Adipose Tissue-Derived Stromal Cells Alter the Mechanical Stability and Viscoelastic Properties of Gelatine Methacryloyl Hydrogels

A 3D Epithelial-Mesenchymal Co-Culture Model of the Airway Wall Using Native Lung Extracellular Matrix

Chronic obstructive pulmonary disease (COPD) is a chronic lung disease characterized by ongoing inflammation, impaired tissue repair, and aberrant interplay between airway epithelium and fibroblasts, resulting in an altered extracellular matrix (ECM) composition. The ECM is the three-dimensional (3D) scaffold that provides mechanical support and biochemical signals to cells, now recognized not only as a consequence but as a potential driver of disease progression. To elucidate how the ECM influences pathophysiological changes occurring in COPD, in vitro models are needed that incorporate the ECM. ECM hydrogels are a novel experimental tool for incorporating the ECM in experimental setups. We developed an airway wall model by combining lung-derived ECM hydrogels with a co-culture of primary human fibroblasts and epithelial cells at an air-liquid interface. Collagen IV and a mixture of collagen I, fibronectin, and bovine serum albumin were used as basement membrane-mimicking coatings. The model was initially assembled using porcine lung-derived ECM hydrogels and subsequently with COPD and non-COPD human lung-derived ECM hydrogels. The resulting 3D construct exhibited considerable contraction and supported co-culture, resulting in a differentiated epithelial layer. This multi-component 3D model allows the investigation of remodelling mechanisms, exploring ECM involvement in cellular crosstalk, and holds promise as a model for drug discovery studies exploring ECM involvement in cellular interactions.

Hybrid biofabricated blood vessel for medical devices testing

Current in vitro and in vivo tests applied to assess the safety of medical devices retain several limitations, such as an incomplete ability to faithfully recapitulate human features, and to predict the response of human tissues together with non-trivial ethical aspects. We here challenged a new hybrid biofabrication technique that combines bioprinting and Fast Diffusion-induced Gelation strategy to generate a vessel-like structure with the attempt to spatially organize fibroblasts, smooth-muscle cells, and endothelial cells. The introduction of Fast Diffusion-induced Gelation minimizes the endothelial cell mortality during biofabrication and produce a thin endothelial layer with tunable thickness. Cell viability, Von Willebrand factor, and CD31 expression were evaluated on biofabricated tissues, showing how bioprinting and Fast Diffusion-induced Gelation can replicate human vessels architecture and complexity. We then applied biofabricated tissue to study the cytotoxicity of a carbothane catheter under static condition, and to better recapitulate the effect of blood flow, a novel bioreactor named CuBiBox (Customized Biological Box) was developed and introduced in a dynamic modality. Collectively, we propose a novel bioprinted platform for human in vitro biocompatibility testing, predicting the impact of medical devices and their materials on vascular systems, reducing animal experimentation and, ultimately, accelerating time to market.

Enhanced hepatotoxicity assessment through encapsulated HepG2 spheroids in gelatin hydrogel matrices: Bridging the gap from 2D to 3D culture

Conventional 2D drug screening often fails to accurately predict clinical outcomes. We present an innovative approach to improve hepatotoxicity assessment by encapsulating HepG2 spheroids in gelatin hydrogel matrices with different mechanical properties. Encapsulated spheroids exhibit sustained liver-specific functionality, enhanced expression of drug-metabolizing enzymes, and increased drug sensitivity compared to 2D cultures. The platform detects critical variations in drug response, with significant differences in IC50 values between 2D and spheroid cultures ranging from 1.3-fold to > 13-fold, particularly for acetaminophen. Furthermore, drug-metabolizing enzyme expression varies across hydrogel concentrations, suggesting a role for matrix mechanical properties in modulating hepatocyte function. This novel spheroid-hydrogel platform offers a transformative approach to hepatotoxicity assessment, providing increased sensitivity, improved prediction, and a more physiologically relevant environment. The use of such advanced in vitro models can accelerate drug development, reduce animal testing, and contribute to improved patient safety and clinical outcomes.

