Ting-Hsuan Chen

Nanochromatography Driven by the Coffee Ring Effect

The coffee ring phenomenon has long been known for its ability to concentrate particles at the rim of a dried liquid droplet, yet little is known about its particle separation capability. Here, we elucidate the physics of particle separation during coffee ring formation, which is based on a particle-size selection mechanism near the contact line of an evaporating droplet. On the basis of this mechanism, we demonstrate nanochromatography of three relevant biological entities (proteins, micro-organisms, and mammalian cells) in a liquid droplet, with a separation resolution on the order of ∼100 nm and a dynamic range from ∼10 nm to a few tens of micrometers. These findings have direct implications for developing low-cost technologies for disease diagnostics in resource-poor environments.

Left-Right Symmetry Breaking in Tissue Morphogenesis via Cytoskeletal Mechanics

Rationale: Left-right (LR) asymmetry is ubiquitous in animal development. Cytoskeletal chirality was recently reported to specify LR asymmetry in embryogenesis, suggesting that LR asymmetry in tissue morphogenesis is coordinated by single- or multi-cell organizers. Thus, to organize LR asymmetry at multiscale levels of morphogenesis, cells with chirality must also be present in adequate numbers. However, observation of LR asymmetry is rarely reported in cultured cells. Objectives: Using cultured vascular mesenchymal cells, we tested whether LR asymmetry occurs at the single cell level and in self-organized multicellular structures. Methods and Results: Using micropatterning, immunofluorescence revealed that adult vascular cells polarized rightward and accumulated stress fibers at an unbiased mechanical interface between adhesive and nonadhesive substrates. Green fluorescent protein transfection revealed that the cells each turned rightward at the interface, aligning into a coherent orientation at 20° relative to the interface axis at confluence. During the subsequent aggregation stage, time-lapse videomicroscopy showed that cells migrated along the same 20° angle into neighboring aggregates, resulting in a macroscale structure with LR asymmetry as parallel, diagonal stripes evenly spaced throughout the culture. Removal of substrate interface by shadow mask-plating, or inhibition of Rho kinase or nonmuscle myosin attenuated stress fiber accumulation and abrogated LR asymmetry of both single-cell polarity and multicellular coherence, suggesting that the interface triggers asymmetry via cytoskeletal mechanics. Examination of other cell types suggests that LR asymmetry is cell-type specific. Conclusions: Our results show that adult stem cells retain inherent LR asymmetry that elicits de novo macroscale tissue morphogenesis, indicating that mechanical induction is required for cellular LR specification.

Branching patterns emerge in a mathematical model of the dynamics of lung development.

The development of the lung is a highly stereotypical process, including the structured deployment of three distinct modes of branching: first side branching and then tip splitting with and without 90° rotation of the branching plane. These modes are supposedly under genetic control, but it is not clear how genes could act to produce these spatial patterns. Here, we show that cascades of branching events emerge naturally; the branching cascade can be explained by a relatively simple mathematical model, whose equations model the reaction and diffusion of chemical morphogens. Our low‐dimensional model gives a qualitative understanding of how generic physiological mechanisms can produce branching phenomena, and how the system can switch from one branching pattern to another using low‐dimensional ‘control knobs’. The model makes a number of experimental predictions, and explains several phenomena that have been observed but whose mechanisms were unknown.

Patterns of periodic holes created by increased cell motility

The reaction and diffusion of morphogens is a mechanism widely used to explain many spatial patterns in physics, chemistry and developmental biology. However, because experimental control is limited in most biological systems, it is often unclear what mechanisms account for the biological patterns that arise. Here, we study a biological model of cultured vascular mesenchymal cells (VMCs), which normally self-organize into aggregates that form into labyrinthine configurations. We use an experimental control and a mathematical model that includes reacting and diffusing morphogens and a third variable reflecting local cell density. With direct measurements showing that cell motility was increased ninefold and threefold by inhibiting either Rho kinase or non-muscle myosin-II, respectively, our experimental results and mathematical modelling demonstrate that increased motility alters the multicellular pattern of the VMC cultures, from labyrinthine to a pattern of periodic holes. These results suggest implications for the tissue engineering of functional replacements for trabecular or spongy tissue such as endocardium and bone.

A Wettability Switchable Surface Driven by Electrostatic Induced Surface Morphology Change Without Energy Interference On Reagents in Droplets

A novel surface wettability switchable device was successfully demonstrated by changing the surface morphology to induce contact angle change. The surface morphology transformation carried out from the deflection of thin PDMS membrane, driven by electrostatic force, can dynamically change the surface contact angle from 131 ° to 152 ° based on the contact area variation. The electrostatic energy can be throughout shielded out from droplets thanks to ground shielding effect. Since there is no direct physical or chemical (thermal, electrical, UV light etc.) interference from this actuation mean to biological solutions, the proposed method has great potential on microscale droplet transportation and is suitable to many applications especially digital fluidic systems.

