Gecko Gaskets for Self-Sealing and High Strength Reversible Bonding of Microfluidics †

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Introduction
Elastomers have been used for microfluidics as they offer easier processing parameters and decent range of properties to choose from. With more and more complex microfluidic systems being developed for either larger number of processing steps or multiplexing, valves will remain an integral part of the microfluidics, and elastomers are the only class of polymers which can support the well-established valves 1,2 . Though, PDMS continues to be a standard material for elastomeric microfluidic devices in an academic setting, its weaknesses as a material for industrial application and manufacturing scalability have been well known for a while [3][4][5] .
Partially as a response to these limitations of PDMS, alternative materials such as SEBS thermoplastic elastomer are more frequently being used in microfluidics as they combine similar mechanical properties with thermoplastic processing techniques [6][7][8] .
Apart from bonding to similar materials, with microfluidics increasingly being integrated with microelectromechanical systems (MEMS) and microelectronics, 9 bonding of microfluidic devices to dissimilar surfaces is becoming increasingly important. The bonding processes for microfluidics are often one of the most complex and low yield steps of the manufacturing process in an academic and commercial setting. While reversible bonding is quite weak, Irreversible bonding via plasma actuation, thermal, solvent, or adhesive techniques 10 can be quite strong, but often is a complex process and needs customizing for different substrates and may even be incompatible in presence of biological reagents 11, 12 . MEMS and microelectronic components are the major cost consuming entities on a modern lab on a chip system. While sensor regeneration is viable 13,14 , sterilization of elastomeric microfluidic channels remains a challenge owing to absorption of reagents and the resultant swelling in presence of sterilizing solvents, thereby preventing reuse. Disposable microfluidics with reusable electrode systems involving reversible bonding have been proposed 15,16 , is one step towards reducing the cost.
Reversible bonding is an alternative which provides for a stick-and-play concept of multi-device integration while allowing more functionality to the devices and lowering cost 17 . A typical reversible bond involves a simple contacting of two cleaned surfaces and self-sealing by the adhesive property of at least one elastomeric surface (to ensure adequate molecular contact and sealing strength via van der Waals adhesion). This kind of reversible bonding is typically very weak and is suitable for low pressure flows (<5psi) 18 . Reversible bonding of microfluidics is generally limited to laboratory use for surface patterning of immunoglobins 19 and cell culturing but in some cases as above, reversible bonding becomes inevitable for the final device. Suction aspiration, magnets, 20 adhesive tapes 21 etc., based techniques have been used for reversible bonding. Adhesive tapes are fairly reliable but the lack of compatible polymers, and possible contamination due to the adhesives, prevent its wide acceptance. In contrast to these previous techniques, we show that strong reversible bonding can be achieved by employing special contact geometries adapted from the field of bio-inspired dry adhesives 22 . Using this concept, we have tried to explore some of the commonly used elastomers for microfluidics. Focusing on thermoplastic elastomers, we also demonstrate that the fabrication process can be completed extremely quickly with modified soft lithography techniques, which is both scalable for large scale manufacturing and achievable with minimal laboratory equipment.
Geckos adhere to different surfaces by van der Waals forces. These forces are extremely weak, but when acting over larger areas, these forces are large enough to exhibit more than 1.4MPa of normal adhesion 23 . Synthetic versions of these gecko-inspired adhesives have been fabricated by multiple research groups for various applications, including high normal strength designs for climbing robots 24 and pick and place tools 25 . Considering its application to microfluidic devices, a non-directional adhesive with maximum normal adhesion is desired. The fiber geometry and the material properties govern the adhesion strength of the fiber 26 .
Carbone G. et al. have theoretically demonstrated that a flat punch contact tip type of geometry (as is used in virtually all reported microfluidic channel designs) is prone to stress concentrations at the contact edge, and crack propagates from outside to inside 26 . This can be avoided by changing the tip geometry to accommodate an overhang cap as has been demonstrated by Spuskanyuk et al. 27 . Reversible adhesion strengths approaching 1 MPa in the axial loading direction has been reported under this condition. A 30 fold increase in adhesion strength compared to flat punch controls has been reported in experimental results by other researchers 28 . Double sided dry adhesives can be manufactured by various means 29,30 to improve functionality and use in bonding applications.
The design of the microfluidic channels involves creating a gasket to contain the fluidics, surrounded by the dry adhesive fibers. The gasket is a sweep of the adhesive fiber geometry defining the path of the desired channel. Aside from its primary function as the sidewalls of the microfluidic channel it also doubles up as a part of adhesive infrastructure. The surrounding fibers enhance the net adhesion and also help to make the whole geometry tolerant to defects, and surface variation. If the gasket and fibers didn't have the mushroom shaped geometry, the adhesive properties would be worse than a standard reversible design.

