Symposium Organizers
Virginia L. Ferguson University of Colorado
John X.-J. Zhang The University of Texas-Austin
Conrad Stoldt University of Colorado
Carl P. Frick Abbott Vascular
OO1: Interface Science and Engineering I
Session Chairs
Karl Boehringer
John Zhang
Monday PM, November 26, 2007
Room 206 (Hynes)
9:00 AM - **OO1.1
Future Opportunities in Materials Design.
John Prater 1
1 Materials Science Division, US Army Research Office, Research Triangle Park, North Carolina, United States
Show Abstract9:30 AM - **OO1.2
Calcite Shape Modulation Through the Lattice Mismatch Between the Self-assembled Monolayer Template and the Nucleated Crystal Face.
Boaz Pokroy 1 , Joanna Aizenberg 1
1 School of Engineering and Applied Sciences , Harvard, Cambridge, Massachusetts, United States
Show AbstractThe nucleation and growth of crystals formed by organisms in the course of biomineralization are precisely controlled by the organism. This control is achieved by specific macromolecules, both in the form of templates and growth-modifying additives. The most abundant biogenic crystal is calcite – the most stable polymorph of calcium carbonate.Numerous studies have shown that the shape and morphology of synthetic calcite crystals can be modulated by the inorganic or organic solution additives. Hardly any studies have to date addressed the concept of controlling the shape of these crystals by organic interfaces. We show here that self-assembled monolayers (SAMs) that template calcite nucleation have two pronounced effects: in addition to inducing the highly oriented crystal growth (the phenomenon that we have extensively described in our previous studies), each SAM induces a clear modification of the calcite shape from its equilibrium rhombohedron. We show that this change in shape originates from the anisotropy of the lattice mismatch strains that develop between the nucleating crystal face and the organic SAM. We present a model that gives qualitative predictions for the shape of crystals grown on a variety of SAM substrates, as a function of the strain, and show that these shapes correlate extremely well with the experimental results. Our study provides a new, interesting concept – the influence of the mismatch strains at the organic/inorganic interfaces on the shape of the templated crystals. We believe that this mechanism might be utilized by organisms in the biomineralization process and that it can be further employed by man to finely tune the shapes of templated crystals by mismatch engineering at the organic/inorganic interface.
10:00 AM - OO1.3
Control of Calcite Crystallisation via Self-Assembled Monolayers.
Colin Freeman 1 , John Harding 1 , Dorothy Duffy 2
1 Engineering Materials, University of Sheffield, Sheffield United Kingdom, 2 Physics and Astronomy, University College London, London United Kingdom
Show AbstractMany organisms (including ourselves) grow minerals either internally (bones and teeth) or externally (shells). These organisms are able to exert high levels of control over crystal growth to produce high energy surfaces and complex shapes. This control is achieved by using large organic molecules, such as peptides and polysaccharides or arrays of molecules. Understanding and emulating these processes is a major challenge. Experimentally, selective surface growth is observed through using different organised molecular structures (e.g. self-assembled monolayers) that present a regular surface at which crystallisation can occur. By adjusting the packing density, ionisation, orientation and identity of the terminal functional groups in the self-assembled monolayers (SAMs) the expression of particular crystal faces can be controlled e.g [1].The mechanisms of selection are still not well understood and much debate remains over the contibutions of different factors [2-5]. Using theoretical models can provide us with the advantage of atomic scale visualisation and direct control over the environment. We have used potential based molecular dynamics to simulate a system that has been experimentally demonstrated to exert control over crystal growth [1,2]: the crystallisation of amorphous calcium carbonate on SAMs terminated with carboxylic acid groups. We have simulated the formation of a calcite nanoparticle at the SAM surface over the timescale of the simulations. Our calculations explore the effects of different monolayer properties: ionisation; head group orientation; charge density and epitaxy to the calcite crystal. The results demonstrate that ionisation of the monolayer is vital to stimulate crystallisation. However, we also report how the other factors contribute and can enhanceor inhibit the rate of crystallisation to differing degrees. When viewed together the results suggest that localised charge density effects may actually be hidden with epitaxial arguments and be the main control over the crystallisation of the calcite.[1] J. Aizenberg, A. J. Black, G. M. Whitesides, J. Am. Chem. Soc., 121, (1999), 4500[2] Y.-J. Han, J. Aizenberg, Angew. Chem. Int. Ed., 42, (2003, 3668[3] D. Volkmer, N. Mayr, M. Fricke, J. Chem. Soc., Dalton Trans., (2006), 4889[4] D. M. Duffy, J. H. Harding, Langmuir, 21, (2005), 3850[5] A. M. Travaille, L. Kaptijn, P. Verwer, B. Hulsken, J. A. A. W. Elemans, R. J. M. Nolte, H. van Kempen, J. Am. Chem. Soc, 125, (2003), 11571
10:15 AM - OO1.4
The Electrolytic Precipitation of Calcium Phosphate on Titanium Alloy.
Yen Pang Liu 1 , Fereshteh Ebrahimi 1
1 MSE, UF, Gainesville, Florida, United States
Show AbstractCalcium phosphate coatings are deposited on metallic medical implants for better bone fixation. Plasma-spraying is a commercially used technique for fabricating calcium phosphate coatings. However, thermally induced degradation of the substrate and thermal decomposition of the coating limit the application of this technique. Electrochemical technique has received much attention because it can be applied on irregular substrate shapes and under low temperatures. Calcium phosphate can be deposited in several crystal structures among which hydroxyapatite (HA) is the structure found in natural bones. The objective of this research was to establish electrodeposition parameters that result in formation of HA. We electrodeposited calcium phosphate on Ti6Al4V alloy substrate using different deposition parameters to investigate the precipitation process of calcium phosphate. The coatings were characterized for their crystal structure, composition, morphology, and crystal size. Consistent with previously reported results, increases in electrolyte temperature and current density promoted the formation of HA. Above 60oC the coating consisted of 100% HA and brushite was found at lower temperatures. Detailed analysis revealed the presence of a thin hydroxyapatite layer next to the substrate surface and under the brushite coating. Short time depositions confirmed that hydroxyapatite layer forms first before the nucleation of brushite can take place. In this presentation the mechanisms of crystallization of calcium phosphate using the electrodeposition technique will be discussed.
10:30 AM - **OO1.5
Neuroscience on a Chip.
Albert Folch 1
1 , University of Washington, Seattle, Washington, United States
Show AbstractCell culture methodology has remained basically unchanged for almost a century: it consists essentially of depositing a large population of cells on a homogeneous substrate and immersing the cells in a homogeneous fluid medium. This approach is becoming increasingly expensive to scale up and cannot mimic, neither in space nor in time, the rich biochemical and biophysical complexity of the cellular microenvironment, including the substrate, the surrounding cells, and the medium. In studies of nerve cells, the highly branched, multicellular tissue architecture makes it particularly challenging to mimic neuronal interactions in dissociated cell cultures.Microtechnology offers the attractive possibility of modulating the microenvironment of single cells and, for the same price, obtain data at high throughput for a small cost. Microfluidic devices and substrate micropatterning combined promise to play a key role for several reasons: 1) the dimensions of microchannels or patterns can be comparable to or smaller than a single cell; 2) the unique physicochemical behavior of liquids confined to microenvironments enables new strategies for delivering compounds to cells on a subcellular level; 3) the devices occupy a small footprint (thus more microscopy data can be acquired per image) and a small volume (thus they consume small quantities of precious/hazardous reagents, reducing the cost of operation/disposal); and 4) they can be mass-produced in low-cost, portable units. Not surprisingly, in recent years there has been an eruption of microfabricated implementations of a variety of traditional bioanalysis techniques. I will review the latest efforts of our laboratory in the development of cell-based microdevices for neurobiology studies, such as neuromuscular synaptogenesis, axon growth, and olfaction.
