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Hydrostatic pressure (HP) and osmotic stress (OS) play an important role in various biological processes, such as cell proliferation and differentiation. In contrast to canonical mechanical signals transmitted through the anchoring points of the cells with the extracellular matrix, the physical and molecular mechanisms that transduce HP and OS into cellular functions remain elusive. Three-dimensional cell cultures show great promise to replicate physiologically relevant signals in well-defined host bioreactors with the goal of shedding light on hidden aspects of the mechanobiology of HP and OS. This review starts by introducing prevalent mechanisms for the generation of HP and OS signals in biological tissues that are subject to pathophysiological mechanical loading. We then revisit various mechanisms in the mechanotransduction of HP and OS, and describe the current state of the art in bioreactors and biomaterials for the control of the corresponding physical signals., Biomaterials Advances, 145, ISSN:2772-9508

The primary aim of this article is to review the clinical challenges related to the supply of power in implanted left ventricular assist devices (LVADs) by means of transcutaneous drivelines. In effect of that, we present the preventive measures and post-operative protocols that are regularly employed to address the leading problem of driveline infections. Due to the lack of reliable wireless solutions for power transfer in LVADs, the development of new driveline configurations remains at the forefront of different strategies that aim to power LVADs in a less destructive manner. To this end, skin damage and breach formation around transcutaneous LVAD drivelines represent key challenges before improving the current standard of care. For this reason, we assess recent strategies on the surface functionalization of LVAD drivelines, which aim to limit the incidence of driveline infection by directing the responses of the skin tissue. Moreover, we propose a class of power transfer systems that could leverage the ability of skin tissue to effectively heal short diameter wounds. In this direction, we employed a novel method to generate thin conductive wires of controllable surface topography with the potential to minimize skin disruption and eliminate the problem of driveline infections. Our initial results suggest the viability of the small diameter wires for the investigation of new power transfer systems for LVADs. Overall, this review uniquely compiles a diverse number of topics with the aim to instigate new research ventures on the design of power transfer systems for IMDs, and specifically LVADs.

Metastasis, the leading cause of mortality in cancer patients, depends upon the ability of cancer cells to invade into the extracellular matrix that surrounds the primary tumor and to escape into the vasculature. To investigate the features of the microenvironment that regulate invasion and escape, we generated solid microtumors of MDA-MB-231 human breast carcinoma cells within gels of type I collagen. The microtumors were formed at defined distances adjacent to an empty cavity, which served as an artificial vessel into which the constituent tumor cells could escape. To define the relative contributions of matrix degradation and cell proliferation on invasion and escape, we used pharmacological approaches to block the activity of matrix metalloproteinases (MMPs) or to arrest the cell cycle. We found that blocking MMP activity prevents both invasion and escape of the breast cancer cells. Surprisingly, blocking proliferation increases the rate of invasion but has no effect on that of escape. We found that arresting the cell cycle increases the expression of MMPs, consistent with the increased rate of invasion. To gain additional insight into the role of cell proliferation in the invasion process, we generated microtumors from cells that express the fluorescent ubiquitination-based cell cycle indicator. We found that the cells that initiate invasions are preferentially quiescent, whereas cell proliferation is associated with the extension of invasions. These data suggest that matrix degradation and cell proliferation are coupled during the invasion and escape of human breast cancer cells and highlight the critical role of matrix proteolysis in governing tumor phenotype.

The architectural features of branching morphogenesis demonstrate exquisite reproducibility among various organs and species despite the unique functionality and biochemical differences of their microenvironment. The regulatory networks that drive branching morphogenesis employ cell-generated and passive mechanical forces, which integrate extracellular signals from the microenvironment into morphogenetic movements. Cell-generated forces function locally to remodel the extracellular matrix (ECM) and control interactions among neighboring cells. Passive mechanical forces are the product of in situ mechanical instabilities that trigger out-of-plane buckling and clefting deformations of adjacent tissues. Many of the molecular and physical signals that underlie buckling and clefting morphogenesis remain unclear and require new experimental strategies to be uncovered. Here, we highlight soft material systems that have been engineered to display programmable buckles and creases. Using synthetic materials to model physicochemical and spatiotemporal features of buckling and clefting morphogenesis might facilitate our understanding of the physical mechanisms that drive branching morphogenesis across different organs and species.

Recent approaches have utilized microfabricated platforms to examine combinations of microenvironmental signals that regulate stem and progenitor cell differentiation. However, the majority of these efforts have focused on the biochemical properties of extracellular matrix (ECM) or soluble factors without simultaneously exploring the biomechanical effects of cell-substrate interactions. To address this need, we combined a high-throughput approach for the analysis of combinatorial ECM cues with substrates of modular stiffness and traction force microscopy. This integrated approach enabled the characterization of cell-generated traction stress and phenotypic expression in response to ECM cues. We investigated the impact of substrate stiffness and ECM composition on the differentiation of bipotential mouse embryonic liver (BMEL) progenitor cells. We observed that hepatocyte differentiation was primarily regulated by ECM composition, and cholangiocyte differentiation was cooperatively influenced by ECM proteins and stiffness properties. In particular, stiffness-mediated cholangiocyte differentiation was observed for cells cultured on fibronectin, while collagen IV promoted differentiation independent of substrate stiffness. We demonstrated the influence of cell contractility and traction stress in early cholangiocyte specification and further uncovered the roles of ERK and ROCK in this differentiation process. Overall, these findings illustrate the involvement of biomechanical signals in liver progenitor differentiation. Further, this approach could enable investigations for a broad range of cell types and ECM proteins, providing an integrated platform for evaluating the combinatorial effects of biochemical and biophysical signals in cell differentiation.

