博士 教授 博士生导师
主要研究方向为智柔体、复合材料力学、微纳米力学。发表SCI学术论文160余篇；获国家发明专利8项。担任Proceedings of the Royal Society A 编委、International Journal of Fracture编委、International Journal of Computational Materials Science and Engineering编委、Journal of Zhejiang University-SCIENCE A (Applied Physics & Engineering)编委、Mechanics of Soft Materials编委、《固体力学学报英文版》编委、《力学进展》编委、《机器人》青年编委。
Hydrogel-polymer hybrids have been widely used for various applications such as biomedical devices and flexible electronics. However, the current technologies constrain the geometries of hydrogel-polymer hybrid to laminates consisting of hydrogel with silicone rubbers. This greatly limits functionality and performance of hydrogel-polymer–based devices and machines. Here, we report a simple yet versatile multimaterial 3D printing approach to fabricate complex hybrid 3D structures consisting of highly stretchable and high–water content acrylamide-PEGDA (AP) hydrogels covalently bonded with diverse UV curable polymers. The hybrid structures are printed on a self-built DLP-based multimaterial 3D printer. We realize covalent bonding between AP hydrogel and other polymers through incomplete polymerization of AP hydrogel initiated by the water-soluble photoinitiator TPO nanoparticles. We demonstrate a few applications taking advantage of this approach. The proposed approach paves a new way to realize multifunctional soft devices and machines by bonding hydrogel with other polymers in 3D forms.
National Science Review，2020，8（2）：nwaa254
Hydrogels—natural or synthetic polymer networks that swell in water—can be made mechanically, chemically and electrically compatible with living tissues. There has been intense research and development of hydrogels for medical applications since the invention of hydrogel contact lenses in 1960. More recently, functional hydrogel coatings with controlled thickness and tough adhesion have been achieved on various substrates. Hydrogel-coated substrates combine the advantages of hydrogels, such as lubricity, biocompatibility and anti-biofouling properties, with the advantages of substrates, such as stiffness, toughness and strength. In this review, we focus on three aspects of functional hydrogel coatings: (i) applications and functions enabled by hydrogel coatings, (ii) methods of coating various substrates with different functional hydrogels with tough adhesion, and (iii) tests to evaluate the adhesion between functional hydrogel coatings and substrates. Conclusions and outlook are given at the end of this review.
hydrogel coatings,， coating methods,， coating tests,， adhesion,， hydrogel applications
International Journal of Plasticity，2021，137（）：102901
The Mullins-type damage behaviors in double network hydrogels have attracted a broad research interest in recent years. However, most of current works focus on characterizing and modeling the uniaxial deformation behaviors of these materials. In this work, we combine experimental and theoretical approaches to investigate the anisotropic damage behaviors of double network hydrogels revealed in multi-axial deformation conditions. We demonstrate that an isotropic damage model based on the eight-chain model and the network alteration theory fails to capture the stress response in multi-axial loading tests. An anisotropic damage theory based on the microsphere model has also been developed, while both the affine and non-affine approaches are adopted to obtain the micro-macro mapping. The results show that the affine microsphere model cannot describe the experimental results in pure shear and unequal biaxial tests. Remarkably, the non-affine microsphere model with three parameters captures all the important features of the experimental observations. This is because the non-affine model accurately predicts the directional damage of the primary cross-linked network. The non-affine microsphere model is also able to describe the damage cross-effect in double network hydrogels. The developed theoretical framework can promote the fundamental understanding of the anisotropic damage behaviors in various types of tough gels.
