Biomechanical cues such as shear stress, stretching, compression, and matrix elasticity are vital in the establishment of next generation physiological in vitro tissue models. Matrix elasticity, for instance, is known to guide stem cell differentiation, influence healing processes and modulate extracellular matrix (ECM) deposition needed for tissue development and maintenance. To better understand the biomechanical effect of matrix elasticity on the formation of articular cartilage analogs in vitro, this study aims at assessing the redifferentiation capacity of primary human chondrocytes in three different hydrogel matrices of predefined matrix elasticities. The hydrogel elasticities were chosen to represent a broad spectrum of tissue stiffness ranging from very soft tissues with a Young’s modulus of 1 kPa up to elasticities of 30 kPa, representative of the perichondral-space. In addition, the interplay of matrix elasticity and transforming growth factor beta-3 (TGF-β3) on the redifferentiation of primary human articular chondrocytes was studied by analyzing both qualitative (viability, morphology, histology) and quantitative (RT-qPCR, sGAG, DNA) parameters, crucial to the chondrotypic phenotype. Results show that fibrin hydrogels of 30 kPa Young’s modulus best guide chondrocyte redifferentiation resulting in a native-like morphology as well as induces the synthesis of physiologic ECM constituents such as glycosaminoglycans (sGAG) and collagen type II. This comprehensive study sheds light onto the mechanobiological impact of matrix elasticity on formation and maintenance of articular cartilage and thus represents a major step toward meeting the need for advanced in vitro tissue models to study both re- and degeneration of articular cartilage.

Knowledge on the availability of dissolved oxygen inside microfluidic cell culture systems is vital for recreating physiological-relevant microenvironments and for providing reliable and reproducible measurement conditions. It is important to highlight that in vivo cells experience a diverse range of oxygen tensions depending on the resident tissue type, which can also be recreated in vitro using specialized cell culture instruments that regulate external oxygen concentrations. While cell-culture conditions can be readily adjusted using state-of-the-art incubators, the control of physiological-relevant microenvironments within the microfluidic chip, however, requires the integration of oxygen sensors. Although several sensing approaches have been reported to monitor oxygen levels in the presence of cell monolayers, oxygen demands of microfluidic three-dimensional (3D)-cell cultures and spatio-temporal variations of oxygen concentrations inside two-dimensional (2D) and 3D cell culture systems are still largely unknown. To gain a better understanding on available oxygen levels inside organ-on-a-chip systems, we have therefore developed two different microfluidic devices containing embedded sensor arrays to monitor local oxygen levels to investigate (i) oxygen consumption rates of 2D and 3D hydrogel-based cell cultures, (ii) the establishment of oxygen gradients within cell culture chambers, and (iii) influence of microfluidic material (e.g., gas tight vs. gas permeable), surface coatings, cell densities, and medium flow rate on the respiratory activities of four different cell types. We demonstrate how dynamic control of cyclic normoxic-hypoxic cell microenvironments can be readily accomplished using programmable flow profiles employing both gas-impermeable and gas-permeable microfluidic biochips.

In the advent of affordable photo- and soft-lithography using polydimethylsiloxane (PDMS), low cost multi-step microfabrication methods have become available to a broad scientific community today. Although these methods are frequently applied for microfluidic prototype production in academic and industrial settings, fast design iterations and rapid prototyping within a few minutes with a high degree of flexibility are nearly impossible. To reduce microfluidic concept-to-chip time and costs, a number of alternative rapid prototyping techniques have recently been introduced including CNC micromachining, 3D printing and plotting out of numeric CAD designs as well as micro-structuring of thin PDMS sheets and pressure sensitive adhesives. Although micro-structuring of pressure sensitive adhesives promises high design flexibility, rapid fabrication and simple biochip assembly, most adhesives are toxic for living biological systems. Since an appropriate bio-interface and proper biology-material interaction is key for any cell chip and organ-on-a-chip system, only a limited number of medical-grade materials are available for microfluidic prototyping. In this study, we have characterized four functional biomedical-grade pressure sensitive adhesives for rapid prototyping (e.g. less than 1 hour) applications including structuring precision, physical and optical properties as well as biocompatibilities. While similar biocompatibility was found for all four adhesives, significant differences in cutting behavior, bonding strength to glass and polymers as well as gas permeability was observed. Practical applications included stability testing of multilayered, membrane-integrated organ-on-a-chip devices under standard cell culture conditions (e.g. 2–3 weeks at 37 °C and 100% humidity) and a shear-impact up to 5 dynes/cm². Additionally, time- and shear-dependent uptake of non-toxic fluorescently labelled nanoparticles on human endothelial cells are demonstrated using micro-structured adhesive-bonded devices. Our results show that (a) both simple and complex microdevices can be designed, fabricated and tested in less than 1 hour, (b) these microdevices are stable for weeks even under physiological shear force conditions and (c) can be used to maintain cell monolayers as well as 3D cell culture systems.