Strategies of functionalized GelMA-based bioinks for bone regeneration: Recent advances and future perspectives

Gelatin methacryloyl (GelMA) hydrogels is a widely used bioink because of its good biological properties and tunable physicochemical properties, which has been widely used in a variety of tissue engineering and tissue regeneration. However, pure GelMA is limited by the weak mechanical strength and the lack of continuous osteogenic induction environment, which is difficult to meet the needs of bone repair. Moreover, GelMA hydrogels are unable to respond to complex stimuli and therefore are unable to adapt to physiological and pathological microenvironments. This review focused on the functionalization strategies of GelMA hydrogel based bioinks for bone regeneration. The synthesis process of GelMA hydrogel was described in details, and various functional methods to meet the requirements of bone regeneration, including mechanical strength, porosity, vascularization, osteogenic differentiation, and immunoregulation for patient specific repair, etc. In addition, the response strategies of smart GelMA-based bioinks to external physical stimulation and internal pathological microenvironment stimulation, as well as the functionalization strategies of GelMA hydrogel to achieve both disease treatment and bone regeneration in the presence of various common diseases (such as inflammation, infection, tumor) are also briefly reviewed. Finally, we emphasized the current challenges and possible exploration directions of GelMA-based bioinks for bone regeneration.

Bottom-Up Extrusion-Based Biofabrication of the Osteoid Niche

Bone regeneration remains a clinical challenge given the transplantation incidence rate and the associated economic burden. Bottom-up osteoid tissue engineering has the potential to offer an alternative approach to current clinical solutions that suffer from various drawbacks. In this paper, deposition-based bioprinting is exploited while the effect is explored of both the crosslinking mechanism (gelatin methacryloyl (GelMA) versus gelatin norbornene (DS 91) crosslinked with thiolated gelatin (GelNBSH)) and the degree of substitution (GelNBSH versus norbornene-norbornene-modified gelatin (DS 169) crosslinked with thiolated gelatin (GelNBNBSH)) on the presented biophysical cues as well as on the osteogenic differentiation. The incorporation of tris(2-carboxyethyl)phosphine (TCEP) to the step-growth inks allows the production of reproducible and biocompatible scaffolds based on thiol-ene chemistry. Dental pulp stem cell encapsulation in GelNBNBSH biofabricated constructs shows a favorable response due to the combination of its stress relaxation and substrate rigidity (bulk compressive modulus of 11-30 kPa) as reflected by a sevenfold increase in calcium production compared to the tissue engineering standard GelMA. This work is the first to exploit a controlled biocompatible and cell-interactive thiolated macromolecular crosslinker (GelSH + TCEP) allowing the extrusion-based biofabrication of low concentration (5 w/v%) modified osteogenic gelatin-based inks (GelNBNBSH + TCEP).

Current possibilities and future opportunities provided by three-dimensional lung ECM-derived hydrogels

Disruption of the complex interplay between cells and extracellular matrix (ECM), the scaffold that provides support, biochemical and biomechanical cues, is emerging as a key element underlying lung diseases. We readily acknowledge that the lung is a flexible, relatively soft tissue that is three dimensional (3D) in structure, hence a need exists to develop in vitro model systems that reflect these properties. Lung ECM-derived hydrogels have recently emerged as a model system that mimics native lung physiology; they contain most of the plethora of biochemical components in native lung, as well as reflecting the biomechanics of native tissue. Research investigating the contribution of cell:matrix interactions to acute and chronic lung diseases has begun adopting these models but has yet to harness their full potential. This perspective article provides insight about the latest advances in the development, modification, characterization and utilization of lung ECM-derived hydrogels. We highlight some opportunities for expanding research incorporating lung ECM-derived hydrogels and potential improvements for the current approaches. Expanding the capabilities of investigations using lung ECM-derived hydrogels is positioned at a cross roads of disciplines, the path to new and innovative strategies for unravelling disease underlying mechanisms will benefit greatly from interdisciplinary approaches. While challenges need to be addressed before the maximum potential can be unlocked, with the rapid pace at which this field is evolving, we are close to a future where faster, more efficient and safer drug development targeting the disrupted 3D microenvironment is possible using lung ECM-derived hydrogels.

