Synthetic Soft Materials Research Articles

Tailoring smart hydrogels through manipulation of heterogeneous subdomains

The mechanical interactions among integrated cellular structures in soft tissues dictate the mechanical behaviors and morphogenetic deformations observed in living organisms. However, replicating these multifaceted attributes in synthetic soft materials remains a challenge. In this work, we develop a smart hydrogel system featuring engineered stiff cellular patterns that induce strain-driven heterogeneous subdomains within the hydrogel film. These subdomains arise from the distinct mechanical responses of the pattern and film domains under applied mechanical forces. Unlike previous studies that incorporate reinforced inclusions into soft matrices to tailor material properties, our method manipulates the localization, integration, and interaction of these subdomain building blocks within the soft film. This enables extensive tuning of both local and global behaviors. Notably, we introduce a subdomain-interface mechanism that allows for the concurrent customization and decoupling of mechanical properties and shape transformations within a single material system—an achievement rarely accomplished with current synthetic soft materials. Additionally, our use of in-situ imaging characterizations, including full-field strain mapping via digital imaging correlation and reciprocal-space patterns through fast Fourier transform analysis of real-space pattern domains, provides rapid real-time monitoring tools to uncover the underlying principles governing tailored multiscale heterogeneities and intricate behaviors.

Lattice Symmetry-Guided Charge Transport and Recombination in Supramolecular Assemblies

Synthetic soft materials mimicking biological structures are promising for many applications in fields such as photovoltaics and photocatalysis. For these applications, a deep understanding of the charge- and energy-transfer mechanisms is required to build more efficient devices. In this work, we investigate self-assembled 2D supramolecular structures using angle-resolved photoluminescence (PL) imaging techniques and decompose the optical transition dipoles in the supramolecular structures. Combining these optical studies with Monte Carlo simulations, we reveal the important role played by lattice symmetry in charge dissociation, transport, and recombination, as well as the exciton spin states. These mechanistic findings have important implications for engineering soft materials towards efficient energy conversion and photocatalytic applications.

Designing Ultratough Single-Network Hydrogels with Centimeter-Scale Fractocohesive Lengths via Inelastic Crack Blunting.

Fractocohesive length, defined as the ratio of fracture toughness to work of fracture, measures the sensitivity of materials to fracture in the presence of flaws. The larger the fractocohesive length, the more flaw-tolerant and crack-resistant the hydrogel. For synthetic soft materials, the fractocohesive length is short, often on the scale of 1 mm. Here, highly flaw-insensitive (HFI) single-network hydrogels containing an entangled inhomogeneous polymer network of widely distributed chain lengths are designed. The HFI hydrogels demonstrate a centimeter-scale fractocohesive length of 2.21cm, which is the highest ever recorded for synthetic hydrogels, and an unprecedented fracture toughness of ≈13300 J m-2 . The uncommon flaw insensitivity results from the inelastic crack blunting inherent to the highly inhomogeneous network. When the HFI hydrogel is stretched, a large number of short chains break while coiled long chains can disentangle, unwind, and straighten, producing large inelastic deformation that substantially blunts the crack tip in a plastic manner, thereby deconcentrating crack-tip stresses and blocking crack extension. The flaw-insensitive design strategy is applicable to various hydrogels such as polyacrylamide and poly(N,N-dimethylacrylamide) hydrogels and enables the development of HFI soft composites.

Harnessing multifunctional collagen mimetic peptides to create bioinspired stimuli responsive hydrogels for controlled cell culture.

