Dual-phase, surface tension-based fabrication method for generation of tumor millibeads

Establishment of a tissue-engineered colon cancer model for comparative analysis of cancer cell lines

To overcome the limitations of in vitro two-dimensional (2D) cancer models in mimicking the complexities of the native tumor milieu, three-dimensional (3D) engineered cancer models using biomimetic materials have been introduced to more closely recapitulate the key attributes of the tumor microenvironment. Specifically, for colorectal cancer (CRC), a few studies have developed 3D engineered tumor models to investigate cell-cell interactions or efficacy of anti-cancer drugs. However, recapitulation of CRC cell line phenotypic differences within a 3D engineered matrix has not been systematically investigated. Here, we developed an in vitro 3D engineered CRC (3D-eCRC) tissue model using the natural-synthetic hybrid biomaterial PEG-fibrinogen and three CRC cell lines, HCT 116, HT-29, and SW480. To better recapitulate native tumor conditions, our 3D-eCRC model supported higher cell density encapsulation (20 × 106  cells/mL) and enabled longer term maintenance (29 days) as compared to previously reported in vitro CRC models. The 3D-eCRCs formed using each cell line demonstrated line-dependent differences in cellular and tissue properties, including cellular growth and morphology, cell subpopulations, cell size, cell granularity, migration patterns, tissue growth, gene expression, and tissue stiffness. Importantly, these differences were found to be most prominent from Day 22 to Day 29, thereby indicating the importance of long-term culture of engineered CRC tissues for recapitulation and investigation of mechanistic differences and drug response. Our 3D-eCRC tissue model showed high potential for supporting future in vitro comparative studies of disease progression, metastatic mechanisms, and anti-cancer drug candidate response in a CRC cell line-dependent manner.

Droplet Microfluidics-Based Fabrication of Monodisperse Poly(ethylene glycol)-Fibrinogen Breast Cancer Microspheres for Automated Drug Screening Applications

Spheroidal cancer microtissues are highly advantageous for a wide range of biomedical applications, including high-throughput drug screening, multiplexed target validation, mechanistic investigation of tumor-extracellular matrix (ECM) interactions, among others. Current techniques for spheroidal tissue formation rely heavily on self-aggregation of single cancer cells and have substantial limitations in terms of cell-type-specific heterogeneities, uniformity, ease of production and handling, and most importantly, mimicking the complex native tumor microenvironmental conditions in simplistic models. These constraints can be overcome by using engineered tunable hydrogels that closely mimic the tumor ECM and elucidate pathologically relevant cell behavior, coupled with microfluidics-based high-throughput fabrication technologies to encapsulate cells and create cancer microtissues. In this study, we employ biosynthetic hybrid hydrogels composed of poly(ethylene glycol diacrylate) (PEGDA) covalently conjugated to natural protein (fibrinogen) (PEG-fibrinogen, PF) to create monodisperse microspheres encapsulating breast cancer cells for 3D culture and tumorigenic characterization. A previously developed droplet-based microfluidic system is used for rapid, facile, and reproducible fabrication of uniform cancer microspheres with either MCF7 or MDA-MB-231 (metastatic) breast cancer cells. Cancer cell-type-dependent variations in cell viability, metabolic activity, and 3D morphology, as well as microsphere stiffness, are quantified over time. Particularly, MCF7 cells grew as tight cellular clusters in the PF microspheres, characteristic of their epithelial morphology, while MDA-MB-231 cells displayed elongated and invasive morphology, characteristic of their mesenchymal and metastatic nature. Finally, the translational potential of the cancer microsphere platform toward high-throughput drug screening is also demonstrated. With high uniformity, scalability, and control over engineered microenvironments, the established cancer microsphere model can be potentially used for mechanistic studies, fabrication of modular cancer microtissues, and future drug-testing applications.

