On the reproducibility of extrusion-based bioprinting: round robin study on standardization in the field

The outcome of three-dimensional (3D) bioprinting heavily depends, amongst others, on the interaction between the developed bioink, the printing process, and the printing equipment. However, if this interplay is ensured, bioprinting promises unmatched possibilities in the health care area. To pave the way for comparing newly developed biomaterials, clinical studies, and medical applications (i.e. printed organs, patient-specific tissues), there is a great need for standardization of manufacturing methods in order to enable technology transfers. Despite the importance of such standardization, there is currently a tremendous lack of empirical data that examines the reproducibility and robustness of production in more than one location at a time. In this work, we present data derived from a round robin test for extrusion-based 3D printing performance comprising 12 different academic laboratories throughout Germany and analyze the respective prints using automated image analysis (IA) in three independent academic groups. The fabrication of objects from polymer solutions was standardized as much as currently possible to allow studying the comparability of results from different laboratories. This study has led to the conclusion that current standardization conditions still leave room for the intervention of operators due to missing automation of the equipment. This affects significantly the reproducibility and comparability of bioprinting experiments in multiple laboratories. Nevertheless, automated IA proved to be a suitable methodology for quality assurance as three independently developed workflows achieved similar results. Moreover, the extracted data describing geometric features showed how the function of printers affects the quality of the printed object. A significant step toward standardization of the process was made as an infrastructure for distribution of material and methods, as well as for data transfer and storage was successfully established.

Flow-induced glycocalyx formation and cell alignment of HUVECs compared to iPSC-derived ECs for tissue engineering applications

The relevance of cellular in vitro models highly depends on their ability to mimic the physiological environment of the respective tissue or cell niche. Static culture conditions are often unsuitable, especially for endothelial models, since they completely neglect the physiological surface shear stress and corresponding reactions of endothelial cells (ECs) such as alignment in the direction of flow. Furthermore, formation and maturation of the glycocalyx, the essential polysaccharide layer covering all endothelial surfaces and regulating diverse processes, is highly dependent on applied fluid flow. This fragile but utterly important macromolecular layer is hard to analyze, its importance is often underestimated and accordingly neglected in many endothelial models. Therefore, we exposed human umbilical vein ECs (HUVECs) and human induced pluripotent stem cell-derived ECs (iPSC-ECs) as two relevant EC models in a side-by-side comparison to static and physiological dynamic (6.6 dyn cm⁻²) culture conditions. Both cell types demonstrated an elongation and alignment along the flow direction, some distinct changes in glycocalyx composition on the surface regarding the main glycosaminoglycan components heparan sulfate, chondroitin sulfate or hyaluronic acid as well as an increased and thereby improved glycocalyx thickness and functionality when cultured under homogeneous fluid flow. Thus, we were able to demonstrate the maturity of the employed iPSC-EC model regarding its ability to sense fluid flow along with the general importance of physiological shear stress for glycocalyx formation. Additionally, we investigated EC monolayer integrity with and without application of surface shear stress, revealing a comparable existence of tight junctions for all conditions and a reorganization of the cytoskeleton upon dynamic culture leading to an increased formation of focal adhesions. We then fabricated cell sheets of EC monolayers after static and dynamic culture via non-enzymatic detachment using thermoresponsive polymer coatings as culture substrates. In a first proof-of-concept we were able to transfer an aligned iPSC-EC sheet to a 3D-printed scaffold thereby making a step in the direction of vascular modelling. We envision these results to be a valuable contribution to improvements of in vitro endothelial models and vascular engineering in the future.

A parallelized, perfused 3D triculture model of leukemia for in vitro drug testing of chemotherapeutics