Fabrication
The mold fabrication process is a two part process creating a master mold in the first process and a daughter mold in the next, which is used for creating the required devices. An overall theme of the fabrication process was to provide a simple, quick and relatively low cost manufacturing alternative. In order to achieve integrated overhanging cap features, we exploit the use of PMMA as a poor selectivity photoresist for deep UV lithography 31 . The fabrication process and design guidelines are discussed in detail in ESI †. A schematic of the fabrication process is shown in Fig. 1(a).
While curable polymers are certainly compatible with the manufacturing technique 32 , the use of thermoplastic elastomers for this method shows much more promise from an industrial

Burst Pressure Test Setup
In order to study the feasibility of using the gecko-inspired adhesives integrated with microfluidic devices for pressure driven flows, a blister burst pressure test was conducted. The adhesion strength of fibers is a function of the height of the fibers, the cap diameter, cap thickness, cap overhang, the surface roughness of the fibers and the effective contact area (fill factor). For larger reservoirs, the pressure in the reservoir is acting over the size of the structure and needs to be contained by the gasket structure and surrounding areas of fibers. For the adhesive integrated microfluidics tested here, the dimensions of the fibers are, cap thickness:3.2µm, cap diameter and gasket thickness: 100µm, height: 72µm. Normal adhesion strength of SEBS fibers on polystyrene has been measured to be up to 1.4MPa for single fibers loaded normally 23 but in the case of microfluidics, the increased size of the gasket means that there would be a higher likelihood of defects or flaws that will trigger adhesion failure at lower pressures.

Please do not adjust margins
Please do not adjust margins The blister (Fig. 2(c)) is mounted on a desired substrate. The pressure supply is routed to the blister via a pressure sensor (Measurement Specialities-M5100) as demonstrated in Fig.2(a) The pressure sensor is connected to a power supply (National Instruments (NI)™, PXI 4110) and the output connected to a digital multimeter (NI™, PXI 4070). The maximum pressure beyond which leakages were detected was identified as the maximum burst pressure. The pressure reading is compa8red to a calibrated pressure-voltage curve against a sealed substrate, and a difference between the values at a particular pressure supply identifies the onset of leakage. As a secondary visual check, a mixture of soap and water is inserted around the perimeter of supporting fibers which produces easily observed bubbles during a test if a leak has occurred.
When subjected to pressures as low as 10psi, the blisters made of Kraton start to visibly inflate and fail at ~20psi against PMMA and ~40psi against G1657. This backing layer was on the order of 100 µm thick in this instance, and the large displacements before failure demonstrate the possible use of the concept for microfluidic valves in future implementations. (See Fig.S2  †) A polystyrene rigid backing was used in order to avoid this issue in other cases where large inflation is undesirable, and it also helps transfer the load to fibers much further from the reservoir, permitting still higher contained pressures. The burst pressure tests were performed against many commonly used materials for microfluidic devices, PS, PMMA, glass (microscope slides), silicon and also Kraton G1657 with PS backing. A 5 minute annealing treatment of the assembly at 85°C (<T g of any of the mating materials) as also tested for some designs to test the effect on adhesion strength.

Results
The fabricated microfluidic devices can be instantly bonded to MEMS systems or flat surfaces to yield a functional device. The devices do not require any clamps and are leak proof as long as they are bonded to clean, relatively smooth surfaces. A large scale fabrication would involve using the silicone mold for all the devices in a variety of polymers and each fabrication cycle is complete in less than 90 seconds which includes the fabrication setup. The silicone molds demonstrate excellent durability even after 100s of molding cycles.
The feasibility of integration with various substrates and channel geometries is illustrated as in Fig 3. The adhesive integrated microfluidic devices demonstrate a wide range of adhesion strengths depending on the substrate (Fig.4). The maximum pressures of 95psi that can be sustained do not imply irreversibility, but is merely a limitation of the pressure source. The devices can be peeled off by applying coupled normalshear forces. Reversible bonded PDMS has a pressure endurance limit of up to 5psi and irreversibly sustains a maximum of 74 ± 2psi. 34 .

World to chip interface
With the thin microfluidic devices, the world to chip interface becomes quite challenging. A fair number of engineered solutions have been proposed involving sockets 35 , magnetic luers 36 , apart from the standard adhesive or plasma bonded adapters, press or screw fit fasteners, and luer locks for rigid chips. Though fairly reliable in their own way, a simple extension of the dry adhesive concept to the adapters can be provide for stick and play interface to microfluidic chips. Providing a dry adhesive integrated O-ring on one side of the chip or on the adapter can ensure a reliable world to chip interface ( Fig.5(a,b)). The adapters demonstrate good reliability at low aspect ratios and can be detached by bending them at a small angle. The rigid backing layer can also be modified to include ports to be used with elastomeric tubing (Fig.5(c)) or thicker elastomer samples can be integrated with New England pins for tubing connections (Fig.5(d)).

Conclusions
We present a novel reversible bonding technique for bonding of microfluidic devices, yet can sustain reasonably high pressures (up to ~100 psi). Depending on the substrate, the concept provides for variable adhesion strength which is a function of material properties and the geometries of the adhesive infrastructure. We also demonstrated a rapid mass manufacturing technique for fabrication of adhesive integrated microfluidic devices using thermoplastic elastomers with a compression molding process. Compared to the standard PDMS reversible bonding, the reversible adhesion using this technique has been demonstrated to be over 10 times stronger, and is almost on par with plasma treated PDMS-glass bonding, with short thermal anneals and a rigid backing layer. This is the first time to our knowledge that the high normal adhesive strength architecture of mushroom shaped adhesives has been applied to the containment of fluids with high reversible bonding strength.
With the added flexibility of using stick and play interconnects, the concept can provide for low cost manufacturing of microfluidic devices and be a great alternative in resource limited applications.