Laser printing polymer particles for biomaterial applications

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 possibility of arranging a variety of differently colored toner particles 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 prerequisite 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

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Tubular structures are created by removing the non-fixed supporting structure after printing. The matrix structure provides stability for the 3D object even after this.

At the Fraunhofer IGB and the Institute of Interfacial Engineering and Plasma Technology at the University of Stuttgart a new method is being investigated, in which the use of different 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 melting 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. 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

Über Suspensionspolymerisation erzeugte sphärische Polymethylmethacrylat-Tonerpartikel.

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 biomaterials 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), which show a similarly low glass transition temperature of approx. 40 °C. The controllable glass transition point enables the sintering temperature to be optimized so as to achieve a high number of covalent bonds during the three-dimensional fixation 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

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Confocal laser microscope image of the fluoresceinamine-functionalized particle surfaces. Overlaying the two-dimensional image planes provides a three-dimensional image of the functionalized particles. Scale: x-axis = 27.5 μm, y-axis = 27.5 μm and z-axis = 3.5 μm.

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 click functions like thiol, azide and alkyne groups, which are available for a chemical 2D fixation via the thiol-ene reaction or the 1,3 dipolar cycloaddition.

The Figure shows the successful functionalization of the toner particles via specific binding of a fluorescent dye. Viewing under a laser microscope can show that the dye is only located on the hydrolyzed particle surface, while no emission is detected in the interior of the particles (Fig. left, above). Overlaying different two-dimensional image planes enables a three-dimensional representation of the fluorescent particle surfaces to be realized (Fig. left, below).

 

Application as biomaterial and outlook

In order to test the suitability of the manufactured polymer toner particles as support materials for tissue engineering, we examined the cytocompatibility compared to human fibroblasts and keratinocytes. These cells indicated high viability on all polymers with a glass transition point above room temperature, which is comparable with growth on commercial cell-culture plates. The functionalization of the surfaces leads to increased cell proliferation that is up to 178 percent above that of the reference material. Printing tests with the functionalized surface toner material have been successfully carried out by our project partner Fraunhofer IPA using an electrophotographic printer. The particle systems developed to date allow us to print reactive structures on level surfaces and thereby bind materials that do not otherwise readily adhere.

References

[1] Speyerer, C.; Güttler, S.; Borchers, K.; Tovar, G.; Hirth, T.; Weber A. (2012) Surface functionalization of toner particles for the assembly of three-dimensional objects via click chemistry, ChemieIngenieurTechnik 84: 322-327

[2] Speyerer, C.; Borchers, K.; Hirth, T.; Tovar, G.T.M.; Weber, A. (2013) Surface etching of methacrylic microparticles via basic hydrolysis and introduction of functional groups for click chemistry, Journal of Colloid and Interface Science 397:185-191

Funding

We would like to thank the Volkswagen Foundation and the Chemical Industry Fund (FCI) for funding this research.

Project partner

  • Fraunhofer IPA, Stuttgart
  • IGVP, Universität Stuttgart