@article{BoehmStahlhutWeichholdetal.2022,
  author    = {B{\"o}hm, Christoph and Stahlhut, Philipp and Weichhold, Jan and Hrynevich, Andrei and Teßmar, J{\"o}rg and Dalton, Paul D.},
  title     = {The Multiweek Thermal Stability of Medical-Grade Poly(ε-caprolactone) During Melt Electrowriting},
  series = {Small},
  volume    = {18},
  journal   = {Small},
  number    = {3},
  doi       = {10.1002/smll.202104193},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-257741},
  year      = {2022},
  abstract  = {Melt electrowriting (MEW) is a high-resolution additive manufacturing technology that places unique constraints on the processing of thermally degradable polymers. With a single nozzle, MEW operates at low throughput and in this study, medical-grade poly(ε-caprolactone) (PCL) is heated for 25 d at three different temperatures (75, 85, and 95 °C), collecting daily samples. There is an initial increase in the fiber diameter and decrease in the jet speed over the first 5 d, then the MEW process remains stable for the 75 and 85 °C groups. When the collector speed is fixed to a value at least 10\% above the jet speed, the diameter remains constant for 25 d at 75 °C and only increases with time for 85 and 95 °C. Fiber fusion at increased layer height is observed for 85 and 95 °C, while the surface morphology of single fibers remain similar for all temperatures. The properties of the prints are assessed with no observable changes in the degree of crystallinity or the Young's modulus, while the yield strength decreases in later phases only for 95 °C. After the initial 5-d period, the MEW processing of PCL at 75 °C is extraordinarily stable with overall fiber diameters averaging 13.5 ± 1.0 µm over the entire 25-d period.},
  language  = {en}
}
@article{SeifertGrollWeichholdetal.2021,
  author    = {Seifert, Annika and Groll, J{\"u}rgen and Weichhold, Jan and Boehm, Anne V. and M{\"u}ller, Frank A. and Gbureck, Uwe},
  title     = {Phase Conversion of Ice-Templated α-Tricalcium Phosphate Scaffolds into Low-Temperature Calcium Phosphates with Anisotropic Open Porosity},
  series = {Advanced Engineering Materials},
  volume    = {23},
  journal   = {Advanced Engineering Materials},
  number    = {5},
  doi       = {10.1002/adem.202001417},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-256311},
  year      = {2021},
  abstract  = {The current study aims to extend the material platform for anisotropically structured calcium phosphates to low-temperature phases such as calcium-deficient hydroxyapatite (CDHA) or the secondary phosphates monetite and brushite. This is achieved by the phase conversion of highly porous α-tricalcium phosphate (α-TCP) scaffolds fabricated by ice-templating into the aforementioned phases by hydrothermal treatment or incubation in phosphoric acid. Prior to these steps, α-TCP scaffolds are either sintered for 8 h at 1400 °C or remain in their original state. Both nonsintered and sintered α-TCP specimens are converted into CDHA by hydrothermal treatment, while a transformation into monetite and brushite is achieved by incubation in phosphoric acid. Hydrothermal treatment for 72 h at 175 °C increases the porosity in nonsintered samples from 85\% to 88\% and from 75\% to 88\% in the sintered ones. An increase in the specific surface area from (1.102 ± 0.005) to (9.17 ± 0.01) m2 g-1 and from (0.190 ± 0.004) to (2.809 ± 0.002) m2 g-1 due to the phase conversion is visible for both the nonsintered and sintered samples. Compressive strength of the nonsintered samples increases significantly from (0.76 ± 0.11) to (5.29 ± 0.94) MPa due to incubation in phosphoric acid.},
  language  = {en}
}
@article{KadeTandonWeichholdetal.2021,
  author    = {Kade, Juliane C. and Tandon, Biranche and Weichhold, Jan and Pisignano, Dario and Persano, Luana and Luxenhofer, Robert and Dalton, Paul D.},
  title     = {Melt electrowriting of poly(vinylidene fluoride-co-trifluoroethylene)},
  series = {Polymer International},
  volume    = {70},
  journal   = {Polymer International},
  number    = {12},
  doi       = {10.1002/pi.6272},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-257654},
  pages     = {1725-1732},
  year      = {2021},
  abstract  = {Poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-co-TrFE)) is an electroactive polymer with growing interest for applications in biomedical materials and flexible electronics. In this study, a solvent-free additive manufacturing technique called melt electrowriting (MEW) has been utilized to fabricate well-defined microperiodic structures of the copolymer (P(VDF-co-TrFE)). MEW of the highly viscous polymer melt was initiated using a heated collector at temperatures above 120 °C and required remarkably slow collector speeds below 100 mm min\(^{-1}\). The fiber surface morphology was affected by the collector speed and an increase in β-phase was observed for scaffolds compared to the unprocessed powder. Videography shows vibrations of the P(VDF-co-TrFE) jet previously unseen during MEW, probably due to repeated charge buildup and discharge. Furthermore, piezo-force microscopy measurements demonstrated the electromechanical response of MEW-fabricated fibers. This research therefore achieves the melt electrohydrodynamic processing of fibers with micrometer resolution into defined structures with an important electroactive polymer.},
  language  = {en}
}
@article{HolzmeisterWeichholdGrolletal.2021,
  author    = {Holzmeister, Ib and Weichhold, Jan and Groll, J{\"u}rgen and Zreiqat,, Hala and Gbureck, Uwe},
  title     = {Hydraulic reactivity and cement formation of baghdadite},
  series = {Journal of the American Ceramic Society},
  volume    = {104},
  journal   = {Journal of the American Ceramic Society},
  number    = {7},
  doi       = {10.1111/jace.17727},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-259457},
  pages     = {3554-3561},
  year      = {2021},
  abstract  = {In this study, the hydraulic reactivity and cement formation of baghdadite (Ca\(_{3}\)ZrSi\(_{2}\)O\(_{9}\)) was investigated. The material was synthesized by sintering a mixture of CaCO\(_{3}\), SiO\(_{2}\), and ZrO\(_{2}\) and then mechanically activated using a planetary mill. This leads to a decrease in particle and crystallite size and a partial amorphization of baghdadite as shown by X-ray powder diffraction (XRD) and laser diffraction measurements. Baghdadite cements were formed by the addition of water at a powder to liquid ratio of 2.0 g/ml. Maximum compressive strengths were found to be ~2 MPa after 3-day setting for a 24-h ground material. Inductively coupled plasma mass spectrometry (ICP-MS) measurements showed an incongruent dissolution profile of set cements with a preferred dissolution of calcium and only marginal release of zirconium ions. Cement formation occurs under alkaline conditions, whereas the unground raw powder leads to a pH of 11.9 during setting, while prolonged grinding increased pH values to approximately 12.3.},
  language  = {en}
}