Wenjin Xing,Youhong Tang

Institute for NanoScale Science and Technology,College of Science and Engineering,Flinders University,South Australia 5042,Australia

Keywords:Hydrogel Nanofillers Mechanical properties Nanocomposite Design Characterisation

ABSTRACT Nanocomposite hydrogels are the combination of known components that are a hydrogel and nanometre-sized fillers,typically leading to improved mechanical properties or new functionalities.With simplicity of design and ease of synthesis,recent advances have highlighted that this family of hydrogels holds the significant promise of application in diverse biomedical and engineering fields.The elaborate design and investigation as well as suitable application of nanocomposite hydrogels require the synergy of mechanics,materials science,engineering,and biology.Despite similarities in design and fabrication,the data of mechanical properties for nanocomposite hydrogels scatter in a large space.It is worthwhile comparing various nanocomposite hydrogels for similarities and differences in mechanical properties to aid in designing novel hydrogels with extreme properties,and guide practical applications.This review aims to fill,in the literature,the missing gap of addressing mechanical measurement methods and comparison of mechanical properties in this ever-evolving broad area of research.Finally,the challenges and future research opportunities are highlighted.

1.Introduction

Increasingly popular,hydrogels are aggregates of hydrophilic polymer networks and water.However,most conventional hydrogels are brittle and fragile,greatly confining their application scope.For a loadbearing application in diverse scenarios,adequate mechanical properties are inevitably crucial.A notable category of hydrogels that are mechanically strong and tough is DN(see Abbreviations for the definition of acronyms)hydrogels,comprised of a rigid and brittle first network,and a soft and ductile second network covalently crosslinked [1,2].The synergy of effective energy dissipation and high stretchability creates such tough hydrogels with notch insensitivity [3].Tough DN hydrogels have been applied in fields like tissue engineering,sensors,and actuators [4,5].

Another important category of hydrogels that continue to develop is nanocomposite hydrogels.These hydrogels integrate hydrogel networks and nanofillers.Both the hydrogel network and nanofiller have a characteristic length at the nanoscale,but they may exhibit distinct mechanical properties at larger scales.The intentionally introduced nanofillers not only offer unique functions,e.g.,electronic conductivity[6] and magnetic response [7],but more importantly bring about necessary interactions between the polymer chains and nanofillers thereby improving the performance of the pure hydrogels.Nanocomposite hydrogels can be tailored in a grand design space to possess superior properties.A broad range of inorganic and organic nanofillers can be considered.The nanofillers can be present in diverse forms in the composite system.In addition,the hydrogel building network can be formed with either synthetic or natural polymers.The building network is chemically or physically crosslinked by nanofillers.As rapidly developing materials,nanocomposite hydrogels have been applied in a wide range of biomedical and engineering applications,including sensors[8],actuators[9],conductors[10],coatings[11],drug delivery[12],wound dressing [13],tissue engineering [14],and antimicrobial applications[15].Many practical biomedical applications require multifunctional nanocomposite hydrogels with good mechanical properties,and dynamic interactions with cellular environments [16].

There have been excellent reviews in this field,to name a few,for fabrication and application [17,18],and for hydrogels containing a particular type of nanofiller [19],for biomedical applications [20],and for tissue engineering[21];however,a detailed summary of mechanical measurement methods,and critical comparison of mechanical properties among different nanocomposite hydrogels remain lacking in most review studies,highlighting the significance of the present review.Besides,fracture,fatigue and adhesion in nanocomposite hydrogels are also shortly discussed from a mechanical point of view.

The review is organised as follows.Section 2 will discuss the design of nanocomposite hydrogels using a variety of polymers and nanofillers.Section 3 will outline common mechanical testing methods with an emphasis on how to quantify fracture toughness.Section 4 will showcase mechanical properties of nanocomposite hydrogels by giving specific data extracted from the literature,and briefly explaining enhancement mechanisms.These properties include strain at break,stiffness,strength,toughness and hysteresis,and self-healing.A critical comparison of mechanical properties is made by plotting data in three property diagrams.Section 5 will discuss mechanics fracture,fatigue,and adhesion in the context of nanocomposite hydrogels.Directions for pursuing superior properties are suggested.Section 6 will illustrate a few promising applications of nanocomposite hydrogels,followed by concluding remarks.

2.Design

Different designs of nanocomposite hydrogels can be realised by selecting the constituent materials,nanofiller and hydrophilic polymer.Considerable progress has been made in designing and fabricating nanocomposite hydrogels.In this section,we review some of the systems that have been developed in the literature.As mentioned in Thoniyot et al.[17],there are five common fabrication methods for a nanocomposite hydrogel with a uniform distribution of nanofillers.Three of them are most relevant to the reviewed studies,including(i)gelation of a nanofiller suspension,(ii) reactive nanoparticle formation within a preformed hydrogel[22],and(iii)physical crosslinking using nanofillers to generate a hydrogel.

Regarding the material,a nanocomposite hydrogel can be made either from natural,synthetic,or combined polymers.Natural polymers like starch,alginate,fibrin,proteins,chitosan,and PLA,are more promising considering their abundance,low cost,excellent biocompatibility,and environmental friendliness [23,24].On the other hand,the synthetic polymers often involve the deployment of chemicals difficult to produce and degrade,and a complicated preparation process[25].They also demonstrate limited practical applications especially in the implant-related fields [26].The synthetic polymers are generally chemically stronger as compared to the natural polymers,leading to long-lasting durability but slow degradation rate[27](Ahmed,2015).To form a three-dimensional architecture in a hydrogel,one can crosslink hydrophilic linear polymer chains via chemical reactions,or physical interactions such as entanglements [28],hydrogen bonding [29],and crystalline domains[30].

2.1.Inorganic nano fillers

The inorganic nanofiller candidates consist of silica,nanoclays,GO,calcium phosphate,HA nanoparticles,CNTs,bioactive glass ceramic,calcium phosphate,metal-based nanoparticles,and others.These nanomaterials may be dimensionally different,thus resulting in varying reinforcing effects in the composite hydrogel.Here,we listed a few of them as examples.Nanoclays are anisotropic,stiff,and plate-like,consisting of 1-nm thick mineral silicate layers.The nanoclays distributed in the hydrogel matrix serve as reversible multifunctional crosslinkers to the strengthen and toughen hydrogels[31–33].The polymer chains can absorb and desorb on the surface of nanoclays,developing mutual physical interactions.Like nanoclays,GO features an extremely high surface-to-volume ratio with a large lateral dimension.In addition,layered GO platelets show outstanding stiffness(～32 GPa)and strength(～120 MPa) [34].Their mechanical and chemical properties (e.g.,surface groups,and hydrophilicity)are also highly tunable using various modification strategies.Due to these attractive attributes,GO has been explored as nanofillers for improving mechanical properties of neat hydrogels.