11:00 AM - OO1:Interfaces I
BREAK
11:30 AM - **OO1.6
Self-assembly from Nano to Milli Scales.
Karl Boehringer 1
1 Electrical Engineering, University of Washington, Seattle, Washington, United States
Show AbstractSelf-assembly is a well-established concept in physics, chemistry, and molecular biology. Only recently have researchers come to understand how self-assembly can be extended to engineered components, thus providing a promising alternative to conventional manufacturing. Instead of pick-and-place, positioning of components is accomplished by minimization of potential or interfacial energies. In this presentation, we discuss the principles and applications of engineered self-assembly, including 'programmable' self-assembly of silicon microcomponents and a 'protein laser printer.'
12:00 PM - OO1.7
Growth of Phospholipid Membrane Systems on Self-organized Semiconductor Templates.
Christian Teichert 1 , Gerald Trummer 1 , Daniel Pressl 1 , Gregor Hlawacek 1
1 Institute of Physics, University of Leoben, Austria, Leoben Austria
Show AbstractSpontaneous pattern formation during epitaxial growth or ion erosion of semiconductor wafers offers an elegant route towards large-area nanostructured surfaces. In the case of semiconductor heteroepitaxy strain relief leads to the formation of nanofaceted three-dimensional crystallites, which may self-organize into quasiperiodic arrays [1]. For low energy ion erosion of compound semiconductors, close-packed arrays of hemispherical dots with diameters between 30 nm - 50 nm are observed due to the interplay of sputtering and surface diffusion effects [2]. Since these self-organized nanostructure arrays cover the entire substrate, they can serve as large-area nanopatterned templates for subsequent deposition of all kinds of materials as was recently demonstrated for the deposition of magnetic thin films [3]. Here, we use atomic force microscopy (AFM) to study the formation of solid-supported lipid bilayers on a variety of nanofaceted self-organized SiGe films on Si(001) and ion eroded semiconductor surfaces in comparison to smooth Si(001) wafers. 1-Palmitoyl-2-Oleoyl-sn-Glycero-3-Phosphoethanolamine (POPE) and 1,2-Dipalmitoyl-sn-Glycero-3-Phosphocholine (DPPC) were used as model systems. The resulting film morphology and the change of surface roughness have been investigated as a function of initial roughness and morphology of the substrate. Phase imaging is used to distinguish between the soft lipid layers and the hard semiconductor substrate. On SiGe templates showing a dislocation network it was found that the ridge trench structures appearing at the surface guide the terrace edges of DPPC layers. When the lipid coverage is very low, 100 nm x 100 nm pits of {105} faceted SiGe film act as preferential deposition sites for lipid bilayers resulting in ordered arrays of small POPE islands. [1] C. Teichert, Phys. Rep. 365 (2002) 335.[2] S. Facsko, et. al., Science 285 (1999) 1551; T. Bobek, et al., Phys. Rev. B 68 (2003) 085324.[3] C. Teichert, Appl. Phys. A 76 (2003).
12:15 PM - OO1.8
Characterization of DNA Oligonucleotides Immobilized on Arsenic-terminated GaAs Surfaces (001).
Joon Hyuk Yang 1 , Lourdes Salamanca-Riba 1 , Mohamad Al-Sheikhly 1
1 Materials science and engineering, University of Maryland, College Park, Maryland, United States
Show AbstractImmobilization of DNA on a variety of substrates has recently become an important field because of its potential applications in molecular electronics, biosensors, and other diagnostic applications. We have successfully attached thiolated, single-stranded 8-bases, 28-base and 100-base DNA to As-terminated GaAs by means of covalent bonding between the sulfur and arsenic. X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) with a fluid imaging cell were used to investigated the morphology of the oligos and their orientation with the surface of the GaAs as a function of length in the presence and absence of water and ionic strength of the solution. Some undesired bonding between N- and O- from the DNA and the As from the GaAs substrate also takes place even for the short ssDNA oligos. 6-mercapto-1-hexanol (MCH) was added enhance the brush-like structure of the probes via displacement reaction of the N-As and O-As bonding by the stronger/preferred S-As bonding. Our XPS results show that with increasing MCH concentration, the ratio of atomic concentration of S/As increases while the ratios of atomic concentration of N/As and O/As decrease. AFM results show a change in the surface morphology of the probes in the presence or absence of a good solvent such as water. In the absence of water the DNA oligos collapse forming relatively large size bundles of probes while in the presence of water the bundle size decreases and the oligos orient at a higher angle with the surface of the GaAs substrate. In addition, the effect of the ionic strength of the solution, which is controlled by [NaCl], to establish charge screening will be discussed. This work was supported by the Department of Energy-Innovations under the Nuclear Infrastructure and Education (INIE) program
12:30 PM - OO1.9
Dynamics of Chain Molecules Adsorbed on Lipid Bilayers.
Shashishekar Adiga 1 , Tapan Desai 2
1 Materials Science Division, Argonne National Laboratory, Argonne, Illinois, United States, 2 Material Sciences Department, Idaho National Laboratory, Idaho Falls, Idaho, United States
Show AbstractThe problem of translational diffusion of adsorbed chain molecules on phospholipid bilayers is not only of fundamental interest in biology but also in applied areas such as drug delivery and smart nanostructures. Experimental investigation of diffusion of DNA molecules adsorbed (dilute coverage) on supported lipid bilayers has showed that the center of mass diffusion coefficient scales with the degree of polymerization as D ~ N-1 [1]. Recently, simulations of a single adsorbed polymer chain at a solid-liquid interface have revealed that D ~ N-x, with x = 3/4 and 1 for smooth and corrugated surfaces, respectively [2]. These simulations substantiate that the diffusion of polymer adsorbed on lipid bilayer is similar to the corrugated surface case where friction of the adsorbed molecule dominates and hydrodynamic coupling between the monomers becomes less important. Here, coarse grained molecular dynamics simulations are performed to investigate the dynamics of polymer chains adsorbed on lipid bilayers. In particular, we will discuss the dynamics of polymer chains at bilayer-water interface with respect to scaling of diffusion coefficient, radius of gyration and relaxation time with degree of polymerization. We will also discuss the effect of polymer adsorption on diffusion of lipid molecules and investigate the phenomenon of slaved diffusion as reported by recent experiments [3].References[1] B. Maier and J. O. Radler, Phys. Rev. Lett. 82,1911 (1999).[2] T. G. Desai, P. Keblinski, S. K. Kumar and S. Granick, Phys. Rev. Lett. 98, 218301 (2007).[3] L. Zhang and S. Granick, Proc. Nat. Acad. Sci. 102, 1918 (2005).
12:45 PM - OO1.10
Effect of Ligand Density Gradient on the Adhesion Kinetics of Biological Membranes.
Alireza Sarvestani 1 , Esmaiel Jabbari 1
1 Chemical Engineering, University of South Carolina, Columbia, South Carolina, United States
Show AbstractThe attachment of cells to substrates is mediated by the interactions between the transmembrane receptors and surface ligands. The kinetics of adhesion is controlled by mobility of the receptors along the membrane and the concentration of ligands on the substrate. A mathematical model is proposed to study the effect of ligand density gradient on the adhesion kinetics of a single giant vesicle with mobile transmembrane receptors. A numerical method has been used to solve the corresponding diffusion equation with varying boundary condition due to the gradient in ligand density. The extent of adhesion, the speed of expansion at the front edge of adhesion zone, and distribution of the binders along the membrane are evaluated as a function of time. For a surface with increasing ligand concentration at a certain direction, it is shown that at each time point, the rate of expansion of adhesion zone is lower and the vesicle-substrate contact area is smaller than the corresponding values in the case of adhesion on a surface with uniform ligand density. In addition, it is shown that the displacement of the front edge of adhesion zone does not scale with the square root of the elapsed time, as observed in adhesion on substrates with uniform ligand concentration.
