# Microfabrication Bonding Process Optimization for a 3D Multi-Layer PDMS Suspended Microfluidics  
[PERSON]  
1 Optical Bio-Microsystems Lab, Micro Nano Bio Integration Center, Department of Mechanical and Industrial Engineering, Concordia University, Montreal, QC H3G 1M8, Canada; [EMAIL_ADDRESS] (M.M.); [EMAIL_ADDRESS] (E.Y.M.)  
[PERSON]  
1 Optical Bio-Microsystems Lab, Micro Nano Bio Integration Center, Department of Mechanical and Industrial Engineering, Concordia University, Montreal, QC H3G 1M8, Canada; [EMAIL_ADDRESS] (M.M.); [EMAIL_ADDRESS] (E.Y.M.)  
[PERSON]  
2 Robotic Assisted Minimally Invasive Surgery Lab, Department of Mechanical and Industrial Engineering, Concordia University, Montreal, QC H3G 1M8, Canada; [EMAIL_ADDRESS]  
[PERSON]  
1 Optical Bio-Microsystems Lab, Micro Nano Bio Integration Center, Department of Mechanical and Industrial Engineering, Concordia University, Montreal, QC H3G 1M8, Canada; [EMAIL_ADDRESS] (M.M.); [EMAIL_ADDRESS] (E.Y.M.)  
Footnote 1: [[https://www.mdpi.com/journal/applsci](https://www.mdpi.com/journal/applsci)]([https://www.mdpi.com/journal/applsci](https://www.mdpi.com/journal/applsci))  
Footnote 2: [[https://www.mdpi.com/journals/pdf/1](https://www.mdpi.com/journals/pdf/1)]([https://www.mdpi.com/journals/pdf/1](https://www.mdpi.com/journals/pdf/1))  
bonding techniques for polymer-based microchannels, namely adhesive [11] and plasma bonding [12].  
Due to several advantages of PDMS, such as biocompatibility, optical transparency, and excellent mechanical properties, PDMS has been widely utilized for microfluidic devices [13; 14]. Fabricating multi-layer microfluidic systems made with PDMS is still a challenge. Three-dimensional printing techniques as alternative approaches were proposed for one-step 3D fabrications with PDMS [15; 16; 17; 18; 19]. However, this method requires a substrate for printing microfluidic systems; thus, the fabrication process has to be optimized for printing PDMS-suspended microfluidics, such as microcantilevers.  
Moreover, there was a problem in soft lithography, specifically when electronic elements are required to be embedded in microchannels. In order to fabricate this kind of microfluidics, a thin layer of the polymer such as PDMS has been used as an adhesive [20], which needs an exact control of the PDMS layer's thickness for device performance or the channel may be blocked or deformed. Using uncured or partially cured PDMS as an adhesive has almost doubled the bonding strength compared to bonding strengths with the plasma technique [21].  
Another issue with multiple layer microfluidics is trapped air bubbles between substrates during bonding. However, since PDMS is a gas-permeable polymer, air bubbles can escape if at least one substrate is not of a hard material such as glass. The trapped air bubbles can degrade the bonding strength, which causes leakage and dysfunction during microfluidic testing. In order to avoid trapped air bubbles, placing a grid of empty channels to such the air bubbles out of microfluidics to free air was suggested by [20]. The grid of air channels is fabricated during the molding step, while microchannel mold is fabricated by using soft lithography.  
In this study, we have proposed and tested an optimum number of air-vent channels between PDMS layers to remove trapped air bubbles. This allows using inexpensive plasma bonding method, which provides the required bonding strength for our experiments. Moreover, the PDMS curing temperature has been optimized to possess some stickiness during the bonding step. Since the thickness of the thin layer is around 200 \(\upmu\)m in our fabrication procedure, using uncured or semi-cured PDMS may damage our thin substrates.