A predominant part of morbidity and mortality in industrial countries can be attributed to deceases of the cardiovascular system. In the last decades great efforts have been undertaken to develop materials for artificial vascular constructs. However, bio-inert materials like ePTFE or PET – even though they are suitable for large caliber vascular substitutes – fail for the application as narrow blood vessel replacements like coronary bypasses.

For this, our aim is to design new biocompatible materials to overcome these problems. Our approach is to use elastic photopolymers for the artificial vascular grafts. By means of additive manufacturing technologies (AMTs) like digital light processing (DLP) very complex structures are realizable. Hence AMTs offer the possibility to create cellular structures within the artificial grafts that might favor the ingrowth of new tissue.

The mechanical properties as well as the biodegradability of materials designated for small caliber blood vessel replacement materials are of crucial importance.

In previous studies different monomer formulations were screened concerning their suitability for this application. Cyanoethyl acrylate-based photopolymer hydrogels fulfilled the basic material requirements for vascular tissue regeneration [1] but still have some drawbacks especially concerning the suture tear resistance. Therefore we reconsidered the network architecture of the photopolymers and also included the concept of thiol-ene chain transfer reactions [2]. A commercially available urethane diacrylate (UDA) was chosen as base monomer since urethane groups are known to have good cell-adhesion behavior [3] and poly-UDAs show adequate mechanical performance. In combination with different reactive diluents (e.g. 2-hydroxyethyl acrylate) and dithiols (e.g. 3,6-dioxa-1,8-octane-dithiol) the properties of the photopolymers can be tailored. By the incorporation of cleavable bonds into the backbone of the polymers by the addition of cleavable chain transfer agents (e.g. ethylene glycol bisthioglucylate) the photoelastomers should be degradable.

The mechanical properties of the new materials were assessed by conventional tensile test and additionally by a special suture tear resistance test. Degradation tests were performed in phosphate buffered saline (PBS) at elevated temperatures.

The optimized photoelastomers are in good accordance with natural blood vessels concerning their mechanical properties, show excellent biocompatibility in in-vitro tests with human umbilical vein endothelial cells, degrade similar to PLA and were successfully manufactured with DLP-AMT.

[1] S. Baudis, C. Heller, R. Liska, J. Stampfl, H. Bergmeister, G. Weigel: "(Meth)acrylate-based photoelastomers as tailored biomaterials for artificial vascular grafts"; Journal of Polymer Science Part A: Polymer Chemistry, 47 (2009), 10; 2664 - 2676.

[2] S. Baudis, B. Steyrer, T. Pulka, H. Wilhelm, G. Weigel, H. Bergmeister, J. Stampfl, R. Liska: "Photopolymerizable Elastomers for Vascular Tissue Regeneration"; Macromolecular Symposia, 296 (2010), 1; 121 - 126.

[3] S. Baudis, F. Nehl, S. Ligon, A. Nigisch, H. Bergmeister, D. Bernhard, J. Stampfl, R. Liska: "Elastomeric degradable biomaterials by photopolymerization-based CAD-CAM for vascular tissue engineering"; Biomedical Materials, 6 (2011), 5.

Frontal polymerization is a method of macromolecular synthesis, characterized by a steady state polymerization front that constantly propagates through the reaction vessel.

The process can be triggered by a one time energy input via an external heat source or the heat of a photopolymerization reaction. Due to the exothermic behavior of polymerization reactions, the local reaction can initiate further polymerization in the surrounding monomer by cleavage of a thermal initiator. At low heat loss, it is possible to generate a self-sustaining, thermal front which propagates step by step through a theoretically unlimited monomer volume.¹

[1] Pojman, J. A., Editors-in-Chief: Matyjaszewski, K. and Möller, M., Frontal Polymerization, in Polymer Science: A Comprehensive Reference. 2012, Elsevier: Amsterdam. p. 957-980.

For large scale industrial processing the use of water as green solvent, as well as the application of energy efficient process techniques is of high interest for a variety of polymer syntheses. UV initiated frontal polymerization of acrylic acid in aqueous formulations, for the production of hydrogels, fits perfectly both requirements. While UV-triggering of the thermal process offers numerous advantages the formation of bubbles during the frontal reaction itself and during the initiation can be fatal.

