Mechanical Properties Of Soft Materials Research Articles

Nanoscopic mechanical anisotropy in hydrogel surfaces.

The bulk mechanical properties of soft materials have been studied widely, but it is unclear to what extent macroscopic behavior is reflected in nanomechanics. Using an atomic force microscopy (AFM) imaging method called force spectroscopy mapping (FSM), it is possible to map the nanoscopic spatial distribution of Young's modulus, i.e. "stiffness," and determine if soft or stiff polymer domains exist to correlate nano- and macro-mechanics. Two model hydrogel systems typically used in cell culture and polymerized by a free radical polymerization process, i.e. poly (vinyl pyrrolidone) (PVP) and poly(acrylamide) (PAam) hydrogels, were found to have significantly different nanomechanical behavior despite relatively similar bulk stiffness and roughness. PVP gels contained a large number of soft and stiff nanodomains, and their size was inversely related to crosslinking density and changes in crosslinking efficiency within the hydrogel. In contrast, PAam gels displayed small nanodomains occuring at low frequency, indicating relatively uniform polymerization. Given the responsiveness of cells to changes in gel stiffness, inhomogeneities found in the PVP network indicate that careful nanomechanical characterization of polymer substrates is necessary to appreciate complex cell behavior.

Nanoscale non-equilibrium dynamics and the fluctuation–dissipation relation in an ageing polymer glass

Response functions and fluctuations measured locally in complex materials should equally well characterize mesoscopic-scale dynamics. The fluctuation-dissipation-relation (FDR), relates the two in equilibrium, a fact used regularly, for example, to infer mechanical properties of soft matter from the fluctuations in light scattering. In slowly-evolving non-equilibrium systems, such as aging spin and structural glasses, sheared soft matter, and active matter, a form of FDR has been proposed in which an effective temperature, Teff, replaces the usual temperature, and universal behavior is found in mean-field models and simulations. Thus far, only experiments on spin-glasses and liquid crystals have succeeded in accessing the strong aging regime, where Teff >T and possible scaling behavior are expected. Here we test these ideas through measurements of local dielectric response and polarization noise in an aging structural glass, poly-vinyl-acetate . The relaxation-time spectrum, as measured by noise, is compressed, and by response, is stretched, relative to equilibrium, requiring an effective temperature with a scaling behavior similar to that of certain mean-field spin-glass models.

Vibro-magnetometry: Theoretical aspects and simulations

In this article, we introduce the theory for vibro-magnetometry (VM) with computational simulations for an idealized experimental setup. A method based on acoustic radiation force and magnetic measurement has been proposed for interrogating the mechanical vibrations of a target immersed in fluid medium. In this method, ultrasound radiation is used to exert a low-frequency (in kHz range) force on a rigid magnetized target immersed in a viscoelastic medium. In response, the target vibrates sinusoidally in a pattern determined by viscoelastic properties of the medium. The magnetic field resulting from target vibration is related to both the ultrasonic and low-frequency (kHz range) mechanical characteristics of the medium. We report the relationship between the magnetic field signal and the incident ultrasonic pressure field in terms of the mechanical parameters of the medium. Simulations were conducted to demonstrate a simple approach based on using amplitude-modulated ultrasound to generate a dynamic acoustic radiation force on a magnetic target. The magnetic field generated by vibration of this target is then obtained and used to estimate the radiation-force-induced displacement as a function of time. It was observed that the intensity of the dynamic component of the magnetic field caused by the acoustic excitation is high enough to be registered by a conventional magnetic sensor. When a low stress is applied on a reactive magnetic target embedded in the medium, the subsequent oscillating magnetic field is measured by a dedicated magnetic sensor, yielding the applicable mechanical information of the host medium. The proposed methodology presents a powerful tool for evaluation of acoustic radiation force as well as the mechanical properties of soft materials.

Microrheology of Soft Matter

Soft matter such as liquid crystals, polymer solutions and colloidal dispersions are complex fluids that exhibit viscoelasticity. Their macroscopic mechanical properties have been studied intensively in the field of rheology. Recently, the structures and dynamics of soft matter at micro- and nanoscale have attracted considerable attention from researchers. The study of microrheology, which deals with the mechanical properties of soft matter at the mesoscopic scale, has made remarkable progress owing to theoretical and technical developments. In this review, we present the basic methods of microrheology and their practical applications to soft matter. We also stress the importance of these methods for understanding fundamental physical problems in nonequilibrium systems at the mesoscopic scale.