Silica nanoparticles as inorganic fillers in a hydrogel network are widely used because of their tunable size,uniform nanostructure,hydrophilic nature,and stability in aqueous solutions.Like most other inorganic nanofillers,the high surface to volume ratio facilitates strong physical interactions,and efficient stress transfer.Silica nanoparticles acting like initiators can also be used to graft hydrophilic monomers and then grow chains [35].Sometimes,excellent biocompatibility is a leading factor when considering the type of inorganic nanofiller.Resembling the chemical composition and structure of bone minerals,HA nanoparticles are an ideal choice of inorganic nanofiller in constructing nanocomposite hydrogels for tissue engineering[36].The morphology of HA presents needle-like crystals with an average size of approximately 30–40 nm in width and 180–240 nm in length[37].

Metallic and metal oxides nanoparticles such as silver,gold,ZnO,CuO,and TiO2are mostly conductive and magnetic in nature.They are much preferred as nanofillers due to providing either excellent electrical conductivity and stimuli responsiveness,or antimicrobial and antifungal properties,when loaded into hydrogels [15,38,39].Some metal-based nanoparticles can strongly interact with the polymer chains to enhance the mechanical performance of hydrogels [40].However,metal-based nanoparticles are often hard to be chemically modified to facilitate their uniform dispersion in hydrogel-forming precursors.Therebyin situformation of these nanofillers has been advocated as an effective means to minimise nanofillers aggregation[41].

2.2.Organic nano fillers

As a common green polymer,cellulose nanofillers can present in different architectures such as nanocrystal,nanofibre,nanowhisker,and other nanobundles [43].Besides their high stiffness and strength,cellulose nanofillers contain abundant hydroxyl groups and has a large specific surface area.For example,BC featured with nano-sized fibrils is a fibre ribbon of 30–100 nm long and 3–8 nm thick[42].The combination of these attributes can significantly improve the mechanical properties of the composite hydrogel [44].HBPs are highly branched dendritic macromolecules containing many reactive repeating units[45].They possess unique chemical properties such as high solubility,low viscosity,a large free volume,and more importantly,highly tunable topology and functional groups [46].Serving as physical crosslinkers,the HBP nanoparticles can strengthen and toughen the nanocomposite hydrogel by using abundant functional groups to develop supramolecular interactions with the polymer chains[47].Except HBPs,PS nanoparticles are among possible options for being organic nanofillers.They act as the crosslinkers that can interact with initiator molecules and initiate thein situfree-radical polymerisation on their surface[48].

3.Mechanical properties characterisation

The knowledge of their mechanical properties is of crucial importance before the deployment of these hydrogels as load bearing components in actual biomedical or engineering scenarios.The mechanical properties of great interest include stiffness,compliance,strength,yielding strength,hysteresis,toughness,work of fracture,fractocohesive length,and fatigue threshold(see Appendix A for the brief definition of these notions).Some of these properties are deformation or fracture-mode relevant.For example,stiffness may represent Young's modulus under tension/compression,or shear modulus under shear.Toughness may refer to Mode-I fracture under opening,Mode-II fracture under in-plane sliding,or Mode-III under out-of-plane tearing.However,for soft materials like hydrogels,tensile/compressive properties and mode-I fracture are of great interest and significance.

3.1.Mechanical tester

To quantify these material properties on a mechanical tester (e.g.,Instron),data collection and mechanical calculations are necessary.The mechanical tester itself can record the applied force and corresponding displacement.The nominal stress is obtained from dividing the force by the initial cross-sectional area of the specimen,and the strain from the ratio of the displacement and the initial gauge length between the grips.Owing to the change in cross-section during deformation,the true stress should be much larger in tension or much smaller in compression than the nominal stress.Tensile,slicing,and compressive experimental setups with a mechanical tester are shown in Fig.1a.In fact,there are a few commonly encountered practicTal difficulties.Since hydrogels are wet,low-friction,and possibly fragile under high pressure,it is nearly impossible to fix the hydrogel at both ends without slippage in a tensile test.To alleviate this issue,sandpapers or glues are mostly employed in practice [49,50].Mounted screws are also used to tighten the grips to ensure no slippage of the specimen.To prevent the test specimen from fracturing at the grips,dog bone-shaped specimens are preferred to make results more trustful[51,52].Hydrogels contain a large amount of “free” water that evaporates gradually in the air.To prevent water loss during the measurement,the as-prepared specimen can be coated with silicone oil [53] or produce a moisture environment using a custom-built humidity chamber [54].In terms of the loading rate that may affect the mechanical response for a hydrogel with sacrificial bonds,different values are recommended in the literature for a quasi-static response,typically in the range of 0.01–0.2 s-1[2,55,56].

Since the values of strength and strain at break are usually little reproducible for a set of samples,the quantity of toughness or fracture energy is much favoured [57].There exist different measurement methods and corresponding interpretations for the toughness of hydrogels [58].Pure shear,single edge crack,and tearing tests are the most frequently used ones in practice [52,55,59].In the pure shear test(Fig.1b),the toughness is calculated using the strain energy density of an intact specimen at the critical stretch times the specimen height.The critical stretch is obtained at crack propagation onset from pulling an edge-notched specimen of the same geometry.For the pure shear test to be valid,the length of the sample must be much greater than the height(at least 5 times).In the single edge crack test(Fig.1c),a short edge notch was cut into the middle of a rectangular specimen with a large height-to-width ratio[60,61].Both the pure shear and single edge crack tests need the calculation of stored strain energy density by integrating,up to the critical stretch,the nominal stress-strain curve of an unnotched specimen.However,the latter test is valid only for small crack lengths and for small to moderate strains [58].In the tearing test configuration(Fig.1d),the crack is deformed by out-of-plane shear loading,instead of dominant in-plane tensile loading as in the pure shear and single edge crack test configurations.If the elastic deformation of the two separating arms is suppressed or neglected,the corresponding toughness is simply calculated as two times the constant tearing force divided by the specimen thickness [52,62].Compared to the other two,the tearing test requires only one specimen to obtain the toughness of the hydrogel.Moreover,inextensible backing layers can be specially introduced to not only guide the tearing path,but also localise active deformation in a region around the tearing front.

