Thermoresponsive nanocomposite hydrogels with high mechanical strength and toughness based on a dual crosslinking strategy

The practical application of thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) hydrogels are severely limited by their poor mechanical properties. Herein, we reported a series of dual crosslinked (DC) PNIPAM hydrogels with superior mechanical properties prepared by simple copolymerization of N-isopropylacrylamide and sodium acrylate (SA) in the laponite RDS suspension, following by a soaking process in multivalent metal cations (e.g., Ca²⁺, Al³⁺, Fe³⁺) aqueous solutions to form ionic coordination interactions with  COO groups of copolymer side chains. The effect of laponite RDS, AANa (sodium acrylate), and metal cation (e.g., Fe³⁺) concentrations on the mechanical properties and deswelling properties of the DC hydrogels are evaluated. The DC hydrogel prepared with 10 w/v% laponite RDS, 0.25 mol/L AANa and 0.45 mol/L Fe³⁺ possesses the best mechanical properties (ca. 1.1 MPa of tensile strength, 9.1 MPa of compression strength at 80% of compression strain, 1.4 MPa of elastic modulus and 1.3 MJ/m³ of toughness). Moreover, we also discovered that the DC hydrogels crosslinked by Fe³⁺ showed better mechanical properties due to the larger charge and ion radius of Fe³⁺.     

A Dual‐Crosslinked Strategy to Construct Physical Hydrogels with High Strength,Toughness, Good Mechanical Recoverability,and Shape‐Memory Ability

A novel type of physical hydrogel based on dual‐crosslinked strategy is successfully synthesized by micellar copolymerization of stearyl methacrylate, acrylamide, and acrylic acid, and subsequent introduction of Fe³⁺. Strong hydrophobic associations among poly(stearyl methacrylate) blocks form the first crosslinking point and ionic coordination bonds between carboxyl groups and Fe³⁺ serve as the second crosslinking point. The mechanical properties of the hydrogel can be tuned in a wide range by controlling the densities of two crosslinks. The optimal hydrogel shows excellent mechanical properties (tensile strength of ≈6.8 MPa, elastic modulus of ≈8.0 MPa, elongation of ≈1000%, toughness of 53 MJ m^?3) and good self‐recovery property. Furthermore, owing to stimuli responsiveness of physical interaction, this hydrogel also shows a triple shape memory effect. The combination of two different physical interactions in a single network provides a general strategy for designing of high‐strength hydrogels with functionalities.     

Synthesis of poly(acrylic acid)–Fe3+/gelatin/poly(vinyl alcohol) triple‐network supramolecular hydrogels with high toughness,high strength and self‐healing properties

Hydrogels with good mechanical and self‐healing properties are of great importance for various applications. Poly(acrylic acid)–Fe³⁺/gelatin/poly(vinyl alcohol) (PAA‐Fe³⁺/Gelatin/PVA) triple‐network supramolecular hydrogels were synthesized by a simple one‐pot method of copolymerization, cooling and freezing/thawing. The PAA‐Fe³⁺/Gelatin/PVA triple‐network hydrogels exhibit superior toughness, strength and recovery capacity compared to single‐ and double‐network hydrogels. The mechanical properties of the synthesized hydrogels could be tailored by adjusting the compositions. The PAA‐Fe³⁺/Gelatin/PVA triple‐network hydrogel with 0.20 mmol Fe³⁺, 3% gelatin and 15% PVA could achieve good mechanical properties, the tensile strength and elongation at break being 239.6 kPa and 12.8 mm mm^?1, respectively, and the compression strength reaching 16.7 MPa under a deformation of about 91.5%. The synthesized PAA‐Fe³⁺/Gelatin/PVA triple‐network hydrogels have good self‐healing properties owing to metal coordination between Fe³⁺ and carboxylic groups, hydrogen bonding between the gelatin chains and hydrogen bonding between the PVA chains. Healed PAA‐Fe³⁺(0.20)/Gelatin3%/PVA15% triple‐network hydrogels sustain a tensile strength of up to 231.4 kPa, which is around 96.6% of the tensile strength of the original samples. Therefore, the synthesized triple‐network supramolecular hydrogels would provide a new strategy for gel research and expand the potential for their application. © 2019 Society of Chemical Industry     

