Sustainable three-dimensional tissue model of human adipose tissue

Development and analysis of scaffold-free adipose spheroids

Adipose tissue plays a crucial role in metabolic syndrome, autoimmune diseases, and many cancers. Because of adipose's role in so many aspects of human health, there is a critical need for in vitro models that replicate adipose architecture and function. Traditional monolayer models, despite their convenience, are limited, showing heterogeneity and functional differences compared to 3D models. While monolayer cultures struggle with detachment and inefficient differentiation, healthy adipocytes in 3D culture accumulate large lipid droplets, secrete adiponectin, and produce low levels of inflammatory cytokines. The shift from monolayer models to more complex 3D models aims to better replicate the physiology of healthy adipose tissue in culture. This study introduces a simple and accessible protocol for generating adipose organoids using a scaffold-free spheroid model. The method, utilizing either 96-well spheroid plates or agarose micromolds, demonstrates increased throughput, uniformity, and ease of handling compared to previous techniques. This protocol allows for diverse applications, including drug testing, toxin screening, tissue engineering, and co-culturing. The choice between the two methods depends on the experimental goals, with the 96-well plate providing individualized control and the micromold offering scale advantages. The outlined protocol covers isolation, expansion, and characterization of stromal vascular fraction cells, followed by detailed steps for spheroid formation and optional downstream analyses.

Mesenchymal Stem Cell-based Scaffolds in Regenerative Medicine of Dental Diseases

Biomedical engineering breakthroughs and increased patient expectations and requests for more comprehensive care are propelling the field of regenerative dentistry forward at a fast pace. Stem cells (SCs), bioactive compounds, and scaffolds are the mainstays of tissue engineering, the backbone of regenerative dentistry. Repairing damaged teeth and gums is a significant scientific problem at present. Novel therapeutic approaches for tooth and periodontal healing have been inspired by tissue engineering based on mesenchymal stem cells (MSCs). Furthermore, as a component of the MSC secretome, extracellular vesicles (EVs) have been shown to contribute to periodontal tissue repair and regeneration. The scaffold, made of an artificial extracellular matrix (ECM), acts as a supporting structure for new cell development and tissue formation. To effectively promote cell development, a scaffold must be non-toxic, biodegradable, biologically compatible, low in immunogenicity, and safe. Due to its promising biological characteristics for cell regeneration, dental tissue engineering has recently received much attention for its use of natural or synthetic polymer scaffolds with excellent mechanical properties, such as small pore size and a high surface-to-volume ratio, as a matrix. Moreover, as a bioactive material for carrying MSC-EVs, the combined application of scaffolds and MSC-EVs has a better regenerative effect on dental diseases. In this paper, we discuss how MSCs and MSC-derived EV treatment may be used to regenerate damaged teeth, and we highlight the role of various scaffolds in this process.

Adipocyte-endothelial cell interplay in adipose tissue physiology

Adipose tissue (AT) expansion through hyperplasia or hypertrophy requires vascular remodeling that involves angiogenesis. There is quite some evidence that obese white AT (WAT) displays altered vasculature. Some studies suggest that this is associated with hypoxia, which is thought to play a role in inducing inflammatory activation of the excessively expanding WAT. Increasing evidence, based on genetic manipulations or treatments with inhibitory or activator pharmaceuticals, demonstrates that AT angiogenesis is crucial for AT metabolic function, and thereby for whole body metabolism and metabolic health. Despite some contradiction between studies, disturbance of WAT angiogenesis in obesity could be an important factor driving WAT dysfunction and the comorbidities of obesity. Endothelial cells (ECs) contribute to healthy WAT metabolism via transport of fatty acids and other plasma components, secretory signaling molecules, and extracellular vesicles (EVs). This communication is crucial for adipocyte metabolism and underscores the key role that the AT endothelium plays in systemic energy homeostasis and healthy metabolism. Adipocytes communicate towards the neighboring endothelium through several mechanisms. The pro-inflammatory status of hypertrophic adipocytes in obesity is reflected in ECs activation, which promotes chronic inflammation. On the other hand, adiponectin secreted by the adipocytes is important for healthy endothelial function, and adipocytes also secrete other pro- or anti-angiogenic effector molecules and a wealth of EVs - however, their detailed roles in signaling towards the endothelium are yet poorly understood. To conclude, targeting AT angiogenesis and promoting the healthy communication between adipocytes and ECs represent potentially promising strategies to treat obesity and its comorbidities.

Human visceral adipose tissue microvascular endothelial cell isolation and establishment of co-culture with white adipocytes to analyze cell-cell communication

Communication between adipocytes and endothelial cells (EC) is suggested to play an important role in the metabolic function of white adipose tissue. In order to generate tools to investigate in detail the physiology and communication of EC and adipocytes, a method for isolation of adipose microvascular EC from visceral adipose tissue (VAT) biopsies of subjects with obesity was developed. Moreover, mature white adipocytes were isolated from the VAT biopsies by a method adapted from a previously published Membrane aggregate adipocytes culture (MAAC) protocol. The identity and functionality of the cultivated and isolated adipose microvascular EC (AMvEC) was validated by imaging their morphology, analyses of mRNA expression, fluorescence activated cell sorting (FACS), immunostaining, low-density lipoprotein (LDL) uptake, and in vitro angiogenesis assays. Finally, we established a new trans filter co-culture system (membrane aggregate adipocyte and endothelial co-culture, MAAECC) for the analysis of communication between the two cell types. EC-adipocyte communication in this system was validated by omics analyses, revealing several altered proteins belonging to pathways such as metabolism, intracellular transport and signal transduction in adipocytes co-cultured with AMvEC. In reverse experiments, induction of several pathways including endothelial development and functions was found in AMvEC co-cultured with adipocytes. In conclusion, we developed a robust method to isolate EC from small quantities of human VAT. Furthermore, the MAAECC system established during the study enables one to study the communication between primary white adipocytes and EC or vice-versa and could also be employed for drug screening.

ECM proteins and cationic polymers coating promote dedifferentiation of patient-derived mature adipocytes to stem cells

Reprogramming of mature adipocytes is an attractive research area due to the plasticity of these cells. Mature adipocytes can be reprogrammed in vitro, transforming them into dedifferentiated fat cells (DFATs), which are considered a new type of stem cell, and thereby have a high potential for use in tissue engineering and regenerative medicine. However, there are still no reports or findings on in vitro controlling the dedifferentiation. Although ceiling culture performed in related studies is a relatively simple method, its yield is low and does not allow manipulation of mature adipocytes to increase or decrease the dedifferentiation. In this study, to understand the role of physicochemical surface effects on the dedifferentiation of patient-derived mature adipocytes, the surfaces of cell culture flasks were coated with extracellular matrix, basement membrane proteins, and cationic/anionic polymers. Extracellular matrix such as fibronectin and collagen type I, and basement membrane proteins such as collagen type IV and laminin strongly promoted dedifferentiation of mature adipocytes, with laminin showing the highest effect with a DFAT ratio of 2.98 (±0.84). Interestingly, cationic polymers also showed a high dedifferentiation effect, but anionic polymers did not, and poly(diallyl dimethylammonium chloride) showed the highest DFAT ratio of 2.27 (±2.8) among the cationic polymers. Protein assay results revealed that serum proteins were strongly adsorbed on the surfaces of the cationic polymer coating, including inducing high mature adipocyte adhesion. This study demonstrates for the first time the possibility of regulating the transformation of mature adipocytes to DFAT stem cells by controlling the physicochemical properties of the surface of conventional cell culture flasks.