3.2.Dynamic mechanical analysis/rheometer

Besides measuring mechanical properties of a hydrogel using a mechanical tester,we can also employ a DMA instrument or rheometer to measure its dynamic mechanical response,and to characterise its viscoelastic properties.In the DMA(Fig.1e),a sinusoidal stress is applied to a hydrogel specimen and the resulting strain is measured.Based on the assumption of linear viscoelastic behaviour,the complex modulus,storage,and loss moduli are determined from the data.The phase lag δ between stress and strain is used to determine tan(δ)(loss factor),equal to the ratio of the loss modulus to the storage modulus.The storage modulus is a measure of the hydrogel's elastic behaviour,conceptually related to stiffness(Young's modulus)determined on a mechanical tester;however,they are not necessarily the same.The loss modulus is a measure of energy dissipation in the hydrogel under cyclic loading.Storage,loss moduli,and tan(δ) are functions of temperature,frequency,or time,which can be analysed to reveal characteristic viscoelastic properties of the hydrogel of interest.The DMA has been used to study the viscous behaviour or the role of sacrificial bonds for DN hydrogels [55,63],nanocomposite hydrogels [64,65],supramolecular hydrogels [66],and self-healing hydrogels [52,67].In general,the loss modulus exhibits temperature-and frequency-dependent values that are closely related to the transient physical interactions.So,both mechanical testers and DMA instruments can be utilised to capture the mechanical response of interest for hydrogels and derive relevant mechanical properties.They can be adopted to characterise hysteresis,fatigue,and adhesion as well.For instance,loading-unloading tests are often performed to characterise the mechanical hysteresis of a hydrogel.The hysteresis reflects the capability of elastic energy dissipation(Fig.1f).

4.Mechanical properties

Many nanofillers can develop physical and/or chemical interactions with polymer chains.In these dynamic interactions,both the rate and degree of association can be controlled by the specific crosslinking moiety[68],thus resulting in much different mechanical and viscoelastic responses.Other factors that affect the mechanical response consist of the crosslinking density,polymer concentration,density of elastically active chains,chain rigidity,and surrounding conditions (e.g.,pH,temperature,solvent,and loading rates).Toughness is an important material property whose reliable quantification requires the presence of a predefined crack (Fig.1g).Note that some reports used work of fracture to characterise fracture resistance,which is converted for direct comparison to toughness by multiplying a fractocohesive length assumed 0.5 mm for all nanocomposite hydrogel systems in this contribution.

4.1.Strain at break

Haraguchi et al.[32]showed that for a nanoclay/PDMAA composite hydrogel,the strain at break(1200–1600%)was slightly decreased with the increasing content of nanoclay.However,with more polymer content,strain at break was enhanced rapidly from close to 0%–1600%,then reaching a plateau.This revealed that a critical polymer content despite low was necessary for the formation of a basic structural network through developing a sufficient number of effective crosslinking sites.Zhang et al.[69] mixed GO and PVA to make nanocomposite hydrogels via the freeze/thaw method,showing a gradual increase in strain at break up to 165% as the GO content was increased viscoelasticity,crystallization,and fillers in elastomers do profoundly affect strength to 1.0 wt%.As compared to that of the neat PVA hydrogel,62%increase was observed[70,71].In the work of a CNCs/PSBMA-co-PAAm composite hydrogel by Yang and Yuan [72],the strain at break reached a maximum value of 1127%at a CNCs concentration of 60 mg/mL,being 2 times higher than that without CNCs,suggesting the important role of physical crosslinking network induced by CNCs as a reinforcement.In the work by Hu and Chen[73],an LDH/PAAm nanocomposite hydrogel was reported with a striking elongation performance.At low contents of LDH nanofillers,the hydrogels were able to withstand more than 4000% strain without breaking,one order of magnitude higher than that of conventional chemically crosslinked PAAm hydrogels.The ultrahigh stretchability was likely resulted from the physically crosslinked network and the great flexibility and mobility of the polymer chain strands among the LDH nanofillers.Interestingly,an unexpected yielding phenomenon was seen during tensile deformations,possibly due to the unusual hierarchical porous structure.

Fig.1.Various mechanical measurement methods for fracture toughness and dynamic properties.(a) Hydrogel specimens under tension,slice,or compression on a mechanical testing platform.(b)Pure shear.(c)Single edge crack.(d)Tearing.In the schematics,the red solid or dashed lines represent predefined or potential crack paths.(e) DMA tests supporting different loading modes that follow a sinusoidal/oscillatory profile.(f) Mechanical hysteresis that reflects the capability of elastic energy dissipation in hydrogels.WD(area indicated by light green)and W(area indicated by yellow dashed lines)denote the dissipated energy density and total strain energy density respectively.(g) Estimating fracture toughness of hydrogels using the pure shear configuration:a:the force-displacement curve for a hydrogel sheet without containing a pre-cut,and b:for a hydrogel sheet containing a pre-cut.Lc minus the initial clamp distance L refers to the critical displacement at which the precut starts extending forward.Fig.1(a) is reproduced with permission from Refs.[1,3].Copyright ? 2003,Wiley-VCH,Copyright ? 2017,Springer Nature.

4.2.Stiffness and strength

Haraguchi et al.[32,74] showed that for a nanoclay/PDMAA or nanoclay/PNIPA composite hydrogel,the stiffness and tensile strength were increased almost proportionally to the nanoclay content (Fig.2I).With more polymer content,the modulus was increased almost linearly without reaching a saturation within the tested parameter range,whereas the strength was clearly increased significantly,and then decreased at a critical polymer content.Pan et al.[75] fabricated a GO loaded copolymerised hydrogel of PAM-co-DAC exhibiting outstanding stiffness (up to 1100 KPa) and strength (up to 2100 KPa) due to the effective multiple interactions,and both the high stiffness and strength of GO.This system contained ionic bonds between DAC chain segments and GO,and the hydrogen bonds between AM chain segments and GO.After the addition of GO,the strength reached 4 times of that without GO(mass ratio of AM and DAC is 1:2).Further increase in the content of GO led to increased strength but slightly decreased strain at break.The stiffness was monotonically varied between 67 and 1056 kPa by modifying the GO content and the mass ratio of AM and DAC.In the study of Zhang et al.[69],GO/PVA hydrogels exhibited significantly enhanced tensile strength (132%) and compressive strength (36%) at the addition of 0.8 wt%of GO.This improvement was attributed mostly to the developed strong interactions between the hydroxyl and carboxyl functional groups on the basal planes and edges of GO,and the hydroxyl groups on the PVA,facilitating a better internal load transfer mechanism.

