Another approach to bond thermoplastic polymers is laser welding. In this process, the substrates are held in contact and a laser beam is transferred through the whole platform to reach and melt the materials at the interface for bonding. Similar to the ultrasonic welding, this process also creates localized heating and can rapidly bond the layers together. However, the transparency of the substrates is an important matter in laser welding. The top layer should be sufficiently thin and transparent to the laser wavelength so that the laser does not lose the required energy for melting the interface while it is passing through the top layer. On the other hand, the bottom layer should be absorbent to the laser beam so as to be able to heat the interface by absorbing the laser energy.