Dual Cross‐Linked Hydrogels with High Strength,Toughness, and Rapid Self‐Recovery Using Dynamic Metal–Ligand Interactions

A dual cross‐linking design principle enables access to hydrogels with high strength, toughness, fast self‐recovery, and robust fatigue resistant properties. Imidazole (IMZ) containing random poly(acrylamide‐co‐vinylimidazole) based hydrogels are synthesized in the presence of Ni²⁺ ions with low density of chemical cross‐linking. The IMZ‐Ni²⁺ metal–ligand cross‐links act as sacrificial motifs to effectively dissipate energy during mechanical loading of the hydrogel. The hydrogel mechanical properties can be tuned by varying the mol% of vinylimidazole (VIMZ) in the copolymer and by changing the VIMZ/Ni²⁺ ratio. The resultant metallogels under optimal conditions (15 mol% VIMZ and VIMZ/Ni²⁺ = 2:1) show the best mechanical properties such as high tensile strength (750 kPa) and elastic modulus (190 kPa), combined with high fracture energy (1580 J m^?2) and stretchability (800–900% strain). The hydrogels are pH responsive and the extent of energy dissipation can be drastically reduced by exposure to acidic pH. These hydrogels also exhibit excellent anti‐fatigue properties (complete recovery of dissipated energy within 10 min after ten successive loading–unloading cycles at 400% strain), high compressive strength without fracture (17 MPa at 96% strain), and self‐healing capability due to the reversible dissociation and re‐association of the metal ion mediated cross‐links.     

High strength and toughness of double physically cross-linked hydrogels composed of polyvinyl alcohol and calcium alginate

The weak mechanical properties of hydrogels, especially physically cross-linked hydrogels are usually a major factor to hinder their application. To solve this problem, in this work, we prepared a high strength and toughness of double physically cross-linked (PDN) hydrogels composed of crystalline domain cross-linked polyvinyl alcohol (PVA) and Ca²⁺-cross-linked alginate (Alg). With a further annealing treatment, the noncovalent cross-linked network via the formed crystalline promote the as-prepared PDN PVA/Alg hydrogel to exhibit well mechanical properties with the tensile strength of ~1.94 MPa, elongation at break of ~607% and Young's modulus of ~0.45 MPa (above 70 wt% of water content). By analyzing the mechanism of improving the hydrogel mechanical properties, it is found that annealing can effectively improve the crystallinity of PVA in the hydrogel, and then greatly improve the mechanical properties of the hydrogel. This provides a general method for improving the mechanical properties of PVA PDN hydrogels. In addition, the PDN PVA/Alg hydrogel was also proved to have good ionic conductivity of 1.70 S m⁻¹. These desirable properties make the prepared physically cross-linked hydrogels promising materials for medical and biosensing fields.     

Dual-responsive shape memory hydrogels with self-healing and dual-responsive swelling behaviors

Shape memory hydrogels (SMHs) can fix the hydrogels in a provisional shape and restore the initial shape under external stimulation. Herein, a dual-responsive shape memory hydrogel with dual-responsive swelling and self-healing properties is presented in this work. The SMHs were fabricated by one-step emulsion copolymerization of acrylic acid (AAc), acrylamide (AAm) and stearyl methacrylate (SMA). Sodium alginate (SA) was introduced as an interpenetrating polymer in the network. With ionic cross-linking between -COO⁻ and Fe³⁺ or saline-reinforced hydrophobic association, the hydrogels can be fixed in a provisional shape, which can be restored by immersing the hydrogels in vitamin C solution or pure water, respectively. When the as-prepared hydrogels were immersed in FeCl₃ solutions, additional ionic cross-linking between Fe³⁺ and -COO⁻ could be formed, thus constructing the dual physically cross-linked (DPC) network, which endows the hydrogels with excellent fracture stress (2.6 MPa) and toughness (5.47 MJ/m³). Besides, the reversible physical cross-linkings endowed the hydrogel with outstanding self-healing capability. Furthermore, the pH and saline responsive swelling properties of the SMHs are additional fantastic properties. Therefore, we believe that this simple strategy provides a great opportunity for the preparation of SMHs with multiple intellectual performances.     