Fig.2.Effects of nanofillers on the mechanical properties of a nanocomposite hydrogel.(I) Varying mechanical properties with the nanoclay content of nanoclay/PNIPA hydrogels,tensile strength (a),modulus (b),and elongation at break (c).(II) Varying fracture energy (toughness) (a),and fractocohesive length (b),Lfc,as a function of hydrogel type.The hydrogel labels ranging from 1 to 6 in order represent PAA,PAH2_14,PAH3_7.5,PAH3_14,PAH3_20,and PAH4_14.PAHn represents the HBP/PAA hydrogel with the n-th generation of HBP.(III)DMA results of silica/PDMAA hydrogels.tan θ is plotted as a function of applied strain at a frequency of 1 Hz.The level of viscous dissipation was increased with the amount of silica.(IV) Tensile stress-strain curves of the original and self-healed ZrH nanocomposite hydrogels for various incubation times at room temperature(a),and the corresponding self-healing efficiency as a function of healing time(b).Panel(I)is reproduced with permission from Ref.[74].Copyright ? 2002,American Chemical Society.Panel (II) is reproduced with permission from Ref.[47].Copyright ? 2021,MDPI.Panel (III) is reproduced with permission from Ref.[60].Copyright ? 2010,American Chemical Society.Panel (IV) is reproduced with permission from Ref.[76].Copyright ? 2017,Elsevier Ltd.

Pang et al.[37]fabricated a composite hydrogel of nano-sized HA and PVA.They found that the tensile strength was increased from 1.43 to 2.34 MPa (increasing 64%) with the increasing HA content from 0 to 4.5 wt%,because of the high surface energy of HA nanoparticles.However,the strength started declining after the HA content attained a certain percentage,due to the inevitable agglomeration of HA nanoparticles.The stiffness of the HA/PVA hydrogel presented a rising trend until the content of HA reached 1.5 wt% and then somewhat decreased with the HA content.In terms of raising the concentration of PVA polymer,both the strength and stiffness were monotonically enhanced,which was caused by the restricted movement of polymer chains in a confined space,and consequently greater hydrogen bonding and the crystallinity degree.For example,the stiffness was increased 35% as the PVA concentration varied from 10 to 15 wt%.Xing et al.[47] compared the mechanical properties of nanocomposite hydrogels with PAA and HBPs of different generations.These HBPs in the form of nanoparticles are featured with abundant hydroxyl end groups developing hydrogen bonding with PAA chains.The authors demonstrated that the third and fourth generations of HBP were more effective than the second one in terms of strengthening and toughening effects (Fig.2II).They ascribed such a difference to the amount of the exposed end functional groups in the outer shell of nanoparticles,despite the same total amount of hydroxyl groups,and the similar hyperbranched architecture for the considered HBP generations.In addition,compared to that of the pure PAA hydrogel,the strengths of the composite hydrogels with the third and fourth HBPs were 2.3 and 2.5 times.This improvement was caused by the incorporation of HBP nanoparticles as organic fillers,creating physical interactions.Another salient finding in this work was that the stiffness corresponding to all generations was almost unaltered in comparison to that of the pure PAA,owing to the weak characteristic of hydrogen bonds in this specific system.This example illustrates that a nanofiller,relying on its nature,may have negligible effects on the overall stiffness of the composite hydrogel.

4.3.Toughness and hysteresis

Fracture toughness as a material property represents the maximum driving force required for crack extension of a unit area.Haraguchi et al.[77] showed that with increasing nanoclay content,the toughness of a nanoclay/PNIPA composite hydrogel was almost linearly enhanced to 2310 J/m2,3300 times that of a conventional chemically-crosslinked hydrogel,evidencing the reinforcing power of nanoclay as the crosslinker.The nanocomposite hydrogel with high content nanoclay exhibited somewhat irregular stress-strain profile,distinct from that of the chemically crosslinked hydrogel.Tang et al.[79] quantified using tear tests the toughness values of the dually-crosslinked GO/PAAm hydrogels with the GO contents of 0.5,1,2,and 4 mg/mL,equal to 341,344,577,and 964 J/m2,respectively.The toughness increase associated with the GO concentration depended on the number of elastically effective PAAm chains.The authors also illustrated increased crosslinking density in GO/PAAm hydrogels by theoretical calculations,and experimental swelling tests.The enhanced energy dissipation mostly originated from the peeling-off of physically adsorbed PAAm chains from GO nanosheets and breaking of chemically grafted PAAm chains from GO nanosheets.The higher the GO content,the more intense the physical interaction per volume.Furthermore,the toughness enhancement was also closely related to the used amount of chemical crosslinker.A modest amount could stabilise the network and promote stress transfer,but simultaneously avoid stress concentration on the shortest chains in the network.This work reveals the significance of employing physical and chemical dual crosslinking in the nanocomposite hydrogel.

Carlsson et al.[80] mechanically tested silica/PDMAA hybrid hydrogels.The silica nanoparticles largely improved the elastic behaviour of originally brittle self-crosslinked matrix.When sufficient silica was added,the physical interactions between silica nanoparticles became comparatively important,resulting in a modulus increasing up to 20–40 kPa at 20–25 vol% silica.The nanocomposite hydrogel demonstrated a significant strain rate dependence,hysteresis,and residual deformation with increasing amounts of added silica.As the nanoparticles were added,the fraction of the softening due to viscoelastic relaxation processes was greatly increased.The dissipative capability and hysteresis increased with increasing amounts of silica both at intermediate strains(below 50%)and at large strains(up to 500%).The authors thought that the dissipative mechanisms were not only because of Mullins effect induced by the presence of nanoparticles,but also due to the very low chemical crosslinking density.The observed increase in residual strain after unloading due to the presence of silica was consistent with that PDMAA and silica nanoparticles form non-permanent and reversible interactions.For intermediate strains,there was no significant change in the initial stiffness with a quantitative recovery independent of the nanoparticle content.At large strains,internal damage appeared more markedly with increasing amount of silica,leading to appreciable loss in initial stiffness in the cyclic loading-unloading test.Lin et al.[60](2010)investigated the large strain and fracture properties of silica/PDMAA nanocomposite hydrogels.The DMA analysis revealed that the level of viscous dissipation was increased with the amount of silica,and the cycles of greater amplitude(Fig.2III).The observed hysteresis loops in the composite hydrogels could be attributed to the de-adsorption and re-adsorption of the PDMAA chains during the time scale of compression testing.The addition of silica likely disturbed the homogeneity of the network,increasing the number of particles in the system and broadening the distribution of elastic chain lengths within the hydrogel.The toughness was much improved within the range of 10–80 J/m2with increasing the content of silica.Interestingly,the fracture tests also suggested that the macroscopic strain at break was mainly controlled by the average extent of chemical crosslinking.

