Showing papers by "Zhigang Suo published in 2019"

Stretchable materials of high toughness and low hysteresis.

Abstract: In materials of all types, hysteresis and toughness are usually correlated. For example, a highly stretchable elastomer or hydrogel of a single polymer network has low hysteresis and low toughness. The single network is commonly toughened by introducing sacrificial bonds, but breaking and possibly reforming the sacrificial bonds causes pronounced hysteresis. In this paper, we describe a principle of stretchable materials that disrupt the toughness-hysteresis correlation, achieving both high toughness and low hysteresis. We demonstrate the principle by fabricating a composite of two constituents: a matrix of low elastic modulus, and fibers of high elastic modulus, with strong adhesion between the matrix and the fibers, but with no sacrificial bonds. Both constituents have low hysteresis (5%) and low toughness (300 J/m2), whereas the composite retains the low hysteresis but achieves high toughness (10,000 J/m2). Both constituents are prone to fatigue fracture, whereas the composite is highly fatigue resistant. We conduct experiment and computation to ascertain that the large modulus contrast alleviates stress concentration at the crack front, and that strong adhesion binds the fibers and the matrix and suppresses sliding between them. Stretchable materials of high toughness and low hysteresis provide opportunities to the creation of high-cycle and low-dissipation soft robots and soft human-machine interfaces.

Fatigue of hydrogels

Abstract: Hydrogels have been developed since the 1960s for applications in personal care, medicine, and engineering. Evidence has accumulated that hydrogels under prolonged loads suffer fatigue. Symptoms include change in properties, as well as nucleation and growth of cracks. This article is the first review on the fatigue of hydrogels. Emphasis is placed on the chemistry of fatigue—concepts and experiments that link symptoms of fatigue to processes of molecules. Symptoms of fatigue are characterized by testing samples with and without precut cracks, subject to prolonged static and cyclic loads. We describe the use of energy release rate for samples with precut cracks, under the conditions of large-scale inelasticity, for hydrogels of complex rheology. Highlighted are three experimental setups: pure shear, tear, and peel, where energy release rate is readily obtained for materials of arbitrary rheology. We describe chemistries of bonds and topologies of networks. Noncovalent bonds and some covalent bonds are reversible: they reform after breaking under relevant conditions. Most covalent bonds are irreversible. Each topology of networks is a way to connect reversible and irreversible bonds. We review experimental data of hydrogels of five representative topologies of networks. We compare the Lake-Thomas threshold, the cyclic-fatigue threshold, and the static-fatigue threshold. Fatigue is a molecular disease. All symptoms of fatigue originate from one fundamental cause: molecular units of a hydrogel change neighbors under prolonged loads. Fatigue correlates with rheology, according to which we distinguish poroelastic fatigue, viscoelastic fatigue, and elastic-plastic fatigue. Many hydrogels have sacrificial bonds that act as tougheners. We distinguish tougheners of two types according to their stress-relaxation behavior under a prolonged static stretch. A liquid-like toughener relaxes to zero stress, and increases neither static-fatigue threshold nor cyclic-fatigue threshold. A solid-like toughener relaxes to a nonzero stress, increases static-fatigue threshold, but does not increase cyclic-fatigue threshold. We outline a strategy to create hydrogels of high endurance. Because of the molecular diversity among hydrogels, the chemistry of fatigue holds the key to the discovery of hydrogels of properties previously unimagined. It is hoped that this review helps to connect chemists and mechanicians.

Self-Healing, Adhesive, and Highly Stretchable Ionogel as a Strain Sensor for Extremely Large Deformation

Abstract: Fabricating a strain sensor that can detect large deformation over a curved object with a high sensitivity is crucial in wearable electronics, human/machine interfaces, and soft robotics. Herein, an ionogel nanocomposite is presented for this purpose. Tuning the composition of the ionogel nanocomposites allows the attainment of the best features, such as excellent self-healing (>95% healing efficiency), strong adhesion (347.3 N m-1 ), high stretchability (2000%), and more than ten times change in resistance under stretching. Furthermore, the ionogel nanocomposite-based sensor exhibits good reliability and excellent durability after 500 cycles, as well as a large gauge factor of 20 when it is stretched under a strain of 800-1400%. Moreover, the nanocomposite can self-heal under arduous conditions, such as a temperature as low as -20 °C and a temperature as high as 60 °C. All these merits are achieved mainly due to the integration of dynamic metal coordination bonds inside a loosely cross-linked network of ionogel nanocomposite doped with Fe3 O4 nanoparticles.

