@article{FlachsEmmerichRothetal.2019, author = {Flachs, Dennis and Emmerich, Florian and Roth, Gian-Luca and Hellmann, Ralf and Thielemann, Christiane}, title = {Laser-bonding of FEP/FEP interfaces for a flexiblemanufacturing process of ferroelectrets}, series = {Journal of Physics: Conference Series}, volume = {1407}, journal = {Journal of Physics: Conference Series}, number = {012107}, doi = {10.1088/1742-6596/1407/1/012107}, pages = {1 -- 5}, year = {2019}, abstract = {This paper presents an optimized laser-bonding process for piezoelectric energy-harvesters based on thin fluorinated-ethylene-propylene (FEP) foils, using an ultra-short-pulse(USP) laser. Due to the minimized thermal stress in the material during bonding, achieved bypulse durations of few picoseconds, we created seams down to 40μm width without generatingholes in the 12.5μm thick FEP-foils. Using a galvanometer scanning system allowed for fastbonding-speed up to several centimeters per second, making the process also suitable for largestructures and areas. The achieved bond strength of the seams under influence of shearingstress was examined using tensile testing, which showed a sufficient strength of about 25 \% of the maximum strength of an unbonded, single layer of FEP.}, subject = {Ultrakurzzeitlaser}, language = {en} } @article{FlachsZhukovZechetal.2024, author = {Flachs, Dennis and Zhukov, Sergey and Zech, Isabella and Schreck, Timo and Belle, Stefan and Seggern, Heinz von and Kupnik, Mario and Altmann, Alexander Anton and Thielemann, Christiane}, title = {Enzymatic Self-Degradable PLA-Based Electrets}, series = {Journal of Polymers and the Environment}, volume = {2024}, journal = {Journal of Polymers and the Environment}, number = {1}, doi = {Journal of https://doi.org/10.1007/s10924-024-03240-6}, pages = {1 -- 11}, year = {2024}, abstract = {In recent years, the demand for sustainable and degradable materials and electronic devices has increased significantly. Among a range of biodegradable polymers, poly(lactic acid) (PLA) is a good alternative to conventional petrol-based polymers because of its attractive mechanical properties and its easy processability. Recently, PLA has also been described as a promising dielectric material with piezoelectric and electret properties. We expect that PLA—after further optimization— will play an important role as a material for environmentally friendly sensors in the future, where first applications such as air filters and pressure sensors have already been proposed. However, degradation under normal ambient conditions is very slow, and an accelerated and controllable degradation process is highly desirable for any type of PLA-based sensors. Enzymatic hydrolysis with embedded enzymes has been proposed as an approach to accelerate and control degradation. In this work, we investigate the properties of PLA in terms of dielectric and mechanical properties with a focus on its ability to store charges after the enzyme proteinase K (Trit. album) has been incorporated. Results reveal that proteinase K has a positive effect on the charge stability of solvent-cast PLA electrets after complete evaporation of the solvent. Furthermore, we observed a concentration-dependent acceleration of mass loss in a Tris-HCl buffer. A fast degradation within only one day occurred at a concentration of 6 wt\% proteinase K.}, subject = {Biologisch abbaubarer Kunststoff}, language = {en} } @article{FlachsEmmerichThielemann2022, author = {Flachs, Dennis and Emmerich, Florian and Thielemann, Christiane}, title = {Fabrication process for FEP piezoelectrets based on photolithographically structured thermoforming templates}, series = {Microsystem Technologies}, volume = {2022}, journal = {Microsystem Technologies}, number = {November}, doi = {doi.org/10.1007/s00542-022-05405-6}, pages = {0 -- 0}, year = {2022}, abstract = {Piezoelectrets fabricated from fluoroethylenepropylene (FEP)-foils have shown drastic increase of their piezoelectric properties during the last decade. This led to the development of FEP-based energy harvesters, which are about to evolve into a technology with a power-generation-capacity of milliwatt per square-centimeter at their resonance frequency. Recent studies focus on piezoelectrets with solely negative charges, as they have a better charge stability and a better suitability for implementation in rising technologies, like the internet of things (IOT) or portable electronics. With these developments heading towards applications of piezoelectrets in the near future, there is an urgent need to also address the fabrication process in terms of scalability, reproducibility and miniaturization. In this study, we firstly present a comprehensive review of the literature for a deep insight into the research that has been done in the field of FEP-based piezoelectrets. For the first time, we propose the employment of microsystem-technology and present a process for the fabrication of thermoformed FEP piezoelectrets based on thermoforming SU-8 templates. Following this process, unipolar piezoelectrets were fabri� cated with air void dimensions in the range of 300-1000 lm in width and approx. 