4.4.Self-healing

Unlike biological tissues that can spontaneously self-heal,most synthetic materials are not self-healable following mechanical damage.One of the self-healing strategies employs an extrinsic capsule based concept,where encapsulated healing agents are introduced within the polymer[81].However,this type of self-healing technique involves an appropriate polymerisation process of monomers,and then requires a catalyst[82].Also,the depletion of healing agents is a key issue,locally decreasing healing efficiency [83].Therefore,the intrinsic self-healing attribute in hydrogels is highly desired in most application scenarios due to the inherent capability to continuously restore their load transferring efficiency by repairing damage.The majority of nanocomposite hydrogels features dynamic and reversible physical interactions,thus exhibiting intrinsic self-healing characteristics.Lin et al.[60]found that silica/PDMAA nanocomposite hydrogels fully recovered their initial elastic moduli over a series of successive loading-unloading cycles during the time scale of the experiment at room temperature because of no permanent damage accumulated in the network structure.Zhong et al.[84] healed the freshly cut hydrogel that used vinyl-hybrid silica nanoparticles as multivalent covalent crosslinkers,without the use of any chemicals,by simply placing two parts together.This self-healing stemmed from the diffusion and ionic bonding of the Fe3+ions and grafting PAA chains on the surface of silica nanoparticles at the contact interface.The healed hydrogel could be stretched to more than fifteen times its initial length and displayed good tensile strength after a long period of incubation at high temperatures(e.g.,12 h and 50°C).Tang et al.[79]observed fast self-healing properties in GO/PAAm hydrogels,achieving 76% and 62% of stiffness and toughness recovery after 30 min of resting at room temperature respectively.The authors also found that the self-healing efficiency was decreased as the chemical crosslinker (MBA)content was increased,due to the much-restricted movement of polymer chains.

Li et al.[85] incorporated Fe3O4nanoparticles into a catechol-modified polymer network of 4-arm catechol-terminated PEG to obtain hydrogels crosslinked via reversible metal-coordination bonds at Fe3O4nanoparticle surfaces.These hydrogels were able to after 10 min fully recover their storage modulus following a 1000% shear strain at room temperature.Jiang et al.[76] employed ZrH nanoparticles as multifunctional physical crosslinkers.The nanocomposite hydrogel without chemical crosslinking could self-heal without any external stimuli at room temperature.The reconstruction of the fractured network at the contact interface was established upon after enough chain diffusion physical entanglements among polymer chains,and dynamic hydrogen bonding between functional groups on polymer chains and ZrH nanoparticles.The authors demonstrated that the healing efficiency was significantly dependent on the content of ZrH and incubation time(Fig.2IV).The highest healing efficiency of 86% in strain and of 75% in strength was reached at the incubation time of 24 h and at the weight content of 4 wt% for ZrH.Multiple types of supramolecular interactions,hydrogen bonding,π-π stacking,and complexing interactions,were formed in a hybrid system of functionalised single-wall CNTs,PVA of short chain length,and PDA chains in the presence of borates[8].These dynamic interactions and the unique structure allowed for remarkable self-healing of efficiency 99%in electrical resistance within about 2 s.

4.5.Comparison of mechanical properties

The mechanical properties among nanocomposite hydrogels and some other representative hydrogels are plotted and shown in Fig.3.The extracted data are corresponding to a moderate content of nanofiller.It is apparent that mechanical properties of nanocomposite hydrogels are improved as compared to those of their neat counterparts.Without considering few exceptions,the following general findings can be suggested.For nanocomposite hydrogels,the values of strain at break spread out in a rather wide range of 200–4000%(Fig.3a).Likewise,the values of tensile strength are widely scattered ranging from 50 to 2000 kPa (Fig.3a).The stiffness values cluster in a narrow range of 10–100 kPa (Fig.3b).The toughness values (Mode-I) are randomly distributed between 200 and 4000 J/m2(Fig.3c).It is worth mentioning that some toughness values may be somewhat overestimated when converted from work of fracture with the assumed fractocohesive length of 0.5 mm.Furthermore,the universal positive stiffness-strength correlation in materials science holds for nanocomposite hydrogels,as indicated by the grey ellipse oriented about 45°from the stiffness axis in Fig.3b.That is to say,the greater the stiffness,the greater the hydrogel strength.On the other hand,the conflict between strain at break and strength is met akin to most hard solids,as indicated by the grey ellipse oriented about 135°from the strain axis in Fig.3a.Additionally,no obvious trade-off between stiffness and toughness occurs in Fig.3c for nanocomposite hydrogels,which seemingly break the well-known stiffness-threshold conflict in single-network polymers[56,86].

The mechanical properties somewhat depend on the types of nanofiller and polymer.For example,the PVA based nanocomposites show much higher stiffness and strength than other nanocomposites(right-and top-most region in Fig.3b).This is due to the effects of high crystallinity besides nanofiller interactions.BC and HA nanofillers result in both higher stiffness and strength relative to other nanofiller types.When compared to other tough hydrogels without nanofillers,many nanocomposite hydrogels appear weak in some mechanical aspects.For example,despite moderately stretchable,PA hydrogels exhibit a combination of high toughness,stiffness,and strength.DN hydrogels [55,97]show a rather high toughness around 10000 J/m2,outperforming all nanocomposite hydrogels.However,it is impressive that a few nanocomposite hydrogel examples achieve a combination of excellent mechanical properties,comparable to those of skeletal muscle.Tough hydrogels need crack blunting to deconcentrate crack-tip stresses at large deformations.The degree of blunting can be roughly estimated by the ratio of fracture strength to stiffness [101,102].Strong blunting likely takes place when this ratio exceeds a factor of 2 (dashed grey line in Fig.3b).Interestingly,the majority of surveyed nanocomposite hydrogels falls above the dashed grey line,which is contrary for neat hydrogels,indicating the significance of nanofillers as effective tougheners to delay crack propagation.

Fig.3.Comparison of mechanical properties among nanocomposite hydrogels and some other representative hydrogels:(a) strain at break vs.strength;(b) stiffness vs.strength;(c) stiffness vs.toughness (Mode-I fracture).Identical colours refer to the same type of polymer,and identical shapes the same type of nanofiller.Hydrogels without any nanofillers are distinguished using colourful rings.In (b),the dashed grey line of a slope 2 is drawn to identify hydrogels that likely exhibit either weak blunting or strong blunting scenarios observed during pulling a pre-cut hydrogel sheet.In (c),the horizontal line represents the fracture toughness separation line of 5000 J/m2.These diagrams are plotted using extracted data corresponding to a moderate nanofiller content from the literature:PAA[47],silica/PAA[35],HBP3/PAA [47],PAAm [55],nanoclay-S/PAAm [87],nanoclay-MMT/PAAm [88],LDH/PAAm [73],CNDs/PAAm [89],HA/PAAm [36],GO/PAAm [79],BC/PAAm[90],PVA[7],GO/PVA[69],CNTs/PVA[91],HA/PVA[37],BC/PVA[42],nanoclay/PDMAA[32],nanoclay/PNIPA[77],OCAPS/PNIPA[92],silica/PEG diacrylate [93],CNTs/GelMA [94],CNTs/PAAm-co-PAA [95],TiO2/PDMAA-co-PAA [96],Zr(OH)4/PAMPS-co-PAAm [76],alginate [55],PAAm-alginate [55],PVA-PAAm [97],muscle [98–100],PA [59].

