Ultrasonic vibration can also be utilized as the heating source, which significantly expedites the bonding process . This process offers high bonding strength in a very short time, making it a good strategy for rapid prototyping. By controlling the pressure, power, and time, the bonding process can be optimized. Ultrasonic welding enables us to locally bond specific regions of the microfluidic layers. Thus, unlike conventional thermal bonding where the temperature of the entire chip is kept above the glass transition temperature, which can denature the preloaded biomolecules or possibly deform the channels, here, the generated heat energy is focused on energy directors (ED) at the specific regions that need to be bonded. The energy directors are essentially convex structures around the channels which are melted during ultrasonic vibration and stick the layers together. The melting flow, however, could leave a small gap between the bonding layers or could flow into the channels and clog the microchannels. In order to tackle this problem, Liang et al.'s incorporated a CO2 laser ablation system in the ultrasonic welding process 113 . Using the CO2 laser cutter, they formed grooves and laser bulges as the energy director. Therefore, during the welding process, the melted laser bulges could flow into the grooves and consequently prevent the gap and clogging issues (Figure ). Viehrig et al. also used laser ablation to engrave grooves on an aluminum-based master mold created by micro-milling technique . The grooves were 50 µm in both depth and width and served as energy directors in ultrasonic welding. Ultrasonic welding can also be employed for bonding microfluidic connectors and luers to the device 114 .