Showing papers by "Zhigang Suo published in 2021"

Fracture, fatigue, and friction of polymers in which entanglements greatly outnumber cross-links.

Abstract: In gels and elastomers, the role of entanglements on deformation has been studied, but their effects on fracture, fatigue, and friction are less well understood. In this study, we synthesized polym...

Functional hydrogel coatings.

Abstract: Hydrogels-natural or synthetic polymer networks that swell in water-can be made mechanically, chemically and electrically compatible with living tissues. There has been intense research and development of hydrogels for medical applications since the invention of hydrogel contact lenses in 1960. More recently, functional hydrogel coatings with controlled thickness and tough adhesion have been achieved on various substrates. Hydrogel-coated substrates combine the advantages of hydrogels, such as lubricity, biocompatibility and anti-biofouling properties, with the advantages of substrates, such as stiffness, toughness and strength. In this review, we focus on three aspects of functional hydrogel coatings: (i) applications and functions enabled by hydrogel coatings, (ii) methods of coating various substrates with different functional hydrogels with tough adhesion, and (iii) tests to evaluate the adhesion between functional hydrogel coatings and substrates. Conclusions and outlook are given at the end of this review.

Fabricating hydrogels to mimic biological tissues of complex shapes and high fatigue resistance

Abstract: Summary Biological tissues, such as heart valves and vocal cords, function through complex shapes and high fatigue resistance. Achieving both attributes with synthetic materials is hitherto an unmet challenge. Here we meet this challenge with hydrogels of heterogeneous structures. We fabricate a three-dimensional hydrogel skeleton by stereolithography and a hydrogel matrix by cast. Both the skeleton and matrix are elastic and stretchable, but the skeleton is much stiffer than the matrix, and their polymer networks entangle topologically. When such a hydrogel is stretched, the compliance of the matrix deconcentrates stress in the skeleton and amplifies fatigue resistance. We fabricate a homogeneous hydrogel and a heterogeneous hydrogel, each in the shape of a human heart valve. Subject to cyclic pressure, the former fractures in ∼560 cycles but the latter is intact after 50,000 cycles. Soft materials of complex shapes and high fatigue resistance open broad opportunities for applications.

Hydrogel-mesh composite for wound closure.

Abstract: During operations, surgical mesh is commonly fixed on tissues through fasteners such as sutures and staples. Attributes of surgical mesh include biocompatibility, flexibility, strength, and permeability, but sutures and staples may cause stress concentration and tissue damage. Here, we show that the functions of surgical mesh can be significantly broadened by developing a family of materials called hydrogel-mesh composites (HMCs). The HMCs retain all the attributes of surgical mesh and add one more: adhesion to tissues. We fabricate an HMC by soaking a surgical mesh with a precursor, and upon cure, the precursor forms a polymer network of a hydrogel, in macrotopological entanglement with the fibers of the surgical mesh. In a surgery, the HMC is pressed onto a tissue, and the polymers in the hydrogel form covalent bonds with the tissue. To demonstrate the concept, we use a poly(N-isopropylacrylamide) (PNIPAAm)/chitosan hydrogel and a polyethylene terephthalate (PET) surgical mesh. In the presence a bioconjugation agent, the chitosan and the tissue form covalent bonds, and the adhesion energy reaches above 100 J⋅m-2 At body temperature, PNIPAAm becomes hydrophobic, so that the hydrogel does not swell and the adhesion is stable. Compared with sutured surgical mesh, the HMC distributes force over a large area. In vitro experiments are conducted to study the application of HMCs to wound closure, especially on tissues under high mechanical stress. The performance of HMCs on dynamic living tissues is further investigated in the surgery of a sheep.

Topoarchitected polymer networks expand the space of material properties

Abstract: Many living tissues achieve functions through architected constituents with strong adhesion. An Achilles tendon, for example, transmits force, elastically and repeatedly, from a muscle to a bone through staggered alignment of stiff collagen fibrils in a soft proteoglycan matrix. The collagen fibrils align orderly and adhere to the proteoglycan strongly. However, synthesizing architected materials with strong adhesion has been challenging. Here we fabricate architected polymer networks by sequential polymerization and photolithography, and attain adherent interface by topological entanglement. We fabricate tendon-inspired hydrogels by embedding hard blocks in topological entanglement with a soft matrix. The staggered architecture and strong adhesion enable high elastic limit strain and high toughness simultaneously. This combination of attributes is commonly desired in applications, but rarely achieved in synthetic materials. We further demonstrate architected polymer networks of various geometric patterns and material combinations to show the potential for expanding the space of material properties.