OO2: Biological Adhesion and Cohesion
Session Chairs
Virginia L. Ferguson
Daniel Fletcher
Monday PM, November 26, 2007
Room 206 (Hynes)
2:30 PM - **OO2.1
Lotus Effect: Roughness-Induced Superhydrophobic Surfaces.
Bharat Bhushan 1
1 MEMS/NEMS (NLIM), Ohio State University, Columbus, Ohio, United States
Show AbstractOne of the requirements for materials in micro/nanoscale applications is to decrease wetting, for water repellency and to facilitate fluid flow. When two bodies are brought in contact, any liquid present at the interface forms menisci, which increases adhesion/friction and the magnitude is dependent upon the contact angle. For non-wetting liquids, the contact angle with a rough surface is greater than with a flat surface and may approach 180o, as reported for leaves of water-repellent plants, such as lotus. These leaves are superhydrophobic due to the presence of micro/nanobumps and a thin wax film on the surface of the leaf. For fluid flow applications, superhydrophobic surfaces should have high contact angle and low contact angle hysteresis. It is desirable that surfaces form a stable composite interface with air pockets between solid and liquid[1]. We have fully characterized various leaf surfaces on the micro- and nanoscale and have attempted to separate out the effects of the micro- and nanobumps and the wax on the hydrophobicity[2]. The next logical step in realizing superhydrophobic surfaces that can be produced in the lab is to learn from the hydrophobic leaves and design surfaces based on that learning. We have explored the effect of micro- and nanopatterning on hydrophobicity, adhesion and friction for two different polymers with three types of patterned asperities, low aspect ratio, high aspect ratio, and lotus replica[3,4]. Scale dependence on adhesion and friction was also studied using AFM tips of various radii. We have also explored wetting properties for silicon surfaces patterned with pillars of two different diameters and heights with varying pitch values deposited with a hydrophobic coating[5]. We showed how static contact angles, hysteresis angle, and tilt angle vary with different pitch values on the patterned silicon surfaces. We studied the effect of droplet size on contact angle by using evaporation studies. A transition criterion was developed to predict when air pockets cease to exist. We have used an environmental scanning electron microscopy (ESEM) to study the effect of droplet size of about 20 μm radius on the contact angle.[1]Nosonovsky, M. and Bhushan, B., “Roughness Optimization for Biomimetic Superhydrophobic Surfaces,” Microsyst. Technol. 11, 535-549 (2005)[2]Bhushan, B. and Jung, Y. C., “Micro- and Nanoscale Characterization of Hydrophobic and Hydrophilic Leaf Surfaces,” Nanotechnology 17, 2758-2772 (2006)[3]Burton, Z. and Bhushan, B., “Hydrophobicity, Adhesion and Friction Properties of Nanopatterened Polymers and Scale Dependence for MEMS/NEMS,” Nano Letters 5, 1607-1613 (2005)[4]Jung, Y. C. and Bhushan, B., “Contact Angle, Adhesion and Friction Properties of Micro- and Nanopatterned Polymers for Superhydrophobicity,” Nanotechnology 17, 4970-4980 (2006)[5]Bhushan, B., Nosonovsky, M., and Jung, Y. C., “Towards Optimization of Patterned Superhydrophobic Surfaces,” J. R. Soc. Interf. 4, 643-648 (2007).
3:00 PM - OO2.2
Adhesion and Friction Force Coupling of Gecko Setal Arrays: Implications for Patterned Adhesive Surfaces.
Boxin Zhao 1 , Noshir Pesika 1 , Kenny Rosenberg 1 , Hongbo Zeng 1 , Yu Tian 1 2 , Patricia McGuiggan 1 , Kellar Autumn 3 , Jacob Israelachvili 1
1 Chemical Engineering Deaprtment and Materials Reserach Laboratory, Univerity of california, Santa Barbara, Santa Barbara, California, United States, 2 State Key Lab of Tribology, Department of Precision Instruments, Tsinghua University, Beijing China, 3 Department of Biology, Lewis & Clark College, Portland, Oregon, United States
Show AbstractThe extraordinary climbing ability of geckos is partly attributed to the fine structure of their toe pads which contain arrays consisting of thousands of micron-sized stalks (setae) which are in turn terminated by millions of finger-like pads (spatulae) having nano-scale dimensions. Using a Surface Forces Apparatus (SFA) we have investigated the dynamic sliding characteristics of setal arrays subjected to various loading, unloading, and shearing conditions at different angles. Setal arrays were glued onto silica substrates and, once installed into the SFA, brought towards a polymeric substrate surface, then sheared. Lateral shearing of the arrays was initiated along both the ‘gripping’ and ‘releasing’ directions of the setae on the foot pads. We find that the anisotropic microstructure of the setal arrays gives rise to quite different adhesive and tribological properties when sliding along these two directions, depending also on the angle that the setae subtend to the surface. Thus, dragging the setal arrays along the gripping direction leads to strong adhesion and friction forces (as required during contact and attachment), whereas when shearing along the releasing direction, both forces fall to almost zero (as required during rapid detachment). The results and analysis provide new insights into the biomechanics of adhesion and friction forces in animals, the coupling between these two forces, and the specialized structures that allow them to optimize these forces along different directions during movement. Our results also have practical implications for designing reversible and responsive adhesives, and articulated robotic mechanisms.
3:15 PM - OO2.3
Mechanics of Single-Cell Adhesion Strength Investigated Using a Microfluidic Device.
Kevin Christ 1 , Kyle Williamson 2 , Kristyn Masters 2 , Kevin Turner 1
1 Mechanical Engineering, University of Wisconsin-Madison, Madison, Wisconsin, United States, 2 Biomedical Engineering, University of Wisconsin-Madison, Madison, Wisconsin, United States
Show AbstractThe strength of cell-substrate adhesion is essential in physiological and disease processes as well as the development of implantable biomaterials. Despite the importance of cell adhesion, there remains a lack of understanding of the fundamental factors that determine the adhesion strength. In this work the role of cell shape and area, focal adhesion arrangement, loading rate, and extracellular matrix (ECM) protein concentration on the dynamic adhesion strength is investigated at the single-cell scale using a microfluidic adhesion assay and computational modeling. NIH 3T3 fibroblasts were sparsely seeded and cultured on collagen coated glass surfaces in a 50 micron high microchannel. Adhesion strength was measured by applying a monotonically increasing shear-stress, via a controlled flow, and monitoring individual cell detachment events using optical microscopy. The small dimensions of the microchannel allow high shear stresses to be achieved under laminar flow conditions, thus permitting controlled measurements of well-spread and strongly adhered cells. As the technique measures the adhesion of individual cells, the heterogeneity of adhesion strength in a population can be captured using this assay. The strengths of NIH 3T3 fibroblasts on collagen coated surfaces of varying density were measured using the microfluidc assay and the strength values were correlated to measurements of cell shape, area, ECM protein concentration, and focal adhesion arrangement. Results show that cell geometry plays a key role in the stress required for cell detachment and that increased cell spreading results in higher adhesion strengths. Furthermore, several distinct failure modes were observed when cells detach from the surface, and adhesion strength was found to correlate strongly with these failure modes. To interpret the experimentally observed trends, computational fluid dynamics models were used to predict the stress distribution on the cell induced by the flow and were coupled to a finite element simulation to estimate the loads at the cell-substrate interface at failure. These experimental and modeling results will enhance the understanding of the underlying mechanics of cell-substrate adhesion strength.
3:30 PM - OO2.4
Computational Analysis of Adhesion Forces during Indentation of Cells Using Atomic Force Microscopy.
