Fibrous biomimetic and biohybrid carbon scaffolds for 3D cell growth

The formation of three dimensional tissue (3D) in the laboratory highly depends on the biomimetic environment, the engineered extracellular matrix (scaffold), the cell type as well as the biologically active components.The primary focus of this study is to fabricate and functionalize these highly porous carbon-based scaffolds and to understand biological and cellular responses toward them. To exploit the unique spatial features of the highly porous network for bone tissue engineering, bioactive ceramic nanoparticles (hydroxyapatite (HA), bioactive glass (BG)) are successfully incorporated into CNT-based scaffolds. The methods presented in this thesis provide a novel concept to generate biocompatible and bioactive fibrous carbon scaffolds that mimic the ECM with the additional feature of conductivity. The generated scaffolds can serve as groundbreaking fiber systems for 3D cell growth, which pave the way toward further investigations of diverse tissue engineering and bioapplications.Die Entstehung eines dreidimensionalen (3D) Gewebes im Labor haängt stark von der biomimetischen Umgebung, der geformten extrazellulären Matrix (Gerüst), dem Zelltyp sowie von den biologisch aktiven Komponenten ab. Der Hauptschwerpunkt dieser Studie ist es, sowohl solche hochporösen kohlenstoffbasierten Gerüste herzustellen und zu funktionalisieren, als auch die biologischen und zellulären Reaktionen auf diese Gerüste verstehen zu lernen. Um die einzigartigen Eigenschaften dieser hochporösen Netzwerke für die Knochengewebezüchtung zu nutzen, wurden bioaktive, keramische Nanopartikel (Hydroxyapetite (HA), bioaktives Glass (BG)) erfolgreich in die CNT-basierten Gerüste eingebaut. Die hier präsentierten Methoden zeigen neue Konzepte der Herstellung von biokompatiblen und -aktiven Kohlenstoff Matrixen mit dem Vorteil der elektrischen Leitfähigkeit. Diese imitieren mit ihrer fibrösen Struktur die Extrazelluläre Matrix im Gewebe und generieren ein bahnbrechendes 3D Sytem für die Zellkultivierung. Im Bereich des Tissue Engineering und der biologischen Anwendungen können diese Strukturen somit als Grundlage zukünftiger Forschung dienen

Microengineered Hollow Graphene Tube Systems Generate Conductive Hydrogels with Extremely Low Filler Concentration

The fabrication of electrically conductive hydrogels is challenging as the introduction of an electrically conductive filler often changes mechanical hydrogel matrix properties. Here, we present an approach for the preparation of hydrogel composites with outstanding electrical conductivity at extremely low filler loadings (0.34 S m-1, 0.16 vol %). Exfoliated graphene and polyacrylamide are microengineered to 3D composites such that conductive graphene pathways pervade the hydrogel matrix similar to an artificial nervous system. This makes it possible to combine both the exceptional conductivity of exfoliated graphene and the adaptable mechanical properties of polyacrylamide. The demonstrated approach is highly versatile regarding porosity, filler material, as well as hydrogel system. The important difference to other approaches is that we keep the original properties of the matrix, while ensuring conductivity through graphene-coated microchannels. This novel approach of generating conductive hydrogels is very promising, with particular applications in the fields of bioelectronics and biohybrid robotics

Microarchitected Compliant Scaffolds of Pyrolytic Carbon for 3D Muscle Cell Growth

The integration of additive manufacturing technologies with the pyrolysis of polymeric precursors enables the design-controlled fabrication of architected 3D pyrolytic carbon (PyC) structures with complex architectural details. Despite great promise, their use in cellular interaction remains unexplored. This study pioneers the utilization of microarchitected 3D PyC structures as biocompatible scaffolds for the colonization of muscle cells in a 3D environment. PyC scaffolds are fabricated using micro-stereolithography, followed by pyrolysis. Furthermore, an innovative design strategy using revolute joints is employed to obtain novel, compliant structures of architected PyC. The pyrolysis process results in a pyrolysis temperature- and design-geometry-dependent shrinkage of up to 73%, enabling the geometrical features of microarchitected compatible with skeletal muscle cells. The stiffness of architected PyC varies with the pyrolysis temperature, with the highest value of 29.57 ± 0.78 GPa for 900 °C. The PyC scaffolds exhibit excellent biocompatibility and yield 3D cell colonization while culturing skeletal muscle C2C12 cells. They further induce good actin fiber alignment along the compliant PyC construction. However, no conclusive myogenic differentiation is observed here. Nevertheless, these results are highly promising for architected PyC scaffolds as multifunctional tissue implants and encourage more investigations in employing compliant architected PyC structures for high-performance tissue engineering applications