Mechanical Properties of Ballistic Gelatin at High Deformation Rates

The characterization of soft or low impedance materials is of increasing importance since these materials are commonly used in impact and energy absorbing applications. The increasing role of numerical modeling in understanding impact events requires high-rate material properties, where the mode of loading is predominantly compressive and large deformations may occur at high rates of deformation. The primary challenge in measuring the mechanical properties of soft materials is balancing the competing effects of material impedance, specimen size, and rate of loading. The traditional Split Hopkinson Pressure Bar approach has been enhanced through the implementation of polymeric bars to allow for improved signal to noise ratios and a longer pulse onset to ensure uniform specimen deformation. The Polymeric Split Hopkinson Pressure Bar approach, including the required viscoelastic bar analysis, has been validated using independent measurement techniques including bar-end displacement measurement and high speed video. High deformation rate characterization of 10% and 20% ballistic gelatin, commonly used as a soft tissue simulant, has been undertaken at nominal strain rates ranging from 1,000 to 4,000/s. The mechanical properties of both formulations of gelatin exhibited significant strain rate dependency. The results for 20% gelatin are in good agreement with previously reported values at lower strain rates, and provide important mechanical properties required for this material.

RECENT PROGRESS OF NANO-MECHANICAL MAPPING

Nano-mechanical mapping by atomic force microscopy has been developed as an useful application to measure mechanical properties of soft materials at nanometer scale. To date, the Hertzian theory was used for analyzing force- distance curves as the simplest model among several contact mechanics between elastic bodies. However, the preexisting methods based on this theory do not consider the adhesive interaction in principle, which cannot be neglected in the ambient condition. A new analytical method was introduced to estimate the elasticity and the adhesive energy simultaneously by means of the JKR theory, describing adhesive contact between elastic materials. Poly(dimethylsiloxane) (PDMS) and isobutylene-co-isoprene rubber (IIR) were analyzed to verify the applicable limit of the JKR analysis. For elastic samples such as PDMS, the force-deformation plots obtained experimentally were consistent with JKR theoretical curves. Meanwhile, for viscoelastic samples, especially for IIR, the experimental plots revealed large deviations from JKR curves depending on scanning velocity and maximum loading force. Some nano-rheological arguments were employed based on the difference between these specimens.

Cavitation and fracture behavior of polyacrylamide hydrogels

The mechanical properties of gels present qualitatively contradictory behavior; they are commonly soft but also notoriously brittle. We investigate the elasticity and fracture behavior of swollen polymer networks using a simple experimental method to induce cavitation within a gel and adapt scaling theories to capture the observed transition from reversible to irreversible deformations as a function of polymer volume fraction. It is shown quantitatively that the transition from reversible cavitation to irreversible fracture depends on the polymer volume fraction and an initial defect length scale. The use of cavitation experiments permits characterization of network properties across length scales ranging from µm to mm. We anticipate that these results may significantly enhance the understanding of mechanical properties of soft materials, both synthetic and biological.

Mechanical characterization of soft viscoelastic gels via indentation and optimization-based inverse finite element analysis

Polymer gels are widely accepted as candidate materials for tissue engineering, drug delivery, and orthopedic load-bearing applications. In addition, their mechanical and physical properties can be tailored to meet a wide range of design requirements. For soft gels whose elastic modulus is in the kPa range, mechanical characterization by bulk mechanical testing methods presents challenges, for example, in sample preparation, fixture design, gripping, and/or load measurement accuracy. Nanoindentation, however, has advantages when characterizing the mechanical properties of soft materials. This study was aimed at investigating the application of an inverse finite element analysis technique to identify material parameters of polymer gels via nanoindentation creep testing, optimization, and finite element simulation. Nanoindentation experiments were conducted using a rigid circular flat punch, and then simulated using the commercial software ABAQUS™. The optimization (error minimization) procedure was integrated in the parameter determination process using a Matlab™ shell program, which makes this approach readily adaptable to other test geometries and material models. The finite element results compare well with a derived analytical viscoelastic solution for a rigid circular flat punch on a Kelvin–Voigt half-space.

Active and Passive Microrheology in Equilibrium and Nonequilibrium Systems

Quantitatively measuring the mechanical properties of soft matter over a wide range of length and time scales, especially if a sample is as complex as typical biological materials, remains challenging. Living cells present a further complication because forces are generated within these nonequilibrium materials that can change material properties. We have here developed high-bandwidth techniques for active one- and two-particle microrheology to tackle these issues. By combining active micromanipulation of probe particles with an optical trap with high-resolution tracking of thermal motions of the very same particles by laser interferometry, we can both measure the mechanical properties of and, at the same time, identify nonequilibrium forces in soft materials. In both simple liquids and equilibrium cytoskeletal actin networks, active microrheology (AMR) proves to be less noise sensitive than and offers extended bandwidth (0.1−100 kHz) compared to passive microrheology (PMR), which merely tracks thermal mo...