Leukemia patients undergo chemotherapy to combat the leukemic cells (LCs) in the bone marrow. During therapy not only the LCs, but also the blood-producing hematopoietic stem and progenitor cells (HSPCs) may be destroyed. Chemotherapeutics targeting only the LCs are urgently needed to overcome this problem and minimize life-threatening side-effects. Predictive in vitro drug testing systems allowing simultaneous comparison of various experimental settings would enhance the efficiency of drug development. Here, we present a 3D human leukemic bone marrow model perfused using a magnetic, parallelized culture system to ensure media exchange. Chemotherapeutic treatment of the acute myeloid leukemia cell line KG-1a in 3D magnetic hydrogels seeded with mesenchymal stem/stromal cells (MSCs) revealed a greater resistance of KG-1a compared to 2D culture. In 3D tricultures with HSPCs, MSCs and KG-1a, imitating leukemic bone marrow, HSPC proliferation decreased while KG-1a cells remained unaffected post treatment. Non-invasive metabolic profiling enabled continuous monitoring of the system. Our results highlight the importance of using biomimetic 3D platforms with proper media exchange and co-cultures for creating in vivo-like conditions to enable in vitro drug testing. This system is a step towards drug testing in biomimetic, parallelized in vitro approaches, facilitating the discovery of new anti-leukemic drugs.

Fused Deposition Modeling of Microfluidic Chips in Transparent Polystyrene

Polystyrene (PS) is one of the most commonly used thermoplastic materials worldwide and plays a ubiquitous role in today's biomedical and life science industry and research. The main advantage of PS lies in its facile processability, its excellent optical and mechanical properties, as well as its biocompatibility. However, PS is only rarely used in microfluidic prototyping, since the structuring of PS is mainly performed using industrial-scale replication processes. So far, microfluidic chips in PS have not been accessible to rapid prototyping via 3D printing. In this work, we present, for the first time, 3D printing of transparent PS using fused deposition modeling (FDM). We present FDM printing of transparent PS microfluidic channels with dimensions as small as 300 µm and a high transparency in the region of interest. Furthermore, we demonstrate the fabrication of functional chips such as Tesla-mixer and mixer cascades. Cell culture experiments showed a high cell viability during seven days of culturing, as well as enabling cell adhesion and proliferation. With the aid of this new PS prototyping method, the development of future biomedical microfluidic chips will be significantly accelerated, as it enables using PS from the early academic prototyping all the way to industrial-scale mass replication.

Nitric Oxide in the Control of the in vitro Proliferation and Differentiation of Human Hematopoietic Stem and Progenitor Cells

Hematopoietic stem and progenitor cell (HSPC) transplantation is the best-studied cellular therapy and successful in vitro control of HSPCs has wide clinical implications. Nitric oxide (NO) is a central signaling molecule in vivo and has been implicated in HSPC mobilization to the blood stream in mice. The influence of NO on HSPC behavior in vitro is, however, largely obscure due to the variety of employed cell types, NO administration systems, and used concentration ranges in the literature. Additionally, most studies are based on murine cells, which do not necessarily mimic human HSPC behavior. Thus, the aim of the present study was the systematic, concentration-dependent evaluation of NO-mediated effects on human HSPC behavior in vitro. By culture in the presence of the long-term NO donor diethylenetriamine/nitric oxide adduct (DETA/NO) in a nontoxic concentration window, a biphasic role of NO in the regulation of HSPC behavior was identified: Low DETA/NO concentrations activated classical NO signaling, identified via increased intracellular cyclic guanosine monophosphate (cGMP) levels and proteinkinases G (PKG)-dependent vasodilator-stimulated phosphoprotein (VASP) phosphorylation and mediated a pro-proliferative response of HSPCs. In contrast, elevated NO concentrations slowed cell proliferation and induced HSPC differentiation. At high concentrations, s-nitrosylation levels were elevated, and myeloid differentiation was increased at the expense of lymphoid progenitors. Together, these findings hint at a central role of NO in regulating human HSPC behavior and stress the importance and the potential of the use of adequate NO concentrations for in vitro cultures of HSPCs, with possible implications for clinical application of in vitro expanded or differentiated HSPCs for cellular therapies.

Iron Nanoparticle Composite Hydrogels for Studying Effects of Iron Ion Release on Red Blood Cell In Vitro Production

