Optoelectronic Heating for Fabricating Microfluidic Circuitry

This work reports on optoelectronic-based heaters that can transduce low-power optical images into high-power heating to melt frozen liquids and form desired microfluidic circuitry. The mechanism of optoelectronic heating (OEH) was studied and characterized. OEH relies on photocurrent heating in the illuminated parts of actuating images. Resolution was affected by dark current heating. Photocurrents and dark currents were measured and related to the operating parameters. Successful melting of a frozen media within seconds with 2？mW light patterns and a 4？V operating voltage was demonstrated with feature sizes down to 200？μm × 200？μm. Strategies to increase resolution were addressed. It was shown that the size and location of heating areas can be reliably and rapidly reconfigured by changing the actuating image. 1. Introduction Microelectromechanical system (MEMS) techniques have enabled thermal control at the microscale and thereby opened up the way to new applications. Microheating has been applied to various chemical and biological processes [1, 2], where accurate control of temperature is often essential, with a particular focus on polymerase chain reaction [3]. It has also been popular for microfluidic flow control; thermal pumps [4] and valves [5] offer elegant solutions for device integration. They are based on thermal expansion or phase transition of specific materials. Other thermal phenomena include the control of viscosity for droplet generation [6], temperature gradient focusing of particles [7], and thermocapillary pumping [8]. Local microheating is usually achieved with thin-film resistive heaters that are deposited and patterned using MEMS fabrication techniques. Optical heating is a convenient alternative. Existing optical heating often requires a high-power light source and has been successfully demonstrated in chemical and biological applications [9, 10]. Optothermal valves and pumps have also been implemented in microfluidic platforms [11, 12]. They are typically based on thermal expansion of bubbles or nanocomposite materials in prefabricated areas. Biocompatible thermoreversible gelation polymers offer more flexibility [13]. Those polymers solidify when heated and are biocompatible. They can be mixed with biochemical samples and be used as ubiquitous valves activated upon irradiation [14]. Nonetheless, all those devices are reliant on the optical absorption of liquids, which varies with chemical composition. Photoabsorbing substrates are a convenient alternative. They have been applied to optothermal valves [15] and droplet