Applications Of Soft Materials Research Articles

Versatile data-adaptive hyperelastic energy functions for soft materials

Applications of soft materials are customarily linked to complex deformation scenarios and material nonlinearities. In the bioengineering field, soft materials typically mimic the low stiffness of biological matter subjected to extreme deformations. Computational frameworks surge as a versatile tool to assist the design of functional applications. The constitutive model lies at the core of such frameworks. In this regard, the customary extreme non-linear behavior of elastomers poses an additional challenge to thoroughly capture the material behavior. Here, data-driven methodologies hold considerable promise for enhancing constitutive modeling when contrasted with phenomenological approaches. In this investigation, we introduce a versatile data-adaptive method tailored to the modeling of hyperelastic soft materials at finite strains. Specifically, our method substitutes an a priori chosen model for the strain energy function by a flexible interpolant defined on a discretized invariant space. Within this framework, the interpolation values assume the role of material parameters and are determined through finite element model updating to conform to measured experimental data — comprising full-field displacements coming from Digital-Image-Correlation and global reaction forces. We validate the method on uniaxial experimental tests of soft elastomers, encompassing ELASTOSILTM, DOWSILTM, and V HBTM. Overall, we aim to establish a new route for the construction of hyperelastic energy functions, untethered from any predefined existing models or assumptions regarding the shape of the energy.

Fatigue-resistant adhesion through high energy barriers

The applications of soft materials in various fields often require interfacial adhesion to sustain prolonged static or cyclic loads, whereas most existing adhesives are susceptible to fatigue failure. Unlike in a quasistatic debonding process, which depends more on the average resistance, the key to preventing fatigue crack propagation is to build up high energy barriers locally. Herein, we invoke three types of structural designs to induce large energy barriers at the interface to achieve fatigue-resistant adhesion. By varying the local bending stiffness of the backing layer, locally altering the fracture mode through kirigami patterns, or hindering crack initiation with simple edge notches, we enhanced the fatigue thresholds of various adhesives against peeling by several orders of magnitude, reaching record-breaking values. To verify the proposed mechanism and reveal the details of these remarkable enhancements, we develop theoretical models to study the peeling processes. Based entirely on structural design, the proposed mechanism is non-material-specific and universally applicable to various intermolecular interactions under any harsh environment, such as high temperature, high humidity, and physiological environments. We envision that the strategy and methodologies presented can pave the avenue of future adhesion designs for both durability and reliability.

Room temperature stable twist-bend nematic materials without crystallization over 1 year

The twist-bend nematic (NTB) phase of achiral liquid crystals (LCs) manifests a unique self-assembled heliconical structure with nanometer-scale pitch length, mirroring the chiral symmetry-breaking phenomena in nature, thus sparking widespread research interest. However, the ingenious NTB phase is only stable at high temperatures within a very limited temperature interval, often undergoing inevitable crystallization at low temperatures. Herein, room temperature supercooled NTB material systems composed of meticulously designed LC dimer mixtures with varying molecular curvatures and central flexibility were developed, resulting in complete resistance to crystallization even after 1 year of storage. Furthermore, the proposed NTB material systems demonstrated exceptional compatibility with common nematic LCs, facilitating the tailoring of overall physical parameters, particularly to achieve a sufficiently low bend elastic constant with excellent stability. This work represents a paradigmatic advancement forward in realizing stable NTB phase materials with a broad temperature range and resistance to crystallization, thereby tackling the enduring and seemingly insurmountable challenge while providing impetus for further exploration of their applications in soft matter, crystallography, and advanced photonics.

Coarse-Grained Artificial Intelligence for Design of Brush Networks.

