Publications

Living cells are known to be in thermodynamically nonequilibrium, which is largely brought about by intracellular molecular motors. The motors consume chemical energies to generate stresses and reorganize the cytoskeleton for the cell to move and divide. However, since there has been a lack of direct measurements characterizing intracellular stresses, questions remained unanswered on the intricacies of how cells use such stresses to regulate their internal mechanical integrity in different microenvironments. This report describes a new experimental approach by which we reveal an environmental rigidity-dependent intracellular stiffness that increases with intracellular stress - a revelation obtained, surprisingly, from a correlation between the fluctuations in cellular stiffness and that of intracellular stresses. More surprisingly, by varying two distinct parameters, environmental rigidity and motor protein activities, we observe that the stiffness-stress relationship follows the same curve. This finding provides some insight into the intricacies by suggesting that cells can regulate their responses to their mechanical microenvironment by adjusting their intracellular stress.

The ability to manipulate and control active microparticles is essential for designing microrobots for applications. This paper describes the use of electric and magnetic fields to control the direction and speed of induced-charge electrophoresis (ICEP) driven metallic Janus microrobots. A direct current (DC) magnetic field applied in the direction perpendicular to the electric field maintains the linear movement of particles in a 2D plane. Phoretic force spectroscopy (PFS), a phase-sensitive detection method to detect the motions of phoretic particles, is used to characterize the frequency-dependent phoretic mobility and drag coefficient of the phoretic force. When the electric field is scanned over a frequency range of 1 kHz–1 MHz, the Janus particles exhibit an ICEP direction reversal at a crossover frequency at ~30 kH., Below this crossover frequency, the particle moves in a direction towards the dielectric side of the particle, and above this frequency, the particle moves towards the metallic side. The ICEP phoretic drag coefficient measured by PFS is found to be similar to that of the Stokes drag. Further investigation is required to study microscopic interpretations of the frequency at which ICEP mobility switched signs and the reason why the magnitudes of the forward and reversed modes of ICEP are so different.

A colloidal suspension of active Brownian particles (ABPs) driven by controllable forces into directed or persistent motions can serve as a model for understanding the biological systems. Experiments and numerical simulations are established to investigate the motions of an ABP, a single, induced-charge electrophoretic (ICEP) metallic Janus particle, confined in a quadratic potential well. On the one hand, 1-D position histograms of the trapped active particle, behaving differently from that of a Boltzmann distribution, reveal a splitting from a single peak of the ABP positional distribution to a bimodal distribution. Decoupling the thermal and non-thermal contributions from the overall histogram is non-trivial. However, the two contributions can be examined by convoluting numerically generated thermal and non-thermal contributions into a full histogram. On the other hand, temporal fluctuations analyzed by the power spectral density (PSD), reveal two unique frequencies characterizing the stiffness of the trap and the rotational diffusion of the particle, respectively. Connections between the spatial and temporal fluctuations are obtained by the separate analysis of the temporal and spatial fluctuations of an ABP trapped in a quadratic potential well. This study reveals how thermal and nonthermal fluctuations play against each other in a confined environment.

Active colloidal particles dissipate energies in a fashion different from Brownian particles, in that active particles undergo directed motions characterized by a persistent length. An outstanding question, debated to date, pertains to how a quantitative and well-defined means can be established to quantify the differences between the statistical behavior of an active particle and a passive Brownian particle. To address this question, we set out to investigate the motions of a single, induced-charge electrophoretic (ICEP) metallic Janus particles in a quadratic potential of an optical trap, by experiments and numerical simulations. The positions of the particle under different driving forces were measured by experiments and simulated numerically using a generalized Langevin equation. The 1-D positional histograms of the active particle, distinctively different from that of a Boltzmann distribution, reveal splitting of the positional distribution of a single peak centered at the bottom of the well into two symmetrical peaks, whose centers move away from the center to a distance increasing with the driven force. Kurtosis of the particle’s spatial distribution is used as a way to quantify the deviation from Gaussian distribution and it was found that this deviation is a function of the particle’s rotational relaxation, the stiffness of the trap and the driving force. The temporal fluctuations of the active particle in the well are analyzed by their power spectral density (PSD).

