@phdthesis{Boehm2015, author = {B{\"o}hm, Christoph}, title = {Loewner equations in multiply connected domains}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-129903}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2015}, abstract = {The first goal of this thesis is to generalize Loewner's famous differential equation to multiply connected domains. The resulting differential equations are known as Komatu--Loewner differential equations. We discuss Komatu--Loewner equations for canonical domains (circular slit disks, circular slit annuli and parallel slit half-planes). Additionally, we give a generalisation to several slits and discuss parametrisations that lead to constant coefficients. Moreover, we compare Komatu--Loewner equations with several slits to single slit Loewner equations. Finally we generalise Komatu--Loewner equations to hulls satisfying a local growth property.}, subject = {Biholomorphe Abbildung}, language = {en} } @article{RymaTylekLiebscheretal.2021, author = {Ryma, Matthias and Tylek, Tina and Liebscher, Julia and Blum, Carina and Fernandez, Robin and B{\"o}hm, Christoph and Kastenm{\"u}ller, Wolfgang and Gasteiger, Georg and Groll, J{\"u}rgen}, title = {Translation of collagen ultrastructure to biomaterial fabrication for material-independent but highly efficient topographic immunomodulation}, series = {Advanced materials}, volume = {33}, journal = {Advanced materials}, number = {33}, doi = {10.1002/adma.202101228}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-256381}, year = {2021}, abstract = {Supplement-free induction of cellular differentiation and polarization solely through the topography of materials is an auspicious strategy but has so far significantly lagged behind the efficiency and intensity of media-supplementation-based protocols. Consistent with the idea that 3D structural motifs in the extracellular matrix possess immunomodulatory capacity as part of the natural healing process, it is found in this study that human-monocyte-derived macrophages show a strong M2a-like prohealing polarization when cultured on type I rat-tail collagen fibers but not on collagen I films. Therefore, it is hypothesized that highly aligned nanofibrils also of synthetic polymers, if packed into larger bundles in 3D topographical biomimetic similarity to native collagen I, would induce a localized macrophage polarization. For the automated fabrication of such bundles in a 3D printing manner, the strategy of "melt electrofibrillation" is pioneered by the integration of flow-directed polymer phase separation into melt electrowriting and subsequent selective dissolution of the matrix polymer postprocessing. This process yields nanofiber bundles with a remarkable structural similarity to native collagen I fibers, particularly for medical-grade poly(ε-caprolactone). These biomimetic fibrillar structures indeed induce a pronounced elongation of human-monocyte-derived macrophages and unprecedentedly trigger their M2-like polarization similar in efficacy as interleukin-4 treatment.}, language = {en} } @article{BoehmTandonHrynevichetal.2022, author = {B{\"o}hm, Christoph and Tandon, Biranche and Hrynevich, Andrei and Teßmar, J{\"o}rg and Dalton, Paul D.}, title = {Processing of Poly(lactic-co-glycolic acid) Microfibers via Melt Electrowriting}, series = {Macromolecular Chemistry and Physics}, volume = {223}, journal = {Macromolecular Chemistry and Physics}, number = {5}, doi = {10.1002/macp.202100417}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-318444}, year = {2022}, abstract = {Polymers sensitive to thermal degradation include poly(lactic-co-glycolic acid) (PLGA), which is not yet processed via melt electrowriting (MEW). After an initial period of instability where mean fiber diameters increase from 20.56 to 27.37 µm in 3.5 h, processing stabilizes through to 24 h. The jet speed, determined using critical translation speed measurements, also reduces slightly in this 3.5 h period from 500 to 433 mm min\(^{-1}\) but generally remains constant. Acetyl triethyl citrate (ATEC) as an additive decreases the glass transition temperature of PLGA from 49 to 4 °C, and the printed ATEC/PLGA fibers exhibits elastomeric behavior upon handling. Fiber bundles tested in cyclic mechanical testing display increased elasticity with increasing ATEC concentration. The processing temperature of PLGA also reduces from 165 to 143 °C with increase in ATEC concentration. This initial window of unstable direct writing seen with neat PLGA can also be impacted through the addition of 10-wt\% ATEC, producing fiber diameters of 14.13 ± 1.69 µm for the first 3.5 h of heating. The investigation shows that the initial changes to the PLGA direct-writing outcomes seen in the first 3.5 h are temporary and that longer times result in a more stable MEW process.}, language = {en} } @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} } @phdthesis{Boehm2023, author = {B{\"o}hm, Christoph}, title = {Thermal Stability of the Polyesters PCL and PLGA during Melt Electrowriting}, doi = {10.25972/OPUS-30613}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-306139}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2023}, abstract = {The focus of this thesis was to investigate how PCL and PLGA react to the heat exposure that comes with the MEW process over a defined timespan. To assess the thermal stability of PCL during MEW over 25 d, an automated collection of fibers has been used to determine the CTS on each day of heating for three different temperatures. PCL is exceptionally stable over 25 d at 75 °C, whereas for 85 °C and 95 °C a slight upward trend during the last 10 d could be observed, which is an indication for thermal degradation. Same trend could be observed for diameter of fibers produced at a fixed collector speed. For all temperatures, CTS during the first 5 d decreased due to inhomogeneities of the melt. Physical analysis of the fibers by XRD and mechanical testing showed no significant changes. To investigate the chemical details of the thermal durability, PCL was artificially aged over 25 d at 75 °C, 85 °C and 95 °C. Data from GPC analysis and rheology revealed that PCL is degrading steadily at all three temperatures. Combined with GC-MS analysis, two different mechanisms for degradation could be observed: random chain scission and unzipping. Additional GPC experiment using a mixture of PCL and a fluorescence labelled PCL showed that PCL was undergoing ester interchange reactions, which could explain its thermal stability. PLGA was established successfully as material for MEW. GPC results revealed that PLGA degraded heavily in the one-hour preheating period. To reduce the processing temperature, ATEC was blended with PLGA in three mixtures. This slowed down degradation and a processing window of 6 h could be established. Mechanical testing with fibers produced with PLGA and all three blends was performed. PLGA was very brittle, whereas the blends showed an elastic behavior. This could be explained by ester interchange reactions that formed a loosely crosslinked network with ATEC.}, subject = {Degradation}, language = {en} }