Clinical Applicability of Visible Light-Mediated Cross-linking for Structural Soft Tissue Reconstruction

Visible light-mediated cross-linking has utility for enhancing the structural capacity and shape fidelity of laboratory-based polymers. With increased light penetration and cross-linking speed, there is opportunity to extend future applications into clinical spheres. This study evaluated the utility of a ruthenium/sodium persulfate photocross-linking system for increasing structural control in heterogeneous living tissues as an example, focusing on unmodified patient-derived lipoaspirate for soft tissue reconstruction. Freshly-isolated tissue is photocross-linked, then the molar abundance of dityrosine bonds is measured using liquid chromatography tandem mass spectrometry and the resulting structural integrity assessed. The cell function and tissue survival of photocross-linked grafts is evaluated ex vivo and in vivo, with tissue integration and vascularization assessed using histology and microcomputed tomography. The photocross-linking strategy is tailorable, allowing progressive increases in the structural fidelity of lipoaspirate, as measured by a stepwise reduction in fiber diameter, increased graft porosity and reduced variation in graft resorption. There is an increase in dityrosine bond formation with increasing photoinitiator concentration, and tissue homeostasis is achieved ex vivo, with vascular cell infiltration and vessel formation in vivo. These data demonstrate the capability and applicability of photocrosslinking strategies for improving structural control in clinically-relevant settings, potentially achieving more desirable patient outcomes using minimal manipulation in surgical procedures.

Study on electrospinning of wheat gluten: A review

Electrospinning has attracted extensive attention among various nanofabrication technologies owing to its ability to produce nanofiber structures with unique properties, such as high specific surface area and porosity, as well as tunable fiber morphology and mechanical properties. The most representative spinning raw materials include natural polymers and synthetic polymers. Owing to the sustainable development strategies, more and more researchers focus on natural polymers. Among natural polymers, wheat gluten (WG) nanofibers have recently attracted much attention owing to its high specific surface area, superior biocompatibility, and unique viscoelasticity. This review summarizes the composition and characteristics of WG, the physical and chemical indicators of a WG electrospinning solution, the main influencing factors in the WG electrospinning process and a characterizations of WG nanofibers. Finally, the review also outlines the applications of WG nanofibers in drug release, biological scaffold, and active food packaging.

Biofabrication of vascularized adipose tissues and their biomedical applications

Recent advances in adipose tissue engineering and cell biology have led to the development of innovative therapeutic strategies in regenerative medicine for adipose tissue reconstruction. To date, the many in vitro and in vivo models developed for vascularized adipose tissue engineering cover a wide range of research areas, including studies with cells of various origins and types, polymeric scaffolds of natural and synthetic derivation, models presented using decellularized tissues, and scaffold-free approaches. In this review, studies on adipose tissue types with different functions, characteristics and body locations have been summarized with 3D in vitro fabrication approaches. The reason for the particular focus on vascularized adipose tissue models is that current liposuction and fat transplantation methods are unsuitable for adipose tissue reconstruction as the lack of blood vessels results in inadequate nutrient and oxygen delivery, leading to necrosis in situ. In the first part of this paper, current studies and applications of white and brown adipose tissues are presented according to the polymeric materials used, focusing on the studies which could show vasculature in vitro and after in vivo implantation, and then the research on adipose tissue fabrication and applications are explained.

Development of a More Environmentally Friendly Silk Fibroin Scaffold for Soft Tissue Applications

A push for environmentally friendly approaches to biomaterials fabrication has emerged from growing conservational concerns in recent years. Different stages in silk fibroin scaffold production, including sodium carbonate (Na₂CO₃)-based degumming and 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP)-based fabrication, have drawn attention for their associated environmental concerns. Environmentally friendly alternatives have been proposed for each processing stage; however, an integrated green fibroin scaffold approach has not been characterized or used for soft tissue applications. Here, we show that the combination of sodium hydroxide (NaOH) as a substitute degumming agent with the popular "aqueous-based" alternative silk fibroin gelation method yields fibroin scaffolds with comparable properties to traditional Na₂CO₃-degummed aqueous-based scaffolds. The more environmentally friendly scaffolds were found to have comparable protein structure, morphology, compressive modulus, and degradation kinetics, with increased porosity and cell seeding density relative to traditional scaffolds. Human adipose-derived stem cells showed high viability after three days of culture while seeded in each scaffold type, with uniform cell attachment to pore walls. Adipocytes from human whole adipose tissue seeded into scaffolds were found to have similar levels of lipolytic and metabolic function between conditions, in addition to a healthy unilocular morphology. Results indicate that our more environmentally friendly methodology for silk scaffold production is a viable alternative and well suited to soft tissue applications.

High-throughput drug screening models of mature adipose tissues which replicate the physiology of patients' Body Mass Index (BMI)

Obesity is a complex and incompletely understood disease, but current drug screening strategies mostly rely on immature in vitro adipose models which cannot recapitulate it properly. To address this issue, we developed a statistically validated high-throughput screening model by seeding human mature adipocytes from patients, encapsulated in physiological collagen microfibers. These drop tissues ensured the maintenance of adipocyte viability and functionality for controlling glucose and fatty acids uptake, as well as glycerol release. As such, patients' BMI and insulin sensitivity displayed a strong inverse correlation: the healthy adipocytes were associated with the highest insulin-induced glucose uptake, while insulin resistance was confirmed in the underweight and severely obese adipocytes. Insulin sensitivity recovery was possible with two type 2 diabetes treatments, rosiglitazone and melatonin. Finally, the addition of blood vasculature to the model seemed to more accurately recapitulate the in vivo physiology, with particular respect to leptin secretion metabolism.

Mechanical Memory Impairs Adipose-Derived Stem Cell (ASC) Adipogenic Capacity After Long-Term In Vitro Expansion

Introduction

Adipose derived stem cells (ASCs) hold great promise for clinical applications such as soft tissue regeneration and for in vitro tissue models and are notably easy to derive in large numbers. Specifically, ASCs provide an advantage for in vitro models of adipose tissue, where they can be employed as tissue specific cells and for patient specific models. However, ASC in vitro expansion may unintentionally reduce adipogenic capacity due to the stiffness of tissue culture plastic (TCPS).