Yong-Wei Zhang 1 , Chun-Yu Zhang 1
1 Materials Science and Engineering, National University of Singapore, Singapore Singapore
Show AbstractThe mechanical responses of a cell to external stimuli and the adhesion of a cell to another cell or to an extracellular matrix are of great importance in many biological processes and biotechnological applications. Indentation using atomic force microscopy (AFM) has been emerging as a powerful technique to study the biological and mechanical behavior of living cells due to its capability of characterizing both the mechanical response of cells and the cellular adhesion behavior as well. By pushing the AFM tip (or a microbead attached to the end of the cantilever) against the sample to a certain depth and then retracting it, a complete force-indentation curve can be collected. By using an appropriate indentation model, the mechanical properties of cells may be determined from the indentation loading curve and the adhesion behavior may be obtained from the unloading curveIn the present work, a kinetics model was developed to investigate the indentation of an Atomic Force Microscopic (AFM) tip on a cell with adhesion mediated by receptor-ligand binding. Parametric studies were conducted to investigate the effects of indentation rate, indentation depth, indenter size and the mechanical properties of cells on the adhesion force. It was found that the presence of the adhesion between the cell and AFM tip may affect both the loading curve and unloading curve, which may in turns change the extracted elastic modulus value using the conventional indentation models. It was found that an increase in the receptor-ligand reaction rate may lead to a transition from an exponential decay of the maximum adhesion force with the indentation rate to an exponential growth of the maximum adhesion force with the indentation rate. It was also shown that the maximum adhesion force was sensitive to the indenter size while its dependence on indentation depth was related to both the receptor-ligand reaction rate and the viscoelasticity of cells.
3:45 PM - OO2.5
Biofilm Cohesiveness Measurement Using a Novel Atomic ForceMicroscopy Methodology.
Greg Haugstad 1 , Francois Ahimou 2 3 , Michael Semmens 2 , Paige Novak 2
1 Institute of Technology Characterization Facility, University of Minnesota, Minneapolis, Minnesota, United States, 2 Department of Civil Engineering, University of Minnesota, Minneapolis, Minnesota, United States, 3 , 3M Company, Maplewood, Minnesota, United States
Show AbstractBiofilms can be undesirable, as in those covering medical implants, and beneficial, such as when they are used for waste treatment. Because cohesive strength is a primary factor affecting the balance between growth and detachment, its quantification is essential in understanding, predicting, and modeling biofilm development. In this study, we developed a novel atomic force microscopy (AFM) method for reproducibly measuring, in situ, the cohesive energy levels of moist 1-day biofilms. The technique is performed with off-the-shelf instrumentation. The biofilm was grown from an undefined mixed culture taken from activated sludge. The volume of biofilm displaced and the corresponding frictional energy dissipated were determined as a function of biofilm depth, resulting in the calculation of the cohesive energy density. Our results showed that cohesive energy increased with biofilm depth, from 0.1 nJ/m3 to 2.0 nJ/m3. This observation was reproducible, with four different biofilms showing the same behavior. Cohesive energy also increased from when calcium (10 mM) was added to the reactor during biofilm cultivation. These results agree with previous reports on calcium increasing the cohesiveness of biofilms. In addition, the protein and polysaccharide concentrations within the biofilm depths, as well as the dissolved oxygen (DO) concentration profiles within the biofilms, were measured. It was found that biofilm cohesion, though increasing with depth, did not with age. Level of biofilm cohesive energy per unit volume was strongly correlated with biofilm polysaccharide concentration, which increased with depth in the membrane-aerated biofilm. In a 12-day-old biofilm, DO also increased with depth and may therefore be linked to polysaccharide production. In contrast, protein concentration was relatively constant within the biofilm and did not appear to influence cohesion.
4:00 PM - OO2: Adhesion
BREAK
OO3: Cells and Interfaces
Session Chairs
Conrad R. Stoldt
Joel Voldman
Monday PM, November 26, 2007
Room 206 (Hynes)
4:30 PM - **OO3.1
Microtechnological Approaches to Studying Embryonic Stem Cell Self-renewal.
Joel Voldman 1
1 , Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractEmbryonic stem cells are cells that have the unusual ability to self-renew for extended periods of time in culture yet are pluripotent in that they can differentiate into any cell type in the adult animal. The decision to self-renew, differentiate, or commit apoptosis is determined in large part by diffusible and contact-mediated signals that come from the microenvironment. We are developing microtechnology to manipulate these signals in order to study the factors involved in embryonic stem cell self-renewal. These tools include microfluidic perfusion, by which we can modulate diffusible signaling between cells, and matrix-independent cell patterning via bio-flipchips, which allows control over diffusible and juxtacrine cell-cell signaling. These tools provide new approaches for studying stem cell biology, and, more broadly, cell-cell signaling in multi-cellular systems.
5:00 PM - OO3.2
Buckling of Microtubules in Living Cells Modulated by Surrounding Cytoplasm and Filament Network.
Teng Li 1 2
1 Department of Mechanical Engineering, University of Maryland, College Park, Maryland, United States, 2 Maryland NanoCenter, University of Maryland, College Park, Maryland, United States
Show AbstractThe mechanics of living cells is largely determined by their cytoskeleton, a dynamic network of microtubules and protein filaments in the cytoplasm. The difference in the rigidity of various cytoskeletal components is huge. For example, the bending rigidity of microtubules is about 100 times that of actin filaments. As the stiffest cytoskeletal component, the microtubules mostly bear compressive load. Experiments show that stimulation of cell contraction causes the microtubules to buckle. Although the persistence length of the microtubules is tens of times larger than the typical size of a cell, the microtubules in vivo often severely buckle into short wavelength. By contrast, other in vitro experiments show that isolated microtubules exhibit a classic Euler buckling instability, resulting in the formation of a single long-wavelength arc. Here we describe a mechanics model to explain the above inconsistency and quantify the wavelength, growth rate and amplitude of the microtubule buckling.In living cells, the stiff microtubules are surrounded not only by the soft elastic filament network, but also by the viscous cytoplasm. The buckling of microtubules causes not only the elastic deformation of the filament network, but also the viscous flow of the cytoplasm. In turn, both these processes influence the buckling wavelength and amplitude of the microtubules. For example, buckling at long wavelength requires viscous mass transportation over long distance, which is unlikely to occur incipiently. We study the coupled effect of the viscous cytoplasm and the elastic filament network on the microtubule buckling. The critical wavelength and the growth rate of the microtubule buckling are studied by a linear perturbation analysis. Furthermore, the buckling of the microtubule reaches a kinetically constrained equilibrium state, from which the buckling amplitude of the microtubule can be determined. Our quantitative mechanics model sheds light on developing new and robust methods to measure various in vivo mechanical properties of subcellular structures, e.g., bending rigidity of microtubules, elastic modulus of filament network, and viscosity of cytoplasm. The model can also be readily generalized to study the deformation of hard engineering materials at soft bio-interfaces.
5:15 PM - OO3.3
Interaction Forces and Mechanics of Cellular Membranes using Novel Atomic Force Microscopy Probes.
Benjamin Almquist 1 , Nicholas Melosh 1
1 Materials Science & Engineering, Stanford University, Palo Alto, California, United States
Show AbstractThe cell membrane is one of the most vital components of a cell, the gatekeeper into and out of the cytoplasm; studies ranging from neural activity to drug discovery to fundamental cell physiology rely upon controlling electronic or chemical flow across this barrier. Integration of inorganic structures with the cell membrane is poorly understood, and current techniques involve creating holes in or puncturing cell membranes to control access into the cell. However, functionalized materials with nanoscale hybrophobic layers may be able to directly fuse the lipid membrane edge to an inorganic structure, enabling non-disruptive electrical and chemical access into the cell. We have tested whether nanoscale inorganic probes integrate into the hydrophobic core of a lipid bilayer using an AFM probe with hydrophobically functionalized bands 5-20 nm thick at the end of the tip. We quantitatively measure the adhesion strength between the probe and the lipid bilayer, and correlate this molecule-membrane force with the molecular structure. We find the thickness of the nanoscale band and the identity of the hydrophobic molecules alters the ability to fuse to the membrane.
