Session V9: Tough Hydrogels II

Mechanically tunable elastomeric hydrogels made from melt-fabricated photoreactive block copolymer micelles

Recently, our group has developed a range of novel elastomeric hydrogels using thermoplastic elastomer design concepts. These have been traditionally formed using two-component blends of AB diblock and ABA triblock copolymer designed to self-assemble into micelle-like domains in the melt. Vitrification of the micelle cores (A blocks) followed by swelling in aqueous media leads to an elastic network of spheres tethered by the population of bridging ABA chains in the blend. The concentration of ABA used has a strong influence on the mechanical properties exhibited by the hydrogels. We have built on this by replacing the traditional AB with a photoreactive AB-p. This construct provides flexibility to install specific concentrations of ABA tethering molecules at any point in the fabrication process as well as at any location simply through intensity-controlled, spatially directed irradiation with UV light. Increasing UV exposure time results in greater ABA concentrations, reinforcing the area of exposure. In this presentation we explore the influence of patterned ABA installation on shape, surface topography, and mechanical properties of the resulting hydrogels.   

Three-dimensional computational model of self-reinforcing polymer gels

We utilize the gel lattice-spring approach to develop the 3D computational model of polymer gels that become stronger and tougher in response to a mechanical deformation. The polymer chains are assumed to incorporate the folded domains that encompass the reactive functional groups (cryptic sites). Under deformation, the domains unfold and expose the cryptic sites, which can then form labile bonds with the linker chains grafted to the network. Once the deformation is removed, the linkers detach from the cryptic sites, and unfolded domains go back to the folded configuration thus hiding the cryptic sites. The gel behavior under applied force is described by the equations of elasticity of the polymer network coupled to the kinetic equations for the folding and binding transitions. The model equations take into account the effects of finite chain extensibility on the gel elasticity and mechanosensitive reaction rates. Elasticity of the transient network is described using the Flory model. The developed 3D computational model could be used for designing novel polymer gel-based materials that exhibit self-strengthening under deformation.   

Time-dependent Fracture Behaviour of Polyampholyte Hydrogels

Recently, we report that polyampholytes, polymers bearing randomly dispersed cationic and anionic repeat groups, form tough and self-healing hydrogels with excellent multiple mechanical functions. The randomness makes ionic bonds with a wide distribution of strength, via inter and intra chain complexation. As the breaking and reforming of ionic bonds are time dependent, the hydrogels exhibit rate dependent mechanical behaviour. We systematically studied the tearing energy by tearing test with various tearing velocity under different temperature, and the linear viscoelastic behaviour over a wide range of frequency and temperature. Results have shown that the tearing energy markedly increase with the crack velocity and decrease with the measured temperature. In accordance with the prediction of Williams, Landel, and Ferry (WLF) rate-temperature equivalence, a master curve of tearing energy dependence of crack velocity can be well constructed using the same shift factor from the linear viscoelastic data. The scaling relation of tearing energy as a function of crack velocity can be predicted well by the rheological data according to the developed linear fracture mechanics.   

Robust Bonding of Tough Double Network Hydrogel to Bone

Tough Double Network (DN) hydrogels are one of candidates as next-generation artificial cartilage from the viewpoints of low friction, water storage capability and toughness. For practical use, the hydrogel must be strongly fixed at the joint. However, strong fixation of such hydrogel to other materials (tissues) has not been achieved yet because the surface property of hydrogel is almost equal to water due to its high water content. Therefore, robust adhesion for fixation and low friction for lithe motion are trade-off relation. Here, we report robust fixation of hydroxyapatite (HAp) mineralized DN hydrogel to the bone without any toxicity. HAp is main inorganic component of bone tissues and has osteoconductive capability. After 4 weeks implantation of HAp/DN gel into rabbit femoral groove, The robust fixation between bone and HAp/DN gel, more than strength of gel matrix, was achieved. The methodology is universal for new biomaterials, which should be fixed on bone, such as ligament and tendon systems.   

