Microthermoforming as a novel technique for manufacturing scaffolds in tissue engineering (CellChips)

Assessment of Cell-Material Interactions in Three Dimensions through Dispersed Coaggregation of Microsized Biomaterials into Tissue Spheroids

In biomaterials R&D, conventional monolayer cell culture on flat/planar material samples, such as films, is still commonly employed at early stages of the assessment of interactions of cells with candidate materials considered for a biomedical application. In this feasibility study, an approach for the assessment of 3D cell-material interactions through dispersed coaggregation of microparticles from biomaterials into tissue spheroids is presented. Biomaterial microparticles can be created comparatively quickly and easily, allow the miniaturization of the assessment platform, and enable an unhindered remodeling of the dynamic cell-biomaterial system at any time. The aggregation of the microsized biomaterials and the cells is supported by low-attachment round-bottom microwells from thin polymer films arranged in densely packed arrays. The study is conducted by the example of MG63 osteoblast-like and human mesenchymal stem/stromal cells, and a small library of model microbiomaterials related to bone repair and regeneration. For the proof of concept, example interactions including cell adhesion to the material, the hybrid spheroids' morphology, size, and shape, material-associated cell death, cell metabolic activity, cell proliferation, and (osteogenic) differentiation are investigated. The cells in the spheroids are shown to respond to differences in the microbiomaterials' properties, their amounts, and the duration of interaction with them.

Flat and microstructured polymeric membranes in organs-on-chips

In recent years, organs-on-chips (OOCs) have been developed to meet the desire for more realistic in vitro cell culture models. These systems introduce microfluidics, mechanical stretch and other physiological stimuli to in vitro models, thereby significantly enhancing their descriptive power. In most OOCs, porous polymeric membranes are used as substrates for cell culture. The polymeric material, morphology and shape of these membranes are often suboptimal, despite their importance for achieving ideal cell functionality such as cell-cell interaction and differentiation. The currently used membranes are flat and thus do not account for the shape and surface morphology of a tissue. Moreover, the polymers used for fabrication of these membranes often lack relevant characteristics, such as mechanical properties matching the tissue to be developed and/or cytocompatibility. Recently, innovative techniques have been reported for fabrication of porous membranes with suitable porosity, shape and surface morphology matching the requirements of OOCs. In this paper, we review the state of the art for developing these membranes and discuss their application in OOCs.

A Microcavity Array-Based 4D Cell Culture Platform

(1) Background: We describe a 4D cell culture platform with which we tried to detect and to characterize migration dynamics of single hematopoietic stem cells in polymer film microcavity arrays integrated into a microtiter plate. (2) Methods: The system was set up with CD34-expressing KG-1a cells as a surrogate for hematopoietic stem cells. We then evaluated the system as an artificial hematopoietic stem cell niche model comprised of a co-culture of human hematopoietic stem cells from cord blood (cord blood CD34+ cells, hHSCs) and human mesenchymal stromal cells (hMSCs) from bone marrow over a period of 21 days. We used a software-based cell detection method to count single hematopoietic stem cells (HSCs) in microcavities. (3) Results: It was possible to detect single HSCs and their migration behavior within single microcavities. The HSCs displayed a pronounced migration behavior with one population of CD34-expressing cells located at the bottom of the microcavities and one population located in the middle of the microcavities at day 14. However, at day 21 the two populations seemed to unite again so that no clear distinction between the two was possible anymore. (4) Conclusions: Single cell migration detection was possible but microscopy and flow cytometry delivered non-uniform data sets. Further optimization is currently being developed.

The effect of pressure and temperature on microthermoforming thermoplastic films integrated in the injection moulding process

Injection moulding is a widespread large-scale production technology for the manufacturing of thermoplastic parts, with small wall thicknesses limiting the feasible flow length. Introducing microthermoforming into the injection moulding process with dynamic mould temperature control enables the production of film-based, plane microstructured parts with further three-dimensional functional structures (e.g. for handling or for fitting in devices/assembly groups). Investigations show that considerable forming is possible with pressures up to 140 bar and forming temperatures far below the glass transition temperature of 50-μm-thick polycarbonate films in cycle times of <3 min. Generally speaking, the novel technology is expected to allow for multifunctional, thin-walled microstructured parts at large scales with short cycle times.

