Siyoung Q. Choi

Interfacial microrheology of DPPC monolayers at the air–water interface

We present systematic measurements of the surface rheology of monolayers of liquid-condensed (LC) dipalmitoylphosphatidylcholine (DPPC) at the air–water interface. Using microfabricated, ferromagnetic ‘microbuttons’ as new microrheological probes, we measure the linear viscoelastic moduli of LC DPPC monolayers as both surface pressure and frequency are varied. Visualization of this interface reveals that the interlocked liquid crystalline domains that comprise an LC-DPPC monolayer give rise to a viscoelastic solid response. Two distinct behaviors arise as surface pressure is increased: for low surface pressures (8 mN m−1 ≤ Π ≤ 12–14 mN m−1), the monolayer behaves like a two-dimensional emulsion, with a surface elastic modulus G ′s that is relatively constant, as would be expected from a line tension-mediated elasticity. The surface viscosity increases exponentially with Π, as would be expected for a condensed liquid monolayer. Above 12–14 mN m−1, however, both moduli increase exponentially with Π, albeit with a weaker slope—a response that would not be expected from line-tension-mediated elasticity. This transition would be consistent with a second-order phase transition between the LC and solid-condensed phase, as has been observed in other phospholipid monolayers. Finally, we employ a controlled-stress (creep) mode to find a stress-dependent viscosity bifurcation, and thus the yield stress of this monolayer.

Effect of cholesterol nanodomains on monolayer morphology and dynamics

Significance Replacement lung surfactants have dramatically reduced premature infant mortality owing to respiratory distress syndrome. However, clinical lung surfactant varies widely in composition, and even the existence of cholesterol in native lung surfactants remains controversial. Improving replacement surfactants will require an understanding of each molecular component’s role in a film’s static and dynamic properties. Here, we show that small cholesterol fractions reduce the viscosity of model lung surfactant interfaces by orders of magnitude while leaving compressibility and collapse unchanged, offering control over surfactant spreadability. Lipid–cholesterol nanodomain complexes are observed, which act as line-active sources of free area to reduce surface viscosity. At low mole fractions, cholesterol segregates into 10- to 100-nm-diameter nanodomains dispersed throughout primarily dipalmitoylphosphatidylcholine (DPPC) domains in mixed DPPC:cholesterol monolayers. The nanodomains consist of 6:1 DPPC:cholesterol “complexes” that decorate and lengthen DPPC domain boundaries, consistent with a reduced line tension, λ. The surface viscosity of the monolayer, ηs, decreases exponentially with the area fraction of the nanodomains at fixed surface pressure over the 0.1- to 10-Hz range of frequencies common to respiration. At fixed cholesterol fraction, the surface viscosity increases exponentially with surface pressure in similar ways for all cholesterol fractions. This increase can be explained with a free-area model that relates ηs to the pure DPPC monolayer compressibility and collapse pressure. The elastic modulus, G′, initially decreases with cholesterol fraction, consistent with the decrease in λ expected from the line-active nanodomains, in analogy to 3D emulsions. However, increasing cholesterol further causes a sharp increase in G′ between 4 and 5 mol% cholesterol owing to an evolution in the domain morphology, so that the monolayer is elastic rather than viscous over 0.1–10 Hz. Understanding the effects of small mole fractions of cholesterol should help resolve the controversial role cholesterol plays in human lung surfactants and may give clues as to how cholesterol influences raft formation in cell membranes.

Synthesis of Multifunctional Micrometer‐Sized Particles with Magnetic, Amphiphilic, and Anisotropic Properties

Well-defi ned, functional colloids provide critical model systems for a variety of fundamental phenomena in materials and soft matter, and enable a broad range of technological applications from optics [ 1 ] to biotechnology. [ 2 ] The ability to fabricate large quantities of colloids with high uniformity and specifi ed shape and function has thus been greatly desired. For example, magnetic colloids can be externally manipulated and controlled, and have been used in applications ranging from photonic crystals, [ 3 ] cell sorting, [ 2 ] biosensors, [ 4 ] drug delivery, [ 5 ] biomedical applications, [ 6 ] and single-molecule biophysics. [ 7 ] Magnetic colloids that can be remotely controlled by applied fi elds have also been exploited in m-ink, [ 8 ] microrheological probes, [ 9–11 ] and a variety of self-organizing systems. [ 12 , 13 ] In a similar fashion, ferrofl uids and magneto-rheological fl uids are frequently used to make useful materials because of their tuneable dynamic response. [ 14 , 15 ]

Sustained plasmid DNA release from dissolving mineral coatings

Calcium phosphate (CaP) minerals such as hydroxyapatite are able to bind a diverse range of biological molecules due to the presence of anions and cations in their crystal structure. The well-characterized ability of CaP minerals to bind and release plasmid DNA, coupled with the ability of biodegradable CaP coatings to form on the surface of common biomaterials, provides a potential mechanism for controlled release of plasmid DNA from various biomaterials. In this study we hypothesized that the release of plasmid DNA from CaP coatings formed on poly(lactide-co-glycolide) (PLG) substrates would be dependent on both the intrinsic properties of the CaP mineral coating and the surrounding solution conditions. Experiments were designed to consider two general parameters: (i) the stability of various CaP mineral coatings in solution environments that are relevant to physiological conditions; (ii) the relationship between mineral stability and sustained plasmid DNA release. Our results corroborate previous studies that have demonstrated a direct relationship between intrinsic mineral composition and mineral stability. In addition, we further demonstrate that ion composition and pH of the surrounding solution environment can significantly influence mineral stability. In turn, mineral stability significantly influenced release of plasmid DNA from mineral coatings in vitro, and the DNA release efficiency could be tuned by controlling the mineral properties in various solution environments. These CaP mineral coatings may be a useful platform for plasmid DNA delivery applications using various biomaterial platforms.