Tunable three-dimensional engineered prostate cancer tissues for in vitro recapitulation of heterogeneous in vivo prostate tumor stiffness

In this manuscript we report the establishment and characterization of a three-dimensional in vitro, coculture engineered prostate cancer tissue (EPCaT) disease model based upon and informed by our characterization of in vivo prostate cancer (PCa) xenograft tumor stiffness. In prostate cancer, tissue stiffness is known to impact changes in gene and protein expression, alter therapeutic response, and be positively correlated with an aggressive clinical presentation. To inform an appropriate stiffness range for our in vitro model, PC-3 prostate tumor xenografts were established. Tissue stiffness ranged from 95 to 6,750 Pa. Notably, xenograft cell seeding density significantly impacted tumor stiffness; a two-fold increase in the number of seeded cells not only widened the tissue stiffness range throughout the tumor but also resulted in significant spatial heterogeneity. To fabricate our in vitro EPCaT model, PC-3 castration-resistant prostate cancer cells were co-encapsulated with BJ-5ta fibroblasts within a poly(ethylene glycol)-fibrinogen matrix augmented with excess poly(ethylene glycol)-diacrylate to modulate the matrix mechanical properties. Encapsulated cells temporally remodeled their in vitro microenvironment and enrichment of gene sets associated with tumorigenic progression was observed in response to increased matrix stiffness. Through variation of matrix composition and culture duration, EPCaTs were tuned to mimic the wide range of biomechanical cues provided to PCa cells in vivo; collectively, a range of 50 to 10,000 Pa was achievable. Markedly, this also encompasses published clinical PCa stiffness data. Overall, this study serves to introduce our bioinspired, tunable EPCaT model and provide the foundation for future PCa progression and drug development studies. STATEMENT OF SIGNIFICANCE: The development of cancer models that mimic the native tumor microenvironment (TME) complexities is critical to not only develop effective drugs but also enhance our understanding of disease progression. Here we establish and characterize our 3D in vitro engineered prostate cancer tissue model with tunable matrix stiffness, that is inspired by this study's spatial characterization of in vivo prostate tumor xenograft stiffness. Notably, our model's mimicry of the TME is further augmented by the inclusion of matrix remodeling fibroblasts to introduce cancer-stromal cell-cell interactions. This study addresses a critical unmet need in the field by elucidating the prostate tumor xenograft stiffness range and establishing a foundation for recapitulating the biomechanics of site-of-origin and soft tissue metastatic prostate tumors in vitro.

Emulsion-based encapsulation of pluripotent stem cells in hydrogel microspheres for cardiac differentiation

Cardiovascular disease is the leading cause of death worldwide, and current treatments are ineffective or unavailable to majority of patients. Engineered cardiac tissue (ECT) is a promising treatment to restore function to the damaged myocardium; however, for these treatments to become a reality, tissue fabrication must be amenable to scalable production and be used in suspension culture. Here, we have developed a low-cost and scalable emulsion-based method for producing ECT microspheres from poly(ethylene glycol) (PEG)-fibrinogen encapsulated mouse embryonic stem cells (mESCs). Cell-laden microspheres were formed via water-in-oil emulsification; encapsulation occurred by suspending the cells in hydrogel precursor solution at cell densities from 5 to 60 million cells/ml, adding to mineral oil and vortexing. Microsphere diameters ranged from 30 to 570 μm; size variability was decreased by the addition of 2% poly(ethylene glycol) diacrylate. Initial cell encapsulation density impacted the ability for mESCs to grow and differentiate, with the greatest success occurring at higher cell densities. Microspheres differentiated into dense spheroidal ECTs with spontaneous contractions occurring as early as Day 10 of cardiac differentiation; furthermore, these ECT microspheres exhibited appropriate temporal changes in gene expression and response to pharmacological stimuli. These results demonstrate the ability to use an emulsion approach to encapsulate pluripotent stem cells for use in microsphere-based cardiac differentiation.

Tunable hydrogels for controlling phenotypic cancer cell states to model breast cancer dormancy and reactivation

During metastasis, disseminated tumor cells (DTCs) from the primary tumor infiltrate secondary organs and reside there for varying lengths of time prior to forming new tumors. The time delay between infiltration and active proliferation, known as dormancy, mediates the length of the latency period. DTCs may undergo one of four fates post-infiltration: death, cellular dormancy, dormant micrometastasis, or invasive growth which, is in part, mediated by extracellular matrix (ECM) properties. Recapitulation of these cell states using engineered hydrogels could facilitate the systematic and controlled investigation of the mechanisms by which ECM properties influence DTC fate. Toward this goal, we implemented a set of sixteen hydrogels with systematic variations in chemical (ligand (RGDS) density and enzymatic degradability) and mechanical (elasticity, swelling, mesh size) properties to investigate their influence on the fate of encapsulated metastatic breast cancer cells, MDA-MB-231. Cell viability, apoptosis, proliferation, metabolic activity, and morphological measurements were acquired at five-day intervals over fifteen days in culture. Analysis of the phenotypic metrics indicated the presence of four different cell states that were classified as: (1) high growth, (2) moderate growth, (3) single cell, restricted survival, dormancy, or (4) balanced dormancy. Correlating hydrogel properties with the resultant cancer cell state indicated that ligand (RGDS) density and enzymatic degradability likely had the most influence on cell fate. Furthermore, we demonstrate the ability to reactivate cells from the single cell, dormant state to the high growth state through a dynamic increase in ligand (RGDS) density after forty days in culture. This tunable engineered hydrogel platform offers insight into matrix properties regulating tumor dormancy, and the dormancy-proliferation switch, and may provide future translational benefits toward development of anti-dormancy therapeutic strategies.