Microcavity arrays as an in vitro model system of the bone marrow niche for hematopoietic stem cells

In previous studies human mesenchymal stromal cells (MSCs) maintained the "stemness" of human hematopoietic progenitor cells (HPCs) through direct cell-cell contact in two-dimensional co-culture systems. We establish a three-dimensional (3D) co-culture system based on a custom-made chip, the 3(D)-KITChip, as an in vitro model system of the human hematopoietic stem cell niche. This array of up to 625 microcavities, with 300 μm size in each orientation, was inserted into a microfluidic bioreactor. The microcavities of the 3(D)-KITChip were inoculated with human bone marrow MSCs together with umbilical cord blood HPCs. MSCs used the microcavities as a scaffold to build a complex 3D mesh. HPCs were distributed three-dimensionally inside this MSC network and formed ß-catenin- and N-cadherin-based intercellular junctions to the surrounding MSCs. Using RT(2)-PCR and western blots, we demonstrate that a proportion of HPCs maintained the expression of CD34 throughout a culture period of 14 days. In colony-forming unit assays, the hematopoietic stem cell plasticity remained similar after 14 days of bioreactor co-culture, whereas monolayer co-cultures showed increasing signs of HPC differentiation and loss of stemness. These data support the notion that the 3D microenvironment created within the microcavity array preserves vital stem cell functions of HPCs more efficiently than conventional co-culture systems.

Microthermoforming Integrated in the Injection Molding Process for Fabrication of Film-Based Microstructured Parts

Thermoplastic micro parts are commonly manufactured by the large-scale production technology of micro injection molding, with small wall thicknesses limiting the feasible flow length. In contrast, microthermoforming is an inexpensive technology to manufacture thin-walled microstructured parts, but connecting three-dimensional solid bodies, e.g. functional structures, is impossible. The novel combination of processing techniques, i.e. microthermoforming integrated in the injection molding process with dynamic mold temperature control enables the use of non-adhesive polymer melt to form thermoplastic films via back molding and employ adhesive melt to connect further structures. Additional heating the film with the back molding component represents a particular benefit in terms of reaching the required film temperature and leads to low temperatures prevailing in the mold. Simulative tools facilitate the understanding of film temperatures after back molding and can be used in a further step for the adaption of homogenous temperature distributions across areally distributed microstructures. Generally speaking, the technology is expected to allow for multifunctional, thin-walled microstructured parts produced in short cycle times.

Characterization of a chip-based bioreactor for three-dimensional cell cultivation via Magnetic Resonance Imaging

We describe the characterization of a chip-based platform (3(D)-KITChip) for the three-dimensional cultivation of cells under perfusion conditions via magnetic resonance imaging (MRI). Besides the chip, the microfluidic system is comprised of a bioreactor housing, a medium supply, a pump for generating active flow conditions as well as a gas mixing station. The closed circulation loop is ideally suited for a characterization via MRI since the small bioreactor setup with active perfusion, driven by the pump from outside the coils, not only is completely MRI-compatible but also can be transferred into the magnetic coil of an experimental animal scanner. We have found that the two halves of the chip inside the bioreactor are homogeneously perfused with cell culture medium both with and without cells inside the 3(D)-KITChip. In addition, the homogeneity of perfusion is nearly independent from the flow rates investigated in this study, and furthermore, the setup shows excellent washout characteristics after spiking with Gadolinium-DOTA which makes it an ideal candidate for drug screening purposes. We, therefore, conclude that the 3(D)-KITChip is well suited as a platform for high-density three-dimensional cell cultures, especially those requiring a defined medium flow and/or gas supply in a precisely controllable three dimensional environment, like stem cells.

Promotion of osteoblast differentiation in 3D biomaterial micro-chip arrays comprising fibronectin-coated poly(methyl methacrylate) polycarbonate

Due to the architecture of solid body tissues including bone, three-dimensional (3D) in vitro microenvironments appear favorable, since herein cell growth proceeds under more physiological conditions compared to conventional 2D systems. In the present study we show that a 3D microenvironment comprising a fibronectin-coated PMMA/PC-based micro-chip promotes differentiation of primary human osteoblasts as reflected by the densely-packed 3D bone cell aggregates and expression of biomarkers indicating osteoblast differentiation. Morphogenesis and fluorescence dye-based live/dead staining revealed homogenous cell coverage of the microcavities of the chip array, whereat cells showed high viability up to 14 days. Moreover, Azur II staining proved formation of uniform sized multilayered aggregates, exhibiting progressive intracellular deposition of extracellular bone matrix constituents comprising fibronectin, osteocalcin and osteonectin from day 7 on. Compared to 2D monolayers, osteoblasts grown in the 3D chip environment displayed differential mostly higher gene expression for osteocalcin, osteonectin, and alkaline phosphatase, while collagen type I remained fairly constant in both culture environments. Our results indicate that the 3D microenvironment, based on the PMMA biomaterial chip array promotes osteoblast differentiation, and hereby renders a promising tool for tissue-specific in vitro preconditioning of osteoblasts designated for clinically-oriented bone augmentation or regeneration.

