Extracting activity from the non-equilibrium fluctuations of a micro-swimmer
Prof. Wylie Ahmed, California State University, Fullerton
Chlamydomonas reinhardtii are a widely-studied microswimmer that propel themselves by converting chemical energy to mechanical motion of their flagellum in a breast stroke motion. Fluid dynamics approaches have revealed much about the importance of hydrodynamics at the micron-scale and its role in microswimmer transport. However, the stochastic dynamics which are dominated by active non-thermal fluctuations are not well understood. We use optical tweezers and the photon momentum method to directly measure the stochastic forces generated by a trapped Chlamydomonas microswimmer. We model the microswimmer using the generalized Langevin equation approach with active stochastic forcing. Our combined experimental and theoretical approach, based on microrheological techniques, isolates the active force spectrum generated by Chlamydomonas to quantify their nonequilibrium dynamics. We seek to use this framework to test recent developments in stochastic thermodynamics.
Developing Force-Activated Covalent Bond Transformations via Polymer Mechanochemistry
Prof. Maxwell Robb, California Institute of Technology
Polymer mechanochemistry is a rapidly emerging area of research that investigates the use of mechanical force to stimulate covalent chemical transformations. Polymers transduce mechanical stress to force-sensitive molecules called mechanophores that can be designed to undergo a wide variety of chemical reactions. Force-induced chemical reactions that produce a change in optical properties such as color or luminescence are particularly useful because they enable molecular level stress sensing by simple visual detection methods. Mechanically triggered release of small molecules is also an underdeveloped, but potentially powerful approach for sensing and delivery applications. Our recent developments on a mechanophore design strategy that decouples mechanical activation from a functional response will be presented. This mechanically gated reaction methodology creates opportunities for stimuli-responsive polymers that address a variety of useful technologies.
Dynamic stiffening and softening of a system of colloids cross-linked via polymers
Prof. Moumita Das, Rochester Institute of Technology
With the goal of ultimately deciphering the design principles for biomimetic materials that can autonomously stiffen and soften, we investigate colloids as a model system that can dynamically transition from fluid-like (sol) to gel-like (gel) when crosslinked with polymers. The model was first developed with colloids only, interacting via a Lennard-Jones potential and undergoing Brownian dynamics, with experimentally relevant parameters, to test and refine the simulation. We then added polymer crosslinkers that connect the colloids via a spring force, and investigated the resulting collective properties, such as the time needed for the formation of system spanning networks and the elastic moduli, for various colloid densities, interaction strengths, and cross-linker rest lengths and densities. Using experimental parameters for polystyrene spheres and Bovine Serum Albumin (BSA) crosslinkers, we predicted the behavior of real systems. Finally, we replaced the passive, one-shot crosslinkers in our system by active cross-linkers that can dynamically and attach and detach, and characterized how the degree of order and themechanical response of the system change with time. Our results provide insights into the design of self-sustaining soft materials that can dynamically stiffen and soften, and how the properties of such materials can be tuned.
The Hierarchical Assembly of Spider Silk: Connecting the Molecular and Nanometer Length Scales
Prof. Gregory Holland, San Diego State University
Spider silk is mother nature’s super fiber with mechanical and physical properties that far exceed the man-made polymers currently used to fabricate the materials in the world around us. Our research team has been using a combination of advanced physical characterization techniques to understand the silk producing process from the spinning dope to final fiber formation. Using a combination of diffusion NMR and cryo-TEM, we recently illustrated the presence of hierarchical micellar silk protein superstructures within the silk dope that are 100’s of nm’s in diameter. These pre-assemblies are likely a critical requirement for the spider silk spinning process. We are now tracking the disassembly of these silk protein micelles in 4M urea with 3D solution NMR to determine the intermolecular contacts that facilitate micelle organization. Recent results will be discussed that indicate the Gly-Ala units flanking the b-sheet forming poly(Ala) runs are perturbed following incubation in denaturant.