Methods

Here, we expanded freshly isolated ASCs on soft and stiff substrates for 4 passages before adipogenic differentiation. At the last passage we swapped the substrate from stiff to soft, or soft to stiff to determine if short term exposure to a different substrate altered adipogenic capacity.

Results

Expansion on stiff substrates reduced adipogenic capacity by 50% which was not rescued by swapping to a soft substrate for the last passage. Stiff substrates had greater nuclear area and gene expression of nesprin-2, a protein that mediates the tension of the nuclear envelope by tethering it to the actin cytoskeleton. Upon swapping to a soft substrate, the nuclear area was reduced but nesprin-2 levels did not fully recover, which differentially regulated cell commitment transcriptional factors.

Conclusion

Therefore, in vitro expansion on stiff substrates must be carefully considered when the end-goal of the expansion is for adipose tissue or soft tissue applications.

Hypoxia induces stress fiber formation in adipocytes in the early stage of obesity

In obese adipose tissue (AT), hypertrophic expansion of adipocytes is not matched by new vessel formation, leading to AT hypoxia. As a result, hypoxia inducible factor-1⍺ (HIF-1⍺) accumulates in adipocytes inducing a transcriptional program that upregulates profibrotic genes and biosynthetic enzymes such as lysyl oxidase (LOX) synthesis. This excess synthesis and crosslinking of extracellular matrix (ECM) components cause AT fibrosis. Although fibrosis is a hallmark of obese AT, the role of fibroblasts, cells known to regulate fibrosis in other fibrosis-prone tissues, is not well studied. Here we have developed an in vitro model of AT to study adipocyte-fibroblast crosstalk in a hypoxic environment. Further, this in vitro model was used to investigate the effect of hypoxia on adipocyte mechanical properties via ras homolog gene family member A (RhoA)/Rho-associated coiled-coil kinases (ROCK) signaling pathways. We confirmed that hypoxia creates a diseased phenotype by inhibiting adipocyte maturation and inducing actin stress fiber formation facilitated by myocardin-related transcription factor A (MRTF-A/MKL1) nuclear translocation. This work presents new potential therapeutic targets for obesity by improving adipocyte maturation and limiting mechanical stress in obese AT.

Engineering a 3D Vascularized Adipose Tissue Construct Using a Decellularized Lung Matrix

Critically sized defects in subcutaneous white adipose tissue result in extensive disfigurement and dysfunction and remain a reconstructive challenge for surgeons; as larger defect sizes are correlated with higher rates of complications and failure due to insufficient vascularization following implantation. Our study demonstrates, for the first time, a method to engineer perfusable, pre-vascularized, high-density adipose grafts that combine patient-derived adipose cells with a decellularized lung matrix (DLM). The lung is one of the most vascularized organs with high flow, low resistance, and a large blood-alveolar interface separated by a thin basement membrane. For our work, the large volume capacity within the alveolar compartment was repurposed for high-density adipose cell filling, while the acellular vascular bed provided efficient graft perfusion throughout. Both adipocytes and hASCs were successfully delivered and remained in the alveolar space even after weeks of culture. While adipose-derived cells maintained their morphology and functionality in both static and perfusion DLM cultures, perfusion culture offered enhanced outcomes over static culture. Furthermore, we demonstrate that endothelial cells seamlessly integrate into the acellular vascular tree of the DLM with adipocytes. These results support that the DLM is a unique platform for creating vascularized adipose tissue grafts for large defect filling.

3D Adipose Tissue Culture Links the Organotypic Microenvironment to Improved Adipogenesis

Obesity and type 2 diabetes are strongly associated with adipose tissue dysfunction and impaired adipogenesis. Understanding the molecular underpinnings that control adipogenesis is thus of fundamental importance for the development of novel therapeutics against metabolic disorders. However, translational approaches are hampered as current models do not accurately recapitulate adipogenesis. Here, a scaffold-free versatile 3D adipocyte culture platform with chemically defined conditions is presented in which primary human preadipocytes accurately recapitulate adipogenesis. Following differentiation, multi-omics profiling and functional tests demonstrate that 3D adipocyte cultures feature mature molecular and cellular phenotypes similar to freshly isolated mature adipocytes. Spheroids exhibit physiologically relevant gene expression signatures with 4704 differentially expressed genes compared to conventional 2D cultures (false discovery rate < 0.05), including the concerted expression of factors shaping the adipogenic niche. Furthermore, lipid profiles of >1000 lipid species closely resemble patterns of the corresponding isogenic mature adipocytes in vivo (R2 = 0.97). Integration of multi-omics signatures with analyses of the activity profiles of 503 transcription factors using global promoter motif inference reveals a complex signaling network, involving YAP, Hedgehog, and TGFβ signaling, that links the organotypic microenvironment in 3D culture to the activation and reinforcement of PPARγ and CEBP activity resulting in improved adipogenesis.

The Materiobiology of Silk: Exploring the Biophysical Influence of Silk Biomaterials on Directing Cellular Behaviors

Biophysical properties of the extracellular environment dynamically regulate cellular fates. In this review, we highlight silk, an indispensable polymeric biomaterial, owing to its unique mechanical properties, bioactive component sequestration, degradability, well-defined architectures, and biocompatibility that can regulate temporospatial biochemical and biophysical responses. We explore how the materiobiology of silks, both mulberry and non-mulberry based, affect cell behaviors including cell adhesion, cell proliferation, cell migration, and cell differentiation. Keeping in mind the novel biophysical properties of silk in film, fiber, or sponge forms, coupled with facile chemical decoration, and its ability to match functional requirements for specific tissues, we survey the influence of composition, mechanical properties, topography, and 3D geometry in unlocking the body's inherent regenerative potential.

Endothelial cell crosstalk improves browning but hinders white adipocyte maturation in 3D engineered adipose tissue

Central to the development of adipose tissue (AT) engineered models is the supporting vasculature. It is a key part of AT function and long-term maintenance, but the crosstalk between adipocytes and endothelial cells is not well understood. Here, we directly co-culture the two cell types at varying ratios in a 3D Type I collagen gel. Constructs were evaluated for adipocyte maturation and function and vascular network organization. Further, these constructs were treated with forskolin, a beta-adrenergic agonist, to stimulate lipolysis and browning. Adipocytes in co-cultures were found to be less mature than an adipocyte-only control, shown by smaller lipid droplets and downregulation of key adipocyte-related genes. The most extensive vascular network formation was found in the 1:1 co-culture, supported by vascular endothelial growth factor (VEGF) upregulation. After forskolin treatment, the presence of endothelial cells was shown to upregulate PPAR coactivator 1 alpha (PGC-1α) and leptin, but not uncoupling protein 1 (UCP1), suggesting a specific crosstalk that enhances early stages of browning.