Growing numbers of complex surgical interventions increase the need for blood transfusions, which cannot be fulfilled by the number of donors. Therefore, the interest in producing erythrocytes from their precursors-the hematopoietic stem and progenitor cells (HSPCs)-in laboratories is rising. To enable this, in vitro systems are needed, which allow analysis of the effects of essential factors such as iron on erythroid development. For this purpose, iron ion-releasing systems based on poly(ethylene glycol) (PEG)-iron nanocomposites are developed to assess if gradual iron release improves iron bioavailability during in vitro erythroid differentiation. The nanocomposites are synthesized using surfactant-free pulsed laser ablation of iron directly in the PEG solution. The iron concentrations released from the material are sufficient to influence in vitro erythropoiesis. In this way, the production of erythroid cells cultured on flat PEG-iron nanocomposite hydrogel pads can be enhanced. In contrast, erythroid differentiation is not enhanced in the biomimetic macroporous 3D composite scaffolds, possibly because of local iron overload within the pores of the system. In conclusion, the developed iron nanoparticle-PEG composite hydrogel allows constant iron ion release and thus paves the way (i) to understand the role of iron during erythropoiesis and (ii) toward the development of biomaterials with a controlled iron release for directing erythropoiesis in culture.

Limited Potential or Unfavorable Manipulations? Strategies Toward Efficient Mesenchymal Stem/Stromal Cell Applications

Despite almost 50 years of research and over 20 years of preclinical and clinical studies, the question of curative potential of mesenchymal stem/stromal cells (MSCs) is still widely discussed in the scientific community. Non-reproducible treatment outcomes or even absence of treatment effects in comparison to control groups challenges the potential of these cells for routine application both in tissue engineering and in regenerative medicine. One of the reasons of such outcomes is non-standardized and often disadvantageous ex vivo manipulation of MSCs prior therapy. In most cases, clinically relevant cell numbers for MSC-based therapies can be only obtained by in vitro expansion of isolated cells. In this mini review, we will discuss point by point possible pitfalls in the production of human MSCs for cell therapies, without consideration of material-based applications. Starting with cell source, choice of donor and recipient, as well as isolation methods, we will then discuss existing expansion protocols (two-/three-dimensional cultivation, basal medium, medium supplements, static/dynamic conditions, and hypoxic/normoxic conditions) and influence of these strategies on the cell functionality after implantation. The role of potency assays will also be addressed. The final aim of this mini review is to illustrate the heterogeneity of current strategies for gaining MSCs for clinical applications with their strengths and weaknesses. Only a careful consideration and standardization of all pretreatment processes/methods for the different applications of MSCs will ensure robust and reproducible performance of these cell populations in the different experimental and clinical settings.

Potential of electrospun cationic BSA fibers to guide osteogenic MSC differentiation via surface charge and fibrous topography

Large or complex bone fractures often need clinical treatments for sufficient bone repair. New treatment strategies have pursued the idea of using mesenchymal stromal cells (MSCs) in combination with osteoinductive materials to guide differentiation of MSCs into bone cells ensuring complete bone regeneration. To overcome the challenge of developing such materials, fundamental studies are needed to analyze and understand the MSC behavior on modified surfaces of applicable materials for bone healing. For this purpose, we developed a fibrous scaffold resembling the bone/bone marrow extracellular matrix (ECM) based on protein without addition of synthetic polymers. With this biomimetic in vitro model we identified the fibrous structure as well as the charge of the material to be responsible for its effects on MSC differentiation. Positive charge was introduced via cationization that additionally supported the stability of the scaffold in cell culture, and acted as nucleation point for mineralization during osteogenesis. Furthermore, we revealed enhanced focal adhesion formation and osteogenic differentiation of MSCs cultured on positively charged protein fibers. This pure protein-based and chemically modifiable, fibrous ECM model allows the investigation of MSC behavior on biomimetic materials to unfold new vistas how to direct cells' differentiation for the development of new bone regenerating strategies.

3D models of the bone marrow in health and disease: yesterday, today and tomorrow

The complex interaction between hematopoietic stem cells (HSCs) and their microenvironment in the human bone marrow ensures a life-long blood production by balancing stem cell maintenance and differentiation. This so-called HSC niche can be disturbed by malignant diseases. Investigating their consequences on hematopoiesis requires deep understanding of how the niches function in health and disease. To facilitate this, biomimetic models of the bone marrow are needed to analyse HSC maintenance and hematopoiesis under steady-state and diseased conditions. Here, 3D bone marrow models, their fabrication methods (including 3D bioprinting) and implementations recapturing bone marrow functions in health and diseases, are presented.

