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The aim of this thesis was the preparation of a biomaterial ink for the fabrication of chemically crosslinked hydrogel scaffolds with low micron sized features using melt electrowriting (MEW). By developing a functional polymeric material based on 2-alkyl-2-oxazine (Ozi) and 2-alkyl-2-oxazoline (Ox) homo- and copolymers in combination with Diels-Alder (DA)-based dynamic covalent chemistry, it was possible to achieve this goal. This marks an important step for the additive manufacturing technique melt electrowriting (MEW), as soft and hydrophilic structures become available for the first time. The use of dynamic covalent chemistry is a very elegant and efficient method for consolidating covalent crosslinking with melt processing. It was shown that the high chemical versatility of the Ox and Ozi chemistry offers great potential to control the processing parameters. The established platform offers straight forward potential for modification with biological cues and fluorescent markers. This is essential for advanced biological applications. The physical properties of the material are readily controlled and the potential for 4D-printing was highlighted as well. The developed hydrogel architectures are excellent candidates for 3D cell culture applications. In particular, the low internal strength of some of the scaffolds in combination with the tendency of such constructs to collapse into thin strings could be interesting for the cultivation of muscle or nerve cells. In this context it was also possible to show that MEW printed hydrogel scaffolds can withstand the aspiration and ejection through a cannula. This allows the application as scaffolds for the minimally invasive delivery of implants or functional tissue equivalent structures to various locations in the human body.
Miniaturized satellites on a nanosatellite scale below 10kg of total mass contribute most to the number of launched satellites into Low Earth Orbit today. This results from the potential to design, integrate and launch these space missions within months at very low costs. In the past decade, the reliability in the fields of system design, communication, and attitude control have matured to allow for competitive applications in Earth observation, communication services, and science missions. The capability of orbit control is an important next step in this development, enabling operators to adjust orbits according to current mission needs and small satellite formation flight, which promotes new measurements in various fields of space science. Moreover, this ability makes missions with altitudes above the ISS comply with planned regulations regarding collision avoidance maneuvering.
This dissertation presents the successful implementation of orbit control capabilities on the pico-satellite class for the first time. This pioneering achievement is demonstrated on the 1U CubeSat UWE–4. A focus is on the integration and operation of an electric propulsion system on miniaturized satellites. Besides limitations in size, mass, and power of a pico-satellite, the choice of a suitable electric propulsion system was driven by electromagnetic cleanliness and the use as a combined attitude and orbit control system. Moreover, the integration of the propulsion system leaves the valuable space at the outer faces of the CubeSat structure unoccupied for future use by payloads. The used NanoFEEP propulsion system consists of four thruster heads, two neutralizers and two Power Processing Units (PPUs).
The thrusters can be used continuously for 50 minutes per orbit after the liquefaction of the propellant by dedicated heaters. The power consumption of a PPU with one activated thruster, its heater and a neutralizer at emitter current levels of 30-60μA or thrust levels of 2.6-5.5μN, respectively, is in the range of 430-1050mW. Two thruster heads were activated within the scope of in-orbit experiments. The thrust direction was determined using a novel algorithm within 15.7° and 13.2° of the mounting direction. Despite limited controllability of the remaining thrusters, thrust vector pointing was achieved using the magnetic actuators of the Attitude and Orbit Control System.
In mid 2020, several orbit control maneuvers changed the altitude of UWE–4, a first for pico-satellites. During the orbit lowering scenario with a duration of ten days, a single thruster head was activated in 78 orbits for 5:40 minutes per orbit. This resulted in a reduction of the orbit altitude by about 98.3m and applied a Delta v of 5.4cm/s to UWE–4. The same thruster was activated in another experiment during 44 orbits within five days for an average duration of 7:00 minutes per orbit. The altitude of UWE–4 was increased by about 81.2m and a Delta v of 4.4cm/s was applied. Additionally, a collision avoidance maneuver was executed in July 2020, which increased the distance of closest approach to the object by more than 5000m.
