Biaxially oriented polypropylene (BOPP) is the current gold standard for DC thin film capacitors. However, one of its major disadvantages is the poor high-temperature stability. It is the aim of this study to contribute to the understanding of the failure of BOPP at elevated temperatures. Therefore, capacitor-grade PP was extruded into a cast film using a twin-screw extruder and a water-cooled chill roll. BOPP films were produced using a laboratory stretching machine. The films were tested for their short-term dielectric strength and voltage endurance between 23 and 140°C. DSC, shrinkage and TSDC measurements were conducted to analyze the morphological and electrical changes in the material at elevated temperatures. An increase in temperature caused a reduction in dielectric strength and voltage endurance. The onset of space charge release by thermal trap activation coincided with a slow deterioration in both properties. The decrease was further enhanced as crystalline regions of low stability started to melt and orientation in amorphous regions started to be relaxed. This shows that trap states and morphological changes in the operating temperature range of thin film capacitors between 60 and 100°C should be the focus of future research.

Abstract Background/Introduction Cardiac tissue engineering is a promising strategy to generate human cardiac tissues for modelling cardiac diseases, screening for therapeutic drugs, and repairing the injured heart. Yet, several issues remain to be resolved including the generation of tissues with high cardiomyocyte density. Purpose Determining the effects of the induction of human-induced pluripotent stem cell-derived (hiPSC) cardiomyocyte proliferation post-fabrication. Methods hiPSCs were differentiated into cardiomyocytes, embedded with or without CHIR990121 at three concentrations in a collagen pre-gel, and cast. The engineered cardiac tissues were then cultured in the absence or presence of CHIR99021 for up to 35 days. Hydrogels and engineered cardiac tissues were analysed utilizing rheology and assays to determine viability, proliferation, calcium flow, and contractility. Results Here, we show that the integration of CHIR99021 in collagen I hydrogels promotes proliferation of hiPSC-cardiomyocytes post-fabrication improving contractility of and calcium flow in engineered cardiac tissues. Presence of CHIR99021 has no effect on the gelation kinetic or the mechanical properties of collagen I hydrogels. Analysis of cell density and proliferation based on Ki-67 staining indicates that integration of CHIR99021 together with external CHIR99021 stimulation increases hiPSC-cardiomyocyte number by ∼2-fold within 7 days post-fabrication. Analysis of the contractility of engineered cardiac tissues after another 3 days in the absence of external CHIR99021 shows that CHIR99021-induced hiPSC-cardiomyocyte proliferation results in synchronized calcium flow, rhythmic beating, increases speed of contraction and contraction amplitude, and reduces peak-to-peak time. The CHIR99021-stimulated engineered cardiac tissues exhibited spontaneous rhythmic contractions for at least 35 days. Conclusion Collectively, our data demonstrate the potential of induced cardiomyocyte proliferation to enhance engineered cardiac tissues by increasing cardiomyocyte density and reducing arrhythmia. Funding Acknowledgement Type of funding sources: Public grant(s) – National budget only. Main funding source(s): Deutsche Forschungsgemeinschaft

When characterizing the viscoelastic properties of polymers, shear rheological measurements are commonly the method of choice. These properties are known to affect extrusion and nozzle-based processes such as fiber melt spinning, cast film extrusion and 3D-printing. However, an adequate characterization of shear thinning polymers can be challenging and still insufficient to not only describe but predict process relevant influences. Furthermore, the evaluation of rheological model systems in literature is mostly based on stress–relaxation experiments, which are rarely available for various polymeric materials. Therefore, a simple approach is presented, that can be used to evaluate and benchmark a wide range of rheological model systems based on commonly accessible frequency sweep data. The approach is validated by analyzing alginate PH176 solutions of various concentrations, a thermoplastic poly-urethane (TPU) Elastollan 1180A melt, the liquid silicon rubber Elastosil 7670 and a polycaprolactone (PCL) fiber-alginate composite system. The used rheological model systems, consisting of simple springs and dashpots, are suitable for the description of complex, viscoelastic material properties that can be observed for polymer solutions and gel-like systems. After revealing a suitable model system for describing those material properties, the determination and evaluation of relevant model parameters can take place. We present a detailed guideline for the systematic parameter revelation using alginate solutions of different concentrations as example. Furthermore, a starting point for future correlations of strut spreading in 3D-bioprinting and model parameters is revealed. This work establishes the basis for a better understanding and potential predictability of key parameters for various fabrication techniques.

Volumetric muscle loss caused by trauma or after tumour surgery exceeds the natural regeneration capacity of skeletal muscle. Hence, the future goal of tissue engineering (TE) is the replacement and repair of lost muscle tissue by newly generating skeletal muscle combining different cell sources, such as myoblasts and mesenchymal stem cells (MSCs), within a three-dimensional matrix. Latest research showed that seeding skeletal muscle cells on aligned constructs enhance the formation of myotubes as well as cell alignment and may provide a further step towards the clinical application of engineered skeletal muscle.