Thermoforming of film-based biomedical microdevices

For roughly ten years now, a new class of polymer micromoulding processes comes more and more into the focus both of the microtechnology and the biomedical engineering community. These processes can be subsumed under the term "microthermoforming". In microthermoforming, thin polymer films are heated to a softened, but still solid state and formed to thin-walled microdevices by three-dimensional stretching. The high material coherence during forming is in contrast to common polymer microreplication processes where the material is processed in a liquid or flowing state. It enables the preservation of premodifications of the film material. In this progress report, we review the still young state of the art of microthermoforming technology as well as its first applications. So far, the applications are mainly in the biomedical field. They benefit from the fact that thermoformed microdevices have unique properties resulting from their special, unusual morphology. The focus of this paper is on the impact of the new class of micromoulding processes and the processed film materials on the characteristics of the moulded microdevices and on their applications.

The famous versus the inconvenient - or the dawn and the rise of 3D-culture systems

One of the greatest impacts on in vitro cell biology was the introduction of three-dimensional (3D) culture systems more than six decades ago and this era may be called the dawn of 3D-tissue culture. Although the advantages were obvious, this field of research was a "sleeping beauty" until the 1970s when multicellular spheroids were discovered as ideal tumor models. With this rebirth, organotypical culture systems became valuable tools and this trend continues to increase. While in the beginning, simple approaches, such as aggregation culture techniques, were favored due to their simplicity and convenience, now more sophisticated systems are used and are still being developed. One of the boosts in the development of new culture techniques arises from elaborate manufacturing and surface modification techniques, especially micro and nano system technologies that have either improved dramatically or have evolved very recently. With the help of these tools, it will soon be possible to generate even more sophisticated and more organotypic-like culture systems. Since 3D perfused or superfused systems are much more complex to set up and maintain compared to use of petri dishes and culture flasks, the added value of 3D approaches still needs to be demonstrated.

The famous versus the inconvenient - or the dawn and the rise of 3D-culture systems

One of the greatest impacts on in vitro cell biology was the introduction of three-dimensional (3D) culture systems more than six decades ago and this era may be called the dawn of 3D-tissue culture. Although the advantages were obvious, this field of research was a "sleeping beauty" until the 1970s when multicellular spheroids were discovered as ideal tumor models. With this rebirth, organotypical culture systems became valuable tools and this trend continues to increase. While in the beginning, simple approaches, such as aggregation culture techniques, were favored due to their simplicity and convenience, now more sophisticated systems are used and are still being developed. One of the boosts in the development of new culture techniques arises from elaborate manufacturing and surface modification techniques, especially micro and nano system technologies that have either improved dramatically or have evolved very recently. With the help of these tools, it will soon be possible to generate even more sophisticated and more organotypic-like culture systems. Since 3D perfused or superfused systems are much more complex to set up and maintain compared to use of petri dishes and culture flasks, the added value of 3D approaches still needs to be demonstrated.

The three-dimensional cultivation of the carcinoma cell line HepG2 in a perfused chip system leads to a more differentiated phenotype of the cells compared to monolayer culture

We describe a polymer chip with a grid-like architecture that it is intended for the three-dimensional cultivation of cells with an active nutrient and gas supply. The chip is typically made from polymethyl methacrylate or polycarbonate but can also be manufactured from biodegradable polymers, such as poly(lactic-co-glycolic acid). Different designs of the chip can be realized. In this study, we evaluated a chip with 506 microcontainers of the size of 300 x 300 x 300 microm that are capable of housing up to 6 million cells, and its suitability as a tissue-specific culture system for the carcinoma cell line HepG2 instead of primary liver cells. Related to an earlier study, where we could show the principal suitability of the system for rat primary cells, we here investigated the system's suitability for the human carcinoma cell line HepG2. The carcinoma cells were used in two different types of chip-containing bioreactors. By confocal laser scanning microscopy, we could show that cellular integrity in the chip culture was maintained and that there were no signs of apoptosis as confirmed by the absence of K18 fragmentation. Gene expression analysis of some liver-specific genes revealed a significantly higher expression of the phase II metabolism genes uridine-diphosphate- glucosyl-transferase (UGT1A1) and glutathione-S-transferase (GSTpi1) as a marker. Therefore, we conclude that by using a three-dimensional instead of a conventional monolayer culture system, hepatocellular carcinoma cells display a phenotype that resembles more closely the tissue of origin.

Flexible fluidic microchips based on thermoformed and locally modified thin polymer films

This paper presents a fundamentally new approach for the manufacturing and the possible applications of lab on a chip devices, mainly in the form of disposable fluidic microchips for life sciences applications. The new technology approach is based on a novel microscale thermoforming of thin polymer films as core process. The flexibility not only of the semi-finished but partly also of the finished products in the form of film chips could enable future reel to reel processes in production but also in application. The central so-called 'microthermoforming' process can be surrounded by pairs of associated pre- and postprocesses for micro- and nanopatterned surface and bulk modification or functionalisation of the formed films. This new approach of microscale thermoforming of thin polymer film substrates overlaid with a split local modification of the films is called 'SMART', which stands for 'substrate modification and replication by thermoforming'. In the process, still on the unformed, plane film, the material modifications of the preprocess define the locations where later, then on the spatially formed film, the postprocess generates the final local modifications. So, one can obtain highly resolved modification patterns also on hardly accessible side walls and even behind undercuts. As a first application of the new technology, we present a flexible chip-sized scaffold for three dimensional cell cultivation in the form of a microcontainer array. The spatially warped container walls have been provided with micropores, cell adhesion micropatterns and thin film microelectrodes.