Migration Assay for Leukemic Cells in a 3D Matrix Toward a Chemoattractant

In leukemia, leukemic cells hijack the hematopoietic stem cell (HSC) microenvironment in the bone marrow-the so-called stem cell niche-by flooding the niche with clonal progeny of leukemic cells. They can exploit signaling pathways which are critical for HSC development to support their own survival, homing, and maintenance. These interactions of leukemic cells with the microenvironment have an impact on therapy progress and patient outcome. Therefore, signals for homing and anchorage of leukemic cells to the bone marrow have to be investigated by using tools that allow the migration of cells toward critical signals. Here, we describe an in vitro migration assay for leukemic cells toward a chemoattractant in a 3D environment exemplified by migration of the cell line OCI-AML3 to a CXC motif chemokine ligand 12 (CXCL12) gradient. For this purpose, a chemotaxis slide is filled with a hydrogel system mimicking the extracellular matrix in vivo. The cells are encapsulated into the hydrogel network during polymerization, and a CXCL12 gradient is introduced in the enclosed chambers to trigger migration. Cell migration in the 3D network of the hydrogel is monitored by time-lapse microscopy. We describe the experimental setup and the tools for cell tracking and data analysis.

Biomimetic 3D in vitro model of biofilm triggered osteomyelitis for investigating hematopoiesis during bone marrow infections

In this work, we define the requirements for a human cell-based osteomyelitis model which overcomes the limitations of state of the art animal models. Osteomyelitis is a severe and difficult to treat infection of the bone that develops rapidly, making it difficult to study in humans. We have developed a 3D in vitro model of the bone marrow, comprising a macroporous material, human hematopoietic stem and progenitor cells (HSPCs) and mesenchymal stromal cells (MSCs). Inclusion of biofilms grown on an implant into the model system allowed us to study the effects of postoperative osteomyelitis-inducing bacteria on the bone marrow. The bacteria influenced the myeloid differentiation of HSPCs as well as MSC cytokine expression and the MSC ability to support HSPC maintenance. In conclusion, we provide a new 3D in vitro model which meets all the requirements for investigating the impact of osteomyelitis.

STATEMENT OF SIGNIFICANCE

Implant-associated osteomyelitis is a persistent bacterial infection of the bone which occurs in many implant patients and can result in functional impairments or even entire loss of the extremity. Nevertheless, surprisingly little is known on the triangle interaction between implant material, bacterial biofilm and affected bone tissue. Closing this gap of knowledge would be crucial for the fundamental understanding of the disease and the development of novel treatment strategies. For this purpose, we developed the first biomaterial-based system that is able to mimic implant-associated osteomyelitis outside of the body, thus, opening the avenue to study this fatal disease in the laboratory.

Magnetic Macroporous Hydrogels as a Novel Approach for Perfused Stem Cell Culture in 3D Scaffolds via Contactless Motion Control

There is an urgent need for 3D cell culture systems that avoid the oversimplifications and artifacts of conventional culture in 2D. However, 3D culture within the cavities of porous biomaterials or large 3D structures harboring high cell numbers is limited by the needs to nurture cells and to remove growth-limiting metabolites. To overcome the diffusion-limited transport of such soluble factors in 3D culture, mixing can be improved by pumping, stirring or shaking, but this in turn can lead to other problems. Using pumps typically requires custom-made accessories that are not compatible with conventional cell culture disposables, thus interfering with cell production processes. Stirring or shaking allows little control over movement of scaffolds in media. To overcome these limitations, magnetic, macroporous hydrogels that can be moved or positioned within media in conventional cell culture tubes in a contactless manner are presented. The cytocompatibility of the developed biomaterial and the applied magnetic fields are verified for human hematopoietic stem and progenitor cells (HSPCs). The potential of this technique for perfusing 3D cultures is demonstrated in a proof-of-principle study that shows that controlled contactless movement of cell-laden magnetic hydrogels in culture media can mimic the natural influence of differently perfused environments on HSPCs.

