Institut für Funktionsmaterialien und Biofabrikation
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Melt electrowriting, a high-resolution additive manufacturing technique, is used in this study to process a magnetic polymer-based blend for the first time. Carbonyl iron (CI) particles homogenously distribute into poly(vinylidene fluoride) (PVDF) melts to result in well-defined, highly porous structures or scaffolds comprised of fibers ranging from 30 to 50 µm in diameter. This study observes that CI particle incorporation is possible up to 30 wt% without nozzle clogging, albeit that the highest concentration results in heterogeneous fiber morphologies. In contrast, the direct writing of homogeneous PVDF fibers with up to 15 wt% CI is possible. The fibers can be readily displaced using magnets at concentrations of 1 wt% and above. Combined with good viability of L929 CC1 cells using Live/Dead imaging on scaffolds for all CI concentrations indicates that these formulations have potential for the usage in stimuli-responsive applications such as 4D printing.
The development of novel fibrous biomaterials and further processing of medical devices is still challenging. For instance, titanium(IV) oxide is a well-established biocompatible material, and the synthesis of TiO\(_x\) particles and coatings via the sol-gel process has frequently been published. However, synthesis protocols of sol-gel-derived TiO\(_x\) fibers are hardly known. In this publication, the authors present a synthesis and fabrication of purely sol-gel-derived TiO\(_x\) fiber fleeces starting from the liquid sol-gel precursor titanium ethylate (TEOT). Here, the α-hydroxy-carboxylic acid lactic acid (LA) was used as a chelating ligand to reduce the reactivity towards hydrolysis of TEOT enabling a spinnable sol. The resulting fibers were processed into a non-woven fleece, characterized with FTIR, \(^{13}\)C-MAS-NMR, XRD, and screened with regard to their stability in physiological solution. They revealed an unexpected dependency between the LA content and the dissolution behavior. Finally, in vitro cell culture experiments proved their potential suitability as an open-mesh structured scaffold material, even for challenging applications such as therapeutic medicinal products (ATMPs).
As one kind of “smart” material, thermogelling polymers find applications in biofabrication, drug delivery and regenerative medicine. In this work, we report a thermosensitive poly(2-oxazoline)/poly(2-oxazine) based diblock copolymer comprising thermosensitive/moderately hydrophobic poly(2-N-propyl-2-oxazine) (pPrOzi) and thermosensitive/moderately hydrophilic poly(2-ethyl-2-oxazoline) (pEtOx). Hydrogels were only formed when block length exceeded certain length (≈100 repeat units). The tube inversion and rheological tests showed that the material has then a reversible sol-gel transition above 25 wt.% concentration. Rheological tests further revealed a gel strength around 3 kPa, high shear thinning property and rapid shear recovery after stress, which are highly desirable properties for extrusion based three-dimensional (3D) (bio) printing. Attributed to the rheology profile, well resolved printability and high stackability (with added laponite) was also possible. (Cryo) scanning electron microscopy exhibited a highly porous, interconnected, 3D network. The sol-state at lower temperatures (in ice bath) facilitated the homogeneous distribution of (fluorescently labelled) human adipose derived stem cells (hADSCs) in the hydrogel matrix. Post-printing live/dead assays revealed that the hADSCs encapsulated within the hydrogel remained viable (≈97%). This thermoreversible and (bio) printable hydrogel demonstrated promising properties for use in tissue engineering applications.
Silk fibroin is commonly used as scaffold material for tissue engineering applications. In combination with a mineralization with different calcium phosphate phases, it can also be applied as material for bone regeneration. Here, we present a study which was performed to produce mineralized silk fibroin scaffolds with controlled macroporosity. In contrast to former studies, our approach focused on a simultaneous gelation and mineralization of silk fibroin by immersion of frozen silk fibroin monoliths in acidic calcium phosphate solutions. This was achieved by thawing frozen silk fibroin monoliths in acidic calcium phosphate solution, leading to the precipitation of monocalcium phosphate within the silk fibroin matrix. In the second approach, a conversion of incorporated -tricalcium phosphate particles into brushite was successfully achieved. Furthermore, a controlled cryostructuring process of silk fibroin scaffolds was carried out leading to the formation of parallel-oriented pores with diameters of 30-50 mu m.