The ability to synthesize elastomeric materials with programmable mechanical properties is vital for advanced soft matter applications. Due to the inherent complexity of hierarchical structure-property correlations in brush-like polymer networks, the application of conventional theory-based, so-called Human Intelligence (HI) approaches becomes increasingly difficult. Herein we developed a design strategy based on synergistic combination of HI and AI tools which allows precise encoding of mechanical properties with three architectural parameters: degrees of polymerization (DP) of network strands, nx, side chains, nsc, backbone spacers between side chains, ng. Implementing a multilayer feedforward artificial neural network (ANN), we took advantage of model-predicted structure-property cross-correlations between coarse-grained system code including chemistry specific characteristics S = [l, v, b] defined by monomer projection length l and excluded volume v, Kuhn length b of bare backbone and side chains, and architecture A = [nsc, ng, nx] of polymer networks and their equilibrium mechanical properties P = [G, β] including the structural shear modulus G and firmness parameter β. The ANN was trained by minimizing the mean-square error with Bayesian regularization to avoid overfitting using a data set of experimental stress-deformation curves of networks with brush-like strands of poly(n-butyl acrylate), poly(isobutylene), and poly(dimethylsiloxane) having structural modulus G < 50 kPa and 0.01 ≤ β ≤ 0.3. The trained ANN predicts network mechanical properties with 95% confidence. The developed ANN was implemented for synthesis of model networks with identical mechanical properties but different chemistries of network strands.

Ultrafast and facile construction of programmable, multidimensional wrinkled-patterned polyacrylamide/sodium alginate hydrogels for human skin-like tactile perception

Customizable structures and patterns are becoming powerful tools for biomimetic design and application of soft materials. The construction of long-range ordered self-wrinkled structures on multi-dimensional and complex-shaped surfaces with facile, fast and efficient strategies still faces serious challenges. During the stretch-recovery process, the carboxyl groups in the polyacrylamide/sodium alginate dual network gel form robust coordination with Fe3+ to achieve a hard shell layer, resulting in a modulus mismatch between the inner soft layer and the outer hard layer, thereby forming a wrinkled surface. This flexible strategy allows simultaneous construction of complex topologies from 1D to 3D wits well-organized microstructure and controllable dimensions. The mechanism of the influence of ion treating time and pre-stretching ratio on wrinkle wavelength was explored in detail. The finite element simulations matched well with the experimental results. Due to the unique surface and dual crosslinking network, the self-wrinkled hydrogel maintains a high sensitivity of up to 67.47 kPa−1 in 1000 compression cycles. As a high-sensitivity pressure sensor integrated into the detection system, it can be efficiently applied to the contact dynamic tactile perception and monitoring of various movement behaviors of the human body.

A mixed finite element method for large deformation of flexoelectric materials

Flexoelectricity is a universal and size-dependent electromechanical phenomenon coupling the electric field with strain gradient in dielectric materials. we establishe a mixed variational principle and develop a mixed finite element method (FEM). This allows us to use C0-continuous interpolation functions in FEM instead of C1 that is difficult to construct but has to be used in conventional FEM to investigate flexoelectricity. A good agreement between the analytical solutions and our numerical results of an infinite cylinder problem validates the present developed mixed FEM. We further investigate the effects of geometrically nonlinearity on flexoelectricity in soft dielectrics under large deformation. It is found that the flexoelectricity can be greatly enhanced due to the large deformation of soft dielectrics even with much smaller flexoelectric coefficients than popular ceramics. We also demonstrate the necessarity of adopting finite strain model or considering geometrical nonlinearity to precisely describing flexoelectric response in soft dielectrics. The findings in this work also suggest tremendous potential application of soft materials in the field of electromechanical sensors via flexoelectricity.

Are MXenes suitable for soft multifunctional composites?

MXenes are a family of two-dimensional (2D) nanomaterials known for their high electrical and thermal conductivity, as well as high aspect ratios. Recent research has focused on dispersing MXenes within compliant polymer matrices, aiming to create flexible and stretchable composites that harness MXenes' exceptional conductivity and aspect ratios. Experimental findings demonstrate the potential of MXene polymer composites (MXPCs) as flexible electrical, thermal conductors, and high dielectric materials, with promising applications in soft matter engineered systems. However, the 2D structure of MXene inclusions and their relatively large elastic modulus can impart increased stiffness to the polymer matrix, posing limitations on the mechanical flexibility of these functional materials. Here, we introduce a modeling platform to predict the mechanics and functionality of MXene elastomer composites and assess their suitability as soft multifunctional materials. Our investigation primarily focuses on understanding the influence of MXenes' size, layered structure, and percolation arrangements on the effective properties of the resulting composites. Through our model, we successfully determined the elastic modulus, thermal conductivity, and dielectric constant of MXene elastomer composites, and our results exhibit strong agreement with those obtained through finite element analysis. By utilizing this framework, we can theoretically identify the necessary microstructures of MXenes and guide the experiments, enabling the creation of MXPCs with the desired synergistic mechanical and functional properties.