Digital Logic for Soft Devices

Abstract: Although soft devices (grippers, actuators, and elementary robots) are rapidly becoming an integral part of the broad field of robotics, autonomy for completely soft devices has only begun to be developed. Adaptation of conventional systems of control to soft devices requires hard valves and electronic controls. This paper describes completely soft pneumatic digital logic gates having a physical scale appropriate for use with current (macroscopic) soft actuators. Each digital logic gate utilizes a single bistable valve-the pneumatic equivalent of a Schmitt trigger-which relies on the snap-through instability of a hemispherical membrane to kink internal tubes and operates with binary high/low input and output pressures. Soft, pneumatic NOT, AND, and OR digital logic gates-which generate known pneumatic outputs as a function of one, or multiple, pneumatic inputs-allow fabrication of digital logic circuits for a set-reset latch, two-bit shift register, leading-edge detector, digital-to-analog converter (DAC), and toggle switch. The DAC and toggle switch, in turn, can control and power a soft actuator (demonstrated using a pneu-net gripper). These macroscale soft digital logic gates are scalable to high volumes of airflow, do not consume power at steady state, and can be reconfigured to achieve multiple functionalities from a single design (including configurations that receive inputs from the environment and from human users). This work represents a step toward a strategy to develop autonomous control-one not involving an electronic interface or hard components-for soft devices.

A soft ring oscillator

Abstract: Periodic actuation of multiple soft, pneumatic actuators requires coordinated function of multiple, separate components. This work demonstrates a soft, pneumatic ring oscillator that induces temporally coordinated periodic motion in soft actuators using a single, constant-pressure source, without hard valves or electronic controls. The fundamental unit of this ring oscillator is a soft, pneumatic inverter (an inverting Schmitt trigger) that switches between its two states ("on" and "off") using two instabilities in elastomeric structures: buckling of internal tubing and snap-through of a hemispherical membrane. An odd number of these inverters connected in a loop produces the same number of periodically oscillating outputs, resulting from a third, system-level instability; the frequency of oscillation depends on three system parameters that can be adjusted. These oscillatory output pressures enable several applications, including undulating and rolling motions in soft robots, size-based particle separation, pneumatic mechanotherapy, and metering of fluids. The soft ring oscillator eliminates the need for hard valves and electronic controls in these applications.

Polyacrylamide hydrogels. I. Network imperfection

Abstract: A real polymer network is never perfect, but the quantification of network imperfection has been elusive. Here we quantify the network imperfection of a polyacrylamide hydrogel through its mechanical properties. We find that the ultimate properties—strength, extensibility, and work of fracture—have narrow scatter, comparable to the scatter of modulus. Despite the narrow scatter, the work of fracture is about four orders of magnitude lower than that of the perfect network. This reduction in work of fracture is a measure of network imperfection. The toughness of the hydrogel is about two orders of magnitude higher than that of the perfect network. When the effects of other inelastic processes are minimized, this amplification in toughness is likely due to distributed chain scission, and is another measure of network imperfection. We find that the ultimate properties of the hydrogel are insensitive to cuts up to about 1 mm. We trace this cut insensitivity to network imperfection.

Agile and Resilient Insect-Scale Robot.