Shape sculpting and shifting in Soft Matter
Prof. Timothy Atherton, Tufts University
Many systems in Soft Matter involve strong deformation and shape change, including lipid membranes, liquid crystal droplets, expansion in hydrogels and soft colloids. A particularly interesting, and highly challenging, class of problems arises where the final shape is unknown and must be solved for together with other quantities such as order parameters, electric fields, etc. In this talk, I’ll introduce my group's efforts to develop generic computational methods to solve these problems and illustrate the results with examples drawn from across Soft Matter. The tools developed are highly accessible for undergraduate research and therefore of particular interest to Cottrell Scholars.
Creating and Discovering Complexity in Ultra-Soft Colloids
Prof. Andrew Lyon, Chapman University
Ultra-soft, dual-responsive colloidal particles prepared via surfactant-free precipitation polymerization display complex phase transition behavior due to the interplay between charged and thermoresponsive polymer segments as a function of polymer ionization. The intertwining of the polymer thermodynamics and mechanics, while interesting, can prevent the rational design of multi-functional particles for specific biomedical applications. Spatial segregation of the different polymer chemistries/functions, e.g. as a core-shell particle, is an attractive approach, provided mechanical coupling of the core and shell components does not negatively impact the desired 'softness' of the colloidal surface. Once established, these synthetic approaches can be elaborated upon to create greater colloid complexity/functionality by designing core-shell particles with specific chemoligation groups in spatially controlled locations within the particle.
A new type of interaction between active colloids
Prof. John Gibbs, Northern Arizona University
We present a new type of tunable interaction between self-propelled active colloids. Janus spheres with a half-coating of a catalyst are actively driven in a solution of hydrogen peroxide. Interactions between such particles are dominated by effects including hydrodynamics and excluded volume, and are known to give rise to phase separation where dense, close-packed “solids” coexist with a dilute “gas” phase. We utilize anisotropic magnetic interactions between particles in order to engineer contactless particle-particle interactions that may be purely attractive or purely repulsive, and due to their contactless nature, show a rich collective behavior not seen in previous systems.
Conjugation of Polynorbornene to Cell Surfaces
Dr. Derek Church, University of California, San Diego
Polymer conjugation to biologics such as proteins and cell surfaces have a bevy of potential applications ranging from therapeutics to biocatalytic materials. While conjugates consisting of polyethylene glycol have been widely studied, only recently have polymers synthesized via living polymerization techniques been incorporated at the biological interface. Our contribution to this growing field has been the utilization of ring opening metathesis polymerization (ROMP) for the synthesis of novel polynorbornene protein conjugates. ROMP is an attractive living polymerization methodology due to its high functional group tolerance, facile block copolymer formation and its amenability to ambient conditions. Herein is described the first examples of polynorbornene cell conjugates. These have been synthesized via a ‘graft-to’ methodology using non-covalent cell membrane insertion to anchor the polymer to the cell surface. Cell polymer conjugates with different polymer backbone structure, molecular weight, functional groups as well as block copolymer topology have been explored and will be discussed.
Diffusive dynamics of beads, molecules and droplets: soft matter probed with novel microscopy methods
Prof. Ryan McGorty, University of San Diego
Common methods to study the dynamics of soft matter systems include optical microscopy with image analysis and dynamic light scattering. In recent years, a new method which combines elements of imaging and scattering techniques, differential dynamic microscopy (DDM), has been successfully demonstrated on a range of systems. Here, I will discuss how we use DDM to measure the dynamics of colloidal particles, bacteriophages, DNA molecules and active particles in various complex fluids. I will also describe our recent extensions of DDM. We have developed methods to extend DDM for measuring dynamics faster than the camera frame rate, three-dimensional dynamics and fluctuations at a liquid-liquid interface. Finally, I will discuss our attempts to measure the dynamics of systems driven out of equilibrium through either shear or changes in temperature.