Double network hydrogel Mullins effect Microsphere model Anisotropic damage
Advanced Engineering Materials，2020，22（12）：2000640
The soft actuators similar to the periodic motion of biological organisms are accomplished based on assembling multiple unidirectional memory effect shape memory alloys (SMAs) components or introducing biasing elements, which give rise to the complex structure and difficult control of soft actuators. Herein, inspired by the power‐amplified biological systems, beneficial with the advantages of 3D integrated molding technology, an integrated SMA‐polydimethylsiloxane (PDMS) composite structure (SPCS) is proposed, which can achieve periodic heterogeneous actuation only by controlling the SMA phase transformation and the PDMS strain potential energy distribution states. To test the feasibility of the mechanism, a theoretical model of SPCS deformation is conducted. The results of numerical feasibility analysis show that the factors affecting SPCS deformation mainly involve the excitation current strength of SMA, PDMS structure thickness and its distribution state. The experimental results show that the current intensity mainly affects the deformation rate of SPCS, and the thickness of PDMS is not only the key to realize the periodic deformation of SPCS but also the orderly arrangement of PDMS structure thickness is helpful for SPCS to achieve periodic heterogeneous deformation. These demonstrate that the proposed mechanism can inspire the design of soft actuators, smart wearable equipment, and medical devices.
Geotextiles and Geomembranes，2020，48（5）：724-734
The basic pore unit model is extended to predict the strained pore size characteristics of woven slit-film geotextiles subjected to unequal biaxial tensile strains. The strained per cent open area (POA) and analytical pore size are expressed as functions of the weft strain and the warp strain to weft strain ratio. The influence of the biaxial tensile strain on pore size characteristics is evaluated in three woven slit-film polypropylene geotextile samples using image analysis under the warp strain to weft strain ratios of 1, 2, 3 and 4. It is shown that the experimental POA and O95 increased significantly with increasing strain at different warp strain to weft strain ratios, and the PSD curves moved toward the direction of large open sizes. The analytical models of POA and pore size can accurately predict the increasing trend of POA and O95. Moreover, unequal biaxial tensile strains can significantly change the shape of the pores, which may influence the results of the pore size obtained by indirect methods. A larger warp strain to weft strain ratio can lead to a larger change in the pore shape when the length to width ratios of initial pores are close to 1.
Geosynthetics Pore size characteristic Unequal biaxial tensile strain Analytical model Pore shape Image analysis
Mechanics of Materials，2020，150（）：103575
An electro-mechanically coupled visco-hyperelastic-plastic constitutive model is established in this work to describe the cyclic deformation of dielectric elastomers (DEs) by addressing the significant ratchetting and cyclic stress-softening behaviors of DEs resulting from the coupled visco-hyperelasticity, plasticity and electro-mechanical effects. First, a visco-hyperelastic-plastic constitutive model is constructed in the framework of large deformation to incorporate the visco-hyperelasticity and plasticity of DEs, simultaneously. The typical Ogden's formulation is employed for the description of basic hyperelastic response; multiple relaxation mechanisms are adopted to capture the time-dependent viscoelastic part; and a finite plastic flow rule based on the arc-length description is proposed to describe the plastic one. Then, within the framework of nonlinear electro-mechanically coupling approach and by assuming quasi-linear dielectric behavior, the visco-hyperelastic-plastic constitutive model is extended to an electro-mechanically coupled one. Finally, the proposed models are, respectively, validated by comparing the predicted results with corresponding experimental ones of VHB™4910 DE. It is found that the pure mechanical and electro-mechanically coupled cyclic deformation of VHB™4910, including the ratchetting and cyclic stress-softening as well as their dependence on the loading level, loading rate and phase-angle difference of cyclic electro-mechanical loading, can be reasonably predicted by the proposed constitutive models.
Materials Today Physics，2020，14（）：100219
Stretchable tactile sensor (STS) is promising for wearable electrical devices, human-machine interfaces, and electronic skin. However, developing a STS based on piezoresistive composite high-pressure sensitivity and dynamic stability remains challenging because stretching deformation destroys the original dispersed state of conductive fillers. This interference of stretching strain on the pressure sensing greatly reduces device performance. Here, we realize an STS based on a piezoresistive composite with different elastic modulus in its functional regions. The composite contains high elastic modulus region (59.1 MPa) of vertically aligned columns of urchin-shaped nanoparticles, and low elastic modulus region (2.4 MPa) of pure matrix. The sensor exhibits high-pressure sensitivity (12.05 kPa−1) owing to the increased conductive contact area between urchin-shaped nanoparticles in the high elastic modulus region. While stretching to 400% strain, the sensor exhibits excellent dynamic stability via strain accommodation in the low elastic modulus region. Our design to separate sensing from multiple stimulus by elastic modulus regulation is easy operative and universal. In addition, the sensor has a low hysteresis coefficient (5.25%), a good detection limit (22 mg), a low response/recovery time (<50 ms), and an excellent mechanical durability (cycled 10,000 times). Finally, we demonstrate the use of our STS for several important stretchable electronic applications to show the feasibility of our design.
