LED Based Opto-wetting Platforms for Micro Mixing

The digital microfluidics utilizes the gradient in wettability for droplet transportation. This paper reports a novel technique of LED induced wetting (Opto-wetting) where a single low power LED (Light emitting diode) emitting light of wavelength 395nm peak with luminous intensity of 300mcd (milli candela) is used to move two droplets of 20 mu L each on a photo responsive substrate of azobenzene (C12H10N2) coated PDMS (Poly Dimethyl Siloxane) substrate. The PDMS substrate of size 5cmx2cmx3.5cm were modified by thin film coating of Azobenzene dissolved in olive oil solution. The LED was aligned vertically on the top of the substrate. The pulsed operation of LED induces a spatial gradient of surface energy due to the reversible process of photo isomerization of Azobenzene molecules coated on the surface. The Photo-isomerization changes the conformation of the molecules thereby changing its surface energy.The change in surface energy of the substrate induces a change in contact angle of the droplet which initiates its movement. Two water droplets each of 20 mu L were dispensed on the substrate using micro syringe. Two droplets which were initially at a few cm distance apart were gradually moved towards the focus of light and merged together. The urine droplets (20 mu L) were moved and mixed with the red dye reagent (20 mu L) by controlling the light intensity of the LED. This concept provides a cost effective technique of droplet manipulation in the Lab on a chip domain where various multiplexed operations on proteins & DNA droplets can be done for point of care diagnostics.