Looking inside polyMOFs: Revealing the structural interior of polymer-MOF hybrid materials
Dr. Kyle Bentz, University of California, San Diego
The hybridization of metal-organic frameworks (MOFs) and polymers offers tremendous promise for implementing the myriad functionalities of MOFs with the facile processability and durability of synthetic polymers. Indeed, significant strides have been achieved towards this goal with the demonstration that amorphous polymers composed of ditopic ligands can be successfully converted to highly crystalline frameworks, termed polyMOFs. Since these pioneering reports, efforts have been taken by our group to create polyMOFs from more complex block polymers. We recently reported on a library of block co-polyMOFs in which the underlying polymer architecture provided exquisite control of the resulting crystal morphology. However, much was unknown about the internal structure of the materials, including the molecular-level morphology and assembly mechanisms. We use a variety of techniques to interrogate the internal structure of polyMOFs, including high-resolution TEM, SEM, energy-dispersive X-ray spectroscopy (EDS), and small-angle synchrotron X-ray scattering (SAXS) to gain remarkable insight into these materials.
Dynamics of the crosslinked composite network of actin and microtubule filaments
Leila Farhadi and Jennifer Ross, UMass Amherst
Cytoskeleton is made of interacting protein filaments such as actin and microtubule and crosslinking proteins. We use fluorescent microscopy to study the dynamics of semiflexible actin filaments and rigid microtubules in the presence of the crosslinkers in vitro. MAP65 proteins link the microtubules when actin filaments are crosslinked by NeutrAvidin-Biotinylated actin complex molecules. The mobility of this composite network is studied using intensity fluctuations of actin and microtubule filaments when different concentrations of crosslinkers are used in this system.
Complex mechanics of an intrinsically disordered protein
Hoang Truong and Omar Saleh, UCSB
Intrinsically disordered proteins (IDPs) lack a well-defined structure, making it difficult to investigate their conformations. Since IDPs have a large conformational entropy, we explore entropic elasticity as a means to study their conformations. We use a high-resolution single-molecule magnetic tweezer to stretch single polypeptide construct of an IDP derived from neurofilaments. At different force regimes, we access both equilibrium and nonequilibrium elastic responses. Sudden jumps from high to moderate forces cause the polypeptide to undergo long-term, nonequilibrium logarithmic relaxation. At low force, the construct exhibits equilibrium elasticity – worm-like chain behavior with a persistence length longer than expected for a fully unfolded polypeptide. Together, these results suggest the IDP has a residual structure comprising an ensemble of intramolecular interactions; this structure stiffens the chain at low force, while at high force it mechanically unfolds over a range of time scales, leading to slow relaxation. We probe this structure formation mechanism by measuring changes in elastic response caused by salts and denaturants. Our data reveals a complex structure and a rich elastic behavior in a nominally disordered chain.
Oscillating Network of Interlinked Beads
Jeffrey Wang, Eliana Petreikis, Rae Robertson-Anderson, USD
Autonomous oscillating mechanical systems offer great potential in the field of active materials. One pathway in creating this system is affixing KaiC proteins to a network of micron-sized beads, thereby creating a colloid that transitions from gel-like when crosslinked to fluid-like when unliked. By utilizing the regular oscillating nature of these proteins, we can create a material that exhibits active properties which can be tuned according to time and molecular mechanics. Here, we demonstrate exploratory work in the creation of these interlinked systems using a variety of linker proteins across a platform of experimental models. We investigate basic proof-of-concept and demonstrate imaging under both bright-field and fluorescent microscopy. Furthermore, we exhibit specific quantitative and qualitative observations that offer promising pathways for future research.
The Intermediate Scale of Active Matter
Anthony Estrada and Wylie Ahmed, CSUF
Active matter systems have typically been studied at the microscopic scale, where thermal noise has a strong effect, but inertia is neglected. This field is often studied at the 100-meter scale, where thermal noise is neglected, yet inertia dominates. We seek to bridge these scales by studying the behavior of a meter-scale system where both noise and inertia affect the systems dynamics. On this scale, our active particles follow the laws of statistical mechanics, and exhibit common features of larger and smaller active matter, such as scale-free Brownian motion and Diffusion. Using a HEXBUG self-propelled device surrounded by a circular boundary, we create an active particle at an intermediate length scale. We will present our experimental setup, preliminary single-particle results, and our plan to investigate the collective behavior of these intermediate-sized active particles in large numbers. With combined experimental and theoretical approach, we hope to establish a connectivity between various levels of scale of active matter systems.
