Despite growing effort to advance materials towards a low fibrotic progression, all implants elicit adverse tissue responses. Pre-clinical biomaterial assessment relies on animals testing, which can be complemented by in vitro tests to address the Russell and Burch’s 3R aspect of reducing animal burden. However, a poor correlation between in vitro and in vivo biomaterial assessments confirms a need for suitable in vitro biomaterial tests. The aim of the study was to identify a test setting, which is predictive and might be time- and cost-efficient. We demonstrated how sensitive in vitro biomaterial assessment based on human primary macrophages depends on test conditions. Moreover, possible clinical scenarios such as lipopolysaccharide contamination, contact to autologous blood plasma, and presence of IL-4 in an immune niche influence the outcome of a biomaterial ranking. Nevertheless, by using glass, titanium, polytetrafluorethylene, silicone, and polyethylene representing a specific material-induced fibrotic response and by comparison to literature data, we were able to identify a test condition that provides a high correlation to state-of-the-art in vivo studies. Most important, biomaterial ranking obtained under native plasma test conditions showed a high predictive accuracy compared to in vivo assessments, strengthening a biomimetic three-dimensional in vitro test platform.

Pacemaker systems are an essential tool for the treatment of cardiovascular diseases. However, the immune system’s natural response to a foreign body results in the encapsulation of a pacemaker electrode and an impaired energy efficiency by increasing the excitation threshold. The integration of the electrode into the tissue is affected by implant properties such as size, mechanical flexibility, shape, and dimensionality. Three-dimensional, tissue-like electrode scaffolds render an alternative to currently used planar metal electrodes. Based on a modified electrospinning process and a high temperature treatment, a conductive, porous fiber scaffold was fabricated. The electrical and immunological properties of this 3D electrode were compared to 2D TiN electrodes. An increased surface of the fiber electrode compared to the planar 2D electrode, showed an enhanced electrical performance. Moreover, the migration of cells into the 3D construct was observed and a lower inflammatory response was induced. After early and late in vivo host response evaluation subcutaneously, the 3D fiber scaffold showed no adverse foreign body response. By embedding the 3D fiber scaffold in human cardiomyocytes, a tissue-electrode hybrid was generated that facilitates a high regenerative capacity and a low risk of fibrosis. This hybrid was implanted onto a spontaneously beating, tissue-engineered human cardiac patch to investigate if a seamless electronic-tissue interface is generated. The fusion of this hybrid electrode with a cardiac patch resulted in a mechanical stable and electrical excitable unit. Thereby, the feasibility of a seamless tissue-electrode interface was proven.

The measurement of transepithelial electrical resistance (TEER) is a common technique to determine the barrier integrity of epithelial cell monolayers. However, it is remarkable that absolute TEER values of similar cell types cultured under comparable conditions show an immense heterogeneity. Based on previous observations, we hypothesized that the heterogeneity of absolute TEER measurements can not only be explained by maturation of junctional proteins but rather by dynamics in the absolute length of cell junctions within monolayers. Therefore, we analyzed TEER in epithelial cell monolayers of Caco2 cells during their differentiation, with special emphasis on both changes in the junctional complex and overall cell morphology within monolayers. We found that in epithelial Caco2 monolayers TEER increased until confluency, then decreased for some time, which was then followed by an additional increase during junctional differentiation. In contrast, permeability of macromolecules measured at different time points as 4 kDA fluorescein isothiocyanate (FITC)-dextran flux across monolayers steadily decreased during this time. Detailed analysis suggested that this observation could be explained by alterations of junctional length along the cell borders within monolayers during differentiation. In conclusion, these observations confirmed that changes in cell numbers and consecutive increase of junctional length have a critical impact on TEER values, especially at stages of early confluency when junctions are immature.

The culture of human induced pluripotent stem cells (hiPSCs) at large scale becomes feasible with the aid of scalable suspension setups in continuously stirred tank reactors (CSTRs). Innovative monitoring options and emerging automated process control strategies allow for the necessary highly defined culture conditions. Next to standard process characteristics such as oxygen consumption, pH, and metabolite turnover, a reproducible and steady formation of hiPSC aggregates is vital for process scalability. In this regard, we developed a hiPSC-specific suspension culture unit consisting of a fully monitored CSTR system integrated into a custom-designed and fully automated incubator. As a step towards cost-effective hiPSC suspension culture and to pave the way for flexibility at a large scale, we constructed and utilized tailored miniature CSTRs that are largely made from three-dimensional (3D) printed polylactic acid (PLA) filament, which is a low-cost material used in fused deposition modelling. Further, the monitoring tool for hiPSC suspension cultures utilizes in situ microscopic imaging to visualize hiPSC aggregation in real-time to a statistically significant degree while omitting the need for time-intensive sampling. Suitability of our culture unit, especially concerning the developed hiPSC-specific CSTR system, was proven by demonstrating pluripotency of CSTR-cultured hiPSCs at RNA (including PluriTest) and protein level.

Because of a rising use of in vitro models as an alternative to animal models, the precise assessment of tissue-specific parameters of such in vitro test systems has become a critical part of ensuring predictive results. Impedance spectroscopy as a noninvasive method serves as a reliable and efficient tool for quality control because it only minimally interferes with the system during investigation. In this study, we present a refined impedance measurement system using nanostructured titanium nitride (TiN) electrodes. This advanced material was used to investigate tissue maturation and changes in the barrier integrity of an intestinal Transwell-based in vitro model. The reduction of noise facilitated a more detailed data extraction and biological interpretation. Compared to standard stainless steel electrodes, at a typical measurement frequency of 12.5 Hz, the maximum electrode impact on the signal could be reduced from over 75% to less than 5%. This allowed the accurate determination of transepithelial electrical resistance values from Caco-2 in vitro tissue models without a further mathematical analysis based on a computer simulation. The novel design of a 3D-printed measurement attachment equipped with nanostructured TiN electrodes was used to continuously monitor the barrier integrity of the Caco-2 cells during a permeability assay. Moreover, because of low process temperatures, the TiN coatings for enhanced impedance measurement sensitivity could also be deposited onto several other materials, e.g., commercially available cell culture equipment such as standard disposable multiwell plate dishes. In conclusion, we developed a novel method to improve the electrode properties for impedance spectroscopy, which can be easily implemented into standardized end-point measurement to qualify a variety of in vitro test systems.