The mechanical properties of the tumor microenvironment (TME) play a critical role on the progression of breast cancer metastasis. However, the complex architecture of the TME conceals the individual effects of different biophysical and biochemical factors on tumor invasion and intravasation. To investigate this question, we engineered a robust breast tumor model of solid-like 3D aggregate of human breast cancer cells with interstitial fluid pressure (IFP), and further integrated it with an empty cavity to emulate the presence of an impaired capillary vessel. In brief, we embed MDA-MB-231 human breast cancer cells in one of two neighboring collagen type I cavities that are molded within polydimethylsiloxane (PDMS) channels. This multicellular aggregate is subject to selected gradients of hydrostatic pressure through opposing reservoirs of culture media that are located at the base (Pbase) and the tip (Ptip) of the tumor. We found that breast cancer cells disseminate from the multicellular aggregate and escape into the proximal cavity under Ptip > Pbase. The separation distance between the aggregate and the cavity influences the features of tumor escape. Tumor models that were seeded within a distance of less than 150 μm from the cavity demonstrated significantly shorter time (t1/2~ 4 days) for the escape of 50% of the tumor population than those seeded between 150 and 300 μm. In contrast, less than 50% of the tumors that were seeded longer than 300 μm apart of the cavity successfully escaped after ~ 2 weeks under Ptip > Pbase. In addition, we found that cells escaped into the cavity through three major modes: a) single-cell migration, b) multicellular invasion, and c) tumor growth. Single-cell migration was the dominant route of escape in collagen gels of low concentration (2.5 mg/ml). In contrast, tumor growth and multicellular invasion were the dominant modes of escape in collagen gels of high concentration (4mg/ml). Moreover, the tumor invasions were found to be preferentially directed normal to the surface of the tumor, and to be drastically eliminated in effect of pharmacological inhibition of matrix metalloproteinases (MMPs). These preliminary findings will be put together with additional quantitative studies to correlate tumor-cavity separation with the different modes of tumor escape. Overall, our engineered breast tumor model composes a unique platform to investigate the biophysical and biochemical mechanisms of the tumor microenvironment that drive tumor invasion and intravasation into the circulatory system. Citation Format: Andreas P. Kourouklis, Usman Ghani, Siyang Han, Yoseph Dance, Allison K. Simi, Joe Tien, Celeste M. Nelson. Tumor invasion and escape from an engineered solid-like aggregate of human breast cancer cells into a cavity [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 4526.

Microfabricated cellular microarrays, which consist of contact-printed combinations of biomolecules on an elastic hydrogel surface, provide a tightly controlled, high-throughput engineered system for measuring the impact of arrayed biochemical signals on cell differentiation. Recent efforts using cell microarrays have demonstrated their utility for combinatorial studies in which many microenvironmental factors are presented in parallel. However, these efforts have focused primarily on investigating the effects of biochemical cues on cell responses. Here, we present a cell microarray platform with tunable material properties for evaluating both cell differentiation by immunofluorescence and biomechanical cell–substrate interactions by traction force microscopy. To do so, we have developed two different formats utilizing polyacrylamide hydrogels of varying Young's modulus fabricated on either microscope slides or glass-bottom Petri dishes. We provide best practices and troubleshooting for the fabrication of microarrays on these hydrogel substrates, the subsequent cell culture on microarrays, and the acquisition of data. This platform is well-suited for use in investigations of biological processes for which both biochemical (e.g., extracellular matrix composition) and biophysical (e.g., substrate stiffness) cues may play significant, intersecting roles.

Collapsed blood or lymphatic vessels in the tumor microenvironment often cause fluid buildup, leading to heterogeneous flow throughout the tissue. Here, we used a three-dimensional (3D) engineered tumor model to investigate how fluid flow specifically influences invasion and chemoresistance of breast cancer cells. To mimic breast tumors, we cultured aggregates of MDA-MB-231 human breast cancer cells embedded in 3D collagen channels. Collagen channels were flanked on the ends by two media reservoirs. By changing the relative heights of media in the reservoirs, we controlled the pressure-induced flow experienced by the tumor cell aggregate. We found that the direction of flow through the collagen channel determined the invasive phenotype of the engineered tumor. These analyses will be repeated with the addition of chemotherapy drugs taxol or 5-fluorouracil in the media to determine the effect of fluid flow on chemotherapeutic response. Our engineered tumor model provides insight into how physical forces influence the invasive phenotype of cancer cells. Citation Format: Allison K. Simi, Andreas P. Kourouklis, Alexandra S. Piotrowski-Daspit, Joe Tien, Celeste M. Nelson. The role of pressure-driven flow in invasion and chemoresistance of cancer cells in an engineered breast tumor model [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 40.

In working towards the goal of mimicking multiple features of the extracellular matrix (ECM), we developed a strategy to create adhesive protein patterns on polymer films with tunable viscous characteristics. The block copolymer films are generated by interfacial self-assembly with the presentation of dopant homopolymer, since the concentration of the latter is known to affect the lateral mobility of the film. The supported polymer films are subsequently surfacemodified by microcontact printing using fibronectin (FN), yielding material surfaces which can potentially display independent control over mechanical and adhesive properties. This work demonstrates a new method for the design of materials with the potential to recapitulate the viscous component of the ECM in future in vitro studies.