Stretchable E-skin Elastic modulus regulation Urchin-shaped conductive magnetic nanoparticles Pressure sensitivity Stretching insesnitivity
Extreme Mechanics Letters，2020，40（）：100926
Multi network elastomers (MNEs), composed of a single sacrificial network and other matrix networks, exhibit appealing mechanical properties. In this paper, we develop a constitutive model for MNEs prepared by multiple swellings. Firstly, the swelling process is analyzed. The free energy of a swollen elastomer consists of the strain energy of deformed polymer chains and the energy of mixing. We analyze the mechanical equilibrium and chemical potential equilibrium during the swelling of MNEs. The degree of swelling of MNEs is affected by the preparation conditions, like the component of the solution, as well as the microscopic physical quantities, such as the chain length and density of polymer chains of the networks. Secondly, the free energy of the completed MNE subjected to external loading is composed of the strain energy of each network. The chains of the sacrificial network (first network) can be destroyed gradually when the applied load increases. We adopt the network alteration theory to describe the above progressive damage of the sacrificial network. In addition, the matrix networks are fully elastic. We verify this model using the experimental data including stress–stretch curves of elastomers with double and triple networks, as well as the step cycle curves of triple network elastomers (TNEs). This model predicts well for the swelling-induced pre-stretches of each network under specified preparation conditions. It relates the stress to the stretch of MNEs under external loading, and indicates the stress contribution of each network, and the damage evolution of the sacrificial network. Our model is instructive for designing MNEs with desired mechanical properties.
Multi network elastomer Swelling Free energy Damage
J. Appl. Mech. ，2020，87（11）：110801
In this paper, we review constitutive models for soft materials. We specifically focus on physically based models accounting for hyperelasticity, visco-hyperelasticity, and damage phenomena. For completeness, we include the thermodynamically based viscohyperelastic and damage models as well as the so-called mixed models. The models are put in the frame of statistical mechanics and thermodynamics. Based on the available experimental data, we provide a quantitative comparison of the hyperelastic models. This information can be used as guidance in the selection of suitable constitutive models. Next, we consider visco-hyperelasticity in the frame of the thermodynamic theory and molecular chain dynamics. We provide a concise summary of the viscohyperelastic models including specific strain energy density function, the evolution laws of internal variables, and applicable conditions. Finally, we review the models accounting for damage phenomenon in soft materials. Various proposed damage criteria are summarized and discussed in connection with the physical interpretations that can be drawn from physically based damage models. The discussed mechanisms include the breakage of polymer chains, debonding between polymer chains and fillers, disentanglement, and so on.
constitutive modeling of soft materials,， mechanical properties of soft materials
J. Mater. Chem. C，2020，8（）：7688-7697
Conductive hydrogel based soft electronics with superior mechanical/electrical properties and biocompatibility have great potential for sensing and stimulation at device–human interfaces, in which one piece of the functional gel is usually used as a multi-sensor to chemicals, mechanical deformations, etc. Also, it is important to develop a facile strategy for patterning intricate circuits and conductive components in a hydrogel system to afford integrated functions. Demonstrated here is a hybrid conductive hydrogel system that can be facilely patterned and integrated with complex circuits, which enables monitoring of multiple signals, including tensile strain, out-of-plane pressure, and temperature. The conductive hydrogel was fabricated by a stencil-aided printing of percolated silver nanowires (AgNWs) on a tough supramolecular hydrogel with robust interfacial bonding. The obtained hydrogel-based electronics exhibited remarkable electrical and mechanical properties, with a sheet resistance of 0.76 Ω sq−1, breaking strain of over 600%, breaking stress up to 3.3 MPa, and self-healing ability, superior to most existing conductive hydrogels. The strain sensors exhibited a gauge factor up to 58.2, enabling monitoring various subtle human motions. Multiple sensing units can be facilely fabricated in this approach using a well-designed silhouette mask. The powerful functions of the integrated electronics were manifested by the detection of complex stress or temperature fields.