Tough,self-recovery and self-healing polyampholyte hydrogels

This article reviews the recently developed tough, self-recovery, and self-healing polyampholyte hydrogels. Polyampholyte hydrogels are synthesized using one-step radical copolymerization of cationic and anionic monomers with equal charges at high monomer concentration. The random copolymerization process makes the ionic monomers randomly distributing along the backbones, resulting in the formation of ionic bonds with a wide strength distribution via inter and intra chain complexation in the polymer network, weak bond and strong bonds. The strong bonds serve as permanent cross-linking, integrating the hydrogels to impart the elastic behavior, while the weak bonds can break upon the loading, dissipating energy to give the toughness, and re-form again after unloading to enable the self-recovery behavior. Accordingly, polyampholyte hydrogels have condensed polymers in water (ca 40?50 wt %). They are strongly viscoelastic and have a high toughness (fracture energy of 4000 J/m²), a wide range of tuning modulus (0.01 to 8 MPa), 100% self-recovery, and a high self-healing efficiency after cutting.     

Highly stretchable,tough, and self‐recoverable and self‐healable dual physically crosslinked hydrogels with synergistic “soft and hard” networks

Strength, toughness and self‐recoverability are among the most important properties of hydrogels for tissue‐engineering applications. Yet, it remains a challenge to achieve these desired properties from the synthesis of a single‐polymer hydrogel. Here, we report our one‐pot, a monomer‐polymerization approach to addressing the challenge by creating dual physically crosslinked hybrid networks, in particular, synergistic “soft and hard” polyacrylic acid‐Fe³⁺ hydrogels (SHPAAc‐Fe³⁺). Favorable mechanical properties achieved from such SHPAAc‐Fe³⁺ hydrogels included high tensile strength (about 1.08 MPa), large elongation at break (about 38 times), excellent work of extension (about 19 MJ m^?3), and full self‐recoverability (100% recovery of initial properties within 15 min at 50°C and within 60 min in ambient conditions, respectively). In addition, the hydrogels exhibited good self‐healing capabilities at ambient conditions (about 40% tensile strength recovery without any external stimuli). This work demonstrates that dual physical crosslinking combining hydrophobic interaction and ionic association can be achieved in single‐polymer hydrogels with significantly improved mechanical performance but without sacrificing favorable properties. POLYM. ENG. SCI., 59:145–154, 2019. © 2018 Society of Plastics Engineers     

Boron Nitride Nanosheets Strengthened PVA/Borax Hydrogels with Highly Efficient Self-Healing and Rapid pH-Driven Shape Memory Effect

Self-healing hydrogels often possess poor mechanical properties which largely limits their applications in many fields. In this work, boron nitride nanosheets are introduced into a network of the poly(vinyl alcohol)/borax (PVA/borax) hydrogels to enhance the mechanical properties of the hydrogel without compromising the self-healing abilities. The obtained hydrogels exhibit excellent mechanical properties with a tensile strength of 0.410 ± 0.007 MPa, an elongation at break of 1712%, a Young's Modulus of 0.860 ± 0.023 MPa, and a toughness of 3.860 ± 0.075 MJ m⁻³. In addition, the self-healing efficiency of the hydrogels is higher than 90% within 10 min at room temperature. Benefiting from the excellent self-healing properties, the shapeability of the hydrogel fragments is observed using different molds. In addition, the hydrogels display rapid pH-driven shape memory effects and can recover to their original shape within 260 s. Overall, this work provides a new approach to hydrogels with integrated excellent mechanical properties, self-healing abilities, and rapid pH-driven shape memory effects.     

Highly Mechanical and Fatigue‐Resistant Double Network Hydrogels by Dual Physically Hydrophobic Association and Ionic Crosslinking

Double network (DN) hydrogels with high strength and toughness are considered as promising soft materials. Herein, a dual physically cross‐linked hydrophobic association polyacrylamide (HPAAm)/alginate‐Ca²⁺ DN hydrogel is reported, consisting of a HPAAm network and a Ca²⁺ cross‐linked alginate network. The HPAAm/alginate‐Ca²⁺ DN hydrogel exhibits excellent mechanical properties with the fracture stress of 1.16 MPa (3.0 and 1.7 times higher than that of HPAAm hydrogel and HPAAm/alginate hydrogel, respectively), fracture strain of 2604%, elastic modulus of 71.79 kPa, and toughness of 14.20 MJ m^?3. HPAAm/alginate‐Ca²⁺ DN hydrogels also demonstrate self‐recovery, notch‐insensitivity, and fatigue resistance properties without any external stimuli at room temperature through reversible physical bonds consisting of hydrophobic association and ionic crosslinking. As a result, the dual physical crosslinking would offer an avenue to design DN hydrogels with desirable properties for broadening current applications of soft materials.     