3D models of the hematopoietic stem cell niche under steady-state and active conditions

Hematopoietic stem cells (HSCs) in the bone marrow are able to differentiate into all types of blood cells and supply the organism each day with billions of fresh cells. They are applied to cure hematological diseases such as leukemia. The clinical need for HSCs is high and there is a demand for being able to control and multiply HSCs in vitro. The hematopoietic system is highly proliferative and thus sensitive to anti-proliferative drugs such as chemotherapeutics. For many of these drugs suppression of the hematopoietic system is the dose-limiting toxicity. Therefore, biomimetic 3D models of the HSC niche that allow to control HSC behavior in vitro and to test drugs in a human setting are relevant for the clinics and pharmacology. Here, we describe a perfused 3D bone marrow analog that allows mimicking the HSC niche under steady-state and activated conditions that favor either HSC maintenance or differentiation, respectively, and allows for drug testing.

Direct Mapping of RAFT Controlled Macromolecular Growth on Surfaces via Single Molecule Force Spectroscopy

Single molecule force spectroscopy (SMFS) is employed to gain insight into reversible addition-fragmentation chain transfer (RAFT) polymerization processes with living characteristics on glass surfaces. Surface-initiated (SI)-RAFT was selected to grow poly(hydroxyethyl methacrylate) (PHEMA). After aminolysis of the RAFT chain termini, thiol moieties serve as anchoring points for the gold tip of an atomic force microscope. The results allow to directly monitor the macromolecular growth of the surface-initiated polymerization. The obtained SMFS-based molecular weight distribution data of the polymers present on the surface indicate that the RAFT chain extension proceeds linearly with time up to high conversions. The current study thus adds SMFS as a valuable tool for the investigation of SI-RAFT polymerizations.

Biophysical regulation of hematopoietic stem cells

Blood is renewed throughout the entire life. The stem cells of the blood, called hematopoietic stem cells (HSCs), are responsible for maintaining a supply of all types of fresh blood cells. In contrast to other stem cells, the clinical application of these cells is well established and HSC transplantation is an established life-saving therapy for patients suffering from haematological disorders. Despite their efficient functionality throughout life in vivo, controlling HSC behaviour in vitro (including their proliferation and differentiation) is still a major task that has not been resolved with standard cell culture systems. Targeted HSC multiplication in vitro could be beneficial for many patients, because HSC supply is limited. The biology of these cells and their natural microenvironment - their niche - remain a matter of ongoing research. In recent years, evidence has come to light that HSCs are susceptible to physical stimuli. This makes the regulation of HSCs by engineering physical parameters a promising approach for the targeted manipulation of these cells for clinical applications. Nevertheless, the biophysical regulation of these cells is still poorly understood. This review sheds light on the role of biophysical parameters in HSC biology and outlines which knowledge on biophysical regulation identified in other cell types could be applied to HSCs.

Fabrication of biofunctionalized, cell-laden macroporous 3D PEG hydrogels as bone marrow analogs for the cultivation of human hematopoietic stem and progenitor cells

In vitro proliferation of hematopoietic stem cells (HSCs) is yet an unresolved challenge. Found in the bone marrow, HSCs can undergo self-renewing cell division and thereby multiply. Recapitulation of the bone marrow environment in order to provide the required signals for their expansion is a promising approach.Here, we describe a technique to produce biofunctionalized, macroporous poly(ethylene glycol) diacrylate (PEGDA) hydrogels that mimic the spongy 3D architecture of trabecular bones, which host the red, blood-forming bone marrow. After seeding these scaffolds with cells, they can be used as simplified bone marrow analogs for the cultivation of HSCs. This method can easily be conducted with standard laboratory chemicals and equipment. The 3D hydrogels are produced via salt leaching and biofunctionalization of the material is achieved by co-polymerizing the PEGDA with an RGD peptide. Finally, cell seeding and retrieval are described.

Surface Properties of Nanostructured Bio-Active Interfaces: Impacts of Surface Stiffness and Topography on Cell-Surface Interactions