A series of electro-mechanically coupled cyclic tests at large deformation are carried out to characterize the cyclic deformation of a laterally constrained dielectric elastomer (DE) in this work. In the strain-controlled cyclic tests of the dielectric elastomer (e.g., VHB 4910 DE) with a constant or cyclic voltage, cyclic stress softening occurs and is influenced by the phase-angle difference between the applied cyclic strain and cyclic voltage. In the stress-controlled cyclic tests of VHB 4910 DE with a constant or cyclic voltage, ratchetting (a cyclic accumulation of inelastic strain) takes place; the ratchetting strain is considerably enhanced by applying higher voltage level, higher stress level and lower stress rate, and is also affected by the phase-angle difference between the applied cyclic stress and cyclic voltage. Moreover, the remarkable recovery of residual strain after the cyclic tests demonstrates that the cyclic stress softening and ratchetting of VHB 4910 DE mainly stem from the viscoelasticity. The comprehensive experimental observations are very useful to develop, calibrate and validate an electro-mechanically coupled constitutive model of dielectric elastomers.
Dielectric elastomer Electro-mechanical coupling Cyclic deformation Ratchetting Viscoelasticity
Journal of the Mechanics and Physics of Solids，2020，137（）：103859
High-rate loading induced cavitation of soft materials, including brain and muscle, has increasingly been studied due to its importance in a variety of biomedical applications, such as potential impact-related traumatic brain injury and high-intensity focused ultrasound therapy. The size of cavitation nuclei or defects is crucial to the onset of cavitation. However, it remains challenging to introduce gaseous defects with controllable size into soft materials and to understand the underlying mechanism of impact-bubble interaction in soft materials. Here, we set up a drop-tower system to perform impact loading tests on gelatin samples with controllable monodispersed microbubbles, allowing us to mimic the general cavitation behavior of human tissues. The microbubbles are inserted into the soft matrix as cavitation nuclei by taking advantage of microfluidic flow focusing. A high-speed camera paired with a signal acquisition system is utilized to determine the critical pressure at which cavitation initiates for different defect sizes. Meanwhile, we propose a theoretical model based on surface tension to predict the time-dependent spherically symmetric cavitation of a pre-existing gas bubble in elastomers under high-speed loading. Our experiments and theoretical modeling demonstrate the strong effect of gas-filled defects on cavitation and indicate the necessity and significance of controlling defect size in mimicking targeted organs using soft gels. Our work provides a promising route to predict cavitation-induced injury in tissues with variable materials stiffness and initial defect size.
Cavitation Defect Size effect Impact Soft materials
Advanced Optical Materials，2020，8（9）：2000031
Recently, hydrogels with coloration have attracted researchers from various fields, such as camouflage, anti‐counterfeiting, and soft display. However, existing thermochromic hydrogels are limited by their weak color display performance and insufficient sensitivity. Here proposed is a new kind of thermochromic hydrogel which possesses bright colors, fast response time, and reliable results across a long lifespan. This hydrogel is prepared by embedding temperature‐responsive microcapsules containing three components: color developing agent, leuco agent, and solvent. The critical temperature for color changing of the microcapsule is determined by the phase transfer temperature of the solvent. The macroscopic coloration and discoloration of this hydrogel are fast. Soft display and camouflage are proposed for the practical applications in which the thermochromic hydrogel acts as the chameleon phase and the electrothermal filaments as the driving phase. The thermochromic hydrogel can be readily integrated into heat‐sensing devices and smart clothes.
