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S. Orman, C. Hofstetter, F. Reinauer, A. Aksu, R. Liska, S. Baudis:
"PCL based toughness enhancers for photopolymer based bone substitutes";
Vortrag: 5th European Symposium of Photopolymer Science, Mulhouse, France; 03.09.2018 - 06.09.2018; in: "5th European Symposium of Photopolymer Science", 5th European Symposium of Photopolymer Science, (2018), S. 10.

Patient-specific implants to treat bone defects are an appealing approach, esp. in the case of oral and maxillofacial traumata. In the last decade, considerable progress was made to exploit the power of lithography-based additive manufacturing techniques (L-AMTs) in this field.1 An important innovation was the establishment of vinyl ester (incl. vinyl carbonates) as reactive species in photopolymer precursors.2 Their crucial advantage compared to common precursors, acrylates and methacrylates, is their low cytotoxicity. Moreover, favorable poly(vinyl alcohol) and low molecular weight carboxylic acids or alcohols are degradation products of vinyl ester based photopolymers, which demonstrated a promising capacity for (bone) tissue regeneration. The comparatively low reactivity of vinyl esters and the high brittleness of crosslinked photopolymers was improved by application of thiol-ene chemistry,3 however, still there is demand to enhance the toughness of these photopolymers in order to catch up with titanium, which is considered to be the golden standard for bear loaded bone implants.
In this work, we investigated the capacity of poly(caprolactone) (PCL) based toughness enhancers. PCL was incorporated as inert additive, as co-monomers (PCL divinyl carbonate, DVC), and as thiol component also including urethane functionalities (PCL ur¬ethane dithiol, UDT) (Figure 1). DVC was synthesized from α,ω-di¬hydroxy PCL by a well-known pathway using vinyl chlo¬ro¬for¬ma¬te in a Schotten-Baumann reaction with pyridine as base.2 UDT was prepared by a Micheal-type oligoaddition using commercially available PCL urethane diacrylate with an excess of 3,6-di¬oxa-1,8-oc¬tane dithiol and trimethylamine as catalyst. The reference system consisted of divinyl adipate (DVA) with trimethylpropane tri(3-mercaproptopionate) (TMPMP) in a tough¬ness optimized formulation, which was modified by addition of different amounts of synthesized PCL based additives (Figure 1). The reference system and formulation with toughness enhancers were characterized by real¬time-near infrared-photorheology (RT-NIR-Photorheology) in or¬der to investigate important cha¬racteristic parameter, including gel point, double bond conversion (at gel point and overall), and shrin¬kage stress.4 Moreover, photopolymer specimens were tested by means of tensile tests, dynamic mechanical thermal analysis (DMTA), and Dynstat impact resistance test in order to study and compare the material properties of final photopolymers. All additives led to an improvement in impact toughness. Unmodified as well as DVC modified PCL effected an increase in elongation at break, while elastic modulus and tensile strength hardly changed. In case of UDT the impact resistance and elongation at break were improved even more, however, tensile strength as well as glass transition temperature were lowered considerably, which is owing to the decreased network density and the rather soft thioether bonds formed during the thiol-ene photopolymerization. The best overall performance was found for DVC in concentrations of 10-15 wt%. These formulations showed high impact strength and elongation at break, increased glass transition temperature at similar reactivities, final conversions and tensile strengths.
Future work will be dedicated to further optimization of the formulations, the investigation of more complex compositions with combinations of different additives, and establishing 3D printing of an ideal formulation by L AMTs.

Patient-specific implants, bone defects, lithography-based additive manufacturing techniques, vinyl ester (incl. vinyl carbonates) as reactive species in photopolymer precursors