Adipose Tissue-Derived Components: From Cells to Tissue Glue to Treat Dermal Damage

In recent decades, adipose tissue transplantation has become an essential treatment modality for tissue (volume) restoration and regeneration. The regenerative application of adipose tissue has only recently proven its usefulness; for example, the method is useful in reducing dermal scarring and accelerating skin-wound healing. The therapeutic effect is ascribed to the tissue stromal vascular fraction (tSVF) in adipose tissue. This consists of stromal cells, the trophic factors they secrete and the extracellular matrix (ECM), which have immune-modulating, pro-angiogenic and anti-fibrotic properties. This concise review focused on dermal regeneration using the following adipose-tissue components: adipose-tissue-derived stromal cells (ASCs), their secreted trophic factors (ASCs secretome), and the ECM. The opportunities of using a therapeutically functional scaffold, composed of a decellularized ECM hydrogel loaded with trophic factors of ASCs, to enhance wound healing are explored as well. An ECM-based hydrogel loaded with trophic factors combines all regenerative components of adipose tissue, while averting the possible disadvantages of the therapeutic use of adipose tissue, e.g., the necessity of liposuction procedures with a (small) risk of complications, the impossibility of interpatient use, and the limited storage options.

From Macro to Micro: Comparison of Imaging Techniques to Detect Vascular Network Formation in Left Ventricle Decellularized Extracellular Matrix Hydrogels

Background: Angiogenesis is a crucial process in physiological maintenance and tissue regeneration. To understand the contribution of angiogenesis, it is essential to replicate this process in an environment that reproduces the biochemical and physical properties which are largely governed by the extracellular matrix (ECM). We investigated vascularization in cardiac left ventricular ECM hydrogels to mimic post-myocardial repair. We set out to assess and compare different destructive and non-destructive methods, optical as well as non-optical, to visualize angiogenesis and associated matrix remodeling in myocardial ECM hydrogels. Methods: A total of 100,000, 300,000, and 600,000 Human Pulmonary Microvascular Endothelial Cells (HPMEC) were seeded in left ventricular cardiac ECM hydrogel in 48-well plates. After 1, 7, and 14 days of culture, the HPMEC were imaged by inverted fluorescence microscopy and 3D confocal laser scanning microscopy (Zeiss Cell Discoverer 7). In addition, cell-seeded ECM hydrogels were scanned by optical coherence tomography (OCT). Fixed and paraffin-embedded gels were thin-sectioned and assessed for ECM components via H&E, picrosirius red histochemical staining, and immunostaining for collagen type I. ImageJ-based densitometry was used to quantify vascular-like networks and GraphPad was used for statistical analyses. Results: Qualitative analyses were realized through fluoromicrographs obtained by the confocal laser scanning microscope which allowed us to visualize the extensive vascular-like networks that readily appeared at all seeding densities. Quantification of networks was only possible using fluoromicrographs from inverted microscopy. These showed that, after three days, the number of master junctions was seeding density-dependent. The resolution of optical coherence tomography was too low to distinguish between signals caused by the ECM and cells or networks, yet it did show that gels, irrespective of cells, were heterogeneous. Interestingly, (immuno)histochemistry could clearly distinguish between the cast cardiac-derived matrix and newly deposited ECM in the hydrogels. The H&E staining corroborated the presence of vascular-like network structures, albeit that sectioning inevitably led to the loss of 3D structure. Conclusions: Except for OCT, all methods had complementary merit and generated qualitative and quantitative data that allowed us to understand vascular network formation in organ-derived ECM hydrogels.

Studies on oxygen availability and the creation of natural and artificial oxygen gradients in gelatin-methacryloyl hydrogel 3D cell culture

Three-dimensional (3D) cultivation platforms allow the creation of cell models, which more closely resemble in vivo-like cell behavior. Therefore, 3D cell culture platforms have started to replace conventional two-dimensional (2D) cultivation techniques in many fields. Besides the advantages of 3D culture, there are also some challenges: cultivation in 3D often results in an inhomogeneous microenvironment and therefore unique cultivation conditions for each cell inside the construct. As a result, the analysis and precise control over the singular cell state is limited in 3D. In this work, we address these challenges by exploring ways to monitor oxygen concentrations in gelatin methacryloyl (GelMA) 3D hydrogel culture at the cellular level using hypoxia reporter cells and deep within the construct using a non-invasive optical oxygen sensing spot. We could show that the appearance of oxygen limitations is more prominent in softer GelMA-hydrogels, which enable better cell spreading. Beyond demonstrating novel or space-resolved techniques of visualizing oxygen availability in hydrogel constructs, we also describe a method to create a stable and controlled oxygen gradient throughout the construct using a 3D printed flow-through chamber.