Noncovalent Spiropyran Coatings for Photoinduced Wettability Switching

The noncovalent binding of spiropyran to candle-soot-covered surfaces is investigated for wettability switching using a coating procedure realized with a drop casting process of using 0.001 mol/L spiropyran in a 5 : 1 toluene-acetone mixture. Scanning electron microscopy images reveal a resulting surface with spiropyran flakes in the candle soot. A reversible switching with UV light and blue or green light is achieved, starting from an initial contact angle of 130°  ± 9.68°. The highest contact angle difference is 41° and reversibility has been shown for several switching cycles. Hence, our methods provide an easy-to-use strategy to generate surfaces with switchable wettability

Increasing the Efficiency of Thermoresponsive Actuation at the Microscale by Direct Laser Writing of pNIPAM

Thermoresponsive hydrogels such as poly(N-isopropylacrylamide) (pNIPAM) are highly interesting materials for generating soft actuator systems. Whereas the material has so far mostly been used in macroscopic systems, we here demonstrate that pNIPAM is an excellent material for generating actuator systems at the micrometer scale. Two-Photon Direct Laser Writing was used to precisely structure thermoresponsive pNIPAM hydrogels at the micrometer scale based on a photosensitive resist. We systematically show that the surface- to-volume ratio of the microactuators is decisive to their actuation efficiency. The phase transition of the pNIPAM was also demonstrated by nanoindentation experiments. We observed that the mechanical properties of the material can easily be adjusted by the writing process. Finally, we found that not only the total size and surface structure of the microactuator plays an important role, but also the crosslinking of the polymer itself. Our results demonstrate for the first time a systematic study of pNIPAM-based microactuators, which can easily be extended to systems of microactuators that act cooperatively, e.g., in microvalves

Bioactive Carbon-Based Hybrid 3D Scaffolds for Osteoblast Growth

Bone, nerve, and heart tissue engineering place high demands on the conductivity of three-dimensional (3D) scaffolds. Fibrous carbon-based scaffolds are excellent material candidates to fulfill these requirements. Here, we show that highly porous (up to 94%) hybrid 3D framework structures with hierarchical architecture, consisting of microfiber composites of self-entangled carbon nanotubes (CNTs) and bioactive nanoparticles are highly suitable for growing cells. The hybrid 3D structures are fabricated by infiltrating a combination of CNTs and bioactive materials into a porous (∼94%) zinc oxide (ZnO) sacrificial template, followed by the removal of the ZnO backbone via a H2 thermal reduction process. Simultaneously, the bioactive nanoparticles are sintered. In this way, conductive and mechanically stable 3D composites of free-standing CNT-based microfibers and bioactive nanoparticles are formed. The adopted strategy demonstrates great potential for implementing low-dimensional bioactive materials, such as hydroxyapatite (HA) and bioactive glass nanoparticles (BGN), into 3D carbon-based microfibrous networks. It is demonstrated that the incorporation of HA nanoparticles and BGN promotes the biomineralization ability and the protein adsorption capacity of the scaffolds significantly, as well as fibroblast and osteoblast adhesion. These results demonstrate that the developed carbon-based bioactive scaffolds are promising materials for bone tissue engineering and related applications

A tunable scaffold of microtubular graphite for 3D cell growth

Aerographite (AG) is a novel carbon-based material that exists as a self-supportive 3D network of interconnected hollow microtubules. It can be synthesized in a variety of architectures tailored by the growth conditions. This flexibility in creating structures presents interesting bioengineering possibilities such as the generation of an artificial extracellular matrix. Here we have explored the feasibility and potential of AG as a scaffold for 3D cell growth employing cyclic RGD (cRGD) peptides coupled to poly(ethylene glycol) (PEG) conjugated phospholipids for surface functionalization to promote specific adhesion of fibroblast cells. Successful growth and invasion of the bulk material was followed over a period of 4 days