Design tough and resilient hydrogels for artificial cartilage and heart valve

As swollen polymer networks in water, hydrogels are usually brittle. However, hydrogels with high toughness play critical roles in many plant and animal tissues as well as in diverse applications such as tissue regeneration. Here, we demonstrate that the general principle for the design of tough hydrogels is to implement mechanisms to dissipate substantial mechanical energy but still maintain high elasticity of hydrogels. A matrix that combines various mechanisms is constructed for the first time to guide the design of next-generation tough hydrogels. We highlight that a particularly promising strategy for the design is to implement multiple mechanisms across multiple length scales into nano-, micro-, meso-, and macrostructures of hydrogels. Thereafter, we use the multiscale multimechanism approach to design a tough and resilient hydrogel capable of 3D printing and cell encapsulation. A multiscale model is further developed to quantitatively explain the experimental results and guide the design of future hydrogels

Designing toughness and strength for soft materials

Soft materials, such as hydrogels, elastomers, and plastics, are pervasive in nature and society. For example, except for teeth, nails, and bones, all other components of the human body are hydrogels, and we eat, wear, and use soft materials as foods, clothes, shoes, and car tires, just to name a few, in our daily life. Although amorphous polymer chains endow soft materials with high flexibility or “softness,” they can also lead to inferior mechanical performances, such as low fracture toughness and low strength, frequently hampering applications and innovations of soft materials. Over recent years, intensive efforts have been devoted to the development of soft materials that possess extraordinary mechanical properties, especially by seeking inspiration from nature and biology. For instance, synthetic hydrogels can now be made much tougher than articular cartilages and more adhesive than mussel glues. In PNAS, Wu et al. report the fabrication of bioinspired fibers with both high toughness and high strength by drawing from a hydrogel at ambient temperatures and pressures. This nascent progress in the field raises a generic question. What are the fundamental principles for the design of soft materials to achieve certain mechanical properties in nature and engineered systems? Compared with hard materials such as steels and ceramics, which have been explored over centuries, such principles are still elusive in the field of soft materials.National Science Foundation (U.S.) (Grant CMMI-1661627)United States. Office of Naval Research (Grant N00014-14-1-0619)Massachusetts Institute of Technology. Institute for Soldier Nanotechnologie

Harnessing large deformation and instabilities of soft dielectrics: theory, experiment, and application

Widely used as insulators, capacitors, and transducers in daily life, soft dielectrics based on polymers and polymeric gels play important roles in modern electrified society. Owning to their mechanical compliance, soft dielectrics subject to voltages frequently undergo large deformation and mechanical instabilities. The deformation and instabilities can lead to detrimental failures in some applications of soft dielectrics such as polymer capacitors and insulating gels, but can also be rationally harnessed to enable novel functions such as artificial muscle, dynamic surface patterning, and energy harvesting. According to mechanical constraints on soft dielectrics, we classify their deformation and instabilities into three generic modes: (i) thinning and pull-in, (ii) electro-creasing to cratering, and (iii) electro-cavitation. We then provide a systematic understanding of different modes of deformation and instabilities of soft dielectrics by integrating state-of-the-art experimental methods and observations, theoretical models, and applications. Based on the understanding, a systematic set of strategies to prevent or harness the deformation and instabilities of soft dielectrics for diverse applications are discussed

Multimodal Surface Instabilities in Curved Film–Substrate Structures

Structures of thin films bonded on thick substrates are abundant in biological systems and engineering applications. Mismatch strains due to expansion of the films or shrinkage of the substrates can induce various modes of surface instabilities such as wrinkling, creasing, period doubling, folding, ridging, and delamination. In many cases, the film-substrate structures are not flat but curved. While it is known that the surface instabilities can be controlled by film-substrate mechanical properties, adhesion and mismatch strain, effects of the structures' curvature on multiple modes of instabilities have not been well understood. In this paper, we provide a systematic study on the formation of multimodal surface instabilities on film-substrate tubular structures with different curvatures through combined theoretical analysis and numerical simulation. We first introduce a method to quantitatively categorize various instability patterns by analyzing their wave frequencies using fast Fourier transform (FFT). We show that the curved film-substrate structures delay the critical mismatch strain for wrinkling when the system modulus ratio between the film and substrate is relatively large, compared with flat ones with otherwise the same properties. In addition, concave structures promote creasing and folding, and suppress ridging. On the contrary, convex structures promote ridging and suppress creasing and folding. A set of phase diagrams are calculated to guide future design and analysis of multimodal surface instabilities in curved structures. Keywords: instability, curvature, film–substrate structure, morphogenesisUnited States. Office of Naval Research (N00014-14-1-0528)Massachusetts Institute of Technology. Institute for Soldier NanotechnologiesNational Science Foundation (U.S.) (CMMI1253495

Tunable stiffness of electrorheological elastomers by designing mesostructures

Electrorheological elastomers have broad and important applications. While existing studies mostly focus on microstructures of electrorheological elastomers, their mesoscale structures have been rarely investigated. We present a theory on the design of mesostructures of electrorheological elastomers that consist of two phases with different permittivity. We show that the deformation of elastomers can reorient their mesostructures, which consequently results in variations of their effective permittivity, leading to stiffening, softening, or instability of the elastomer. Optimal design of the mesostructures can give giant tunable stiffness. Our theoretical model is further validated by results from numerical simulations.The work was supported by NSF (CMMI-1253495, CMMI-1200515, and DMR-1121107). C.C. acknowledged the financial support from the Australian National Universality by Dean’s Travel Grant Award and Vice Chancellor’s Travel Grant

