Growing “Nanofruit” Textures on Photo-Crosslinked SU-8 Surfaces through Layer-by-Layer Grafting of Hyperbranched Poly(Ethyleneimine)

We report the growth of hierarchical nanostructures on a photo-crosslinked SU-8 surface through alternating layer-by-layer Michael addition reactions between poly(ethyleneimine) (PEI) and dipentaerythritol pentaacrylate (SR399) to generate grafted hyperbranched poly(ester amines) (HPEA). Initial surface treatment with acryloyl chloride (AC) in toluene partially swelled the polymer network, resulting in the formation of “nano-cranberry” domes (∼100 nm in diameter), whose size and density could be varied by the choice of solvent, concentration, and reaction time of acryloyl chloride. The subsequent layer-by-layer grafting of hyperbranched PEI led to the formation of surfaces evocative of “nano-strawberries”, “nano-raspberries” and “nano-pineapples”, each of which consisted of ∼16−21 nm “nanoseeds” grown on top of and between the nanodomes. The complexity and coverage of the seeds can be manipulated by the generation and molecular weight of PEI grafts. In contrast, when the cross-linked SU-8 surface was initially treated with (aminopropyl)triethoxy silane (APTES), a smooth surface with uniform coverage of nanomotifs (19.4 ± 5.6 nm) was obtained, independent of the PEI generation. Because the hierarchical hyperbranched poly(ester amines) “nanofruits” are grown from a photopatterned substrate, it will allow us to harness the underlying micropatterns for various applications, including surface wetting, adhesion, and biomimetic mineralization.