Für die Fügung der Interkonnektoren einer Hochtemperaturbrennstoffzelle wurden in der hier vorliegenden Arbeit glaskeramische Lote entwickelt und untersucht. Es konnte ein hochviskoses Glas
gefunden werden, das trotz fehlendem Erweichen bei der Fügung eine stabile, gasdichte und elektrisch isolierende glaskeramische Fügung ausbildet. Auch während des Betriebs kommt es zu keinem Erweichen der Fügung. Weiter treten keine feststellbaren Reaktionen mit den potentiellen Reaktionspartnern, den Stahlelementen, auf. Es konnte eine Korrelation dieses Reaktionsverhaltens
mit dem Kristallisationsverhalten der Glaskeramik gefunden werden. Das Verhalten des Glaslotes
wurde über mehrere tausend Stunden unter Betriebsbedingungen beziehungsweise betriebsimulierenden Bedingungen untersucht. Dabei konnte die Kristallisationsentwicklung beschrieben werden.
Ein weiterer Aspekt der Arbeit war die Untersuchung des Einflusses der einzelnen Faktoren, denen
ein Glaslot während seines Einsatzes von der Fügung bis zum Betrieb ausgesetzt ist, wie die
Fügetemperatur, die Viskosität der eingesetzten glasbildenden Schmelze oder die Dualgasatmosphäre im Betrieb, auf das Gefüge und die Diffusion.
Hierbei konnte gezeigt werden, dass die Fügetemperatur mit Abstand den größten Einfluss auf die
Stabilität der Glaslotschicht hat. Diese bedingt nicht nur die Kinetik des Fließens und die Benetzung
des Stahls durch das Glas, sondern vor allem, welche Kristallphasen gebildet werden und
wie das finale Gefüge im Hinblick auf Kristallitgröße und –verteilung aussieht. So kommt es bei
höheren Temperaturen zu einem größeren Restglasphasenanteil und einem geringeren Kristallitanteil, was wiederum die Diffusion der Stahlelemente in die Glaslotschicht begünstigt.
Diese Doktorarbeit beschäftigt sich mit dem Wirkmechanismus der elektrischen Leitfähigkeit in Blei-Säure-Batterien. Obwohl ihm eine zentrale Rolle beim „Kohlenstoff-Effekt“ zugeordnet wird, ist der Wirkmechanismus der elektrischen Leitfähigkeit bislang vergleichsweise wenig untersucht worden und konnte dementsprechend noch nicht vollständig aufgeklärt werden. Mit dem Anspruch, diese Forschungslücke zu schließen, zielt die vorliegende Doktorarbeit darauf ab, den Einfluss der elektrischen Leitfähigkeit auf die Performance der Blei-Säure-Batterie systematisch herauszuarbeiten und so einen Beitrag zur Generierung neuer Entwicklungsansätze zu leisten, z. B. in Form von maßgeschneiderten Additiven. Bislang ist noch unklar, ob allein die elektrische Leitfähigkeit des Aktivmaterials relevant ist oder diese auch durch Additive beeinflusst wird. Das liegt vor allem daran, dass geeignete Messmethoden fehlen und deshalb der Einfluss von Additiven auf die elektrische Leitfähigkeit des Aktivmaterials wenig untersucht wurde. Deswegen zielt diese Arbeit auch darauf ab, eine neuartige Messmethode zu entwickeln, um die elektrische Leitfähigkeit des Aktivmaterials im laufenden Betrieb bestimmen zu können. Aufgrund der Vorkenntnisse und Vorarbeiten am Fraunhofer ISC werden die Untersuchungen dabei auf die negative Elektrode limitiert. Insgesamt unterteilt sich die Doktorarbeit in die zwei Abschnitte.
Im ersten Abschnitt werden elektrisch isolierende Stöber-Silica als Additive im negativen Aktivmaterial eingesetzt, um den Einfluss der elektrischen Leitfähigkeit des Additivs auf die elektrochemischen Eigenschaften der Batterie herauszustellen. Untersucht wird dabei die u.a. die Doppelschichtkapazität, die Wasserstoffentwicklung und die dynamische Ladeakzeptanz.
