Session J22: Focus Session: Biological and Bio-Inspired Adhesive Polymers II

Switchable adhesion of liquid crystalline elastomers

Liquid crystal elastomers (LCEs) are rubbery materials that composed of liquid crystalline polymers (LCPs) crosslinked into a network. The rod-like mesogens incorporated into the LCPs are have random orientations in the high temperature isotropic phase, but can adopt the canonical liquid crystalline phases as the temperature is lowered. LCEs have not yet found a key application, however, these materials are highly dissipative. I will describe a proposed application of reversibly switchable pressure sensitive adhesives (PSAs). The quality of their adhesion can be measured by the tack energy. To investigate their performance as switchable PSAs we compare the tack energy for the director aligned parallel, and perpendicular to the substrate normal, with that for the isotropic state using a finite element model that incorporates cavitation within the adhesive layer. The constitutive properties of the LCE are modelled using the nematic dumbbell model. We find that the tack energy depends on the director orientation, with parallel orientation of the nematic having higher tack energy than both the isotropic and the perpendicular director orientation [1]. I will report on how this model compares with recent experiments on LCE PSAs. [1] Soft Matter, 2013,9, 1151-1163   

Measurement of depletion-induced force in microtubule bundles

Microtubule (MT) bundles formed in the presence of non-adsorbing polymers - poly-ethylene glycol (PEG) or Dextran - are widely used in experimental active matter systems. However, many properties of such MT bundles have not been studied experimentally. In this work, we combine optical trapping techniques with an umbrella sampling method in order to measure the depletion force acting on individual microtubule in the axial direction within the bundle. We find depletion force is independent of bundle overlap length and measure its magnitude to be on the order of tens of $\frac{k_{B}T}{\mu m}$. We explore the dependence of the depletion force on concentration of depletant (PEG 20K) as well as $K^{+}$ ions (necessary for screening electrostatic repulsion between MT filaments). We also verify additivity of depletion interaction and confirm that force is increased by a factor of two for three-MT bundles. Additionally, our experimental technique allows us to probe interactions between MTs within the bundle. Experimental data suggests that filaments in the bundle interact only hydrodynamically when depletant concentrations are low enough; however, we observe onset of solid-like friction when osmotic pressure is increased above a certain threshold.   

Hybrid metal-coordinate transient networks: using bio-inspired building blocks to engineer the mechanical properties of physical hydrogels

Recently, metal-coordinate complex crosslinks have been suggested to contribute to the self-healing properties of mussel byssi. Two specific amino acid derivatives - 3,4 dihydroxy-L-phenylalanine (dopa) and histidine (his) - are known to form coordinate complexes with trivalent and divalent ions (respectively) in aqueous solutions. We show here that, by functionalizing poly(ethylene glycol) polymers with dopa and his we are (1) able to characterize the fundamental kinetics and energetics of each specific metal-ligand pair using small amplitude oscillatory shear rheology and (2) create hybrid networks using various mixtures of metals and ligands. From this information, we can design gels with specific target mechanical properties by tailoring the amounts and types of metal-ligand crosslinks present in the gel network, resulting in the ability to engineer the mechanical relaxation spectrum. This work provides basic understanding necessary to intelligently design materials which incorporate metal-ligand crosslinks in more complex architectures.   

Predictive relationships between crosslinker unbinding kinetics, gel stiffness, and plasticity in adhesive biopolymers

We determine the viscoelastic responses of rigid rod polymer networks that have been strongly bonded by labile crosslinkers. Experimentally, we use microtubules, extremely stiff biopolymers that play important roles in maintaining the strength and organization of cells. We generate controllable adhesive bonds using well-characterized protein chemistries, such as biotin-streptavidin bonds, or using recombinant microtubule-associated proteins. Networks are visualized using confocal scanning fluorescence microscopy or transmission electron microscopy, and custom-built, high-force magnetic tweezers devices are used to apply localized forces to the gels. For rigid crosslinkers, we find that at short time scales, the networks respond nonlinearly to applied force, with stiffening at small forces, followed by a softening regime, which we attribute to the force-induced unbinding of crosslinkers. At long time scales, force-induced bond breakage leads to local network rearrangement and significant bead creep. Interestingly, the material retains its elastic modulus even under conditions of significant plastic flow, suggesting that crosslinker breakage is balanced by the formation of new bonds. These results provide important insight into the determinants of gel toughness, elasticity, and plastic deformation in rigid networks, but also suggest new avenues for materials optimization based on modulation of crosslinker kinetics. In particular, the incorporation of crosslinkers that break under force, but are competent to reform when the force is removed, significantly enhance gel toughness while minimizing material fatigue under cyclic loading.   