Selective Wettability Assisted Nanoliter Sample Generation via Electrowetting-Based Transportation

A novel method for nanoliter sample generation is demonstrated. In this method, an electrowetting-based platform (EWOD) was used to transport a water droplet which was sandwiched between two hydrophobic plates, and the transportation was carried out by the direct electrical control of planar electrodes on the bottom glass substrate. In contrast to the air environment in ordinary EWOD, silicone oil was employed to surround the water droplet to reduce the surface hysteresis; therefore, the fluidic operations including cutting and transportation became easier to manipulate. While the droplet was moving through the electrodes, a nanoliter sample was produced within the circular hydrophilic area which was patterned on the upper ITO coated plate. Hence, based on the definition of selective wettability areas, the sample volume is capable to be well generated and controlled. Besides, in order to optimize the dimensions of electrode, the dimensional criterion for complete sample generation was investigated. The result shows that larger electrode width permits more flexibility to determine the radius of hydrophilic circle. Upon this mechanism, the hydrophilic circle of 0.25 mm diameter is able to generate the tiny sample of 3.9 nanoliter. Since the dimensions of the hydrophilic circle could be easily patterned in tens microns, this method has the potential to achieve the picoliter sample via similar procedure. Consequently, according to the tiny sample generation and reduction of hysteresis, such method is well-suitable for the versatile applications.© 2007 ASME

Three dimensional tubular structure self-assembled by vascular mesenchymal cells at stiffness interfaces of hydrogels.

In this study, we report a rational and robust methodology to construct three dimensional (3D) tubular-structures solely by self-assembly of vascular mesenchymal cells (VMCs). Using the cell-laden hyaluronic acid hydrogel surrounded by cell-free gel with a higher stiffness, VMCs spontaneously migrated across the interface and assembled into 3D tubes, which composes of numerous cells. Based on turing instability which describes the reaction-diffusion processes of inhibitors and activators, this result of 3D tubular structure formation agrees with theoretical predictions from simulations of the reaction-diffusion of morphogens and cells under the initial conditions of patterned cell-laden hydrogel. We showed that this combination of theoretical prediction and experiments is able to produce multi-cellular 3D tubes with desired dimensions and determinate orientation in hydrogel mimicking the 3D features of tubular tissue. This work provides a reliable methodology for creating tubular structures with controllable sizes inside the 3D hydrogel through multi-cellular self-organization.

Tissue regeneration: from synthetic scaffolds to self-organizing morphogenesis.

Regenerative medicine offers therapeutic approaches to treating non-regenerative diseases such as spinal cord injury and heart disease. Owing to the limited donor tissue available, cell-based therapy using cultured cells with supporting scaffolds has been proposed to rebuild damaged tissue. Early attempts at repairing skin and cartilage achieved significant success thanks to the simplicity of the tissue architecture, which later fueled enthusiasm for applying the same strategy to other types of tissue. However, more complex tissue functions require a more extensive vasculature and heterogeneous cell arrangements, which together constitute a significant hurdle in practical applications. Accordingly, recent years an increased interest has been in the use of decellularized matrices that retain the natural microarchitecture as the scaffold. However, although a number of engineering approaches have been suggested, self-organizing behavior such as cell proliferation, migration, and differentiation may still disorganize and frustrate the artificial attempts. This mini-review first provides examples of the early history of tissue engineering using skin and cartilage as examples, and then elaborates on the key technologies used to fabricate synthetic acellular scaffolds and cell/scaffold constructs with more complicated architectures. It also summarizes the progress achieved in the use of decellularized matrices for cell seeding as well as the recent success seen in self-organizing two- and three-dimensional tissue formation with the aid of biomathematical modeling. The review concludes by proposing the future integration of biomathematics, developmental biology, and engineering in concert with the self-organization approach to tissue regeneration.

A Parametric Design Study of an Electrochemical Sensor

The development of highly sensitive biosensors for the detection of biomolecules, such as the biomarker interleukin-8 for the early detection of oral cancer, requires optimization of sensor design. To augment the performance of an electrochemical sensor, this study used a microscale, aptamer-based electrochemical sensor for detecting botulinum neurotoxin aptamer hybridization. We first used top—down lithographic processing to define the pattern of the electrodes and then used bottom—up manufacturing to modify the surface molecular properties for reducing nonspecific binding. We systemically examined the effects of the design parameters of an aptamer-based electrochemical sensor. Specifically, five key design parameters were examined: the area of the working electrode (WE), the area of the counter electrode (CE), the separation distance between the WE and CE, the overlap length between the WE and CE, and the aptamer concentration. Through an analysis of the signal and noise generated across variations of the different parameters, the significance of each parameter in sensor performance was determined. In particular, we found that the area of the WE was the only key parameter that influenced the performance of the sensor. The output signal level increased with the area of the WE and the signal-to-noise ratio was about constant in the tested range (i.e., from 0.02 to 4 mm2).

Discrete Element Model for Suppression of Coffee-Ring Effect

When a sessile droplet evaporates, coffee-ring effect drives the suspended particulate matters to the droplet edge, eventually forming a ring-shaped deposition. Because it causes a non-uniform distribution of solid contents, which is undesired in many applications, attempts have been made to eliminate the coffee-ring effect. Recent reports indicated that the coffee-ring effect can be suppressed by a mixture of spherical and non-spherical particles with enhanced particle-particle interaction at air-water interface. However, a model to comprehend the inter-particulate activities has been lacking. Here, we report a discrete element model (particle system) to investigate the phenomenon. The modeled dynamics included particle traveling following the capillary flow with Brownian motion, and its resultant 3D hexagonal close packing of particles along the contact line. For particles being adsorbed by air-water interface, we modeled cluster growth, cluster deformation, and cluster combination. We found that the suppression of coffee-ring effect does not require a circulatory flow driven by an inward Marangoni flow at air-water interface. Instead, the number of new cluster formation, which can be enhanced by increasing the ratio of non-spherical particles and the overall number of microspheres, is more dominant in the suppression process. Together, this model provides a useful platform elucidating insights for suppressing coffee-ring effect for practical applications in the future.