5:30 PM - OO3.4
Influence of EGF on HaCaT (Human Epithelial) Cells by Simultaneous AFM and 3-D Optical Deconvolution.
Kate Bagnoli 1 , Ali Langston 1 , Brian Aneskievich 2 , Bryan Huey 1
1 Institute of Materials Science, University of Connecticut, Storrs, Connecticut, United States, 2 Pharmaceutical Sciences, University of Connecticut, Storrs, Connecticut, United States
Show AbstractThe widely applied optical technique of 3-D fluorescence microscopy and deconvolution has been uniquely combined with the high force sensitivity and spatial resolution of atomic force microscopy (AFM). These complimentary methods allow novel simultaneous optical and mechanical measurements of biological cells, particularly the cell shape, volume, and membrane compliance. In this study, living HaCaT (human epithelial) cells are monitored in vitro at 37 C, both with and without exposure to epidermal growth factor (EGF). Finally, results applicable to more general cell-AFM studies include optically characterizing living cell dimensions during concurrent AFM measurements. This reveals extensive cell deformation before statistically significant forces are detected with the AFM, suggesting that 3-D optics provide a more accurate measure of cell height than AFM imaging.
5:45 PM - OO3.5
Sol-gel Encapsulated Gold-Silica Nanoshells for SERS Detection of Peptide – Gold Binding.
Sandra Bishnoi 1 , Yu-Jen Lin 1
1 BCPS, Illinois Institute of Technology, Chicago, Illinois, United States
Show AbstractThe study of the interfaces between biological molecules and nanomaterials has become increasingly important to improve drug delivery and biological sensing. Surface enhanced Raman scattering (SERS) is a powerful technique that could be used to investigate these interfaces, but suffers from reproducibility issues under in vitro conditions. In order to increase our understanding of the mechanism of interactions between gold nanoparticles and "gold binding peptides," we have created sol-gel substrates for surface enhanced Raman scattering studies. By taking advantage of the large enhancements obtained with gold-silica nanoshells (~1010 over normal Raman),1 robust, SERS substrates with high reproducibility have been obtained. Using a protocol commonly used to immobilize proteins,2 we have created “biologically friendly” SERS sensors for the study of the interactions between gold binding peptides and gold nanoparticles. Specifically, by combining tetraethyl orthosilicate (TEOS), methyltrimethoxysilicate (MTMS), phosphate buffer, and gold nanoparticles we have created sol-gels with minimal fluorescence and Raman backgrounds. Since organic solvents have been eliminated in the synthesis, these porous materials are ideal for studying the attachment of various biological molecules to gold surfaces.1 Jackson, J. B.; Halas, N. J. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 17930-17935.2 Soyoun Kim, Youngdeuk Kim, Philseok Kim, Jeongmin Ha, Kyunyoung Kim, Mijin Sohn, Jin-San Yoo, Jungeun Lee, Jung-ah Kwon, and Kap No Lee. Anal. Chem. 2006, 78(21), 7392 -7396.
Symposium Organizers
Virginia L. Ferguson University of Colorado
John X.-J. Zhang The University of Texas-Austin
Conrad Stoldt University of Colorado
Carl P. Frick Abbott Vascular
OO4: Interface Science and Engineering II
Session Chairs
Mehmet Sarikaya
Conrad R. Stoldt
Tuesday AM, November 27, 2007
Room 206 (Hynes)
9:30 AM - **OO4.1
Molecular Biomimetics – Genetically Linking Biology and Material Using Designed Peptides for Technology and Medicine.
Mehmet Sarikaya 1
1 Materials Science and Engineering, University of Washington, Seattle, Washington, United States
Show AbstractIn technology and medicine, biological entities, such as proteins, enzymes, DNA, cell etc., are interfaced with solids mostly either by synthetic linkers (e.g., thiols and silanes), often producing non-natural interfaces, or serendipitously via non-specific interactions, both of which pose eventual issues that limit their wide-spread and lasting implementations. In organisms, however, solids are interfaced with proteins and peptides that provide chemical and mechanical coupling, linking the biology of the organisms and the solid materials (e.g., mineral or metal) and provide an entity that functions as a hybrid (e.g., bone, shells and spicules). Using biocombinatorics, we have adapted molecular biology protocols to select short (7-12 aa) peptides that specifically bind to individual solids, e.g., metals, ceramics, semiconductors. Through molecular characterization, e.g., quantitative binding, experimental and computational peptide-structure determination coupled with bioinformatics, it is now possible to control peptide evolution through sequence similarity, multimerization and architectural control to design next and higher generation peptides for specific applications including directed enzyme or nanoparticles immobilization, developing biocompatible surfaces and interfaces, and biosynthesis/biofabrication. We plan to present a summary of molecular biomimetics activities from our Center and collaborative groups directed towards both nanotechnology and medicine. Supported by NSF-UW/MRSEC, AFOSR-Bioinspired Materials, NSF-BioMat, and NIH.
10:00 AM - OO4.2
Guiding Neurons with Micropatterned Solid Substrates.
J. Nevill 1 , Luke Lee 1 , Mu-ming Poo 2 , Sarah Heilshorn 3
1 Bioengineering, University of California, Berkeley, California, United States, 2 Molecular and Cell Biology, University of California, Berkeley, California, United States, 3 Materials Science and Engineering, Stanford University, Stanford, California, United States
Show AbstractDuring development, young neurons sprout several processes (called neurites) that probe their environmental niche. Based on a combination of intracellular signaling molecules and extracellular cues, one neurite develops into the axon (the portion of the neuron that can transmit electrical and chemical signals) while the remaining neurites become dendrites. While this developmental program is tightly regulated in vivo, currently there are no methods to control neuronal polarity in vitro. We have designed a protocol to pattern various guidance cues on surfaces to guide the development of rat hippocampal neurons. Soft lithography was used to create microfluidic devices that generate patterned regions of guidance cues physically adsorbed to the culture substrate within a polypeptide matrix. This protocol is compatible with both small-molecule guidance cues as well as protein guidance cues such as the neurotrophins semaphorin 3a and brain-derived neurotrophic factor. Our patterned surfaces are able to specify axon initiation as well as guide the path-finding of the developing axon. These patterns are currently being used to study the development of human embryonic stem cell derived neurons.Complex patterns of multiple bioactive molecules on a single solid substrate can also be created using these microfluidic techniques. As proof of principle, arrays of three bioactive molecules were patterned on glass substrates. Rat hippocampal neurons plated onto these substrates aligned into ordered arrays, initiated axons at a specified location, and sprouted neurites that followed predetermined paths. This is a first step towards guiding the formation of simple neuronal circuits in vitro. The ability to guide neuronal development on patterned substrates has potential application in bioMEMs devices, living neural networks, biosensors, and scaffolds for tissue engineering of nerve grafts.
10:15 AM - OO4.3
Neuronal Adhesion and Growth on Modified Diamond-Like-Carbon Substrates.