Self-assembly of anionic surfactant and its effect on oil–water interface: Implications for enhanced oil recovery

Surfactant micellization is a self-assembly process governed by complex interplay of solvent-mediated interactions. However, quantitative modeling of micelle formation and its effect on oil–water interface are not yet completely understood. In this work, dissipative particle dynamics simulations were used to investigate the self-assembly behavior of anionic surfactants of alcohol alkoxy sulfate (AAS) and its structural influence on the interfacial properties of oil–water at the mesoscopic scale. Our results indicate that with the increase of surfactant concentration, the morphology of surfactant aggregates varied from spherical micelles to rod-shaped micelles, and finally formed worm-like micelles. The larger the number of propylene oxide (PO) groups, the easier the formation of large micelles and the more tightness of formed micelles. On the oil–water interface, the interfacial tension of solutions decreased with increasing surfactant concentration below the critical micelle concentration, whereas a positive correlation was found for the tightness of surfactant arrangement. When surfactants formed micelles, the overall coverage area of AAS decreased and a lower root mean square end distance of individual PO group directly affected interfacial properties of AAS. The obtained results could provide a solid theoretical basis for the rational design of efficient surfactants in oil recovery and soft matter applications.

Enhanced Accumulation of Colloidal Particles in Microgrooved Channels via Diffusiophoresis and Steady-State Electrolyte Flows.

The delivery of colloidal particles in dead-end microstructures is very challenging, since these geometries do not allow net flows of particle-laden fluids; meanwhile, diffusive transport is slow and inefficient. Recently, we introduced a novel particle manipulation strategy, based on diffusiophoresis, whereby the salt concentration gradient between parallel electrolyte streams in a microgrooved channel induces the rapid (i.e., within minutes) and reversible accumulation, retention, and removal of colloidal particles in the microgrooves. In this study, we investigated the effects of salt contrast and groove depth on the accumulation process in silicon microgrooves and determined the experimental conditions that lead to a particle concentration peak of more than four times the concentration in the channel bulk. Also, we achieved an average particle concentration in the grooves of more than twice the concentration in the flowing streams and almost 2 orders of magnitude larger than the average concentration in the grooves in the absence of a salt concentration gradient. Analytical sufficient and necessary conditions for particle accumulation are also derived. Finally, we successfully tested the accumulation process in polydimethylsiloxane microgrooved channels, as they are less expensive to fabricate than silicon microgrooved substrates. The controlled and enhanced accumulation of colloidal particles in dead-end structures by solute concentration gradients has potential applications in soft matter and living systems, such as drug delivery, synthetic biology, and on-chip diagnostics.

Structural Preservation and Analysis of Biological Macromolecules in Cryogenic Microscopy for Future Drug Discovery

Lately, cryogenic electron microscopy (cryo-EM) has emerged as a cutting-edge biophysical technique for structural analysis of biological macromolecules and assemblies. In structural analysis, structural preservation of samples such as isolated cell organelles, viruses, bacteria, eukaryotic cells and other samples for soft-matter applications is extremely important. Due to the advantage of cryo-EM technique to preserve the structures of biological macromolecules, there is a tremendous research interest in Cryo-EM among biotechnologists, medical professionals and biomaterials scientists that seek an unaltered view on the architecture of a vast array of clinically important biological specimens. In this perspective, we have described a brief overview of the recent progress in this field.

Hybrid double-network dual-crosslinked hydrogel with self-healing ability and mechanical stability. Synthesis, characterization and application for motion sensors

The growing list of potential applications of soft materials increases the demand for, among others, new multifunctional hydrogels. We propose a hybrid double-network hydrogel based on synthetic poly(acrylic acid) and alginate, a natural polymer. These polymer networks were double crosslinked: chemically and physically. The ferrous ion was used to catalyze the decomposition of the initiator and later, after its oxidation by the initiator to the ferric ion, was coordinated by the carboxylic groups of the polymer networks and acted as a physicochemical crosslinker. The composition of the hydrogel was optimized to get improved mechanical and electrical properties, and the self-healing ability. The introduction of the coordination bond between Fe3+ and the carboxylic groups of AA and alginate molecules created the ability of self-healing of the hydrogels; they could regenerate without any external stimulus and regain their electrical conductivity and mechanical properties.

