Session Q39: Focus Session: Materials and Functional Structures for Biological Interfaces - Micro and Nanofluidics

Integration of Materials and Functions in Microfluidic Devices

The physical and chemical properties of a surface determine how that surface interacts with its surrounding environment. Despite the large number of potential schemes feasible for surface modification, the covalent attachment of polymers remains the most promising approach to tailor important properties of lab-on-chip (LOC) devices such as adhesion, wettability and biocompatibility. This presentation deals with ``surface related'' issues that must be addressed in the development of LOC systems. An innovative approach that allows the attachment of polymer molecules to surfaces of different composition such as glass, silicon and polymer materials will be presented. Examples of interfaces modified by ``smart coatings'' able to give an appropriate and predictable response to outside conditions and decorated with biologically relevant biomolecules will be discussed.   

Dynamic micromolds to fabricate multi-layered hydrogel microstructures

Hydrogel microstructures can be used to mimic living systems and create drug carriers. Living materials can be encapsulated within multi-layered microgels to replicate native tissues. Furthermore, multiple drugs can be immobilized within different layers of microgels to create multifunctional drug carriers. Photolithography is a commonly used method to create these multi-layered microgels, but it is not applicable to non-photocrosslinkable materials. Also, conventional micromolding methods do not allow creating multi-layered microgels due to the static environment of the microstructures. Herein, we created dynamic micromolds by exploiting the thermoresponsiveness of poly(N-isopropylacrylamide). These micromolds allowed sequential molding of microgels at different temperatures. Different cell types were spatially immobilized in different layers of microgels to replicate native tissue complexity. Furthermore, fluorescent microbeads were spatially immobilized within different microgel layers to show a concept of drug carriers which could encapsulate various drugs. These dynamic micromolds could be potentially useful in creating multi-layered hydrogel microstructures in order to mimic biological systems and fabricate multifunctional drug carriers.   

Implementation of a Peltier-based cooling device for localized deep cortical deactivation during in vivo object recognition testing

While the application of local cortical cooling has recently become a focus of neurological research, extended localized deactivation deep within brain structures is still unexplored. Using a wirelessly controlled thermoelectric (Peltier) device and water-based heat sink, we have achieved inactivating temperatures ($<$20 C) at greater depths ($>$8 mm) than previously reported. After implanting the device into Long Evans rats' basolateral amygdala (BLA), an inhibitory brain center that controls anxiety and fear, we ran an open field test during which anxiety-driven behavioral tendencies were observed to decrease during cooling, thus confirming the device's effect on behavior. Our device will next be implanted in the rats' temporal association cortex (TeA) and recordings from our signal-tracing multichannel microelectrodes will measure and compare activated and deactivated neuronal activity so as to isolate and study the TeA signals responsible for object recognition. Having already achieved a top performing computational face-recognition system, the lab will utilize this TeA activity data to generalize its computational efforts of face recognition to achieve general object recognition.   

Droplet Microfluidics for Artificial Lipid Bilayers

Droplet interface bilayer is a versatile approach that allows formation of artificial lipid bilayer membrane at the interface of two lipid monolayer coated aqueous droplets in a lipid filled oil medium. Versatility exists in the form of voltage control of DIB area, ability of forming networks of DIBs, volume control of droplets and lipid-oil, and ease of reformation. Significant effect of voltage on the area and capacitance of DIB as well as DIB networks are characterized using simultaneous optical and electrical recordings. Mechanisms behind voltage-induced effects on DIBs are investigated. Photo induced effect on the DIB membrane porosity is obtained by incorporating UVC-sensitive photo-polymerizable lipids in DIB. Photo-induced effects can be extended for in-vitro studies of triggered release of encapsulated contents across membranes. A droplet based low voltage digital microfluidic platform is developed to automate DIB formation, which could potentially be used for forming arrays of lipid bilayer membranes.   

Density fluctuations in nanochannel-confined DNA

The dynamic behavior of a polymer chain in dense solution is typically described within the framework of reptation, which assumes that polymers primarily move along tubes formed by other chains. We have studied the dynamic density fluctuations of single DNA molecules confined to nanofabricated channels that mimic reptation tubes, and found that the classical harmonic spring model yields a satisfactory description. In particular, we have recovered the expected dispersion relationship. By looking at fluctuation amplitudes, we have also found that the description demands a minimum spring length approximately equal to the size of self-avoiding DeGennes blobs.   