DNA Gel with dynamic cross-links

The mechanical properties of a living cell are strongly related to the cytoskeletal network, which is comprised of diverse protein filaments connected by cross-linking proteins, some of which are dynamic. Gels comprised of dynamic cross-linkers exhibit unique mechanical properties not seen in those using permanent cross-linkers [1,2]. To investigate the effect of a dynamic cross-linker on mechanical properties of a material, we have synthesized biopolymer gels with a well-known semi-flexible biopolymer, DNA, and probed the mechanics of the system using microrheological techniques. We discuss these results in comparison to cytoskeletal systems, and seek to establish universal principles of dynamic cross-link based gels. - References 1. S. M. V. Ward, A. Weins, M. R. Pollak, D. a Weitz, Dynamic viscoelasticity of actin cross-linked with wild-type and disease-causing mutant alpha-actinin-4., Biophys. J. 95, 4915--23 (2008). 2. C. P. Broedersz et al., Cross-Link-Governed Dynamics of Biopolymer Networks, Phys. Rev. Lett. 105, 238101 (2010).   

Bio-inspired adhesion: local chemical environments impact adhesive stability

3,4-dihydroxyphenylalanine (Dopa) is an amino acid that is naturally synthesized by marine mussels and exhibits the unique ability to strongly bind to surfaces in aqueous environments. However, the Dopa functional group undergoes auto-oxidation to a non-adhesive quinone form in neutral to basic pH conditions, limiting the utilization of Dopa in biomedical applications. In this work, we performed direct surface force measurements with in situ electrochemical control across a Dopa-rich native mussel foot protein (mfp-5), as well as three simplified model peptide sequences. We find that the neighboring peptide residues can significantly impact the redox stability of Dopa functional groups, with lysine residues imparting a substantial degree of Dopa redox stabilization. Surprisingly, the local chemical environments only minimally impact the magnitude of the adhesion forces measured between molecularly-smooth mica and gold surfaces. Our results provide molecular level insight into approaches that can be used to mitigate the detrimental impact of Dopa auto-oxidation, thus suggesting new molecular design strategies for improving the performance of Dopa-based underwater adhesives.   

Multi-scale models for cell adhesion

The interactions of membrane receptors during cell adhesion play pivotal roles in tissue morphogenesis during development. Our lab focuses on developing multi-scale models to decompose the mechanical and chemical complexity in cell adhesion. Recent experimental evidences show that clustering is a generic process for cell adhesive receptors. However, the physical basis of such receptor clustering is not understood. We introduced the effect of molecular flexibility to evaluate the dynamics of receptors. By delivering new theory to quantify the changes of binding free energy in different cellular environments, we revealed that restriction of molecular flexibility upon binding of membrane receptors from apposing cell surfaces (trans) causes large entropy loss, which dramatically increases their lateral interactions (cis). This provides a new molecular mechanism to initialize receptor clustering on the cell-cell interface. By using the subcellular simulations, we further found that clustering is a cooperative process requiring both trans and cis interactions. The detailed binding constants during these processes are calculated and compared with experimental data from our collaborator's lab.   

Semiflexible networks with labile crosslinkers: Bundling, rheology, ripping, and healing

Networks of semiflexible filaments may be cross-linked by molecules that unbind and then rebind in different places throughout the network. The structure of such networks in equilibrium is dynamic. That structure will also evolve in time either in the relaxation towards equilibrium, or in response to external perturbations such as applied stress. Cross linker mobility leads to new rheological features that depend on e.g., the degree of filament bundling, and allows for new dissipative mechanisms related to cross linker unbinding and rebinding in the networks under applied mechanical load. In this talk, I present the results of analytic calculations and numerical simulations exploring the effect of labile cross linkers on the rheology and structural evolution of semiflexible networks. Specifically, I discuss the fluctuation-induced or Casimir interactions between cross linkers in a semiflexible filament network. I also report on the linear response of such network to applied shear, particularly for the case where the cross linkers induce filament bundling. In that case, there is a universal high-frequency bundle rheology distinct from that of semiflexible filament networks. Cross linker unbinding leads to new dissipative mechanisms, and there is a new low frequency, non-Newtonian rheological regime associated with bundle dissolution. Finally, I comment on the nonlinear response of these networks to applied stress, examining the role of cross linker unbinding (and rebinding) on the energy dissipation in, and the plastic deformation of the network under a time-independent applied load.