Transition of surface–interface creasing in bilayer hydrogels

Zhou, Zhiheng; Li, Ying; Wong, W.H.; Guo, T.F.; Tang, Shan; Luo, Jiayan

doi:10.1039/c7sm01013c

Cited by 16 publications

(10 citation statements)

References 60 publications

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“…global harmonic waves) and/or creasing (i.e. localized sharp folds) [10,11,12,13,14,15,16,17,18,19,20], as illustrated in Figure 1. While creasing mainly occurs as the first mode of instability when homogeneous gels are exposed to large swelling [12,18], wrinkling patterns are first and foremost found in gels having a gradient stiffness through the thickness, caused by a variation in the crosslinking density within the gel [14,21,22], or by the deposition of a thin and stiff film at the outer surface of the gel, possibly to alter properties like permeability, stability or biocompatibility [23,24,25].…”

Section: Introductionmentioning

confidence: 99%

Buckling initiation in layered hydrogels during transient swelling

Ilseng

Prot

Skallerud

et al. 2019

Journal of the Mechanics and Physics of Solids

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Subjected to compressive stresses, soft polymers with stiffness gradients can display various buckling patterns. These compressive stresses can have different origins, like mechanical forces, temperature changes, or, for hydrogel materials, osmotic swelling. Here, we focus on the influence of the transient nature of osmotic swelling on the initiation of buckling in confined layered hydrogel structures. A constitutive model for transient hydrogel swelling is outlined and implemented as a user-subroutine for the commercial finite element software Abaqus. The finite element procedure is benchmarked against linear perturbation analysis results for equilibrium swelling showing excellent correspondence. Based on the finite element results we conclude that the initiation of buckling in a two-layered hydrogel structure is highly affected by transient swelling effects, with instability emerging at lower swelling ratios and later in time with a lower diffusion coefficient. In addition, for hard-on-soft systems the wavelength of the buckling pattern is found to decrease as the diffusivity of the material is reduced for gels with a relatively low stiffness gradient between the substrate and the upper film. This study highlights the difference between equilibrium and transient swelling when it comes to the onset of instability in hydrogels, which is believed to be of importance as a fundamental aspect of swelling as well as providing input to guiding principles in the design of specific hydrogel systems.while the flux of the solvent molecules in a gel was given in Equation (6). By comparing Equation (A.1) to Equation (A.3) and Equation (A.2) to Equation (6), we find that the equivalents of temperature, specific heat capacity, mass density, and conductivity can be identified asμ, ∂C ∂μ , 1 J , and CD J = J−1 vJ D respectively. These values are calculated as internal variables in the Fortran code using the USDFLD subroutine in Abaqus. The internal heat source in Abaqus, r, is set to zero.

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Section: Introductionmentioning

confidence: 99%

Buckling initiation in layered hydrogels during transient swelling

Ilseng

Prot

Skallerud

et al. 2019

Journal of the Mechanics and Physics of Solids

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“…The initial chemical potential μ0 is negative in the formula. Readers are referred to our recent work for more details [51]. The first Piola–Kirchhoff (PK) stress can be derived as Pij=∂W∂Fij=KTυNvϕ01/3Fij+[]1−Nvϕ0J−μ0+▵μKT+χϕ02J2+lnJ−ϕ0JJF−1ji.…”

Section: Numerical Analysismentioning

confidence: 99%

“…( a ) The mechanical test machine and the cylindrical specimen through which we obtained the force–strain curves for hydrogels (reproduced from Ref. [51] with permission from The Royal Society of Chemistry.) ( b ) The finite element (FE) mesh used to calibrate the material parameters of the hydrogel.…”

Section: Figurementioning

confidence: 99%

Surface Instability of Bilayer Hydrogel Subjected to Both Compression and Solvent Absorption

Zhou

Guo

et al. 2018

Polymers

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Abstract:The bilayered structure of hard thin film on soft substrate can lose stability and form specific patterns, such as wrinkles or creases, on the surface, induced by external stimuli. For bilayer hydrogels, the surface morphology caused by the instability is usually controlled by the solvent-induced swelling/shrinking and mechanical force. Here, two important issues on the instability of bilayer hydrogels, which were not considered in the previous studies, are focused on in this study. First, the upper layer of a hydrogel is not necessarily too thin. Thus we investigated how the thickness of the upper layer can affect the surface morphology of bilayer hydrogels under compression through both finite element (FE) simulation and theoretical analysis. Second, a hydrogel can absorb water molecules before the mechanical compression. The effect of the pre-absorption of water before the mechanical compression was studied through FE simulations and theoretical analysis. Our results show that when the thickness of the upper layer is very large, surface wrinkles can exist without transforming into period doublings. The pre-absorption of the water can result in folds or unexpected hierarchical wrinkles, which can be realized in experiments through further efforts.

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“…Additionally, in an effort to improve mechanical properties and stability of bilayer hydrogels in environments with different pH, this study also compared PAA-PVA thin films made by conventional techniques to PAA-PVA bilayers fabricated as IDNs. Delamination, 29,30 the impact of IDN hydrogels in actuation of the bilayers, and their corresponding drug release profile are characterized. Additionally, since bending deflections have previously been reported by measuring bending angle 31 or displacement distance from the zero order, 32 the bending strain of the developed bilayers are measured directly using a strain gauge in order to better quantify bilayer efficacy under stimulus.…”

Section: Introductionmentioning

confidence: 99%

Polyvinyl alcohol-poly acrylic acid bilayer oral drug delivery systems: A comparison between thin films and inverse double network bilayers

et al. 2019

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Polymer network structures in the design of bilayer hydrogels for oral drug delivery systems contribute to the mechanical stability as well as the stimulus responsive drug release. We report a strategy to improve the stability of polyelectrolyte layer interfaces in bilayer hydrogels by employing concepts from tough interpenetrating hydrogel network design. Polyvinyl alcohol as the neutral substrate and Polyacrylic acid as the pH-sensitive layer were used to prepare bilayers capable of pH-sensitive bending behavior, and inverse double network structures were compared to conventional thin film coated bilayers in this study. The morphology of the bilayers observed after exposure to stimulus revealed that the polyacrylic acid layer was fragmented in conventional thin films. The delamination of the coating also had an adverse impact on bending behavior. Additionally, bilayers with inverse double network structure had a better pH-sensitive swelling behavior which was lacking in regular thin films. Controllable, pH-specific drug release behavior was a direct benefit of using the inverse double network structure, and was demonstrated by controlled vancomycin release under different pHs chosen to simulate the physiological milieu of exposure corresponding to hydrogel passage along the gastrointestinal tract. Additionally, the inverse double networks showed improved mucoadhesive properties and mechanical stability compared to the thin film bilayers. The use of inverse double network structured hydrogels has many potential applications in improving bilayer hydrogel stability; specifically, for drug delivery systems, designing hydrogel origamis, and to prepare soft material actuators.

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Transition of surface–interface creasing in bilayer hydrogels

Cited by 16 publications

References 60 publications

Buckling initiation in layered hydrogels during transient swelling

Buckling initiation in layered hydrogels during transient swelling

Surface Instability of Bilayer Hydrogel Subjected to Both Compression and Solvent Absorption

Polyvinyl alcohol-poly acrylic acid bilayer oral drug delivery systems: A comparison between thin films and inverse double network bilayers

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