Human Adipose Derived Cells in Two- and Three-Dimensional Cultures: Functional Validation of an In Vitro Fat Construct

Obesity, defined as a body mass index of 30 kg/m² or above, has increased considerably in incidence and frequency within the United States and globally. Associated comorbidities including cardiovascular disease, type 2 diabetes mellitus, metabolic syndrome, and nonalcoholic fatty liver disease have led to a focus on the mechanisms promoting the prevention and treatment of obesity. Commonly utilized in vitro models employ human or mouse preadipocyte cell lines in a 2-dimensional (2D) format. Due to the structural, biochemical, and biological limitations of these models, increased attention has been placed on "organ on a chip" technologies for a 3-dimensional (3D) culture. Herein, we describe a method employing cryopreserved primary human stromal vascular fraction (SVF) cells and a human blood product-derived biological scaffold to create a 3D adipose depot in vitro. The "fat-on-chip" 3D cultures have been validated relative to 2D cultures based on proliferation, flow cytometry, adipogenic differentiation, confocal microscopy/immunofluorescence, and functional assays (adipokine secretion, glucose uptake, and lipolysis). Thus, the in vitro culture system demonstrates the critical characteristics required for a humanized 3D white adipose tissue (WAT) model.

Mechanisms of action, chemical characteristics, and model systems of obesogens

There is increasing evidence for the role of environmental endocrine disrupting contaminants, coined obesogens, in exacerbating the rising obesity epidemic. Obesogens can be found in everyday items ranging from pesticides to food packaging. Although research shows that obesogens can have effects on adipocyte size, phenotype, metabolic activity, and hormone levels, much remains unknown about these chemicals. This review will discuss what is currently known about the mechanisms of obesogens, including expression of the PPARs, hormone interference, and inflammation. Strategies for identifying obesogenic chemicals and their mechanisms through chemical characteristics and model systems will also be discussed. Ultimately, research should focus on improving models to discern precise mechanisms of obesogenic action and to test therapeutics targeting these mechanisms.

Collagen Stiffness and Architecture Regulate Fibrotic Gene Expression in Engineered Adipose Tissue

Adipose tissue (AT) has a dynamic extracellular matrix (ECM) surrounding adipocytes that allows for remodeling during metabolic fluctuations. During the progression of obesity, AT has increased ECM deposition, stiffening, and remodeling, resulting in a pro-fibrotic dysfunctional state. Here, the incorporation of ethylene glycol-bis-succinic acid N-hydroxysuccinimide ester (PEGDS) allows for control over 3D collagen hydrogel stiffness and architecture to investigate its influence on adipocyte metabolic and fibrotic function. Upon stiffening and altering ECM architecture, adipocytes did not alter their expression of key adipokines, leptin, and adiponectin. However, they do increase actin cytoskeletal fiber formation, pro-fibrotic gene expression, ECM deposition, and remodeling within a stiffer, 3D collagen hydrogel. For example, COL6A3 gene expression is upregulated approximately twofold, resulting in increased deposition of pericellular collagen VI alpha 3 surrounding adipocytes. Furthermore, inhibition of actin contractility results in a reversal of pro-fibrotic gene expression and ECM deposition, indicating that adipocytes are mediating mechanical cues through actin cytoskeletal networks. This study demonstrates that ECM stiffness and architecture plays a critical regulatory role in adipocyte fibrotic function and contributes to the overall pro-fibrotic dysfunctional state of AT during the progression of obesity and AT fibrosis.

iTRAQ-Based Quantitative Proteomic Comparison of 2D and 3D Adipocyte Cell Models Co-cultured with Macrophages Using Online 2D-nanoLC-ESI-MS/MS

The demand for novel three-dimensional (3D) cell culture models of adipose tissue has been increasing, and proteomic investigations are important for determining the underlying causes of obesity, type II diabetes, and metabolic disorders. In this study, we performed global quantitative proteomic profiling of three 3D-cultured 3T3-L1 cells (preadipocytes, adipocytes and co-cultured adipocytes with macrophages) and their 2D-cultured counterparts using 2D-nanoLC-ESI-MS/MS with iTRAQ labelling. A total of 2,885 shared proteins from six types of adipose cells were identified and quantified in four replicates. Among them, 48 proteins involved in carbohydrate metabolism (e.g., PDHα, MDH1/2, FH) and the mitochondrial fatty acid beta oxidation pathway (e.g., VLCAD, ACADM, ECHDC1, ALDH6A1) were relatively up-regulated in the 3D co-culture model compared to those in 2D and 3D mono-cultured cells. Conversely, 12 proteins implicated in cellular component organisation (e.g., ANXA1, ANXA2) and the cell cycle (e.g., MCM family proteins) were down-regulated. These quantitative assessments showed that the 3D co-culture system of adipocytes and macrophages led to the development of insulin resistance, thereby providing a promising in vitro obesity model that is more equivalent to the in vivo conditions with respect to the mechanisms underpinning metabolic syndromes and the effect of new medical treatments for metabolic disorders.

The importance of the neuro-immuno-cutaneous system on human skin equivalent design

The skin is a highly complex organ, responsible for sensation, protection against the environment (pollutants, foreign proteins, infection) and thereby linked to the immune and sensory systems in the neuro-immuno-cutaneous (NIC) system. Cutaneous innervation is a key part of the peripheral nervous system; therefore, the skin should be considered a sensory organ and an important part of the central nervous system, an 'active interface' and the first connection of the body to the outside world. Peripheral nerves are a complex class of neurons within these systems, subsets of functions are conducted, including mechanoreception, nociception and thermoception. Epidermal and dermal cells produce signalling factors (such as cytokines or growth factors), neurites influence skin cells (such as via neuropeptides), and peripheral nerves have a role in both early and late stages of the inflammatory response. One way this is achieved, specifically in the cutaneous system, is through neuropeptide release and signalling, especially via substance P (SP), neuropeptide Y (NPY) and nerve growth factor (NGF). Cutaneous, neuronal and immune cells play a central role in many conditions, including psoriasis, atopic dermatitis, vitiligo, UV-induced immunosuppression, herpes and lymphomas. Therefore, it is critical to understand the connections and interplay between the peripheral nervous system and the skin and immune systems, the NIC system. Relevant in vitro tissue models based on human skin equivalents can be used to gain insight and to address impact across research and clinical needs.

In vitro tissue-engineered adipose constructs for modeling disease

BACKGROUND

Adipose tissue is a vital tissue in mammals that functions to insulate our bodies, regulate our internal thermostat, protect our organs, store energy (and burn energy, in the case of beige and brown fat), and provide endocrine signals to other organs in the body. Tissue engineering of adipose and other soft tissues may prove essential for people who have lost this tissue from trauma or disease.