Cytoskeletal Crosslinking Produces Varying Degrees of Anomalous Diffusion
Sylas Anderson and Rae Robertson-Anderson, USD
The diffusion of microscopic particles through the cell, important to processes such as viral infection, gene delivery, and vesicle transport, is largely controlled by the complex cytoskeletal network – comprised of semiflexible actin filaments and rigid microtubules – that pervades the cytoplasm. By varying the types of crosslinking between actin and microtubules, the cytoskeleton can display a host of different structural and dynamic properties that in turn impact the diffusion of particles through the cross-linked composite networks. Here we couple single-particle tracking with differential dynamic microscopy to characterize the transport of microsphere tracers diffusing through composite in vitro networks, using various types of cross-linking by connecting the filaments with the permanent cross-linkers biotin and NeutrAvidin. We find that particles exhibit anomalous subdiffusion in all networks, however; subdiffusive characteristics are markedly more pronounced in networks that have more cross-linking between different types of filaments.
Force Fluctuations and Effective Viscosity in Active Systems
Hunter Seyforth and Wylie Ahmed, CSUF
Active systems such as active colloids, bacteria, and enzymes have been observed to influence diffusion in fluid baths. My goal is to observe and measure how the viscosity of a fluid bath is affected in the presence of active particles and how this is different from passive systems. To do so, I use Differential Dynamic Microscopy, which is an analysis tool that uses the fluctuations of light intensity between frames to quantify the diffusion of the system. Optical tweezers will also be used to trap colloids and measure the forces experienced by the colloids due to thermal and active forces. These methods will allow us to quantify the effective viscosity and diffusion of simple fluid systems that have active force generators in the bath and provide insight into the differences between passive and active systems.
Actin crosslinking in actin-microtubule composites reveals non-monotonic dependence on stiffness, viscosity and mobility
Shea Ricketts and Rae Robertson-Anderson, USD
Semiflexible actin filaments and rigid microtubules are key biopolymers comprising the cytoskeleton and enabling the multifunctional mechanics of cells. Interactions between actin and microtubules, either steric interactions or chemical crosslinking by smaller binding proteins, allows for precise structural and mechanical tunability. Here we use optical tweezer microrheology and fluorescence microscopy to determine how chemical crosslinking of actin filaments impacts the mechanics and mobility of actin-microtubule composites from the steady state to nonlinear regime. Specifically we map mechanical properties including viscosity, nonlinear force response, stiffness, force relaxation and mobility to the ratio of crosslinkers to actin in composites (R). We find elasticity and stiffness display a nonmonotic dependence on crosslinking, initially increasing until R = 0.02 and then decreasing at R = 0.08 to a response similar to unlinked networks. Using mobility analysis, we show the non-monotonic dependence on actin crosslinking influences microtubule mobility which in turn controls composite stiffness and elasticity.
Active Brownian Particles Studied with Differential Dynamic Microscopy
Kira Tran and Ryan McGorty, USD
We study active Brownian particles or microswimmers using differential dynamic microscopy (DDM). We use titanium dioxide particles that become active in certain solutions when exposed to UV light. We quantify the diffusivity of particles as a function of UV light intensity. Sample slide thickness and particle concentration are also observed to influence the activity of our particles. At certain concentrations, slide thicknesses, and UV light intensities, the titanium dioxide particles clump together, whereas in other conditions, we observe that clumps of particles break apart. We use DDM and other image analysis routines to quantify the observed behavior.