Frederik Claeyssens 1 , Ed Regan 2 , Eric Mayer 3 , James Uney 2 , Andrew Dick 3 , Joe McGeehan 4 , Stephen Kelly 2
1 School of Chemistry, University of Bristol, Bristol United Kingdom, 2 Henry Wellcome Laboratories for Integrative Neuroscience and Endocrinology, University of Bristol, Bristol United Kingdom, 3 Bristol Eye Hospital, University of Bristol, Bristol United Kingdom, 4 Department of Electrical and Electronic Engineering, University of Bristol, Bristol United Kingdom
Show AbstractMaterials that produce minimal immunogenic and thrombogenic responses in vivo are potentially useful for the coating of many orthopaedic implants and implantable devices. The physical properties of the material also bear much importance on functional “life-expectancy” of implants, as wear debris can cause inflammatory responses and release of potentially cytotoxic substances such as metal ions. Diamond-Like-Carbon (DLC) is a hard, wear resistant and chemically inert material that can be deposited onto the surfaces of implants. DLC has been shown to support in vitro growth of multiple cell types including osteoblasts, monocytes, macrophages and neutrophils, without significant alteration in cellular metabolism.[1,2] In vivo tests have demonstrated the high level of biocompatibility of DLC: including year-long implantation of DLC-coated titanium into rabbit skeletal muscle without detectable damage to surrounding tissue.[3]DLC can be deposited via various methods including cathodic arc deposition,[4] Radio Frequent Sputtering [5] and Pulsed Laser Deposition (PLD).[6] Further surface modification of or the incorporation of dopant materials in the deposited films can be used to alter the physical characteristics and haemocompatibility of DLC. Phosphorous doping [7] and surface fluorination[8] have been shown to improve the antithrombogenic properties of DLC.In this study the adhesion, survival and morphology of neuroblastoma N2a cell lines and rat primary cortical neurons is described on DLC thin films on glass substrates deposited at room temperature by PLD. Three different types of DLC films were studied (i) nascent DLC films, (ii) surface oxidised DLC films and (iii) phosphorus doped DLC films. The interaction of cells with DLC substrates is assessed using an adhesion score method and measures of cell metabolic activity are determined using an MTT assay. Morphology of neurons was determined using scanning electron microscopy. These measures were compared to address the neuro-compatibility of the different DLC surfaces. This is correlated with the properties of these materials, assessed with Raman spectroscopy and contact angle measurements. Furthermore, the possibility of patterned growth of neurons on these substrates is explored. [1]W.J. Ma et al., Biomaterials 28 (2007) 1620-1628[2]A. Singh et al., Biomaterials 24 (2003) 5083-5089[3]M. Mohanty et al., Biomolecular Engineering 19 (2002) 125-128[4]B. Bhusban, Diamond and Related Materials 8 (1999) 1985-2015[5]S. Ulrich et al., Surface & Coatings Technology 97 (1997) 45-59[6]A.A. Voevodin et al., Surface & Coatings Technology 82 (1996) 199-213[7]S.C.H. Kwok et al., Diamond and Related Materials 14 (2005) 78-85 [8]T. Saito et al., Diamond and Related Materials 14 (2005) 1116-1119
10:30 AM - OO4.4
Carbon Micro-arrays as a Substrate for Patterned Cell Growth.
Genis Turon Teixidor 1 , Manish Kulkarni 2 , PremPrakash Tripathi 3 , Jamuna Subramaniam 3 , Ashutosh Sharma 2 , Marc Madou 1
1 Mechanical and Aerospace Engineering Department, University of California, Irvine, Irvine, California, United States, 2 Chemical Engineering , Indian Institute of Technology, Kanpur, Kanpur, Uttar Pradesh, India, 3 Biological Sciences and Bioengineering, Indian Institute of Technology, Kanpur, Kanpur, Uttar Pradesh, India
Show Abstract10:45 AM - OO4.5
Magnetically Patterned Co-Cultures for Probing Cell-Cell Interactions.
Edward Felton 1 , Daniel Reich 1 , Yoojin An 2 , Christopher Chen 2
1 Physics and Astronomy, Johns Hopkins University, Baltimore, Maryland, United States, 2 Bioengineering, University of Pennsylvania, Philadelphia, Pennsylvania, United States
Show AbstractThe ability to selectively pattern cultures of biological cells has led to novel approaches to several areas of interest, from tissue engineering to the study of cell-cell interactions. We have introduced ferromagnetic nanowires as a tool for applying forces to cells; their high remanent magnetization allows cells bound to nanowires to be manipulated in low-strength magnetic fields. Interaction of these nanowires with patterned ferromagnetic microstructures allows cells bound to the nanowires to be guided precisely to predetermined positions on substrates. Surface functionalization techniques that control cell adhesion can then be used to retain the cells at these positions for periods of several days. We have exploited these techniques to organize cells into a variety of patterns with a single cell type, and have created arrays of cells in which heterotypic cell pairs are magnetically trapped at each array site. The cell pair yields obtained with this magnetic method are significantly higher than has been obtained by random seeding using non-magnetic techniques. This method of producing isolated heterotypic cell pairs is potentially useful in studies of cell-cell interactions between different cell types.
11:00 AM - OO4:Interfaces
BREAK
11:30 AM - OO4.6
Co-patterning Mobile and Immobilized Proteins on Surfaces to Model Cell-cell Interactions with Cell-surface Contacts.
Darrell Irvine 1 , Bonnie Huang 1
1 Mat. Sci. & Eng./Biological Eng., MIT, Cambridge, Massachusetts, United States
Show AbstractPatterned surfaces are powerful tools for the study of cell functions, and we have a long-standing interest in using patterned surfaces to study T cell biology. T cells are activated by forming a long-lasting cell-cell contact with an antigen presenting cell (APC), forming an immunological synapse at the cell-cell interface. We experimentally modeled T cell-APC interactions by studying the interaction of T cells with synthetic surfaces as artificial APCs. Glass substrates were photolithographically patterned with arrays of ‘activation sites’, 1-10 micron patches of arbitrary geometry displaying T cell receptor (TCR) ligands, surrounded by co-patterned adhesion proteins. A biotinylated polyelectrolyte photoresist previously developed in our laboratory was used, which allows for sequential deposition of proteins into two different regions of a patterned surface (foreground and background) under mild aqueous conditions. Each activation site models a discrete APC; T cells seeded on these surfaces adhere and migrate, and form synapses when they encounter activation sites. In prior work, we immobilized TCR ligands in the activation sites via short poly(ethylene glycol) tethers. However, the protein ligands displayed on the surfaces of APCs are mobile and diffuse in the 2D plane of the membrane; further it is known that T cells binding to ligands on the APC surface cluster these ligands using their cytoskeletons. To provide 2D mobility to ligands in activation sites, we developed a strategy to photolithographically pattern patches of supported lipid membranes as the activation sites, with immobilized adhesion proteins on the surrounding background. Liposomes were fused with photolithographically-pattered resist films, followed by a ‘lift-off’ step to remove lipid from the background regions of the pattern. T cell ligands were anchored to the membrane patches through biotinylated lipids incorporated in the deposited membranes. The mobility of lipids and membrane patch-tethered TCR ligands was characterized using fluorescence recovery after photobleaching techniques, and revealed fluidity comparable to typical supported lipid bilayers on glass. Microscopy studies of T cells seeded on these lipid membrane patch arrays showed that cells halted migration and elevated intracellular calcium on contact with lipid patches bearing TCR ligands, mimicking their response to live APCs. Within minutes of contact, T cells clustered their ligands when interacting with patterned lipid membrane patches. Ongoing studies are comparing the effect of these biophysical changes in ligand presentation (mobile vs. immobilized ligand) on endpoint functional T cell responses (proliferation, cytokine secretion). These surfaces, combining mobile ligands on activation sites with immobilized ligands on the ‘background’, may be of general utility for studying cell-cell interactions and the interface of synthetic materials with living systems.
11:45 AM - OO4.7
Role of Biopolymers on the Crystal Structure and Growth of Hydroxyapatite in Biopolymer/Hydroxyapatite Nanocomposite Bone Biomaterials.
Devendra Verma 1 , Patrick Dunlap 1 , Kalpana Katti 1 , Dinesh Katti 1
1 Civil Engineering, North Dakota State University, Fargo, North Dakota, United States
Show AbstractTuesday, Nov 26Transferred Poster OO6.2 to OO4.7 @ 10:45 AMRole of Biopolymers on the Crystal Structure and Growth of Hydroxyapatite in Biopolymer/Hydroxyapatite Nanocomposite Bone Biomaterials. Devendra Verma
12:00 PM - OO4.8
Controlling Hemostasis with Inorganic Surfaces.
