Ethan Tumarkin

Probing Dynamic Generation of Hot-Spots in Self-Assembled Chains of Gold Nanorods by Surface-Enhanced Raman Scattering

Further progress in the applications of self-assembled nanostructures critically depends on developing a fundamental understanding of the relation between the properties of nanoparticle ensembles and their time-dependent structural characteristics. Following dynamic generation of hot-spots in the self-assembled chains of gold nanorods, we established a direct correlation between ensemble-averaged surface-enhanced Raman scattering and extinction properties of the chains. Experimental results were supported with comprehensive finite-difference time-domain simulations. The established relationship between the structure of nanorod ensembles and their optical properties provides the basis for creating dynamic, solution-based, plasmonic platforms that can be utilized in applications ranging from sensing to nanoelectronics.

High-throughput combinatorial cell co-culture using microfluidics

Co-culture strategies are foundational in cell biology. These systems, which serve as mimics of in vivo tissue niches, are typically poorly defined in terms of cell ratios, local cues and supportive cell-cell interactions. In the stem cell niche, the ability to screen cell-cell interactions and identify local supportive microenvironments has a broad range of applications in transplantation, tissue engineering and wound healing. We present a microfluidic platform for the high-throughput generation of hydrogel microbeads for cell co-culture. Encapsulation of different cell populations in microgels was achieved by introducing in a microfluidic device two streams of distinct cell suspensions, emulsifying the mixed suspension, and gelling the precursor droplets. The cellular composition in the microgels was controlled by varying the volumetric flow rates of the corresponding streams. We demonstrate one of the applications of the microfluidic method by co-encapsulating factor-dependent and responsive blood progenitor cell lines (MBA2 and M07e cells, respectively) at varying ratios, and show that in-bead paracrine secretion can modulate the viability of the factor dependent cells. Furthermore, we show the application of the method as a tool to screen the impact of specific growth factors on a primary human heterogeneous cell population. Co-encapsulation of IL-3 secreting MBA2 cells with umbilical cord blood cells revealed differential sub-population responsiveness to paracrine signals (CD14+ cells were particularly responsive to locally delivered IL-3). This microfluidic co-culture platform should enable high throughput screening of cell co-culture conditions, leading to new strategies to manipulate cell fate.

Microfluidic Encapsulation of Cells in Polymer Microgels

In this Concept article, recent advances in microfluidic platforms for the generation of cell-laden hydrogel particles (microgels) are reported. Advances in the continuous microfluidic encapsulation of cells in droplets and microgels are critically reviewed, and currently used methods for the encapsulation of cells in polymer microgels are discussed. An outlook on current applications and future directions in this field of research are also presented. This article will be of interest to chemists, materials scientists, cell biologists, bioengineers, and pharmacologists.

High-throughput generation of hydrogel microbeads with varying elasticity for cell encapsulation

Elasticity of cellular microenvironments strongly influences cell motility, phagocytosis, growth and differentiation. Currently, the relationship between the cell behaviour and matrix stiffness is being studied for cells seeded on planar substrates, however in three-dimensional (3D) microenvironments cells may experience mechanical signalling that is distinct from that on a two-dimensional matrix. We report a microfluidic approach for high-throughput generation of 3D microenvironments with different elasticity for studies of cell fate. The generation of agarose microgels with different elastic moduli was achieved by (i) introducing into a microfluidic droplet generator two streams of agarose solutions, one with a high concentration of agarose and the other one with a low concentration of agarose, at varying relative volumetric flow rate ratios of the two streams, and (ii) on-chip gelation of the precursor droplets. At 37 degreesC, the method enabled a approximately 35-fold variation of the shear elastic modulus of the agarose gels. The application of the method was demonstrated by encapsulating two mouse embryonic stem cell lines within the agarose microgels. This work establishes a foundation for the high-throughput generation of combinatorial microenvironments with different mechanical properties for cell studies.

Small, Stable, and Monodispersed Bubbles Encapsulated with Biopolymers

A microfluidic route to producing small (<10 µm) bubbles with a narrow size distribution and long-time (at least, up to one month) stability is reported. The bubbles are encapsulated with a protein-polysaccharide shell. The strategy includes the following events, occurring in sequence: (i) a microfluidic generation of bubbles from a mixture of CO(2) and a minute amount of gases with low solubility in water, in an aqueous solution of lysozyme and sodium alginate; (ii) the dissolution of CO(2) leading to the shrinkage of bubbles and a local increase in acidity of the medium; (iii) the deposition of lysozyme at the gas-water interface triggered by the local decrease in pH; (iv) the deposition of alginate onto the lysozyme shell, due to the electrostatically driven complexation of alginate with lysozyme.