The demand for synthetic soft materials with bioinspired structures continues to grow. Material applications range from in vitro and in vivo tissue mimics to therapeutic delivery systems, where well-defined synthetic building blocks offer precise and reproducible property control. This work examines a synthetic assembling peptide, specifically a multifunctional collagen mimetic peptide (mfCMP) either alone or with reactive macromers, for the creation of responsive hydrogels that capture aspects of soft collagen-rich tissues. We first explored how buffer choice impacts mfCMP hierarchical assembly, in particular, peptide melting temperature, fibril morphology, and ability to form physical hydrogels. Assembly in physiologically relevant buffer resulted in collagen-like fibrillar structures and physically assembled hydrogels with shear-thinning (as indicated through strain-yielding) and self-healing properties. Further, we aimed to create fully synthetic, composite peptide-polymer hydrogels with dynamic responses to various stimuli, inspired by the extracellular matrix (ECM). Specifically, we established mfCMP-poly(ethylene glycol) (PEG) hydrogel compositions that demonstrate increasing non-linear viscoelasticity in response to applied strain as the amount of assembled mfCMP content increases. Furthermore, the thermal responsiveness of mfCMP physical crosslinks was harnessed to manipulate the composite hydrogel mechanical properties in response to changes in temperature. Finally, cells relevant in wound healing, human lung fibroblasts, were encapsulated within these peptide-polymer hydrogels to explore the impact of increased mfCMP, and the resulting changes in viscoelasticity, on cell response. This work establishes mfCMP building blocks as versatile tools for creating hybrid and adaptable systems with applications ranging from injectable shear-thinning materials to responsive interfaces and synthetic ECMs for tissue engineering.

Investigating Fundamental Principles of Nonequilibrium Assembly Using Temperature-Sensitive Copolymers.

Both natural biomaterials and synthetic materials benefit from complex energy landscapes that provide the foundation for structure-function relationships and environmental sensitivity. Understanding these nonequilibrium dynamics is important for the development of design principles to harness this behavior. Using a model system of poly(ethylene glycol) methacrylate-based thermoresponsive lower critical solution temperature (LCST) copolymers, we explored the impact of composition and stimulus path on nonequilibrium thermal hysteretic behavior. Through turbidimetry analysis of nonsuperimposable heat-cool cycles, we observe that LCST copolymers show clear hysteresis that varies as a function of pendent side chain length and hydrophobicity. Hysteresis is further impacted by the temperature ramp rate, as insoluble states can be kinetically trapped under optimized temperature protocols. This systematic study brings to light fundamental principles that can enable the harnessing of out-of-equilibrium effects in synthetic soft materials.

Interactions of Bacterial Quorum Sensing Signals with Model Lipid Membranes: Influence of Membrane Composition on Membrane Remodeling.

We report the influence of membrane composition on the multiscale remodeling of multicomponent lipid bilayers initiated by contact with the amphiphilic bacterial quorum sensing signal N-(3-oxo)-dodecanoyl-l-homoserine lactone (3-oxo-C12-AHL) and its anionic headgroup hydrolysis product, 3-oxo-C12-HS. We used fluorescence microscopy and quartz crystal microbalance with dissipation (QCM-D) to characterize membrane reformation that occurs when these amphiphiles are placed in contact with supported lipid bilayers (SLBs) composed of (i) 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) containing varying amounts of cholesterol or (ii) mixtures of DOPC and either 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE, a conical zwitterionic lipid) or 1,2-dioleoyl-sn-glycero-3-phospho-l-serine (DOPS, a model anionic lipid). In general, we observe these mixed-lipid membranes to undergo remodeling events, including the formation and subsequent collapse of long tubules and the formation of hemispherical caps, upon introduction to biologically relevant concentrations of 3-oxo-C12-AHL and 3-oxo-C12-HS in ways that differ substantially from those observed in single-component DOPC membranes. These differences in bilayer reformation and their associated dynamics can be understood in terms of the influence of membrane composition on the time scales of molecular flip-flop, lipid packing defects, and lipid phase segregation in these materials. The lipid components investigated here are representative of classes of lipids that comprise both naturally occurring cell membranes and many useful synthetic soft materials. These studies thus represent a first step toward understanding the ways in which membrane composition can impact interactions with this important class of bacterial signaling molecules.