5.Discussion

Owing to their very large surface-to-volume ratios and potential surface functionalisation,nano-sized fillers always show special mechanical,physical and chemical properties.The hydrogel network provides high porosity and permeability that accommodate water molecules and even some nanofillers of small size.The nanocomposite system,perhaps with chemical crosslinking,definitively contains physical interactions between nanofillers and polymer chains.Those physical interactions,to different extents,improve the mechanical properties(Fig.2I,II),primarily depending on the characteristics of employed nanofillers such as shape,size,mechanical attributes,and surface chemistry[103].Nanofillers possess a large surface-to-volume ratio,and high surface reaction activity,facilitating the physical interactions.However,nanofiller dispersion is also a key factor determining the mechanical reinforcement.If nanofillers become agglomerated despite their small sizes,they do not play the expected role[104,105].

5.1.Mechanics

Considering the fact that water is negligibly viscous,many singlenetwork hydrogels without fillers of any kind and crystallinities are featured with low viscoelasticity.The introduction of nanofillers do augment the macroscopic viscoelasticity of elastomers [71] and hydrogels [106] with dynamic bonds,thus affecting mechanical strength and toughness[47].The sole polymer network behaves as an entropic spring,imparting elasticity [107].On the other hand,the introduction of nanofillers enables inelasticity in some situations.In some cases of nanocomposite hydrogels,covalent crosslinking has been introduced to endow high resilience and elasticity and preserve the network topology.

The functionality of a crosslink refers to the number of polymer chains interconnected at the crosslinking point.Common covalent crosslinks usually have relatively low functionality normally less than 10.In contrast,nanofillers can offer multifunctional crosslinking points on the order of several tens and even hundreds to the network(Fig.4a and c),thus leading to increased density of elastically active chains per volume.For a nanocomposite hydrogel with a nanoclay/silica-like filler,thein-situpolymerisation process starts on the surface of the nanofillers with the initiator adsorbed [31,35].The absorbed polymer chains after growth that are flexible usually have chain lengths of inhomogeneous distribution and take nearly random conformations.Being deformed,a portion of polymer chains can detach from the nanofiller crosslinkers to partially release the stored elastic energy in the polymer network[108].The dynamic chain entanglement-disentanglement process also dissipate energy [35].In some other cases,the polymer chains can form diverse sacrificial bonds with nanofillers via supramolecular chemistries(Fig.4b).A single adsorption strength or sacrificial bond is weaker than a covalent bond;however,a large population of such bonds can well distribute stresses and significantly contribute to energy dissipation.As a result,the nanocomposite hydrogel shows higher mechanical properties than the chemically crosslinked hydrogel.

Fig.4.Interactions between nanofillers and polymer chains.(a) Structural model for nanoclay/PDMAA nanocomposite hydrogels.Dic is interparticle distance of exfoliated nanoclay platelets.χ,g1,and g2 represent cross-linked chain,grafted chain,and looped chain.(b) Preparation and illustration of the network structure of GO/PAAm nanocomposite hydrogels with both chemically and physically crosslinked sites between GO nanosheets and PAAm chains.(c)Elastic rearrangement model and typical stress-strain curves for silica/PAA hydrogels.Covalent crosslinks between silica and polymer chains led to an elastic network,whereas physical entanglements among polymer chains were related to the viscoelastic properties.During deformation,coiled polymer chains were unfolded and rearranged.The dynamic entanglement-disentanglement process of polymer chains contributed to effective energy dissipation.Panel (a) is reproduced with permission from Ref.[31].Copyright ?2003 American Chemical Society.Panel (b) is reproduced with permission from Ref.[79].Copyright ? 2017 Royal Society of Chemistry.Panel (c) is reproduced with permission from Ref.[35].Copyright ? 2013 Royal Society of Chemistry.

Nanocomposite hydrogels exhibit high macroscopic viscoelasticity.Because of the kinetics of bond dissociation and reformation,the sacrificial bonds formed between nanofillers and polymer chains are responsible for the viscoelastic character of the nanocomposite hydrogel[109].There exists a characteristic time scale so that these viscoelastic hydrogels have intrinsic time dependent dissipative mechanisms,and different mechanical responses at varying strain rates [52,110].The polymer network is able to self-reorganize to recover an equivalent network of elastically active chains,and the physical interactions may avoid irreversible damage of the covalent network by releasing stresses,leading to self-healing.Experimental evidence has confirmed that fast stress relaxation and effective energy dissipation in nanocomposite hydrogels are key design or selection parameters for regulating cell fate and activities[111,112].

5.2.Fracture

Of great interest,fracture is a material separation phenomenon,featuring a geometrical and displacement discontinuity.The crack tip concentrates stresses under loading.Moreover,the propagation of the crack impairs the structural integrity,reducing the service life of a component or device.For a crosslinked polymer with a single covalent network,the classical Lake-Thomas model [86] states that the energy necessary to break the network to extend a crack by a unit area,known as the intrinsic fracture toughness,is approximately on the order of 10 J/m2.This value is much lower than that of the tough hydrogels developed in the literature.In addition,a covalent polymer network cannot achieve simultaneously both high toughness and high stiffness[56].This conflict can be resolved by utilising the concept of nanocomposite hydrogels as a general method.Before getting into fracture in nanocomposite hydrogels,it is beneficial to recall the toughening principle in DN hydrogels.The first network in DN hydrogels serves as a sacrificial network contributing to a large amount of energy dissipation,whereas the second network keeps the basic structural integrity even in the presence of cracks[55].Likewise,for the nanocomposite hydrogels,the internal physical or sacrificial interactions greatly improve the fracture toughness as compared to that of the sole covalent network.To resist the crack growth,the nanocomposite hydrogels dissipate energy through two mechanisms:scission of a layer of polymer chains in the direction of crack growth,and breakage of sacrificial bonds in the surrounding of the crack tip(Fig.5I(A),5II(a)).This surrounding can be called local damage zone as in Ref.[118]or as bulk dissipation zone in Ref.[113].The material element inside this so-called local damage zone undergoes a loading-unloading cycle as the crack tip advances.This macroscopic hysteresis loop reflects bulk energy dissipation during fracture(Fig.5I(B)).The continuous breaking of physical interactions at the same time leads to the softening behaviour of a nanocomposite hydrogel during unloading,as seen from hysteresis loops in cyclic tensile testing[47,79,114].It should be noted that it is necessary to account for chemical crosslinking to form a dual crosslinked network,enabling a fast shape and damage recovery[115].