MAIN TEXT

In this review, we discuss the applications of tissue-engineered adipose tissue specifically for disease modeling applications. We provide a basic background to adipose depots and describe three-dimensional (3D) in vitro adipose models for obesity, diabetes, and cancer research applications.

CONCLUSIONS

The approaches to engineering 3D adipose models are diverse in terms of scaffold type (hydrogel-based, silk-based and scaffold-free), species of origin (H. sapiens and M. musculus) and cell types used, which allows researchers to choose a model that best fits their application, whether it is optimization of adipocyte differentiation or studying the interaction of adipocytes and other cell types like endothelial cells. In vitro 3D adipose tissue models support discoveries into the mechanisms of adipose-related diseases and thus support the development of novel anti-cancer or anti-obesity/diabetes therapies.

Comparing the use of differentiated adipose-derived stem cells and mature adipocytes to model adipose tissue in vitro

In vitro models of human adipose tissue may serve as beneficial alternatives to animal models to study basic biological processes, identify new drug targets, and as soft tissue implants. With this approach, we aimed to evaluate adipose-derived stem cells (ASC) and mature adipocytes (MA) comparatively for the application in the in vitro setup of adipose tissue constructs to imitate native adipose tissue physiology. We used human primary MAs and human ASCs, differentiated for 14 days, and encapsulated them in collagen type I hydrogels to build up a three-dimensional (3D) adipose tissue model. The maintenance of the models was analyzed after seven days based on a viability staining. Further, the expression of the adipocyte specific protein perilipin A and the release of leptin and glycerol were evaluated. Gene transcription profiles of models based on dASCs and MAs were analyzed with regard to native adipose tissue. Compared to MAs, dASCs showed an immature differentiation state. Further, gene transcription of MAs suggests a behavior closer to native tissue in terms of angiogenesis, which supports MAs as preferred cell type. In contrast to native adipose tissue, genes of de novo lipogenesis and tissue remodeling were upregulated in the in vitro attempts.

2D and 3D In Vitro Co-Culture for Cancer and Bone Cell Interaction Studies

Co-culture assays are used to study the mutual interaction between cells in vitro. This chapter describes 2D and 3D co-culture systems used to study cell-cell signaling crosstalk between cancer cells and bone marrow adipocytes, osteoblasts, osteoclasts, and osteocytes. The chapter provides a step-by-step guide to the most commonly used cell culture techniques, functional assays, and gene expression.

Biomaterials to Mimic and Heal Connective Tissues

Connective tissue is one of the four major types of animal tissue and plays essential roles throughout the human body. Genetic factors, aging, and trauma all contribute to connective tissue dysfunction and motivate the need for strategies to promote healing and regeneration. The goal here is to link a fundamental understanding of connective tissues and their multiscale properties to better inform the design and translation of novel biomaterials to promote their regeneration. Major clinical problems in adipose tissue, cartilage, dermis, and tendon are discussed that inspire the need to replace native connective tissue with biomaterials. Then, multiscale structure-function relationships in native soft connective tissues that may be used to guide material design are detailed. Several biomaterials strategies to improve healing of these tissues that incorporate biologics and are biologic-free are reviewed. Finally, important guidance documents and standards (ASTM, FDA, and EMA) that are important to consider for translating new biomaterials into clinical practice are highligted.

Development of in vitro three-dimensional co-culture system for metabolic syndrome therapeutic agents

AIMS

There are many obstacles to overcome in the development of new drugs for metabolic diseases, including efficacy and toxicity problems in later stages of drug development. To overcome these problems and predict efficacy and toxicity in early stages, we constructed a new model of insulin resistance in terms of communication between 3T3-L1 adipocytes and RAW264.7 macrophages by three-dimensional (3D) culture.

RESULTS

In this study, results focused on the functional resemblance between 3D co-culture of adipocytes and macrophages and adipose tissue in diabetic mice. The 3D mono-culture preadipocytes showed good cell viability and induced cell differentiation to adipocytes, without cell confluence or cell-cell contact and interaction. The 3D co-cultured preadipocytes with RAW264.7 macrophages induced greater insulin resistance than two-dimensional and 3D mono-cultured adipocytes. Additionally, we demonstrated that 3D co-culture model had functional metabolic similarity to adipose tissue in diabetic mice. We utilized this 3D co-culture system to screen PPARγ antagonists that might have potential as therapeutic agents for diabetes as demonstrated by an in vivo assay.

CONCLUSION

This in vitro 3D co-culture system could serve as a next-generation platform to accelerate the development of therapeutics for metabolic diseases.

Silicon-Enhanced Adipogenesis and Angiogenesis for Vascularized Adipose Tissue Engineering

The enhancement of adipogenic differentiation of bone marrow mesenchymal stem cells (BMSCs) and sufficient vascularization remain great challenges for the successful reconstruction of engineered adipose tissue. Here, the bioactive effects of silicon (Si) ions on adipogenic differentiation of human BMSCs (HBMSCs) and the stimulation of vascularization during adipose tissue regeneration are reported. The results show that Si ions can enhance adipogenic differentiation of HBMSCs through the stimulation of the expression of adipogenic differentiation switches such as peroxisome proliferator-activated receptor γ and CCAAT/enhancer-binding protein α. Furthermore, Si ions can enhance both angiogenesis and adipogenesis, and inhibit dedifferentiation of cocultured adipocytes by regulating the interactions between HBMSC-derived adipocytes and human umbilical vein endothelial cells, in which the promotion of the expression of insulin-like growth factor 1 and vascular endothelial growth factor plays vital roles. The in vivo studies further demonstrate that the designed composite hydrogel with the ability to release bioactive Si ions clearly stimulates neovascularization and adipose tissue regeneration. The study suggests that Si ions released from biomaterials are important chemical cues for adipogenic differentiation and biomaterials with the ability to release Si ions can be designed for adipose tissue engineering.

Short-term post-implantation dynamics of in vitro engineered human microvascularized adipose tissues

Engineered adipose tissues are developed for their use as substitutes for tissue replacement in reconstructive surgery. To ensure a timely perfusion of the grafted substitutes, different strategies can be used such as the incorporation of an endothelial component. In this study, we engineered human adipose tissue substitutes comprising of functional adipocytes as well as a natural extracellular matrix using the self-assembly approach, without the use of exogenous scaffolding elements. Human microvascular endothelial cells (hMVECs) were incorporated during tissue production in vitro and we hypothesized that their presence would favor the early connection with the host vascular network translating into functional enhancement after implantation into nude mice in comparison to the substitutes that were not enriched in hMVECs. In vitro, no significant differences were observed between the substitutes in terms of histological aspects. After implantation, both groups presented numerous adipocytes and an abundant matrix in addition to the presence of host capillaries within the grafts. The substitutes thickness and volume were not significantly different between groups over the short-term time course of 14 days (d). For the microvascularized adipose tissues, human CD31 staining revealed a human capillary network connecting with the host microvasculature as early as 3 d after grafting. The detection of murine red blood cells within human CD31+ structures confirmed the functionality of the human capillary network. By analyzing the extent of the global vascularization achieved, a tendency towards increased total capillary network surface and volume was revealed for prevascularized tissues over 14 d. Therefore, applying this strategy on thicker reconstructed adipose tissues with rate-limiting oxygen diffusion might procure added benefits and prove useful to provide voluminous substitutes for patients suffering from adipose tissue loss or defects.