Microfabrication of chip-sized scaffolds for three-dimensional cell cultivation

Using microfabrication technologies is a prerequisite to create scaffolds of reproducible geometry and constant quality for three-dimensional cell cultivation. These technologies offer a wide spectrum of advantages not only for manufacturing but also for different applications. The size and shape of formed cell clusters can be influenced by the exact and reproducible architecture of the microfabricated scaffold and, therefore, the diffusion path length of nutrients and gases can be controlled.1 This is unquestionably a useful tool to prevent apoptosis and necrosis of cells due to an insufficient nutrient and gas supply or removal of cellular metabolites. Our polymer chip, called CellChip, has the outer dimensions of 2 x 2 cm with a central microstructured area. This area is subdivided into an array of up to 1156 microcontainers with a typical dimension of 300 m edge length for the cubic design (cp- or cf-chip) or of 300 m diameter and depth for the round design (r-chip).2 So far, hot embossing or micro injection moulding (in combination with subsequent laborious machining of the parts) was used for the fabrication of the microstructured chips. Basically, micro injection moulding is one of the only polymer based replication techniques that, up to now, is capable for mass production of polymer microstructures.3 However, both techniques have certain unwanted limitations due to the processing of a viscous polymer melt with the generation of very thin walls or integrated through holes. In case of the CellChip, thin bottom layers are necessary to perforate the polymer and provide small pores of defined size to supply cells with culture medium e.g. by microfluidic perfusion of the containers. In order to overcome these limitations and to reduce the manufacturing costs we have developed a new microtechnical approach on the basis of a down-scaled thermoforming process. For the manufacturing of highly porous and thin walled polymer chips, we use a combination of heavy ion irradiation, microthermoforming and track etching. In this so called "SMART" process (Substrate Modification And Replication by Thermoforming) thin polymer films are irradiated with energetic heavy projectiles of several hundred MeV introducing so-called "latent tracks" Subsequently, the film in a rubber elastic state is formed into three dimensional parts without modifying or annealing the tracks. After the forming process, selective chemical etching finally converts the tracks into cylindrical pores of adjustable diameter.

A chip-based platform for the in vitro generation of tissues in three-dimensional organization

We describe a multi-purpose platform for the three-dimensional cultivation of tissues. The device is composed of polymer chips featuring a microstructured area of 1-2 cm(2). The chip is constructed either as a grid of micro-containers measuring 120-300 x 300 x 300 microm (h x l x w), or as an array of round recesses (300 microm diameter, 300 microm deep). The micro-containers may be separately equipped with addressable 3D-micro-electrodes, which allow for electrical stimulation of excitable cells and on-site measurements of electrochemically accessible parameters. The system is applicable for the cultivation of high cell densities of up to 8 x 10(6) cells and, because of the rectangular grid layout, allows the automated microscopical analysis of cultivated cells. More than 1000 micro-containers enable the parallel analysis of different parameters under superfusion/perfusion conditions. Using different polymer chips in combination with various types of bioreactors we demonstrated the principal suitability of the chip-based bioreactor for tissue culture applications. Primary and established cell lines have been successfully cultivated and analysed for functional properties. When cells were cultured in non-perfused chips, over time a considerable degree of apoptosis could be observed indicating the need for an active perfusion. The system presented here has also been applied for the differentiation analysis of pluripotent embryonic stem cells and may be suitable for the analysis of the stem cell niche.

3D tissue culture substrates produced by microthermoforming of pre-processed polymer films

We describe a new technology based on thermoforming as a microfabrication process. It significantly enhances the tailoring of polymers for three dimensional tissue engineering purposes since for the first time highly resolved surface and bulk modifications prior to a microstructuring process can be realised. In contrast to typical micro moulding techniques, the melting phase is avoided and thus allows the forming of pre-processed polymer films. The polymer is formed in a thermoelastic state without loss of material coherence. Therefore, previously generated modifications can be preserved. To prove the feasibility of our newly developed technique, so called SMART = Substrate Modification And Replication by Thermoforming, polymer films treated by various polymer modification methods, like UV-based patterned films, and films modified by the bombardment with energetic heavy ions, were post-processed by microthermoforming. The preservation of locally applied specific surface and bulk features was demonstrated e.g. by the selective adhesion of cells to patterned microcavity walls.