Templated Macroporous Polyethylene Glycol Hydrogels for Spheroid and Aggregate Cell Culture

Macroporous cell-laden hydrogels have recently gained recognition for a wide range of biomedical and bioengineering applications. There are various approaches to create porosity in hydrogels, including lyophilization or foam formation. However, many do not allow a precise control over pore size or are not compatible with in situ cell encapsulation. Here, we developed novel templated macroporous hydrogels by encapsulating uniform degradable hydrogel microspheres produced via microfluidics into a hydrogel slab. The microspheres degraded completely leaving macropores behind. Microsphere degradation was dependent on the incubation medium, microsphere size, microsphere confinement in the hydrogel as well as cell encapsulation. Uniquely, the degradable microspheres were biocompatible and when laden with cells, the cells were deposited in the macropores upon microsphere degradation and formed multicellular aggregates. The hydrogel-encapsulated cell aggregates were used in a small drug screen to demonstrate the relevance of cell-matrix interactions for multicellular spheroid drug responsiveness. Hydrogel-grown spheroid cultures are increasingly important in applications such as in vitro tumor, hepatocellular, and neurosphere cultures and drug screening; hence, the templated cell aggregate-laden hydrogels described here would find utility in various applications.

Engineered In Vitro Models of Tumor Dormancy and Reactivation

Metastatic recurrence is a major hurdle to overcome for successful control of cancer-associated death. Residual tumor cells in the primary site, or disseminated tumor cells in secondary sites, can lie in a dormant state for long time periods, years to decades, before being reactivated into a proliferative growth state. The microenvironmental signals and biological mechanisms that mediate the fate of disseminated cancer cells with respect to cell death, single cell dormancy, tumor mass dormancy and metastatic growth, as well as the factors that induce reactivation, are discussed in this review. Emphasis is placed on engineered, in vitro, biomaterial-based approaches to model tumor dormancy and subsequent reactivation, with a focus on the roles of extracellular matrix, secondary cell types, biochemical signaling and drug treatment. A brief perspective of molecular targets and treatment approaches for dormant tumors is also presented. Advances in tissue-engineered platforms to induce, model, and monitor tumor dormancy and reactivation may provide much needed insight into the regulation of these processes and serve as drug discovery and testing platforms.

Design of spherically structured 3D in vitro tumor models -Advances and prospects

Three-dimensional multicellular tumor models are receiving an ever-growing focus as preclinical drug-screening platforms due to their potential to recapitulate major physiological features of human tumors in vitro. In line with this momentum, the technologies for assembly of 3D microtumors are rapidly evolving towards a comprehensive inclusion of tumor microenvironment elements. Customized spherically structured platforms, including microparticles and microcapsules, provide a robust and scalable technology to imprint unique biomolecular tumor microenvironment hallmarks into 3D in vitro models. Herein, a comprehensive overview of novel advances on the integration of tumor-ECM components and biomechanical cues into 3D in vitro models assembled in spherical shaped platforms is provided. Future improvements regarding spatiotemporal/mechanical adaptability, and degradability, during microtumors in vitro 3D culture are also critically discussed considering the realistic potential of these platforms to mimic the dynamic tumor microenvironment. From a global perspective, the production of 3D multicellular spheroids with tumor ECM components included in spherical models will unlock their potential to be used in high-throughput screening of therapeutic compounds. It is envisioned, in a near future, that a combination of spherically structured 3D microtumor models with other advanced microfluidic technologies will properly recapitulate the flow dynamics of human tumors in vitro.

STATEMENT OF SIGNIFICANCE

The ability to correctly mimic the complexity of the tumor microenvironment in vitro is a key aspect for the development of evermore realistic in vitro models for drug-screening and fundamental cancer biology studies. In this regard, conventional spheroid-based 3D tumor models, combined with spherically structured biomaterials, opens the opportunity to precisely recapitulate complex cell-extracellular matrix interactions and tumor compartmentalization. This review provides an in-depth focus on current developments regarding spherically structured scaffolds engineered into in vitro 3D tumor models, and discusses future advances toward all-encompassing platforms that may provide an improved in vitro/in vivo correlation in a foreseeable future.