April Sawvel 1 , Sarah Baker 1 , Galen Stucky 1
1 Chemistry, University of California Santa Barbara, Santa Barbara, California, United States
Show AbstractThe ability to control bioprocesses via interface interactions with inorganic materials is evident in biomineralization1, supported enzyme activity2, 3, and protein folding or denaturation4. The development of advanced wound-dressing materials is an emerging field of materials chemistry that exploits the tunable surface properties of inorganic materials to modulate the intrinsic coagulation response5-8. Inorganic hemostatic agents are estimated to have saved hundreds of lives in the current military conflicts5 and are finding an increasing number of civilian applications9.Our work has resulted in the further development of rapid-acting hemostatic agents and in a better understanding of how to use inorganic surfaces to control coagulation. We have tested a variety of inorganic materials that vary with respect to their surface and materials properties, and have identified the key materials properties necessary to stimulate a pro- or anti-coagulant response. The materials studied were thoroughly characterized with respect to their physical properties and rheological studies were used to assess the blood clotting response in vitro. In addition to investigating the physical properties of these materials, we have also studied the dependence of intrinsic coagulation on particle size and morphology. As a consequence of systematic investigations into the coagulation response to various inorganic materials, we have been able to develop superior hemostatic agents and are working towards a more detailed understanding of the enzymatic and cellular response to inorganic surfaces.1. Zaremba, C. M.; Belcher, A. M.; Fritz, M.; Li, Y.; Mann, S.; Hansma, P. K.; Morse, D. E.; Speck, J. S.; Stucky, G. D. Chem. Mater. 1996, 8, 679.2. Han, Y.-J.; Watson, J. T.; Stucky, G. D.; Butler, A. J. Mol. Catal. B: Enzym. 2002, 17, 1.3. Carrado, K. A.; Macha, S. M.; Tiede, D. M. Chem. Mater. 2004, 16, 2559.4. Charache, P.; MacLeod, C. M.; White, P. Journal of General Physiology 1962, 45, 1117.5. Harris, E. Nature 2007, 446, 369.6. Ostomel, T. A.; Shi, Q.; Stucky, G. D. J. Am. Chem. Soc. 2006, 128, 8384.7. Ellis-Behnke, R. G.; Liang, Y.-X.; Tay, D. K. C.; Kau, P. W. F.; Schneider, G. E.; Zhang, S.; Wu, W.; So, K.-F. Nanomedicine: Nanotechnology, Biology and Medicine 2006.8. Fischer, T. H.; Thatte, H. S.; Nichols, T. C.; Bender-Neal, D. E.; Bellinger, D. A.; Vournakis, J. N. Biomaterials 2005, 26, 5433.9. Marshall, J. New Scientist 2006, 28.
12:15 PM - OO4.9
A Robust Coating for Nano Titanium Dioxide which Blocks Photocatalytic Activity.
Chien-Hsiu Lin 1 , Wilson Lee 1 , Nadine Pernodet 1 , Bingquan Li 2 , Eli Hatchwell 1 , Miriam Rafailovich 1
1 Material Science, SUNY-Stony Brook, Stony Brook, New York, United States, 2 Department of Earth & Environmental Engineering, Columbia University, New York, New York, United States
Show Abstract12:30 PM - **OO4.10
The Cellular Cytoskeleton as a Mechanically Adaptive Material.
Daniel Fletcher 1
1 Bioengineering & Biophysics, University of California-Berkeley, Berkeley, California, United States
Show AbstractThe ability of cells to generate forces and resist deformation is critical for organism development, maintenance, and repair. Within the cell, physical stresses are mediated by an interconnected and adaptive network of filaments known collectively as the cytoskeleton. Composed of actin filaments, intermediate filaments, and microtubules, the cytoskeleton provides an internal framework that organizes the cytoplasm, enables dynamic shape changes, and can mechanically link the cell to its external environment. Among the most dynamic structures within the cytoskeleton are actin filament networks, whose growth plays a critical role in cellular process ranging from crawling motility to endocytosis. Monomeric actin polymerizes into non-covalent polymer filaments that are organized by actin-binding proteins into branched and cross-linked actin filament networks. In dendritic actin networks that drive lamellipodial protrusions, new filaments are formed by the branching complex Arp2/3 at locations near the cell membrane, while old filaments are terminated by a capping protein present throughout the network. The repeated steps of branching and capping result in the formation of a highly oriented network that drives membrane deformations. Despite extensive biochemical knowledge of proteins necessary for actin network formation, little is known about how the cytoskeleton adapts to external forces. This talk will present recent work investigating adaptive properties of dendritic actin networks using a series of atomic force microscopy (AFM)-based experimental tools integrated with epi-fluorescence microscopy. In our experiments, actin networks are reconstituted in vitro and directly probed with AFM under controlled loading conditions to measure both static and dynamic properties. First, we find that these networks exhibit three regimes of elasticity under compression: linear, non-linear stress stiffening, and non-linear stress softening. The data is consistent with a network of filaments that experience buckling of individual elements under large stresses. Second, we find that growth of an actin network under load alters its growth velocity in a history-dependent manner, suggesting that network density is altered. These data challenge existing models of actin network assembly and offer insight into the design of adaptive materials.
OO5: Tissue Scaffolds and Implant Materials
Session Chairs
Tuesday PM, November 27, 2007
Room 206 (Hynes)
2:30 PM - **OO5.1
Designing Shape Memory Polymers Networks for Biomedical Applications.
Ken Gall 1 , Chris Yakacki 1 , Alicia Ortega 1 , Alan Greenberg 1
1 Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States
Show AbstractShape memory polymers have potential applications in minimally invasive surgery through the delivery of compacted devices that deploy into functional form once implanted. The objective of this talk is to overview the design of shape memory polymer networks for various biomedical applications. We will examine chemical and structural factors important to the thermo-mechanical behaviour of shape memory polymers during deformation, storage, and recovery. We will begin by discussing structure-property requirements for relatively compliant cardiovascular applications and relatively rigid orthopaedic applications. We will subsequently consider (1) the effect of temperature on deformability of the networks, (2) the effects of crosslinking density on recoverable strain limits, and (3) the effects of glass transition temperature and rubbery modulus on recovery time and stress. The talk will end with conclusions and avenues for future research.
3:00 PM - OO5.2
Microfabrication of Biomimetic Cardiac Tissue Engineering Scaffolds.
George Engelmayr 1 , Mingyu Cheng 1 , Christopher Bettinger 2 4 , Jeffrey Borenstein 4 , Robert Langer 1 3 , Lisa Freed 1
1 Harvard-MIT Division of Health Sciences and Technology, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 4 MEMS Technology Group, Charles Stark Draper Laboratory, Cambridge, Massachusetts, United States, 3 Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractTissue engineered cardiac grafts (TECG) comprised of heart cells and scaffolds are conceptually appealing for use in the repair of congenital and acquired lesions of the heart. To date, however, scaffold materials applied to TECG have only been capable of promoting a limited subset of the anisotropic structural-mechanical properties of native myocardium. For example, collagen sponge, while sufficiently compliant to allow for contractility, fails to promote native-like cell alignment due to its isotropic microstructure, and generally exhibits insufficient strength to support high tensile loading. In contrast, nonwoven poly(glycolic acid) has a relatively high tensile strength and can induce some degree of cell elongation along fibers; however its high initial stiffness can impede robust contractility. Toward recapitulating biomimetic structural-mechanical properties in TECG, biologically-inspired scaffolds with controlled mechanical anisotropy and cell-orienting pore structures were microfabricated by excimer laser microablation of the bioresorbable elastomer poly(glycerol sebacate) (PGS). Patterns of square and/or rectangular holes were drilled through an ~250 µm thick PGS membrane to yield grid-like scaffold structures with 50 µm wide struts and geometrically well-defined open pores. PGS scaffolds with 1:1 aspect ratio (200 x 200 µm) square pores, 2:1 aspect ratio (400 x 200 µm) rectangular pores, and other novel anisotropic pore structures were fabricated and assessed for their utility in fabricating TECG. PGS scaffolds with 2:1 aspect ratio and other anisotropic pore structures were found to exhibit mechanical properties, including anisotropy, similar to native ventricular myocardium, as assessed by tensile testing of oriented scaffolds and native ventricles. Upon seeding with neonatal rat heart cells, all PGS scaffolds yielded spontaneously contractile TECG which could be paced by electrical field stimulation at frequencies of 1 to 4 Hz. On culture day 7, PGS scaffolds with 2:1 aspect ratio pores exhibited a significantly lower excitation voltage threshold when the electric field was applied parallel versus perpendicular to the preferred strut direction (0.75 ± 0.06 vs. 0.91 ± 0.01 volts; p < 0.05), and a significantly higher index of cellular orientation (scale 0-1) than scaffolds with 1:1 aspect ratio pores (0.48 ± 0.04 vs. 0.10 ± 0.03; p < 0.0001) as assessed by F-actin labeling and image analysis of confocal micrographs. Toward scaling-up to thicker TECG, a prototype bi-layered PGS scaffold (~400 µm thick) with a fully interconnected three-dimensional pore network was fabricated by a combination of excimer laser microablation and lamination, and was also demonstrated to yield a contractile TECG. In conclusion, PGS scaffolds microfabricated to include specific anisotropic pore structures can overcome the principal structural-mechanical limitations of previous scaffolds to promote the formation of more biomimetic cardiac grafts.
