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2013|14 Annual Report Fraunhofer IGB

9 4 The challenge of manufacturing cell-compatible 3D objects Electrophotography has developed into one of the leading digital technologies in graphic printing. The process, which is also known as xerography and laser printing, offers the pos- sibility of arranging a variety of differently colored toner par- ticles with high resolution and thereby individually designing a paper substrate. However, the printing process is currently largely limited to two-dimensional (2D) applications, although the large solid content of toner particles provides a good pre- requisite for the fast construction of three-dimensional (3D) objects. The first commercial 3D laser printing applications are aimed at the construction of simple molded parts. The layered laser printing of cytocompatible objects like artificial arteries or other tubular structures represents a special challenge. Fixation by a chemical reaction of the toner particles At the Fraunhofer IGB and the Institute of Interfacial Engineer- ing and Plasma Technology at the University of Stuttgart a new method is being investigated, in which the use of differ- ent toner components ensures that the spatial arrangement of complex structures is maintained. To do this, the structure was first printed in layers as a three-dimensional block. After each application of particles, instead of the conventional melt- ing fixation, a chemical reaction between the particle surfaces to be fixed occurs. The stability of the 3D objects rests on the formation of covalent bonds and does not require complete melting of the individual particles. The complex geometry of the object is created by support materials, which consist of non-crosslinking toner components and can selectively be removed after printing (Fig. 1). Subsequently the stability of porous or tubular structures, that are required as support materials in applications such as tissue engineering, is assured by the presence of a stable matrix material. Optimized glass transition temperature and particle size Because of their construction, laser printers function very well with particles in the size range of 3 µm to 30 µm, as well as with polymers with softening temperatures below 110°C. We produce toner particles that are suitable for use as biomateri- als out of poly (methyl methacrylates) (PMMA). The selection of the comonomer composition enables the glass transition temperature of the amorphous poly (methyl methacrylate) to be varied over a large temperature range of between –48°C and 110°C. Low reaction temperatures of 20°C lead, within 24 h to spherical poly (methyl methacrylate) (Fig. 2), which show a similarly low glass transition temperature of approx. 40°C. The controllable glass transition point enables the sin- tering temperature to be optimized so as to achieve a high number of covalent bonds during the three-dimensional fixa- tion of the polymer particles. The covalent bonds between the polymer chains prevent the formation of a solvate sheath, while the non-bonded polymer particles can be selectively dissolved. These are therefore suitable as removable support material for porous and tubular structures in 3D printing. The desired particle size can be very accurately set to between 3 μm and 30 μm by UV-initiated suspension polymerization. Modification of the particle surface via click chemistry The poly (methyl acrylate) surfaces are modified with the aid of polymeric analogous conversions. The main chains of the polymer surface remain unchanged during this, while the side chains are chemically modified. The activated surface enables the progressive functionalization of the toner particles with LASER PRINTING POLYMER PARTICLES FOR BIOMATERIAL APPLICATIONS Dr. rer. nat. Achim Weber CHEMISTRY 21 50 μm 2 μm bulk removable filling interface

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