We measured the intrinsic electrophoretic drag coefficient of a single charged particle by optically trapping the particle and applying an AC electric field, and found it to be markedly different from that of the Stokes drag. The drag coefficient, along with the measured electrical force, yield a mobility-zeta potential relation that agrees with the literature. By using the measured mobility as input, numerical calculations based on the Poisson–Nernst–Planck equations, coupled to the Navier–Stokes equation, reveal an intriguing microscopic electroosmotic flow near the particle surface, with a well-defined transition between an inner flow field and an outer flow field in the vicinity of electric double layer's outer boundary. This distinctive interface delineates the surface that gives the correct drag coefficient and the effective electric charge. The consistency between experiments and theoretical predictions provides new insights into the classic electrophoresis problem, and can shed light on new applications of electrophoresis to investigate biological nanoparticles.

We demonstrate wavelength-independent optical limiting based on colloidal phase transitions induced by the dielectrophoretic force from focused electromagnetic radiation. The focused radiation acts as an optical trap that increases the particle density. The increased density then leads to a colloidal gas-solid phase transition and an aggregate that effectively blocks the incoming radiation when it passes a threshold power. The process is reversible, with the colloidal particles returning to a homogenous distribution after the incoming radiation is removed. We demonstrate the effect using polystyrene nanoparticles mixed with pluronics and polyethylene glycol polymers in low-concentration KCl salt solutions. We observe the light-induced phase separation under confocal fluorescent microscope, and we provide a proof-of-principle demonstration of optical limiting using a 100 μm thick colloid cell.

We have shown that materials other than hydrogels commonly used in tissue engineering can be effective in enabling differentiation of dental pulp stem cells (DPSC). Here we demonstrate that a hydrophobic elastomer, polyisoprene (PI), a component of Gutta-percha, normally used to obturate the tooth canal, can also be used to initiate differentiation of the pulp. We showed that PI substrates without additional coating promote cell adhesion and differentiation, while their moduli can be easily adjusted either by varying the coating thickness or incorporation of inorganic particles. DPSC plated on those PI substrates were shown, using SPM and hysitron indentation, to adjust their moduli to conform to differentially small changes in the substrate modulus. In addition, optical tweezers were used to separately measure the membrane and cytoplasm moduli of DPSC, with and without Rho kinase inhibitor. The results indicated that the changes in modulus were attributed predominantly to changes within the cytoplasm, rather than the cell membrane. CLSM was used to identify cell morphology. Differentiation, as determined by qRT-PCR, of the upregulation of OCN, and COL1α1 as well as biomineralization, characterized by SEM/EDAX, was observed on hard PI substrates in the absence of induction factors, i.e. dexamethasone, with moduli 3–4 MPa, regardless of preparation. SEM showed that even though biomineralization was deposited on both spun cast thin PI and filled thick PI substrates, the minerals were aggregated into large clusters on thin PI, and uniformly distributed on filled thick PI, where it was templated within banded collagen fibers.

A temperature sensitive sol-gel transition induced by the self-assembly of amphiphilic copolymers and its application in industry have been the objects of increasing study. We demonstrate here a two-step, reversible addition-fragmentation chain transfer (RAFT) polymerization of an ABA-type copolymer consisting of poly(N,N-dimethylacrylamide)-b-poly(diacetone acrylamide)-b-poly(N,N-dimethylacrylamide) (PDMAA-b-PDAAM-b-PDMAA). This copolymer can be easily dispersed in water, and this dispersion is critical for its lower critical solution temperature (LCST)-type sol-gel transition, which was monitored using dynamic light scattering (DLS), transmission electron microscopy (TEM), and rheology analysis, in addition to temperature-dependent 1H nuclear magnetic resonance (1H NMR) and Fourier transform infrared spectroscopy (FTIR). Results revealed an abnormal sphere-to-worm micellar transition of this ABA copolymer at the LCST point, which could be affected by the length of the PDAAM block (B-block), the length as well as the distribution of the PDMAA block (A-block), and the concentration of the copolymer dispersion. Thus, copolymer dispersion could be feasibly used for drug loading at a low temperature, which could then be transformed into a gel at an elevated temperature. The loading and controllable release of the model drug of paracetamol into and out of a copolymer gel was further determined. The sustained release behavior was also studied using the Rigter-Peppas model.