Recent advances in materials and applications for bioelectronic and biorobotic systems

AbstractThe ultimate goal of the advancements in bioelectronics and robotics is the creation of seamless interfaces between artificial devices and biological structures. Current efforts in this area have been focused on designing biocompatible, mechanically compliant, and minimally invasive electronic and robotic systems for a range of applications, such as motor control and sweat sensing. The purposeful design of bioelectronic and robotic systems using the principles of biomimicry enables the creation of biocompatible and life‐like machines and electronics. The success of such approaches relies on the new development and applications of soft materials, as well as methods of actuation and sensing that are inspired, either by composition, function, or properties, of the naturally occurring organisms. A combination of rigid structural components, soft actuators, and flexible sensors can enable the integration of such devices with biological organisms and eventually human users. In this review, we highlight the recent advances in biomimetic soft robotics and bioelectronics. We describe the soft robotic fabrication toolbox and modern solution in bioelectronics that, in our opinion, will enable the fusion of these fields by creating robotic bioelectronic systems. Future development in this area will require substantial integration of adaptable and responsive components at the biointerfaces.

From micelle-like aggregates to extremely-stretchable, fatigue-resistant, highly-resilient and self-healable hydrogels

The applications of hydrogels can be greatly expanded by the improvement of extensibility and toughness. Despite remarkable efforts in tough hydrogels, little is known about the fracture strain above 100 mm/mm due to the inhomogeneous chemical networks and inefficient energy-dissipative mechanism. Herein, micelle-like aggregates were obtained from the self-assembly of dually alkyl-modified polyethylene glycol (PEG) in water, serving as dynamic junctions to fabricate hydrogel via the aqueous radical polymerization of acrylamide (AM) monomers in water with the assistance of methylene bisacrylamide. Unlike hairy micelles, interactions of aggregates and hydrogel framworks were improved obviously through pronounced entanglements between PEG and PAM macromolecules. Meanwhile, the reversible break/reformation of micelle-like aggregates upon stretching empowered the gel an efficient energy-dissipative mechanism. Therefore, hydrogels with extreme stretchability (tensile strain = 150 mm/mm) and high toughness (toughness = 24.8 MJ/m3) were generated. Remarkably, their mechanics could be easily modulated by varying alkyl terminals and molecular weights of PEG to control the strength of hydrophobic domains and physical entanglements, respectively. Two examples were displayed: ultra-sensitive gels for wearable sensors and mechano-responsive molecule release. Impressively, the gauge factor was 60.29 under large deformation (strain > 20 mm/mm), outperforming most conductive hydrogels. As the first research to tune the macroscopic property of the hydrogel by delicately adjusting the microstructures of micelles, and interactions between micelles and gel network, this work not only expands the applications of soft materials but also sheds light on fabricating super-stretchable matters, making it promising candidates for flexible sensors, artificial skins, and drug delivery systems.

Stripe patterns orientation resulting from nonuniform forcings and other competitive effects in the Swift–Hohenberg dynamics