Abstract: A key challenge in bioinspired insect-scale running robots is to make them both agile and resilient. In this study, we develop a dielectric elastomer actuated soft robot that mimics inchworms. We use an elastomer to make the soft body, a stretchable dielectric to provide electrostatic actuation of high power density, and multizone actuation to achieve ratcheting locomotion. We fabricate the body, muscles, and feet in a single piece, with no internal open space. The robot runs four times its body length per second and turns at a radius about three times its body length in 0.3 s. The robot survives compression 30,000 times its own weight and survives collision with a rigid surface at a speed of 30 m/s. The robot can climb a slope of 30°. Walking on a horizontal plane, the robot carries a payload four times its own weight. The robot can operate on land, underwater, and in vacuum. The simplicity in design and fabrication will enable the robot to serve as a model system to investigate insect-scale actuation and locomotion, as well as the social behavior of swarms of robots. The robot also provides a platform to integrate wireless charging, mobile communication, and stretchable electronics.

Printing Hydrogels and Elastomers in Arbitrary Sequence with Strong Adhesion

Abstract: Many recently demonstrated devices require the integration of hydrogels and hydrophobic elastomers. Extrusion print is a promising method for rapid prototyping but existing approaches do not fulfill a basic requirement: print integrated structures of a hydrogel and an elastomer, in arbitrary sequence, with strong adhesion. This paper demonstrates an approach to fulfill this requirement. During print, the ink of each material flows through a nozzle under a pressure gradient but retains the shape against gravity and capillarity. During cure, covalent bonds form to link monomer units into polymer chains, crosslink the polymer chains into the polymer networks of the hydrogel and the elastomer, as well as interlink the two polymer networks into an integrated structure. The approach covalently interlinks the hydrogel network and the elastomer network by adding an interlink initiator in one of the inks. An adhesion energy above 5000 J m−2 is demonstrated. Printed morphing structures survive swelling and printed artificial axons survive repeated hits of a hammer. This approach opens a road to the development of soft devices for broad applications in medicine and engineering.

Fracture Toughness and Fatigue Threshold of Tough Hydrogels

Abstract: Hydrogels of numerous chemical compositions have achieved high fracture toughness on the basis of one physical principle. As a crack advances in such a hydrogel, a polymer network of strong bonds r...

Design Molecular Topology for Wet-Dry Adhesion.

Abstract: Recent innovations highlight the integration of diverse materials with synthetic and biological hydrogels. Examples include brain-machine interfaces, tissue regeneration, and soft ionic devices. Existing methods of strong adhesion mostly focus on the chemistry of bonds and the mechanics of dissipation but largely overlook the molecular topology of connection. Here, we highlight the significance of molecular topology by designing a specific bond-stitch topology. The bond-stitch topology achieves strong adhesion between preformed hydrogels and various materials, where the hydrogels have no functional groups for chemical coupling, and the adhered materials have functional groups on the surface. The adhesion principle requires a species of polymer chains to form a bond with a material through complementary functional groups and form a network in situ that stitches with the polymer network of a hydrogel. We study the physics and chemistry of this topology and describe its potential applications in medicine and engineering.

