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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.
Die „Malen nach Zahlen“ Methode zur Verbesserung der Präparation einer Vollgusskrone der Studenten
(2021)
Einleitung: Es gibt derzeit keine kommerziell erhältliche Lösung zur Verbesserung des Erlernens einer Kronenpräparation an Modellzähnen. Um diese Lücke zu schließen und die Betreuer von zahnmedizinischen Kursen zu unterstützen, wurde ein druckbarer und kostengünstiger Zahn zur strukturierten Selbsteinschätzung entwickelt. Das Ziel dieser Studie war es, diesen druckbaren Zahn unter realistischen vorklinischen Situationen zu testen. Materialien und Methoden: Es wurde ein zweifarbiger, zweischichtiger Übungszahn entwickelt. Dieser Zahn bestand aus einer korrekten Präparationsschicht und der Zahnkrone. Alle gedruckten Zähne wurden mit einem Stereolithografiedrucker hergestellt. 35 freiwillige Zahnmedizinstudenten des zweiten vorklinischen Kurses im zweiten Jahr, wurden nach dem Zufallsprinzip in zwei Gruppen aufgeteilt. Alle Studenten hatten Erfahrung mit Modellzähnen. Die erste Gruppe trainierte an vier Standard-Modellzähnen. Die zweite Gruppe verwendete Modellzähne für den ersten und vierten Versuch und gedruckte Zähne für den zweiten und dritten Versuch. Die Präparationen der Studenten wurden mit einem In-Lab-Scanner gescannt und die Oberflächenabweichungen im Gegensatz zu einer perfekten Präparation gemessen. Die Unterschiede zwischen dem ersten und vierten Versuch wurden berechnet. Der Nutzen des gedruckten Zahnes wurde durch einen Fragebogen mit deutschen Schulnoten von den Studierenden bewertet (1 = Ausgezeichnet, 2 = Gut, 3 = Befriedigend, 4 = Ausreichend, 5 = Schlecht, 6 = Unbefriedigend). Ergebnisse: Der Arbeitsablauf war praktikabel und kostengünstig in der Herstellung der der gedruckten Zähne. Die Gesamtbewertung des gedruckten Zahns im Fragebogen war gut (Ø 2,1 ± 0,22). Die Studenten berichteten verschiedene Vorteile dieser Methode im Freitext. Der Vergleich der Präparation zwischen dem ersten und vierten Versuch zeigte, dass mit den gedruckten Zähnen eine signifikant bessere Präparation erreicht wurde. Die vollständige Präparation hatte Medianwerte von 0,05 mm (Gruppe1: Standardmodellzahn) und -0,03 mm (Gruppe2: gedruckter Zahn) (P = .005). Aufgeteilt in einzelne Flächen waren die vestibulären und okklusalen Bereiche signifikant besser. Für die vestibuläre Fläche ergaben sich folgende Werte 0,11 mm (Gruppe1) und -0,04 mm (Gruppe2) (P = .018). Für die Okklusalfläche ergab sich eine Abweichung von 0,13 mm (Gruppe1) und -0,05 mm (Gruppe2) (P = .009). Schlussfolgerungen: Das Ziel dieser Studie wurde erfüllt. Der gedruckte Zahn wurde erfolgreich in einem vorklinischen Kurs getestet. Die Machbarkeit dieses Lehrkonzepts wurde durch den Fragebogen und die Analyse der Präparationsform bestätigt. Ein signifikanter Unterschied zu einem Standard-Modellzahn war messbar. Die Studenten hatten die Möglichkeit eine korrekte Kronenpräparation an einem standardisierten zweischichtigen Zahn mit eingebauter Präparation zu üben. Dieser gedruckte Zahn ermöglichte es den Studenten die Kronenpräparation selbst zu kontrollieren.
In order to mimic the extracellular matrix for tissue engineering, recent research approaches often involve 3D printing or electrospinning of fibres to scaffolds as cell carrier material. Within this thesis, a micron fibre printing process, called melt electrospinning writing (MEW), combining both additive manufacturing and electrospinning, has been investigated and improved. Thus, a unique device was developed for accurate process control and manufacturing of high quality constructs. Thereby, different studies could be conducted in order to understand the electrohydrodynamic printing behaviour of different medically relevant thermoplastics as well as to characterise the influence of MEW on the resulting scaffold performance.