The Stochastic Force Spectrum of a Micro-Swimmer
Corbyn Jones and Wylie Ahmed, CSUF
We seek to understand the dynamics of micro-swimmers by quantifying the stochastic forces generated by their motion. We are currently working with Chlamydomonas reinhardtii—a green algae commonly used to study microscopic locomotion. Our approach is to use optical tweezers and a direct force calibration known as the photon momentum method (PMM) to measure micro-swimmer forces. The power spectral density (PSD) of the force dynamics is analyzed, providing information about the frequency content of the force signals. A simple stochastic model based on the generalized Langevin equation predicts the power spectral density to have a Lorentzian-type curvature. We compare our experimental data to the theoretical model to test if the model can predict our experimentally measured PSD. This approach allows the calculation of thermodynamic quantities such as work, power, efficiency, etc. to describe the microscopic motion. Our analysis seeks to apply concepts from stochastic thermodynamics to understand micro-swimmer dynamics.
Non-equilibrium fluctuations of a chlamydomonas microswimmer
Mauricio Gomez and Wylie Ahmed, CSUF
Optical tweezers are a useful tool to study out of equilibrium system fluctuations in microscopic systems. Using a recently developed technique, the Photon Momentum Method, we are able to accurately measure forces of complex objects such as living microswimmers at pico-Newton scales. In addition to thermal forces experienced in equilibrium, the chlamydomonas actively generate forces to propel itself through the surrounding media. To systematically study such a system, we trap chlamydomonas using optical tweezers and measure the forces generated. We calculate the stochastic force spectrum and apply a theoretical framework to extract the active forces. We also investigate the probability distribution of force fluctuations using the van Hove Correlation function which provides information on the non-equilibrium dynamics of the system.
Modelling of live cell mitotic spindle as in vitro tactoid
Sumon Sahu and Jennifer Ross, Syracuse University
Self-organization is an important phenomenon of matter observed from nano to macro scales in both living and non-living systems. A fundamentally important active matter system is the mitotic spindle of dividing cells, a highly dynamic yet regulated self-organized structure made out of microtubules and associated proteins used to separate the genetic material into two daughter cells. We use in vitro reconstitution experiments to show that an antiparallel microtubule cross-linker, MAP65, and a depletion agent, methylcellulose, can make spindle-like microtubule organizations. The spindle-like structures are homogeneous birefringent tactoids as found using cross-polarized imaging and shape analysis. We find that depletion agents with different size, charge, and viscosity properties can affect the organization of the tactoids. Using computer simulations, we predict the steady state configuration of tactoid assembly from microtubules with dynamic instability incorporated in order to uncover the physical principles of spindle self-organization.
Diffusion of Microspheres in Concentrated DNA Solutions
Serenity Adalbert and Rae Robertson-Anderson, USD
To learn more about the viscoelastic properties of DNA, we look at fluorescently labelled microspheres in different concentrations of ring/linear DNA. As the beads undergo Brownian Motion, we use Matlab scripts to track them as they diffuse within the sample and calculate the diffusion coefficients.
Dual-Color Differential Dynamics Microscopy
Ruilin You and Ryan McGorty, USD
We introduce dual-color differential dynamic microscopy (DDM) for detecting fast dynamics. DDM has been used extensively to measure the diffusive or ballistic motion of small particles, macromolecules and bacteria. Rather than localizing and tracking individual particles, DDM works by measuring the intensity fluctuations in images across a range of detectable spatial frequencies and provides data similar to that provided by dynamic light scattering. However, DDM is limited by the camera frame rate. Fast dynamics can be measured with high-speed cameras but those are typically expensive. We have developed a dual-color imaging setup which allows us to detect dynamics faster than the camera’s frame rate. We trigger blue and red light at well-defined times within a single image exposure. By analyzing each color channel separately and in combination we detect dynamics that are at least four times faster than the camera frame rate.