Microfluidic Generation of Composite Biopolymer Microgels with Tunable Compositions and Mechanical Properties

To develop an understanding of the nature of complex, spatiotemporal interactions between cells and the extracellular matrix (ECM), artificial ECMs formed from hydrogels with a particular spectrum of properties are being developed at a rapid pace. We report the microfluidic generation of small, monodisperse composite agarose-gelatin hydrogel modules (microgel particles) that can be used for cell encapsulation and can serve as instructive cellular microenvironments. The agarose component of the microgels gelled under reduced temperature, while gelatin modified with phenolic hydroxyl groups underwent peroxidase-catalyzed gelation. Microgel composition, structure, morphology, and rigidity were tuned in a high-throughput manner. The results of this work are important for the generation of libraries of cell-laden polymer microgels for single-cell analysis, tissue engineering, and fundamental studies of the role of local microenvironments in cell fate.

Characterization of the mechanical properties of microgels acting as cellular microenvironments

There is an increasing effort in the search and characterization of materials that can function as artificial three-dimensional cellular microenvironments. In particular, the encapsulation of cells in micrometer-size hydrogel particles (microgels) offers the ease of cell visualization, the transport of nutrients, and the ability to generate large combinatorial libraries of cellular microenvironments. In the present work, we report an atomic force microscopy (AFM) method for the characterization of an average Youngs modulus and the stress relaxation time of micrometer-size polymer microgel particles. We studied the mechanical properties of agarose microgels with different polymer concentration at room temperature and 37 °C (the temperature of cell culture). We examined the temporal variation in the Young modulus of cell-free and cell-laden microgels under physiological conditions. Furthermore, we correlated the mechanical properties of the microgels laden with a single carcinoma associated fibroblast (CAF) cell with the spatial distribution of collagen around the cell. This work is potentially useful for studies of changes in cellular microenvironments, when cells locally degrade the matrix or secrete molecules that change the mechanical properties of the matrix.

Temperature mediated generation of armoured bubbles

This communication describes a novel strategy for the continuous microfluidic generation of highly monodispersed particle-coated microbubbles using temperature-dependent dissolution of carbon dioxide.

Nanofibrillar thermoreversible micellar microgels

By using microfluidics, we generated micellar nanofibrillar microgels from solutions of worm-like micelles of poly(N-isopropyl acrylamide)-block-polystyrene. The microgels were formed under physiological conditions (pH = 7.4, 37 °C) and rapidly dissociated upon cooling to 25–27 °C, thereby enabling cell encapsulation and release for further characterization. Our work offers a new concept for the generation of dynamic artificial three-dimensional microenvironments for studies of cell fate.

Medical management of Salmonella enteritidis prosthetic valve endocarditis with multiple infectious foci

(0.9 × 0.85 × 1.1 cm) valve, thickening of the proximal aorta suggestive of a peri-annular aortic root abscess, and a large echogenic mass on the right ventricular pacemaker lead [2]. Abdominal imaging demonstrated evidence of splenic infarcts. Due to an unacceptably high surgical risk, a trial of medical management was sought with an aim to reduce disease burden followed by chronic suppressive therapy. He completed a 6 week course of ceftriaxone followed by oral trimethoprim/sulfamethoxazole (TMP/SMX) 800 mg/160 mg twice daily as chronic suppressive therapy. His symptoms resolved and he remains clinically well after 12 months of follow-up while adherent to TMP/SMX. This represents the first reported case of S. enteriditis endocarditis involving two mechanical valves, an aortic root graft, and a pacemaker lead. Indications for surgery include an infected cardiac implantable electronic device [3], large valvular vegetations (> 10 mm), and a peri-annular abscess [4]. A review by Gönen et al. found that 10 of 13 reported cases of S. enteritidis prosthetic valve endocarditis required surgical intervention. Of the four cases involving two prosthetic valves, three underwent valve replacement, and one died pre-operatively [5]. Our case demonstrates that in selected individuals, treatment with intravenous antibiotic followed by oral chronic suppressive therapy is an option. TMP/SMX was chosen based on its excellent oral bioavailability, relatively narrow spectrum of activity, and low cost. Finally, this case highlights a potential occupational risk for developing Salmonella bacteremia. Patients with a history of immunosuppression or prosthetic implant should be counseled on the risk of salmonellosis through certain occupational exposures. To the Editor,