Photooxidatively crosslinked acellular tumor extracellular matrices as potential tumor engineering scaffolds

Acellular tumor extracellular matrices (ECMs) have limitations when employed as three-dimensional (3D) scaffolds for tumor engineering. In this work, methylene blue-mediated photooxidation was used to crosslink acellular tumor ECMs. Photooxidative crosslinking greatly increased the stiffness of acellular tumor ECM scaffolds but barely altered the Amide III band of the secondary structure of polypeptides and proteins. MCF-7, HepG2 and A549 cells cultured on photooxidatively crosslinked acellular tumor ECM scaffolds exhibited greater cell number per scaffold, more IL-8 and VEGF secretion, and increase migration and invasion abilities than cells cultured on uncrosslinked acellular tumor ECM scaffolds. The three tumor cell lines cultured on the stiffer photooxidatively crosslinked acellular matrices acquire mesenchymal properties (mesenchymal shift) and dedifferentiated phenotypes. Furthermore, the malignant phenotypes induced in vitro when cultured on the crosslinked scaffold promoted the in vivo tumor growth of BALB/c nude mice. Finally, the dedifferentiated cancer cells, including MCF-7, HepG2 and A549 cells, were less sensitive to chemotherapeutics. Thus, photooxidatively crosslinked acellular tumor ECMs have potentials as 3D tumor engineering scaffolds for cancer research.

STATEMENT OF SIGNIFICANCE

Natural material scaffolds have been successfully used as 3D matrices to study the in vitro tumor cell growth and mimic the in vivo tumor microenvironment. Acellular tumor ECMs are developed as 3D scaffolds for tumor engineering but have limitations in terms of elastic modulus and cell spheroid formation. Here we use methylene blue-mediated photooxidation to crosslink acellular tumor ECMs and investigate the influence of photooxidative crosslinking on structural, mechanical and biological characteristics of acellular tumor ECM scaffolds. It is the first study to evaluate the feasibility of photooxidatively crosslinked acellular tumor ECMs as 3D scaffolds for cancer research and the results are encouraging. Moreover, this study provides new research areas in regard to photodynamic therapy (PDT) for Cancer.

A Microvascularized Tumor-mimetic Platform for Assessing Anti-cancer Drug Efficacy

Assessment of anti-cancer drug efficacy in in vitro three-dimensional (3D) bioengineered cancer models provides important contextual and relevant information towards pre-clinical translation of potential drug candidates. However, currently established models fail to sufficiently recapitulate complex tumor heterogeneity. Here we present a chip-based tumor-mimetic platform incorporating a 3D in vitro breast cancer model with a tumor-mimetic microvascular network, replicating the pathophysiological architecture of native vascularized breast tumors. The microfluidic platform facilitated formation of mature, lumenized and flow-aligned endothelium under physiological flow recapitulating both high and low perfused tumor regions. Metastatic and non-metastatic breast cancer cells were maintained in long-term 3D co-culture with stromal fibroblasts in a poly(ethylene glycol)-fibrinogen hydrogel matrix within adjoining tissue chambers. The interstitial space between the chambers and endothelium contained pores to mimic the “leaky” vasculature found in vivo and facilitate cancer cell-endothelial cell communication. Microvascular pattern-dependent flow variations induced concentration gradients within the 3D tumor mass, leading to morphological tumor heterogeneity. Anti-cancer drugs displayed cell type- and flow pattern-dependent effects on cancer cell viability, viable tumor area and associated endothelial cytotoxicity. Overall, the developed microfluidic tumor-mimetic platform facilitates investigation of cancer-stromal-endothelial interactions and highlights the role of a fluidic, tumor-mimetic vascular network on anti-cancer drug delivery and efficacy for improved translation towards pre-clinical studies.