Organ-on-a-chip technology has the potential to accelerate pharmaceutical drug development, improve the clinical translation of basic research, and provide personalized intervention strategies. In the last decade, big pharma has engaged in many academic research cooperations to develop organ-on-a-chip systems for future drug discoveries. Although most organ-on-a-chip systems present proof-of-concept studies, miniaturized organ systems still need to demonstrate translational relevance and predictive power in clinical and pharmaceutical settings. This review explores whether microfluidic technology succeeded in paving the way for developing physiologically relevant human in vitro models for pharmacology and toxicology in biomedical research within the last decade. Individual organ-on-a-chip systems are discussed, focusing on relevant applications and highlighting their ability to tackle current challenges in pharmacological research.

Reengineering functional vascular networks in vitro remains an integral part in tissue engineering, since the incorporation of non-perfused tissues results in restricted nutrient supply and limited waste removal. Microfluidic devices are routinely used to mimic both physiological and pathological vascular microenvironments. Current procedures either involve the investigation of growth factor gradients and interstitial flow on endothelial cell sprouting alone or on the heterotypic cell-cell interactions between endothelial and mural cells. However, limited research has been conducted on the influence of flow on co-cultures of these cells. Here, we exploited the ability of microfluidics to create and monitor spatiotemporal gradients to investigate the influence of growth factor supply and elution on vascularization using static as well as indirect and direct flow setups. Co-cultures of human adipose-derived stem/stromal cells and human umbilical vein endothelial cells embedded in fibrin hydrogels were found to be severely affected by diffusion limited growth factor gradients as well as by elution of reciprocal signaling molecules during both static and flow conditions. Static cultures formed pre-vascular networks up to a depth of 4 mm into the construct with subsequent decline due to diffusion limitation. In contrast, indirect flow conditions enhanced endothelial cell sprouting but failed to form vascular networks. Additionally, complete inhibition of pre-vascular network formation was observable for direct application of flow through the hydrogel with decline of endothelial cell viability after seven days. Using finite volume CFD simulations of different sized molecules vital for pre-vascular network formation into and out of the hydrogel constructs, we found that interstitial flow enhances growth factor supply to the cells in the bulk of the chamber but elutes cellular secretome, resulting in truncated, premature vascularization.

The understanding that systemic context and tissue crosstalk are essential keys for bridging the gap between in vitro models and in vivo conditions led to a growing effort in the last decade to develop advanced multi-organ-on-a-chip devices. However, many of the proposed devices have failed to implement the means to allow for conditions tailored to each organ individually, a crucial aspect in cell functionality. Here, we present two 3D-print-based fabrication methods for a generic multi-organ-on-a-chip device: One with a PDMS microfluidic core unit and one based on 3D-printed units. The device was designed for culturing different tissues in separate compartments by integrating individual pairs of inlets and outlets, thus enabling tissue-specific perfusion rates that facilitate the generation of individual tissue-adapted perfusion profiles. The device allowed tissue crosstalk using microchannel configuration and permeable membranes used as barriers between individual cell culture compartments. Computational fluid dynamics (CFD) simulation confirmed the capability to generate significant differences in shear stress between the two individual culture compartments, each with a selective shear force. In addition, we provide preliminary findings that indicate the feasibility for biological compatibility for cell culture and long-term incubation in 3D-printed wells. Finally, we offer a cost-effective, accessible protocol enabling the design and fabrication of advanced multi-organ-on-a-chip devices.