A cellulose-based self-healing composite eutectogel with reversibility and recyclability for multi-sensing

Flexible sensors have been widely applied in wearable devices recently. These devices are prone to abrasion, crack, and accidental damage, which result in function failure or even abandonment aggravating environmental degradation. Therefore, it is a huge challenge to endow the flexible sensors with self-healability, recyclability as well as high mechanical property. Herein, a novel recyclable, highly stretchable, self-healing eutectogel was fabricated by freeze-thaw of polyvinyl alcohol (PVA), gelatin and dialdehyde carboxymethyl cellulose (DCMC) in a deep eutectic solvent (DES) with strain rate, humid and temperature sensing capability. The composite eutectogel consisted of imine bond and hydrogen bond reversible networks, which endowed the eutectogel with faster self-healability, as well as the green and simple recyclability. It performed satisfying tensile strength (up to ∼1.25 MPa) with unexceptionable fracture elongation (∼1400%). By leveraging the DES to inhibit freezing and dehydration, the eutectogel was fabricated with a wide working temperature (−20.9 to 64.1 °C) while its weight and sensing performance remained stable after 30 days in open air. The composite eutectogel sensor manifested reliable mechanical and electrical properties under repetitive deformation operation and sensitive responsiveness to real-time variation of rate, temperature, and humidity, which well mimics human breathing in time. Amazingly, based on the hydrolysis of the imine and hydrogen bonds, the recyclable eutectogels retained vast majority of mechanical strength, adhesion, and conductivity. Compared to most synthetic soft materials, the composite eutectogels provide a promising strategy for the development of sustainable multifunctional soft materials that have high environmental adaptability and practical applications.

Tough, aorta-inspired soft composites

Spatial variations in fiber alignment (and, therefore, in mechanical anisotropy) play a central role in the excellent toughness and fatigue characteristics of many biological materials. In this work, we examine the effect of fiber alignment in soft composites, including both "in-plane" and "out-of-plane" fiber arrangements. We take inspiration from the spatial variations of fiber alignment found in the aorta to three-dimensionally (3D) print soft, tough silicone composites with an excellent combination of stiffness, toughness, and fatigue threshold, regardless of the direction of loading. These aorta-inspired composites exhibit mechanical properties comparable to skin, with excellent combinations of stiffness and toughness not previously observed in synthetic soft materials.

Bioinspired Ultra Tear-Resistant Elastomer with a Slidable Double-Network Structure.

Tear resistance is of vital importance in the fabrication and application of synthetic soft materials. However, the paradox of simultaneously improving the tearing energy and elasticity remains a huge challenge for conventional approaches. Here, inspired by the skin, we successfully constructed an extraordinary tear-resistant, superelastic elastomer by the introduction of nanosized polycyclodextrin into the elastomer network to form a slidable interpenetrate double network structure. The tearing energy of the SDEP elastomer is up to 274 KJ/m2, which is comparable to metals and alloys and increased more than 100 times compared with the chemically cross-linked elastomer. The fracture strain exceeded 3300%, which is hardly achieved by other materials with high tearing energy. This comprehensive improvement of antitearing and super elasticity property was achieved by (i) a slide ring effect to dissipate energy and blunt a crack tip; (ii) straightening and reorientation of the slidable double network to deflect the advancing of a crack tip; (iii) a double network sharing the load. These results provide a novel strategy to fabricate elastic, tear-resistant soft material, which may contribute to the practical application as tear-resistant flexible electronics and irregular-shaped stretchable devices.