By providing a new sulfate-sulfonyl thermal initiator we are able to report the first bubble free steady state thermal front reaction in water initiated by UV-light, which could be investigated reproducible with regard to the initiation process.¹

[1] Potzmann, P. M., Lopez Villanueva, F. J. and Liska, R., "UV initiated bubble free frontal polymerization in aqueous conditions" Macromolecules accepted.

Photo curing of epoxy resins by common techniques is limited to very thin layers (like in coatings or printing inks) due to the little penetration depth of the UV-light. Therefore the combination of frontal polymerization and radical induced cationic polymerization¹ leads to a superior method of epoxy photocuring. With radical induced cationic frontal polymerization (RICFP) the bulk curing even of complex shapes is possible. The initiation can be done by locally application of either UV-light or also heat.

Current research is based on literature from Mariani et. al.² who developed a RICFP system for cycloaliphatic epoxy resins. The usage of C-C labile compounds like benzopinacol enabled us to also cure the industrial most used epoxy resin: bisphenol-A-diglycidylether (BADGE).³

 The Video shows a radical induced cationic frontal polymerization of a BADGE resin formulation containing benzopinacol and the commercial available cationic photoinitiator IOC-8 SbF

[1] Ledwith, A., Possibilities for promoting cationic polymerization by common sources of free radicals. Polymer, 1978. 19(10): p. 1217-1219.

[2] Mariani, A., Bidali, S., Fiori, S., Sangermano, M., Malucelli, G., Bongiovanni, R. and Priola, A., UV-ignited frontal polymerization of an epoxy resin. Journal of Polymer Science Part A: Polymer Chemistry, 2004. 42(9): p. 2066-2072.

[3] Bomze, D., Knaack, P. and Liska, R., Successful radical induced cationic frontal polymerization of epoxy-based monomers by C-C labile compounds. Polymer Chemistry, 2015, 6, 8161 - 8167.

Generally, low penetration depth of light into the material limits the application of photo-polymerizable resins to thin films. Curing of thick bulk materials could open a wider field of applications. A promising approach to increase the curing depth is the so called “frontal polymerization” [1]. In this case, the polymerization is induced at the surface of a formulation and a polymerization front migrates through the material. Previous works enabled migrating fronts with peroxides as thermal initiators. Unfortunately, the usage of peroxides which feature good storage stability afford high initiation temperatures, and peroxides which decompose at lower temperatures show low shelf life stability, which strongly limits the field of applications.

We developed a new class of functional peroxides which are characterized by an radical induced destabilization during polymerization. By adding them in up to 1wt% in classical monomer formulations sufficient storage stability was observed. Practical photocuring experiments showed that with some peroxides even an unlimited migrating front could be observed [2]. So a first proof of our concept could be achieved and a very promising class of frontal polymerization agents is waiting for further examination and new applications.

Photopolymers exhibiting biocompatibility and biodegradability are promising materials for the application in the field of Tissue Engineering. The possibility of structuring compounds via Additive Manufacturing Technologies (AMT) such as Microstereolithography, Digital Light Processing (DLP) or Two-photon Induced Photopolymerization (TPIP) enables the fabrication of constructs with complex geometries and high resolutions of about 10µm - or even 200nm in the case of TPIP - in order to mimic the cellular structures of natural materials such as bone.

State of the art compounds for these applications are (meth)acrylate-based. However, methacrylates exhibit low photoreactivity and acrylates are prone to side-reactions with proteins inside the human body giving explanation for the high irritation and sometimes toxicity. Furthermore, the formation of poly(acrylic acid) upon hydrolytic degradation provokes a local decrease of the pH value and can cause the formation of non-excretable precipitate in the presence of calcium ions. Hence, these materials are less suitable for biomedical applications.

Taking into account these adverse effects, new monomers with different polymerizable groups such as vinyl esters, vinyl carbonates and vinyl carbamates have been synthesized [1-3]. Therefore, various alcohols and amines with differing functionality, molecular weight and hydrophilicity were converted into the respective vinyl ester, -carbonate or -carbamate by one step syntheses with good yields. These new monomers were found to be significantly less cytotoxic than acrylates. Moreover, polymers derived from these classes of monomers give water-soluble and biocompatible poly(vinyl alcohol) as degradation product. Photo-Differential Scanning Calorimetry studies of the new monomer classes showed that their reactivity is between acrylates and methacrylates. Mechanical properties such as Young’s modulus and hardness were determined by nanoindentation experiments and were found to be similar to their (meth)acrylate counterparts. Additionally, basic studies towards the suitability of these new materials for biomedical applications were accomplished by measuring their cytotoxicity towards osteoblast-like cells and by conducting cell culture tests. To evaluate the degradation behavior, studies with model substances as well as the actual polymers were conducted under accelerated conditions. It was found that especially hydrophilic vinyl ester-based materials were readily degrading and exhibited significantly higher degradation rates than poly(lactic acid).

