Nature Commun. 2, 527 (2011)

Many microfluidic systems have been developed, but they are typically made in block-like geometries, rather than curved ones that would resemble the vascularized three-dimensional systems found in nature. David Gracias and co-workers from Johns Hopkins University in Baltimore have now prepared microfluidic networks that reversibly curve into complex structures.

Credit: © 2011 NPG

The assembly relies on SU-8, a polymer that can be photo-patterned: UV irradiation, followed by a heating step, induces crosslinking. Non-irradiated regions remain free from crosslinks and are subsequently removed by an organic solvent. Through carefully controlled irradiation, the researchers created a gradient of crosslinking across the thickness of a planar SU-8 film. Once the solvent is removed, the less crosslinked side of the hydrophobic film undergoes a larger contraction than the highly crosslinked one, spontaneously bending the film. This process is reversible and reproducible, through successive de- and re-solvation with the original solvent. The curving can be precisely controlled by the UV exposure, film morphology and heating conditions, which enabled the researchers to assemble various differently curving segments into complex three-dimensional architectures.

These SU-8 films served as support layers for the organization of hollow channels of polydimethylsiloxane (PDMS) into three-dimensional bent geometries. Both materials are widely used in microfluidics, making the resulting SU-8/PDMS networks well suited to fluid transportation. The transport of a green and a red dye (fluorescein and Rhodamine B, respectively) through a dual-channel cylindrical device was clearly observed by fluorescence. Furthermore, the devices are biologically inert and remain curved in cell culture media, which means they are promising for bio-applications.