Chiral emergence in multistep hierarchical assembly of achiral conjugated polymers

Intimately connected to the rule of life, chirality remains a long-time fascination in biology, chemistry, physics and materials science. Chiral structures, e.g., nucleic acid and cholesteric phase developed from chiral molecules are common in nature and synthetic soft materials. While it was recently discovered that achiral but bent-core mesogens can also form chiral helices, the assembly of chiral microstructures from achiral polymers has rarely been explored. Here, we reveal chiral emergence from achiral conjugated polymers, in which hierarchical helical structures are developed through a multistep assembly pathway. Upon increasing concentration beyond a threshold volume fraction, dispersed polymer nanofibers form lyotropic liquid crystalline (LC) mesophases with complex, chiral morphologies. Combining imaging, X-ray and spectroscopy techniques with molecular simulations, we demonstrate that this structural evolution arises from torsional polymer molecules which induce multiscale helical assembly, progressing from nano- to micron scale helical structures as the solution concentration increases. This study unveils a previously unknown complex state of matter for conjugated polymers that can pave way to a field of chiral (opto)electronics. We anticipate that hierarchical chiral helical structures can profoundly impact how conjugated polymers interact with light, transport charges, and transduce signals from biomolecular interactions and even give rise to properties unimagined before.

Developing biomaterials to mediate the spatial distribution of integrins.

Innovation in material design to regulate cell behavior and function is one of the primary tasks in materials science. Integrins, a family of cell surface-adhesion receptors that mechanically connect the extracellular matrix (ECM) to the intracellular cytoskeleton, have long served as primary targets for the design of biomaterials because their activity is not only critical to a wide range of cell and tissue functions but also subject to very tight and complex regulations from the outside environment. To review the recent progress of material innovations targeting the spatial distribution of integrins, we first introduce the interaction mechanisms between cells and the ECM by highlighting integrin-based cell adhesions, describing how integrins respond to environmental stimuli, including variations in ligand presentation, mechanical cues, and topographical variations. Then, we overview the current development of soft materials in guiding cell behaviors and functions via spatial regulation of integrins. Finally, we discuss the current limitations of these technologies and the advances that may be achieved in the future. Undoubtedly, synthetic soft materials that mediate the spatial distribution of integrins play an important role in biomaterial innovations for advancing biomedical applications and addressing fundamental biological questions.

Anisotropically Fatigue-Resistant Hydrogels.

Nature builds biological materials from limited ingredients, however, with unparalleled mechanical performances compared to artificial materials, by harnessing inherent structures across multi-length-scales. In contrast, synthetic material design overwhelmingly focuses on developing new compounds, and fails to reproduce the mechanical properties of natural counterparts, such as fatigue resistance. Here, a simple yet general strategy to engineer conventional hydrogels with a more than 100-fold increase in fatigue thresholds is reported. This strategy is proven to be universally applicable to various species of hydrogel materials, including polysaccharides (i.e., alginate, cellulose), proteins (i.e., gelatin), synthetic polymers (i.e., poly(vinyl alcohol)s), as well as corresponding polymer composites. These fatigue-resistant hydrogels exhibit a record-high fatigue threshold over most synthetic soft materials, making them low-cost, high-performance, and durable alternatives to soft materials used in those circumstances including robotics, artificial muscles,etc.

Biosynthetic self-healing materials for soft machines.

Self-healing materials are indispensable for soft actuators and robots that operate in dynamic and real-world environments, as these machines are vulnerable to mechanical damage. However, current self-healing materials have shortcomings that limit their practical application, such as low healing strength (below a megapascal) and long healing times (hours). Here, we introduce high-strength synthetic proteins that self-heal micro- and macro-scale mechanical damage within a second by local heating. These materials are optimized systematically to improve their hydrogen-bonded nanostructure and network morphology, with programmable healing properties (2-23 MPa strength after 1 s of healing) that surpass by several orders of magnitude those of other natural and synthetic soft materials. Such healing performance creates new opportunities for bioinspired materials design, and addresses current limitations in self-healing materials for soft robotics and personal protective equipment.