3:15 PM - OO5.3
Polyurethane-Hyaluronic Acid Copolymers as Hemocompatible, Bioactive Materials.
Fangmin Xu 1 , John Nacker 2 , Wendy Crone 1 2 3 , Kristyn Masters 1
1 Biomedical Engineering, University of Wisconsin-Madison, Madison, Wisconsin, United States, 2 Engineering Mechanics Program, University of Wisconsin-Madison, Madison, Wisconsin, United States, 3 Engineering Physics, University of Wisconsin - Madison, Madison, Wisconsin, United States
Show AbstractAfter several decades of research into hemocompatible biomaterials, there remain surprisingly few materials that can be used in blood-contacting applications. We have synthesized copolymers of polyurethane (PU) with hyaluronic acid (HA), a glycosaminoglycan that is naturally anti-thrombotic, with the goal of creating materials that incorporate an inherently non-thrombogenic, bioactive component into the bulk polymer structure. Low molecular weight HA was incorporated into the polymer backbone as a chain extender during PU synthesis, and both the physical and biological properties of the resulting copolymer were directly controlled by the HA content. Increases in HA content led to a linear increase in hydrophilicity and corresponding increase in surface energy and decrease in protein adsorption compared to polyurethane controls. Increasing HA content also led to significantly increased elastic modulus. Incorporation of even small amounts of HA resulted in negligible platelet adhesion to the PU-HA copolymers, representing a 20-fold decrease in platelet adhesion compared to PU controls. Red blood cell adhesion also significantly decreased with increasing HA content, a result that correlated well with theoretical trends predicted using work of adhesion calculations. Preliminary studies of endothelial cells seeded on the materials confirmed that the PU-HA materials were also highly cytocompatible and supported cell adhesion and viability. Thus, we have demonstrated the synthesis of a unique bioactive polymer whose physical and biological properties are easily tailored, and whose potent anti-thrombogenic properties demonstrate its great promise for use in vascular applications.
3:30 PM - OO5.4
Mechanical Properties of Nanoparticle Hydroxyapatite/gelatin Constructs.
Steven Fox 1 , Inessa Stanishevskaya 1 , Shafiul Chowdhury 1 , Andrei Stanishevsky 1
1 , University of Alabama at Birmingham, Birmingham, Alabama, United States
Show Abstract3:45 PM - OO5.5
Osteogenic Differentiation of Mesenchymal Stem Cells on Biomineralized Collagenenous Scaffolds for Bone Tissue Engineering.
Harold Castano 1 , Chuanbin Mao 1
1 Chemistry & Biochemistry, University of Oklahoma, Norman, Oklahoma, United States
Show AbstractIn this work we study the osteogenic differentiation of mesenchymal stem cells (MSCs) seeded on biomineralized small intestine submucosal (SIS) membranes and evaluate in vivo bone regeneration on MSC-seeded SIS mmebranes. SIS membrane is a naturally occurring 3D collagenous biomaterial derived from the submucosal layer of porcine small intestine. It is predominantly composed of type I collagen fibers and also rich in growth factors that can support the growth of different types of cells. Thus SIS membrane is a potential biomaterial for bone tissue regeneration. We first studied the biomineralization of the SIS membrane in order to integrate the bone mineral hydroxylapatite into the collagenous scaffold. We then evaluated the attachment, proliferation, and differentiation of rat MSCs toward the osteoblastic phenotype seeded on the biomineralized SIS membranes. Rat critical size cranial defect was used as an animal model to study the in vivo bone regeneration on MSC-seeded SIS membranes.
4:00 PM - OO5:Scaffold/Imp
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4:30 PM - OO5.6
Hydrogel Nanoparticle Aggregates at the Wound-Dressing Interface.
John St. John 1 , Lynda Waller 1 , Daniel Moro 1 , Daniel Hatef 2 , Spencer Brown 2
1 Research, ULURU Inc., Addison, Texas, United States, 2 Plastic Surgery, University of Texas Southwestern Medical School, Dallas, Texas, United States
Show AbstractA full thickness dermal wound represents a dynamic tissue environment where normal cellular function has been dramatically challenged to force repair of a compromised surface. We report on the development of a new class of advanced wound dressings composed of dehydrated hydrogel nanoparticles. The hydrogel nanoparticle powder when applied to a wound surface adsorbs exudate and the dehydrated nanoparticles hydrate and irreversibly aggregate forming a film with intimate contact to the moist granulating wound bed.The nanoparticles were formed from poly-2-hydroxyethyl methacrylate and poly-2-hydroxypropl methacrylate with uniform particle sizes. The nanoparticles were subjected to rigorous purification, lyophilization and sterilized with gamma radiation. Application of the powder to a moist wound surface resulted in the immediate formation of a solid hydrogel film that covered and sealed the wound. The hydrogel nanoparticle wound dressing allowed high volume transpiration of wound exudate and did not require a secondary occlusive dressing for the duration of application to 30 days. Data for gross wound healing and appearance for the dressings applied to full and partial thickness surgically induced porcine wounds shown excellent results. Wound healing data were also shown for application of the powder to surgically debrided chemical burns.Histolological observations for biopsies taken at the wound/dressing interface over time showed changes both in tissue morphology and the interaction between new tissue and the nanoparticle wound dressings. Differences between the hydrogel nanoparticle dressing and control dressings with traditional moist wound healing protocols were observed for both wound healing and for the granulating tissue. Data for the local acute immune response were shown with TNFα levels obtained by microdialysis near the wound site, and the nanoparticle dressing showed no elevation over controls. Of more critical importance to advanced wound healing is the possible introduction of growth factors or other bioactive agents from the nanoparticle dressing at the wound surface.. Growth factors were added to nanoparticle formulations and resulatant morphologic changes in the healing wound were observed relative to the respectice growth factor. Data for gross wound healing were correlated with histological and immunohistochemistry staining to show the influence of different growth factor release profiles from the solid material on the subsequent phases of wound healing. Therefore, these data showed that bioactive agents were delivered to the wound healing surface from a solid material in a bioactive form that retained biological activity.The hydrogel nanoparticle wound dressing is currently under commercial production. Advanced hydrogel nanoparticle wound dressings containing growth factors are under preclinical development.
4:45 PM - OO5.7
On the Release of Nickel from Dynamically Loaded Orthodontic Wires.
Thorsten Peitsch 1 , Arndt Klocke 2 , Oleg Prymak 1 , Matthias Epple 1
1