3D bioprinting of mineralizing cyanobacteria as novel approach for the fabrication of living building materials

Living building materials (LBM) are gaining interest in the field of sustainable alternative construction materials to reduce the significant impact of the construction industry on global CO2 emissions. This study investigated the process of three-dimensional bioprinting to create LBM incorporating the cyanobacterium Synechococcus sp. strain PCC 7002, which is capable of producing calcium carbonate (CaCO3) as a biocement. Rheology and printability of biomaterial inks based on alginate-methylcellulose hydrogels containing up to 50 wt% sea sand were examined. PCC 7002 was incorporated into the bioinks and cell viability and growth was characterized by fluorescence microscopy and chlorophyll extraction after the printing process. Biomineralization was induced in liquid culture and in the bioprinted LBM and observed by scanning electron microscopy, energy-dispersive X-ray spectroscopy, and through mechanical characterization. Cell viability in the bioprinted scaffolds was confirmed over 14 days of cultivation, demonstrating that the cells were able to withstand shear stress and pressure during the extrusion process and remain viable in the immobilized state. CaCO3 mineralization of PCC 7002 was observed in both liquid culture and bioprinted LBM. In comparison to cell-free scaffolds, LBM containing live cyanobacteria had a higher compressive strength. Therefore, bioprinted LBM containing photosynthetically active, mineralizing microorganisms could be proved to be beneficial for designing environmentally friendly construction materials

Die Herstellung von Feuerleichtsteinen über die Gefrier-Direktschäumungsmethode

Refractories are a key component for cost effective and energetic sustainable processes. Depending on the operation temperature of industrial furnace thermal characteristics of porous refractory linings have to be adjusted. There are a lot of possible porosity adjusting methods, e.g. placeholder, replica and gas injection/development approaches. Now, the freeze-foaming as direct foaming technique shall be introduced as an environmentally friendly way to refractories. In this presented work an aqueous ceramic mullite suspension is foamed within minutes just by the reduction of the ambient pressure in a freeze drying device. The foam structure suddenly freezes when the suspension temperature, related to the vacuum pressure, reaches the liquid-solid equilibrium line (p, T-diagram of water). The porous structure is then dried by sublimating the frozen water. The resulting bricklike lightweight refractories exhibit a high amount of open porosity and dense struts. Just 5 mass-% to 10 mass-% organic additives, required for a stable foaming, minimize the effect of crack formation during the sintering step and provide an environmentally friendly processing route to the final product. The pore morphology is being determined by X-ray computed tomographic images and mercury porosimetry. Measurements of the thermal conductivity, compressive strength and creep have been carried out to evaluate the freeze-foaming process as a promising approach for manufacturing refractories

Novel ceramic composites for personalized 3D-structures

The objective of the presented work is to further introduce a new hybrid shaping technique and develop novel porous, near-net shaped composite structures e.g. for personalized bone replacement materials. The suspension-based additive manufacturing technique Lithography-based Ceramic Manufacturing provides a high structural resolution and the manufacturing of dense (> 99%) ceramic components with a high performance compared to other available AM techniques. On the other hand the so-called Freeze Foaming offers the possibility to achieve mainly open porous and interconnected sponge-like structures provably allowing the ingrowth and differentiation of human mesenchymal stem cells (hMSCs). The near-net shaping feasibility of this foaming technique was now used to foam the inner contours of complex LCM-manufactured ceramic shell structures in shape of a femoral bone model. After a co-sintering step they combine to structural composites with dense and porous features in one single 3D-structure. This contribution therefore provides insides into a new line of technology comprising a new freedom of degree in personalization and application such as bone replacement materials

Investigation of the structure formation of Freeze Foams on the example of biocompatible ceramics: Presentation held at 5th Celluar Materials - CellMAT, Bad Staffelstein, 24.-26.10.2018

The objective of the presented work is a detailed investigation of the so-called Freeze Foaming process, which is a relatively new shaping method to produce cellular, open porous and interconnected components, e.g. ceramics for potential bone substitutes. Freeze Foaming is based on the ambient pressure reduction of an aqueous suspension in a freeze dryer. Due to the decreasing pressure, the suspension inflates and forms a proto foam until it crosses the triple point at which it consolidates by instantaneous freezing, followed by sublimation to a dry ceramic foam. The aim of a recent project is to identify process-influencing factors and their effects on the pore morphology of Freeze Foams to derive the very principles of the foaming process. For evaluating individual effects on the foam structure, a stable and reproducible model suspension was developed using hydroxyapatite as ceramic material. Characterization was carried out with respect to solid content and viscosity. This model suspension enables a stable foaming behavior resulting in a reproducible porosity of shaped Freeze Foams. In a first series of experiments to evaluate the effect of particular pore forming mechanisms the air content of the suspension and the pressure reduction rate of the freeze dryer were varied. Process monitoring by vacuum pressure and temperature recording inside the foams allowed clearly distinguishing between two pore forming factors: air and water vapor. Micro computed tomography as a nondestructive testing method and mercury porosimetry were carried out to investigate porosity, pore size and shape. Scanning electron microscopy was used to analyze the microstructure of shaped foams. As result, not only rising dissolved and entrapped air as well as vapor were individually identified as pore formers, but also ice crystals, which seem to be crucial for the development of a Freeze Foam’s microstructure. Due to sublimation during freeze-drying, microporous struts are formed, which (inter)connect the foam cells. The morphology of these micropores was found to strongly depend on the air content of the suspension. Whereas foams made of a degassed suspension showed dendritic micropores usually obtained by freeze casting of ceramics, non-degassed suspensions resulted into a more globular pore morphology. Thus, a targeted adjustment of these properties would allow directly influencing the biocompatibility and mechanical stability of potential ceramic bone substitutes