Im zweiten Abschnitt steht die elektrische Leitfähigkeit des negativen Aktivmaterials im Fokus. Es wird zunächst eine neue Messmethodik entwickelt, die ihre in-situ- und operando-Bestimmung ermöglicht. Nach einer umfassenden Evaluierung und der Betrachtung verschiedener Betriebsparameter wird die Methodik für eine erste proof-of-concept-Messreihe angewendet, um den Einfluss von Additiven auf die elektrische Leitfähigkeit des negativen Aktivmaterials zu untersuchen.
In the past decade, poly(2-oxazoline)s (POx) and very recently poly(2-oxazine)s (POzi) based amphiphiles have shown great potential for medical applications. Therefore, the major aim of this thesis was to further explore the pharmaceutical and biomedical applications of POx/POzi based ABA triblock and AB diblock copolymers, respectively with the special emphasis on structure property relationship (SPR). ABA triblock copolymers (with shorter side chain length in the hydrophobic block) have shown high solubilizing capacity for hydrophobic drugs. The issue of poor aqueous solubility was initially addressed by developing a (micellar) formulation library of 21 highly diverse, hydrophobic drugs with POx/POzi based ABA triblock copolymers. Theoretically, the extent of compatibility between polymers and drug was determined by calculating solubility parameters (SPs). The SPs were thoroughly investigated to check their applicability in present systems. The selected formulations were further characterized by various physico-chemical techniques. For the biomedical applications, a novel thermoresposive diblock copolymer was synthesized which has shown promising properties to be used as hydrogel bioink or can potentially be used as fugitive support material. The most important aspect i.e. SPR, was studied with respect to hydrophilic block in either tri- or di-block copolymers. In triblock copolymer, the hydrophilic block played an important role for ultra high drug loading, while in case of diblock, it has improved the printability of the hydrogels. Apart from the basic research, the therapeutic applications of two formulations i.e. mitotane (commercially available as tablet dosage form for adrenocortical carcinoma) and BT-44 (lead compound for nerve regeneration) were studied in more detail.
Adrenocortical carcinoma (ACC) is a malignant tumor originating from the adrenal gland cortex with a heterogeneous but overall dismal prognosis in advanced stages. For more than 50 years, mitotane has remained a cornerstone for the treatment of ACC as adjuvant and palliative therapy. It has a very poor aqueous solubility of 0.1 mg/l and high partition coefficient in octanol/water (log P) value of 6. The commercially available dosage form is 500 mg tablets (Lysodren®). Even at doses up to 6 g/day (12 tablets in divided doses) for several months, > 50% patients do not achieve therapeutic plasma concentration > 14 mg/l due to poor water solubility, large volume of distribution and inter/intra-individual variability in bioavailability. This article aims to give a concise update of the clinical challenges associated with the administration of high-dose mitotane oral therapy which encompass the issues of poor bioavailability, difficult-to-predict pharmacokinetics and associated adverse events. Moreover, we present recent efforts to improve mitotane formulations. Their success has been limited, and we therefore propose an injectable mitotane formulation instead of oral administration, which could bypass many of the main issues associated with high-dose oral mitotane therapy. A parenteral administration of mitotane could not only help to alleviate the adverse effects but also circumvent the variable oral absorption, give better control over therapeutic plasma mitotane concentration and potentially shorten the time to achieve therapeutic drug plasma concentrations considerably.
Mitotane as tablet form is currently the standard treatment for adrenocortical carcinoma. It has been used for 5 decades but suffers from highly variable responses in patients, subsequent adverse effects and overall lower response rate. This can be fundamentally linked to the exceedingly poor water solubility of mitotane itself. In terms of enhancing water solubility, a few research groups have attempted to develop better formulations of mitotane to overcome the issues associated with tablet dosage form. However, the success rate was limited, and these formulations did not make it into the clinics. In this article, we have comprehensively reviewed the properties of these formulations and discuss the reasons for their limited utility. Furthermore, we discuss a recently developed mitotane nanoformulation that led us to propose a novel approach to mitotane therapy, where intravenous delivery supplements the standard oral administration. With this article, we combine the current state of knowledge as a single piece of information about the various problems associated with the use of mitotane tablets, and herein we postulate the development of a new injectable mitotane formulation, which can potentially circumvent the major problems associated to mitotane's poor water solubility.