Extension and Diffusion of DNA in Nanochannels

Nanochannels are an ideal platform for studying the basic physics of confined polymers, using DNA as the model polymer. While the scaling laws for strong (Odijk) and weak (de Gennes) confinement were established decades ago, recent experiments have illuminated the complex physics arising between these limiting cases. We will first present Monte Carlo simulation data on the extension of DNA in nanochannels. Our results provide clear evidence for the existence of two transition regimes between the Odijk and de Gennes regimes, thereby resolving the apparent contradiction between these scaling theories and the corresponding experiments by Austin and coworkers. We will then present results for the diffusivity of DNA in nanochannels and explain their connection to the different regimes of extension. By using Monte Carlo sampling of the Kirkwood diffusivity and a numerical solution for the confined Green's function, we have calculated the diffusivity for DNA contour lengths ranging over three orders of magnitude and nanochannel sizes over two orders of magnitude. By using a DNA model that accurately reproduces the free solution radius of gyration and diffusivity over a range of molecular weights, we can directly connect the simulation data and experiments.   

DNA conformation and dynamics in quasi-2D and -1D confinement

We investigate the structure and correlation length of DNA molecules in sub-100nm quasi-2D slits and 1D square channels. In strong slit confinement, the segmental correlation length of DNA molecules separates into two components -- in the confined and unconfined dimensions. In the confined dimension, the segmental correlation length is controlled by the slit height. In the unconfined dimension, the segmental correlation length increases as the slit height decreases. In the nano-channel, segmental correlation length increases beyond the chain contour length as channel height decreases below channel persistence length. We generalize how this affects the entropic elasticity of confined DNA molecules and how it affects chain thermodynamic properties.   

Mapping the yeast genome by melting in nanofluidic devices

Optical mapping of DNA provides large-scale genomic information that can be used to assemble contigs from next-generation sequencing, and to detect re-arrangements between single cells. A recent optical mapping technique called denaturation mapping has the unique advantage of using physical principles rather than the action of enzymes to probe genomic structure. The absence of reagents or reaction steps makes denaturation mapping simpler than other protocols. Denaturation mapping uses fluorescence microscopy to image the pattern of partial melting along a DNA molecule extended in a channel of cross-section $\sim$100nm at the heart of a nanofluidic device. We successfully aligned melting maps from single DNA molecules to a theoretical map of the yeast genome (11.6Mbp) to identify their location. By aligning hundreds of molecules we assembled a consensus melting map of the yeast genome with 95\% coverage.   

Spatially Varying Nanoconfinement as a Probe of Polymer Physics

Complex nanofluidic systems have the capability to unveil a rich landscape of new polymer physics. One-dimensional channels and two-dimensional slits have been used for precise measurements of persistence length and to verify scaling laws. Recently, devices with spatially varying confinement have been used to gain further control over single molecule polymer conformation. We use a system consisting of a nanofluidic slit embedded with a lattice of pits acting as entropic traps. Single DNA polymers in this system self-organize into discrete conformational states. We have shown that this system can be used to define stable DNA configurations at equilibrium and to fine-tune diffusion to a local minimum corresponding to stable conformational states. Measurements of mean occupancy with varying device parameters can be fit to theory, giving information about the confinement free energy of DNA in a nanoslit (a subject of controversy) and the strength of excluded volume interactions. Measurements of the excluded volume interaction provide information about the strength of intersegmental repulsive electrostatic interactions, quantified by the notion of effective width. The scaling of width with respect to salt concentration is observed in single DNA molecules for the first time.   

The Nanofluidic Staircase: A Brownian Motor for Polymer Characterization and Transport

A coarse-grained molecular dynamics simulation is used to study the motion of a polymer chain in a nanofluidic staircase (Stavis et al., Nanotechnology, 20(16), 2009), a device which consists of a collection of nanoslits of increasing depth arranged in step-like fashion in a fluidic channel. The slit depths span the Odijk ($H [Preview Abstract]

Geometrically induced polarization and alignment of cells on nanopillar arrays

Topological features at the nano and microscale can trigger mammalian cell growth and differentiation. In this work, we describe geometrical tuning of ordered arrays of nanopillars and micropillars that elicit specialized morphologies in adherent cells. Systematic analysis of the effects of the pillar radius, height, and spacing reveals that stem cells assume either flattened, polarized, or stellate morphologies in direct response to interpillar spacing. Notably, on patterns of pitch near a critical spacing (dcrit = 2 ?m for C3H10T1/2 cells), cells exhibit rounding of the cell body, pronounced polarization, and extension of narrow axon-like cell projections aligned with the square or hexagonal lattice of the NP array. This morphology persists for various stem cell lines and primary mesenchymal stem cells. The neuron-like morphological characteristics suggest that NP arrays can be utilized in tissue engineering applications that require directed axon growth. The ability of nano and micropillars to support various morphogenetic trends will allow rational design of scaffolds that may be useful for stem cell lineage specification, formation of patterned neural networks, and enhancement of implant integration with adjoining tissue.