Polyacrylamide hydrogels. III. Lap shear and peel

Abstract: Lap shear and peel are common tests for soft materials. Their results, however, are rarely compared. Here we compare lap shear and peel as tests for measuring toughness. We prepare specimens for both tests by using stiff layers to sandwich a layer of a polyacrylamide hydrogel. We introduce a cut in the hydrogel by scissors, pull one stiff layer at constant velocity, and record the force. In lap shear, the force peaks and then drops to zero, the cut grows unstably through the entire hydrogel, and the peak force is used to determine toughness. In peel, the force peaks and then drops to a plateau, the cut grows in the hydrogel in steady state, and the plateau force is used to determine toughness. Our experimental data show that the average values of toughness determined by lap shear and peel are comparable. The peak forces in both tests scatter significantly, but the plateau force in peel scatters narrowly. Consequently, toughness determined by lap shear scatters more than toughness determined by peel. We hypothesize that the peak forces scatter mainly due to the statistical variation of the cuts made by scissors, and test the hypothesis using two additional sets of experiments. First, after a cut is made by scissors, we pre-peel the specimen to extend the cut somewhat, and then measure toughness by lap shear and peel. The peak force in lap shear scatters less, and the peak force in peel is removed. Second, we prepare cuts using spacers of various thicknesses, and find that the peak forces in both lap shear and peel vary with the thickness of the spacer. These findings clarify the use of lap shear and peel to characterize soft materials.

Flaw-sensitivity of a tough hydrogel under monotonic and cyclic loads

Abstract: Rupture of emerging tough hydrogels has received much attention in recent years. It is a fundamental question what length of initial flaws can significantly affect the rupture of a tough hydrogel. Here we study the rupture of a tough hydrogel, using samples with and without initial cuts, under monotonic and cyclic loads. We prepare six samples under the same conditions, either without initial cut, or with initial cuts of the same length, and load them monotonically to inspect the statistical variation of measured rupture stress, rupture stretch and work of rupture. We cycle the six samples in parallel to the same amplitude of stretch and record the number of cycles to rupture of each sample. The average number of cycles to rupture decreases with the amplitude of stretch; and an endurance stretch exists, below which the samples can sustain indefinite number of cycles without rupture. We find that when the initial cut is long, the endurance stretch decreases with the initial cut length. When the initial cut is short, the endurance stretch is insensitive to the initial cut length. We interpret this finding by a material-specific length, the endurance fractocohesive length. We compare the endurance fractocohesive lengths of the hydrogel and other materials, including elastomers, plastics, metals, and ceramics. It is hoped that similar experiments will be soon conducted for other hydrogels to guide their development.

All-Solid Ionic Eye

Abstract: We describe the materials, design, experimental measurements, and simulations of a bio-inspired all-solid tunable optical device: ionic eye. A dielectric elastomer functions as an electroactive material. An ionogel functions as an ionic conductor. Both materials are stretchable and transparent. The ionic eye achieves a ∼50% relative change of the focal length, beyond that of the human eye. Our analysis also points out that the ionic eye can respond rapidly (3.6 ms) and be miniaturized in size. This all-solid deformable lens eliminates the risk of leakage of currently used encapsulated fluid lenses and can be integrated into other devices for diverse applications.

Photoinitiator-grafted polymer chains for integrating hydrogels with various materials

Abstract: Summary Hydrogels are commonly integrated with other materials. In the one-pot synthesis of a hydrogel coating, polymerization, crosslink, and interlink are concurrent. This concurrency, however, is often inapplicable for integrating hydrogels to other materials. For example, a permeable substrate will absorb small molecules in the solution, causing side reactions and even toxicity. Here, we report a method to break the concurrency by using photoinitiator-grafted polymer chains (PGPCs). A type of photoinitiator is copolymerized with various monomers. The PGPCs are uncrosslinked during synthesis, have long shelf lives in dark storage, and can be applied to a substrate by brush, cast, spin, dip, spray, or print. Under ultraviolet light, the polymer chains crosslink into a network and interlink with the substrate. The cured PGPC hydrogels are characterized by mechanical tests. Furthermore, the PGPCs are demonstrated to adhere wet materials, form hydrophilic coatings on hydrophobic substrates, and pattern functional groups on permeable substrates.

Transduction between magnets and ions

Abstract: A time-varying magnetic field generates an electric field in an electrolyte, in which ions move. This magnetoionic transduction is studied here in several arrangements. The electrolyte is a hydrogel containing mobile ions, and is in contact with two metallic electrodes. An alternating electric current applied to a metal coil generates a time-varying magnetic field. In response, ions in the hydrogel move. The two hydrogel/electrode interfaces are non-faradaic and accumulate excess ions of opposite signs, which attract and repel electrons in the two electrodes. When the two electrodes are connected to a voltmeter of internal resistance much larger than that of the hydrogel, an open-circuit voltage is measured, linear in the alternating current applied to the metal coil. A metal coil and a hydrogel coil form an ionotronic transformer, in which an alternating electric current in the metal coil induces an alternating ionic current in the hydrogel coil. Such a transformer can be used for noncontact power transmission, with a voltage high enough to turn on many light-emitting diodes in series. The hydrogel is soft, and readily conforms to a curved surface, such as a glove on a human hand. Motion of the hand can be detected by noncontact magnetoionic transduction.

Elastomeric temperature sensor

Abstract: A stretchable temperature sensor includes one or more elastomeric ionic conducting layers; at least two electronic conducting elements, wherein the one or more ionic conducting layers and one or more electronic conducting elements are configured and arranged to provide at least one electrical double layer at a first contact area between the ionic conducting layer and a first electronic conducting element in a sensing end and at least one electrical double layer at a contact area between the ionic conducting layer and a second electronic conducting element in an open end of the temperature sensor; wherein the second electronic conducting element provides a connection at the open end to an external circuit for measuring a signal generated in response to a temperature condition at the sensing end.