Motivated by the great potential offered by the combination of additive manufacturing technology and hydrogels, especially in the field of tissue engineering and regenerative medicine, a series of novel hybrid hydrogel inks were developed based on the recently described thermogelling poly(2-oxazoline)s-block-poly(2-oxazine)s diblock copolymers, which may help to expand the platform of available hydrogel inks for this transformative 3D printing technology (Fig. 5.1).
In the present thesis, the first reported thermogelling polymer solely consisting of POx and POzi, i.e., the diblock copolymer PMeOx-b-PnPrOzi comprising a hydrophilic block (PMeOx) and a thermoresponsive block (PnPrOzi), was selected and used as a proof-of-concept for the preparation of three novel hybrid hydrogels. Therefore, three batches of the diblock copolymers with a DP of 100 were synthesized for the study of three different hybrid hydrogels with a special focus on their suitability as (bio)inks for extrusion-based 3D printing. The PMeOx-b-PnPrOzi diblock copolymer solution shows a temperature induced reversible gelation behavior above a critical polymer concentration of 20 wt%, as described for the Pluronic F127 solution but with a unique gelation mechanism, working through the formation of a bicontinuous sponge-like structure from the physically crosslinked vesicles. Specially, its intrinsic shear thinning behavior and excellent recovery property with a certain yield point make it a promising ink candidate for extrusion-based printing technology.
Increasing the polymer concentration is the most traditional approach to improve the printability of an ink material, and serve as the major strategy available to improve the printability of PMeOx-b-PnPrOzi systems prior to this work. From the analysis of rheological properties related to printability, it came a conclusion that increasing the copolymer concentration does improve the hydrogel strength and thus the printability. However, such improvement is very limited and usually leads to other problems such as more viscous systems and stringent requirements on the printers, which are not ideal for the printing process and applications especially in the cell-embedded biofabrication field.
POx-b-POzi/clay Hybrid Hydrogel
An alternative method proposed to improve the printability of this thermoresponsive hydrogel ink is through nanoclay (Laponite XLG) addition, i.e., the first hybrid hydrogel system of PMeOx-b-PnPrOzi/clay (also named shortly as POx-b-POzi/clay) in this thesis. To optimize the viscoelastic properties of the ink material, Laponite XLG acted as a reinforcement additive and a physically crosslinker was blended with the copolymers. Compared with the pristine copolymer solution of PMeOx-b-PnPrOzi, the hybrid PMeOx-b-PnPrOzi/clay solution well retained the temperature induced gelation performance of the copolymers.
The obtained hybrid hydrogels exhibited a rapid in situ reversible thermogelation at a physiological relevant Tgel of around 15 ℃ and a rapid recovery of viscoelastic properties within a few seconds. More importantly, with the addition of only a small amount of 1.2 wt% clay, it exhibited obviously enhanced shear thinning character (n = 0.02), yield stress (240 Pa) and mechanical strength (storage modulus over 5 kPa). With this novel hybrid hydrogel, real three-dimensional constructs with multiple layers and various geometries are generation with greatly enhanced shape fidelity and resolution. In this context, the thermogelling properties of the hybrid hydrogels over a copolymer concentration range of 10-20 wt% and a clay concentration of 0-4 wt% were systematically investigated, and from which a printable window was obtained from the laboratory as a reference.
In fact, the printing performance of an ink is not only determined by the intrinsic physicochemical properties of the material, but is also influenced by the external printing environments as well as the printer parameter settings. All the printing experiments in this study were conducted under a relatively optimized conditions obtained from preliminary experiments. In future work, the relationship between material rheology properties, printer parameters and printing performance could be systematically explored. Such a fundamental study will help to develop models that allows the prediction and comparison of printing results from different researches based on the parameters available through rheology, which is very beneficial for further development of more advanced ink systems.