Bioprinting of 3D Adipose Tissue Models Using a GelMA-Bioink with Human Mature Adipocytes or Human Adipose-Derived Stem Cells

Adipose tissue is related to the development and manifestation of multiple diseases, demonstrating the importance of suitable in vitro models for research purposes. In this study, adipose tissue lobuli were explanted, cultured, and used as an adipose tissue control to evaluate in vitro generated adipose tissue models. During culture, lobule exhibited a stable weight, lactate dehydrogenase, and glycerol release over 15 days. For building up in vitro adipose tissue models, we adapted the biomaterial gelatin methacryloyl (GelMA) composition and handling to homogeneously mix and bioprint human primary mature adipocytes (MA) and adipose-derived stem cells (ASCs), respectively. Accelerated cooling of the bioink turned out to be essential for the homogeneous distribution of lipid-filled MAs in the hydrogel. Last, we compared manual and bioprinted GelMA hydrogels with MA or ASCs and the explanted lobules to evaluate the impact of the printing process and rate the models concerning the physiological reference. The viability analyses demonstrated no significant difference between the groups due to additive manufacturing. The staining of intracellular lipids and perilipin A suggest that GelMA is well suited for ASCs and MA. Therefore, we successfully constructed physiological in vitro models by bioprinting MA-containing GelMA bioinks.

A Beginner’s Guide to the Characterization of Hydrogel Microarchitecture for Cellular Applications

The extracellular matrix (ECM) is a three-dimensional, acellular scaffold of living tissues. Incorporating the ECM into cell culture models is a goal of cell biology studies and requires biocompatible materials that can mimic the ECM. Among such materials are hydrogels: polymeric networks that derive most of their mass from water. With the tuning of their properties, these polymer networks can resemble living tissues. The microarchitectural properties of hydrogels, such as porosity, pore size, fiber length, and surface topology can determine cell plasticity. The adequate characterization of these parameters requires reliable and reproducible methods. However, most methods were historically standardized using other biological specimens, such as 2D cell cultures, biopsies, or even animal models. Therefore, their translation comes with technical limitations when applied to hydrogel-based cell culture systems. In our current work, we have reviewed the most common techniques employed in the characterization of hydrogel microarchitectures. Our review provides a concise description of the underlying principles of each method and summarizes the collective data obtained from cell-free and cell-loaded hydrogels. The advantages and limitations of each technique are discussed, and comparisons are made. The information presented in our current work will be of interest to researchers who employ hydrogels as platforms for cell culture, 3D bioprinting, and other fields within hydrogel-based research.

Viscoelastic properties of plasma-agarose hydrogels dictate favorable fibroblast responses for skin tissue engineering applications

Dermal wound healing relies on the properties of the extracellular matrix (ECM). Thus, hydrogels that replicate skin ECM have reached clinical application. After a dermal injury, a transient, biodegradable fibrin clot is instrumental in wound healing. Human plasma, and its main constituent, fibrin would make a suitable biomaterial for improving wound healing and processed as hydrogels albeit with limited mechanical strength. To overcome this, plasma-agarose (PA) composite hydrogels have been developed and used to prepare diverse bioengineered tissues. To date, little is known about the influence of variable agarose concentrations on the viscoelastic properties of PA hydrogels and their correlation to cell biology. This study reports the characterization of the viscoelastic properties of different concentrations of agarose in PA hydrogels: 0 %, 0.5 %, 1 %, 1.5 %, and 2 % (w/v), and their influence on the cell number and mitochondrial activity of human dermal fibroblasts. Results show that agarose addition increased the stiffness, relaxation time constants 1 (τ1) and 2 (τ2), and fiber diameter, whereas the porosity decreased. Changes in cell metabolism occurred at the early stages of culturing and correlated to the displacement of fast (τ1) and intermediate (τ2) Maxwell elements. Fibroblasts seeded in low PA concentrations spread faster during 14 d than cells cultured in higher agarose concentrations. Collectively, these results confirm that PA viscoelasticity and hydrogel architecture strongly influenced cell behavior. Therefore, viscoelasticity is a key parameter in the design of PA-based implants.