Inorganic coatings for optimized non-viral transfection of stem cells

“Biomimetic” approaches for heterogeneous growth of inorganic coatings have become particularly widespread in biomedical applications, where calcium phosphate (CaP) mineral coatings are used to improve biomedical implants. Changes in coating properties can influence the effects of mineral coatings on adjacent cells, but to date it has not been practical to systematically vary inorganic coating properties to optimize specific cell behaviors. Here, we present an approach to grow CaP mineral coatings in an enhanced throughput format to identify unprecedented capabilities in non-viral gene delivery. Subtle changes in coating properties resulted in widely variable transfection, and optimized coatings led to greater than 10-fold increases in transgene expression by multiple target cell types when compared to standard techniques. The enhanced transfection observed here is substrate-mediated, and related to the characteristics of the local environment near the surface of dissolving mineral coatings. These findings may be particularly translatable to medical device applications.

Multifunctional Mixed SAMs That Promote Both Cell Adhesion and Noncovalent DNA Immobilization

The ability of DNA strands to influence cellular gene expression directly and to bind with high affinity and specificity to other biological molecules (e.g., proteins and target DNA strands) makes them a potentially attractive component of cell culture substrates. On the basis of the potential importance of immobilized DNA in cell culture and the well-defined characteristics of alkanethiol self-assembled monolayers (SAMs), the current study was designed to create multifunctional SAMs upon which cell adhesion and DNA immobilization can be independently modulated. The approach immobilizes the fibronectin-derived cell adhesion ligand Arg-Gly-Asp-Ser-Pro (RGDSP) using carbodiimide activation chemistry and immobilizes DNA strands on the same surface via cDNA-DNA interactions. The surface density of hexanethiol-terminated DNA strands on alkanethiol monolayers (30.2-69.2 pmol/cm2) was controlled using a backfill method, and specific target DNA binding on cDNA-containing SAMs was regulated by varying the soluble target DNA concentration and buffer characteristics. The fibronectin-derived cell adhesion ligand GGRGDSP was covalently linked to carboxylate groups on DNA-containing SAM substrates, and peptide density was proportional to the amount of carboxylate present during SAM preparation. C166-GFP endothelial cells attached and spread on mixed SAM substrates and cell adhesion and spreading were specifically mediated by the immobilized GGRGDSP peptide. The ability to control the characteristics of noncovalent DNA immobilization and cell adhesion on a cell culture substrate suggests that these mixed SAMs could be a useful platform for studying the interaction between cells and DNA.

Environmental parameters influence non-viral transfection of human mesenchymal stem cells for tissue engineering applications

Non-viral transfection is a promising technique that could be used to increase the therapeutic potential of stem cells. The purpose of this study was to explore practical culture parameters of relevance in potential human mesenchymal stem cell (hMSC) clinical and tissue engineering applications, including type of polycationic transfection reagent, N/P ratio and dose of polycation/pDNA polyplexes, cell passage number, cell density and cell proliferation. The non-viral transfection efficiency was significantly influenced by N/P ratio, polyplex dose, cell density and cell passage number. hMSC culture conditions that inhibited cell division also decreased transfection efficiency, suggesting that strategies to promote hMSC proliferation may be useful to enhance transfection efficiency in future tissue engineering studies. Non-viral transfection treatments influenced hMSC phenotype, including the expression level of the hMSC marker CD105 and the ability of hMSCs to differentiate down the osteogenic and adipogenic lineages. The parameters found here to promote hMSC transfection efficiency, minimize toxicity and influence hMSC phenotype may be instructive in future non-viral transfection studies and tissue engineering applications.

Processable high internal phase Pickering emulsions using depletion attraction

High internal phase emulsions have been widely used as templates for various porous materials, but special strategies are required to form, in particular, particle-covered ones that have been more difficult to obtain. Here, we report a versatile strategy to produce a stable high internal phase Pickering emulsion by exploiting a depletion interaction between an emulsion droplet and a particle using water-soluble polymers as a depletant. This attractive interaction facilitating the adsorption of particles onto the droplet interface and simultaneously suppressing desorption once adsorbed. This technique can be universally applied to nearly any kind of particle to stabilize an interface with the help of various non- or weakly adsorbing polymers as a depletant, which can be solidified to provide porous materials for many applications.

Influence of molecular coherence on surface viscosity.

Adding small fractions of cholesterol decreases the interfacial viscosity of dipalmitoylphosphatidylcholine (DPPC) monolayers by an order of magnitude per wt %. Grazing incidence X-ray diffraction shows that cholesterol at these small fractions does not mix ideally with DPPC but rather induces nanophase separated structures of an ordered, primarily DPPC phase bordered by a line-active, disordered, mixed DPPC-cholesterol phase. We propose that the free area in the classic Cohen and Turnbull model of viscosity is inversely proportional to the number of molecules in the coherence area, or product of the two coherence lengths. Cholesterol significantly reduces the coherence area of the crystals as well as the interfacial viscosity. Using this free area collapses the surface viscosity data for all surface pressures and cholesterol fractions to a universal logarithmic relation. The extent of molecular coherence appears to be a fundamental factor in determining surface viscosity in ordered monolayers.

Optical vortex arrays from smectic liquid crystals.

We demonstrate large-area, closely-packed optical vortex arrays using self-assembled defects in smectic liquid crystals. Self-assembled smectic liquid crystals in a three-dimensional torus structure are called focal conic domains. Each FCD, having a micro-scale feature size, produces an optical vortex with consistent topological charge of 2. The spiral profile in the interferometry confirms the formation of an optical vortex, which is predicted by Jones matrix calculations.