Polyampholyte hydrogel electrolytes for flexible and self-healing aqueous supercapacitor for low temperature applications

Quenched polyampholytes provide a novel class of tough hydrogel that has self-healing ability, strong adhesion, and mechanical flexibility. In this study, we show that the polyampholyte hydrogels can be utilized as an aqueous gel electrolyte material that is especially useful for low temperature operations; at --30 \textdegree C, energy density of 10.5 Wh/kg at a power density of 500 W/kg was achieved. The high performance at the low temperature is associated to the concept of non-freezable water near the hydrophilic polymer chains. A comparison between differential scanning calorimetry (DSC) measurements for polyampholytes that contained KOH and neat KOH solution revealed that increased amount of water molecules become non-freezable when the solution is contained in the hydrogel networks. In addition, the crosslinked network structure of the polyampholyte chains disrupts the crystalline growth of ice, resulting in `slush-like' ice formation. The interplay between the increased amount of unfrozen water and the limited growth of ice crystals leads to the enhanced supercapacitor performance at low temperatures.   

Harnessing phase separation of tough hydrogel for optical modulation

Optically switchable materials, such as photo-electrochromic and liquid crystalline materials, has driven the advance of tunable optics devices. One interesting example is a reversible tuning of transparency that reflects electrical, thermal, or mechanical stimuli. Conventional materials the transparency tuning include electrochromic oxides (rigid, but dimensionally non-conformable) and liquid crystals (conformable, but must be encapsulated to prevent flowing). In our study, we developed a tough hydrogel based strategy to combine the merits of the rigid and the liquid materials. Here, soft polyampholytes (PA) hydrogel undergoes a transition between opacity and transparency at the upper critical solution temperature (UCST) due to its interaction with surround water molecules. In order to fine tune the UCST temperature in practically useful range (from 40 to 65 \textdegree C), we adjusted the hydrophilicity/phobicity of PA by modulating monomer concentration when synthesizing the random copolymer. We discuss the origin of the phase behavior. In addition, we developed a stretchable, high-contrast, optically tunable stretchable window which consists of the PA hydrogel and a printed stretchable electric heater.   

Structural Relaxation in Supramolecular Hydrogels After Uniaxial Elongation Probed by SANS

The relationship between stress relaxation and the microstructure of a supramolecular random amphiphilic copolymer-based hydrogel was probed using small angle neutron scattering (SANS). The amphiphilic random copolymer separates into microphases and in water forms a highly swollen hydrogel that is physically crosslinked by domains of\textit{ 2-(N-ethylperfluorooctanesulfonamido)ethyl acrylate (FOSA)}. SANS with contrast matching enables the size of the FOSA crosslinks and the distance between crosslinks to be probed. The hydrogel was uniaxially stretched to determine the stress relaxation and monitor the structural changes. In the first 5 min, approximately 60{\%} of the stress decays, followed by a slow decay in the stress over multiple hours. From SANS the hydrogels were found to exhibit significant anisotropy in their scattering profiles after stretching, which is attributed chain stretching and deformation of the FOSA domains during elongation. The decay of this structural anisotropy correlates well with the long relaxation times associated with the stress relaxation. These measurements provide insight into the stress-driven microstructure changes in reversible physical associations in swollen networks.   

Structure-Property Relationships in Tough, Superabsorbent Thermoplastic Elastomers for Hemorrhage Control

Between 2001 and 2009, uncontrolled hemorrhaging from major trauma accounted for the deaths of roughly 80{\%} of wounded soldiers with potentially survivable injuries. Modern hemostatic materials are limited in their ability to deliver therapeutic agents, causing tissue damage themselves, or being difficult to remove intact. The goal of this study is to create a mechanically robust polymer that takes up as much as 1000 wt{\%} water in seconds while maintaining sufficient toughness to be removed intact from the wound intact. A thermoplastic elastomer scaffold in which physical crosslinks provide mechanical toughness might provide an appropriate combination of fast swelling and excellent toughness if the matrix material can be engineered to be strongly hydrophilic and swell rapidly. In this work, a commercial SBS triblock copolymer has been modified with poly(acrylic acid) side chains, resulting in materials that are superabsorbent but retain good mechanical properties when saturated. Although SAXS experiments failed to show any significant changes in morphology, even with 800 wt{\%} water uptake, preliminary SANS experiments using selectively deuterated materials and swelling with D2O show significant changes in morphology. Our most recent findings will be presented.   