Spatio-temporal pattern formation in complex systems presents rich nonlinear dynamics which leads to the emergence of periodic nonequilibrium structures. One of the most prominent equations for the theoretical and numerical study of the evolution of these textures is the Swift–Hohenberg (SH) equation, which presents a bifurcation parameter (forcing) that controls the dynamics by changing the energy landscape of the system, and has been largely employed in phase-field models. Though a large part of the literature on pattern formation addresses uniformly forced systems, nonuniform forcings are also observed in several natural systems, for instance, in developmental biology and in soft matter applications. In these cases, an orientation effect due to forcing gradients is a new factor playing a role in the development of patterns, particularly in the class of stripe patterns, which we investigate through the nonuniformly forced SH dynamics. The present work addresses amplitude instability of stripe textures induced by forcing gradients, and the competition between the orientation effect of the gradient and other bulk, boundary, and geometric effects taking part in the selection of the emerging patterns. A weakly nonlinear analysis suggests that stripes are stable with respect to small amplitude perturbations when aligned with the gradient, and become unstable to such perturbations when when aligned perpendicularly to the gradient. This analysis is vastly complemented by a numerical work that accounts for other effects, confirming that forcing gradients drive stripe alignment, or even reorient them from preexisting conditions. However, we observe that the orientation effect does not always prevail in the face of competing effects, whose hierarchy is suggested to depend on the magnitude of the forcing gradient. Other than the cubic SH equation (SH3), the quadratic–cubic (SH23) and cubic–quintic (SH35) equations are also studied. In the SH23 case, not only do forcing gradients lead to stripe orientation, but also interfere in the transition from hexagonal patterns to stripes.

Speckle tweezers for manipulation of high and low refractive index micro-particles and nano-particle loaded vesicles

Abstract Several fundamental research and applications in biomedicine and microfluidics often require controlled manipulation of suspended micro- and nanoscale particles. Speckle tweezers (ST) by incorporating randomly distributed light fields have been used to control micro-particles with refractive indices higher than their medium and to perform manipulation tasks such as guiding and sorting. Indeed, compared to periodic potentials, ST represents a wider possibility to be operated for such tasks. Here, we extend the usefulness of ST into micro-particles of low index with respect to the surrounding. Repelling of such particles by high intensity regions into lower intensity regions makes them to be locally confined, and the confinement can be tuned by changing the average grain intensity and size of the speckle patterns. Experiments on polystyrenes and liposomes validate the procedure. Moreover, we show that ST can also manipulate the nano-particle (NP)-loaded liposomes. Interestingly, the different interactions of NP-loaded and empty liposomes with ST enable collective manipulation of their mixture using the same speckle pattern, which may be explained by inclusion of the photophoretic forces on NPs. Our results on the different behaviors between empty and non-empty vesicles may open a new window on controlling collective transportation of drug micro-containers along with its wide applications in soft matter.

A Dual Physical Cross‐Linking Strategy to Construct Tough Hydrogels with High Strength, Excellent Fatigue Resistance, and Stretching‐Induced Strengthening Effect

AbstractHydrogels with excellent stiffness, toughness, anti‐fatigue, and self‐recovery properties are regarded as promising water‐containing materials. In this work, a dual physically cross‐linked (DPC) sodium alginate (SA)/poly[acrylamide (AAm)‐acrylic acid (AAc)‐octadecyl methacrylate (OMA)]‐Fe3+ hydrogel is reported, which is constructed by hydrophobic association (HA) and ionic coordination (IC). The optimal DPC hydrogel demonstrates excellent mechanical performance: tensile modulus of 0.65 MPa, tensile strength of 3.31 MPa, elongation at break of 1547%, and toughness of 27.8 MJ m–3. SA/P(AAm‐AAc‐OMA)‐Fe3+ DPC hydrogels also exhibit prominent anti‐fatigue and self‐recovery performance (99.1–109.7% modulus recovery and 90.4–108.9% dissipated energy recovery after resting for 5 min without additional stimuli at ambient temperature) through the reconstruction of reversible physical cross‐linking. Some of the SA/P(AAm‐AAc‐OMA)‐Fe3+ DPC hydrogels even exhibit a stretching‐induced strengthening effect, which is similar to the performance of muscle—“the more training, the more strength.” Hence, the combination of HA and IC will provide an effective approach to design DPC hydrogels with desirable mechanical performances and a longer service life for wider applications of soft materials.

Adhesion-Induced Instability Regulates Contact Mechanics of Soft Thin Elastic Films

Adhesive contact of soft materials plays an essential role in flexible electronics, soft robots, and biological systems. Classical contact mechanics theories assume that the contact region between a spherical indenter and a flat surface is circular. In this paper, however, we show that fingering instability and inner cavitation may appear and disappear during the loading-unloading process when a soft thin elastic film is indented with a spherical indenter. We further demonstrate that this adhesion-induced instability can enhance the resistance to deformation and thus significantly increase contact force. Finally, we find an important dimensionless number, defined as the ratio of adhesion energy to elastic energy, and thus reveal that the competition between adhesion energy and elastic energy yields the critical condition for adhesion-induced instability. Thus, our findings bridge the gap between contact mechanics and interfacial instabilities of soft materials and may have many important implications for the applications of soft materials.