Generation of Gellan Gum-Based Adipose-Like Microtissues

Adipose tissue is involved in many physiological processes. Therefore, the need for adipose tissue-like analogues either for soft tissue reconstruction or as in vitro testing platforms is undeniable. In this work, we explored the natural features of gellan gum (GG) to recreate injectable stable adipose-like microtissues. GG hydrogel particles with different percentages of polymer (0.5%, 0.75%, 1.25%) were developed and the effect of obtained mechanical properties over the ability of hASCs to differentiate towards the adipogenic lineage was evaluated based on the expression of the early (PPARγ) and late (FABP4) adipogenic markers, and on lipids formation and accumulation. Constructs were cultured in adipogenic induction medium up to 21 days or for six days in induction plus nine days in maintenance media. Overall, no significant differences were observed in terms of hASCs adipogenic differentiation within the range of Young’s moduli between 2.7 and 12.9 kPa. The long-term (up to six weeks) stability of the developed constructs supported its application in soft tissue reconstruction. Moreover, their ability to function as adipose-like microtissue models for drug screening was demonstrated by confirming its sensitivity to TNFα and ROCK inhibitor, respectively involved in the repression and induction of the adipogenic differentiation.

Micro- and nano-patterned elastin-like polypeptide hydrogels for stem cell culture

We show that submicron-sized patterns can be imprinted into soft, recombinant-engineered protein hydrogels (here elastin-like proteins, ELP) by transferring wavy patterns from polydimethylsiloxane (PDMS) molds. The high-precision topographical tunability of the relatively stiff PDMS is translated to a bio-responsive, soft material, enabling topographical cell response studies at elastic moduli matching those of tissues. Aligned and unaligned wavy patterns with mold periodicities of 0.24-4.54 μm were imprinted and characterized by coherent anti-Stokes Raman scattering and atomic force microscopy. The pattern was successfully transferred down to 0.37 μm periodicity (width in ELP: 250 ± 50 nm, height: 70 ± 40 nm). The limit was set by inherent protein assemblies (diameter: 124-180 nm) that formed due to lower critical solution temperature behavior of the ELP during molding. The width/height of the ELP ridges depended on the degree of hydration; from complete dehydration to full hydration, ELP ridge width ranged from 79 ± 9% to 150 ± 40% of the mold width. The surface of the ridged ELP featured densely packed protein aggregates that were larger in size than those observed in bulk/flat ELP. Adipose-derived stem cells (ADSCs) oriented along hydrated aligned patterns with periodicities ≥0.60 μm (height ≥170 ± 100 nm), while random orientation was observed for smaller distances/amplitudes, as well as flat and unaligned wavy ELP surfaces. Hence, micro-molding of ELP is a promising approach to create tissue-mimicking, hierarchical architectures composed of tunable micron-sized structures with nano-sized protein aggregates, which opens the way for orthogonal screening of cell responses to topography and cell-adhesion ligands at relevant elastic moduli.

Silk based bioinks for soft tissue reconstruction using 3-dimensional (3D) printing with in vitro and in vivo assessments

In the field of soft tissue reconstruction, custom implants could address the need for materials that can fill complex geometries. Our aim was to develop a material system with optimal rheology for material extrusion, that can be processed in physiological and non-toxic conditions and provide structural support for soft tissue reconstruction. To meet this need we developed silk based bioinks using gelatin as a bulking agent and glycerol as a non-toxic additive to induce physical crosslinking. We developed these inks optimizing printing efficacy and resolution for patient-specific geometries that can be used for soft tissue reconstruction. We demonstrated in vitro that the material was stable under physiological conditions and could be tuned to match soft tissue mechanical properties. We demonstrated in vivo that the material was biocompatible and could be tuned to maintain shape and volume up to three months while promoting cellular infiltration and tissue integration.

Biodegradable polymer scaffolds

Tissue engineering aims to repair the damaged tissue by transplantation of cells or introducing bioactive factors in a biocompatible scaffold. In recent years, biodegradable polymer scaffolds mimicking the extracellular matrix have been developed to promote the cell proliferation and extracellular matrix deposition. The biodegradable polymer scaffolds thus act as templates for tissue repair and regeneration. This article reviews the updated information regarding various types of natural and synthetic biodegradable polymers as well as their functions, physico-chemical properties, and degradation mechanisms in the development of biodegradable scaffolds for tissue engineering applications, including their combination with 3D printing.

Development of a Three-Dimensional Adipose Tissue Model for Studying Embryonic Exposures to Obesogenic Chemicals

Obesity is a rising issue especially in the United States that can lead to heart problems, type II diabetes, and respiratory problems. Since the 1970s, obesity rates in the United States have more than doubled in adults and children. Recent evidence suggests that exposure to certain chemicals, termed "obesogens," in utero may alter metabolic processes, predisposing individuals to weight gain. There is a need to develop a three-dimensional human tissue system that is able to model the effects of obesogens in vitro in order to better understand the impact of obesogens on early development. Human embryonic-derived stem cells in three-dimensional collagen embedded silk scaffolds were exposed to three different obesogens: Bisphenol A (BPA), Bisphenol S (BPS), and Tributyltin (TBT). The exposed tissues accumulated triglycerides and increased expression of adipogenic genes (Perilipin (PLIN1), peroxisome proliferator-activated receptor gamma (PPARy), fatty acid binding protein 4 (FABP4)) compared to equivalent control cultures with no obesogen exposure. These cultures were also compared to human adult stem cell cultures, which did not respond the same upon addition of obesogens. These results demonstrate the successful development of a representative tissue model of in utero obesogen exposures. This tissue system could be used to determine mechanisms of action of current obesogens and to screen other potential obesogens.