Dual Cross-Linked Polymethacrylic Acid Hydrogels with Tunable Mechanical Properties and Shape Memory Behavior

Herein, a series of poly(methacrylic acid) hydrogels are prepared via bulk polymerization of methacrylic acid (MAAc) and grafting of Triton X-100 (TX-100). One-pot and extremely simple chemistry consist of only mixing and subsequently heating of commercially available monomer and surfactant. The polymer chains are interconnected through dual physical cross-link points formed by the hydrophobic associations in the center of TX-100 micelles and hydrogen bonds stabilized by hydrophobic α-methyl groups of MAAc. The hydrogels exhibit tunable mechanical properties ranging between softness and stiffness by adjusting the surfactant/monomer molar ratio, such as Young modulus of 0.6−22 MPa, elongation at break of 750−1700%, tensile strength of 0.21−3.6 MPa, and compressive strength of 41−93 MPa. The synergistic effect of high-density hydrogen bonds with hydrophobic associations endows a plastic-like hydrogel with high strength and shape memory (SM) behavior, while a high concentration of micelles with low-density hydrogen bonds endows a stretchable elastic hydrogel. The combination of temperature-induced SM property and wide-ranging mechanical performance will make such hydrogels useful in diverse applications.     

Synthesis and properties of self-healing hydrogel plugging agent

The P(AM-co-AMPS)/SA DN hydrogel was synthesized through aqueous polymerization in this study. It formed a crosslinking network with hydrophobic associations between acrylamide (AM) and lauryl methacrylate (LMA), as well as an ionic bond network involving sodium alginate and Ca²⁺. To enhance its high-temperature resistance, 2-acrylamide-2-methylpropane sulfonic acid (AMPS) was incorporated into the hydrogel formulation. The structure of the hydrogel was characterized using Fourier transform infrared spectrometer (FTIR), thermogravimetric analyzer (TGA), and scanning electron microscopy (SEM) techniques. Results demonstrated that the hydrogel exhibited excellent temperature resistance and possessed a porous structure. Mechanical testing revealed a high tensile strength of 110 kPa, elongation at break of 995.31%, along with good fatigue resistance and self-recovery performance during multiple cyclic stretching. Healing experiments indicated that the healing strength of the hydrogel was influenced by temperature variations. Furthermore, pressure plugging tests were conducted on steel models with crack widths of 0.5 and 1 mm, respectively; it was found that the 0.8%P(AM-co-AMPS)/SA DN hydrogel could withstand pressures up to 4.5 MPa at a temperature of 70°C. This novel hydrogel material exhibits remarkable mechanical properties along with certain self-healing capabilities, making it suitable for leak control applications.     

Agar/PAAc-Fe<Superscript>3+</Superscript> hydrogels with pH-sensitivity and high toughness using dual physical cross-linking

As promising structural materials, various tough hydrogels have been developed recently by incorporating various kinds of bonds. An important challenge is to use dual physical cross-linking to develop both toughness and self-recovery in a single material. Here we report smart, strain-responsive hydrogels composed of a fully physically linked agarose/poly(acrylic acid)-ferric ion (agar/PAAc-Fe³⁺) double network (DN) with high toughness and pH-sensitivity. These hydrogels were fabricated in a one-pot reaction to generate dual physical cross-linking through, first, a hydrogen-bonded cross-linked agarose network, and, second, a physically linked PAAc-Fe³⁺ network via Fe³⁺ coordination interactions. The DN hydrogels possessed high toughness, with breaking strain of 1130%, fast self-recovery properties in ambient conditions (100% recovery in 30 min) and self-healing properties (the healed hydrogels can be manually stretched up to 700% of their original length after self-healing for 60 h from the cut-off state). In addition, the hydrogels exhibited pH-sensitivity due to the dissociation of ionic coordinate bonds between –COO^? ions of the PAAc chains and Fe³⁺ ions. Double-layer hydrogel strips with two different concentrations of PAAc formed a “C”-shaped material when initially immersed in pH 7 solution and then soaked in a pH 3 solution. These characteristics make the hydrogels attractive candidates for tissue engineering, soft actuators and flexible electronics.     