Latex, an aqueous dispersion of sub-micron polymer particles, is widely used as polymer binder in waterborne coatings and adhesives. Drying of a latex is inhomogeneous, during which the spatial distribution of particles is non-uniform and changes with time, usually resulting in a compromise of the integrity of a dried film. To study drying inhomogeneity of latex, we developed a system integrating optical coherence tomography (OCT) with gravimetric and video analysis (OCT-Gravimetry-Video method) to non-destructively monitor the drying process of non-film-forming latexes consisting of hard polystyrene spheres over time. OCT structural and speckle images of the latex’s internal structure show the packing process of particles, the detachment of latex and the formation of apparent shear bands in cross-sectional views. Video recordings show the formation of cracks and the propagation of the drying boundary in the horizontal direction. The drying curve, measured by gravimetry, shows the drying rate and the water content of the latex at each drying stage. Furthermore, we find that the particle size affects packing and cracking phenomena remarkably. The OCT-Gravimetry-Video method serves as a general and robust approach to investigate the drying process of waterborne latex system. This method can be employed for fundamental studies of colloids and for evaluations of industrial latex products.

Dielectrophoresis (DEP) has been widely used to manipulate nanoparticles in microfluidic applications. However, determination of the DEP force of nanoparticles by theoretical models is not easy due to complications caused by the polarization of electrical double layer. Additionally, there is a lack of suitable experimental techniques to quantify the DEP force of nanoparticles. This article reports a statistical mechanics-based experimental method to determine the DEP potential energy of a single particle by measuring the equilibrium number density of particles in a DEP force field. Results show that at high frequencies, the measured potentials agree with the Maxwell-Wagner-O'Konski (MWO) theory. At frequencies lower than the crossover frequency (ωco), the measured potential values are larger than MWO theory's predictions. When an effective particle radius (particle radius plus Debye length) is used to replace the particle radius, MWO theory fits better with the measured potentials on both sides of ωco. Also, the measured ωco was found inversely proportional to the effective particle radius, which agrees with MWO theory. The new DEP potential spectroscopy is not limited to the size or shape of particles, opening doors to investigate the DEP response functions of quantum dots and proteins in an alternating current electric field.

Optical tweezers use a highly focused laser beam to form a stable trap to confine one or more micron- or nano-sized particles in three-dimensional space, enabling noninvasive manipulation, without any mechanical contact, of microscopic probe particles embedded in a sample. Since its first demonstration in 1986 by Ashkin et al., single-beam optical tweezers have been used to manipulate microscopic objects such as colloidal particles, biomolecules, and biological cells. In addition, optical tweezers have also been used as pico-Newton force transducers to measure the strength of molecular bonds and to determine the transmission of forces in the microscopic environment of complex fluids. Combining the ability to manipulate microparticles with force measurement, optical tweezers have been used to study the micromechanical properties of soft materials, such as colloidal crystals, liquid crystals, carbon nanotube suspensions, actin-coated lipid vesicles, living cells, cytoskeletal networks, DNA networks, polymer solutions, collagen gels, human erythrocyte membranes, and even individual strands of DNA molecules.

An optical binding force between two nearby colloidal particles trapped by two coherent laser beams is measured by phase-sensitive detection. The binding force is long-range and spatially oscillatory. For identical linearly-polarized incident beams, the oscillation period is equal to the optical wavelength. For mutually perpendicular polarizations, a new force appears with half-wavelength periodicity, caused by double inter-particle scattering. This force is observable only with cross-polarized incident beams, for which the stronger single-scattering forces are forbidden by parity.