Advanced Functional Materials，2020，30（12）：1909473
Many emerging technologies such as wearable batteries and electronics require stretchable functional structures made from intrinsically less deformable materials. The stretch capability of most demonstrated stretchable structures often relies on either initially out‐of‐plane configurations or the out‐of‐plane deflection of planar patterns. Such nonplanar features may dramatically increase the surface roughness, cause poor adhesion and adverse effects on subsequent multilayer processing, thereby posing a great challenge for flexible devices that require smooth surfaces (e.g., transparent electrodes in which flat‐surface‐enabled high optical transmittance is preferred). Inspired by the lamellar layouts of collagenous tissues, this work demonstrates a planar bilayer lattice structure, which can elongate substantially via only in‐plane motion and thus maintain a smooth surfaces. The constructed bilayer lattice exhibits a large stretchability up to 360%, far beyond the inherent deformability of the brittle constituent material and comparable to that of state‐of‐the‐art stretchable structures for flexible electronics. A stretchable conductor employing the bilayer lattice designs can remain electrically conductive at a strain of 300%, demonstrating the functionality and potential applications of the bilayer lattice structure. This design opens a new avenue for the development of stretchable structures that demand smooth surfaces.
Extreme Mechanics Letters，2020，35（）：100643
Nature abounds with structures capable of changing shapes upon exposure to stimuli. These shape-morphing structures (for instance, the conifer pinecone) are often comprised of passive hard phases embedded in active soft materials that are stimuli-responsive. Although enthusiasm in mimicking the shape-changing phenomena in natural systems has induced an impetus to develop various artificial self-shaping structures, the effect of hard phases on the shape transformation and its underlying mechanics have not been systematically investigated. In this paper, we conduct finite element modeling to simulate the shape transformation of hybrid composite sheets consisting of fibers embedded in the hydrogel matrix, which is a widely employed approach to construct self-folding structures. It is revealed that the orientation of the folding axis is dictated by the hard fibers when the fiber modulus is sufficiently larger than that of the hydrogel matrix. Guided by the mechanics analysis, we construct self-folding composite sheets by incorporating 3D-printed stiff polylactic acid (PLA) patterns into soft poly(N-isopropylacrylamide) (PNIPAm) gels. The modulus of PLA exceeds that of PNIPAm by five orders of magnitude. Upon stimulation of elevated temperature, the PNIPAm/PLA composite sheets exhibit directional folding with folding axis parallel to the hard PLA strips, contrasting with most reported hybrid hydrogel sheets in which folding axes were perpendicular to the embedded fibers. Various 3D morphologies, including tubes, helices, scrolls, have been achieved by programming the embedded PLA patterns. We also generate an analog of curled leaves and the results offer mechanistic understandings of the curling process of leaves in nature. We hope this work can provide guidance for designing self-shaping soft machines that are made by integrating hard and soft materials together.
Finite element modeling Composite hydrogels Folding axis Shape transformation
Extreme Mechanics Letters，2020，35（）：100645
A failure criterion is introduced for highly stretchable hyper-elastic materials under tensile-dominated triaxial loading. It is based on energy balance using Griffith approach but is presented by the critical state of load as in traditional size independent strength theories. The basic argument is made of the recent assessment about the cavity nucleation in soft materials: the volumetric energy dissipation in the growth of defect and size-insensibility of the critical load in soft materials when the defect is smaller than the material characteristic length. Thus, using the energy approach as in fracture mechanics, we are able to estimate the critical state of load using only one material parameter, which is subsequently replaced by the critical stress under one specific loading, e.g., the strength of uniaxial loading. This leads us to the failure surface in the normalized principal stress space. The predictions of the present theory agree well with the experimental results collected from the literature, including an order of magnitude difference between the strengths of the uniaxial loading and hydrostatic loading. This addresses a variety of difficulties in the previous failure analyses. For the easiness of practical application, we further analyze the data and obtain a criterion using mean stress and the first invariant of Cauchy–Green deformation tensor, . We are able to obtain the widely mentioned maximum criterion, and also find its limitation: only applicable when one of the principal stresses is zero. For general triaxial loadings, the material can fail at any allowable and the critical mean stress monotonically increases with ; and under a constant mean stress that is above the hydrostatic strength, the material is only safe when is above a mean stress-dependent lower bound but below an unknown up-bound.
