Biological Gels Research Articles

Nanomotors

This minireview discusses whether catalytically active macromolecules and abiotic nanocolloids, that are smaller than motile bacteria, can self-propel. Kinematic reversibility at low Reynolds number demands that self-propelling colloids must break symmetry. Methods that permit the synthesis and fabrication of Janus nanocolloids are therefore briefly surveyed, as well as means that permit the analysis of the nanocolloids’ motion. Finally, recent work is reviewed which shows that nanoagents are small enough to penetrate the complex inhomogeneous polymeric network of biological fluids and gels, which exhibit diverse rheological behaviors.

Surface effects on nanoindentation of soft solids by different indenters

For soft materials like biological tissues and gels, surface energy and hyperelasticity have significant influences on their mechanical response to external load. In this paper, we investigate the effects of surface energy on nanoindentation of hyperelastic solids by using conical, flat and spherical indenters. The hyperelastic behavior of soft solids is characterized by the neo-Hookean model, and the influence of surface energy is analyzed through finite element simulations. For the three typical indenters, the explicit relations between compressive load and indent depth are obtained considering both finite deformation and surface energy. When the contact radius is comparable with the ratio of surface energy density to elastic modulus, surface energy will evidently alter the contact pressure, surface profile, and overall response. Compared to the linear elastic predictions, the neo-Hookean hyperelasticity tends to increase the indent depth, while surface energy has a reverse effect. The obtained results are helpful to accurately characterize the mechanical response of soft solids via nanoindentation tests.

Mechanoelectrical transduction in the hydrogel-based biomimetic sensors

The study addresses the phenomenon of mechanoelectrical transduction in polyelectrolyte hydrogels and, in particular, the search of the driving force for the change of the electrical potential of a gel under the applied mechanical stretch. Polyelectrolyte gels of calcium and magnesium salts of polymethacrylic acid were synthesized by the radical polymerization in water solution. Their electrical potential measured by microcapillary electrodes was negative and fall within 100⬜140mV range depending on the nature of a counterion and the networking density of a gel. The rectangular samples (↼10mm in length and 2Ô2mm in cross-section) of gel-based sensors underwent the dynamic axial deformation, and the simultaneous monitoring of their geometrical dimensions and the electrical potential was performed. Sensor elongation resulted in the overall increase of gel volume, and it was always accompanied by the gel potential change toward the depolarization (diminishing of the negative values). Theoretical model based on the assumption of the total electrical charge conservation in the course of the dynamic deformation of a filament was proposed to describe the dependence of the electrical potential of a gel on its volume. Good agreement between the predictions of the model and the experimental trend was shown. The proposed mechanism of mechanoelectrical transduction based on the stretch-dependant volume changes in polyelectrolyte hydrogels might be useful to understand the nature of mechanical sensing in much more complex biological gels like the cell cytoskeleton.

Strain-enhanced stress relaxation impacts nonlinear elasticity in collagen gels

The extracellular matrix (ECM) is a complex assembly of structural proteins that provides physical support and biochemical signaling to cells in tissues. The mechanical properties of the ECM have been found to play a key role in regulating cell behaviors such as differentiation and malignancy. Gels formed from ECM protein biopolymers such as collagen or fibrin are commonly used for 3D cell culture models of tissue. One of the most striking features of these gels is that they exhibit nonlinear elasticity, undergoing strain stiffening. However, these gels are also viscoelastic and exhibit stress relaxation, with the resistance of the gel to a deformation relaxing over time. Recent studies have suggested that cells sense and respond to both nonlinear elasticity and viscoelasticity of ECM, yet little is known about the connection between nonlinear elasticity and viscoelasticity. Here, we report that, as strain is increased, not only do biopolymer gels stiffen but they also exhibit faster stress relaxation, reducing the timescale over which elastic energy is dissipated. This effect is not universal to all biological gels and is mediated through weak cross-links. Mechanistically, computational modeling and atomic force microscopy (AFM) indicate that strain-enhanced stress relaxation of collagen gels arises from force-dependent unbinding of weak bonds between collagen fibers. The broader effect of strain-enhanced stress relaxation is to rapidly diminish strain stiffening over time. These results reveal the interplay between nonlinear elasticity and viscoelasticity in collagen gels, and highlight the complexity of the ECM mechanics that are likely sensed through cellular mechanotransduction.