Stick‐On Large‐Strain Sensors for Soft Robots

Abstract: DOI: 10.1002/admi.201900985 pneumatic and microfluidic structures,[1–8] dielectric elastomer actuators,[9–11] shape memory alloys,[12,13] responsive hydrogels,[14,15] and living cells.[16] Among them, soft pneumatic robots have attracted attention due to their complex motion, simple control input, and low-impedance interactions. For soft pneumatic robots, sensing techniques are important to improve accuracy and functionality when grasping or manipulating objects of different shapes and sizes. However, the limitations of existing sensing capabilities greatly restrict the applications for this type of soft robot. Traditional rigid sensing components on soft robots can limit the deformation and compliance of the underlying soft robotic structure. Existing strain sensors for soft robots mainly rely on stretchable electronic conductors, either in the form of 1) elastomeric conductors, which consist of elastomers embedded with conducting components such as silver nanowires, silver particles, carbon nanotubes, and graphene, or 2) liquid conductors, such as grease and liquid metal. The limitations of these sensors have been noted in previous studies. The conductivity of elastomeric conductors degrades due to contact defects between separated particles or liquid, especially under large deformation, as well as the resistance-strain hysteresis resulting from cyclic loading.[17] For liquid conductors like conductive paste, the localized plastic deformation resulting from ratcheting during cyclic loading accumulates and deteriorates conductivity as well.[18,19] The change in conductivity leads to signal drifts. In addition, liquid conductors are not biocompatible and need tight sealing to prevent oxidation and leakage, thus requiring additional technical effort. Soft actuators with integrated microchannels filled with conductive fluid have been fabricated via 3D printing.[20,21] But the intricate designs and complex manufacturing requirements dramatically increase the fabrication difficulty. It remains a major challenge to manufacture robust soft strain sensors for soft robots.[22–27] Compared to electronic conductors, ionically conductive hydrogels can be readily used as stretchable conductors,[28–30] and have recently enabled a new family of devices called hydrogel ionotronics.[31] Both the mechanical and electrical properties of hydrogels can be tuned on demand over a wide range. For example, hydrogels can be as soft as living tissues or as tough as natural rubber. As another example, the resistivity of hydrogels can vary from 18.2 MΩ m to 10−1 Ω m, depending on the type and concentration of salt.[32] In fact, hydrogels resemble ideal conductors, as their resistivity is a material Soft robots require sensors that are soft, stretchable, and conformable to preserve their adaptivity and safety. In this work, hydrogels are successfully applied as large-strain sensors for elastomeric structures such as soft robots. Following a simple surface preparation step based on silane chemistry, prefabricated sensors are strongly bonded to elastomers via a “stick-on” procedure. This method separates the construction of the soft robot’s structure and sensors, expanding the potential design space for soft robots that require integrated sensing. The adhesion strength is shown to exceed that of the hydrogel itself, and the sensor is characterized via quasi-static, fatigue, and dynamic response tests. The sensor exhibits exceptional electrical and mechanical properties: it can sense strains exceeding 400% without damage, maintain stable performance after 1500 loading cycles, and has a working bandwidth of at least 10 Hz, which is sufficient for rapidly-actuated soft robots. In addition, the hydrogel-based large-strain sensor is integrated into a soft pneumatic actuator, and the sensor effectively measures the actuator’s configuration while allowing it to freely deform. This work provides “stick-on” large-strain sensors for soft robots and will enable novel functionality for wearable robots, potentially serving as a “sensing skin” through stimuli-responsive hydrogels.

Polyacrylamide hydrogels. II. elastic dissipater

Abstract: A rubber band is an elastic dissipater. It has narrow hysteresis on load and unload, but dissipates all its elastic energy on snap. We hypothesize that the dissipation of elastic energy can be a potent toughening mechanism. A polyacrylamide hydrogel has narrow hysteresis, but appreciable toughness, providing an ideal candidate to test the hypothesis of elastic dissipater. We peel the polyacrylamide hydrogel of various thicknesses, and record the steady peel forces. A transition thickness exists, below which the steady peel force increases linearly with thickness, and above which the steady peel force is independent of thickness. The transition thickness is comparable to the fractocohesive length, defined by the ratio of toughness over the work of fracture, and is ∼1 mm for the polyacrylamide hydrogel. The linear slope is comparable to the work of fracture. The linear extrapolation of the steady peel force to vanishing thickness defines the peel threshold, and the thickness-independent steady peel force defines the toughness. The former is much below the latter. We interpret these experimental findings in terms of elastic dissipater. A crack not only cuts a layer of polymer chains, but also breaks some polymer chains in a zone of thickness comparable to the fractocohesive length. Even though only a small fraction of the chains in the zone rupture, all the elastic energy stored in this zone dissipates and contributes to toughness. The entire zone acts as an elastic dissipater, not just a layer of individual polymer chains. We discuss the importance of the elastic dissipater in creating materials of high toughness, high threshold, and low hysteresis.

Covalent Topological Adhesion

Abstract: Tough adhesion between wet materials (i.e., synthetic hydrogels and biological tissues) is undergoing intense development, but methods reported so far either require functional groups from the wet ...

