Nature has demonstrated that surface patterns can control adhesion and release. For example, the attachment devices of geckos [1,2] and some insects [3,4] are decorated with fibrillar structures designed specifically for locomotion. Inspired by the fibrillar arrays, there have been significant efforts in mimicking these materials [5][6][7][8][9][10][11][12] to develop synthetic analogs as "smart" adhesives, that is, where the geometry of the patterns can tailor the adhesion of the material. However, there are currently two limitations in the design of patterned adhesives. First, because of either materials selection or geometric design, the adhesive properties are not always reversible over the course of multiple attachment-detachment cycles. [5,6,11,12] Second, as the current fabrication strategies are primarily based on lithographic or soft-lithographic approaches, there are issues related to the fabrication of these materials in an efficient and scaleable manner. These two limitations must be addressed in order to successfully adopt these materials as "smart" adhesives.In this work, we present an alternative design strategy for a reusable smart adhesive that uses surface wrinkles as patterns to control the adhesion of a poly(n-butyl acrylate) (PnBA) elastomer. The wrinkle patterning process is based on the swelling of a laterally confined polymer film. We demonstrate the ability to control the wrinkle dimensions as a simple approach to design a "smart" adhesive, where the control of adhesion is determined by the wavelength of the wrinkles based on a mechanism known as contact splitting. Our materials design offers several advantages over previous approaches including: (i) enhanced control of adhesion provided by well-defined surface wrinkle patterns, (ii) convenience and simplicity of the fabrication process without expensive lithography for patterning, and (iii) amenability to patterning a wide variety of polymer systems.Our wrinkled PnBA elastomer is fabricated using a patterning process based on surface wrinkling. The general prerequisite for surface wrinkling is the development of a critical compressive stress; hence, the formation of surfaces wrinkles has been observed for a variety of materials using different external stimuli. [13][14][15][16][17][18][19][20] We use a process that is loosely based on a wrinkling approach developed by Southern and Thomas on the swelling of a laterally-confined elastomer (Fig. 1a). [21] We begin by preparing a PnBA elastomeric film with defined film thickness (h) by depositing a photocurable nBA monomer formulation onto a glass substrate. Next, the same photocurable nBA solution is deposited onto the PnBA film surface, which swells the elastomer. As the PnBA film is pinned to the rigid substrate, the lateral expansion of the polymer film is confined. This osmotic stress coupled with lateral confinement leads to the development of a net compressive force and the formation of an elastic instability, that is, surface wrinkling. Following the brief swelling process, the wrink...

There is a significant medical need for tough biodegradable polymer adhesives that can adapt to or recover from various mechanical deformations while remaining strongly attached to the underlying tissue. We approached this problem by using a polymer poly(glycerol-co-sebacate acrylate) and modifying the surface to mimic the nanotopography of gecko feet, which allows attachment to vertical surfaces. Translation of existing gecko-inspired adhesives for medical applications is complex, as multiple parameters must be optimized, including: biocompatibility, biodegradation, strong adhesive tissue bonding, as well as compliance and conformability to tissue surfaces. Ideally these adhesives would also have the ability to deliver drugs or growth factors to promote healing. As a first demonstration, we have created a gecko-inspired tissue adhesive from a biocompatible and biodegradable elastomer combined with a thin tissue-reactive biocompatible surface coating. Tissue adhesion was optimized by varying dimensions of the nanoscale pillars, including the ratio of tip diameter to pitch and the ratio of tip diameter to base diameter. Coating these nanomolded pillars of biodegradable elastomers with a thin layer of oxidized dextran significantly increased the interfacial adhesion strength on porcine intestine tissue in vitro and in the rat abdominal subfascial in vivo environment. This gecko-inspired medical adhesive may have potential applications for sealing wounds and for replacement or augmentation of sutures or staples.chemical cross-link ͉ medical adhesive ͉ nanotopography ͉ surgical suture

The elastic moduli of polythiophenes, regioregular poly(3-hexylthiophene) (P3HT) and poly-(2,5-bis(3-alkylthiophene-2-yl)thieno[3,2-b]thiophene) (pBTTT), are compared to their field effect mobility showing a proportional trend. The elastic moduli of the films are measured using a buckling-based metrology, and the mobility is determined from the electrical characteristics of bottom contact thin film transistors. Moreover, the crack onset strain of pBTTT films is shown to be less than 2.5%, whereas that of P3HT is greater than 150%. These results show that increased long-range order in polythiophene semiconductors, which is generally thought to be essential for improved charge mobility, can also stiffen and enbrittle the film. This work highlights the critical role of quantitative mechanical property measurements in guiding the development of flexible organic semiconductors.