90 lm in height. For samples with a void size of 1000 lm, a d33-coefficient up to 26,508 pC/N has been achieved, depending on the applied seismic mass. Finally, the properties as energy harvester were characterized. At the best, an electrical power output of 0.51 mW was achieved for an acceleration of 1 � g with a seismic mass of 101 g. Such piezoelectrets with highly defined dimensions show good energy output in relation to volume, with high potential for widespread applications.}, subject = {Mikrosystemtechnik}, language = {en} } @inproceedings{AltmannSuppeltRuhletal.2024, author = {Altmann, Alexander Anton and Suppelt, Sven and Ruhl, Max and Schaumann, Stephan and Latsch, Bastian and Ben Dali, Omar and Zhukov, Sergey and Flachs, Dennis and Zhang, Xiaoqing and Thielemann, Christiane and Seggern, Heinz von and Kupnik, Mario}, title = {Monolithic Wideband Air-Coupled Ultrasonic Transducer Based on Additively Manufactured Ferroelectrets}, series = {2024 IEEE Ultrasonics, Ferroelectrics, and Frequency Control Joint Symposium (UFFC-JS)}, booktitle = {2024 IEEE Ultrasonics, Ferroelectrics, and Frequency Control Joint Symposium (UFFC-JS)}, publisher = {IEEE}, doi = {10.1109/UFFC-JS60046.2024.10793858}, pages = {1 -- 4}, year = {2024}, abstract = {Air-coupled ultrasonic transducers are widely used in non-destructive testing, acoustical sonar systems, and biomedical imaging. These applications require transducers that operate effectively across a broad acoustic frequency spectrum, offer adaptable geometric designs, and increasingly incorporate eco-friendly materials. In this work, we present a monolithic, 3D-printed air-coupled ultrasonic transducer based on ferroelectrets (FEs) and fabricated from biocompatible polylactic acid (PLA). We evaluated the transducer's acoustical performance by measuring the surface velocity of its active area using laser Doppler vibrometry and assessed its robustness during continuous operation over a 19-day period. Additionally, we measured the sound pressure level (SPL) and wideband characteristics in an anechoic chamber across excitation frequencies from 1kHz to 100kHz. At a resonance frequency of 33kHz, our transducer achieved an SPL of 94.3dB and surface velocities up to 37mm/s. The measured bandwidth of 65.2kHz at the -6dB threshold corresponds to a fractional bandwidth of 189\%. The observed exponential decay of the surface velocity, stabilizing at 15\% of its initial amplitude, aligns with the isothermal surface potential decay typically observed in FE films made from PLA. These results demonstrate the effectiveness of the transducer, which features an adaptable backplate for tuning acoustic properties. The low-cost transducer, manufactured from biocompatible PLA, is particularly suited for imaging and biomedical applications furthering green electronics.}, subject = {Ultraschallwandler}, language = {en} } @inproceedings{StehlikovaDoehringStollenwerketal.2021, author = {Stehlikova, Veronika and D{\"o}hring, Thorsten and Stollenwerk, Manfred and Stadtm{\"u}ller, Johannes and Marsikova, Veronika and Hudec, Rene and Flachs, Dennis and Burwitz, Vadim and Hartner, Gisela and Rukdee, Surangkhana and M{\"u}ller, Thomas and Schmidt, Thomas and Inneman, Adolf and Klementova, Mariana and Zeising, Sebastian and Langmeier, Andreas}, title = {Lobster eye type X-ray telescope with chromium-iridium coated tri-layer mirrors}, series = {Proceedings of SPIE}, volume = {2021}, booktitle = {Proceedings of SPIE}, number = {11776}, publisher = {SPIE}, organization = {Czech Technical University in Prague}, doi = {https://doi.org/10.1117/12.2589266}, pages = {1177605-1 -- 1177605-8}, year = {2021}, abstract = {For space-born astronomical X-ray telescopes, iridium-based reflective layer systems are known as highly effective mirrors coatings. During the recent years, Aschaffenburg University of Applied Sciences and the Czech Technical University in Prague jointly developed stress compensated chromium-iridium coatings for this application. To overcome the disturbing reflectivity reduction of the iridium absorption edge around 2 keV photon energy, thin overcoat layers of chromium were applied in addition. Now a prototype of a wide-field, imaging X-ray telescope of Lobster Eye type is assembled at the company RIGAKU. For this purpose a small series of 34 mirrors based on 100 x 50 mm semiconductor