Journal of Physics D: Applied Physics，2020，53（23）：235402
Numerical investigations of magnetorheological elastomer millimeter-scale robots capable of undulatory swimming under magnetic body torques are realized for the first time in low Reynolds number conditions. The simulated results agree well with reported experimental results under the same conditions. The countermovement of the robots increases as the enhancement of magnetic field strength and the reduction of the length of the robots. The swimming gaits of the robots are characterized by a proposed theoretical function differing obviously from Taylor's model. The amplitudes of the points along the neutral layer are inconsistent. These simulations provide rapid and low-cost investigation methods for soft-bodied locomotion actuated by distributed torques.
Composites Part B: Engineering，2019，178（）：107503
To regulate the bending behavior of laminated metal matrix composites, the effects of stacking sequence and notch position were studied by using both numerical simulation and experimental testing. Laminated composites consisting of TiB/Ti6Al4V composite layer and Ti6Al4V ductile layer were fabricated. The results of numerical simulation on specimen with alloy layer as the outermost layer indicate a better bending ductility compared with the other stacking sequence, and this conjecture was also confirmed by the experimental result where a double increase in bending strain was observed. This was attributed to the tunneling crack blunting by adjacent alloy layers. As for the notch position, crack was preferentially to generate in the composite layer first whether the pre-notch existed in alloy layer or in composite layer. If pre-notched on the alloy layer, tunneling crack in the composite layer would extend earlier than the fracture of the notched outermost layer. For specimen with pre-notch in the composite layer, the adjacent alloy layer and the inner composite layer would fracture at the same time after the crack initiation of the notched composite layer. The fracture behavior of laminated composites can be manipulated by conveniently tailoring stacking sequences and notch position.
Layered structures Metal-matrix composites (， MMCs)， Fracture Finite element analysis (， FEA)， Powder processing
ACS Appl. Mater. Interfaces，2020，12（10）：12010–1201
As one of the most promising drug delivery carriers, hydrogels have received considerable attention in recent years. Many previous efforts have focused on diffusion-controlled release, which allows hydrogels to load and release drugs in vitro and/or in vivo. However, it hardly applies to lipophilic drug delivery due to their poor compatibility with hydrogels. Herein, we propose a novel method for lipophilic drug release based on a dual pH-responsive hydrogel actuator. Specifically, the drug is encapsulated and can be released by a dual pH-controlled capsule switch. Inspired by the deformation mechanism of Drosera leaves, we fabricate the capsule switch with a double-layer structure that is made of two kinds of pH-responsive hydrogels. Two layers are covalently bonded together through silane coupling agents. They can bend collaboratively in a basic or acidic environment to achieve the “turn on” motion of the capsule switch. By incorporating an array of parallel elastomer stripes on one side of the hydrogel bilayer, various motions (e.g., bending, twisting, and rolling) of the hydrogel bilayer actuator were achieved. We conducted an in vitro lipophilic drug release test. The feasibility of this new drug release method is verified. We believe this dual pH-responsive actuator-controlled drug release method may shed light on the possibilities of various drug delivery systems.
capsule switch actuator control drug release dual pH-responsive hydrogel
Wearable sensors are gradually enabling decentralized healthcare systems. However, these sensors need to be closely attached to skin, which is unsuitable for long‐term dynamic health monitoring of the patients, such as infants or persons with burn injuries. Here, a wearable capacitive sensor based on the capacitively coupled effect for healthcare monitoring in noncontact mode is reported. It consists of a ring‐shaped top electrode, a disk‐shaped bottom electrode, and a porous dielectric layer with low permittivity. This unique design enhanced the capacitively coupled effect of the sensor, which enables a high noncontact detectivity of capacitance change. When an object approaches the sensor, its capacitance change (ΔC/Ci = −38.7%) is 3–5 times higher than that of previously reported sensors. Meanwhile, the sensor is insensitive to the stretching strain and pressure (ΔC/Ci < 5%) due to the unique ring‐shaped electrode and the incompressible closed cells of the porous dielectric material, respectively. Finally, various human physiological signals (pulse and respiratory) are recorded in noncontact mode, where a person wears loose and soft clothes implanted with the sensor. Thus, it is promising to build smart healthcare clothes based on it to develop wearable decentralized healthcare systems.