Nano- and micro-scale topographical features play a critical role in the induction and maintenance of various cellular properties and functions, including morphology, adhesion, gene regulation, and cell-to-cell communication. In addition, recent studies have indicated that the structure and function of heart tissue are also sensitive to mechanical cues at the nano- and micro-scale. Although fabrication methods exist for generating topographical features on polymeric scaffolds for cell culture, current techniques, especially those with nano-scale resolution, are typically complex, prohibitively expensive and not accessible to most biology laboratories. Here, we present a simple and tunable fabrication method for the production of patterned electrospun fibers that simulate the complex anisotropic and multi-scale architecture of cardiac tissue, to promote cardiac cell alignment. This method is based on the combination of electrospinning and soft lithography techniques, in which electrospun fibers, based on a blend of poly(glycerol sebacate) and poly(caprolactone), were collected on a patterned Teflon-coated silicon wafer with imprinted topographical features. Different surface topographies were investigated, such as squares and grooves, with constant or different interspatial distances. In vitro cell culture studies successfully demonstrated the alignment of both C2C12 myoblasts and neonatal rat cardiomyocytes on fabricated electrospun patterned surfaces. C2C12 cells were cultured over a period of 72 h to study the effect of topographical cues on cell morphology. Cells attached within the first 8 h after seeding and after 24 h most of the cells started to align responding to the topographical cues. Similarly, cardiomyocytes responded to the topographical features by aligning themselves and by expressing Connexin 43 along cellular junctions. Summarizing, we have developed a new method with the potential to significantly promote cardiac tissue engineering by fabricating electrospun fibers with defined topographical features to guide and instruct donor and/or host cells.

We investigate the influence of the reduction of width along the stretching direction, the so-called neck-in effect, on the draw resonance instability in Newtonian film casting using a linear stability analysis of a model of reduced dimensionality including gravity and inertia forces. Proper scaling reveals the aspect ratio, i.e., the ratio of the initial film half-width to the film length, together with the fluidity and the inlet velocity as independent, dimensionless control parameters. Moreover, we introduce the local Trouton ratio as a measure for the type of elongational deformation, which can be uniaxial, planar, or a combination of both. In the case of purely uniaxial or planar deformations, a one-dimensional model is sufficient. The influence of the control parameters on the draw resonance instability, including a threshold to unconditional stability, is visualized by several stability maps. Special cases of viscous-gravity and viscous-inertia models are analyzed separately due to their practical importance. Gravity appears to influence the aspect ratio at which the critical draw ratio is maximum and amplifies the stabilizing effect of the neck-in. Inertia increases the stabilization due to neck-in, eventually leading to a window of unconditional stability within the analyzed region of aspect ratios. The mechanism underlying the complete suppression of draw resonance is presented, using exclusively steady state analysis. Additionally, the stabilizing mechanisms of gravity and neck-in are revealed. Known alternative stability criteria are extended to the case of finite width and their validity is tested in the presence of inertia, gravity, and finite aspect ratios.

The influence of viscosity and inlet velocity on the draw resonance instability of film casting processes is quantitatively analysed. By linear stability analysis of a Newtonian model including inertia and gravity effects, stability curves for different control parameter values are calculated numerically. For this purpose, we propose a scaling law which separates the fluidity, i.e. the reciprocal viscosity and the inlet velocity into two independent dimensionless parameters. This new scaling evidences a minimum of stability, separating two regimes of opposite behaviour: one for which increasing the inlet flow rate has a destabilizing effect due to viscosity and one for which increasing the inlet flow rate has a stabilizing effect due to gravity and inertia; increasing the fluidity has always a stabilizing effect. By fitting the stability curves with an appropriate postulated function, we are able to construct correlations between the critical draw ratio, the fluidity and the inlet velocity. For the first time regimes of negligible inertia or negligible gravity effects are revealed as well as a regime of unconditional stability. The proposed correlations for each of these regimes can further be used as an analytical solvable criterion for determining the onset of draw resonance in film casting.

Alternating current electrophoretic deposition (AC-EPD) of polyacrylic acid (PAA)–titanium oxide (TiO2) nanoparticle composites on stainless steel electrodes was investigated in basic aqueous solution. AC square wave with duty cycle of 80% was applied at a frequency of 1 kHz. FTIR–ATR spectra showed that both AC and direct current (DC) EPD successfully deposited PAA–TiO2 composites. The deposition rate using AC-EPD was lower than that obtained in direct current DC-EPD. However, the microstructure and surface morphology of the deposited composite coatings were different depending on the type of electric field applied. AC-EPD applied for not more than 5 min led to smooth films without bubble formation, while DC-EPD for 1 min or more showed deposits with microstructural defects possibly as result of water electrolysis. AC-EPD was thus for the first time demonstrated to be a suitable technique to deposit organic–inorganic composite coatings from aqueous suspensions, showing that applying a square wave and frequency of 1 kHz leads to uniform PAA–TiO2 composite coatings on conductive materials.