grade silicon substrates has been coated at Aschaffenburg University. The applied tri-layer system consists of a stack of 40 nm chromium, which act as adhesive layer and compensates layer stress, a 30 nm iridium thick reflective layer, and an additional overcoat layer of 6 nm chromium. This layer system have been analysed by AFM and TEM images. The mirrors are assembled into an aluminium frame to build a 2D Lobster Eye type telescope. The designed focal length of this wide field X-ray telescope is two meter. To study the performance of the tri-layer coating system, a twin LE telescope with convenient gold coatings was manufactured also. Performance measurements of both telescopes and under same conditions are planned at the PANTER test facility at the Max-Planck Institute for Extraterrestrial Physics. First experimental results, their comparison with theoretical simulations and the comparison between both models will be presented in this contribution.}, subject = {R{\"o}ntgenteleskop}, language = {en} } @inproceedings{DoehringStollenwerkStadtmuelleretal.2021, author = {D{\"o}hring, Thorsten and Stollenwerk, Manfred and Stadtm{\"u}ller, Johannes and Zeising, Sebastian and Flachs, Dennis and Stehlikova, Veronika and Burwitz, Vadim and Krumrey, Michael K. and Cotroneo, Vincenzo and Klementova, Mariana}, title = {Characterisation of X-ray mirrors based on chromium-iridium tri-layer coatings}, series = {Proceedings of SPIE}, volume = {2021}, booktitle = {Proceedings of SPIE}, number = {11776}, publisher = {SPIE}, organization = {TH Aschaffenburg}, doi = {https://doi.org/10.1117/12.2592551}, pages = {1177607-1 -- 11776076}, year = {2021}, abstract = {Iridium-based layer systems are highly effective mirror coatings for space-born X-ray telescopes. During the recent years, Aschaffenburg University of Applied Sciences and its partners developed stress compensated chromium-iridium coatings for such astronomical applications, using chromium as an adhesive layer between iridium reflective layer and mirror substrate. However, there was room for improvement: To overcome the disturbing reflectivity reduction of the iridium absorption edge around 2 keV photon energy, thin overcoat layers of chromium are applied in addition now. This layer system has been analyzed by atomic force microscopy and transmission electron microscopy images. Furthermore, the reflectivity of such innovative X-ray mirrors based on chromium-iridium tri-layer coatings was recently measured at PTB's four-crystal monochromator beamline at the synchrotron radiation facility BESSY II. The experimental results, obtained for photon energies between 1.9 keV and 11 keV at two grazing incidence angles (0.6 degree and 0.9 degree) and their comparison with corresponding simulations are presented in this contribution. When compared to simulations of pure iridium coatings, a significantly higher reflectivity was achieved especially in the soft X-ray regime between 2 keV and 4 keV. Such chromium-iridium tri-layer coatings have high potential to increase the effective area for X-ray telescopes in Lobster Eye Design, for Wolter-I type X-ray optics, and for silicon pore optics as considered for the ATHENA telescope of the European Space Agency.}, subject = {R{\"o}ntgenspiegel}, language = {en} } @inproceedings{FlachsBernhardThielemann2024, author = {Flachs, Dennis and Bernhard, Levin and Thielemann, Christiane}, title = {Fully 3D-Printed Rotational Energy Harvester Based On Bipolar Charged PLA Electrets}, series = {Micro and Nanotechnology for Power Generation and Energy Conversion Applications (PowerMEMS)}, volume = {2024}, booktitle = {Micro and Nanotechnology for Power Generation and Energy Conversion Applications (PowerMEMS)}, publisher = {IEEE}, isbn = {979-8-3503-8020-0}, doi = {979-8-3503-8020-0}, year = {2024}, abstract = {Rotational energy harvesters have emerged as a promising solution for sustainable power generation in a variety of applications, ranging from small-scale devices to large-scale industrial systems. In this work, we present a fully 3D-printed electret rotational harvester based on the biodegradable and compostable polymer polylactic acid (PLA). The 3D-printed harvester consists of a rotor made from a bipolar-charged PLA electret, electrode pairs of conductive PLA that function as the stator, and a PLA bearing. An output power of 61μ W was achieved at a rotational speed of 400 rpm and a load resistance of 28.2MΩ. To assess the durability of the harvester, particularly the 3D-printed bearing, the extent of mechanical wear was examined after one million rotations at a rotational speed of 400 rpm. The findings indicate that the rollers undergo a 1.2\% loss in mass, yet the functionality remains intact. The charge decay of PLA electrets is influenced by external factors such as elevated humidity and temperature. Future research will focus on improving the charge stability of PLA by addressing these factors, expanding the potential applications of this rotational energy harvester.