Finally, 3D structures were fabricated using AMT techniques and some of these parts made from selected compositions were successfully tested by in vivo-studies in New Zealand white rabbits.

[1] C. Heller, M. Schwentenwein, G. Russmüller, F. Varga, J. Stampfl, R. Liska:
"Vinyl Esters: Low Cytotoxicity Monomers for the Fabrication of Biocompatible 3D Scaffolds by Lithography Based Additive Manufacturing";
Journal of Polymer Science Part A: Polymer Chemistry, 47 (2009), 6941 - 6954.

[2] C. Heller, M. Schwentenwein, G. Russmüller, T. Koch, D. Moser, C. Schopper, F. Varga, J. Stampfl, R. Liska:
"Vinylcarbonates and Vinylcarbamates: Biocompatible Monomers for Radical Photopolymerization";
Journal of Polymer Science Part A: Polymer Chemistry, 49 (2011), 3; 650 - 661.

[3] Liska, Robert; Stampfl, Juergen; Varga, Franz; Gruber, Heinrich; Baudis, Stefan; Heller, Christian; Schuster, Monika; Bergmeister, Helga; Weigel, Guenter; Dworak, Claudia; PCT Int. Appl. (2009), WO 2009065162 A2 20090528

Oxygen inhibition in radical photopolymerization is a major drawback as it leads to tacky surfaces and frequently requires expensive nitrogen inertization. Beside such physical methods there are a large variety of chemical methods available that try to circumvent these disadvantages [1, 2]. However, a recent study that compares most of the strategies known from literature shows that there is up to now only a moderate improvement possible and no general strategy available [3].

[1] S. Ligon, B. Husar, H. Wutzel, R. Holman, R. Liska: "Strategies to Reduce Oxygen Inhibition in Photoinduced Polymerization"; Chemical Reviews, 114 (2014), 1; S. 557 - 589.

[2] M. Höfer, N. Moszner, R. Liska: "Oxygen scavengers and sensitizers for reduced oxygen inhibition in radical photopolymerization"; Journal of Polymer Science Part A: Polymer Chemistry, 46 (2008), 20; S. 6916 - 6927.

[3] B. Husar, S. Ligon, H. Wutzel, H. Hoffmann, R. Liska: "The formulator´s guide to anti-oxygen inhibition additives"; Progress in Organic Coatings, 77 (2014), S. 1789 - 1798.

Polymer electrolyte fuel cells (PEFC) gained a lot of interest in recent years as a potential solution for an eco-friendly energy. Proton exchange membranes (PEM) are one of the main components of PEFCs and require mechanical and chemical stability to ensure high proton conductivity and effective separation of anode and cathode under challenging conditions. Best commercial membranes made from sulfonated fluoropolymers, such as Nafion®, are rather expensive. To improve fuel cell performance at a lower cost, 2-acrylamido-2-methylpropane sulfonic acid (AMPS) was investigated recently. Since polyAMPS (PAMPS) excessively swells or even dissolves in water, we investigated several commercial crosslinkers and new multifunctional monomers to decrease swelling by crosslinking.

PAMPS formulations containing different crosslinkers with enhanced hydrolytical stability and conductivity can be photocured in porous PP-membranes. With this procedure more than 2.5 times the conductivity of Nafion with only 5 wt% crosslinker has been achieved.

We used this novel crosslinkers to integrate them also into asymmetric membranes with interpenetrating proton-conducting morphology for enhanced methanol barrier properties.  With this method 8 times of the performance of Nafion has been achieved.

Radovanovic, P., et al., Journal of Membrane Science 2012, 401-402, 254-261.

The two-photon induced polymerization (2PP) is a novel concept of photopolymer chemistry allowing for elegant 3D direct laser writing (DLW) with resolutions down to several tens of nanometers within photoreactive resins. It has attracted significant attention in hot research fields such micro-electromechanical systems (MEMS), defect engineering in photonic crystals or surface plasmon polaritons. Additionally, 2PP can be applied for the structuring of optical waveguides by inducing a local refractive index increase over the surrounding cladding material.