Fig.5.(I,II)Understanding fracture characteristics and processes and(III)pursuing superior mechanical properties by design.I:(A)A hydrogel with a crack of length a,and a local damage zone around the crack tip.(B)Mechanical dissipation in the damage zone,manifested as a macroscopic hysteresis loop.(C)Decreasing fracture strength when the flaw size is greater than ac defined by the ratio of toughness and work of fracture.(D)High strength achieved by high-functionality crosslinks that accommodate a bunch of hidden polymer chains.II:Two fracture-related characteristic length scales in the context of soft materials.(a)The nonlinear elastic length scale l expresses the size of the crack-tip zone where nonlinear elastic effects are dominant,and the dissipation length ξ expresses the size of the crack-tip failure zone.(b)The l-ξ chart for various hard and soft materials.Note that for soft materials,the relation l ＞ξ holds true.The data were extracted from Refs.[113],and for the nanocomposite hydrogel,ξ=0.5 mm was assumed.(III)Integrating multiple mechanisms that span across multiple length scales is a promising solution to designing next-generation strong and tough hydrogels.Specially,one can account for chemical/physical hybrid crosslinkers at the nanoscale,multi-functional crosslinkers at the micro/nanoscale,and fibre-reinforcement at the macro/mesoscale.Panel (I) is reproduced with permission from Ref.[3].Copyright ? 2017 American Chemical Society.Panel (III) is reproduced with permission from Ref.[115].Copyright ? 2014 Royal Society of Chemistry.

When cracks or notches are small enough,the tough hydrogels can still sustain large deformations without any crack propagation.To elucidate such an effect of notch insensitivity,Chen et al.[116]proposed the concept of fractocohesive length for soft materials,quantified as the ratio of fracture toughness to work of fracture.This critical length scale predicts the transition from flaw-insensitive to flaw-sensitive fracture(Fig.5I(C)),and the evaluation of the size of fracture process zone(Long et al.,2020).In a most recent work by Long et al.(2020),the authors emphasised two length scales involved in fracture of soft materials:nonlinear elastic length and dissipation length characterising respectively the strongly nonlinear elastic and dissipation zones near the crack tip (see Fig.5II(a)).The former is also termed elasto-adhesive length elsewhere [117],defined as the ratio of fracture toughness to stiffness,reflecting large deformation capability whose nonlinear effects on the crack-tip fields need to be considered.The latter indeed coincide with the concept of the fractocohesive length.In the near-tip dissipation zone,the stress concentration is somewhat wiped out,and a characteristic load is transferred to failure processes from the applied external mechanical fields.The quantitative estimates for some typical hard and soft materials are presented in Fig.5II(b),from which one can appreciate that,for soft materials including hydrogels,the strongly nonlinear elastic zone is(much) greater than the dissipation zone.However,these two length scales are not strictly well-defined to date for nanocomposite hydrogels as they are highly viscoelastic and fracture process is complex,dependent on the crack velocity and in turn loading rates.Knowing these different length scales,one can not only clarify hydrogels,but also deal with them without compromising properties by making all possible defects smaller than the fractocohesive length.

5.3.Fatigue and adhesion

When a pre-cut specimen is subject to a cyclic load,the gradual extension of the crack is termed fatigue fracture.It is often characterised using the extension length of the crack per loading cycle as a function of energy release rate.The fatigue threshold is much lower than fracture toughness measured under monotonic loading,even for a tough DN hydrogel.Bai et al.[119] have showed that even self-healing hydrogels are still susceptible to fatigue fracture,using a demonstration example of a pre-cut hydrogel containing both covalently crosslinked polyacrylamide and uncrosslinked PVA.The threshold for fatigue fracture depends on the covalent network but negligibly on physical interactions.This conclusion should apply to precut nanocomposite hydrogels focused in this review that typically possess physical interactions.However,compared to fracture,fatigue performance of nanocomposite hydrogels is still less explored [120],demanding further research to reveal their fatigue performance.

Besides fracture and fatigue,adhesion is another common theme in engineering and daily life.To adhere a soft and wet hydrogel on the surface of diverse materials is always a difficult task [121].Stable adhesion requires strong molecular connections at the interface.Tough adhesion requires significant energy dissipation both locally near the interfacial crack tip in the adhesive and globally in the two adherends[122].The philosophy of nanocomposite hydrogels can be applied in the scene of adhesion [92,123].For example,exploiting the nanofillers'capability of adsorbing polymer chains in adherends should be a straightforward yet effective method.To the best of the authors’knowledge,however,a detailed experimental investigation and mechanistic analysis for the effects on adhesion strength and toughness,of different types of nanofillers in the adhesive-forming precursor seem lacking.

5.4.Pursuing superior properties

Because of highly nonuniform chain lengths and molecular-level imperfections,the polymer chains fracture sequentially,resulting in a low tensile strength [57].By means of employing nanofillers as multifunctional crosslinkers accommodating hidden chains [3],the shorter chains under deformation can pull hidden chain segments from the crosslinkers,leading to their simultaneous fracture with long chains(Fig.5 I(D)).A combination of superior mechanical properties is always the long-pursuing goal for material researchers.The properties diagrams in Fig.3 have shown that there is still large room to fill for nanocomposite hydrogels.Following the suggestions of Zhao[115],a promising design strategy for future strong and tough hydrogels is to integrate multiple pairs of mechanisms across multiple length scales into a hydrogel on the top of nanocomposite hydrogels.Specifically,one can account for chemical/physical hybrid crosslinkers at the nanoscale,multi-functional crosslinkers at the micro/nanoscale,and fibre-reinforcement at the macro/mesoscale [124],as shown in Fig.5III.The introduction of long fibres into nanocomposite hydrogels can significantly improve both their fracture toughness and fatigue threshold via substantial stress de-concentration and energy dissipation [125].However,we should keep in mind that excessive mechanical properties and the associated high crosslinking density may be disadvantageous sometimes.For example,they do not favour the desired biological response in terms of excellent biocompatibility and integration with biological tissues in biomedical applications[126–128].

6.Applications

The past two decades have seen outstanding achievements in the use of nanocomposite hydrogels in biomedical and engineering areas.Fig.6 demonstrates some promising application examples enabled by nanocomposite hydrogels.It should be emphasised that the majority of these applications cannot succeed without involving reasonable mechanical properties for nanocomposite hydrogels discussed in this contribution.Nanocomposite hydrogels can possess a combined set of features -tunable mechanical properties,fast stress relaxation,good injectability,and unique bioactivities,greatly facilitating effective and efficient drug delivery(Fig.6i)[129].Culturing cells on a substrate is widely utilised in biomedical research and tissue engineering [130].Nanocomposite hydrogels have been proved to support normal cell activities including adhesion,growth,proliferation,and migration(Fig.6(ii))[78,131,132].These activities have been found to depend strongly on the settings of nanofiller such as type and concentration.