}, subject = {Elektret}, language = {en} } @article{FlachsEtzelMayeretal.2022, author = {Flachs, Dennis and Etzel, Johannes and Mayer, Margot and Harbecke, Frederic and Belle, Stefan and Rickmeyer, Tim and Thielemann, Christiane}, title = {Characterization of electrically conductive, printable ink based on alginate hydrogel and graphene nanoplatelets}, series = {Biomedical Engineering Advances}, volume = {2022}, journal = {Biomedical Engineering Advances}, number = {4}, doi = {https://doi.org/10.1016/j.bea.2022.100045}, pages = {1 -- 2}, year = {2022}, abstract = {In recent years, there has been an increasing interest in electrically conductive hydrogels for a wide range of biomedical applications, like tissue engineering or biosensors. In this study, we present a cost-effective conductive hydrogel based on alginate and graphene nanoplatelets for extrusion-based bioprinters. The hydrogel is prepared under ambient conditions avoiding high temperatures detrimental for cell culture environments. Investigation of the hydrogel revealed a conductivity of up to 7.5 S/cm, depending on the ratio of platelets. Furthermore, in vitro tests with human embyronic kidney cells - as an example cell type - showed good adhesion of the cells to the surface of the conductive hydrogel. Electrochemical measurements revealed a low electrode impedance which is desirable for the extracellular recording, but also low electrode capacitance, which is unfavorable for electrical stimulation purposes. Therefore, future experiments with the graphene nanoplatelets-based hydrogels will focus on electrodes for biosensors and extracellular recordings of neurons or cardiac myocytes.}, subject = {Hydrogel}, language = {en} } @article{FlachsKoehlerThielemann2018, author = {Flachs, Dennis and K{\"o}hler, Tim and Thielemann, Christiane}, title = {Transparent poly(3,4-ethylenedioxythiophene)-based microelectrodes for extracellular recording}, series = {Biointerphases}, volume = {2018}, journal = {Biointerphases}, number = {13:041008}, doi = {10.1116/1.5041957}, pages = {1 -- 1}, year = {2018}, abstract = {It is well known that at the interface between neuronal tissue and recording electrode low electrical impedance is required. However, if simultaneous optical detection or stimulation is an issue, good optical transmittance of the electrode material is desirable as well. State-of-the-art titanium nitride electrodes provide superior low impedance compared to gold or iridium, but are nontransparent. Transparent electrode materials like the transparent conducting oxide, indium tin oxide (ITO), or graphene offer high light transmittance (>80\%) but reveal relatively high impedance. In this paper, the authors propose the conducting polymer poly(3,4-ethylenedioxythiophene) with the counter ion NO-3 as the electrode material for low impedance and good optical transmittance properties. The polymer is electrochemically deposited onto ITO improving the relatively high impedance of ITO. This multilayer electrode allows not only for electrophysiological recordings of cardiomyocytes but also for monitoring of cell contraction under the microscope. Electrochemical impedance spectroscopy and action potential recordings reveal that the new transparent electrodes are a good compromise in terms of low impedance and transparency if deposition parameters are optimized.}, subject = {Impedanzspektroskopie}, language = {en} } @article{FlachsCiba2016, author = {Flachs, Dennis and Ciba, Manuel}, title = {Cell-based sensor chip for neurotoxicity measurements in drinking water}, series = {L{\´e}kař a technika - Clinician and Technology}, volume = {46}, journal = {L{\´e}kař a technika - Clinician and Technology}, number = {2}, pages = {46 -- 50}, year = {2016}, abstract = {Our drinking water contains residues of pharmaceuticals. A sub-group of these contaminants are neuro-active substances, the antiepileptic carbamazepine being one of the most relevant. For assessment of the neurotoxicity of this drug at a sub-therapeutic level, a cell-based sensor chip platform has been realized and characterized. For this purpose, a microelectrode array chip was designed and processed in a clean room and optimized in terms of low processing costs and good recording properties. For characterization of the system neuronal cells were plated on microelectrode array chips and electrical activity was measured as a function of applied carbamazepine concentration. We found that the relative spike rate decreased with increasing drug concentration resulted in IC50 values of around 36 μM. This value is five orders of magnitude higher than the maximal dose found in drinking water. IC50 values for burst rate, burst duration and synchrony were slightly higher, suggesting spike rate being a more sensitive parameter to carbamazepine.