A unified 3D phase diagram of growth induced surface instabilities

Biological world metabolizes itself with germination, growth, development, and aging every second. A variety of fascinating morphological patterns arise on surfaces of growing, developing or aging tissues, organs and micro--organism colonies. The basic mechanism has been long believed to be the mechanical mismatch due to -differential growth between layers with different biological compositions. These patterns have been observed in separate systems and topologically classified as crease, wrinkle-fold, period-double, ridge, delaminated-buckle, and coexistence states. However, a general and systematic understanding of their initiation and evolution remains largely elusive. We construct a unified 3D phase diagram that predicts initially flat tissue layers can transform to various instability patterns, systematically depending on three physical parameters: mismatch strain, modulus ratio between layers, and adhesion energy on the interface. Our phase diagram matches consistently with our mimic in vitro experiments and documented data in state-of-the-art literature

Design stiff, tough and stretchy hydrogels via nanoscale hybrid crosslinking and macroscale fiber reinforcement

Hydrogels’ applications are limited by their weak mechanical properties. The toughness, modulus, and strength of conventional hydrogels (single network gels) are, respectively, \u3c10 J m‑2, \u3c100 kPa, and \u3c10 kPa, which fail to provide sufficient mechanical properties in large quantities of applications. Here, we designed highly stretchable, tough, yet stiff hydrogels via nanoscale hybrid crosslinking and macroscale fiber reinforcement. We used 3D printing technology to fabricate 3D patterned fibrous structures. Hydrogel composites were constructed by impregnating the PLA fiber mesh with highly stretchable and tough PAAM-alginate hydrogels. Synthetic gels can reach fracture energies of ~9000 J m‑2. However, modulus of these tough hydrogels is only ~100 kPa. Here, we designed fiber reinforced hydrogels, which can reach fracture energy of about 30 000 J m-2 and modulus of ~6 MPa. The enhancement of toughness is due to multiscale toughening mechanism which spans over multiple length scales ranging from nanometers to millimeters. This design of fiber reinforced hydrogel composites can serve as a model to expand the application of hydrogels in both biomedical and robotic areas

Experimental and theoretical investigation of magnetorheological elastomers with layered mesostructures

Magnetorheological elastomer (MRE) is a type of smart material which can vary its shear modulus rapidly, continuously and reversibly, by the external magnetic field. MRE has attracted increasing attention and been widely used in various applications. Generally it has been created by dispersing magnetic filler particles in polymer matrices. Most current studies are focusing on the microstructures of MRE such as the alignments of iron-filler particles and their effects on tunable moduli. However, the mesoscale structures of MREs have been rarely investigated by now. In this study, we present a theory on the design of mesostructures of MRE composites consisting of two phases of materials with different permeability. We show that the deformation of elastomers can reorient their mesostructures, which consequently results in variations of their effective permeability. Such variations change the magnetostatic potential energies of the elastomers under applied fields, leading to stiffening, softening, or instabilities. We further fabricate composite MREs by embedding metal-sheets into PDMS matrix to test the feasibility of the concept for MRE. Experimental results show that giant tunable stiffness of MREs can be achieved by carefully designing and optimizing their anisotropic mesostructures. The effect of metal-sheets at mesoscale and carbonyl iron particles at microscale can be superimposed together to increase the MR effect of the composite even if the microsized particles are uniformly distributed

Hydrogel bioelectronics

Bioelectronic interfacing with the human body including electrical stimulation and recording of neural activities is the basis of the rapidly growing field of neural science and engineering, diagnostics, therapy, and wearable and implantable devices. Owing to intrinsic dissimilarities between soft, wet, and living biological tissues and rigid, dry, and synthetic electronic systems, the development of more compatible, effective, and stable interfaces between these two different realms has been one of the most daunting challenges in science and technology. Recently, hydrogels have emerged as a promising material candidate for the next-generation bioelectronic interfaces, due to their similarities to biological tissues and versatility in electrical, mechanical, and biofunctional engineering. In this review, we discuss (i) the fundamental mechanisms of tissue-electrode interactions, (ii) hydrogels' unique advantages in bioelectrical interfacing with the human body, (iii) the recent progress in hydrogel developments for bioelectronics, and (iv) rational guidelines for the design of future hydrogel bioelectronics. Advances in hydrogel bioelectronics will usher unprecedented opportunities toward ever-close integration of biology and electronics, potentially blurring the boundary between humans and machines.National Science Foundation (U.S.) (CMMI-1661627)United States. Office of Naval Research (N00014-17-1-2920)Massachusetts Institute of Technology. Institute for Soldier Nanotechnologies (W911NF-13-D-0001)Samsung Scholarship FoundationNational Natural Science Foundation (China) (51763010)Science Foundation for Excellent Youth Talents in Jiangxi Province (20162BCB23053)Key Research and Development Program of Jiangxi Province (20171BBH80007)Natural Science Foundation of Jiangxi Province (20171BAB216018)China Scholarship Council (201608360062

Predicting Fracture Energies and Crack-Tip Fields of Soft Tough Materials

Soft materials including elastomers and gels are pervasive in biological systems and technological applications. Whereas it is known that intrinsic fracture energies of soft materials are relatively low, how the intrinsic fracture energy cooperates with mechanical dissipation in process zone to give high fracture toughness of soft materials is not well understood. In addition, it is still challenging to predict fracture energies and crack-tip strain fields of soft tough materials. Here, we report a scaling theory that accounts for synergistic effects of intrinsic fracture energies and dissipation on the toughening of soft materials. We then develop a coupled cohesive-zone and Mullins-effect model capable of quantitatively predicting fracture energies of soft tough materials and strain fields around crack tips in soft materials under large deformation. The theory and model are quantitatively validated by experiments on fracture of soft tough materials under large deformations. We further provide a general toughening diagram that can guide the design of new soft tough materials.Comment: 22 pages, 5 figure