Response of biopolymer networks governed by the physical properties of cross-linking molecules.

In this study, we examine how the physical properties of cross-linking molecules affect the bulk response of bio-filament networks, an outstanding question in the study of biological gels and the cytoskeleton. We show that the stress-strain relationship of such networks typically undergoes linear increase - strain hardening - stress serration - total fracture transitions due to the interplay between the bending and stretching of individual filaments and the deformation and breakage of cross-linkers. Interestingly, the apparent network modulus is found to scale with the linear and rotational stiffness of the crosslinks to a power exponent of 0.78 and 0.13, respectively. In addition, the network fracture energy will reach its minimum at intermediate rotational compliance values, reflecting the fact that most of the strain energy will be stored in the distorted filaments with rigid cross-linkers while the imposed deformation will be "evenly" distributed among significantly more crosslinking molecules with high rotational compliance.

Dynamics of water and sodium in gels under salt-induced phase transition

Synthetic and biological gels undergo a sharp volume phase transition when subjected to a variety of environmental changes. Water and ion dynamics within swollen and compact phases are critical for understanding fundamental concepts in cellular (specifically neuronal) biophysics, for models of bound, free, or ordered water in complex environments; and for practical applications such as the design of gels for drug release, biomimetics, sensors, or actuators. In this work, we find, for the first time, basic physical parameters that shed light on the interaction of gels with water and electrolytes, across a volume phase transition. Water within a gel can be separated into bound and free populations with high exchange rate. We show that free water dynamics in compact gels are the same as those in pure water. Bound water was found to comprise a single layer around the polymers in both phases, with a correlation time three orders of magnitude higher than that of free water. Most importantly, salt-induced phase transition was found to be different from a standard coil-globule transition (e.g., temperature-induced), with no rejection of bound water as the gel compacts. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 1620–1628

Hopping Diffusion of Nanoparticles in Polymer Matrices.

We propose a hopping mechanism for diffusion of large nonsticky nanoparticles subjected to topological constraints in both unentangled and entangled polymer solids (networks and gels) and entangled polymer liquids (melts and solutions). Probe particles with size larger than the mesh size ax of unentangled polymer networks or tube diameter ae of entangled polymer liquids are trapped by the network or entanglement cells. At long time scales, however, these particles can diffuse by overcoming free energy barrier between neighboring confinement cells. The terminal particle diffusion coefficient dominated by this hopping diffusion is appreciable for particles with size moderately larger than the network mesh size ax or tube diameter ae. Much larger particles in polymer solids will be permanently trapped by local network cells, whereas they can still move in polymer liquids by waiting for entanglement cells to rearrange on the relaxation time scales of these liquids. Hopping diffusion in entangled polymer liquids and networks has a weaker dependence on particle size than that in unentangled networks as entanglements can slide along chains under polymer deformation. The proposed novel hopping model enables understanding the motion of large nanoparticles in polymeric nanocomposites and the transport of nano drug carriers in complex biological gels such as mucus.

Multiscale Diffusion Measurements in Biological Gels Using Photoactivatable Fluorescent Nanoparticles

Diffusion through biological gels is crucial for effective nanoparticle drug delivery. In this work, we develop a method to measure diffusivity over a large range of length scales -- from tens of nanometers to tens of microns -- using photoactivatable nanoparticle probes. Nanoparticles composed of block copolymers of poly(lactic-co-glycolic acid) (PLGA) and polyethylene glycol (PEG) were synthesized and confirmed to possess dense PEG coatings that resist bioadhesion, and these particles were conjugated with caged rhodamine to make them photoactivatable. Using confocal microscopy, we activated a region of these particles with a brief, targeted exposure to UV light, uncaging the rhodamine and causing the particles to become fluorescent. We observed their collective diffusion over tens of minutes and tens of microns, thus obtaining a measure of their diffusivity. This technique is complementary to traditional multiple particle tracking (MPT); it extends the range over which particle motion can be directly observed. We confirmed the accuracy of this technique with reference to MPT measurements and the known diffusivity of particles in water. We applied the method to a model fibrin gel system, and found that both our method and MPT measurements show an immobile fraction of particles and mobile fraction that diffuses over long scales. Finally, we examined nanoparticle diffusion in sputum collected from cystic fibrosis patients, and we measured particle diffusion over distances relevant to drug delivery in the lungs. Coupled with traditional MPT, this technique enables multiscale characterization of particle mobility in complex biological fluids