Extracting Mechanical Properties of Fully-Extended Polymer Chains from DN Gels

Mechanical properties of polymer chains are a basic and dominant factor of the mechanical characteristics of polymeric materials. Many studies have been attempted to understand the single chain behavior. In this study, we extracted the mechanical properties of polymer chains in the fully-extended state, such as the force-extension relationship, from analysis of the mechanical behavior of bulk double network (DN) gels. The DN gels show high strength and toughness, which originates from the energy dissipation by polymer chain scission during deformation, called internal fracture. Since the internal fracture phenomenon is observed by the mechanical hysteresis loop during cyclic tensile testing, the mechanical properties of a fully-extended polymer chain is embedded in the hysteresis loop. Here, we extracted a specific single-chain mechanical property of poly(ethylene glycol) from analysis of uniaxial cyclic tensile behaviors of DN gels. The extracted results are also compared to single molecule force-extension curves obtained by atomic force microscopy.   

Tough Hydrogel Robots: High-Speed, High-Force and Opto-sonically Invisible in Water

Sea animals such as leptocephali develop tissues and organs composed of active transparent hydrogels to achieve agile motions and natural camouflage in water. Hydrogel-based actuators that can imitate the capabilities of leptocephali will enable new applications in diverse fields. However, existing hydrogel actuators, mostly osmotic-driven, are intrinsically low-speed and/or low-force; and their camouflage capabilities have not been explored. Here we show that hydraulic actuations of tough hydrogels with designed structures and properties can give soft actuators and robots that are high-speed, high-force, and optically and sonically camouflaged in water. We invent a simple method capable of assembling physically-crosslinked hydrogel parts followed by covalent crosslinking to fabricate large-scale hydraulic hydrogel actuators and robots with robust bodies and interfaces. The hydrogel actuators and robots can maintain their robustness and functionality over multiple cycles of actuations, owning to the anti-fatigue property of the hydrogel under moderate stresses. A multiscale theoretical framework has been developed to guide the design and optimization of the hydrogel robots. We further demonstrate that the agile and transparent hydrogel actuators and robots perform extraordinary functions including swimming, kicking rubber-balls and catching a live fish in water.   

Three-Dimensional Printing of Complex Structures by Freeform Reversible Embedding of Suspended Hydrogels (FRESH)

We demonstrate the additive manufacturing of complex three-dimensional (3D) structures using soft protein and polysaccharide hydrogels that are challenging or impossible to create using traditional fabrication approaches. These structures are built by embedding the printed hydrogel within a secondary hydrogel that serves as a temporary, thermoreversible, and biocompatible support. This process, termed freeform reversible embedding of suspended hydrogels (FRESH), enables 3D printing of hydrated materials with an elastic modulus less than 500 kPa including alginate, collagen, hyaluronic acid and fibrin. A range of crosslinking mechanisms can be used depending on the polymer being printed, including ionic, enzymatic, pH, thermal and light based approaches. CAD models of 3D optical, computed tomography, and magnetic resonance imaging data can be 3D printed at a resolution of 100 $\mu $m and at low cost by leveraging open-source hardware and software tools. Proof-of-concept structures based on femurs, branched coronary arteries, trabeculated embryonic hearts, and human brains are mechanically robust and recreate complex 3D internal and external anatomical architectures. Recent advances have improved the resolution and broadened the range of materials that can be FRESH 3D printed.