Swelling-induced instabilities in microscale, surface-confined poly(N-isopropylacryamide) hydrogels

DuPont, Samuel J.; Cates, Ryan S.; Stroot, Peter G.; Toomey, Ryan

doi:10.1039/c0sm00021c

Cited by 58 publications

(79 citation statements)

References 27 publications

(32 reference statements)

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“…Tall "walls" of hydrogel attached to a rigid substrate swell into a similar meandering pattern suggesting that once delamination occurs, a fold behaves qualitatively similar to a tall wall that is fixed at its base but free to swell along its top. 15,25,26 In Figure 2, the remainder of the film displays folding as well, but at a later stage during swelling. The similarity between the early "programmed" fold and the later "uncontrolled" folds suggests that delamination is the mechanism for both.…”

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Swelling-Induced Delamination Causes Folding of Surface-Tethered Polymer Gels

Velankar

Lai

Vaia

2012

ACS Appl. Mater. Interfaces

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When a polymer film that is weakly attached to a rigid substrate is exposed to solvent, swelling-induced compressive stress nucleates buckle delamination of the film from the substrate. Surprisingly, the buckles do not have a sinusoidal profile, instead, the film near the delamination buckles slides toward the buckles causing growth of sharp folds of high aspect ratio. These folds do not result from a wrinkle-to-fold transition; instead, the film goes directly from a flat state to a folded state. The folds persist even after the solvent evaporates. We propose that patterned delamination and folding may be exploited to realize high-aspect ratio topological features on surfaces through control of a set of boundary constraints arising from the interrelation of film-surface adhesion, film thickness and degree of swellabilty. KEYWORDS: folds, delamination, creases, buckles, wrinkles, swelling The swelling of polymers with solvent can lead to one class of such buckling phenomena in which an osmotic stress applied to a homogeneous body leads to nonhomogeneous deformation.10−12 One of the simplest geometries in which this can happen is the swelling of a cross-linked polymer film that is attached to a rigid substrate. If the extent of swelling is sufficiently large, the film swells nonhomogeneously and the free surface develops cusp-like creases and rounded wrinkles.12−15 Recent research 13,16 has clarified the mechanism of crease formation: lateral film swelling (i.e., swelling perpendicular to the film normal) is constrained by the rigid substrate, and hence the free surface of the film is under compressive stress. It is this compression that induces the creasing instability at the free surface. In this paper, we show that a new buckling phenomenon can also occur in the same situation: if the film is not strongly bound to the rigid substrate, a second kind of buckling instability can lead to large scale folding of the film. This Letter describes the basic phenomenon, clarifies the conditions under which folding does or does not occur, and proposes a schematic model for the process.Experiments were conducted using PDMS (Sylgard 184) films swollen with toluene. Sylgard is a two-part silicone rubber formulation in which the base resin and the cross-linker are usually mixed in a 10:1 ratio. For most of our experiments however, the ratio of the base resin to cross-linker was 10:0.4 which resulted in a lower cross-link density. The shear modulus of these films was 18 kPa (in contrast to 54 kPa for 10:1 ratio), and the films swelled 380% by weight with toluene.17 Films were prepared by spincoating the PDMS mixture onto acrylic sheets, letting them cross-link overnight, and then completing the cross-linking reaction for one hour at 75°C. Film thickness was measured with a profilometer at three locations on the spincoated films. Thickness variation was less than 15%. Sections of the films, roughly 25 × 25 mm in size were peeled off the acrylic sheets and carefully placed (without entrapping air bubbles) on glass slide...

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Swelling-Induced Delamination Causes Folding of Surface-Tethered Polymer Gels

Velankar

Lai

Vaia

2012

ACS Appl. Mater. Interfaces

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“…These materials are widely used in a variety of biomedical applications ranging from tissue engineering, drug delivery, to tissue adhesives [1]. Hydrogels that can change their shape and physical properties in response to various environmental stimuli (e.g., temperature, pH, humidity) are being explored in applications such as soft robotic components, biosensors, and controlled drug delivery [2][3][4]. Recently, we combined ionoprinting techniques pioneered by Palleau et.…”

Section: Introductionmentioning

confidence: 99%

Novel Hydrogel Actuator Based on Biomimetic Chemistry

2014

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Hydrogel actuators were prepared by combining ionoprinting technique with reversible metal ion coordination chemistry found in mussel adhesive proteins. Hydrogels were formulated with biomimetic dopamine moiety, which contains a catechol side chain that is capable of forming mono-, bis-, and tris-complexes with ferric (Fe 3+ ) ions with increasing pH. CatecholFe 3+ complexation increased local crosslinking density, which induced hydrogel bending at the site of Fe 3+ ionoprinting. The effect of pH on the dynamic response of hydrogel actuation was tracked by following the radius of curvature at the ionoprinting site. Both the rate of change and the maximum radius of curvature increased when the pH with increasing pH (2.5-9.5), indicating that pH can be used to modulate hydrogel actuation. Additionally, hydrogels containing Fe 3+ demonstrated higher extent of deswelling when equilibrated at a basic pH. Similarly, dynamic mechanical analysis in the compression mode revealed that both the storage and loss modulus values for Fe 3+ -containing hydrogels increased with increasing pH. These results indicated that bis-and tris-complexes formed at an elevated pH level contributed to a faster rate of actuation and a more condensed network architecture. Hydrogel actuation and deswelling were also observed at pH of 3.5 although to a lesser degree, potentially due to a stronger affinity between network-bound catechol and Fe 3+ ions as compared to complexes formed in a dilute solution.

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“…This effect has been attributed to spatially constrained hydrogel swelling, and was elegantly described to depend on the aspect ratio of the features exhibiting differences in crosslink density (and hence difference in Q). [48, 49] In a photodegradable system, the crosslink density and Q scale with the concentration of photodegradable groups in the system. At any given time, the concentration of photodegradable groups remaining in the system is given by Equation 1, where φ is quantum yield, ε is molar absorptivity, I is light irradiance (intensity), λ is wavelength, N A is Avogadro’s number, h is Planck’s constant, and c is the speed of light.…”

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Complex Dynamic Substrate Control: Dual‐Tone Hydrogel Photoresists Allow Double‐Dissociation of Topography and Modulus

2013

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Hydrogels are widely utilized as artificial extracellular matrices, but current materials are unable to completely recapitulate the geometric, mechanical and dynamic characteristics of natural tissues. Here, we report an approach to dynamically tune both topography and elasticity in a single photoresponsive hydrogel substrate. Upon exposure to spatially controlled doses of light, a topographically and mechanically micropatterned surface forms. Atomic force microscopy was used to investigate changes in topographical feature size and elastic moduli of the hydrogel surface as a function of irradiation time and wavelength. These photodegradable hydrogels can act as both positive and pseudo-negative photoresists, depending on exposure time and wavelength. By carefully controlling the aspect ratio (surface area to depth) of micropatterned features, unique swelling-induced ordered microstructures can be formed on the surface. These dual-tone hydrogel photoresists therefore allow dynamic tunability in both topography and elasticity, enabling the fabrication of complex and anisotropic biomaterials.

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Swelling-induced instabilities in microscale, surface-confined poly(N-isopropylacryamide) hydrogels

Cited by 58 publications

References 27 publications

Swelling-Induced Delamination Causes Folding of Surface-Tethered Polymer Gels

Swelling-Induced Delamination Causes Folding of Surface-Tethered Polymer Gels

Novel Hydrogel Actuator Based on Biomimetic Chemistry

Complex Dynamic Substrate Control: Dual‐Tone Hydrogel Photoresists Allow Double‐Dissociation of Topography and Modulus

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