo, 2011). Other recent review by Baigl (Baigl, 2012) describes in detail strategies to photo-actuate microfluidic systems, such as the application of the chromocapillary effect, light-induced Marangoni effects, optically induced dielectrophoresis, digital optofluidics, light-induced electro-osmosis and optoelectrowetting, among others.

Here we review the possibility of controlling flow in closed microchannels, and manipulating discrete microliter-sized droplets by employing photo- and thermo-responsive materials incorporated in microfluidic units in the form of polymeric actuators, photo-responsive coatings or photo-sensitive surfactants.