The ability to generate microlens arrays in a rapid and costeffective manner allows for the fabrication of a variety of inexpensive functional devices, such as optical refractive elements or smart surfaces that mimic the patterned surfaces in biological systems used to control solid [1][2][3] and liquid adhesion. [4] A variety of strategies have been adopted for fabricating microlens structures. In general, they can be broadly classified into three categories: 1) surface-tension-driven techniques consisting of melt-reflow [5][6][7] and ink-jet printing; [8] 2) imprinting methods; [9,10] and 3) lithographic approaches such as grayscale photolithography [11,12] or interference lithography. [13,14] While these approaches demonstrate the ability to produce microlens arrays with uniform surface profiles, the techniques are either high-cost or require long fabrication times.In this paper, we introduce an alternative and novel approach for fabricating microlens arrays that is based on the confinement of surface wrinkles.[15] We demonstrate the ability to control the size and the arrangement of the microlenses through clever control of the geometric shape and material properties of the wrinkled regions. Our approach offers several advantages over previous methodologies of microlens fabrication, including: 1) the ability to create microlens arrays rapidly; 2) ease of tuning the dimensions of the microlenses; and 3) versatility in the process that allows the formation of microlens arrays on nonplanar substrates. We demonstrate the flexibility of our approach in patterning nonplanar surfaces by patterning a hemispherical surface with an array of microlenses, thereby forming a compound lens (Fig. 1). To fabricate the microlens arrays, we modified our previously developed methodology for generating wrinkle-pattern surfaces (Fig. 2a). [15] We began by selective ultraviolet/ ozone (UVO) oxidation of a crosslinked polydimethylsiloxane (PDMS) film to convert specific regions of the PDMS surface into a silicate thin film. The chemical modification created the necessary elastic-moduli differences on the PDMS surface to allow us to control and define the wrinkle formation. Following the silicate formation, the surface was coated with photopolymerizable n-butyl acrylate (nBA) and then covered with a glass superstrate. The acrylate monomer swelled the PDMS surface globally, but the surface wrinkles occurred only in regions where the moduli mismatch existed-that is, in the oxidized PDMS regions. This selective UVO allowed for the control of the spatial distribution of the wrinkle patterns ( Fig. 2b and c). The wrinkle patterns disappeared upon evaporation of the acrylate swelling agent; however, we stabilized these wrinkle structures through photopolymerization of the nBA. Finally, we lifted away the glass superstrate, which caused cohesive fracture of the polymerized poly(n-butyl acrylate) (PnBA) film. Due to the extreme interfacial moduli mismatch between the PnBA and silicate layers, the fracture path proceeded along the contours o...

Molecular layer-by-layer (mLbL) assembled thin-film composite membranes fabricated by alternating deposition of reactive monomers on porous supports exhibit both improved salt rejection and enhanced water flux compared to traditional reverse osmosis membranes prepared by interfacial polymerization. Additionally, the well-controlled structures achieved by mLbL deposition further lead to improved antifouling performance.

We introduce a new methodology to produce aligned, or patterned, surface wrinkles on a soft elastomer sans topography. The surface buckles orient through the manipulation of the local stress distributions, which we control by defining specific regions of local differences in the elastic moduli of the material.

Polymer gels are remarkable materials with physical structures that can adapt significantly and quite rapidly with changes in the local environment, such as temperature, light intensity, electrochemistry, and mechanical force. An interesting phenomenon observed in certain polymer gel systems is mechanochromism - a change in color due to a mechanical deformation. Mechanochromic photonic gels are periodically structured gels engineered with a photonic stopband that can be tuned by mechanical forces to reflect specific colors. These materials have potential as mechanochromic sensors because both the mechanical and optical properties are highly tailorable via incorporation of diluents, solvents, nanoparticles, or polymers, or the application of stimuli such as temperature, pH, or electric or strain fields. Recent advances in photonic gels that display strain-dependent optical properties are discussed. In particular, this discussion focuses primarily on polymer-based photonic gels that are directly or indirectly fabricated via self-assembly, as these materials are promising soft material platforms for scalable mechanochromic sensors.

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