Novel chitosan composite coatings containing titania nanoparticles (n-TiO2) for biomedical applications were developed by electrophoretic deposition (EPD) from ethanol-water suspensions. The optimal ethanol-water ratio was studied in order to avoid bubble formation during the EPD process and to ensure homogeneous coatings. Different n-TiO2 contents (0.5-10 g L-1) were studied for a fixed chitosan concentration (0.5 g L-1) and the properties of the electrophoretic coatings obtained were characterized. Coating composition was analyzed by thermogravimetric analysis (TG), Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) analysis. Scanning electron microscopy (SEM) was employed to study both the surface and the cross section morphology of the coatings, and the thicknesses (2-6 mu m) of the obtained coatings were correlated with the initial ceramic content. Contact angle measurements, as a preliminary study to predict hypothetic protein attachment on the coatings, were performed for different samples and the influence of a second chitosan layer on top of the coatings was also tested. Finally, the electrochemical behavior of the coatings, evaluated by polarization curves in DMEM at 37 degrees C, was studied in order to assess the corrosion resistance provided by the n-TiO2/chitosan coatings.

The present article investigates the use of a novel electrospun fibrous blend of poly(glycerol sebacate) (PGS) and poly(butylene succinate-butylene dilinoleate) (PBS-DLA) as a candidate for cardiac tissue engineering. Random electrospun fibers with various PGS/PBS-DLA compositions (70/30, 60/40, 50/50, and 0/100) were fabricated. To examine the suitability of these fiber blends for heart patches, their morphology, as well as their physical, chemical, and mechanical properties were measured before examining their biocompatibility through cell adhesion. The fabricated fibers were bead-free and exhibited a relatively narrow diameter distribution. The addition of PBS-DLA to PGS resulted in an increase of the average fiber diameter, whereas increasing the amount of PBS-DLA decreased the hydrophilicity and the water uptake of the nanofibrous scaffolds to values that approached those of neat PBS-DLA nanofibers. Moreover, the addition of PBS-DLA significantly increased the elastic modulus. Initial toxicity studies with C2C12 myoblast cells up to 72 h confirmed nontoxic behavior of the blends. Immunofluorescence analyses and scanning electron microscopy analyses confirmed that C2C12 cells showed better cell attachment and proliferation on electrospun mats with higher PBS-DLA content. However, immunofluorescence analyses of the 3-day-old rat cardiomyocytes cultured for 2 and 5 days demonstrated better attachment on the 70/30 fibers containing well-aligned sarcomeres and expressing high amounts of connexin 43 in cellular junctions indicating efficient cell-to-cell communication. It can be concluded, therefore, that fibrous PGS/PBS-DLA scaffolds exhibit promising characteristics as a biomaterial for cardiac patch applications.

A straightforward method for manufacturing stratified scaffolds with potential application in osteochondral tissue engineering is reported. The method integrates three different techniques widely used for scaffold fabrication: sponge replica method, freeze-drying and electrospinning. 45S5 Bioglass® was used for the fabrication of the rigid bioactive substrate intended to be in contact with bone tissue, chitosan and alginate based solutions were used for building the interface between the Bioglass®-based scaffold and the soft cartilage side of the construct. Finally a chitosan-based electrospun nanofibrous membrane was selected for the upper layer of the scaffold. In this design, the intermediate layer has multiple functions providing adherence between the other scaffold components, preventing delamination and acting as a soft coating for the rigid Bioglass® substrate. Morphological analysis, bioactivity tests in simulated body fluid (SBF), as well as mechanical and wettability tests were performed on all samples to assess the optimal stratified scaffold combination. The best results, in terms of resistance to layer delamination, preservation of the bioactivity and improvement of the mechanical properties, were obtained using a chitosan solution and the freeze gelation technique for fabrication of the intermediate layer. Key results are discussed in the context of the potential application of these novel scaffolds in osteochondral tissue engineering.

Three different poly(hydroxyalkanoates) (PHAs), copolymers of poly(3-hydroxybutyrate) (P3HB), have been used to make composites using two different fillers, bioactive glass (type 45S5 Bioglass®) and calcium sulfate dihydrate. The PHAs used were poly(3-hydroxybutyrate-co-3-hydroxyvalerate) [PHBHV] and two copolymers of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) [PHBHHx]. The aim of the study was the fabrication and characterization of the new composites and the assessment of the influence of the particular filler combination on the physical properties and bioactivity of the films. The thermal behaviour was studied using differential scanning calorimetry while mechanical properties were evaluated using dynamic mechanic thermal analysis and tensile strength test. The mechanical and thermal properties were affected by particles addition. The distribution of the particles in the polymer matrix, observed by scanning electron microscopy, was directly related to the mechanical properties. The surface characteristics were investigated by contact angle measurements and Raman spectroscopy. The extent of formation of hydroxyapatite (HA) upon immersion in simulated body fluid (SBF) depended on the polymer used, the amount of fillers employed and the time of immersion in SBF. Bioactivity was enhanced in the composites with a rise of hydrophilicity. The HA formation was controllable with time in the case of PHBHHx composites.