Additionally,nanocomposite hydrogels can perform with the help of functional nanofillers as smart sensors with adequate resolution,reliability,and sensitivity.One attractive sensing purpose is to monitor both subtle and large human motions(Fig.6(iii)),when these hydrogels also exhibit reliable adhesion capability of being intimately adhered onto the body surface [95,133].Like sensors,nanocomposite hydrogels can be made as actuators by simply incorporating functional nanofillers [134].Unlike rigid actuators,however,these actuators have virtually infinite degrees of freedom,and are able to complete very complicated tasks even in a much confined space by responding to various external stimuli such as light,pH,temperature,magnetic and electric fields [41].Besides the above applications,nanocomposite hydrogels with high electrical conductivity have made achievable various applications in bioelectronics to offer a seamless soft and wet interface between electronics and biology,such as cardiac patches made of gold nanowires/alginate hydrogel[135](Fig.6v).To meet the ever-growing requirements in multifunctionality,structural complexity (e.g.,hierarchy),and compositional diversity,advanced manufacturing technology is vital,among which 3/4D printing of hydrogels stands out[136,137].By means of printing,the physical and mechanical properties of nanocomposite hydrogels can be tuned spatially and temporally[138].

Fig.6.Nanocomposite hydrogel applications in biomedical and engineering areas.(i) Drug delivery enabled by PAM-Mg/PAM-grafted HyA hydrogel.(ii) Cell cultivation supported by nanoclay/PNIPA.(iii)Strain sensors for body motion detection based on CNTs/PAAm hydrogel.(iv)Actuation realised by(NFC/polyaniline/MnFe2O4)/PVA hydrogel possessing conductive and magnetic properties.(v) Functioning of cardiac patches made of gold nanowires/alginate hydrogel.Fig.6(i) is reproduced with permission from Ref.[129].Copyright ? 2018 Wiley-VCH.Fig.6(ii) is reproduced with permission from Ref.[78].Copyright ? 2006 American Chemical Society.Fig.6(iii) is reproduced with permission from Ref.[133].Copyright ? 2020 Elsevier.Fig.6(iv) is reproduced with permission from Ref.[134].Copyright ? 2018 American Chemical Society.Fig.6(v) is reproduced with permission from Ref.[135].Copyright ? Nature Publishing Group.

7.Concluding remarks

Nanocomposites hydrogels that are considered fundamental to numerous applications share considerable similarities in design and fabrication.All of them are based on integrating nanofillers and hydrophilic polymers.However,to meet practical needs we can carefully select synthetic or natural polymers,and as well different materials as nanofillers that are either organic or inorganic,rigid,or compliant,spherical,or planar.The combination diversity tremendously expands the materials space in biomedical and engineering applications.In this review,we have focused on the mechanical properties of various nanocomposite hydrogels.These mechanical properties are finely tunable by controlling the content of nanofiller and polymer.Also,they somewhat depend on the type of these constituents.To design novel hydrogels with extreme properties,the multiscale design paradigm is indispensable.Larger features at meso-/macroscales can significantly improve both fracture and fatigue resistance.We also present and compare mechanical data,aiming to guide practical applications of the existing nanocomposite hydrogels.In cell culture,low stiffness is a prerequisite.In artificial tissues,sufficient stiffness,and strength comparable to those of tissues are preferred.In flexible electronics,high toughness is a necessity.In soft sensors and electronic skins,high strain at break is essential.One can deliberately select appropriate nanocomposite hydrogels to meet their needs according to the plotted property diagrams of nanocomposite hydrogels.

There is still much work that remains to be carried out:(i) A significant amount of literature has reported the tensile or compressive properties for nanocomposite hydrogels.However,fracture toughness was not quantified for them on most occasions using the fracture measurement methods.(ii)Their fatigue threshold has not yet been much explored as well as the effects of nanofillers on adhesion strength and toughness;(iii)Reliable and accurate material constitutive models for nanocomposite hydrogels under large deformation still remain scarce,impeding the direct application of established numerical methods to quantify stress distribution and energy dissipation;(iv)Numerical simulation examples for obtaining mechanical fields around the crack tip and predicting fracture of nanocomposite hydrogel structures are scanty;(v) A deep understanding of the connection between the molecule-level events such as bond kinetics to the continuum-level deformation,inelasticity,and fracture can start with elaborate experimental investigations by employing innovative experimental techniques,including scission-based mechanophores and mechanoradical polymerisation.Finally,we believe that progress in understanding experimentally and numerically the mechanical behaviour of these nanocomposite materials pushes forward new nanotechnology and hydrogel machines.

Declaration of competing interest

The authors declare no known competing financial interests or personal relationships that could affect the work reported in this paper.

Acknowledgements

The authors gratefully acknowledge the financial support provided by Flinders University through the DVCR Research Investment Fund Scheme to provide Research Support for ECR to MCR Academics.

Appendix A.Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.nanoms.2021.07.004.

Appendix A.Definition of mechanical notions

In this section,we cover the definition of important mechanical notions often encountered in the context of soft materials,which are also closely relevant to the mechanical characterisation and interpretation of nanocomposite hydrogels.

Stiffness,defined as the ratio of stress to strain in the linear regime of tensile or compressive stress-strain curves,predicts how much a material specimen in one direction elongates under tension or shortens under compression.The higher the stiffness is,the more difficult it is for the material to deform.For some materials like viscous materials,stiffness is rate dependent.

Compliance,the inverse of stiffness,is defined as the ratio of strain to stress in the linear regime.The higher the compliance is,the more effortless it is for the material to deform.

Strengthis defined by the applied force at fracture divided by the deformed cross-sectional area.In most cases,one replaces for simplicity the deformed cross-sectional area with the original cross-sectional area in the initial configuration.There is no direct relationship between stiffness and strength.A material can have high strength but low stiffness.

Yielding strength,is the stress corresponding to the yield point at which the material begins to deform plastically.It is often difficult to precisely define yielding in the stress-strain curve for a material.

Hysteresisreflects the energy dissipation capability during deformation,whose coefficient is usually defined by the ratio of the energy dissipated to the work ever done,both of which can be obtained from the stress-strain curves in a loading-unloading cycle.

Toughness,one of the most important mechanical properties,expresses a material's resistance to fracture in the presence of a crack or notch.It has the dimension of energy per unit area.

Work of fractureis characterised by the area under the stress-strain curve.It has the dimension of energy per unit volume.High work of fracture usually corresponds to a high toughness.

Fractocohesive lengthis assumed equal to the ratio of fracture toughness to work of fracture.This critical length scale can be adopted for the approximate prediction of the transition from flaw-insensitive to flaw-sensitive fracture.For the specimen with a notch of a depth less than fractocohesive length,the measured load-displacement curve is almost unaltered as compared to that of an intact specimen.

Fatigue thresholdis an amplitude of energy release rate below which the crack does not propagate,but above which it grows rapidly under prolonged cyclic loads.