Mass Production of Microfluidic Devices from SEBS

Hard thermoplastic polymers versus thermoplastic elastomers

If elasticity of material is not needed by functional requirements, it is in general advantageous to mass-produce microfluidic devices for commercial applications from hard thermoplastic materials. This is because individual hard layers are easier to handle and align than highly flexible layers, especiallyif they are thin films.  In addition, absorption of small hydrophobic molecules is much less of an issue in hard thermoplastic polymers than thermoplastic elastomers (TPEs). If, however, it is necessary to manufacturer parts of the microfluidic devices (e. g., septa, valve seats or membranes) or entire devices from elastomers, then polymers with suitable set of elastic and other properties for the targeted application need to be selected.

SEBS thermoplastic elastomers

Styrenic block copolymers based on styrene-ethylene-butylene-styrene (SEBS) are synthetic TPEs which have been developed as a phthalate-free alternative to polyvinyl chloride (PVC) for applications in many fields, including medicine. They contain polystyrene hard domains in a continuous elastomeric ethylene–butylene phase, with the hard domains forming the physical crosslinks and providing the strength to the copolymers, while the elastomer phase gives the network its flexibility and elasticity. The individual polymers retain most of their characteristics, including their respective glass transition temperature (Tg) so that when the hard phase is melted, the material can flow and be processed by injection molding, extrusion, and other methods. Upon cooling, the hard phase solidifies and the material regains its strength and elasticity.

A useful advantage of SEBS thermoplastic elastomers is that they can be co-injection molded (AKA 2K or two-shot injection molding) with hard themoplastic polymers such as polystyrene. In part due to the presence of hard styrene domains in SEBS, co-injection molded hard polystyrene has a very strong adhesion to elastomeric SEBS. This allows, for example, for two-shot injection molding of polystyrene microfluidic devices with SEBS valve seats or septa. Using the design and manufacturing strategy yields structurally strong parts with soft elements. It can also greatly simplify device assembly and improve its performance.

Absorption of small hydrophobic molecules

To achieve desirable material properties and enhance manufacturability and, oil additives are frequently added by resin manufacturers into many SEBS polymers, including medical grade formulations such as Mediprene 500422 M. Not supprisingly, these oil-extended SEBS polymers tend to strongly absorb small hydrophobic molecules. If reducing absorption of small hydrophobic molecules is desirable (e.g., in microfluidic devices for drug development applications), then it is recommended to fabricate microfluidic devices from oil-free SEBS formulations such as Kraton G1645. [pdf]

Optical properties of SEBS TPEs

Optical transparency is important in microfluidic devices for life science applications where it might necessary to provide an optical readout, such as optically interrogate living cells cultured inside the devices.  Optically clear SEBS such as Kraton G1643 or G1645 begin transmitting light at wavelength around 280 nm, a threshold that is sufficient for most fluorescent assays.

Mechanical properties of SEBS

Styrenic block copolymers exhibit anisotropic mechanical properties because they are composed of incompatible block segments that display microdomain structure in the solid state due to microphase separation of the constituent blocks. Microdomain orientation induced by mechanical flow fields occurs in common processing methods such as extrusion, roll milling, planar extension or injection molding, and global microdomain orientation can be preserved and further enhanced by the subsequent annealing. Importantly, microdomain orientation can influence the Young’s modulus, which has been reported to be higher along the polymer flow direction. 

Thus, anisotropic properties of SEBS extruded or injection molded parts can be significant. They should be taken into account when designing the mold. With judicious planning, they can be exploited to enhance performace of finished devices.

SEBS materials as cell culture substrates

Polystyrene is the most commonly used substrate for standard in vitro cell culture. To make it cell adhesive, its surface is usually oxidized using corona discharge or plasma. The same approach is effective for treating SEBS polymers.

In contrast to PDMS, additional modification of the SEBS surface with extracellular matrix proteins, such as fibronectin, is not necessary after oxygen plasma treatment for effective cell attachment in serum-containing medium. Prior to cell seeding, many SEBS polymers can be activated and sterilized for example by oxygen plasma, UV ozone, and ethylene oxide. [pdf]

Gas permeability and water vapor transmission of SEBS TPEs

One of the frequently quoted advantages of PDMS for fabricating microfluidic cell culture devices is high permeability to gases such as oxygen and carbon dioxide. High oxygen and carbon dioxide permeability through the walls of PDMS devices allows for oxygenation and pH control of cell culture medium in the device. However, for oxygenation in the device to be effective, the thickness of the device walls must be rather low.

Kraton G1643 and G1645 thermoplastic elastomers have ~80 times lower oxygen and carbon dioxide permeability than PDMS. However, this difference in oxygen permeability between these polymers may be partially reduced because plasma treatment frequently used for PDMS bonding can reduce permeation of oxygen through the PDMS walls by as muchas 40–80% whereas SEBS polymers do not require plasma activation for bonding and cell adhesion. In addition, the gas permeability of SEBS is ~2 orders of magnitude higher than other thermoplastics typically used for cell culture applications, such as polystyrene. Therefore, while Kraton G1643 and G1645 are less gas permeable than PDMS, they are significantly more permeable than many rigid thermoplastic alternatives.

While high permeation of oxygen and carbon dioxide in microfluidic PDMS cell culture devices is usually considered useful, high transmission rate of water vapor is typically associated with undesirable effects, such as evaporation-mediated osmolarity shifts, even in humidified cell culture incubators. Water vapor transmission rates of G1643 and G1645 are more than 200 times lower than PDMS. Thus, in contrast to PDMS microfluidic devices that suffer from loss of water and osmolarity changes due to high permeation of water vapor through the PDMS walls, this effect should be significantly less pronounced in SEBS devices.

Fabrication of SEBS microfluidic devices by injection molding and extrusion

SEBS polymers offer a way to address many of the problems that limit the utility of PDMS devices for commercial applications in life science area and in drug development in particular. They can be used to extrude or injection mold entire microfluidic devices, or they may be co-extruded or co-injected with rigid thermoplastic polymers (e.g., polystyrene) for mass production of rigid microfluidic devices containing elastomeric components (e.g., septa, valve seats or porous SEBS TPE membranes). Besides the attractive material properties and ability to be processed many time faster by mass production techniques of injection molding (~35 second cycle time) and extrusion, SEBS elastomers are also ~9 times less expensive than PDMS (~ $11 kg−1 at 23-kg bag of Kraton G1643 resin compared to ~ $95 kg−1 at 20-kg kit of Sylgard 184). [pdf]