Remote sensing time series is the collection or acquisition of remote sensing data in a
fixed equally spaced time period over a particular area or for the whole world. Near
daily high spatial resolution data is very much needed for remote sensing applications
such as agriculture monitoring, phenology change detection, environmental
monitoring and so on. Remote sensing applications can produce better and accurate
results if they are provided with dense and accurate time series of data. The current
remote sensing satellite architecture is still not capable of providing near daily
or daily high spatial resolution images to fulfill the needs of the above mentioned
remote sensing applications. Limitations in sensors, high development, operational
costs of satellites and presence of clouds blocking the area of observation are some
of the reasons that makes near daily or daily high spatial resolution optical remote
sensing data highly challenging to achieve. With developments in the optical sensor
systems and well planned remote sensing satellite constellations, this condition
can be improved but it comes at a cost. Even then the issue will not be completely
resolved and thus the growing need for high temporal and high spatial resolution
data cannot be fulfilled entirely. Because the data collection process relies on satellites
which are physical system, these can fail unpredictably due to various reasons
and cause a complete loss of observation for a given period of time making a gap
in the time series. Moreover, to observe the long term trend in phenology change
due to rapidly changing environmental conditions, the remote sensing data from
the present is not just sufficient, the data from the past is also important. A better
alternative solution for this issue can be the generation of remote sensing time series
by fusing data from multiple remote sensing satellite which has different spatial and
temporal resolutions. This approach will be effective and efficient. In this method
a high temporal low spatial resolution image from a satellite such as Sentinel-2 can
be fused with a low temporal and high spatial resolution image from a satellite such
as the Sentinel-3 to generate a synthetic high temporal high spatial resolution data.
Remote sensing time series generation by data fusion methods can be applied to
the satellite images captured currently as well as the images captured by the satellites
in the past. This will provide the much needed high temporal and high spatial
resolution images for remote sensing applications. This approach with its simplistic
nature is cost effective and provides the researchers the means to generate the
data needed for their application on their own from the limited source of data available
to them. An efficient data fusion approach in combination with a well planned
satellite constellation can offer a solution which will ensure near daily time series of
remote sensing data with out any gap. The aim of this research work is to develop
an efficient data fusion approaches to achieve dense remote sensing time series.
After examining suitable parameters for a newly designed system, dynamic SIPGP could be developed. For the first time, SIPGP was performed while applying a constant flow of monomer solution through the reaction system. This added a new parameter: the flow rate (rfl). Accordingly, this parameter was examined, comparing dynamic to static SIPGP. It could be shown, that by applying higher rfl to the system, the contact angle increases, which indicates a slower coating. The flow patterns inside the reactor were then modelled and calculated. These calculations indicated, that, due to higher flow velocities, the contact angle on the coated samples would be lower on the sides of the sample and higher in the middle. This finding was verified by contact angle measurements. The influence of dynamic SIPGP on the temperature inside the reaction chamber during the reaction was examined by temperature sensors inside the reactor. This showed, that the constant flow of monomer solution can be utilized to decrease the warming of the reaction solution during the reaction. Finally it was shown, that dynamic SIPGP can decrease the formation of bulk polymer on the sample, which is forming during the reaction. This enables SIPGP to fabricate more homogeneous coatings by applying a constant monomer flow.
This thesis encompasses the development of the additive manufacturing technology melt electrowriting, in order to achieve the improved applicability in biomedical applications and design of scaffolds. Melt electrowriting is a process capable of producing highly resolved structures from microscale fibres. Nevertheless, there are parameters influencing the process and it has not been clear how they affect the printing result. In this thesis the influence of the processing and environmental parameters is investigated with the impact on their effect on the jet speed, fibre diameter and scaffold morphology, which has not been reported in the literature to date and significantly influences the printing quality. It was demonstrated that at higher ambient printing temperatures the fibres can be hampered to the extent that the individual fibres are completely molten together and increased air humidity intensifies this effect. It was also shown how such parameters as applied voltage, collector distance, feed pressure and polymer temperature influence the fibre diameter and critical translation speed. Based on these results, a detailed investigation of the fibre diameter control and printing of scaffolds with novel architectures was made. As an example, a 20-fold diameter ratio is obtained within one scaffold by changing the collector speed and the feed pressure during the printing process. Although the pressure change caused fibre diameter oscillations, different diameter fibres were successfully integrated into two scaffold designs, which were tested for mesenchymal stromal cell suspension and adipose tissue spheroid seeding. Further design and manufacturing aspects are discussed while jet attraction to the printed structures is illuminated in connection with the fibre positioning control of the multilayer scaffolds. The artefacts that appear with the increasing scaffold height of sinusoidal laydown patterns are counteracted by layer-by-layer path adjustment. For the prediction of a printing error of the first deposited layer, an algorithm is developed, that utilizes an empirical jet lag equation and the speed of fibre deposition. This model was able to predict the position of the printing fibre with up to ten times smaller error than the of the programmed path. The same model allows to qualitatively assess the fibre diameter change along the nonlinear pattern as well as to indicate the areas of the greatest pattern deformation with the growing scaffold height. Those results will be used in the later chapters for printing of the novel MEW structures for biomedical applications. In the final chapter the concept of multimodal scaffold was combined with the suspended fibre printing, for the manufacturing of the MEW scaffolds with controlled pore interconnectivity in three dimensions. Those scaffolds were proven to be a promising substate for the control of the neurite spreading of the chick DRG neurons.