}, subject = {Mikroelektrode}, language = {en} } @inproceedings{KhropostRiethmuellerDoehringetal.2025, author = {Khropost, Diana and Riethm{\"u}ller, Franziska and D{\"o}hring, Thorsten and Flachs, Dennis and H{\"u}lag{\"u}, Deniz and Hertwig, Andreas and Cotroneo, Vincenzo and Gibertini, Eugenio}, title = {Polydopamine: a bio-inspired polymer for X-ray mirror coatings and other technical applications}, series = {EUV and X-ray Optics: Synergy between Laboratory and Space IX}, volume = {2025}, booktitle = {EUV and X-ray Optics: Synergy between Laboratory and Space IX}, number = {13531}, editor = {Hudec, Ren{\´e} and Pina, Ladislav}, publisher = {SPIE}, doi = {10.1117/12.3056298}, pages = {135310H-1 -- 135310H-11}, year = {2025}, abstract = {Although the organic molecule dopamine (3,4-dihydroxyphenethylamine) is commonly known as one of the "hormones of happiness", thin polymer films of polydopamine (PDA) also have interesting technical properties. PDA is a very strong glue that sticks on almost everything, even under water. In nature, PDA is found in the byssal thread cuticles of mussels. When produced by dip-coating, the self-organizing PDA layers grow in a reproducible thickness of single or multiple molecule monolayers of a few nanometres thickness only. Here we present an optimized preparation regime as derived from polymerization analysis through absorption spectroscopy. One application is the use of thin PDA overcoatings to increase the soft X-ray reflectivity of astronomical X-ray mirrors. Furthermore, we give an outlook to other technical applications for this interesting material, presenting this bio-inspired organic polymer as an innovative technical solution for the future, with applications such as PDA-based super-capacitors and its promising role in enhancing separator materials for batteries.}, subject = {R{\"o}ntgenspiegel}, language = {en} } @article{AltmannSuppeltWuestetal.2025, author = {Altmann, Alexander Anton and Suppelt, Sven and W{\"u}st, Philipp and D{\"o}rsam, Jan Helge and Latsch, Bastian and Flachs, Dennis and Blaeser, Andreas and Zhang, Xiaoqing and Thielemann, Christiane and Seggern, Heinz von and Kupnik, Mario}, title = {Self-supporting mid-air 3D printing of single-layer polypropylene structures: Flow rate-dependent analytical modeling and surface characterization}, series = {Additive Manufacturing}, volume = {113}, journal = {Additive Manufacturing}, publisher = {Elsevier BV}, issn = {2214-8604}, doi = {https://doi.org/10.1016/j.addma.2025.105010}, year = {2025}, abstract = {High-precision deposition in material extrusion-based additive manufacturing (MEX-AM), especially in mid-air extrusion, is essential for creating microfluidic channels, integrated sensors, and tissue scaffolds with fine vascular features. However, despite advances in support-free slicing and multi-axis printing, no standardized method exists for fabricating enclosed air cavities with high shape fidelity, particularly in single-layer structures. Reliable deposition at sub-millimeter scales remains challenging, especially when printing low-stiffness polymers such as polypropylene (PP), due to delayed solidification and viscoelastic effects. This study introduces a flow-rate-dependent analytical model to predict extrusion behavior, interfacial bonding, and surface morphology in self-supporting single-layer PP structures. To validate the model, surface roughness, material distribution, and intralayer bonding were quantified using spectral analysis, cross-sectional thickness measurements, and Abbott-Firestone curve evaluation. Results show that reducing the flow rate (relative to 100\% nominal flow through a 0.4 mm nozzle) to 40\% improves surface roughness (Ra) to below 20 m on average across all surfaces, with the lowest Ra of 4.52 ± 0.49 m observed at a 20\% flow rate on the top surfaces enclosing the cavity. The intralayer bonding between adjacent extruded lines increases up to 70\%, improving deposition uniformity. Nozzle-induced smoothing effects enhance surface quality at flow rates of 60\% or lower, while higher flow rates result in increased surface waviness and geometric irregularities. These findings enable the fabrication of enclosed microfluidic channels and functional cavities with well-defined surfaces and reduced mechanical stiffness, suitable for applications involving sensing, controlled deformation, or flexible system integration.}, subject = {Fertigungstechnik}, language = {en} }