Investigation of foam structure formation in the Freeze Foaming process based on in-situ computed tomography

Using the so-called Freeze-Foaming method, it is possible to ecologically create ceramic cellular scaffolds without a need for pore creators or organic scaffolds. One of their main applications can be found in the medical sector. Tailoring these ceramic foams for certain applications requires a defined and reproducible adjustment of their pores and pore morphology.This paper examines the foaming process depending on its process parameters and the used material. In-situ X-ray computed tomography in combination with a specially designed setup for examining the foaming process allowed us to hold the stage of the foam in different phases of development and to analyze the changes in microstructure between each process phase. In this contribution we present the first findings for the production of ceramic foams depending on different process parameters. Keywords: Freeze foaming, Bioceramics, Foaming process, In-situ CT, None destructive testin

Investigation of Targeted Process Control for Adjusting the Macrostructure of Freeze Foams Using In Situ Computed Tomography

Freeze foams are novel and innovative cellular structures that are based on a direct foaming process and that can be manufactured using any material that can be processed by powder technology. The foam formation process is characterized by the highly complex interaction of various process and material parameters that were chosen empirically and that have so far been difficult to reproduce. To allow properties to be specifically tailored towards certain applications, it is necessary to examine the phenomena observed during foam formation as well as the impact of the process and material parameters on the structural constitution to deduce guidelines for manufacturing and quality assessment (e.g., mechanical strength, cell and pore sizes, pore size distribution). The variety of possible applications are a result of the wide spectrum of initial suspensions and especially the foam structure properties derived from process parameters such as the cell geometry, pore size distribution, fraction of open and closed porosity, and the textures of the cell struts. Due to earlier findings, the focus of this paper focuses on adjusting and tailoring the macrostructure (homogenization of the pore sizes and their distribution inside foam cells) to create load- and application-adapted ceramic foams. To this end, an experiment was designed using previously identified pore and characteristic influencers (air and water content, temperature of the suspension, pressure reduction rate) as influencing parameters. Their interconnected impacts on selected target values were examined during the freeze foaming process using an in situ freeze foaming device inside an X-ray

Investigation of Targeted Process Control for Adjusting the Macrostructure of Freeze Foams Using In Situ Computed Tomography

Freeze foams are novel and innovative cellular structures that are based on a direct foaming process and that can be manufactured using any material that can be processed by powder technology. The foam formation process is characterized by the highly complex interaction of various process and material parameters that were chosen empirically and that have so far been difficult to reproduce. To allow properties to be specifically tailored towards certain applications, it is necessary to examine the phenomena observed during foam formation as well as the impact of the process and material parameters on the structural constitution to deduce guidelines for manufacturing and quality assessment (e.g., mechanical strength, cell and pore sizes, pore size distribution). The variety of possible applications are a result of the wide spectrum of initial suspensions and especially the foam structure properties derived from process parameters such as the cell geometry, pore size distribution, fraction of open and closed porosity, and the textures of the cell struts. Due to earlier findings, the focus of this paper focuses on adjusting and tailoring the macrostructure (homogenization of the pore sizes and their distribution inside foam cells) to create load- and application-adapted ceramic foams. To this end, an experiment was designed using previously identified pore and characteristic influencers (air and water content, temperature of the suspension, pressure reduction rate) as influencing parameters. Their interconnected impacts on selected target values were examined during the freeze foaming process using an in situ freeze foaming device inside an X-ray

Investigation of the Foam Development Stages by Non-Destructive Testing Technology Using the Freeze Foaming Process

With a novel Freeze Foaming method, it is possible to manufacture porous cellular components whose structure and composition also enables them for application as artificial bones, among others. To tune the foam properties to our needs, we have to understand the principles of the foaming process and how the relevant process parameters and the foam’s structure are linked. Using in situ analysis methods, like X-ray microcomputed tomography (µCT), the foam structure and its development can be observed and correlated to its properties. For this purpose, a device was designed at the Institute of Lightweight Engineering and Polymer Technology (ILK). Due to varying suspension temperature and the rate of pressure decrease it was possible to analyze the foam’s developmental stages for the first time. After successfully identifying the mechanism of foam creation and cell structure formation, process routes for tailored foams can be developed in future