Matrix Metalloproteases from Adipose Tissue-Derived Stromal Cells Are Spatiotemporally Regulated by Hydrogel Mechanics in a 3D Microenvironment

Adipose tissue-derived stromal cells (ASCs) are of interest in tissue engineering and regenerative medicine (TERM) due to their easy acquisition, multipotency, and secretion of a host of factors that promote regeneration. Retention of ASCs in or around lesions is poor following direct administration. Therefore, for TERM applications, ASCs can be ‘immobilized’ via their incorporation into hydrogels such as gelatine methacryloyl (GelMA). Tweaking GelMA concentration is a common approach to approximate the mechanical properties found in organs or tissues that need repair. Distinct hydrogel mechanics influence the ability of a cell to spread, migrate, proliferate, and secrete trophic factors. Mesenchymal cells such as ASCs are potent remodellers of the extracellular matrix (ECM). Not only do ASCs deposit components, they also secrete matrix metalloproteases (MMPs) which degrade ECM. In this work, we investigated if GelMA polymer concentration influenced the expression of active MMPs by ASCs. In addition, MMPs’ presence was interrogated with regard to ASCs morphology and changes in hydrogel ultrastructure. For this, immortalised ASCs were embedded in 5%, 10%, and 15% (w/v) GelMA hydrogels, photopolymerised and cultured for 14 d. Zymography in situ indicated that MMPs had a variable, hydrogel concentration-dependent influence on ASCs-secreted MMPs. In 5% GelMA, ASCs showed a high and sustained expression of MMPs, while, in 10% and 15% GelMA, such expression was almost null. ASCs morphology based on F-actin staining showed that increasing GelMA concentrations inhibit their spreading. Scanning electron microscopy (SEM) showed that hydrogel ultrastructure in terms of pore density, pore size, and percentage porosity were not consistently influenced by cells. Interestingly, changes in ultrastructural parameters were detected also in cell-free materials, albeit without a clear trend. We conclude that hydrogel concentration and its underlying mechanics influenced MMP expression by ASCs. The exact MMPs that respond to these mechanical cues should be defined in follow-up experiments.

An in vitro model of fibrosis using crosslinked native extracellular matrix-derived hydrogels to modulate biomechanics without changing composition

Extracellular matrix (ECM) is a dynamic network of proteins, proteoglycans and glycosaminoglycans, providing structure to the tissue and biochemical and biomechanical instructions to the resident cells. In fibrosis, the composition and the organization of the ECM are altered, and these changes influence cellular behaviour. Biochemical (i. e. protein composition) and biomechanical changes in ECM take place simultaneously in vivo. Investigating these changes individually in vitro to examine their (patho)physiological effects has been difficult. In this study, we generated an in vitro model to reflect the altered mechanics of a fibrotic microenvironment through applying fibre crosslinking via ruthenium/sodium persulfate crosslinking on native lung ECM-derived hydrogels. Crosslinking of the hydrogels without changing the biochemical composition of the ECM resulted in increased stiffness and decreased viscoelastic stress relaxation. The altered stress relaxation behaviour was explained using a generalized Maxwell model. Fibre analysis of the hydrogels showed that crosslinked hydrogels had a higher percentage of matrix with a high density and a shorter average fibre length. Fibroblasts seeded on ruthenium-crosslinked lung ECM-derived hydrogels showed myofibroblastic differentiation with a loss of spindle-like morphology together with greater α-smooth muscle actin (α-SMA) expression, increased nuclear area and circularity without any decrease in the viability, compared with the fibroblasts seeded on the native lung-derived ECM hydrogels. In summary, ruthenium crosslinking of native ECM-derived hydrogels provides an exciting opportunity to alter the biomechanical properties of the ECM-derived hydrogels while maintaining the protein composition of the ECM to study the influence of mechanics during fibrotic lung diseases. STATEMENT OF SIGNIFICANCE: Fibrotic lung disease is characterized by changes in composition and excessive deposition of extracellular matrix (ECM). ECM fibre structure also changes due to crosslinking, which results in mechanical changes. Separating the changes in composition and mechanical properties has been difficult to date. In this study, we developed an in vitro model that allows alteration of the mechanical changes alone by applying fibre crosslinking in native lung ECM-derived hydrogels. Characterisations of the crosslinked hydrogels indicated the model mimicked mechanical properties of fibrotic lung tissue and reflected altered fibre organisation. This ECM-based fibrosis model provides a method to preserve the native protein composition while altering the mechanical properties providing an important tool, not only for lung but also other organ fibrosis.