In this review we highlight new developments in tough hydrogel materials in terms of their enhanced mechanical performance and their corresponding toughening mechanisms. These mechanically robust hydrogels have been developed over the past 10 years with many now showing mechanical properties comparable with those of natural tissues. By first reviewing the brittleness of conventional synthetic hydrogels, we introduce each new class of tough hydrogel: homogeneous gels, slip-link gels, double-network gels, nanocomposite gels and gels formed using poly-functional crosslinkers. In each case we provide a description of the fracture process that may be occurring. With the exception of double network gels where the enhanced toughness is quite well understood, these descriptions remain to be confirmed. We also introduce material property charts for conventional and tough synthetic hydrogels to illustrate the wide range of mechanical and swelling properties exhibited by these materials and to highlight links between these properties and the network topology. Finally, we provide some suggestions for further work particularly with regard to some unanswered questions and possible avenues for further enhancement of gel toughness.

Actuating materials capable of producing useful movement and forces are recognized as the "missing link" in the development of a wide range of frontier technologies including haptic devices, [1] microelectromechanical systems (MEMS), [2] and even molecular machines. [3] Immediate uses for these materials include an electronic Braille screen, [4] a rehabilitation glove, [5] tremor suppression, [6] and a variable-camber propeller. [7] Most of these applications could be realized with actuators that have equivalent performance to natural skeletal muscle. Although many actuator materials are available, none have the same mix of speed, movement, and force as skeletal muscle. Indeed, the actuator community was challenged to produce a material capable of beating a human in an arm wrestling match. [8] This challenge remains to be met.One class of materials that has received considerable attention as actuators is low-voltage electrochemical systems utilizing conducting polymers [9][10][11] and carbon nanotubes. [12,13] Low-voltage sources are convenient and safe, and power inputs are potentially low. One deficiency of conducting polymers and nanotubes compared with skeletal muscle is their low actuation strains: less than 15 % for conducting polymers and less than 1 % for nanotubes. It has been argued that the low strains can be mechanically amplified (levers, bellows, hinges, etc.) to produce useful movements, [7] but higher forces are needed to operate these amplifiers. In recent studies of the forces and displacements generated from conducting-polymer actuators, it has become obvious that force generation is limited by the breaking strength of the actuator material. [14][15][16] Baughman [17] has predicted that the maximum stress generated by an actuator can be estimated as 50 % of the breaking stress, so that for highly drawn polyaniline (PAni) fibers, stresses on the order of 190 MPa should be achievable. However, in practice the breaking stresses of conducting-polymer fibers when immersed in electrolyte and operated electromechanically are significantly lower than their dry-state strengths. [15,16,18] The reasons for the loss of strength are not well known, but the limitations on actuator performance are severe. The highest reported stress that can be sustained by conducting polymers during actuator work cycles is in the range 20-34 MPa [16,19] for polypyrrole (PPy) films. However, the maximum stress that can be sustained by PPy during actuation appears to be very sensitive to the dopant ion and polymerization conditions used, [16] with many studies showing maximum stress values of less than 10 MPa. [4,[14][15][16]20] The low stress generation from conducting polymers, limited by the low breaking strengths, mean that the application of mechanical amplifiers is also very limited. To improve the mechanical performance, we have investigated the use of carbon nanotubes as reinforcing fibers in a polyaniline (PAni) matrix. Previous work has shown that the addition of singlewalled nanotubes (SWNTs) and multi-walled nanotu...

Novel hydrogels with excellent mechanical properties have prompted applications in biomedical and other fields. The reported tough hydrogels are usually fabricated by complicated chemical and/or physical methods. To develop more facile fabrication methods is very important for the practical applications of tough hydrogels. We report a very simple yet novel method for fabricating tough hydrogels that are totally physically cross-linked by cooperative hydrogen bonding between a pre-existing polymer and an in situ polymerized polymer. In this work, tough hydrogels are prepared by heating aqueous acrylamide (AAm) solution in the presence of poly(Nvinylpyrrolidone) (PVP) but without any chemical initiators or covalent bonding cross-linking agents. Mechanical tests of the asprepared and swollen PVP-in situ-PAAm hydrogels show that they exhibit very high tensile strengths, high tensile extensibility, high compressive strengths, and low moduli. Comparative synthesis experiments, DSC characterization, and molecular modeling indicate that the formation of strong cooperative hydrogen bonding between the pre-existing PVP and the in situ formed PAAm chains contributes to the gel formation and the toughening of the hydrogels. The unique microstructure of the gels with evenly distributed flexible cross-linking sites and long polymer chains attached to them endow the hydrogels with an excellent mechanism of distributing the applied load.