Increasing valence pushes DNA nanostar networks to the isostatic point
Nathaniel Conrad, Omar Saleh and Deborah Fygenson, UCSB
Classical rubber theory says gel elasticity is related to the entropic elasticity of flexible polymeric linkers. The rubber model, however, ignores the role of valence (i.e., number of links emanating from a network junction). Recent work predicts that valence, and particularly the Maxwell isostatic point, plays a key role in determining the mechanics of polymer networks. Here, we demonstrate the prominent role of valence in determining the mechanics of a model system. The system is based on DNA nanostars (DNAns): multi-armed, self-assembled nano-structures that form thermo-reversible equilibrium gels through base pair controlled cross-linking. We measure the linear and nonlinear elastic properties of these gels as a function of DNAns arm number and concentration. We show DNAns gel elasticity is sensitive both to entropic elasticity of network chains and to valence, with an apparent isostatic point between 5 and 6 agreeing with the Maxwell prediction.
Modifying the KaiABC Circadian Clock to Act as an Oscillating Scaffold
Janet Kang and Michael Rust, University of Chicago
The KaiABC system is a bacterial clock that generates ~24 hour rhythms in protein phosphorylation and protein complex formation. Because this system can be reconstituted with purified protein it presents an exciting synthetic biology / bioengineering opportunity to use this protein clock to drive oscillations in desired output systems. We are designing schemes to use KaiB as a time-dependent crosslinker to enable oscillations in material properties of polymer networks. We have prepared site-specifically biotinylated KaiB and are testing the impact of biontinylated KaiB and free streptavidin on the KaiABC clock reactions using a high throughput, high time-resolution plate reader flourescence polarization assay that probes the period, amplitude and waveform of the circadian oscillations by measuring the periodic Kai protein interactions.
Interdisciplinary Exploration of Latch Mediated, Spring Actuated Systems
Andres Cook and Mark Ilton, Harvey Mudd College
Animals such as the mantis shrimp and trapjaw ant store and release energy in elastic materials to achieve kinematic performance far beyond what would be expected from purely muscle-driven motion. We explore the design, performance, and biological function of these latch-mediated, spring-actuated (LaMSA) systems at various levels of abstraction. We examine materials properties pertaining to recoil and energy storage behaviors, the effects of component properties on kinematic performance, and the interpretation of system parameters with respect to biological performance and function. We accomplish this through both experimental and modeling approaches.
Linear and Ring DNA Transport in Crowded Cytoskeleton Networks
Jonathan Garamella, Ryan McGorty and Rae Robertson-Anderson, USD
Molecular crowding is perhaps the most important cellular property given its impact on gene expression, protein function and stability, and molecular transport. Of particular interest is the interaction of DNA with cytoskeletal crowders, which act as barriers to effective intracellular transport and conformational stability required for such processes as transfection, viral infection, and gene therapy. By coupling single-molecule conformational tracking with differential dynamic microscopy, we build on previous studies by investigating the transport and conformation properties of linear and ring DNA in cytoskeleton networks composed of entangled or crosslinked actin and microtubules. While both topologies exhibit anomalous subdiffusion in both entangled and crosslinked networks, they experience divergent effects with respect to crosslinking when looking at the DNA conformation states and the nature of the diffusion, which we probe by studying multiple parameters related to the ergodicity of the transport.
Optical tweezers microrheology reveals the viscoelastic properties of entangled ring-linear DNA blends
Karthik Peddireddy and Rae Robertson-Anderson, USD
Solutions of entangled polymers display complex and intriguing viscoelastic properties that are still poorly understood. While the reptation model can describe the viscoelastic properties of entangled melts of linear polymers, the model is ill-equipped to deal with circular or ring polymers, blends of polymers of varying topologies, or solutions of polymers at concentrations near the critical entanglement concentration. DNA is an excellent model system for resolving this issue as it occurs naturally in linear and circular forms. Here, we use optical tweezers microrheology to measure the linear and nonlinear viscoelastic response of semidilute and entangled blends of circular and linear DNA. We characterize the dependence of viscoelastic properties on the ratio of circular and linear chains in the blend as well as the overall solution concentration. Our results show intriguing non-monotonic properties of blends compared to single-component systems.