Silk as a Biomaterial to Support Long-Term Three-Dimensional Tissue Cultures

Tissue engineering has broad and diverse impacts on a variety of different applications from tissue regeneration to drug screening. While two-dimensional (2-D) cell culture platforms are suitable for tissue interfaces where planar surfaces are relevant, three dimensional (3-D) tissue models have enhanced relevance and sustainability over 2-D devices. The improvements between 2-D and 3-D functions and sustainability are related to the limitations of 2-D systems to support proper cellular morphology and signaling over time, resulting in cell overgrowth or changes in viability. For sustainable (long-term) cultures, 3-D silk protein scaffolds provide biocompatibility, porous features for transport, robust yet tunable mechanical properties, retain size and open porous structures for extended time frames due to slow proteolytic biodegradation, avoid specific cell signaling, and require no chemical cross-linking. Silk degradation can be extended for months to years without premature collapse of structures (that would result in necrosis) to support cell interactions during slow remodeling toward native tissue. Silk can also be fabricated into different material formats, such as hydrogels, tubes, sponges, composites, fibers, microspheres, and thin films, providing versatile platforms and interfaces for a variety of different applications. For sustainable tissue engineering applications, many formats have been used, including silk ionmer hydrogels that have been cultured for up to 8 weeks and porous silk scaffolds that have been cultured for up to 6 months. In this review, we highlight some of our tissue engineering work related to long-term in vitro cultures. While each tissue engineered system (adipose tissue, cortical brain tissue, intestine, kidney tissue, bone) is unique, they all use silk biomaterials as a base scaffolding material to achieve sustainable cultivation. Sustainability is important for studies that extend past a few weeks to study acute and chronic impacts of treatments, disease models, and other related applications in the field of tissue engineering.

The Use of Silk as a Scaffold for Mature, Sustainable Unilocular Adipose 3D Tissue Engineered Systems

There is a critical need for monitoring physiologically relevant, sustainable, human adipose tissues in vitro to gain new insights into metabolic diseases. To support long-term culture, a 3D silk scaffold assisted culture system is developed that maintains mature unilocular adipocytes ex vivo in coculture with preadipocytes, endothelial cells, and smooth muscle cells obtained from small volumes of liquefied adipose samples. Without the silk scaffold, adipose tissue explants cannot be sustained in long-term culture (3 months) due to their fragility. Adjustments to media components are used to tune lipid metabolism and proliferation, in addition to responsiveness to an inflammatory stimulus. Interestingly, patient specific responses to TNFα stimulation are observed, providing a proof-of-concept translational technique for patient specific disease modeling in the future. In summary, this novel 3D scaffold assisted approach is required for establishing physiologically relevant, sustainable, human adipose tissue systems from small volumes of lipoaspirate, making this methodology of great value to studies of metabolism, adipokine-driven diseases, and other diseases where the roles of adipocytes are only now becoming uncovered.

Non-invasive Assessments of Adipose Tissue Metabolism In Vitro

Adipose tissue engineering is a diverse area of research where the developed tissues can be used to study normal adipose tissue functions, create disease models in vitro, and replace soft tissue defects in vivo. Increasing attention has been focused on the highly specialized metabolic pathways that regulate energy storage and release in adipose tissues which affect local and systemic outcomes. Non-invasive, dynamic measurement systems are useful to track these metabolic pathways in the same tissue model over time to evaluate long term cell growth, differentiation, and development within tissue engineering constructs. This approach reduces costs and time in comparison to more traditional destructive methods such as biochemical and immunochemistry assays and proteomics assessments. Towards this goal, this review will focus on important metabolic functions of adipose tissues and strategies to evaluate them with non-invasive in vitro methods. Current non-invasive methods, such as measuring key metabolic markers and endogenous contrast imaging will be explored.

Characterization of In Vitro Engineered Human Adipose Tissues: Relevant Adipokine Secretion and Impact of TNF-α

Representative modelling of human adipose tissue functions is central to metabolic research. Tridimensional models able to recreate human adipogenesis in a physiological tissue-like context in vitro are still scarce. We describe the engineering of white adipose tissues reconstructed from their cultured adipose-derived stromal precursor cells. We hypothesize that these reconstructed tissues can recapitulate key functions of AT under basal and pro-inflammatory conditions. These tissues, featuring human adipocytes surrounded by stroma, were stable and metabolically active in long-term cultures (at least 11 weeks). Secretion of major adipokines and growth factors by the reconstructed tissues was determined and compared to media conditioned by human native fat explants. Interestingly, the secretory profiles of the reconstructed adipose tissues indicated an abundant production of leptin, PAI-1 and angiopoietin-1 proteins, while higher HGF levels were detected for the human fat explants. We next demonstrated the responsiveness of the tissues to the pro-inflammatory stimulus TNF-α, as reflected by modulation of MCP-1, NGF and HGF secretion, while VEGF and leptin protein expression did not vary. TNF-α exposure induced changes in gene expression for adipocyte metabolism-associated mRNAs such as SLC2A4, FASN and LIPE, as well as for genes implicated in NF-κB activation. Finally, this model was customized to feature adipocytes representative of progressive stages of differentiation, thereby allowing investigations using newly differentiated or more mature adipocytes. In conclusion, we produced tridimensional tissues engineered in vitro that are able to recapitulate key characteristics of subcutaneous white adipose tissue. These tissues are produced from human cells and their neo-synthesized matrix elements without exogenous or synthetic biomaterials. Therefore, they represent unique tools to investigate the effects of pharmacologically active products on human stromal cells, extracellular matrix and differentiated adipocytes, in addition to compounds modulating adipogenesis from precursor cells.

Opportunities and challenges in three-dimensional brown adipogenesis of stem cells

The formation of brown adipose tissue (BAT) via brown adipogenesis has become a notable process due to its ability to expend energy as heat with implications in the treatment of metabolic disorders and obesity. With the advent of complexity within white adipose tissue (WAT) along with inducible brown adipocytes (also known as brite and beige), there has been a surge in deciphering adipocyte biology as well as in vivo adipogenic microenvironments. A therapeutic outcome would benefit from understanding early events in brown adipogenesis, which can be accomplished by studying cellular differentiation. Pluripotent stem cells are an efficient model for differentiation and have been directed towards both white adipogenic and brown adipogenic lineages. The stem cell microenvironment greatly contributes to terminal cell fate and as such, has been mimicked extensively by various polymers including those that can form 3D hydrogel constructs capable of biochemical and/or mechanical modifications and modulations. Using bioengineering approaches towards the creation of 3D cell culture arrangements is more beneficial than traditional 2D culture in that it better recapitulates the native tissue biochemically and biomechanically. In addition, such an approach could potentially protect the tissue formed from necrosis and allow for more efficient implantation. In this review, we highlight the promise of brown adipocytes with a focus on brown adipogenic differentiation of stem cells using bioengineering approaches, along with potential challenges and opportunities that arise when considering the energy expenditure of BAT for prospective therapeutics.