External and internal gelation of pectin solutions: microscopic dynamics versus macroscopic rheology

Pectin is a natural biopolymer that forms, in the presence of divalent cations, ionic-bound gels typifying a large class of biological gels stabilized by non-covalent cross-links. We investigate and compare the kinetics of formation and aging of pectin gels obtained either through external gelation via perfusion of free Ca2+ ions, or by internal gelation due to the supply of the same ions from the dissolution of CaCO3 nanoparticles. The microscopic dynamics obtained with photon correlation imaging, a novel optical technique that allows obtaining the microscopic dynamics of the sample while retaining the spatial resolution of imaging techniques, is contrasted with macroscopic rheological measurements at constant strain. Pectin gelation is found to display peculiar two-stage kinetics, highlighted by non-monotonic growth in time of both microscopic correlations and gel mechanical strength. These results are compared to those found for alginate, another biopolymer extensively used in food formulation.

An adaptive algorithm for tracking 3D bead displacements: application in biological experiments

This paper presents a feature-vector-based relaxation method (FVRM) to track bead displacements within a three-dimensional (3D) volume. The FVRM merges the feature vector method, a technique used in tracking bead displacements in biological gels, with the relaxation method, an algorithm employed successfully in tracking bead pairs in fluids. More specifically, the FVRM evaluates the probability of a bead pairing event based on the quasi-rigidity condition between the feature vectors of a bead and its candidate positions within a searching domain. Computational efficiency is improved via the introduction of an adaptive searching domain size and mismatches are reduced via a two-directional matching strategy. The algorithm is validated using simulated 3D bead displacements caused by a force dipole within a linear elastic gel. Results demonstrate a consistently high recovery ratio (above 98%) and low mismatch ratio (below 0.1%) for tracking parameter (mean bead distance/maximum bead displacement) greater than 0.73.

Role of linear viscoelasticity and rotational diffusivity on the collective behavior of active particles

A linear dynamics scheme has been used to quantify the impact of viscoelasticity of the suspending fluid on the collective structure of active particles, including rotational diffusivity. The linear stability examines the response near an isotropic state using a mean-field theory including far-field hydrodynamic interactions of the swimmers. The kinetic model uses three possible constitutive models, the Oldroyd-B, Maxwell, and generalized linear viscoelastic models inspired by fluids like saliva, mucus, and biological gels. The perturbation growth rate has been quantified in terms of wavenumber, translational diffusivity, rotational diffusivity, and material properties of the fluids. A key dimensionless group is the Deborah number, which compares the relaxation time of the fluid with the characteristic timescale of the instability. An advantage of the model formalism is the ability to calculate some properties analytically and others efficiently numerically in the presence of rotational diffusion. The different constitutive equations examined help illustrate when and why the dispersion relation can have a peak at a particular wavenumber. The fluid properties can also change the role of rotational diffusion; diffusion always stabilizes a system in a Newtonian fluid but can destabilize a system in a Maxwell fluid.

Reactions drive conformations. Biomimetic properties and devices, theoretical description.

Life is supported by cellular reactions driving conformational, folding and unfolding macromolecular movements with ionic and aqueous exchange. Here we review the state of the art of the electrochemistry of conducting polymers as elemental material models of biological reactive gels. The different experimental methodologies allowing identification and quantification of the reaction induced molecular (conformation) and macroscopic (relaxation, swelling, shrinking and compaction) structural changes are reviewed. Similar structural effects from other materials and systems, artificial or biological, are presented. Some reaction induced properties and devices mimicking biological functions and organs are described: sensing and tactile artificial muscles, smart membranes, artificial glands or artificial chemical synapses. The new structural chemical kinetics opens the way to modelling and predicting health issues linked to reactive folding or misfolding processes, and to advancing the knowledge of brain molecular mechanisms such as memory. Chemo-electronic conformational memories and dual sensing-actuating, zoomorphic and anthropomorphic gel tools and robots are envisaged.