Design of electrohydrodynamic sprayed polyethylene glycol hydrogel microspheres for cell encapsulation

Electrohydrodynamic spraying (EHS) has recently gained popularity for microencapsulation of cells for applications in cell delivery and tissue engineering. Some of the polymers compatible with EHS are alginate, chitosan, and other similar natural polymers, which are subject to ionotropic or physical gelation. It is desirable to further extend the use of the EHS technique beyond such polymers for wider biofabrication applications. Here, building upon our previous work of making PEG microspheres via EHS, we utilized the principles of EHS to fabricate cell-laden polyethylene glycol (PEG) hydrogel microspheres. The gelation of PEG hydrogel microspheres was achieved by forming covalent crosslinks between multiarm PEG acrylate and dithiol crosslinkers via Michael-type addition. We conducted a detailed investigation of the critical parameters of EHS, such as the applied voltage, inner needle diameter (i.d. needle), and flow rate, to obtain PEG microspheres with high cell viability and tightly-controlled diameters in the range of 70-300 μm. The polydispersity of cell-laden PEG hydrogel microspheres as measured by % coefficient of variation was between 6% and 23% for all conditions tested. We established that our method was compatible with different cell types and that all tested cell types could be encapsulated at high densities of 10⁶-10⁹ and ≥90% encapsulation efficiency. We observed cell aggregation within the hydrogel microspheres at applied voltage >5 kV. Since PEG is a synthetic polymer devoid of cell attachment sites, we could overcome this limitation by tethering Arg-Gly-Asp-Ser (RGDS) peptide to the PEG hydrogel microspheres; upon RGDS tethering, we observed uniform cell dispersion. The microencapsulated cells could be cultured in the PEG hydrogel microspheres of different sizes for up to one week without significant loss in cell viability. In conclusion, the EHS technique developed here could be used to generate cell-laden PEG hydrogel microspheres of controlled sizes for potential applications in cell delivery and organoid cultures.

A three-dimensional spheroidal cancer model based on PEG-fibrinogen hydrogel microspheres

Three-dimensional (3D) in vitro cancer models offer an attractive approach towards the investigation of tumorigenic phenomena and other cancer studies by providing dimensional context and higher degree of physiological relevance than that offered by conventional two-dimensional (2D) models. The multicellular tumor spheroid model, formed by cell aggregation, is considered to be the "gold standard" for 3D cancer models, due to its ease and simplicity of use. Although better than 2D models, tumor spheroids are unable to replicate key features of the native tumor microenvironment, particularly due to a lack of surrounding extracellular matrix components and heterogeneity in shape, size and aggregate forming tendencies. In order to address this issue, we have developed a 3D "tumor microsphere" model, formed by a dual-photoinitiator, aqueous-oil emulsion technique, for the encapsulation of cancer cells within PEG-fibrinogen hydrogel microspheres and for subsequent long-term 3D culture. In comparison to self-aggregated tumor spheroids, the tumor microspheres displayed a higher degree of size and shape homogeneity throughout long-term culture. In sharp contrast to cells in tumor spheroids, cells within tumor microspheres demonstrated significant loss in apico-basal polarity and cellular architecture, cellular and nuclear atypia, increased disorganization, elevated nuclear cytoplasmic ratio and nuclear volume density and reduction in cell-cell junction length, all of which are hallmarks of malignant transformation and tumorigenic progression. Additionally, the tumor microsphere model was extended for the 3D encapsulation and maintenance of a wide range of other cancer cell (metastatic and non-metastatic) types. Taken together, our results reinforce the importance of incorporating a biomimetic matrix in the cellular microenvironment of 3D tumor models and the influential effects of the matrix on the tumorigenic morphology of 3D cultured cells. The tumor microsphere system established in this study has the potential to be used in future investigations of 3D cancer cell-cell and cell-ECM interactions and in drug-testing applications.

Polymeric Biomaterials for In Vitro Cancer Tissue Engineering and Drug Testing Applications

Biomimetic polymers and materials have been widely used in tissue engineering for regeneration and replication of diverse types of both normal and diseased tissues. Cancer, being a prevalent disease throughout the world, has initiated substantial interest in the creation of tissue-engineered models for anticancer drug testing. The development of these in vitro three-dimensional (3D) culture models using novel biomaterials has facilitated the investigation of tumorigenic and associated biological phenomena with a higher degree of complexity and physiological context than that provided by established two-dimensional culture models. In this review, an overview of a wide range of natural, synthetic, and hybrid biomaterials used for 3D cancer cell culture and investigation of cancer cell behavior is presented. The role of these materials in modulating cell-matrix interactions and replicating specific tumorigenic characteristics is evaluated. In addition, recent advances in biomaterial design, synthesis, and fabrication are also assessed. Finally, the advantages of incorporating polymeric biomaterials in 3D cancer models for obtaining efficacy data in anticancer drug testing applications are highlighted.