A novel optical coherence tomography-based micro-indentation technique for mechanical characterization of hydrogels

Depth-sensing micro-indentation has been well recognized as a powerful tool for characterizing mechanical properties of solid materials due to its non-destructive approach. Based on the depth-sensing principle, we have developed a new indentation method combined with a high-resolution imaging technique, optical coherence tomography, which can accurately measure the deformation of hydrogels under a spherical indenter at constant force. The Hertz contact theory has been applied for quantitatively correlating the indentation force and the deformation with the mechanical properties of the materials. Young's moduli of hydrogels estimated by the new method are comparable with those measured by conventional depth-sensing micro-indentation. The advantages of this new method include its capability to characterize mechanical properties of bulk soft materials and amenability to perform creeping tests. More importantly, the measurement can be performed under sterile conditions allowing non-destructive, in situ and real-time investigations on the changes in mechanical properties of soft materials (e.g. hydrogel). This unique character can be applied for various biomechanical investigations such as monitoring reconstruction of engineered tissues.

Mechanical properties and failure of Streptococcus mutans biofilms, studied using a microindentation device

Knowledge of mechanical properties and failure mechanisms of biofilms is needed to determine how biofilms react on mechanical stress. Methods currently available cannot be used to determine mechanical properties of biofilms on a small scale with high accuracy. A novel microindentation apparatus in combination with a confocal microscope was used to determine the viscoelastic properties of Streptococcus mutans biofilms. The apparatus comprises a small glass indenter and a highly sensitive force transducer. It was shown that the present biofilm, grown under still conditions, behaves as a viscoelastic solid with a storage modulus of 1–8 kPa and a loss modulus of 5–10 kPa at a strain of 10%. Biofilm failure was investigated visually through a confocal microscope by dragging the indenter through the biofilm. It was shown that the tensile strength of the biofilm is predominantly determined by the tensile strength of the extracellular polysaccharide matrix. The combination of microindentation and confocal microscopy is a promising technique to determine and characterize the mechanical properties of soft materials in various fields of microbiology.

Experimental and numerical investigations of the deformation of soft materials under tangential loading

The surface ‘tensile test’, in which tangential loads are applied through surface mounted adhesive tapes, is a viable method for the assessment of mechanical properties of soft materials, particularly biological soft materials in vivo. In the present work the deformation pattern and force–displacement relationship in the surface tensile test were experimentally investigated using surface displacement analysis (SDA) and numerically simulated using finite element modelling. The experimental and FE results showed close agreement using silicone rubber as a model material. The force–displacement relationship was found to be dependent on the tape separations. SDA measurements and FE simulation showed that the displacement and strain fields were not uniform and the distribution pattern varies with tape separation. A combined experimental–numerical approach to inversely extract material properties using multiple tests with different length scales is proposed and assessed using a model material.

Second Phase Effects on the Conductivity of Non‐Aqueous Salt Solutions: “Soggy Sand Electrolytes”

Introducing insulating phases into non‐aqueous salt solution leads to remarkable conductivity enhancements, an effect ascribed to breaking up of ion pairs. This new class of electrolytes combines the high ionic conductivities of liquids and beneficial mechanical properties of soft matter (“soggy sand”; see Figure).

Improved Li-Battery Electrolytes by Heterogeneous Doping of Nonaqueous Li-Salt Solutions

The concept of heterogeneous doping is applied to liquid lithium battery electrolytes. For that purpose, surface-acidic oxides (viz. SiO 2 ) are dispersed in LiPF 6 containing ethylene carbonate/dimethyl carbonate (w/w 1:1). If the volume fraction (φ) is so high that the particles touch, the overall conductivity is significantly enhanced, in particular if the grain size is small. The results are explained by adsorption of the anion and therefore dissociating Li + out of the ion pairs. This is in line with the zeta potential measurements. At lower φ, the effect is negligible owing to the repulsion of the colloids. At higher φ the conductivity is lowered because of inhomogeneity effects and eventually by the insulating nature of the admixtures. In addition to the enhanced overall conductivity, these composites show the favorable mechanical properties of soft matter (soggy sand electrolytes) and exhibit high effective viscosities. A significant advantage is that the SiO 2 content can make the use of polymer separators dispensable. Performance tests in Li/Li x C 6 cells do not show any perceptible interaction of SiO 2 with high Li activity materials.

Growth kinetics of a nanoprotuberance under the action of an oscillating nanotip

The atomic force microscope is a versatile tool that allows many routes to be used for investigating the mechanical properties of soft materials on the nanometer scale. In the present work, experiments were performed on polystyrene polymer films of various molecular weight by approaching a vibrating nanotip towards the surface. The variation of the oscillating amplitude of the cantilever is interpreted as the result of the growth process of a nanoprotuberance. The growth rate is found to be dependent of the magnitude of the oscillating amplitude and of the molecular weight. A model is developed describing in a very simple way the action of the tip and a viscoelastic response of the polymer. The numerical simulation helps in understanding the nonlinear relation between the growth rate and the vibrating amplitude of the microlever and describes qualitatively most of the experimental features. For the softer material, experimental situations are found that allow the experimental results to be amenable with an analytical solution. The analytical solution provides a fruitful comparison with the experimental results showing that some of the nanoprotuberance evolution cannot be explained with the approximation used. The presents results show that there exists a new and fascinating route to better understand the mechanical response at the local scale.