Flaw-Insensitive Hydrogels under Static and Cyclic Loads

Abstract: New applications of hydrogels draw growing attention to the development of tough hydrogels. Most tough hydrogels are designed through incorporating large energy dissipation from breaking sacrificial bonds. However, these hydrogels still fracture under prolonged cyclic loads with the presence of even small flaws. This paper presents a principle of flaw-insensitive hydrogels under both static and cyclic loads. The design aligns the polymer chains in a hydrogel at the molecular level to deflect a crack. To demonstrate this principle, a hydrogel of polyacrylamide and polyvinyl alcohol is prepared with aligned crystalline domains. When the hydrogel is stretched in the direction of alignment, an initial flaw deflects, propagates along the loading direction, peels off the material, and leaves the hydrogel flawless again. The hydrogel is insensitive to pre-existing flaws, even under more than ten thousand loading cycles. The critical degree of anisotropy to achieve crack deflection is quantified by experiments and fracture mechanics. The principle can be generalized to other hydrogel systems.

Instant, Tough, Noncovalent Adhesion

Abstract: Noncovalent adhesion has long been developed for numerous applications, including pressure-sensitive adhesives, wound closure, and drug delivery. Recent advances highlight an urgent need: a general principle to guide the development of instant, tough, noncovalent adhesion. Here, we show that noncovalent adhesion can be both instant and tough by separately selecting two types of noncovalent bonds for distinct functions: tougheners and interlinks. We demonstrate the principle using a hydrogel with a covalent polymer network and noncovalent tougheners, adhering another material through noncovalent interlinks. The adhesion is instant if the interlinks form fast. When an external force separates the adhesion, the covalent polymer network transmits the force through the bulk of the hydrogel to the front of the separation. The adhesion is tough if the interlinks are strong enough for many tougheners to unzip. Our best result achieves adhesion energy above 750 J/m2 within seconds. The adhesion detaches in response to a cue, such as a change in pH or temperature. We identify several topologies of noncovalent adhesion and demonstrate them in the form of tape, powder, brush, solution, and interpolymer complex. The abundant diversity of noncovalent bonds offers enormous design space to create instant, tough, noncovalent adhesion for engineering and medicine.

Tearing a hydrogel of complex rheology

Abstract: Tough hydrogels of many chemical compositions are being discovered, and are opening new applications in medicine and engineering. To aid this rapid and worldwide development, it is urgent to study these hydrogels at the interface between mechanics and chemistry. A tough hydrogel often deforms inelastically over a large volume of the sample used in a fracture experiment. The rheology of the hydrogel depends on chemistry, and is usually complex, which complicates the crack behavior. This paper studies a hydrogel that has two interpenetrating networks: a polyacrylamide network of covalent crosslinks, and an alginate network of ionic (calcium) crosslinks. When the hydrogel is stretched, the polyacrylamide network remains intact, but the alginate network partially unzips. We tear a thin layer of the hydrogel at speed v and measure the energy release rate G. The v–G curve depends on the thickness of the hydrogel for thin hydrogels, and is independent of the thickness of the hydrogel for thick hydrogels. The energy release rate approaches a threshold, below which the tear speed vanishes. The threshold depends on the concentration of calcium. The threshold may also depend on the thickness when the thickness is comparable to the size of inelastic zone. The threshold determined by slow tear differs from the threshold determined by cyclic fatigue. We discuss these experimental findings in terms of the mechanics of tear and the chemistry of the hydrogel.

Giant Poisson's Effect for Wrinkle-Free Stretchable Transparent Electrodes.

Abstract: The next generation of flexible electronics will require highly stretchable and transparent electrodes, many of which consist of a relatively stiff metal network (or carbon materials) and an underlying soft substrate. Typically, such a stiff-soft bilayer suffers from wrinkling or folding when subjected to strains, causing high surface roughness and seriously deteriorated optical transparency. In this work, a network with a giant effective Poisson's ratio on a soft substrate is found to be under biaxial tension upon deformation, and thus does not wrinkle or fold, but maintains smooth surfaces and high transparency. Soft tactile sensors employing such network electrodes exhibit high transparency and low fatigue over many stretching cycles. Such a giant Poisson's ratio has the same effect in other systems. This work offers a new understanding of surface instabilities and a general strategy to prevent them not only in flexible electronics, but also in other materials and mechanical structures that require flat surfaces.

