© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1304 www.advmat.de www.MaterialsViews.com wileyonlinelibrary.com COMMUNICATION Simona C. Laza, Marco Polo, Antonio A. R. Neves, Roberto Cingolani, Andrea Camposeo,* and Dario Pisignano* Two-Photon Continuous Flow Lithography Dr. S. C. Laza, Dr. M. Polo, Dr. A. A. R. Neves, Dr. A. Camposeo, Prof. D. Pisignano National Nanotechnology Laboratory of Istituto Nanoscienze-CNR Università del Salento via Arnesano, I-73100 Lecce, Italy E-mail: andrea.camposeo@nano.cnr.it; dario.pisignano@unisalento.it Dr. A. Camposeo, Prof. D. Pisignano Center for Biomolecular Nanotechnologies @ UNILE Istituto Italiano di Tecnologia via Barsanti, I-73010 Arnesano, Italy Prof. D. Pisignano Dipartimento di Ingegneria dell’Innovazione Università del Salento via Arnesano I-73100 Lecce, Italy Prof. R. Cingolani Italian Institute of Technology via Morego 30, I-16163 Genova, Italy DOI: 10.1002/adma.201103357 The functional properties of polymeric particles strongly depend on their shape, size, and chemistry, making them useful in fields as diverse as photonics, [1] microrheology, [2] tissue engi- neering, [3] and biomolecule analysis. [4] For instance, polymer particles are used as carriers in drug delivery, [5] where important issues related to particle flow, degradation and phagocytosis are influenced by their geometry and dimensions. [6] Fundamental properties, including diffusion, suspension rheology, and self- assembly, can be effectively tailored by engineering the particle shape and composition. [7–9] These findings motivate the devel- opment of novel fabrication techniques of micro- and nanopar- ticles with various shape and chemistry, [10,11] possibly exhibiting low surface roughness and custom-designed three-dimensional (3D) geometry. To this aim, numerous methods have been pro- posed, including stretching of spherical particles, [12] particle replication in non-wetting templates, [13] photolithography, [14] and microfluidic approaches. [2,15] In particular, though origi- nally limited to spherical or deformed spherical shapes, lab-on- a-chip fabrication allows one to produce continuously particles with a wide range of morphologies and chemistries. [2] The advent of continuos flow lithography (CFL) [16] from the Doyle’s group has overcome some limitations of microfluidics, enabling the fabrication of complex structures. [4,17–21] In CFL, projection photolithography is generally used for the defini- tion of the particle shape. Self-standing particles are obtained by photopolymerization of a pre-polymer sensitive to UV light during its flow through a microchannel (μCh), whereas chem- ical anisotropy is achieved by polimerizing across the interface of co-flowing pre-polymer solutions, since laminar flow pro- vides poor, diffusion-limited mixing. Particles are synthesized in continuous runs, and their shape can be changed in real time using a digital mirror device (DMD) [22] instead of standard masks. Multifunctional particles can also be produced by hydrodynamic focusing lithography. [21] Overall, CFL methods developed to date achieve excellent results, however two impor- tant issues still remains open for this new class of techniques, namely the synthesis of (i) particles with sub-micrometer fea- tures and (ii) truly 3D particles in single-step processes. On one side, the highest reported resolution is 1.5 μm, [10,22] whereas the control of the particle structure at the sub-micrometer scale is often needed in many applications. On the other side, UV-mask projection and DMDs limit the achievable particle geometry to basically two-dimensional (2D) geometries. The fabrication of 3D objects with arbitrarily complex shapes and feature sizes below the diffraction limit is instead possible by two-photon lithography (TPL). [23,24] This process requires the simultaneous absorption of two photons having half the energy of the involved transition of the resist material, with probability depending on the squared intensity of the excitation laser. Two-photon absorption (TPA) is typically limited to a region very close to the laser focus, where light is intense enough to induce non-linear absorption, thus allowing to achieve sub- diffraction spatial resolution. Photonic crystals, [25] microt- weezers and needles, [26] and in situ scaffolding of cells [27] are demonstrated by TPL. However, this technique on its turn is limited in throughput by its intrinsic seriality, making dif- ficult to pattern large areas or to generate a high number of structures. Alternatives to circumvent such drawbacks include holography, [19,28] though this method can not generate as arbi- trary geometries as in TPL. In this work, we introduce a new approach for microfluidics-based production of polymeric particles, namely the two-photon continuous flow lithography (TP-CFL). This technique takes advantage of TPA high resolu- tion to create objects with sub-micrometer and 3D features, and overcomes serial process limitations of traditional TPL by using multiple beam production under continuous flow. We demon- strate polymeric fibers, helical and bow-tie particles with sub- diffraction resolution and surface roughness as low as 10 nm. A schematic view of the TP-CFL is shown in Figure 1a-b. In the following we indicate the direction of light propagation as z-axis, and the flow direction as y-axis. In Figure 1c–f we display some objects fabricated by TP-CFL. The 3D, free-standing helical structure shown in Figure 1d is obtained by the composition of the circular motion (diameter 20 μm) of the photopolymeriza- tion spot (x 0 , z 0 ), controlled by the piezo-stage perpendicularly to the flow direction (x-z plane), and the flow motion along the μCh length (y-axis, Suppl. data movie 1). Particles with complex shape can be therefore synthesized by TP-CFL with a proper combination of flow conditions and writing-beam positioning within the μCh. This can be controlled either by moving the microfluidic device (as performed here) or by deflecting the Adv. Mater. 2012, 24, 1304–1308