Immense elastic nonlinearities at the demixing transition of aqueous PNIPAM solutions

Elastic nonlinearities are particularly relevant for soft materials because of their inherently small linear elasticity. Nonlinear elastic properties may even take over the leading role for the transformation at mechanical instabilities accompanying many phase transitions in soft matter. Because of inherent experimental difficulties, only little is known about third order (nonlinear) elastic constants within liquids, gels and polymers. Here we show that a key concept to access third order elasticity in soft materials is the determination of mode Gruneisen parameters. We report the first direct observation of third order elastic constants across mechanical instabilities accompanying the liquid–liquid demixing transition of semi-dilute aqueous poly(N-isopropylacrylamide) (PNIPAM) solutions. Immense elastic nonlinearities, leading to a strong strain-softening in the phase-separating PNIPAM solutions, are observed. Molecular mechanisms, which may be responsible for these immense elastic nonlinearities, are discussed. The importance of third order elastic constants in comparison to second order (linear) elastic constants in the demixing PNIPAM solutions evidences the need to focus more on the general role played by nonlinear elasticity at phase transitions within synthetic and biological liquids and gels.

Biopolymer gels with “physical” cross-links: gelation kinetics, aging, heterogeneous dynamics, and macroscopic mechanical properties

Alginate is a natural biopolymer that forms, in the presence of divalent cations, ionic-bound gels typifying a large class of biological gels stabilized by non-covalent cross-links, and displaying a consistent restructuring kinetics. We investigate the kinetics of formation and aging of alginate gels by slow permeation of a curing CaCl2 agent by means of photon correlation imaging, a novel optical technique that allows obtaining the microscopic dynamics of the sample, while retaining at the same time the spatial resolution of imaging techniques. Specifically, the gelling kinetics displays a peculiar non-diffusive behavior, and the subsequent restructuring of the gel structure shares several features in common with the aging of colloidal gels, in particular for what concerns the occurrence of heterogeneous dynamics effects. A comparative analysis of the gel macroscopic mechanical properties at different aging stages further highlights distinctive effects arising from the non-permanent nature of the bonds.

Anisotropic hydrogels fabricated with directional freezing and radiation-induced polymerization and crosslinking method

Biological gels are generally anisotropic in both structures and mechanical properties, but conventional synthetic hydrogels are generally isotropic. We developed a directional freezing and radiation-induced polymerization and crosslinking (DFRPC) method to fabricate anisotropic hydrogels starting from monomers. The obtained poly(2-hydroxyethyl methacrylate) (PHEMA) hydrogels show significant anisotropy in both microstructures and mechanical properties. The hydrogels exhibit an aligned porous structure along the freezing direction and an irregular porous structure in the vertical direction. Their elastic modulus and tensile stress in the parallel direction are significantly higher than those in the vertical direction, and the difference between the two directions decreases as the increase of HEMA concentration. This novel methodology offers a promising way to fabricate a wide variety of anisotropic hydrogels, and the anisotropic hydrogels may find practical applications in biomedical fields.

Mapping Three-Dimensional Stress and Strain Fields within a Soft Hydrogel Using a Fluorescence Microscope

Three-dimensional cell culture is becoming mainstream as it is recognized that many animal cell types require the biophysical and biochemical cues within the extracellular matrices to perform truly physiologically realistic functions. However, tools for characterizing cellular mechanical environment are largely limited to cell culture plated on a two-dimensional substrate. We present a three-dimensional traction microscopy that is capable of mapping three-dimensional stress and strain within a soft and transparent extracellular matrix using a fluorescence microscope and a simple forward data analysis algorithm. We validated this technique by mapping the strain and stress field within the bulk of a thin polyacrylamide gel layer indented by a millimeter-size glass ball, together with a finite-element analysis. The experimentally measured stress and strain fields are in excellent agreements with results of the finite-element simulation. The unique contributions of the presented three-dimensional traction microscopy technique are: 1), the use of a fluorescence microscope in contrast with the confocal microscope that is required for the current three-dimensional traction microscopes in the literature; 2), the determination of the pressure field of an incompressible gel from strains; and 3), the simple forward-data-analysis algorithm. Future application of this technique for mapping animal cell traction in three-dimensional nonlinear biological gels is discussed.