Strong and Degradable Adhesion of Hydrogels

Abstract: Adhesives potentially offer convenient means to close wounds, but existing adhesives do not fulfill many common requirements. Here we demonstrate an approach to develop hydrogel adhesives that are strong initially, remain soft when adhered to soft tissues, and degrade over time. We demonstrate the approach by using a hydrogel that dissipates a large amount of energy during separation, forms strong and interlinks with the soft tissues, and degrades by breaking cross-links. The hydrogel achieves initial adhesion energies of 300–700 J/m2 when adhered to different tissues, can bear huge pressure, and completely degrades in 5 weeks under simulated physiological conditions.

Soft kink valves

Abstract: Completely soft, autonomous fluidic robots require valves made of soft materials. Such a soft valve has been demonstrated recently to enable complex movements of soft robots using a single source of air of constant pressure. This paper studies the mechanics of the valve using a combination of experiments and calculations. The valve is made of an elastomeric tube, subject to axial compression. At critical compression, the tube snaps into a kink and blocks the flow of the air in the tube. At another critical compression, the tube snaps open the kink and lets the air flow in the tube. The instability functions as a digital, on-and-off valve, and this function is unaffected by inaccurate deformation of the ends of the tube. A kinked tube blocks fluid flow in the tube up to a certain pressure. Because the elastomer readily undergoes large and reversible deformation, the kink valve can close and open repeatedly without damage. We map out the functional characteristics of the kink valve—the kink-close compression, the kink-open compression, and the breakthrough pressure—in the design space of material and geometry. It is hoped that this study will stimulate further work to harness diverse elastic instabilities for functions needed in soft robots.

Molecular Staples for Tough and Stretchable Adhesion in Integrated Soft Materials.

Abstract: The integration of soft materials-biological tissues, gels, and elastomers-is a rapidly developing technology of this time. Whereas hard materials are adhered using adhesives of hard polymers since antiquity, these hard polymers are generally unsuited to adhere soft materials, because hard polymers constrain the deformation of soft materials. This paper describes a design principle to use hard polymers to adhere soft materials, such that adhesion remains tough after the adhered soft materials are subject to many cycles of large stretches in the plane of their interface. The two soft materials have stretchable polymer networks, but need not have functional groups for adhesion. The two soft materials are adhered by forming, in situ at their interface, islands of a hard polymer. The adhesion is tough if the islands themselves are strong, and the polymers of the islands are in topological entanglement with the polymer networks of the soft materials. The adhesion is stretchable if the islands are smaller than the flaw sensitivity length. Several methods of forming the hard polymer islands are demonstrated, and the mechanics and chemistry of adhesion are studied. The design principle will enable many hard polymers to form tough and stretchable adhesion between soft materials.

Plasticity retards the formation of creases

Abstract: When an elastomer is compressed, its surface forms creases at a critical strain about 0.35. When a plastically deformable material is compressed, however, its surface remains smooth at much larger strains. As the smooth surface folds locally into a crease, the material around the crease loads and unloads. We show that the hysteresis of plasticity retards the formation of the crease. For a crease growing in an infinite body, the stress field is self-similar as the crease grows, and the critical strain ec is the applied strain needed to maintain the self-similar growth. We simulate the formation of creases using an elastic-plastic model with linear hardening, and characterize the degree of plasticity of the model by the ratio of the tangent modulus to elastic modulus, Et/E. A small value of Et/E leads to a large critical strain for the onset of creases. We further show experimentally that creases can form at a strain of 0.49 for high-density polyethylene (HDPE) with Et/E ∼ 0.025, but cannot form for metals (aluminum, copper, and stainless steel) with Et/E ∼ 0.001.

Topological adhesion of materials

Abstract: A composite material is described, including a first material comprising a first polymeric network; a second material comprising a second polymeric network; and an adhesion polymeric network comprising a plurality of adhesion polymer chains joined together by a bonding force and interwoven with the first and second polymeric networks to adhere the first and second materials together, where the adhesion polymeric network is not covalently bonded with the first or second material. Methods of making such composite material are also described.