Facile fabrication of tough and biocompatible hydrogels from polyvinyl alcohol and agarose

A polyvinyl alcohol (PVA)-agarose (agar) composite hydrogel (M-PVA-agar-60) was developed by simple three cycles of freeze-thawing, followed by successively soaking in ammonium sulfate aqueous solution to induce phase separation and dialyzing against deionized water to remove residual sulfate salts. Due to the synergy of crystalline regions, hydrogen bonding and phase separation domains, the obtained M-PVA-agar-60 hydrogel exhibits excellent mechanical properties (tensile strength = 1.1 MPa, tensile strain = 324% and compressive stress = 12.5 MPa), combined with a high water content of 87.0%. Moreover, the hydrogel hardly expands after immersing in the phosphate-buffered saline aqueous solution at 37°C for a week, and the tensile stress and toughness remain almost the same as their initial values, superior to most reported non-swellable hydrogels. Because of the biocompatible starting materials, absence of toxic chemicals, and dialysis in advance to remove ammonium sulfate, the hydrogel also shows excellent cell compatibility, making it an ideal candidate for tissue engineering materials.     

Facile preparation and characterization of sodium alginate/graphite conductive composite hydrogel

Conductive composite hydrogels based on sodium alginate (SA) and graphite were fabricated by a facile method via dispersing homogeneously conductive graphite into SA hydrogel matrix. The hydrogel was formed by in situ release of Ca²⁺ from Ca–EDTA, thus eliminating the multistep reactions and tedious purification compared to the previous work. Raman spectra, scanning electron microscopy (SEM), X‐ray diffraction (XRD), and thermogravimetric analysis (TGA) were used to characterize the structure, crystalline nature, and thermostability of SA/graphite composite hydrogels. The SA/graphite composite hydrogels exhibited the improved network and layer‐type structure. The thermal stability of the hydrogel decreased slightly after the graphite was incorporated into the SA hydrogel matrix regardless of the content of graphite. The enhanced mechanical strength of SA/graphite composite hydrogel was achieved via increasing the f value (i.e., [Ca²⁺]/[COO^‐ in alginate]) and lowering graphite content. The conductivity of the composite hydrogels could be varied in a broad range, reaching up to 10⁻³ S/cm, mainly depending on the content of graphite and the f value. POLYM. COMPOS., 37:3050–3056, 2016. © 2015 Society of Plastics Engineers     

High-strength,ultrastretchable hydrophobic association hydrogel reinforced by tailored modified carboxymethyl cellulose

A new pathway to construct high-strength, ultrastretchable hydrogels based on tailored modified carboxymethyl cellulose (TMCMC)and hydrophobic association(HPA) system was investigated. TMCMC was prepared from degradation of carboxymethyl cellulose (CMC)/N,N′-methylene bisacrylamide (MBA) chemical cross-linked hydrogels. The residual double bonds of TMCMC, confirmed by ¹H-NMR and FTIR analysis, reacted with acrylamide in HPA to form TMCMC/HPA hydrogels, and the homogeneous and fine spatial network structure of TMCMC/HPA hydrogel could be observed by scanning electron microscopy. Rheological analysis revealed that the coexistence of physical and chemical crosslinking in TMCMC/HPA hydrogels. Further studies showed that TMCMC/HPA hydrogel with MBA content of 0.5 wt% has outstanding mechanical properties, and its fracture stress, elongation and tensile strength reach 1.17 MPa, 3717.05% and 15.68 MJ·m⁻³, respectively. Moreover, the hydrogel displayed good swelling resistance and stable strain electrical signal response.     

Tough and self-healing hydrophobic association hydrogels with cationic surfactant

Hydrophobic association (HA) hydrogels with outstanding mechanical, rheological and recovery properties were successfully synthesized by micellar copolymerization of acrylamide with lauryl methacrylate. The synthesis occurred at room temperature and the synthesis condition was moderate by using the redox initiator system of Ammonium persulfate - sodium bisulfite as initiators. Cationic surfactant (dodecyl trimethyl ammonium bromide) was utilized to form micelles with hydrophobe, served as physical cross-linking points in the 3D networks of hydrogels. The HA hydrogels showed a high tensile strength of 181 kPa, superior stretchability of 2300% and excellent toughness of 2.16 MJ m⁻³. Moreover, they owned extraordinary self-recovery under different conditions. It is hopeful that the hydrogels with superior mechanical strength and self-healing properties would be applied to the fields of biomedicine and engineering. Meanwhile, based on above materials, HA hydrogels could also be synthesized with the combination of hydrophobic association and other synergistic effects, such as latex particles, electrostatic effect and nanoparticles.     