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Asymmetric films formed by flash-welding polyaniline nanofiber mats demonstrate rapid reversible actuation in the presence of select aqueous acids and bases. These continuous single component bending/curling actuators have several advantages over conventional dual component, bimorph actuators including ease of synthesis, large degree of bending, patternability and no delamination. The films are made through a controlled, facile, all aqueous process that yields water dispersed polyaniline nanofibers that are readily cast into films. Flash welding photothermally cross-links and melts the top surface of the nanostructured polymer producing an asymmetric film. The resultant cross-linked surface is quite dense and has a reduced number of protonic acid doping sites available. The film surface is therefore less susceptible to the protonic acid doping which expands the underlying high surface area nanofiber layer. Actuation occurs at a comparable or faster rate than bimorph actuators with an unprecedented > 720°bending relative to the initial flat position for a 2.5 cm length film. The collective movement of the individual nanofibers in the asymmetric film creates a large degree of actuation resembling natural muscle. These bending actuators could be developed for use in microtweezers, microvalves, artificial muscles, chemical sensors and/or patterned actuator structures.Polyaniline and other conducting polymers have been of interest for their actuation properties for more than two decades. [1][2][3][4][5][6][7][8] The actuation is due to the unique chemistry of conducting polymers, which generally swell reversibly with the incorporation of dopant ions and their associated solvent molecules. Previous work on polyaniline actuators has involved dispersing conventional polyaniline in highly polar solvents such as N-methyl pyrrolidinone for casting into fibers, [9][10][11] rods, [12][13][14] sheets, [5] layered bimorphs [15,16] and integrally skinned asymmetric membranes. [7,[17][18][19] Elongation or contraction of polyaniline films and fibers has been induced by oxidation state, electrostatic or conformational changes as well as combinations of all of these to create linear or bending movement depending on the initial structure. Typical bending actuators require the use of two or more different materials bound together to produce a bimorph. One material forms the active part that expands or contracts relative to the inactive part upon stimulation, thus inducing bending. Bending of bimorph structures has generally been limited to < 90°and problems with adhesion between the layers often leads to delamination especially with extended use. [20] Alternative bending structures have been proposed such as active dual layers, which expand and contract cooperatively.[21] Wang, et al. [7] made a major advance by developing integrally skinned asymmetric membrane (ISAM) bending polyaniline actuators. ISAMs use a single material processed so that one side of the film has much higher porosity than the other side. Doping induced swelling ...

Electrically conductive, mechanically tough hydrogels based on a double network (DN) comprised of poly(ethylene glycol) methyl ether methacrylate (PPEGMA) and poly(acrylic acid) (PAA) were produced. Poly(3,4-ethylenedioxythiophene) (PEDOT) was chemically polymerized within the tough DN gel to provide electronic conductivity. The effects of pH on the tensile and compressive mechanical properties of the fully swollen hydrogels, along with their electrical conductivity and swelling ratio were determined. Compressive and tensile strengths as high as 11.6 and 0.6 MPa, respectively, were obtained for hydrogels containing PEDOT with a maximum conductivity of 4.3 S cm -1 . This conductivity is the highest yet reported for hydrogel materials of high swelling ratios. These hydrogels may be useful as soft strain sensors because their electrical resistance changed significantly when cyclically loaded in compression. ABSTRACT: Electrically conductive, mechanically tough hydrogels based on a double network (DN) comprised of poly(ethylene glycol) methyl ether methacrylate (PPEGMA) and poly(acrylic acid) (PAA) were produced. Poly(3,4-ethylenedioxythiophene) (PEDOT) was chemically polymerized within the tough DN gel to provide electronic conductivity. The effects of pH on the tensile and compressive mechanical properties of the fully swollen hydrogels, along with their electrical conductivity and swelling ratio were determined. Compressive and tensile strengths as high as 11.6 and 0.6 MPa, respectively, were obtained for hydrogels containing PEDOT with a maximum conductivity of 4.3 S cm −1 . This conductivity is the highest yet reported for hydrogel materials of high swelling ratios. These hydrogels may be useful as soft strain sensors because their electrical resistance changed significantly when cyclically loaded in compression.

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