Strain-dependent Mechanical Properties Research Articles

Colloidal hydrogels made of gelatin nanoparticles exhibit fast stress relaxation at strains relevant for cell activity

Viscoelastic properties of hydrogels such as stress relaxation or plasticity have been recognized as important mechanical cues that dictate the migration, proliferation, and differentiation of embedded cells. Stress relaxation rates in conventional hydrogels are usually much slower than cellular processes, which impedes rapid cellularization of these elastic networks. Colloidal hydrogels assembled from nanoscale building blocks may provide increased degrees of freedom in the design of viscoelastic hydrogels with accelerated stress relaxation rates due to their strain-sensitive rheology which can be tuned via interparticle interactions. Here, we investigate the stress relaxation of colloidal hydrogels from gelatin nanoparticles in comparison to physical gelatin hydrogels and explore the particle interactions that govern stress relaxation. Colloidal and physical gelatin hydrogels exhibit comparable rheology at small deformations, but colloidal hydrogels fluidize beyond a critical strain while physical gels remain primarily elastic independent of strain. This fluidization facilitates fast exponential stress relaxation in colloidal gels at strain levels that correspond to strains exerted by cells embedded in physiological extracellular matrices (10-50%). Increased attractive particle interactions result in a higher critical strain and slower stress relaxation in colloidal gels. In physical gels, stress relaxation is slower and mostly independent of strain. Hence, colloidal hydrogels offer the possibility to modulate viscoelasticity via interparticle interactions and obtain fast stress relaxation rates at strains relevant for cell activity. These beneficial features render colloidal hydrogels promising alternatives to conventional monolithic hydrogels for tissue engineering and regenerative medicine. Statement of significanceIn the endeavor to design biomaterials that favor cell activity, research has long focused on biochemical cues. Recently, the time-, stress-, and strain-dependent mechanical properties, i.e. viscoelasticity, of biomaterials has been recognized as important factor that dictates cell fate. We herein present the viscoelastic stress relaxation of colloidal hydrogels assembled from gelatin nanoparticles, which show a strain-dependent fluidization at strains relevant for cell activity, in contrast to many commonly used monolithic hydrogels with primarily elastic behavior.

Halide Perovskites: Anisotropic In Situ Strain‐Engineered Halide Perovskites for High Mechanical Flexibility (Adv. Funct. Mater. 4/2021)

In article number 2007131, Yong Soo Cho, Da Bin Kim, and Jung Won Lee propose an in-situ strain engineering method to substantially enhance the bending fracture behavior of flexible halide perovskite films. The strain-dependent mechanical properties with structural simulations are extensively useful for halides-based electronic and optoelectronic devices.

Exceptional mechano-electronic properties in the HfN2 monolayer: a promising candidate in low-power flexible electronics, memory devices and photocatalysis.

The response of the electronic properties of the HfN2 monolayer to external perturbation, such as strain and electric fields, has been extensively investigated using density functional theory calculations for its device-based applications and photocatalysis. The HfN2 monolayer is found to be a semiconductor showing a direct band gap of 1.44 eV, which is widely tunable by 0.9 eV via application of biaxial strain. Furthermore, the tunability in the band edges of the HfN2 monolayer straddling the water redox potential under a biaxial strain of ±10% makes it suitable for solar energy harvesting via photocatalytic applications over a wide range (0-7) of pH. The band gap can be decreased by 29.8% under a biaxial tensile strain of 10%. Upon incorporation of spin orbit coupling (SOC) a large spin splitting at the conduction band (Δc ∼ 314 meV) and a small splitting at the valence band (Δv ∼ 32 meV) are noted, which is attributable to the orbital composition of the band edges. The spin splitting in the band edges is found to be adjustable via biaxial compressive strain. The strain dependent mechanical properties and stability reveal the ability of the HfN2 monolayer to withstand a large magnitude of strain of up to ±10%, thereby bringing about a giant tunability in its Young modulus (Y) from 66 N m-1 to 283 N m-1, which is gainfully exploitable in flexible electronics. The tunability in Y over such a wide range has not been observed in other 2D materials. Moreover, the HfN2 monolayer undergoes a transition from a semiconducting to a metallic state under the application of a normal electric field or gate voltage of 0.48 V Å-1, which may potentially serve as the OFF (semiconducting) and ON (metallic) state in devices. Interestingly, an electric field of such intensity has been realized experimentally using pulsed ac field technology. Such a small gate voltage will greatly lower its power consumption. The electronic origin of this transition from the OFF to the ON state is found to arise from unoccupied NFEG (Nearly Free Electron Gas) states. A HfN2 monolayer based tunnel field effect transistor (t-FET) is proposed herewith as a model device for low-power digital data storage, thereby paving new avenues in flexible electronics and memory devices.

