Materials with three-dimensional (3D) curved structures that contain embedded microfluidic channels are of great interest, providing mimics of natural structures such as leaves and tissues. These are permeated with vasculature systems to enable the efficient local transport of nutrients and waste products. Although self-assembly to 3D geometries can be obtained by introducing structural and mechanical heterogeneity in a material, integrating this with a microfluidic network to enable precise delivery of fluid in curved locations, or to create 3D vasculature, creates significant challenges. Recently, D.H. Gracias and colleagues from John Hopkins University have created such a self-assembled microfluidic network by integrating poly(dimethylsiloxane) (PDMS) channels in differentially photo-cross-linked SU-8 (a negative photoresist) thin films which spontaneously and reversibly curve on desolvation and resolvation.
As reported in the November 8, 2011 issue of Nature Communications (DOI: 10.1038/ncomms1531), the researchers first created heterogeneous SU-8 films with crosslink gradients (CLGs) along their thickness by exposing the films to ultraviolet (UV) light. The films were then soaked in acetone to condition them and to generate stress gradients for self-assembly. The SU-8 films then spontaneously curved on desolvation by drying or the addition of water and then re-flattened when resolvated by organic solvents like acetone.
The radii of curvature strongly depend on UV exposure energy and the film thickness and a wide range of geometries could be obtained by simply varying the CLG using conventional photolithography.
The ability of these SU-8 templates to show reversible curvature could also be exploited to curve thicker polymeric films deposited on the surface of a prepared SU-8 film. Integration of microfluidic channels into the polymer film surface layer then enabled the construction of a curved microfluidic device. Fluid flow through the networks with single and dual channel devices established the functionality of the devices as a pathway for fluid transport to curved locations (see Figure). The SU-8/PDMS devices are bio-inert and remain curved in culture media. They can therefore be used in biological applications to transport biochemical nutrients or growth factors for tissue engineering. Incorporation of lithographically defined pores can also be used to locally release chemicals. The researchers also created a reconfigurable metamaterial suggesting that these self-assembling devices could prove useful as 3D electromagnetic devices.
(a) Microfluidic devices with poly(dimethylsiloxane) (PDMS) inlets/outlets attached to a Si substrate and with PDMS channels integrated with a differentially cross-linked SU-8 film; (b,c) a bright-field image SU-8/PDMS microfluidic device containing a single channel, (b) as patterned on a Si substrate, and (c) after self-assembly; (d) a bright-field image of a self-assembled microfluidic device with dual channels; and (e,f) fluorescence images showing the flow of (e) fluorescein (green), and (f) fluorescein (green)/rhodamine B (red) through single and dual channel devices, respectively. Scale bars are 500 μm (1 mm for b). Reproduced with permission from Nat. Commun. 2:527, DOI: 10.1038/ncomms1531. © 2011 Macmillan Publishers Ltd.
