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Posted: Nov 14th, 2011
Curved microfluidic devices that self-assemble
(Nanowerk Spotlight) Conventional microfluidic devices are fabricated in inherently planar, block-like devices. In contrast, an important feature of naturally self-assembled systems such as leaves and tissues is that they are curved and have embedded fluidic channels that enable the transport of nutrients to, or removal of waste from, specific three-dimensional regions.
Since most microfluidic devices are created using layer-by-layer lithographic patterning and molding methods, it is challenging to create microfluidic networks in curved or folded geometries. However, such networks are important to pattern chemicals in 3D and also to create realistic models of tissues.
Examples of self-assembling microfluidic devices. Left: Fluorescence images showing the flow of fluorescein (green)/rhodamine B (red) through single and dual channel devices, respectively. Right: A hybrid SU-8/PDMS microfluidic device was self-assembled using a CLG-containing SU-8 layer to curve an underlying PDMS channel. (Reprinted with permission from Nature Publishing Group)
"From an fundamental perspective, we show how stress gradients can be created in thin polymer films using photo-crosslinking and conditioning to allow them to reversibly curve/fold and flatten when wet or dry," Gracias explains to Nanowerk. "This reversibility can also be achieved on immersion in water and organic solvents."
The idea of differential crosslinking to achieve curvature or folding is new; previously, researchers have shown that bilayers with two different polymers can be utilized, but the JHU team's methodology uses a single material and provides considerable flexibility in the type and extent of curvature that can be created by varying the intensity and direction of exposure to ultraviolet (UV) light.
Although stresses in polymeric films are often undesirable, Gracias and his team developed a strategy to create a photopatternable stress gradient in these materials so that the films reversibly and reproducibly curved on solvent exchange between water and acetone.
"We controlled the extent and directionality of curvature by varying the ultraviolet exposure energy and direction" says Mustapha Jamal, a graduate student in Gracias' group and first author of the paper. "We could curve rectangular SU-8 structures with radii of curvature as small as 80 µm and with bidirectional curvature. We also developed a multilayer patterning scheme to integrate PDMS-based microfluidic networks with these SU-8 films and realize the self-assembly of curved microfluidic networks."
The team found that the radii of curvature of conditioned SU-8 films were dependent on several controllable processing parameters: mostly ultraviolet exposure energy and film thickness, and to a lesser degree film aspect ratio and post-exposure bake temperature. By controlling these parameters, they were able to fabricate desired curved patterns.
In order to self-assemble microfluidic devices, the researchers used the SU-8 templates as a support layer to curve thicker polymeric films like PDMS. Gracias notes that the overall thickness of the SU-8/PDMS devices that the team fabricated was less than 40 µm, and they were built using planar microfabrication techniques. "We could therefore pattern multiple devices in parallel and with high resolution."
An illustration of a self-assembling microfluidic device with PDMS inlets/outlets attached to a Si substrate and with PDMS channels integrated with a differentially crosslinked SU-8 film. (Reprinted with permission from Nature Publishing Group)
This novel approach addresses a host of problems such as the challenge of creating 3D reconfigurable metamaterials, electronic circuits, polymeric actuators and tissue scaffolds.
"Our methodology can be utilized to create reconfigurable curved and flexible metamaterials which change their optical properties in response to different stimuli," explains Gracias. "Since our approach is compatible with planar lithography methods, we have incorporated optical elements such as split ring resonators which have unique optical resonances. Alternatively, other optical modules or electronic circuits could also be incorporated."
Since many hydrogels can be photopolymerized, this methodology of differential cross-linking can be used to create stress gradients in these materials. Gracias' team are now planning to create biodegradeable vascularized tissue scaffolds using this approach.