Fabrication, characterisation and modelling of uniform and gradient auxetic foam sheets

Large sheets of polyurethane open-cell foam were compressed (or stretched) using pins and a conversion mould whilst undergoing thermal softening and controlled cooling. Sheets (final dimensions 355 × 344 × 20 mm) were fabricated with uniform triaxial compression, with and without through-thickness pins, and also with different compression regimes (uniform triaxial compression or through-thickness compression and biaxial planar tension) in opposing quadrants. The samples fabricated under uniform triaxial compression with and without pins exhibited similar cell structure and mechanical properties. The sheets fabricated with graded compression levels displayed clearly defined quadrants of differing cell structure and mechanical properties. The graded foam quadrants subject to triaxial compression displayed similar cell structure, tangent moduli and negative Poisson's ratio responses to the uniform foams converted with a similar level of triaxial compression. The graded foam quadrants subject to through-thickness compression and biaxial planar tension displayed a slightly re-entrant through-thickness cell structure contrasting with an in-plane structure resembling the fully reticulated cell structure of the unconverted parent foam. This quadrant of graded foam displayed positive and negative Poisson's ratios in tension and compression, respectively, accompanied by high and low in-plane tangent modulus, respectively. The strain-dependent mechanical properties are shown to be fully consistent with expectations from honeycomb theory. The triaxially compressed quadrants of the graded sheet exhibited ∼4 times lower peak acceleration than quadrants with through-thickness compression and biaxial planar tension in 6 J impact tests using a steel hemispherical drop mass.

Strain-dependent dynamic mechanical properties of Kevlar to failure: Structural correlations and comparisons to other polymers

The processing of Kevlar to certain strengths by hot-drawing can benefit by quantitative understanding of the correlation between structural and mechanical properties during the pre-drawing process. Here, we use a novel continuous dynamic analysis (CDA) to monitor the evolution in storage modulus and loss factor of Kevlar 49 fibers as a function of strain via a quasi-static tensile test. Unlike traditional dynamic mechanical analysis, CDA allows the tracking of strain-dependent mechanical properties until failure. The obtained dynamic viscoelastic properties of Kevlar 49 are correlated with structural data obtained from synchrotron radiation analysis and with Raman scattering frequencies. Rate-dependent stress–strain results from Kevlar are compared to Nomex, spider silk, polyester and rubber, and provide insight into how the mechanical properties of Kevlar originate from its characteristic structural features. We find that as the storage modulus of Kevlar is essentially equal to the Young's modulus, the measured quantitative relationships between storage modulus and strain can provide insights into the tuning of the mechanical properties of aramid materials for specific applications.

Characteristics of highly flexible PDMS membranes for long-term mechanostimulation of biological tissue.

Measurement of mechanical properties of soft biological tissue remains a challenging task in mechanobiology. Recently, we presented a bioreactor for simultaneous mechanostimulation and analysis of the mechanical properties of soft biological tissue samples. In this bioreactor, the sample is stretched via deflection of a flexible membrane. It was found that the use of highly compliant membranes increases accuracy of measurements. Here, we describe the production process and characteristics of thin and flexible membranes of polydimethylsiloxane (PDMS) designed to improve the signal-to-noise ratio of our bioreactor. By a spin-coating process, PDMS membranes were built by polymerization of a two component elastomer. The influence of resin components proportion, rotation duration, and speed of the spinning were related to the membrane mechanics. Membranes of 22 mm inner diameter and 33 to 36 microm thickness at homogeneous profiles were produced. Isolated rat diaphragms served as biological tissue samples. Mechanical properties of the membranes remained constant during 24 h of mechanostimulation. In contrast, time- and strain-dependent mechanical properties of the diaphragms were found.

Measuring Molecular Interaction between Actin Filament and Actin Binding Protein Governing Mechanical Properties of Cross-Linked F-Actin Network

Actin binding proteins (ABPs) regulate the assembly of actin filaments (F-actin) into various structures that provide physical support for the cell and play important roles in numerous cellular processes. Although the mechanical properties of F-actin networks have been extensively studied, a full understanding of how molecular interactions between ABPs and actin filaments influence the network mechanical properties of F-actin network is lacking. Here, we study the molecular interaction between ABPs and actin filaments and the mechanical rupture of a cross-linked F-actin network. A single molecule assay was used to measure the rupture force of a complex formed by an ABP filamin linking two actin filaments by pulling one of the filaments with the optical tweezers. The rupture forces for filamin/F-actin complex were 88±34 pN for loading rates of 1209±119 pN/s. About 20% of the force-extension traces exhibited a sawtooth pattern with intervals ∼30 nm comparable to length changes due to unfolding of individual immunoglobulin domains of filamin. We also prepared the F-actin gels and monitored their mechanical responses while an embedded bead was displaced at constant speed. Compared to similar experiments with an entangled F-actin solution, the force-extension curves for the filamin/F-actin network exhibited multiple transitions in the force of 37±17 pN, suggestive of bond rupture. In addition, using the optical tweezers, active microrheology was used to measure the strain-dependent mechanical properties of the filamin/F-actin network. Results demonstrate the network to be highly nonlinear and irreversible. Support from the NIGMS (GM-076689) is gratefully acknowledged.

Negative Poisson ratios and strain-dependent mechanical properties in arterial prostheses

The elastic properties of two artificial arterial prostheses have been measured. One is a fibrillar Polyurethane (PU) material, the other is an expanded polytetrafluoroethylene (ex-PTFE). It is shown that the conventional assumption of incompressibility is invalid for both materials and that changes in the thickness of the prosthetic artery wall must be measured directly. The ex-PTFE shows highly strain-dependent properties, the most significant being that of large negative through-thickness Poisson ratios, up to −11. These results have important consequences for matching the properties of prostheses to natural materials.