Modelling the swelling and osmotic properties of clay soils. Part I: The phenomenological approach

Clay soils with a high montmorillonite content in their mineralogical composition are characterized by swelling and osmotic properties like biological tissues and polyelectrolyte gels. These phenomena are caused by the very high specific surface (≈760 m 2/g) and the negative electric charge of montmorillonite lamellae, which determine an interaction with the charge of the ions present in the pore solution. The interest in modelling the behavior of such soils is related to the evaluation of their performances as hydraulic and contaminant barriers in landfill and soil remediation applications. The theoretical approach of the thermodynamics of irreversible processes is applied to find suitable phenomenological constitutive equations, under the assumptions of a unidimensional geometry, infinitesimal strains of the solid skeleton and isothermal conditions. The approach is related to a saturated porous medium, whose voids are filled by an electrolyte solution containing an unspecified number of ions. To make the approach purely phenomenological, the parameters introduced into the constitutive equations are expected to be measured by macroscopic experimental tests, without any specification of the physical and chemical phenomena that occur at the pore scale. The constitutive equations allow the coupled transport and consolidation problem to be formulated for a clay barrier.

Dynamic impact indentation of hydrated biological tissues and tissue surrogate gels

For both materials engineering research and applied biomedicine, a growing need exists to quantify mechanical behaviour of tissues under defined hydration and loading conditions. In particular, characterisation under dynamic contact-loading conditions can enable quantitative predictions of deformation due to high rate ‘impact’ events typical of industrial accidents and ballistic insults. The impact indentation responses were examined of both hydrated tissues and candidate tissue surrogate materials. The goals of this work were to determine the mechanical response of fully hydrated soft tissues under defined dynamic loading conditions, and to identify design principles by which synthetic, air-stable polymers could mimic those responses. Soft tissues from two organs (liver and heart), a commercially available tissue surrogate gel (Perma-Gel™) and three styrenic block copolymer gels were investigated. Impact indentation enabled quantification of resistance to penetration and energy dissipative constants under the rates and energy densities of interest for tissue surrogate applications. These analyses indicated that the energy dissipation capacity under dynamic impact increased with increasing diblock concentration in the styrenic gels. Under the impact rates employed (2 mm/s to 20 mm/s, corresponding to approximate strain energy densities from 0.4 kJ/m3 to 20 kJ/m3), the energy dissipation capacities of fully hydrated soft tissues were ultimately well matched by a 50/50 triblock/diblock composition that is stable in ambient environments. More generally, the methodologies detailed here facilitate further optimisation of impact energy dissipation capacity of polymer-based tissue surrogate materials, either in air or in fluids.

Isolation, Biochemical Characterization and Anti-adhesion Property of Mucin from the Blue Blubber Jellyfish (<i>Catostylus mosaicus</i>)

Mucins are large glycoproteins that have been identified as the main component of various biological gels and lubricants. Their compositions contribute to the viscoelastic properties of mucus secretions. Gel-forming mucins have now been identified in a variety of organisms from marine molluscs to humans and are thought to cover and protect epithelial cells from attachment and entry of pathogens. We employed a new approach using a combination of tryptic digestion and the fractionation by hydrophobic interaction chromatography to enable the isolation and purification of mucins from the Blue Blubber jellyfish, Catostylus mosaicus . The purified proteins were stained with Alcian blue indicating extensive glycosylation. Amino acid composition analysis of the purified protein found that it was enriched for Ala, Glu, Thr, Pro and Val residues (together constituting ~93 mole % of the protein), which is typical for mucins. Consistent with this, monosaccharide composition analysis revealed extensive O-linked oligosaccharides with N-acetylgalactosamine (GalNAc), galactose (Gal) and N-acetylglucosamine (GlcNAc) as major monosaccharide constituents. The purified C. mosaicus mucins inhibited the attachment of an ocular Pseudomonas aeruginosa (PA) isolate (Paer6294) to human corneal epithelial (HCE) cells grown in vitro .

Bending and twisting of soft materials by non-homogenous swelling

Soft materials such as biological tissues and gels undergo morphological changes, motion, and instabilities when subjected to external stimuli. We examine how thin elastic plates undergo rapid bending and buckling instabilities after non-homogenous exposure to a favorable solvent that swells the network. An unconstrained beam bends along its length, while a circular disc bends and buckles with multiple curvatures. In the case of a disc, a large-amplitude transverse travelling wave rotates azimuthally around the disc. We provide theoretical interpretations inspired by the complementary thermal expansion problem of transient shape changes triggered by non-homogenous time-dependent heating, which allows collapse of time-dependent swelling data onto universal curves. Control of dynamical, swelling-induced shape changes provides new directions for the utilization of soft materials.