Modification of whey protein microgel particles with mono- oligo- and polysaccharides through the Maillard reaction: Effects on structural and techno-functional properties

Many food and drink products have a pH value between 4.0 and 6.0, which is near the isoelectric point (pI) of whey proteins. This study evaluated the modification of whey protein microgel (WPM) through Maillard conjugation with several common saccharides including arabinose, glucose, fructose, lactose, inulin, and maltodextrin in a liquid system, which is considered a potential route to boost the utilization of these protein particles in acidic conditions. Glycation degree (22–31 %), pH decrease, presence of final products, FTIR, and SDS-PAGE confirmed the bonding of carbohydrates to WPMs. The conjugates prepared by lactose, glucose, and inulin, i.e. moderately reactive carbohydrates, rendered lower surface hydrophobicity, higher colloidal stability (∼40−50% at pH 4.0), and improved emulsifying properties in comparison with WPM, heated WPM and other glycoproteins. Foaming capacity of Maillard reaction products was lower or without change relative to the WPM; however, foam prepared by maltodextrin-glycated WPM was substantially more stable (up to 24 h). These findings could open avenues for applicability of these pre-denatured densely-packed particles in a range of food products and soft-matter applications.

Synergistic Interactions of Plant Protein Microgels and Cellulose Nanocrystals at the Interface and Their Inhibition of the Gastric Digestion of Pickering Emulsions.

It is possible that Pickering emulsions can optimize the transport of nutraceuticals, pharmaceuticals, and other bioactive compounds in human physiology. So-called ultrastable Pickering emulsions are often destabilized in the gastric digestion regime if the particles are proteinaceous in nature. The present study seeks to test how the interfacial structure can be engineered via synergistic particle-particle interactions to impact the gastric coalescence of Pickering emulsions. In this study, we designed plant-based protein-particle-stabilized oil-in-water emulsions (PPM-E, with 20 wt % sunflower oil) via pea protein microgels (PPM at 1 wt %). The PPM hydrodynamic diameter is ∼250 nm. In vitro gastric digestion of PPM-E confirmed droplet coalescence within 30 min of pepsin addition. Supposedly surface-active cellulose nanocrystals (CNCs, 1-3 wt %) were added to PPM-E at pH 3.0 to determine if they could act as a barrier to interfacial pepsinolysis due to the CNC and PPM being oppositely charged at this gastric pH value. A combination of confocal microscopy, zeta potential, and Langmuir trough measurements suggested that CNCs and PPMs might form a combined layer at the O/W interface, owing to the electrostatic attraction between them. CNCs at >2 wt % inhibited the pepsinolyis of the adsorbed PPM film and thus droplet coalescence. However, increasing concentrations of CNC also increased the bulk viscosity of the PPM-E and eventually caused gelation of the emulsions, which would also delay their gastric breakdown. In conclusion, tuning the bulk and interfacial structure of Pickering emulsions via synergistic interactions between two types of particles could be an effective strategy to modify the enzymatic breakdown of such emulsions, which would have important applications in pharmaceuticals, foods, and other soft-matter applications.

Crosslinking of hydrophilic polymers using polyperoxides

Hydrogels that can mimic mechanical properties and functions of biological tissue have attracted great interest in tissue engineering and biofabrication. In these fields, new materials and approaches to prepare hydrogels without using toxic starting materials or materials that decompose into toxic compounds remain to be sought after. Here, we report the crosslinking of commercial, unfunctionalized hydrophilic poly(2-ethyl-2-oxazoline) using peroxide copolymers in their melt. The influence of temperature, peroxide copolymer concentration, and duration of the crosslinking process has been investigated. The method allows to create hydrogels from unfunctionalized polymers in their melt and to control the mechanical properties of the resulting materials. The design of hydrogels with a suitable mechanical performance is of crucial importance in many existing and potential applications of soft materials, including medical applications.