Bonding Surface - Xi Hao
Among studies using microfluidic devices, a common challenge is fabrication. Since the microstructures inside the microfluidic devices are at small scales (<1 mm), small obstructions or punctures can drastically alter experimental outcomes. Thus, meticulous optimization of microfabrication steps is vital. These steps usually involve creating layers of the microfluidic chips, and the assembly of those layers.
Bonding surfaces is a process that adheres two surfaces to assemble them into a complete unit. It is a critical fabrication step for microfluidic device assembly as it provides sealing. Creation of microfluidic devices involves a multilayer fabrication process as shown in Figure 1, and since the devices are designed to contain fluids, incomplete or low quality bonding may allow fluid leakage and directly affect the functionality of the devices. According to properties of the surfaces and the requirements for the devices, there are multiple methods to bond surfaces, and a some of the most common methods for bonding surfaces are UV/Ozone treatment, thermal bonding, adhesives and crosslinking.
Methods to Bond Surfaces
Ultraviolet (UV)–Ozone Treatment
UV-Ozone treatment is also known as UV-Ozone Activating (UVO), which is a surface activating process of sample or substrate used in material and device fabrication. In this process, photochemical reaction happens on the surfaces to depolymerize and remove impurities from substrates to promote better interfacial adhesion. Sometimes, it is not necessary to use UV, which in laboratories, we can create a plasma using radio frequency (RF) and achieve the same result as UV-Ozone.
UV light has two types of wavelengths (185nm and 254 nm). Each wavelength plays a different role in UV-Ozone treatment. 185-nm UV light dissociates molecular oxygen into triplet atomic oxygen, then the triplet atomic oxygen combines with molecular oxygen and generates ozone. In addition, the 254-nm UV light will dissociate ozone to molecular oxygen and singlet atomic oxygen. Singlet atomic oxygen has strong oxidation power, and it reacts with substrate surfaces. In this reaction, the surfaces were oxidizing on inorganic substrates such as silicon wafers. In the case of organic materials, chain scission of molecules happens, and organic residue contaminants are gently removed from the substrates as volatile byproduct molecules such as Carbon dioxide, water, and oxygen.
Poly(dimethylsiloxane) (PDMS) and poly(vinylmethylsiloxane) (PVMS) are two types of commonly used polysiloxanes. Both PDMS and PVMS undergo dramatic changes in their properties when exposed to UVO. The surface chemical composition of both PDMS and PVMS at long UVO treatment times changes substantially and features a high density of hydrophilic groups.
These oxidized polysiloxanes will then be able to attach to inorganic surfaces such as silicon wafers of glass slides to form conformal Si-O-Si bonds.
Thermal bonding is a bonding between materials that their physical properties changes with increasing temperatures. These materials are usually called thermoplastics. Thermoplastics are materials that are reversibly moldable above a certain temperature, and solidify upon cooling. Thermal bonding occurs when thermal plastics are heated to and beyond their Glass transition temperature, and pressures are applied.. 
The temperature where the materials start becoming moldable is known as the glass transition temperature. Glass transition is the reversible transition in amorphous materials from a hard and relative brittle state into a soften or viscous state when increasing the temperature.. 
When the thermoplastic materials are heated above its glass transition temperature, the mobility of the polymer chains in the thermoplastic materials start increasing and when the polymer chains are free to move, they can bridge the gap between the two layers and bond them together.. 
Thermal bonding requires extremely clean and flat surfaces and is typically performed in cleanrooms. In application, thermal bonding can be used to bond glass to glass or silicon to silicon. In thermal bonding, similar materials are bonded at elevated temperatures and pressure.. 
When the surfaces are photocrosslinking photoresists, they can bond to each other by cross-linking under present of light. Photocrosslinking photoresist is a type of photoresist, which could crosslink chain by chain when exposed to light, to generate an insoluble network. Photocrosslinking photoresist are usually used for negative photoresist. There are cross-linkers in the photocrosslinking photoresists that will be activated when exposed to light, the crosslinker ratios determine the rate of the bonding process. When there is abundance of light sensitive cross-linkers in the photoresists, the surfaces will bond to each other faster, and vice-versa, if there are less light sensitive cross-linkers in the photoresist the bonding process will be slower.
Adhesive is another example of cross-link bonding. An adhesive layer between surfaces can bond the surfaces onto each other. Many common adhesives are epoxy-based, which they can be either cured by exposure to UV light or by heating. The curing process results in bonding between the surfaces.. 
There are some other methods to create molecular-level cross-linking. For example, organofunctional alkoxysilane molecules can be used to modify the surface of a substrate that contain hydroxyl groups. These hydroxyl groups can attack and displace the alkoxy groups on the silane and form a covalent Si-O-Si bond, and can form bonds across the interface of the substrate, which can be used to bond another substrate like glass or PDMS. These alkoxysilane allows molecular-level cross-linking without a thick layer of glue.. 
 Maboudian, R, "Micro Devices: Stiction and Adhesion" Encyclopedia of Materials: Science and Technology, (2001), DOI: 10.1016/b0-08-043152-6
 S. Sui, Y. Wang, K.W. Kolewe, V. Srajer, R. Henning, J.D. Schiffman, C. Dimitrakopoulos, S.L. Perry, "Graphene-Based Microfluidics for Serial Crystallography" Lab on a Chip, (2016) 16, 3082-3096.DOI: 10.1039/C6LC00451B
 S.L. Perry, J.J.L. Higdon, P.J.A. Kenis, "Design rules for pumping and metering of highly viscous fluids in microfluidics" Lab on a Chip, (2010) 10, 3112-3124. DOI: 10.1039/c0lc00035c
 Vig, J. R, "UV/Ozone Cleaning of Surfaces" The Electrochemical Society, (1990) Vol. 90, DOI: 10.1116/1.573115.
 SAMCO Inc. The Basics of UV-Ozone Cleaning of Surfaces
 A. E. Özçam, K. Efimenko, J. Genzer, "Effect of ultraviolet/ozone treatment on the surface and bulk properties of poly(dimethyl siloxane) and poly(vinylmethyl siloxane) networks" Polymer, (2014) 55, 3107-3119.
 Henniker Plasma. Plasma Treatment of PDMS for Microfluidics
 Giri, K.; Tsao, C. W. "Recent Advances in Thermoplastic Microfluidic Bonding", Micromachines, (2022), DOI: 10.3390/mi13030486
 Perry, S. ChE590E Microfluidics and Analysis. Lecture 2A: Microfabrication Processes. 2018. University of Massachusetts, Department of Chemical Engineering.
 Y. Hou, Z. Wang, C. Cai, X. Hao, D, Li, N. Zhao, Y. Zhao, L. Chen, H. Ma, J. Xu, "Conformal Nanocoatings with Uniform and COntrollable Thickness on Microstructured Surfaces: A General Assembly Route" Adv. Mater. 2018, 1704131. DOI: 10.1002/adma.201704131