High‐Strength,Rapidly Self‐Recoverable,and Antifatigue Nano‐SiO2/Poly(Acrylamide–Lauryl Methacrylate) Composite Hydrogels

Creating load‐bearing hydrogels with superior mechanical strength and toughness is of vital importance for promoting the development of polymer hydrogels toward practical applications. Herein, a type of composite hydrogel is facilely fabricated employing simple and effective UV irradiation one‐pot method by introducing cheap and available nanosilica sol into hydrophobic association poly(acrylamide–lauryl methacrylate) (HAPAM gels). Composite hydrogels exhibit enhanced mechanical strength (compression stress reaching 4.4 MPa) and toughness (compression hysteresis energy achieved is 151.15 kJ m^?3) compared to HAPAM gels. Composite hydrogels also demonstrate rapid self‐recovery behavior (95.91% stress recovery and 92.19% hysteresis energy recovery after restoration for 15 min, respectively) and favorable fatigue‐resistant ability without the help of external stimuli at room temperature based on the cyclic loading–unloading compression measurements. The simple and effective design strategy may help the development of hydrogel materials toward practical applications for soft sensors, tissue engineering, and actuators.     

A Carbodiimide Cross-Linked Silk Fibroin/Sodium Alginate Composite Hydrogel with Tunable Properties for Sustained Drug Delivery

Carbodiimide cross-linked silk fibroin (SF)/sodium alginate (SA) composite hydrogels with superior stability and tunable properties are developed by varying preparation parameters. SF/SA blend ratio modulation allows to achieve composite hydrogel gelation times of 18–65 min, and rheological analysis shows that the speed of gel formation, the hydrogel network's density, and the hydrogels’ compressive properties are closely related to the blend ratio. The G′ of different hydrogels varies substantially from 28 to 413 Pa, and the hydrogel with higher SF content has a greater stiffness. The composite hydrogels present appropriate porosity of 76.63–85.09% and pore size of 316–603 µm. Hydrogel stability improves significantly after cross-linking, and substantial swelling occurs due to the hydrophilicity of SA. The 7/3 and 6/4 SF/SA hydrogels are more resistant to degradation in PBS, and cytotoxicity testing confirmed their biocompatibility. For release studies in vitro, two model compounds are used as drug models, tetracycline hydrochloride, and bovine serum albumin (BSA). Different ratios of SF/SA have a greater influence on the release of BSA. This study provides a practical preparation method for flexible SF/SA composite hydrogels, which can help design hydrogels with specific physicochemical properties for different applications, especially drug delivery.     

Preparation and characterization of tough and highly resilient nanocomposite hydrogels reinforced by surface-grafted cellulose nanocrystals

Despite many strong and tough hydrogels have been fabricated according to the energy dissipating mechanism, they usually lack high resilience due to the presence of large hysteresis. Herein, poly (N-vinylpyrrolidone) grafted cellulose nanocrystal (CNC-g-PVP) was used as special multifunctional physical crosslinkers to fabricate tough and highly resilient nanocomposite hydrogels. CNC-g-PVP with varying loading was incorporated into chemically crosslinked polyacrylamide (PAM) networks by in-situ radical polymerization to give PAM/CNC-g-PVP nanocomposite hydrogels. Robust cooperative hydrogen bonds existed between the surface-grafted PVP chains and the PAM matrix, which could rupture to dissipate energy upon deformation and recover instantly on the removal of stress. This unique energy dissipating mechanism led to excellent mechanical performance of the hydrogels. Their tensile elastic modulus, toughness, and compressive strength are 1.4–1.8, 2.1–3.0, and 1.44–2.73 times of pure PAM hydrogel, respectively. Moreover, the hydrogels exhibit low hysteresis, high resilience (ca. 97%) under cyclic tensile loading-unloading and good recovery of hysteresis (ca. 90%) under cyclic compressive loading-unloading.