For reproducible scaffold printing, a commonly occurring processing instability was investigated and defined as pulsing, or in extreme cases as long beading. Here, processing analysis could be performed with the aim to overcome those instabilities and prevent the resulting manufacturing issues. Two different biocompatible polymers were utilised for this study: poly(ε-caprolactone) (PCL) as the only material available for MEW until then and poly(2-ethyl-2-oxazoline) for the first time. A hypothesis including the dependency of pulsing regarding involved mass flows regulated by the feeding pressure and the electrical field strength could be presented. Further, a guide via fibre diameter quantification was established to assess and accomplish high quality printing of scaffolds for subsequent research tasks.
By following a combined approach including small sized spinnerets, small flow rates and high field strengths, PCL fibres with submicron-sized fibre diameters (fØ = 817 ± 165 nm) were deposited to defined scaffolds. The resulting material characteristics could be investigated regarding molecular orientation and morphological aspects. Thereby, an alignment and isotropic crystallinity was observed that can be attributed to the distinct acceleration of the solidifying jet in the electrical field and by the collector uptake. Resulting submicron fibres formed accurate but mechanically sensitive structures requiring further preparation for a suitable use in cell biology. To overcome this handling issue, a coating procedure, by using hydrophilic and cross-linkable star-shaped molecules for preparing fibre adhesive but cell repellent collector surfaces, was used.
Printing PCL fibre patterns below the critical translation speed (CTS) revealed the opportunity to manufacture sinusoidal shaped fibres analogously to those observed using purely viscous fluids falling on a moving belt. No significant influence of the high voltage field during MEW processing could be observed on the buckling phenomenon. A study on the sinusoidal geometry revealed increasing peak-to-peak values and decreasing wavelengths as a function of decreasing collector speeds sc between CTS > sc ≥ 2/3 CTS independent of feeding pressures. Resulting scaffolds printed at 100 %, 90 %, 80 % and 70 % of CTS exhibited significantly different tensile properties, foremost regarding Young’s moduli (E = 42 ± 7 MPa to 173 ± 22 MPa at 1 – 3 % strain). As known from literature, a changed morphology and mechanical environment can impact cell performance substantially leading to a new opportunity of tailoring TE scaffolds.
Further, poly(L-lactide-co-ε-caprolactone-co-acryloyl carbonate) as well as poly(ε-caprolactone-co-acryloyl carbonate) (PCLAC) copolymers could be used for MEW printing. Those exhibit the opportunity for UV-initiated radical cross-linking in a post-processing step leading to significantly increased mechanical characteristics. Here, single fibres of the polymer composed of 90 mol.% CL and 10 mol.% AC showed a considerable maximum tensile strength of σmax = 53 ± 16 MPa. Furthermore, sinusoidal meanders made of PCLAC yielded a specific tensile stress-strain characteristic mimicking the qualitative behaviour of tendons or ligaments. Cell viability by L929 murine fibroblasts and live/dead staining with human mesenchymal stem cells revealed a promising biomaterial behaviour pointing out MEW printed PCLAC scaffolds as promising choice for medical repair of load-bearing soft tissue.
Indeed, one apparent drawback, the small throughput similar to other AM methods, may still prevent MEW’s industrial application yet. However, ongoing research focusses on enlargement of manufacturing speed with the clear perspective of relevant improvement. Thereby, the utilisation of large spinneret sizes may enable printing of high volume rates, while downsizing the resulting fibre diameter via electrical field and mechanical stretching by the collector uptake. Using this approach, limitations of FDM by small nozzle sizes could be overcome. Thinking visionary, such printing devices could be placed in hospitals for patient-specific printing-on-demand therapies one day. Taking the evolved high deposition precision combined with the unique small fibre diameter sizes into account, technical processing of high performance membranes, filters or functional surface finishes also stands to reason.