Although the printability has been significantly improved by the addition of nanoclay Laponite XLG, the hybrid hydrogels and their printed constructs still suffer from some major limitations. For example, these materials are still thermoresponsive, which will cause the printed constructs to collapse when the environment temperature changes below their Tgel. In addition, the formed hydrogel constructs are mechanical too weak for load-bearing applications, and the allowed incubation time is very limited during media exchange/addition as it will lead to dissolution of the hydrogels due to dilution effects. Therefore, it is essential to establish a second (chemical or physical) crosslinking mechanism that allows further solidification of the gels after printing. It should be kept in mind that the second crosslinking step will eliminate the thermoresponsive behavior of the gels and thus the possibility of cell recovery. In this case, besides through the traditional approach of copolymer modification to realize further crosslinking, like one of the well-known post-polymerization modification approach Diels-Alder reaction,[430] designing of interpenetrating networks (IPN) hydrogels serves as one of the major strategy for advanced (bio)ink preparation.[311] Therefore, the second hybrid hydrogel system of PMeOx-b-PnPrOzi/PDMAA/clay (also named shortly as POx-b-POzi/PDMAA/clay) was developed in this thesis, which is a 3D printable and highly stretchable ternary organic-inorganic IPN hydrogel.
POx-b-POzi/PDMAA/clay Hybrid Hydrogel
The nanocomposite IPN hydrogel combines a thermoresponsive hydrogel with clay described above and in situ polymerized poly(N, N-dimethylacrylamide). Before in situ polymerization, the thermoresponsive hydrogel precursors exhibited thermogelling behavior (Tgel ~ 25 ℃, G' ~ 6 kPa) and shear thinning properties, making the system well-suited for extrusion-based 3D printing. After chemical curing of the 3D-printed constructs by free radical polymerization, the resulting IPN hydrogels show excellent mechanical strength with a high stretchability to a tensile strain at break exceeding 550%. The hybrid hydrogel can sustain a high stretching deformation and recover quickly due to the energy dissipation from the non-covalent interactions. With this hybrid hydrogel, integrating with the advanced 3D-printing technique, various 3D constructs can be printed and cured successfully with high shape fidelity and geometric accuracy.
In this context, we also investigated the possibility of acrylic acid (AA) and 2-hydroxyethylmethacrylate (HEMA) as alternative hydrogel precursors. However, the addition of these two monomers affected the thermogelation of POx-b-POzi in an unfavorable manner, as these monomers competed more effectively with water molecules, preventing the hydration of nPrOzi block at lower temperatures and therefore, the liquefaction of the gels. Furthermore, the influence of the printing process and direction on the mechanical properties of the hydrogel was investigated and compared with the corresponding bulk materials obtained from a mold. No significant effects from the additive manufacturing process were observed due to a homogeneously adhesion and merging between sequentially deposited layers. In the future, further studies on the specific performance differences among hydrogels fabricated at different printing directions/speeds would be of great interest to the community, as this allows for a more accurately control and better predict of the printed structures.
This newly developed hybrid IPN hydrogel is expected to expand the material toolbox available for hydrogel-based 3D printing, and may be interesting for a wide range of applications including tissue engineering, drug delivery, soft robotics, and additive manufacturing in general. However, in this case, the low toxicity from the monomer DMAA and other small molecules residuals in the polymerized hydrogels made this hybrid hydrogel not ideal for bioprinting in the field of biofabrication. For this problem, cyto-/biocompatible monomers such as polyethylene glycol diacrylate (PEGDA) can be used as an alternative, while the overall properties of the hydrogels including mechanical properties should be re-evaluated accordingly. Moreover, the swelling behavior of the hydrogels should also be taken into account, as it may most likely affect the mechanical strength and geometry size of the printed scaffold, but is often be overlooked after printing. For example, regarding the specific hybrid hydrogel POx-b-POzi/PDMAA/clay in this work, an equilibrium swelling ratio of 1100% was determined. The printed hydrogel cuboid experienced a volume increasing over 6-fold after equilibrium swelling in water, and became mechanical fragile due to the formation of a swollen hydrogel network absorbing large amount of water.