The optical-bottle technique is used to measure osmotic bulk moduli of colloid suspensions. The bulk modulus is determined by optically trapping an ensemble of nanoparticles and invoking a steady-state force balance between confining optical-gradient forces and repulsive osmotic-pressure forces. Osmotic bulk moduli are reported for aqueous suspensions of charged polystyrene particles in NaCl solutions as a function of particle concentration and ionic strength, and are compared to those determined by turbidity measurements under the same conditions. Effective particle charges are calculated from the bulk moduli and are found to increase as a function of ionic strength, consistent with previously reported results.

Background

Titanium dioxide (TiO2) is one of the most common nanoparticles found in industry ranging from food additives to energy generation. Approximately four million tons of TiO2 particles are produced worldwide each year with approximately 3000 tons being produced in nanoparticulate form, hence exposure to these particles is almost certain.

Results

Even though TiO2 is also used as an anti-bacterial agent in combination with UV, we have found that, in the absence of UV, exposure of HeLa cells to TiO2 nanoparticles significantly increased their risk of bacterial invasion. HeLa cells cultured with 0.1 mg/ml rutile and anatase TiO2 nanoparticles for 24 h prior to exposure to bacteria had 350 and 250 % respectively more bacteria per cell. The increase was attributed to bacterial polysaccharides absorption on TiO2 NPs, increased extracellular LDH, and changes in the mechanical response of the cell membrane. On the other hand, macrophages exposed to TiO2 particles ingested 40 % fewer bacteria, further increasing the risk of infection.

Conclusions

In combination, these two factors raise serious concerns regarding the impact of exposure to TiO2 nanoparticles on the ability of organisms to resist bacterial infection.

We use optical tweezers-based dielectrophoresis (DEP) force spectroscopy to investigate the roles of the electrical double layer in the AC dielectric response of an individual colloidal particle in an aqueous medium. Specifically, we measure the DEP crossover frequency as a function of particles size, medium viscosity, and temperature. Experimental results were compared to low frequency relaxation mechanisms predicted by Schwarz, demonstrating the dielectrophoretic responses in the frequency range between 10 kHz and 1 MHz were dominated by counterion diffusion within the electric double layer.

Endothelial cells form the inner lining of blood vessels and are exposed to various factors like hemodynamic conditions (shear stress, laminar, and turbulent flow), biochemical signals (cytokines), and communication with other cell types (smooth muscle cells, monocytes, platelets, etc.). Blood vessel functions are regulated by interactions among these factors. The occurrence of a pathological condition would lead to localized upregulation of cell adhesion molecules on the endothelial lining of the blood vessel. This process is promoted by circulating cytokines such as tumor necrosis factor-alpha, which leads to expression of intercellular adhesion molecule-1 (ICAM-1) on the endothelial cell surface among other molecules. ICAM-1 is critical in regulating endothelial cell layer dynamic integrity and cytoskeletal remodeling and also mediates direct cell-cell interactions as part of inflammatory responses and wound healing. In this study, we developed a biomimetic blood vessel model by culturing confluent, flow aligned, endothelial cells in a microfluidic platform, and performed real time in situ characterization of flow mediated localized pro-inflammatory endothelial activation. The model mimics the physiological phenomenon of cytokine activation of endothelium from the tissue side and studies the heterogeneity in localized surface ICAM-1 expression and F-actin arrangement. Fluorescent antibody coated particles were used as imaging probes for identifying endothelial cell surface ICAM-1 expression. The binding properties of particles were evaluated under flow for two different particle sizes and antibody coating densities. This allowed the investigation of spatial resolution and accessibility of ICAM-1 molecules expressed on the endothelial cells, along with their sensitivity in receptor-ligand recognition and binding. This work has developed an in vitro blood vessel model that can integrate various heterogeneous factors to effectively mimic a complex endothelial microenvironment and can be potentially applied for relevant blood vessel mechanobiology studies.