Methacrylated gelatin and mature adipocytes are promising components for adipose tissue engineering

In vitro engineering of autologous fatty tissue constructs is still a major challenge for the treatment of congenital deformities, tumor resections or high-graded burns. In this study, we evaluated the suitability of photo-crosslinkable methacrylated gelatin (GM) and mature adipocytes as components for the composition of three-dimensional fatty tissue constructs. Cytocompatibility evaluations of the GM and the photoinitiator Lithium phenyl-2,4,6-trimethylbenzoylphosphinate (LAP) showed no cytotoxicity in the relevant range of concentrations. Matrix stiffness of cell-laden hydrogels was adjusted to native fatty tissue by tuning the degree of crosslinking and was shown to be comparable to that of native fatty tissue. Mature adipocytes were then cultured for 14 days within the GM resulting in a fatty tissue construct loaded with viable cells expressing cell markers perilipin A and laminin. This work demonstrates that mature adipocytes are a highly valuable cell source for the composition of fatty tissue equivalents in vitro. Photo-crosslinkable methacrylated gelatin is an excellent tissue scaffold and a promising bioink for new printing techniques due to its biocompatibility and tunable properties.

Strategies for improving the physiological relevance of human engineered tissues

This review examines important robust methods for sustained, steady-state, in vitro culture. To achieve 'physiologically relevant' tissues in vitro additional complexity must be introduced to provide suitable transport, cell signaling, and matrix support for cells in 3D environments to achieve stable readouts of tissue function. Most tissue engineering systems draw conclusions on tissue functions such as responses to toxins, nutrition, or drugs based on short-term outcomes with in vitro cultures (2-14 days). However, short-term cultures limit insight with physiological relevance because the cells and tissues have not reached a steady-state.

Long term perfusion system supporting adipogenesis

Adipose tissue engineered models are needed to enhance our understanding of disease mechanisms and for soft tissue regenerative strategies. Perfusion systems generate more physiologically relevant and sustainable adipose tissue models, however adipocytes have unique properties that make culturing them in a perfusion environment challenging. In this paper we describe the methods involved in the development of two perfusion culture systems (2D and 3D) to test their applicability for long term in vitro adipogenic cultures. It was hypothesized that a silk protein biomaterial scaffold would provide a 3D framework, in combination with perfusion flow, to generate a more physiologically relevant sustainable adipose tissue engineered model than 2D cell culture. Consistent with other studies evaluating 2D and 3D culture systems for adipogenesis we found that both systems successfully model adipogenesis, however 3D culture systems were more robust, providing the mechanical structure required to contain the large, fragile adipocytes that were lost in 2D perfused culture systems. 3D perfusion also stimulated greater lipogenesis and lipolysis and resulted in decreased secretion of LDH compared to 2D perfusion. Regardless of culture configuration (2D or 3D) greater glycerol was secreted with the increased nutritional supply provided by perfusion of fresh media. These results are promising for adipose tissue engineering applications including long term cultures for studying disease mechanisms and regenerative approaches, where both acute (days to weeks) and chronic (weeks to months) cultivation are critical for useful insight.

Injectable silk foams for soft tissue regeneration

Soft tissue fillers are needed for restoration of a defect or augmentation of existing tissues. Autografts and lipotransfer have been under study for soft tissue reconstruction but yield inconsistent results, often with considerable resorption of the grafted tissue. A minimally invasive procedure would reduce scarring and recovery time as well as allow the implant and/or grafted tissue to be placed closer to existing vasculature. Here, the feasibility of an injectable silk foam for soft tissue regeneration is demonstrated. Adipose-derived stem cells survive and migrate through the foam over a 10-d period in vitro. The silk foams are also successfully injected into the subcutaneous space in a rat and over a 3-month period integrating with the surrounding native tissue. The injected foams are palpable and soft to the touch through the skin and returning to their original dimensions after pressure is applied and then released. The foams readily absorb lipoaspirate making the foams useful as a scaffold or template for existing soft tissue filler technologies, useful either as a biomaterial alone or in combination with the lipoaspirate.

3D in vitro modeling of the central nervous system

There are currently more than 600 diseases characterized as affecting the central nervous system (CNS) which inflict neural damage. Unfortunately, few of these conditions have effective treatments available. Although significant efforts have been put into developing new therapeutics, drugs which were promising in the developmental phase have high attrition rates in late stage clinical trials. These failures could be circumvented if current 2D in vitro and in vivo models were improved. 3D, tissue-engineered in vitro systems can address this need and enhance clinical translation through two approaches: (1) bottom-up, and (2) top-down (developmental/regenerative) strategies to reproduce the structure and function of human tissues. Critical challenges remain including biomaterials capable of matching the mechanical properties and extracellular matrix (ECM) composition of neural tissues, compartmentalized scaffolds that support heterogeneous tissue architectures reflective of brain organization and structure, and robust functional assays for in vitro tissue validation. The unique design parameters defined by the complex physiology of the CNS for construction and validation of 3D in vitro neural systems are reviewed here.

‘Body-on-a-Chip’ Technology and Supporting Microfluidics

In order to effectively streamline current drug development protocols, there is a need to generate high information content preclinical screens capable of generating data with a predictive power in relation to the activity of novel therapeutics in humans. Given the poor predictive power of animal models, and the lack of complexity and interconnectivity of standard in vitro culture methodologies, many investigators are now moving toward the development of physiologically and functionally accurate culture platforms composed of human cells to investigate cellular responses to drug compounds in high-throughput preclinical studies. The generation of complex, multi-organ in vitro platforms, built to recapitulate physiological dimensions, flow rates and shear stresses, is being investigated as the logical extension of this drive. Production and application of a biologically accurate multi-organ platform, or ‘body-on-a-chip’, would facilitate the correct modelling of the dynamic and interconnected state of living systems for high-throughput drug studies as well as basic and applied biomolecular research. This chapter will discuss current technologies aimed at producing ‘body-on-a-chip’ models, as well as highlighting recent advances and important challenges still to be met in the development of biomimetic single-organ systems for drug development purposes.

Arrayed Hollow Channels in Silk-based Scaffolds Provide Functional Outcomes for Engineering Critically-sized Tissue Constructs

In the field of regenerative medicine there is a need for scaffolds that support large, critically-sized tissue formation. Major limitations in reaching this goal are the delivery of oxygen and nutrients throughout the bulk of the engineered tissue as well as host tissue integration and vascularization upon implantation. To address these limitations we previously reported the development of a porous scaffold platform made from biodegradable silk protein that contains an array of vascular-like structures that extend through the bulk of the scaffold. Here we report that the hollow channels play a pivotal role in enhancing cell infiltration, delivering oxygen and nutrients to the scaffold bulk, and promoting in vivo host tissue integration and vascularization. The unique features of this protein biomaterial system, including the vascular structures and tunable material properties, render this scaffold a robust and versatile tool for implementation in a variety of tissue engineering, regenerative medicine and disease modeling applications.