Cephalopod‐Inspired High Dynamic Range Mechano‐Imaging in Polymeric Materials

AbstractCephalopods, such as squid, cuttlefish, and octopuses, use an array of responsive absorptive and photonic dermal structures to achieve rapid and reversible color changes for spectacular camouflage and signaling displays. Challenges remain in designing synthetic soft materials with similar multiple and dynamic responsivity for the development of optical sensors for the sensitive detection of mechanical stresses and strains. Here, a high dynamic range mechano‐imaging (HDR‐MI) polymeric material integrating physical and chemical mechanochromism is designed providing a continuous optical read‐out of strain upon mechanical deformation. By combining a colloidal photonic array with a mechanically responsive dye, the material architecture significantly improves the mechanochromic sensitivity, which is moreover readily tuned, and expands the range of detectable strains and stresses at both microscopic and nanoscopic length scales. This multi‐functional material is highlighted by creating detailed HDR mechanographs of membrane deformation and around defects using a low‐cost hyperspectral camera, which is found to be in excellent agreement with the results of finite element simulations. This multi‐scale approach to mechano‐sensing and ‐imaging provides a platform to develop mechanochromic composites with high sensitivity and high dynamic mechanical range.

Sliding of adhesive nanoscale polymer contacts

The interaction between friction and adhesion is a defining factor of the macroscopic behavior of natural and synthetic soft materials. In this work, the interfacial normal and shear adhesion strength of contacts between nanoscale polymeric fibers were studied using novel experiments aided by micromachined devices by which the critical normal and tangential pull-off forces between individual polyacrylonitrile (PAN) nanofibers with diameters in the range 400 nm – 4 µm were measured in real-time. The work of adhesion under normal detachment, as computed using the Johnson-Kendall-Roberts (JKR) and the Maugis-Dugdale (M-D) models for elastic adhesive contact, was shown to be independent of the nanofiber diameter and comparable to twice the surface energy of bulk PAN. Under shear detachment, peeling of the contact area was calculated using the JKR and the M-D models combined with linear elastic fracture mechanics (LEFM). The M-D model combined with LEFM could predict the experimentally obtained tangential pull-off force instabilities. The interfacial shear adhesion strength is shown to be constant for a broad range of contact radii (25–140 nm) and approximately equal to the material shear stress at yielding. Thus, shear yielding is shown to be the controlling mechanism during shear detachment of individual polymer nanofibers interacting with strong van der Waals adhesion.

Macroscopically oriented polymeric soft materials: synthesis and functions

Owing to the similarity to biological tissues, soft materials have attracted increasing attention as components of biological tissues. However, there is a large difference between synthetic soft materials and biological tissues. Most synthetic soft materials are structurally isotropic, whereas some biological tissues have anisotropic structures that continue over a macroscopic size scale. As seen in muscle, blood vessels, articular cartilage, and iridophores, such anisotropic structures often play critical roles in producing their superb functions. Thus, the control of “structural anisotropy” is a key concept for the further exploration of functional soft materials science. Using the magnetic orientation of nanoobjects in a colloidal dispersion and subsequent in situ polymerization, we recently succeeded in developing anisotropic soft materials, in which highly oriented structures (order parameter = > 0.95) extend over a macroscopic size scale (~10 cm). In addition, we demonstrated unprecedented unique functions of the resultant materials, including ultralarge mechanical anisotropy similar to articular cartilage, quick and large deformation similar to muscular tissues, and dynamic structural color modulation similar to the skin of tropical fish. Integration of the related studies on soft materials with anisotropically oriented structures would bring the concepts of artificial organs and soft robotics into reality. Magnetic orientation of nanoscopic objects in dispersions and subsequent in situ polymerization produced anisotropic soft materials, in which highly oriented structures (order parameter = > 0.95) extended over a macroscopic size scale (~10 cm). The resultant materials exhibited unprecedented unique functions that are reminiscent of biological tissues, including ultralarge mechanical anisotropy, similar to articular cartilages, quick and large deformation, similar to muscular tissues, and dynamic structural color modulation, similar to the skin of tropical fish.

3D Printing of Anisotropic Hydrogels with Bioinspired Motion.