In the framework of the Austrian Nano Initiative we have developed high performance waveguide materials on the basis of polysiloxane/photopolymer hybrid materials, which we consider to be useful for connecting electrooptical components on flexible circuit boards in the communication technology.

First concepts focused on flexible sol-gel materials [1]. Recent work uses high refractive acrylic and thiol-ene formulations that were cured via 2PP with new less time-consuming pre- and post-processing methods [2]. DLW of waveguide structures was carried out using our novel two-photon initiators B3FL and B3K [3].

[1] S. Krivec, N. Matsko, V. Satzinger, N.U Pucher, N. Galler, T. Koch, V. Schmidt, W. Grogger, R. Liska, H. Lichtenegger: "Silica-Based, Organically Modified Host Material for Waveguide Structuring by Two-Photon-Induced Photopolymerization"; Advanced Functional Materials, 20 (2010), S. 1 - 9.

[2] J. Kumpfmueller, K. Stadlmann, Z. Li, V. Satzinger, J. Stampfl and R. Liska: "Flexible Optical Interconnects via Thiol-ene Two-photon-induced Polymerization"; MRS Proceedings, 2012, 1438.

[3] N. Pucher, A. Rosspeintner, V. Satzinger, V. Schmidt, G. Gescheidt, J. Stampfl, R. Liska, Macromolecules, 42, pp. 6519-6528, (2009).

In the field of dental filling materials [0], our research started with the development of optimized photoinitiator systems that have to absorb in the visible light region. State of the art photoinitiator camphorquinone has been modified with tertiary amine groups to improve the photoreactivity [1]. Especially in aqueous acidic dental primer formulations, this bimolecular photoinitiation system suffers from the solvent cage effect has shows only poor photoreactivity. Phenylglycines avoid the back electron transfer by spontaneous decarboxylation reaction [2]. Limited thermal storage stability forced us to the development of monomolecular initiators that don’t suffer from the solvent cage effect. Bisacyl phosphine oxides (BAPOs) are generally the initiators of choice but very poor solubility, especially in the aqueous formulations, led to the development of hydrophilic BAPOs [3].

The new generation of LED based dental lamps has a much smaller emission spectra than classical halogen lamps. Therefore new, more red-shifted monomolecular initiators were necessary. The germanium-based initiators [4] fulfil all the requirements and are now commercially on the market under the trade name Ivocerin [5].

Another shortcoming of currently used dental filling materials is the limited penetration depth of light in the highly filled composite material. Frontal polymerization using smart peroxides that undergo a destabilization reaction during the polymerization reaction allow a travelling front throughout the system. [6].

[0] Moszner, N.; Salz, U. Macromol. Mater. Eng.; 2007, 292, 245-271

[1] G. Ullrich, D. Herzog, R. Liska, P. Burtscher, N. Moszner: "Photoinitiators with Functional Groups. VII. Covalently Bonded Camphorquinone-Amines"; Journal of Polymer Science Part A: Polymer Chemistry, 42 (2004), S. 4948 - 4963.

[2] G. Ullrich, P. Burtscher, U. Salz, N. Moszner, R. Liska: "Phenylglycine Derivatives as Coinitiators for the Radical Photopolymerization of Acidic Aqueous Formulations"; Journal of Polymer Science Part A: Polymer Chemistry, 44 (2006), S. 115 - 125.

[3] G. Ullrich, B. Ganster, U. Salz, N. Moszner, R. Liska: "Photoinitiators With Functional Groups. IX. Hydrophylic Bisacylphosphine Oxides for Acidic Aqueous Formulations"; Journal of Polymer Science Part A: Polymer Chemistry, 44 (2006), S. 1686 - 1700.

[4] B. Ganster, U. Fischer, N. Moszner, R. Liska: "New Photocleavable Structures. V. Diacylgermane Based Photoinitiators for Visible Light Curing"; Macromolecules, 7 (2008), 41; S. 2394 - 2400.

[5] N. Moszner, U. Fischer, B. Ganster, R. Liska, V. Rheinberger: "Benzoyl germanium derivatives as novel visible light photoinitiators for dental materials"; Dental Materials, 7 (2008), 24; S. 901 - 907.

[6] A. Gugg, C. Gorsche, N. Moszner, R. Liska: "Frontal Polymerization: Polymerization Induced Destabilization of Peracrylates"; Macromolecular Rapid Communications, 32 (2011), S. 1096 - 1100.