POx-b-POzi/Alg/clay Hybrid Hydrogel
In the final part of this dissertation, to enable the cell-loaded bioprinting and long-term cell culture, the third hybrid hydrogel system POx-b-POzi/Alg/clay was introduced by replacing the monomer DMAA to the natural polysaccharides alginate. Initially, detailed rheological characterization and mechanical tests were performed to evaluate their printability and mechanically properties. Subsequently, some simple patterns were printed with the optimized hydrogel precursor solutions for the preliminary filament fusion and collapse test before proceeding to more complex printings. The fibers showed a sufficient stability which allows the creation of large structures with a height of a few centimeters and a suspended filament up to centimeter. Accordingly, various 3D constructs including suspended filaments were printed successfully with high stackability and shape fidelity. The structure after extrusion was physical crosslinked easily by soaking in CaCl2 solution and, thereafter exhibited a good mechanical flexibility and long-term stability. Interestingly, the mechanical strength and geometry size of the generated scaffolds were well maintained over a culture period of weeks in water, which is of great importance for clinical applications. In addition, the post-printing ionic crosslinking of alginate could also be realized by other di/trivalent cations such as Fe3+ and Tb3+.
Subsequently, the cell-laden printing with this hybrid hydrogel and post-printing crosslinking by Ca2+ ions highlighting its feasibility for 3D bioprinting. WST-1 assay of fibroblast suggested no-dose dependent cytocompatibility of the hydrogel precursor solution. The cell distribution was uniform throughout the printed construct, and proliferated with high cell viability during the 21 days culture. The presented hybrid approach, utilizing the beneficial properties of the POx-b-POzi base material, could be interesting for a wide range of bioprinting applications and potentially enabling also other biological bioinks such as collagen, hyaluronic acid, decellularized extracellular matrix or cellulose based bioinks. Although the results look promising and the developed hydrogel is an important bioink candidate, the long-term in vitro cell studies with different cell lines and clinical model establishment are still under investigation, which remains a long road but is of great importance before realizing real clinical application.
Last but not least, the improvement to the printability of thermogelling POx/POzi-based copolymers by the clay Laponite XLG was also demonstrated in another thermogelling copolymer PEtOx-b-PnPrOzi. This suggests that the addition of clay may be a general strategy to improve the printability of such polymers. Despite these advances in this work which significantly extended the (bio)material platform of additive manufacturing technology, the competition is still fierce and more work should be done in the further to reveal the potential and limitations of this kind of new and promising candidate (bio)ink materials. It is also highly expected for further creative works based on the thermogelling POx/POzi polymers, such as crosslinking in Ca2+ solution containing monomer acrylamide to prepare printable and mechanically tough hydrogels, research on POx-based support bath material, and print of clinically more relevant sophisticated structures such as 3D microvascular networks omnidirectionally.
Among external stimuli used to trigger release of a drug from a polymeric carrier, ultrasound has gained increasing attention due to its non-invasive nature, safety and low cost. Despite this attention, there is only limited knowledge about how materials available for the preparation of drug carriers respond to ultrasound. This study investigates the effect of ultrasound on the release of a hydrophobic drug, dexamethasone, from poly(2-oxazoline)-based micelles. Spontaneous and ultrasound-mediated release of dexamethasone from five types of micelles made of poly(2-oxazoline) block copolymers, composed of hydrophilic poly(2-methyl-2-oxazoline) and hydrophobic poly(2-n-propyl-2-oxazoline) or poly(2-butyl-2-oxazoline-co-2-(3-butenyl)-2-oxazoline), was studied. The release profiles were fitted by zeroorder and Ritger-Peppas models. The ultrasound increased the amount of released dexamethasone by 6% to 105% depending on the type of copolymer, the amount of loaded dexamethasone, and the stimulation time point. This study investigates for the first time the interaction between different poly(2-oxazoline)-based micelle formulations and ultrasound waves, quantifying the efficacy of such stimulation in modulating dexamethasone release from these nanocarriers.