The glycocalyx as a physical barrier: evaluating the impact of inert synthetic glycoproteins on cell surface interactions
Daniel Honigfort and Kamil Godula, UCSD
As the primary interface between the cell and its extracellular environment, the glycocaylx dictates interactions with a broad range of stimuli, from beneficial growth factor signaling and lectin recognition to exclusion of harmful pathogens. Recent advances have shown the importance of individual glycans in mediating these interactions, but less focus has been paid to the bulk steric properties imparted by the high glycan density of the glycocalyx. Here, we will discuss the synthesis of artificial glycoprotein mimetics, their assembly into a “spectator” glycocalyx, and their ability to alter the binding and organization of lectins and viruses to the cell membrane.
Probing the fluid-fluid interfacial dynamics of phase-separated colloid-polymer mixtures
Jing Wang and Ryan McGorty, USD
We study the thermally induced capillary waves between the colloid-poor (gas) and colloid-rich (liquid) phase of a colloid-polymer mixture. Using suspensions of ~200 nm colloidal particles and polymers we observe the solution phase separate into two fluid phases with ultralow surface tension. We are therefore able to optically detect the roughness of the interface. To quantify the interfacial dynamics we use a novel extension of differential dynamic microscopy (DDM). We also investigate our fluid-fluid phase separated mixtures in shear flow using bright-field and light-sheet microscopy. We observe elongated liquid domains and find how their size depends on shear rate. Finally, using the temperature-responsive feature of our colloidal particles we study the kinetics of phase separation under shear flow.
Emergence of Dynamic Director in 2D Active Nematic
Jing Xu, UC Merced
Motivated by recent work on active nematics, we employ here a microtubule and kinesin motor protein system to examine collective order in 2D. Distinct from previous studies, our system demonstrate collective order without the aid of any crowding agents. Interestingly, the nematic order in our system undergoes a collective reorganization as a function of time. We demonstrate that the introduction of a dynamic director is necessary for proper characterization of this active nematic system. We found that the rotational dynamics of the director correlates negatively with the fraction of microtubules demonstrating outlier orientations. We observed power-law relationship between the ordered state of the system and the degree to which the rotational dynamics of the director deviated from a simple, linear variation in time.
DNA Encoded Glycan Arrays: Soluble Glycopolymers for Improving Detection of Glycan
Austin Michalak and Kamil Godula, UCSD
Interactions Glycans are present in all cells, and have tremendous functions in biology- however, the glycome is still poorly characterized compared other biological molecules. A primary reason why our understanding of glycans lags other major biomolecules is the lack of detection and amplification methods for glycan binding interactions. To address these limitations in tools to probe glycan interactions, I have developed a glycopolymer probe with cleavable reporter elements, such as a fluorophore or oligonucleotide barcode. In this “soluble microarray”, the use of oligonucleotides as a readout is an attractive alternative to the “gold standard” of traditional microarray analysis. In contrast with the established microarray technologies, DNA is affordable, fast, and easy to work with in many labs. The increased sensitivity rendered by DNA amplification will allow this system to detect low affinity glycan interactions which were previously below limit of detection, and profile whole organisms in a multiplexed manner.
Matrix-mediated glycocalyx engineering for the patterning of growth factor gradients across embryoid bodies
Logan Laubach and Kamil Godula, UCSD
Current three-dimensional (3D) cellular models of development are limited to introduction of soluble factors without the ability to localize GFs to specific layers. FGF2 was recruited to its cell surface receptor by glycosaminoglycan (GAG) mimetics to direct stem cell differentiation into neural rosettes. This provided the basis for testing the incorporation of these GAG mimetics into a 3D cell scaffold, more representative of a developing embryo or organoid. Altering the structure of the GAG mimetics affected their penetration into the EBs. The longer the GAG mimetic, the larger the molecular weight and size, thus taking longer to incorporate into the EB compared to a shorter GAG. The short GAG mimetic recruited FGF2 throughout the EBs showing their ability to localize GFs in a 3D cell scaffold. These GAG mimetics show promise as tools to further explore stem cell differentiation through the recruitment of growth factors in three dimensional cell models.