Motion in biological organisms often relies on the functional arrangement of anisotropic tissues that linearly expand and contract in response to external signals. However, a general approach that can implement such anisotropic behavior into synthetic soft materials and thereby produce complex motions seen in biological organisms remains a challenge. Here, a bioinspired approach is presented that uses temperature‐responsive linear hydrogel actuators, analogous to biological linear contractile elements, as building blocks to create three‐dimensional (3D) structures with programmed motions. This approach relies on a generalizable 3D printing method for building 3D structures of hydrogels using a fugitive carrier with shear‐thinning properties. This study demonstrates that the metric incompatibility of an orthogonally growing bilayer structure induces a saddle‐like shape change, which can be further exploited to produce various bioinspired motions from bending to twisting. The orthogonally growing bilayer structure undergoes a transition from a stretching‐dominated motion to a bending‐dominated motion during its shape transformation. The modular nature of this approach, together with the flexibility of additive manufacturing, enables the fabrication of multimodular 3D structures with complex motions through the assembly of multiple functional components, which in turn consist of simple linear contractile elements.

Needle-shaped ultrathin piezoelectric microsystem for guided tissue targeting via mechanical sensing.

Needles for percutaneous biopsies of tumour tissue can be guided by ultrasound or computed tomography. However, despite best imaging practices and operator experience, high rates of inadequate tissue sampling, especially for small lesions, are common. Here, we introduce a needle-shaped ultrathin piezoelectric microsystem that can be injected or mounted directly onto conventional biopsy needles and used to distinguish abnormal tissue during the capture of biopsy samples, through quantitative real-time measurements of variations in tissue modulus. Using well-characterized synthetic soft materials, explanted tissues and animal models, we establish experimentally and theoretically the fundamental operating principles of the microsystem, as well as key considerations in materials choices and device designs. Through systematic tests on human livers with cancerous lesions, we demonstrate that the piezoelectric microsystem provides quantitative agreement with magnetic resonance elastography, the clinical gold standard for the measurement of tissue modulus. The piezoelectric microsystem provides a foundation for the design of tools for the rapid, modulus-based characterization of tissues.

A two-step method for rate-dependent nano-indentation of hydrogels

Soft, biphasic materials such as hydrogels are commonly used to mimic lubrication and confinement mechanics of biological tissue such as articular cartilage or the cornea. In-depth understanding of such mechanics is crucial for designing synthetic replacements for cartilage, contact-lens materials or soft coatings for medical devices. Using colloidal-probe atomic force microscopy (AFM), surfaces can be investigated at the nanoscale and information on the contact modulus, poro-viscoelastic properties and the permeability can be extracted. Yet, probing the surface of a soft material in a liquid environment is challenging, since the point of contact between a probe and sample surface during finite-rate indentation can be obscured by viscous squeeze-out effects of temporarily confined liquid. To address this issue, we have developed a 2-step indentation method that enables accurate alignment of finite-rate indentation curves with respect to the contact point of quasi-static indentation of soft matter in liquid. In this work, the issue and the method are illustrated by measurements on a commonly used poly(acrylamide) (PAAm) hydrogel. We have shown that liquid squeeze-out may cause non-negligible force offsets that can result in false contact-point determination during finite-rate indentation. The presented method allows accurate alignment of the indentation curves, enables one to accurately study the rate-dependent contact moduli and related stiffening effects, and thus greatly facilitates mechanical characterization of both biological as well as synthetic soft materials.

Mimicking Bone Nanostructure by Combining Block Copolymer Self-Assembly and 1D Crystal Nucleation

The orientation and spatial distribution of nanocrystals in the organic matrix are two distinctive structural characteristics associated with natural bone. Synthetic soft materials have been used to successfully control the orientation of mineral crystals. The spatial distribution of minerals in a synthetic scaffold, however, has yet to be reproduced in a biomimetic manner. Herein, we report using block copolymer-decorated polymer nanofibers to achieve biomineralized fibrils with precise control of both mineral crystal orientation and spatial distribution. Exquisite nanoscale structural control in biomimetic hybrid materials has been demonstrated.