Due to their ability to confer key functions of the native extracellular matrix (ECM) poly(ethylene glycol) (PEG)-based and PEG-modified materials have been extensively used as biocompatible and biofunctionalized substrate systems to study the influence of environmental parameters on cell adhesion in vitro. Given wide-ranging recent evidence that ECM compliance influences a variety of cell functions, the detailed determination and characterization of the specific PEG surface characteristics including topography, stiffness and chemistry is required. Here, we studied two frequently used bio-active interfaces - PEG-based and PEG-modified surfaces - to elucidate the differences between the physical surface properties, which cells can sense and respond to. For this purpose, two sets of surfaces were synthesized: the first set consisted of nanopatterned glass surfaces containing cRGD-functionalized gold nanoparticles surrounded by a passivated PEG-silane layer and the second set consisted of PEG-diacrylate (PEG-DA) hydrogels decorated with cRGD-functionalized gold nanoparticlesAlthough the two sets of nanostructured materials compared here were highly similar in terms of density and geometrical distribution of the presented bio-ligands as well as in terms of mechanical bulk properties, the topography and mechanical properties of the surfaces were found to be substantially different and are described in detail. In comparison to very stiff and ultrasmooth surface properties of the PEG-passivated glasses, the mechanical properties of PEG-DA surfaces in the biologically relevant stiffness range, together with the increased surface roughness at micro- and nanoscale levels have the potential to affect cell behavior. This potential was verified by studying the adhesive behavior of hematopoietic KG-1a and rat embryonic fibroblast (REF52) cells on both surfaces.

Regulation of hematopoietic stem cell behavior by the nanostructured presentation of extracellular matrix components

Hematopoietic stem cells (HSCs) are maintained in stem cell niches, which regulate stem cell fate. Extracellular matrix (ECM) molecules, which are an essential part of these niches, can actively modulate cell functions. However, only little is known on the impact of ECM ligands on HSCs in a biomimetic environment defined on the nanometer-scale level. Here, we show that human hematopoietic stem and progenitor cell (HSPC) adhesion depends on the type of ligand, i.e., the type of ECM molecule, and the lateral, nanometer-scaled distance between the ligands (while the ligand type influenced the dependency on the latter). For small fibronectin (FN)-derived peptide ligands such as RGD and LDV the critical adhesive interligand distance for HSPCs was below 45 nm. FN-derived (FN type III 7-10) and osteopontin-derived protein domains also supported cell adhesion at greater distances. We found that the expression of the ECM protein thrombospondin-2 (THBS2) in HSPCs depends on the presence of the ligand type and its nanostructured presentation. Functionally, THBS2 proved to mediate adhesion of HSPCs. In conclusion, the present study shows that HSPCs are sensitive to the nanostructure of their microenvironment and that they are able to actively modulate their environment by secreting ECM factors.

The significance of integrin ligand nanopatterning on lipid raft clustering in hematopoietic stem cells

Hematopoietic stem cells (HSCs) are the vital, life-long source of all blood cell types. They are found in stem cell niches, specific anatomic locations that offer all the factors and signals necessary for the maintenance of the stem cell potential of HSCs. Much attention has been paid to the biochemical composition of the niches, but only little is known about the influence of physical parameters, such as ligand nanopatterns, on HSCs. To investigate the impact of nanometer-scale spacing between cell ligands on HSC adhesion, integrin distribution and signal transduction, we employed geometrically defined, nanostructured, bio-functionalized surfaces. HSCs proved to be sensitive to the lateral distance between the presented ligands with regard to adhesion and lipid raft clustering, the latter being a prerequisite for the formation of signaling complexes. Furthermore, an extensive redistribution of stem cell markers, integrins and phosphorylated proteins in HSCs was observed. In conclusion, integrin-mediated adhesion and signaling of HSCs proved to depend on the nanostructured presentation of ligands in their environment. In this work, we show that the nanostructure of the matrix is an important parameter influencing HSC behavior that should be integrated into biomaterial-based approaches aiming at HSC multiplication or differentiation.

Impact of substrate elasticity on human hematopoietic stem and progenitor cell adhesion and motility

In the bone marrow, hematopoietic stem cells (HSCs) reside in endosteal and vascular niches. The interactions with the niches are essential for the maintenance of HSC number and properties. Although the molecular nature of these interactions is quite well understood, little is known about the role of physical parameters such as matrix elasticity. Osteoblasts, the major cellular component of the endosteal HSC niche, flatten during HSC mobilization. We show that this process is accompanied by osteoblast stiffening, demonstrating that not only biochemical signals but also mechanical properties of the niche are modulated. HSCs react to stiffer substrates with increased cell adhesion and migration, which could facilitate the exit of HSCs from the niche. These results indicate that matrix elasticity is an important factor in regulating the retention of HSCs in the endosteal niche and should be considered in attempts to propagate HSCs in vitro for clinical applications.